Image forming apparatus

Abstract

To accurately detect the position of a transfer member even when an electric resistance value of the transfer member changes.SOLUTION: An image forming apparatus has: an image carrier 13; a transfer member 25; a moving unit 223 that moves the transfer member 25 to a plurality of positions with respect to the image carrier 13 including a contact position and a separated position; a driving unit 221 that drives the moving unit 223; an application unit 26 that applies voltage to the transfer member 25; detection units 204, 27 that detect at least either one of the voltage that the application unit 26 applies to the transfer member 25 or a current flowing in the transfer member 25 when the application unit 26 applies the voltage to the transfer member 25; and a position detection unit 206 that detects the position of the transfer member 25. The position detection unit 206 sets a second test voltage based on a detection result from the detection unit 204 acquired when the application unit 26 applies a first test voltage to the transfer member 25, and detects the position of the transfer member 25 based on a result of detection of a current value performed by the detection unit 27 acquired when the application unit 26 applies the second test voltage to the transfer member 25.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus using an electrophotographic method or an electrostatic recording method.

Background Art

[0002] Conventionally, in an image forming apparatus using an electrophotographic method or the like, a toner image formed on an image carrier such as a photosensitive drum or an intermediate transfer belt is transferred to a transfer material by applying a transfer voltage to a transfer member that forms a transfer portion by contacting the image carrier. As the transfer member, a transfer roller having an elastic layer formed of an elastic member on a core metal is used.

[0003] In such an image forming apparatus, when the image forming apparatus is left unattended (such as during long-term storage) with the transfer member in contact, local deformation may occur in the transfer member and the image carrier due to the pressure (contact pressure) applied to the contact portion. And depending on the degree of deformation, it may cause image defects due to poor transfer. Therefore, an image forming apparatus may be provided with a configuration (contact / separation mechanism) for separating the transfer member from the image carrier or reducing the contact pressure.

[0004] When adopting the above contact / separation mechanism, a mechanism for detecting the position (contact / separation state) of the transfer member is required. Patent Document 1 discloses a configuration for detecting the position of the transfer member by detecting the current value flowing through the transfer member.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in a configuration that detects the position of a transfer member by detecting the current value flowing through the transfer member, if the electrical resistance value of the transfer member changes, the current value flowing through the transfer member will change, which may lead to an incorrect detection of the transfer member's position.

[0007] Under conditions where the electrical resistance of the transfer material is high, current does not flow easily through the transfer material. Therefore, if the voltage applied to the transfer material is low, there is a possibility of false detection that the transfer material is separated from the image carrier when it is in contact with it. Thus, under conditions where the electrical resistance of the transfer material is high, the voltage applied to the transfer material needs to be increased. On the other hand, under conditions where the electrical resistance of the transfer material is low, current flows easily through the transfer material. Therefore, if the voltage applied to the transfer material is high, there is a possibility that an excessive current will flow when the transfer material is in contact with the image carrier. While measures can be taken to prevent damage to the current detection circuit and the transfer material even when such an excessive current flows, this may lead to increased costs. Thus, under conditions where the electrical resistance of the transfer material is low, the voltage applied to the transfer material needs to be low.

[0008] Factors that can cause variations in the electrical resistance of the transfer material include manufacturing variations, environmental conditions (temperature and humidity), and the degree of use of the transfer material. While it is possible to take measures to suppress variations in the electrical resistance of the transfer material due to these factors, this may lead to increased costs due to changes in the material of the transfer material.

[0009] Therefore, the objective of the present invention is to accurately detect the position of a transfer member even when the electrical resistance value of the transfer member changes. [Means for solving the problem]

[0010] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention is an image forming apparatus comprising: an image carrier that carries a toner image; a transfer member that contacts the image carrier and forms a transfer section for transferring a toner image from the image carrier to a transfer material; a moving unit that moves the transfer member to a plurality of positions relative to the image carrier, including a contact position in contact with the image carrier and a separated position separated from the image carrier; a driving unit that drives the moving unit; an application unit that applies a voltage to the transfer member; a detection unit that detects at least one of the voltage applied to the transfer member by the application unit or the current flowing through the transfer member when the application unit applies a voltage to the transfer member; and a position detection unit that detects the position of the transfer member, wherein the position detection unit sets a second test voltage based on the detection result of the detection unit obtained when the application unit applies a first test voltage to the transfer member, and detects the position of the transfer member based on the detection result of the current value obtained by the detection unit when the application unit applies the second test voltage to the transfer member. [Effects of the Invention]

[0011] According to the present invention, the position of the transfer member can be accurately detected even when the electrical resistance value of the transfer member changes. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus. [Figure 2] This is a block diagram showing the control modes of the main parts of an image forming apparatus. [Figure 3] This is a schematic diagram illustrating the operation of the secondary transfer attachment / detachment mechanism. [Figure 4] This diagram shows the relationship between the position of the fixing roller and the position of the secondary transfer roller. [Figure 5] This is a timing chart diagram illustrating the movement control of the secondary transfer roller. [Figure 6] This is a timing chart diagram illustrating the position detection operation in Example 1. [Figure 7]It is a flowchart diagram of the control in Example 1. [Figure 8] It is a flowchart diagram of the control in Example 1. [Figure 9] It is a flowchart diagram of the control in Example 1. [Figure 10] It is a flowchart diagram of the control in Example 1. [Figure 11] It is a timing chart diagram for explaining the position detection operation in Example 2. [Figure 12] It is a flowchart diagram of the control in Example 2. [Figure 13] It is a block diagram showing another example of the control mode of the image forming apparatus. [Figure 14] It is a timing chart diagram for explaining the method of calculating the electrical resistance value of the secondary transfer roller. [Figure 15] It is a timing chart diagram for explaining the position detection operation in Example 3. [Figure 16] It is a graph diagram for explaining the method of determining the voltage value Vp in Example 3. [Figure 17] It is a flowchart diagram of the control of Example 3. [Figure 18] It is a schematic diagram for explaining the fixing contact and separation mechanism.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0014] [Example 1] 1. Configuration of the Image Forming Apparatus First, the main configuration of the image forming apparatus 100 of this embodiment will be described. FIG. 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem type printer (color image forming apparatus) adopting an intermediate transfer method, which can form a full-color image using the electrophotographic method.

[0015] The image forming apparatus 100 has a plurality of image forming units (stations), the first, second, third, and fourth image forming units Sa, Sb, Sc, and Sd, which each form an image using toners of yellow (Y), magenta (M), cyan (C), and black (Bk), respectively. These four image forming units Sa, Sb, Sc, and Sd are arranged in a line at approximately constant intervals along the direction of movement of the surface onto which the image of the intermediate transfer belt 13, which will be described later, is transferred. In addition, elements with the same or corresponding functions or configurations provided for each color may be described collectively by omitting the a, b, c, and d at the end of the symbols indicating that they are elements for any of the colors. In this embodiment, the image forming unit S is composed of a photosensitive drum 1, a charging roller 2, an exposure device 11, a developing device 8, a primary transfer roller 10, a cleaning device 3, etc., which will be described later.

[0016] The image forming unit S has a photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor) serving as the first image carrier. The photosensitive drum 1 is constructed by laminating multiple layers of functional organic material on a cylindrical metal member, the carrier generation layer which generates charge upon exposure to light, and the charge transport layer which transports the generated charge. The outermost layer has low conductivity and is almost electrically insulating. The photosensitive drum 1 receives a driving force from a drive source (not shown) and rotates at a predetermined peripheral speed (process speed) in the direction of arrow R1 in the figure (counterclockwise).

[0017] The charging roller 2, a roller-type charging member used as a means of charging, contacts the photosensitive drum 1 and rotates in conjunction with the rotation of the photosensitive drum 1. As the charging roller 2 rotates, it charges the surface of the photosensitive drum 1 in a substantially uniform manner. The charging roller 2 is connected to a charging power supply 20, which acts as a charging voltage application unit. A DC voltage is applied to the charging roller 2 from the charging power supply 20 as a charging voltage (charging bias). As a result, the surface of the photosensitive drum 1 is charged by discharges generated in the minute gaps between the charging roller 2 and the photosensitive drum 1, which are formed upstream and downstream of the contact point between the charging roller 2 and the photosensitive drum 1 in the direction of rotation of the photosensitive drum 1.

[0018] The exposure apparatus 11, as an exposure means, consists of a scanner unit that scans laser light using a multifaceted mirror. The exposure apparatus 11 irradiates the photosensitive drum 1 with a scanning beam 12 that has been modulated based on an image signal.

[0019] The developing apparatus 8, as a developing means, comprises a developing container 5, a developing roller 4 as a developing member, and a developing agent application blade 7 as a developing agent regulating member, with toner as the developing agent contained inside the developing container 5. The developing roller 4 is connected to a developing power supply 21, which is a developing voltage application unit. An alternating voltage, which is a superposition of a DC voltage and an AC voltage, is applied to the developing roller 4 from the developing power supply 21 as the developing voltage (developing bias).

[0020] The cleaning device 3, as a cleaning means, includes a cleaning blade 41 as a cleaning member that contacts the photosensitive drum 1, and a cleaning container 42 that contains toner and other materials removed from the photosensitive drum 1 by the cleaning blade 41. The cleaning device 3 recovers toner and other materials remaining on the photosensitive drum 1.

[0021] Furthermore, the photosensitive drum 1, the charging roller 2 which acts on the photosensitive drum 1 as a process means, the developing device 8, and the cleaning device 3 together constitute a process cartridge 9 that can be detachably attached to the main body 101 of the image forming apparatus 100.

[0022] An intermediate transfer belt 13, which is an intermediate transfer body composed of an endless belt as a second image carrier, is positioned opposite the photosensitive drum 1 of each image forming unit S. The intermediate transfer belt 13 is stretched by three tension rollers: a secondary transfer opposing roller (hereinafter also simply referred to as "opposing roller") 15, a tension roller 14, and an auxiliary roller 19, which are tensioning members. The tension roller 14 is biased by a spring (not shown), which is a biasing member acting as a biasing means, in order to maintain appropriate tension (tension) of the intermediate transfer belt 13. The opposing roller 15 rotates in the direction of arrow R2 (clockwise) in the figure, receiving a driving force from a drive source (not shown). The intermediate transfer belt 13 rotates in a circular motion in the direction of arrow R3 (clockwise) in the figure as the opposing roller 15 rotates. The intermediate transfer belt 13 can move at approximately the same speed in the forward direction relative to the photosensitive drum 1 at the portion opposite to the photosensitive drum 1. The auxiliary roller 19, tension roller 14, and opposing roller 15 are electrically grounded (connected to ground). The opposing roller 15 is a roller with an outer diameter of 24.0 mm, constructed by covering an aluminum core (base) with an elastic layer (elastic part) made of EPDM rubber with a wall thickness of 0.5 mm. The opposing roller 15 has an electrical resistance of approximately 1 × 10⁻¹⁰ 5 The electrical resistance is adjusted by dispersing carbon, a conductive agent, in EPDM rubber to achieve an ohm (Ω) resistance.

[0023] On the inner circumferential surface of the intermediate transfer belt 13, primary transfer rollers 10a, 10b, 10c, and 10d, which are roller-type primary transfer members serving as primary transfer means, are provided, corresponding to each of the photosensitive drums 1a, 1b, 1c, and 1d. The primary transfer rollers 10 are positioned opposite the photosensitive drum 1 via the intermediate transfer belt 13, and contact the inner circumferential surface of the intermediate transfer belt 13, rotating in accordance with the movement of the intermediate transfer belt 13. The primary transfer rollers 10 contact the photosensitive drum 1 via the intermediate transfer belt 13, are pressed toward the photosensitive drum 1, and form a primary transfer section (primary transfer nip) N1 where the photosensitive drum 1 and the intermediate transfer belt 13 come into contact. The primary transfer rollers 10 are connected to a primary transfer power supply 22, which serves as a primary transfer voltage application unit. The primary transfer roller 10 is constructed by covering a core (base) made of a nickel-plated steel rod with an outer diameter of 5 mm with an elastic layer (elastic part) made of a foamed elastic material with an outer diameter of 14 mm. The primary transfer roller 10 has an electrical resistance of approximately 1 × 10⁻¹⁰ 6 The electrical resistance of the foamed elastic material is adjusted by incorporating a conductive agent so that it becomes Ω. The electrical resistance of the primary transfer roller 10 is 10 3 ~10 7 Being within the Ω range is preferable for achieving good image formation.

[0024] On the outer circumferential surface of the intermediate transfer belt 13, a secondary transfer roller 25, which is a roller-type secondary transfer member serving as a secondary transfer means, is positioned opposite the opposing roller 15. The secondary transfer roller 25 is movable toward and away from the outer circumferential surface of the intermediate transfer belt 13. The secondary transfer roller 25 is positioned opposite the opposing roller 15 via the intermediate transfer belt 13, and contacts the outer circumferential surface of the intermediate transfer belt 13, rotating in accordance with the movement of the intermediate transfer belt 13. The secondary transfer roller 25 contacts the opposing roller 15 via the intermediate transfer belt 25 and is pressed toward the opposing roller 15, forming a secondary transfer portion (secondary transfer nip) N2 where the intermediate transfer belt 13 and the secondary transfer roller 25 are in contact. The secondary transfer roller 25 is connected to a secondary transfer power supply 26, which serves as a secondary transfer voltage application unit. The secondary transfer power supply 26 is connected to a current detection circuit 27, which serves as a detection unit. The secondary transfer power supply 26 applies a voltage to the secondary transfer roller 25, and the current detection circuit 27 can detect the value of the current flowing through the secondary transfer roller 25. The secondary transfer roller 25 is constructed by covering a metal core (base) with an elastic layer (elastic part) made of foamed elastic material.

[0025] The fixing device 50, as a fixing means, includes a fixing roller (pressure roller) 51 and a cylindrical fixing film (fixing belt) 52 as fixing members (fixing rotating bodies). A heating member 53 is arranged on the inner circumferential surface side of the fixing film 52 to apply heat to the transfer material P via the fixing film 52. The fixing roller 51 is movable toward and away from the outer circumferential surface of the fixing film 52. The fixing roller 51 contacts the heating member 53 via the fixing film 52 and is pressed toward the heating member 53 to form a fixing portion (fixing nip) N3 where the fixing roller 51 and the fixing film 52 are in contact. The fixing roller 51 rotates by receiving a driving force from a fixing motor 221 (Figure 2) as a driving source, and the fixing film 52 rotates in conjunction with the rotation of the fixing roller 51.

[0026] Furthermore, the image forming apparatus 100 is equipped with a control unit (control board, controller) 200 that has electrical circuits for controlling the operation of each part of the image forming apparatus 100. The control unit 200 is equipped with a CPU 211 as a control means, a memory 212 as a storage means for storing various control information, and an input / output unit (I / F) 213 for controlling the exchange of signals between the control unit 200 and each part. The CPU 211 performs control related to the transport of the transfer material P, control related to the driving of the image forming unit S and the intermediate transfer belt 13, control related to image formation, and control related to fault detection. The memory 212 is composed of ROM (including rewritable ROM) and RAM, with control programs and data tables stored in the ROM, and data showing the detection results of various sensors and calculation results related to control stored in the RAM.

[0027] 2. Image Forming Process Next, the image forming operation of the image forming apparatus 100 in this embodiment will be described. When the control unit 200 receives an image signal from an external device (not shown), such as a personal computer, it starts the image forming operation. When the image forming operation starts, each photosensitive drum 1 and opposing roller 15, etc., starts rotating at a predetermined peripheral speed (process speed) due to the driving force from a drive source (not shown). In this embodiment, the process speed is 200 mm / s.

[0028] The surface of the rotating photosensitive drum 1 is uniformly charged by the charging roller 2. During the charging process, a charging voltage, which is a DC voltage with the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment), is applied to the charging roller 2 from the charging power supply 20. The charged surface of the photosensitive drum 1 is scanned and exposed by the exposure device 11, which irradiates it with a scanning beam 12 corresponding to the image information of the color components corresponding to each image forming section S, and an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 8 when toner is supplied, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In the developing device 8, the toner contained in the developing container 5 is negatively charged by the developer application blade 7 and applied to the developing roller 4. Also, during the developing process, a developing voltage containing a DC component with the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the developing roller 4 from the developing power supply 21. As a result, in the developing section where the developing roller 4 and the photosensitive drum 1 come into contact, toner moves from the developing roller 4 to the image portion of the electrostatic latent image on the photosensitive drum 1 and adheres to it. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed area (image area), where the absolute value of the potential has decreased due to exposure after uniform charging treatment (reverse development). In this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner during development, is negative polarity.

[0029] The toner image formed on the photosensitive drum 1 is transferred (primary transfer) to the rotating intermediate transfer belt 13 in the primary transfer section N1 by the action of the primary transfer roller 10. During the primary transfer process, the primary transfer roller 10 is supplied with a primary transfer voltage (primary transfer bias) from the primary transfer power supply 22, which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner. For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially superimposed onto the intermediate transfer belt 13 in a primary transfer process. As a result, four toner images corresponding to the desired color image are formed on the intermediate transfer belt 13.

[0030] The toner image formed on the intermediate transfer belt 13 is transferred (secondary transfer) in the secondary transfer section N2 by the action of the secondary transfer roller 25 onto the transfer material P which is being transported while being held between the intermediate transfer belt 13 and the secondary transfer roller 25. During the secondary transfer process, a secondary transfer voltage (secondary transfer bias), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 25 from the secondary transfer power supply 26. The transfer material (recording medium, recording material, sheet, paper) P, such as paper or an OHP sheet, is housed in a transfer material cassette 16. The transfer material P is fed from the transfer material cassette 16 to the transport roller 18 by the feeding roller 17, and then transported towards the secondary transfer section N2 by the transport roller 18.

[0031] The transfer material P onto which the toner image has been transferred is transported toward the fixing device 50 by the secondary transfer roller 25 and the opposing roller 15. The fixing device 50 heats and pressurizes the transfer material P in the fixing section N3. The unfixed toner image supported on the transfer material P is fixed (melted and solidified) onto the recording material P as the transfer material P passes through the fixing section N3. For example, when forming a full-color image, the four colors of toner on the transfer material P are melted and mixed in the fixing section N3 and fixed onto the transfer material P. After that, the transfer material P is discharged (output) to the outside of the main body 101 of the image forming apparatus 100 and loaded onto the discharge tray 60, which serves as a loading section provided on the top of the main body 101.

[0032] The image forming apparatus 100 is also equipped with sensors such as a resist sensor 110 and an ejection sensor 111 for detecting the transfer material P during the image forming operation described above.

[0033] On the other hand, toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residue toner) is removed from the photosensitive drum 1 and recovered by the cleaning device 3. In addition, a belt cleaning device 30, which serves as an intermediate transfer body cleaning means, is positioned on the outer circumferential surface side of the intermediate transfer belt 13, facing the opposing roller 15 via the intermediate transfer belt 13. Toner remaining on the intermediate transfer belt 13 after the secondary transfer (secondary transfer residue toner) is removed from the intermediate transfer belt 13 and recovered by the belt cleaning device 30. The belt cleaning device 30 is configured to have a cleaning blade 31 that contacts the outer circumferential surface of the intermediate transfer belt 13 at a position facing the opposing roller 15.

[0034] 3. Control Modes Figure 2 is a block diagram showing the control configuration for detecting (determining) the position of the secondary transfer roller 25 in the image forming apparatus 100 of this embodiment. The figure shows the functional blocks in the control unit 200 and the hardware 220 that operates under the control of the control unit 200.

[0035] The control unit 200 includes, as functional blocks, a drive control unit 202, a movement control unit 203, a voltage control unit 204, a current detection control unit 205, and a position detection control unit 206. In this embodiment, each of the above functional blocks is realized in the control unit 200 by the CPU 211 (Figure 1) executing a program stored in the memory 212 (Figure 1). Furthermore, in the control unit 200, the CPU 211 that realizes each of the above functional blocks mainly controls the operation (including acquisition of detection results) of the hardware 220 shown in Figure 2 via the input / output unit 213 (Figure 1) to perform processing related to the detection of the position of the secondary transfer roller 25. The hardware 220 includes a fixing motor 221, a fixing separation cam 222, a secondary transfer separation cam 223, a phase detection sensor 224, a secondary transfer roller 25, a secondary transfer power supply 26, and a current detection circuit 27.

[0036] The movement control unit 203 drives the fixing motor 221 via the drive control unit 202, thereby operating the fixing separation cam 222 and the secondary transfer separation cam 223, and moving the fixing roller 51 and the secondary transfer roller 25, respectively. In other words, the movement control unit 203 changes the position of the fixing roller 51 relative to the fixing film 52 (or heating member 53) and the position of the secondary transfer roller 25 relative to the intermediate transfer belt 13 (or opposing roller 15). The movement control unit 203 also detects the phase (position in the rotational direction) of the fixing separation cam 222, i.e., the position of the fixing roller 51, using the action of the phase detection sensor 224. The fixing separation cam 222 constitutes the fixing contact / separation mechanism 400 (Figure 18), which will be described later. The secondary transfer separation cam 223, as a moving part, constitutes the secondary transfer contact / separation mechanism 300 (Figure 2), which will be described later.

[0037] The position detection control unit 206 detects the position of the secondary transfer roller 25 through the actions of the voltage control unit 204, the current detection control unit 205, and the movement control unit 203. Specifically, as will be described in more detail later, the position detection control unit 206 moves the secondary transfer roller 25 using the movement control unit 203 and applies a voltage to the secondary transfer roller 25 from the secondary transfer power supply 26 using the voltage control unit 204. The position detection control unit 206 then detects the position of the secondary transfer roller 25 based on the detection result of the current value obtained by the current detection control unit 205 from the current detection circuit 27 when the above voltage is applied to the secondary transfer roller 25.

[0038] In this embodiment, the secondary transfer power supply 26 can apply a voltage to the secondary transfer roller 25 that is controlled to be approximately constant (constant voltage control) at a voltage value set by the voltage control unit 204. The voltage control unit 204 can detect (recognize) the voltage value of the voltage applied from the secondary transfer power supply 26 to the secondary transfer roller 25 based on the voltage value set for the secondary transfer power supply 26. In other words, this embodiment includes a voltage control unit 204h and a voltage detection unit that detects the voltage value of the voltage applied to the secondary transfer roller 25. The current detection circuit 27, acting as a current detection unit, detects the current value flowing through the secondary transfer roller 25 when the secondary transfer power supply 26 applies voltage to the secondary transfer roller 25. The current detection control unit 205 acquires the current value detection result from the current detection circuit 27. In this embodiment, the secondary transfer power supply 26 can apply a voltage to the secondary transfer roller 25 that is controlled to be approximately constant (constant current control) at a voltage value detected by the current detection circuit 27.

[0039] 4. Secondary transfer attachment / detachment mechanism Next, the secondary transfer contact / separation mechanism 300, which is a moving mechanism for moving the secondary transfer roller 25 to multiple positions relative to the intermediate transfer belt 13 in this embodiment, will be described. Figure 3 is a schematic diagram illustrating the operation of the secondary transfer contact / separation mechanism 300. Figures 3(a) to (d) each show one end of the secondary transfer roller 25 in the direction of its rotation axis, but the configuration of the other end is the same as shown (approximately symmetrical with respect to the center in the direction of the rotation axis of the secondary transfer roller 25).

[0040] In this embodiment, the secondary transfer contact / separation mechanism 300 consists of a secondary transfer separation cam 223, a fixing motor 221, and a bearing 301 for the secondary transfer roller 25. The secondary transfer separation cam 223 is rotatably provided at both ends of the opposing roller 15 in the direction of its rotation axis. The secondary transfer separation cam 223 is rotatable about a rotation axis coaxial with the rotation axis of the opposing roller 15. The bearing 301 for the secondary transfer roller 25 is provided at both ends of the secondary transfer roller 25 in the direction of its rotation axis and rotatably supports the secondary transfer roller 25. The bearing 301 for the secondary transfer roller 25 has a contact surface 302 that contacts the secondary transfer separation cam 223. The bearing 301 for the secondary transfer roller 25 is biased toward the intermediate transfer belt 13 by a secondary transfer pressing spring 304, which is a biasing member acting as a biasing means.

[0041] In this embodiment, the fixing motor 221 is a drive source for rotating the fixing roller 51 and the fixing film 52, and is also used as a drive source for rotating the fixing separation cam 222 and the secondary transfer separation cam 223. When the fixing motor 221 is rotated in a first direction (hereinafter also referred to as "forward rotation"), the fixing roller 51 and the fixing film 52 rotate, and the transfer material P can be conveyed in the fixing unit N3. On the other hand, when the fixing motor 221 is rotated in a second direction opposite to the first direction (hereinafter also referred to as "reverse rotation"), the fixing separation cam 222 rotates, and the fixing roller 51 can be moved to multiple positions relative to the fixing film 52. In this embodiment, the separation cam 22 moves the fixing roller 51 to a contact position in contact with the fixing film 52 and to a separated position separated from the fixing film 52. Furthermore, the fixing separation cam 222 and the secondary transfer separation cam 223 are driven and connected via a gear train 303, and the fixing separation cam 222 and the secondary transfer separation cam 223 are rotated in conjunction by the fixing motor 221. Therefore, when the fixing motor 221 is rotated in the reverse direction, the secondary transfer separation cam 223 rotates, and the secondary transfer roller 25 can be moved to multiple positions relative to the intermediate transfer belt 13. In this embodiment, the secondary transfer separation cam 223 moves the secondary transfer roller 25 to a contact position in contact with the intermediate transfer belt 13 and to a separated position separated from the intermediate transfer belt 13. As will be described later, in this embodiment, the secondary transfer separation cam 223 is capable of moving the secondary transfer roller 25 to two positions where the contact pressure of the secondary transfer roller 25 against the intermediate transfer belt 13 (or opposing roller 15) is different. Here, the position with relatively high contact pressure (first contact position) is simply called the "contact position," and the position with relatively low contact pressure (second contact position) is also called the "reduced pressure position." In this embodiment, the reduction ratio from the fixing separation cam 222 to the secondary transfer separation cam 223 is 2:1, and when the fixing separation cam 222 is rotated by approximately 180 degrees, the secondary transfer separation cam 223 rotates by approximately 90 degrees. In this embodiment, the phase detection sensor 224 detects the phase of the fixing separation cam 222, and the position of the fixing roller 51 (whether it is in the contact position or the separation position) is detected.In this embodiment, the fixing separation cam 222 and the secondary transfer separation cam 223 are configured to rotate in only one direction due to the reverse rotation of the fixing motor 221.

[0042] From the state where the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 (Figure 3(a)), the fixing motor 221 is rotated in the reverse direction, rotating the secondary transfer separation cam 223 by approximately 90 degrees. As a result, the bearing 301 of the secondary transfer roller 25 is pushed by the secondary transfer separation cam 223 and retracts away from the intermediate transfer belt 13, moving the secondary transfer roller 25 to a separated position (Figure 3(b)) away from the intermediate transfer belt 13. Next, from the state where the secondary transfer roller 25 is in a separated position (Figure 3(b)) away from the intermediate transfer belt 13, the fixing motor 221 is rotated in the reverse direction, rotating the secondary transfer separation cam 223 by approximately 90 degrees. As a result, the bearing 301 of the secondary transfer roller 25 moves towards the intermediate transfer belt 13, moving the secondary transfer roller 25 to a reduced pressure position (Figure 3(c)) where it contacts the intermediate transfer belt 13 in a reduced pressure state. The reduced pressure position is the contact position (second contact position) where the distance between the core metal of the secondary transfer roller 25 and the intermediate transfer belt 13 is greater than the contact position (first contact position) shown in Figure 3(a). Next, from the reduced pressure position (Figure 3(c)) where the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 under reduced pressure, the fixing motor 221 is rotated in the reverse direction, rotating the secondary transfer separation cam 223 by approximately 90 degrees. This moves the bearing 301 of the secondary transfer roller 25 closer to the intermediate transfer belt 13, and the secondary transfer roller 25 moves to the contact position (Figure 3(d)) where it is in contact with the intermediate transfer belt 13. The position of the secondary transfer roller 25 relative to the intermediate transfer belt 13 shown in Figure 3(d) is substantially the same as the position of the secondary transfer roller 25 relative to the intermediate transfer belt 13 shown in Figure 3(a). Next, from the state shown in Figure 3(d), the fixing motor 221 is rotated in the reverse direction, rotating the secondary transfer separation cam 223 by approximately 90 degrees. As a result of this operation, the bearing 301 of the secondary transfer roller 25 does not move substantially, and the secondary transfer roller 25 is maintained in the contact position (Figure 3(a)) where it is in contact with the intermediate transfer belt 13, as shown in Figure 3(a).

[0043] Figure 4 shows the relationship between the position of the fixing roller 51 (contact and separation state with respect to the fixing film 52) and the position of the secondary transfer roller 25 (contact and separation state with respect to the intermediate transfer belt 13). Hereinafter, the states in which the fixing roller 51 and the secondary transfer roller 25 are located at positions A, B, C, and D in Figure 4 will be referred to as "State A," "State B," "State C," and "State D," respectively.

[0044] State A is the state where the fixing roller 51 is in contact position and the secondary transfer roller 25 is in contact position (Figure 3(a)). From state A, rotating the fixing separation cam 222 by approximately 180 degrees and the secondary transfer separation cam 223 by approximately 90 degrees results in state B. State B is the state where the fixing roller 51 is in a separated position and the secondary transfer roller 25 is in a separated position (Figure 3(b)). From state B, rotating the fixing separation cam 222 by approximately 180 degrees and the secondary transfer separation cam 223 by approximately 90 degrees results in state C. State C is the state where the fixing roller 51 is in contact position and the secondary transfer roller 25 is in a reduced pressure position (Figure 3(c)). From state C, rotating the fixing separation cam 222 by approximately 180 degrees and the secondary transfer separation cam 223 by approximately 90 degrees results in state D. State D is the state where the fixing roller 51 is in the separated position and the secondary transfer roller 25 is in the contact position (Figure 3(d)). Then, from state D, rotating the fixing separation cam 222 by approximately 180 degrees and the secondary transfer separation cam 223 by approximately 90 degrees returns to state A.

[0045] Figure 18 is a schematic diagram of the fixing mechanism 400, which serves as a fixing movement mechanism for moving the fixing roller 51 to multiple positions relative to the fixing film 52 in this embodiment. In this embodiment, the fixing mechanism 400 consists of a fixing separation cam 222, a fixing motor 221, and a bearing 401 for the fixing roller 51. The fixing separation cam 222 is rotatably provided opposite to both ends of the fixing roller 51 in the direction of its rotation axis. The fixing separation cam 222 is rotatable about a rotation axis substantially parallel to the rotation axis of the fixing roller 51. The bearing 401 for the fixing roller 51 is provided at both ends of the fixing roller 51 in the direction of its rotation axis and rotatably supports the fixing roller 51. The bearing 401 for the fixing roller 51 has a contact surface 402 that contacts the fixing separation cam 222. The bearing 401 of the fixing roller 51 is biased toward the fixing film 52 by a fixing pressure spring 404, which is a biasing member acting as a biasing means. When the fixing motor 221 is rotated in reverse, the drive is transmitted to the fixing separation cam 222 via the gear train 403, causing the fixing separation cam 222 to rotate. As described above, in this embodiment, the fixing roller 51 can be moved between a contact position and a separation position relative to the fixing film 52 each time the fixing separation cam 222 rotates approximately 180 degrees. In this embodiment, the phase detection sensor 224 is capable of detecting when the fixing separation cam 222 is in a phase that positions the fixing roller 51 in contact with the fixing roller 51, and when the fixing separation cam 222 is in a phase that positions the fixing roller 52 in a separation position. In this embodiment, the phase detection sensor 224 is configured to have an optical sensor that detects a flag 225 provided on the fixing separation cam 222. Hereinafter, the signal that the phase detection sensor 224 inputs to the control unit 200 (movement control unit 203) (acquired by the movement control unit 203) when the fixing separation cam 222 is in a phase (phase range) that positions the fixing roller 52 in contact with it will be referred to as the "contact detection signal". Also, the signal that the phase detection sensor 224 inputs to the control unit 200 (movement control unit 203) (acquired by the movement control unit 203) when the fixing separation cam 222 is in a phase (phase range) that positions the fixing roller 52 in a separated position will be referred to as the "separation detection signal".

[0046] 5. Control of movement of the secondary transfer roller Next, using Figure 5(a), the movement control of the secondary transfer roller 25 by the control unit 200 (movement control unit 203) in this embodiment will be explained. Figure 5(a) is a timing chart showing the states of each part when the positions of the fixing roller 51 and the secondary transfer roller 25 are moved from state A to state B, from state B to state C, from state C to state D, and from state D to state A as shown in Figure 4. In Figure 5(a), t100 to t111 indicate the timings, respectively.

[0047] The movement control unit 203 reverses the rotation of the fixing motor 221 to begin movement from state A to state B (t100). When the movement control unit 203 detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separation detection signal (t101), it waits until time Tf has elapsed. Then, when time Tf has elapsed, the movement control unit 203 stops the fixing motor 221 and completes the movement to state B (t102). Movement from state B to state C, from state C to state D, and from state D to state A is performed similarly. In other words, the movement control unit 203 reverses the rotation of the fixing motor 221 to begin movement between each state (t103, t106, t109). Also, when the movement control unit 203 detects that the signal from the phase detection sensor 224 has switched (t104, t107, t110), it waits until time Tf has elapsed. Then, when time Tf has elapsed, the movement control unit 203 stops the fixing motor 221 and completes the movement between each state (t105, t108, t111).

[0048] In this embodiment, the time Tf from the detection of a switch in the signal from the phase detection sensor 224 until the fixing motor 221 is stopped (the movement of the secondary transfer roller 25 is completed) is set to 100 msec. However, this is not limited to this, and this time Tf can be set appropriately depending on the configuration of the phase detection sensor 224, etc. Note that this time Tf may be set differently for some or all of the movement of the secondary transfer roller 25 to the separated position, the reduced pressure position, and the contact position.

[0049] Furthermore, in this embodiment, the fixing motor 221 is stopped (the movement of the secondary transfer roller 25 is completed) after a predetermined time has elapsed since detecting that the signal from the phase detection sensor 224 has switched, but the system is not limited to this configuration. For example, the fixing motor 221 may be stopped (the movement of the secondary transfer roller 25 is completed) after the rotation distance of the fixing motor 221 reaches a predetermined distance since detecting that the signal from the phase detection sensor 224 has switched.

[0050] 6. Relationship between the position of the secondary transfer roller and the current value. Next, using Figure 5(b), the relationship between the position of the secondary transfer roller 25 in this embodiment and the detection result of the current value obtained by the control unit 200 (current detection control unit 205) from the current detection circuit 27 will be explained. Figure 5(b) is a timing chart diagram showing the voltage applied by the secondary transfer power supply 26 to the secondary transfer roller 25 and the detection result of the current value obtained by the current detection control unit 205 from the current detection circuit 27 during the operation shown in Figure 5(a). In this embodiment, the voltage applied from the secondary transfer power supply 26 to the secondary transfer roller 25 for detecting the position of the secondary transfer roller 25 is a positive polarity DC voltage.

[0051] When a voltage is applied to the secondary transfer roller 25 in states A, B, C, and D, the current values ​​shown in the figure are detected. Current value I1 is the current value when the secondary transfer roller 25 is in contact position (states A and D) or reduced pressure position (state C). Current value I2 is the current value when the secondary transfer roller 25 is in separated position (state B).

[0052] Here, the reduced pressure position (state C) is a reduced pressure state compared to the contact position (states A and D), but the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 (or opposing roller 15). Therefore, in this embodiment, the detection result of the current value acquired by the current detection control unit 205 when a voltage is applied to the secondary transfer roller 25 is the same for the contact position (states A and D) and the reduced pressure position (state C).

[0053] 7. Detection of the position of the secondary transfer roller Next, the detection (determination) of the position of the secondary transfer roller 25 by the control unit 200 (position detection control unit 206) in this embodiment will be described.

[0054] In this embodiment, detecting (determining) the position of the secondary transfer roller 25 means, more specifically, relating the phase of the fixing separation cam 222 at a predetermined time (for example, the present) to the position of the secondary transfer roller 25 (whether it is in contact or separation position). Specifically, it means storing the information necessary for this relationship in a predetermined storage area of ​​the storage means (memory 212 such as RAM). In other words, it is possible to determine how much (time or distance) the fixing separation cam 222 (fixing motor 221) needs to be rotated to position the secondary transfer roller 25 based on the state at the predetermined time. In particular, in this embodiment, it is possible to detect whether the fixing roller 51 is in contact position (state A or state C) based on the detection result of the phase detection sensor 224. And in this embodiment, if the fixing roller 51 is in contact position (state A or state C), it is known that the secondary transfer roller 25 is in contact position or reduced pressure position. Therefore, when the fixing roller 51 is in the contact position, it is possible to detect whether the secondary transfer roller 25 is in the contact position or the reduced pressure position (the state in which the secondary transfer roller 25 is in contact with the intermediate transfer belt 13) based on the detection result of the phase detection sensor 224. On the other hand, in this embodiment, when the fixing roller 51 is in the separated position (state B or state D), it is not possible to determine from the detection result of the phase detection sensor 224 whether the secondary transfer roller 25 is in the contact position or the separated position. Therefore, in this embodiment, the position detection control unit 206 detects the position of the secondary transfer roller 25 (whether it is in the contact position or the separated position) when the fixing roller 51 is in the separated position (state B or state D) based on the current value flowing through the secondary transfer roller 25. As mentioned above, in this embodiment, when the fixing motor 221 is rotated in reverse, the positions of the fixing roller 51 and the secondary transfer roller 25 are sequentially changed to state A, state B, state C, and state D. Therefore, by detecting the position of the secondary transfer roller 25 at at least one predetermined time point when the fixing roller 51 is in a separated position, the position of the secondary transfer roller 25 at any time point before or after that point can be detected based on the detection results of the phase detection sensor 224.

[0055] Specifically, in this embodiment, the position detection control unit 206 performs the following position detection operation (position determination operation) to detect (determine) the position of the secondary transfer roller 25. That is, in at least one of state A or state C, the position detection control unit 206 determines the voltage value required to supply a predetermined current value to the secondary transfer roller 25. State A and state C are states in which the phase detection sensor 224 detects that the fixing roller 51 is in contact position. In addition, in at least one of state B or state D, the position of the secondary transfer roller 25 is detected based on the current value that flows through the secondary transfer roller 25 when the voltage of the above-determined voltage value is applied to the secondary transfer roller 25. State B and state D are states in which the phase detection sensor 224 detects that the fixing roller 51 is in a separated position. In particular, in this embodiment, the current value is obtained for one state B or state D and for the other state, and the position of the secondary transfer roller 25 corresponding to each state is detected by comparing the obtained current values.

[0056] Figure 6 is a timing chart diagram of an example of the position detection operation in this embodiment. The figure shows an example in which the voltage value applied to the secondary transfer roller 25 is determined when detecting the position of the secondary transfer roller 25 in state C, and the position of the secondary transfer roller 25 is detected in state D and state B, respectively. For convenience, it is explained that the voltage value is determined in state C and the position of the secondary transfer roller 25 is detected in states D and B, but it is not known whether the voltage value was determined in state C or state A until the position of the secondary transfer roller 25 is detected. In Figure 6, t200 to t213 indicate the timings.

[0057] The position detection control unit 206 starts applying a voltage (first test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 when the fixing roller 51 is in contact position (t200). The position detection control unit 206 also waits until time Tv1 has elapsed for the voltage output to stabilize (t201). After time Tv1 has elapsed, the position detection control unit 206 controls the secondary transfer power supply 26 via the voltage control unit 204 in order to converge the current value obtained from the current detection circuit 27 via the current detection control unit 205 to a predetermined target current value It. In other words, if the obtained current value is greater than the target current value It, the voltage output value is lowered, and if the obtained current value is less than the target current value It, the voltage output value is increased (t201~t202). When the acquired current value converges to the target current value It (t202), the position detection control unit 206 calculates the average value (average voltage value) Vave of the voltage value set by the voltage control unit 204 on the secondary transfer power supply 26 R times at regular intervals Ts (total time Tr) (t203). The position detection control unit 206 stores this average voltage value Vave in a predetermined memory area (memory 212 such as RAM). Almost simultaneously, the position detection control unit 206 stops the application of voltage (first test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 by the voltage control unit 204 (t203). In this way, the position detection control unit 206 determines the voltage value Vave necessary to supply a predetermined current value It to the secondary transfer roller 25 as the voltage value to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25.

[0058] Here, the position detection control unit 206 determines a voltage value Vave such that the difference between the current value detected when the secondary transfer roller 25 is in a separated position and when the secondary transfer roller 25 is in a contact position is greater than or equal to a certain value, regardless of the electrical resistance value of the secondary transfer roller 25. The position detection control unit 206 also determines a voltage value Vave such that the current flowing through the secondary transfer roller 25 does not become excessive. In other words, the target current value It is set in this manner. This voltage value Vave may be equivalent to the secondary transfer voltage value applied to the secondary transfer roller 25 during secondary transfer, or it may be a voltage value whose absolute value is larger or smaller than the said secondary transfer voltage value. In this embodiment, this voltage value Vave is set so that its absolute value is smaller than the secondary transfer voltage value applied to the secondary transfer roller 25 during secondary transfer.

[0059] Next, the position detection control unit 206 moves the fixing roller 51 to the separated position using the movement control unit 203 in order to detect the current value flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. In other words, the position detection control unit 206 starts the fixing motor 221 to rotate in the reverse direction (t204), and when it detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t205), it waits until time Tf has elapsed. Then, when time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t206). Almost simultaneously with this, the position detection control unit 206 starts applying a voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 using the voltage control unit 204 (t206). This voltage value is the voltage value (average voltage value) Vave determined in t203. After a time Tv1 has elapsed for the voltage output to stabilize (t207), the position detection control unit 206 uses the current detection control unit 205 to acquire the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti). The position detection control unit 206 then calculates the average value (average current value) Iave1 of the current flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in a separated position (t208). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). Almost simultaneously, the position detection control unit 206 uses the voltage control unit 204 to stop the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 (t208).

[0060] Next, the position detection control unit 206 uses the movement control unit 203 to move the fixing roller 51 back to the separated position in order to detect the current value flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. In other words, the position detection control unit 206 waits from the time Tv2 elapsed, after stopping the application of the voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 (t208), until the output of the secondary transfer power supply 26 stops. Then, after time Tv2 has elapsed, the position detection control unit 206 starts the fixing motor 221 to rotate in the reverse direction and starts the movement of the fixing roller 51 so that it goes through the contact position and then back to the separated position (t209). After that, the position detection control unit 206 waits until time Tf elapses, after detecting that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t210). Then, after time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t211). Almost simultaneously, the position detection control unit 206 starts applying a voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t211). This voltage value is the voltage value (average voltage value) Vave determined at t203. After time Tv1 has elapsed for the voltage output to stabilize (t212), the position detection control unit 206 uses the current detection control unit 205 to acquire the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti). Then, the position detection control unit 206 calculates the average value (average current value) Iave2 of the current flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position (t213). The position detection control unit 206 stores this average current value Iave2 in a predetermined memory area (memory 212 such as RAM). Almost simultaneously, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t213).

[0061] The position detection control unit 206 compares the average current value Iave1 at the first position of the secondary transfer roller 25 with the average current value Iave2 at the second position of the secondary transfer roller 25. The position detection control unit 206 then determines that the larger current value is state D (the secondary transfer roller 25 is in contact position) and the smaller current value is state B (the secondary transfer roller 25 is in separation position). The position detection control unit 206 also stores information relating the current position of the secondary transfer roller 25 (contact position or separation position) to the phase of the fixing separation cam 222 in a predetermined memory area (memory 212 such as RAM).

[0062] In this embodiment, the voltage value Vave required to supply a predetermined current to the secondary transfer roller 25 is determined when the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 (in the contact position or reduced pressure position). This voltage value Vave is then determined as the voltage value to be applied to the secondary transfer roller 25 when detecting its position. Therefore, the average current value Iave1 detected when the secondary transfer roller 25 is in the contact position will be close to the target current value It. On the other hand, the average current value Iave2 detected when the secondary transfer roller 25 is in the separated position will be smaller than the average current value Iave1 detected when it is in the contact position.

[0063] Thus, according to this embodiment, the current flowing through the secondary transfer roller 25 while it is in contact with the intermediate transfer belt 13 can be brought close to a predetermined current value It, regardless of the electrical resistance value of the secondary transfer roller 25. Therefore, the position of the secondary transfer roller 25 (whether it is in contact or separated from the belt) can be accurately detected (determined), regardless of the electrical resistance value of the secondary transfer roller 25. Furthermore, it is possible to suppress the flow of excessive current through the secondary transfer roller 25 and simplify the configuration of the current detection circuit 27 and the secondary transfer roller 25.

[0064] Here, as an example, we have described how to determine the voltage value Vave when the secondary transfer roller 25 is in the reduced pressure position (state C). It is also possible to determine the voltage value Vave in the same way when the secondary transfer roller 25 is in the contact position (state A).

[0065] Furthermore, in this embodiment, the obtained average voltage value Vave was determined as the voltage value applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25, but this is not the only way to do so. For example, the voltage value obtained by performing a predetermined process, such as multiplying the obtained average voltage value by a predetermined coefficient, may be determined as the voltage value applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25.

[0066] 8. Procedure for position detection operation Next, the procedure for position detection operation in this embodiment will be explained using Figures 7 to 10. Figure 7 is a flowchart showing the procedure for position detection operation in this embodiment. Figures 8 to 10 are flowcharts showing the procedures for some of the processes performed in the procedure shown in Figure 7. In this embodiment, this position detection operation is performed, for example, during preparatory operations such as when the image forming apparatus 100 is powered on or when it wakes up from sleep mode.

[0067] The position detection control unit 206 checks whether the fixing roller 51 is in contact position (S101). If it is not in contact (No in S101), the movement control unit 203 performs a contact operation to move the fixing roller 51 to the contact position (S102). Figure 8(a) is a flowchart showing the procedure for the contact operation of the fixing roller 51 in S102 of Figure 7. The movement control unit 203 reverses the rotation of the fixing motor 221 (S201) and waits until it detects that the signal from the phase detection sensor 224 has switched from a separation detection signal to a contact detection signal (No in S202). Once the movement control unit 203 detects that the signal from the phase detection sensor 224 has switched from a separation detection signal to a contact detection signal (Yes in S202), it waits until time Tf has elapsed (No in S203). Then, when time Tf has elapsed (Yes in S203), the movement control unit 203 stops the fixing motor 221 (S204) and completes the contact operation of the fixing roller 51.

[0068] Next, the position detection control unit 206 has the voltage control unit 204 apply a voltage (first test voltage) to the secondary transfer roller 25 (S103) and waits until time Tv1 has elapsed (S104, "No"). Then, once time Tv1 has elapsed (S104, "Yes"), the position detection control unit 206 executes coarse adjustment control (S105) and fine adjustment control (S106) to converge the current value flowing through the secondary transfer roller 25 to a predetermined target current value It. Once the current value flowing through the secondary transfer roller 25 has converged to the target current value It through coarse adjustment control (S105) and fine adjustment control (S106), the position detection control unit 206 calculates the average voltage value (average voltage value) Vave (S107). Almost simultaneously, the position detection control unit 206 stops applying the voltage (first test voltage) to the secondary transfer roller 25 via the voltage control unit 204 (S108).

[0069] Fig. 9(a) is a flowchart showing the procedure of coarse control in S105 of Fig. 7. The position detection control unit 206 acquires the current value flowing through the secondary transfer roller 25 by the current detection control unit 205 (S401). When the absolute value of the difference between the target current value It and the detected current value is greater than the threshold value Ith1 (\"No\" in S402), and when the detected current value is greater than the target current value It (\"Yes\" in S403), the position detection control unit 206 lowers the absolute value of the voltage by Vd1 (S404). Also, when the detected current value is less than or equal to the target current value It (\"No\" in S403), the position detection control unit 206 raises the absolute value of the voltage by Vd1 (S405). After that, the position detection control unit 206 waits until the time Ts elapses (\"No\" in S406). Then, when the time Ts has elapsed (\"Yes\" in S406), the position detection control unit 206 acquires the current value again (S401). Thus, when the absolute value of the difference between the target current value It and the detected current value becomes less than or equal to the threshold value Ith1, the position detection control unit 206 ends the coarse control (\"Yes\" in S402). Fig. 9(b) is a flowchart showing the procedure of fine control in S106 of Fig. 7. The position detection control unit 所 206 acquires the current value flowing through the secondary transfer roller 25 by the current detection control unit 205 (S501). When the absolute value of the difference between the target current value It and the detected current value is greater than the threshold value Ith2 (<Ith1) (\"No\" in S502), and when the detected current value is greater than the target current value It (\"Yes\" in S503), the position detection control unit 206 lowers the absolute value of the voltage by Vd2 (<Vd1) (S504). Also, when the detected current value is less than or equal to the target current value It (\"No\" in S503), the position detection control unit 206 raises the absolute value of the voltage by Vd2 (S505). After that, the position detection control unit 206 waits until the time Ts elapses (\"No\" in S506). Then, when the time Ts has elapsed (\"Yes\" in S506), the position detection control unit 206 acquires the current value again (S501). Thus, when the absolute value of the difference between the target current value It and the detected current value becomes less than or equal to the threshold value Ith2, the position detection control unit 206 ends the fine control (\"Yes\" in S502). Fig. 10 is a flowchart showing the procedure of the process for calculating the average voltage value Vave in S107 of Fig. 7.The position detection control unit 206 obtains the current value flowing through the secondary transfer roller 25 from the current detection control unit 205 (S601). If the number of acquired current values ​​is less than R (No in S602) and the detected current value is greater than the target current value It (Yes in S603), the position detection control unit 206 lowers the absolute value of the voltage by Vd2 (S604). Also, if the detected current value is less than the target current value It (No in S603, Yes in S605), the position detection control unit 206 raises the absolute value of the voltage by Vd2 (S606). Note that if the detected current value and the target current value It are equal (No in S605), the position detection control unit 206 does not change the voltage. After that, the position detection control unit 206 waits for time Ts to elapse (No in S607). Then, when time Ts has elapsed (Yes in S607), the position detection control unit 206 acquires the current value again (S601). In this way, when the number of acquired current values ​​reaches R or more (Yes in S602), the position detection control unit 206 calculates the average voltage value Vave (S608). The position detection control unit 206 stores this average voltage value Vave in a predetermined memory area (memory 212 such as RAM). In other words, the position detection control unit 206 determines the voltage value Vave necessary to supply a predetermined current value It to the secondary transfer roller 25 as the voltage value to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25.

[0070] As described above, once the application of voltage (first test voltage) to the secondary transfer roller 25 is stopped (S108), the position detection control unit 206 performs a separation operation to move the fixing roller 51 to the separated position using the movement control unit 203 (S109). Figure 8(b) is a flowchart showing the procedure for the separation operation of the fixing roller 51 in S109 of Figure 7. The movement control unit 203 reverses the rotation of the fixing motor 221 (S301) and waits until it detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separation detection signal (No in S302). Once the movement control unit 203 detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separation detection signal (Yes in S302), it waits until time Tf has elapsed (No in S303). Then, when time Tf has elapsed (Yes in S303), the movement control unit 203 stops the fixing motor 221 (S304) and completes the separation operation of the fixing roller 51.

[0071] Next, the position detection control unit 206 has the voltage control unit 204 apply a voltage of the above voltage value Vave (second test voltage) to the secondary transfer roller 25 (S110), and waits until time Tv1 has elapsed (S111, "No"). Then, once time Tv1 has elapsed (S111, "Yes"), the position detection control unit 206 has the current detection control unit 205 acquire the current values ​​flowing through the secondary transfer roller 25 S times at time intervals Ts (S112~S114). Once the position detection control unit 206 has acquired the current values ​​for S times (S113, "Yes"), it calculates the average value of the acquired current values ​​(average current value) Iave1 (S115). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). Furthermore, the position detection control unit 206 stops the application of voltage (second test voltage) to the secondary transfer roller 25 by the voltage control unit 204 (S116).

[0072] Next, the position detection control unit 206 checks whether the average current value has been calculated twice (S117). If the average current value has not been calculated twice (No in S117), the position detection control unit 206 moves the position of the secondary transfer roller 25 and calculates the second average current value Iave2, similar to the process of calculating the first average current value Iave1 (S109-S116). Once the second average current value Iave2 has been calculated (Yes in S117), the position detection control unit 206 compares the absolute value of the difference between the first average current value Iave1 and the second average current value Iave2 with the error threshold Ierr (S118). If the error threshold Ierr is greater than the absolute value of the difference (Yes in S118), the position detection control unit 206 determines that it has failed to detect the position of the secondary transfer roller 25 (S119). Furthermore, if the absolute value of the above difference is greater than or equal to the error threshold Ierr (No in S118), the position detection control unit 206 compares the first average current value Iave1 with the second current value Iave2 (S120). If the second average current value Iave2 is greater than the first average current value Iave1 (Yes in S120), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the contact position (S121). Also, if the first average current value Iave1 is greater than the second average current value Iave2 (No in S120), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the separation position (S122). In S121 and S122, the position detection control unit 206 stores information relating the current position of the secondary transfer roller 25 with the phase of the fixing separation cam 222 in a predetermined storage area (memory 212 such as RAM).

[0073] In S119, the position detection control unit 206 may display an error on a display unit provided on the operation unit (not shown) of the image forming apparatus 100, or on a display unit of an external device (not shown) such as a personal computer connected to the image forming apparatus 100. Instead of or in addition to displaying an error on the display unit, the unit may also generate sound from a sound-producing unit or emit light from a light-emitting unit. In this case, the position detection operation may be re-executed, for example, until a predetermined number of times is reached.

[0074] In this example, the detected current value was converged to the target current value using a two-stage control system consisting of coarse and fine adjustment. However, this is not the only method; for example, the detected current value could be converged to the target current value using a single-stage control system corresponding to the fine adjustment described above.

[0075] 9. Effects As described above, the image forming apparatus 100 of this embodiment includes an image carrier (intermediate transfer belt) 13 that carries a toner image, a transfer member (secondary transfer roller 25) that contacts the image carrier 13 to form a transfer section (secondary transfer section) N2 that transfers the toner image from the image carrier 13 to the transfer material P, a moving section (secondary transfer separation cam) 223 that moves the transfer member 25 to multiple positions relative to the image carrier 13, including a contact position in contact with the image carrier 13 and a separated position separated from the image carrier 13, a drive section (fixing motor) 221 that drives the moving section 223, an application section (secondary transfer power supply) 26 that applies voltage to the transfer member 25, and an application section 2 The device includes a detection unit (voltage control unit 204, current detection circuit 27) that detects at least one of the voltage applied by the device to the transfer member 25 or the current flowing through the transfer member 25 when a voltage is applied to the transfer member 25 by the application unit 26, and a position detection unit (position detection control unit) 206 that detects the position of the transfer member 25. The position detection unit 206 sets a second test voltage based on the detection result of the detection unit 204 obtained when the first test voltage is applied to the transfer member 25 by the application unit 26, and detects the position of the transfer member 25 based on the current value detection result of the detection unit 27 obtained when the second test voltage is applied to the transfer member 25 by the application unit 26.

[0076] In this embodiment, the position detection unit 206 performs a position detection operation in which the transfer member 25 is moved to multiple positions relative to the image carrier 13 by the moving unit 223 and the position of the transfer member 25 is detected. Based on the detection result obtained by the detection unit 204 when a first test voltage is applied to the transfer member 25 during the position detection operation, the second test voltage to be applied to the transfer member 25 during the position detection operation is set. In addition, in this embodiment, the position detection unit 206 sets the second test voltage based on the voltage value detection result obtained by the detection unit 204 when the voltage value of the first test voltage is adjusted so that the current value flowing through the transfer member 25 approaches a predetermined current value. In particular, in this embodiment, during the position detection operation, the position detection unit 206 moves the transfer member 25 to a first position which is either a contact position or a separated position and to a second position which is the other, and applies the second test voltage to the transfer member 25 by the application unit 26 when the transfer member 25 is in the first position and the second position respectively, and obtains the current value detection result by the detection unit 27. The position detection unit 206 outputs at least one of the following: information indicating that the first position is a contact position or information indicating that the second position is a separated position, if the current value obtained when the transfer member 25 is in the first position is greater than the current value obtained when the transfer member 25 is in the second position. The position detection unit 206 also outputs at least one of the following: information indicating that the first position is a separated position or information indicating that the second position is a contact position, if the current value obtained when the transfer member 25 is in the first position is less than the current value obtained when the transfer member 25 is in the second position. For example, the position detection unit 206 can output this information to the memory 212 and store it in the memory 212. In this embodiment, the position detection unit 206 also outputs information indicating that the detection of the position of the transfer member 25 failed if the difference between the current value obtained when the transfer member 25 is in the first position and the current value obtained when the transfer member 25 is in the second position is less than a predetermined value. For example, the position detection unit 206 can output the information to the display unit of the operation unit provided on the image forming apparatus 100 or to the display unit of an external device connected to the image forming apparatus 100, and display it on the display unit.

[0077] In this embodiment, the position detection unit 206 sets a second test voltage based on the detection result of the detection unit 204 acquired when the transfer member 25 is in contact position. In particular, in this embodiment, the image forming apparatus 100 has a driven part (fixing separation cam) 222 that is driven by a drive unit 221 common to the moving unit 223 to move to a first predetermined position and a second predetermined position, and a sensor (phase detection sensor) 224 that detects the position of the driven part 222. In this embodiment, when the sensor 224 detects that the driven part 222 is in the first predetermined position, the transfer member 25 is in contact position, and when the sensor 224 detects that the driven part 222 is in the second predetermined position, the transfer member 25 is in either the contact position or the separation position. In this embodiment, the position detection unit 206 sets the second test voltage based on the detection result of the detection unit 204 obtained when the sensor 224 detects that the driven unit 222 is in a first predetermined position, and detects the position of the transfer member 25 based on the current value detection result of the detection unit 204 obtained when the sensor 224 detects that the driven unit 222 is in a second predetermined position. In this embodiment, the driven unit 222 is a member that moves the fixing member (fixing roller) 51 that fixes the toner image transferred to the transfer material P onto the transfer material P. In this embodiment, the moving unit 223 can move the transfer member 25 to a first contact position and a second contact position, and the contact pressure of the transfer member 25 against the image carrier 13 is greater when the transfer member 25 is in the first contact position than when the transfer member 25 is in the second contact position.

[0078] Furthermore, according to this embodiment, even if the electrical resistance value of the secondary transfer roller 25 changes (varies), the position of the secondary transfer roller 25 can be accurately detected (determined). In addition, it is possible to suppress excessive current flow in the secondary transfer roller 25, thereby simplifying the configuration of the current detection circuit 27 and the secondary transfer roller 25.

[0079] Furthermore, in this embodiment, in order to determine the voltage value when detecting the position of the secondary transfer roller 25, the state in which the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 can be detected by the phase detection sensor 224 used to detect the position of the fixing roller 51. Thus, according to this embodiment, since no dedicated sensor or the like is provided to detect (determine) the position of the secondary transfer roller, the device configuration can be simplified and miniaturized.

[0080] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0081] 1. Overview of this embodiment In Example 1, during the position detection operation, the voltage value required to supply a predetermined current to the secondary transfer roller 25 was determined, and this voltage value was set as the voltage value applied to the secondary transfer roller 25 when detecting its position. In contrast, in this embodiment, during the position detection operation, the electrical resistance value of the secondary transfer roller 25 is determined based on the current value that flows through the secondary transfer roller 25 when a predetermined voltage is applied to it. Then, based on this electrical resistance value, the voltage value applied to the secondary transfer roller 25 when detecting its position is determined. As a result, in this embodiment, it is not necessary to perform controls such as coarse adjustment control and fine adjustment control described in Example 1 to determine the voltage value required to supply a predetermined current. Therefore, according to this embodiment, the processing time for the position detection operation can be shortened compared to Example 1.

[0082] 2. Detection of the position of the secondary transfer roller Next, the detection (determination) of the position of the secondary transfer roller 25 by the control unit 200 (position detection control unit 206) in this embodiment will be described.

[0083] In this embodiment, the position detection control unit 206 performs the following position detection operation. Specifically, in at least one of state A or state C, the position detection control unit 206 applies a voltage of a predetermined value to the secondary transfer roller 25. At that time, it detects the current value flowing through the secondary transfer roller 25 to determine the electrical resistance value of the secondary transfer roller 25, and based on this electrical resistance value, it determines the voltage value to be applied to the secondary transfer roller 25 when detecting its position. States A and state C are states in which the phase detection sensor 224 detects that the fixing roller 51 is in contact position. In addition, in at least one of state B or state D, the position of the secondary transfer roller 25 is detected based on the current value flowing through the secondary transfer roller 25 when the voltage of the above-determined value is applied to the secondary transfer roller 25. States B and state D are states in which the phase detection sensor 224 detects that the fixing roller 51 is in separated position. In particular, in this embodiment, current values ​​are acquired for one state (state B or state D) and the other state (state D), and the position of the secondary transfer roller 25 corresponding to each state is detected by comparing the acquired current values. In this embodiment, the voltage applied from the secondary transfer power supply 26 to the secondary transfer roller 25 in order to determine the electrical resistance value of the secondary transfer roller 25 and to detect the position of the secondary transfer roller 25 is a positive polarity DC voltage.

[0084] Figure 11 is a timing chart diagram of an example of the position detection operation in this embodiment. The figure shows an example in which the voltage value applied to the secondary transfer roller 25 is determined when detecting the position of the secondary transfer roller 25 in state C, and the position of the secondary transfer roller 25 is detected in state D and state B, respectively. For convenience, it is explained that the voltage value is determined in state C and the position of the secondary transfer roller 25 is detected in states D and B, but it is not known whether the voltage value was determined in state C or state A until the position of the secondary transfer roller 25 is detected. t300 to t312 in Figure 11 indicate the timings.

[0085] The position detection control unit 206, with the fixing roller 51 in contact with the surface, starts applying a predetermined voltage value Vi (first test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t300). The position detection control unit 206 also waits until time Tv1 has elapsed for the voltage output to stabilize (t301). Once time Tv1 has elapsed, the position detection control unit 206 uses the current detection control unit 205 to acquire the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti). The position detection control unit 206 then calculates the average value (average current value) Iave0 of the acquired current values ​​(t302). The position detection control unit 206 stores this average current value Iave0 in a predetermined memory area (memory 212 such as RAM). At approximately the same time, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t302). The position detection control unit 206 also determines the electrical resistance value of the secondary transfer roller 25 and the voltage value to be applied to the secondary transfer roller 25 when detecting its position, as described below. This voltage value only needs to be determined before the application of voltage to the secondary transfer roller 25 begins in order to detect its position, as will be described later.

[0086] The position detection control unit 206 calculates the electrical resistance value Ri of the secondary transfer roller 25 using the following formula 1, based on the voltage value Vi and the calculated average current value Iave0. Ri = Vi ÷ Iave 0 (Equation 1)

[0087] Furthermore, the position detection control unit 206 calculates the voltage value Vp required to supply a predetermined current value Ip to the secondary transfer roller 25 when the secondary transfer roller 25 is in contact with the secondary transfer roller 25, based on the calculated electrical resistance value Ri, using the following formula 2. Vp=Ip×Ri (Formula 2)

[0088] As described above, the position detection control unit 206 determines the electrical resistance value Ri of the secondary transfer roller 25 and further determines the voltage value Vp to be applied to the secondary transfer roller 25 when detecting its position. The position detection control unit 206 stores the determined voltage value Vp in a predetermined storage area (memory 212 such as RAM).

[0089] By setting the voltage value applied to the secondary transfer roller 25 when detecting its position to the above voltage value Vp, the current value detected when the secondary transfer roller 25 is in contact can be made close to the above current value Ip. Here, if the electrical resistance of the secondary transfer roller 25 is low, the average current value Iave0 calculated as described above will be a large value. On the other hand, if the electrical resistance of the secondary transfer roller 25 is high, the average current value Iave0 calculated as described above will be a small value. Regardless of the electrical resistance of the secondary transfer roller 25, the position detection control unit 206 determines a voltage value Vp such that the difference between the current value detected when the secondary transfer roller 25 is in a separated position and the current value detected when the secondary transfer roller 25 is in contact is greater than or equal to a certain value. The position detection control unit 206 also determines a voltage value Vp such that the current flowing through the secondary transfer roller 25 does not become excessive. In other words, the above current value Ip is set in this manner. This voltage value Vp may be equivalent to the secondary transfer voltage value applied to the secondary transfer roller 25 during secondary transfer, or it may be a voltage value that is larger or smaller in absolute value than the said secondary transfer voltage value. In this embodiment, this voltage value Vp is set to be smaller in absolute value than the secondary transfer voltage value applied to the secondary transfer roller 25 during secondary transfer. Furthermore, the above Vi may be equivalent to the secondary transfer voltage value applied to the secondary transfer roller 25 during secondary transfer, or it may be a voltage value that is larger or smaller in absolute value than the said secondary transfer voltage value. In this embodiment, this voltage value Vi is set to be smaller in absolute value than the secondary transfer voltage value (and furthermore, the above voltage value Vp) applied to the secondary transfer roller 25 during secondary transfer.

[0090] Next, the position detection control unit 206 moves the fixing roller 51 to the separated position using the movement control unit 203 in order to detect the current value flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. That is, it starts the fixing motor 221 to rotate in the reverse direction (t303), and when it detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t304), it waits until time Tf has elapsed. Then, when time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t305). At almost the same time, the position detection control unit 206 starts applying the voltage Vp (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 using the voltage control unit 204 (t305). After a time Tv1 has elapsed for the voltage output to stabilize (t306), the position detection control unit 206 acquires the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti) using the current detection control unit 205. The position detection control unit 206 then calculates the average value (average current value) Iave1 of the current flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in a separated position (t307). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). Almost simultaneously, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 using the voltage control unit 204 (t307).

[0091] Next, the position detection control unit 206 uses the movement control unit 203 to move the fixing roller 51 back to the separated position in order to detect the current value flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. In other words, the position detection control unit 206 waits from the time Tv2 elapsed, until the output of the secondary transfer power supply 26 stops, after stopping the application of the voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 (t307) as described above. Then, after time Tv2 has elapsed, the position detection control unit 206 starts the fixing motor 221 to rotate in the reverse direction and starts the movement of the fixing roller 51 so that it goes through the contact position and then back to the separated position (t308). After that, the position detection control unit 206 waits until time Tf elapses, after detecting that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t309). Then, after time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t310). Almost simultaneously, the position detection control unit 206 starts applying the voltage Vp (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t310). After time Tv1 has elapsed for the voltage output to stabilize (t311), the position detection control unit 206 uses the current detection control unit 205 to acquire the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti). Then, the position detection control unit 206 calculates the average value (average current value) Iave2 of the current flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position (t312). The position detection control unit 206 stores this average current value Iave2 in a predetermined memory area (memory 212 such as RAM). At approximately the same time, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t312).

[0092] The position detection control unit 206 compares the average current value Iave1 at the first position of the secondary transfer roller 25 with the average current value Iave2 at the second position of the secondary transfer roller 25. The position detection control unit 206 then determines that the larger current value is state D (the secondary transfer roller 25 is in contact position) and the smaller current value is state B (the secondary transfer roller 25 is in separation position). The position detection control unit 206 also stores information relating the current position of the secondary transfer roller 25 (contact position or separation position) to the phase of the fixing separation cam 222 in a predetermined memory area (memory 212 such as RAM).

[0093] In this embodiment, the electrical resistance value Ri of the secondary transfer roller 25 is determined when the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 (in the contact position or reduced pressure position). Then, based on the electrical resistance value Ri, the voltage value Vp required to supply a predetermined current value Ip to the secondary transfer roller 25 is determined, and this voltage value Vp is determined as the voltage value to be applied to the secondary transfer roller 25 when detecting its position. Therefore, the average current value Iave1 detected when the secondary transfer roller 25 is in the contact position will be close to the predetermined current value Ip. On the other hand, the average current value Iave2 detected when the secondary transfer roller 25 is in the separated position will be a smaller current value than the average current value Iave1 detected when it is in the contact position.

[0094] Thus, according to this embodiment, the current flowing through the secondary transfer roller 25 while it is in contact with the intermediate transfer belt 13 can be brought closer to a predetermined current value Ip, regardless of the electrical resistance value of the secondary transfer roller 25. Therefore, the position of the secondary transfer roller 25 (whether it is in contact or separated from the belt) can be accurately detected (determined), regardless of the electrical resistance value of the secondary transfer roller 25. Furthermore, by suppressing the flow of excessive current through the secondary transfer roller 25, it is possible to simplify the configuration of the current detection circuit 27 and the secondary transfer roller 25.

[0095] Here, as an example, we have described how to determine the voltage value Vp when the secondary transfer roller 25 is in the reduced pressure position (state C). It is also possible to determine the voltage value Vp in the same way when the secondary transfer roller 25 is in the contact position (state A).

[0096] Furthermore, in this embodiment, the electrical resistance value of the secondary transfer roller 25 was determined, but the process is not limited to determining the electrical resistance value itself. Current values ​​and voltage values ​​correlated with the electrical resistance value may also be used in the process.

[0097] 3. Procedure for position detection operation Next, the procedure for position detection in this embodiment will be explained using Figure 12. Figure 12 is a flowchart showing the procedure for position detection in this embodiment.

[0098] The position detection control unit 206 checks whether the fixing roller 51 is in contact position (S701). If it is not in contact (No in S701), the movement control unit 203 performs a contact operation to move the fixing roller 51 to the contact position (S702). The procedure for this contact operation is the same as the procedure shown in Figure 8(a) described in Example 1. Next, the position detection control unit 206 has the voltage control unit 204 apply a voltage of a predetermined voltage value Vi (first test voltage) to the secondary transfer roller 25 (S703) and waits until time Tv1 has elapsed (No in S704). Then, when time Tv1 has elapsed (Yes in S704), the position detection control unit 206 has the current detection control unit 205 acquire the current values ​​flowing through the secondary transfer roller 25 S times at time intervals Ts (S705~S707). The position detection control unit 206, after acquiring S current values ​​(Yes in S706), calculates the average value (average current value) Iave0 of the acquired current values ​​(S708). The position detection control unit 206 stores this average current value Iave0 in a predetermined memory area (memory 212 such as RAM). The position detection control unit 206 also calculates the electrical resistance value Ri using the aforementioned equation 1 (S709), and further calculates the voltage value Vp using the aforementioned equation 2 (S710). The position detection control unit 206 stores this voltage value Vp in a predetermined memory area (memory 212 such as RAM). In other words, the position detection control unit 206 determines the voltage value Vp to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25. Almost simultaneously, the position detection control unit 206 stops the application of voltage (first test voltage) to the secondary transfer roller 25 using the voltage control unit 204 (S711).

[0099] Next, the position detection control unit 206 performs a separation operation to move the fixing roller 51 to a separated position using the movement control unit 203 (S712). The procedure for this separation operation is the same as the procedure shown in Figure 8(b) described in Example 1. Next, the position detection control unit 206 has the voltage control unit 204 apply a voltage of the above voltage value Vp (second test voltage) to the secondary transfer roller 25 (S713) and waits until time Tv1 has elapsed (S714, "No"). Then, once time Tv1 has elapsed (S714, "Yes"), the position detection control unit 206 has the current detection control unit 205 acquire the current values ​​flowing through the secondary transfer roller 25 S times at time intervals Ts (S715~S717). Once the position detection control unit 206 has acquired the current values ​​for S times (S716, "Yes"), it calculates the average value (average current value) Iave1 of the acquired current values ​​(S718). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). The position detection control unit 206 also stops the application of voltage (second test voltage) to the secondary transfer roller 25 by the voltage control unit 204 (S719).

[0100] Next, the position detection control unit 206 checks whether the average current value has been calculated twice (S720). If the average current value has not been calculated twice (No in S720), the position detection control unit 206 moves the position of the secondary transfer roller 25 and calculates the second average current value Iave2, similar to the process for calculating the first average current value Iave1 (S712-S719). Once the second average current value Iave2 has been calculated (Yes in S720), the position detection control unit 206 compares the absolute difference between the first average current value Iave1 and the second average current value Iave2 with the error threshold Ierr (S721). If the error threshold Ierr is greater than the absolute difference (Yes in S721), the position detection control unit 206 determines that it has failed to detect the position of the secondary transfer roller 25 (S722). Furthermore, if the absolute value of the above difference is greater than or equal to the error threshold Ierr (No in S721), the position detection control unit 206 compares the first average current value Iave1 with the second average current value Iave2 (S723). If the second average current value Iave2 is greater than the first average current value Iave1 (Yes in S723), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the contact position (S724). Also, if the first average current value Iave1 is greater than the second average current value Iave2 (No in S723), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the separated position (S725). In S724 and S725, the position detection control unit 206 stores information relating the current position of the secondary transfer roller 25 with the phase of the fixing separation cam 222 in a predetermined storage area (memory 212 such as RAM).

[0101] 4. Effects As described above, in this embodiment, the position detection unit 206 sets a second test voltage based on the current value detection result obtained by the detection unit 27 when a first test voltage of a predetermined voltage value is applied to the transfer member 25, and the predetermined voltage value. In particular, in this embodiment, the position detection unit 206 determines the electrical resistance value of the transfer member 25 based on the current value detection result obtained when the first test voltage is applied to the transfer member 25, and the predetermined voltage value, and sets a second test voltage based on the electrical resistance value.

[0102] Furthermore, according to this embodiment, even when the electrical resistance value of the secondary transfer roller 25 changes, the position of the secondary transfer roller 25 can be accurately detected (determined). In addition, it is possible to suppress excessive current flow to the secondary transfer roller 25, thereby simplifying the configuration of the current detection circuit 27 and the secondary transfer roller 25. Moreover, according to this embodiment, the processing time for the position detection operation can be shortened compared to Embodiment 1.

[0103] [Example 3] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0104] 1. Overview of this embodiment In Examples 1 and 2, the voltage value applied to the secondary transfer roller 25 when detecting its position was determined during the position detection operation. In contrast, in this embodiment, the voltage value applied to the secondary transfer roller 25 when detecting its position during the position detection operation is determined based on the electrical resistance value of the secondary transfer roller 25 obtained before executing the position detection operation. In particular, in this embodiment, the electrical resistance value of the secondary transfer roller 25 obtained during the image forming operation (more specifically, the pre-processing operation (pre-rotation operation), which is a preparatory operation performed before starting the image forming operation and performing secondary transfer of the toner image) is used. As a result, in this embodiment, it is not necessary to perform control to apply voltage to the secondary transfer roller 25 in order to determine the voltage value during the position detection operation. Therefore, according to this embodiment, the processing time of the position detection operation can be shortened compared to Examples 1 and 2.

[0105] 2. Control Modes Figure 13 is a block diagram showing the control configuration for detecting (determining) the position of the secondary transfer roller 25 in the image forming apparatus 100 of this embodiment. The control configuration in this embodiment shown in Figure 13 is substantially the same as the control configurations in embodiments 1 and 2 shown in Figure 2. However, in this embodiment, the control unit 200 further includes a resistance value calculation unit 207 as a functional block. In addition, in this embodiment, the hardware 220 that operates under the control of the control unit 200 includes an environmental sensor 226. The resistance value calculation unit 207 determines the electrical resistance value of the secondary transfer roller 25 during image forming operation through the action of the voltage control unit 204 and the current detection control unit 205. The environmental sensor 226 is an example of an environmental detection means that detects at least one of the temperature or humidity inside or outside the image forming apparatus 100, and in this embodiment, it is composed of a temperature and humidity sensor that detects the temperature and humidity inside the image forming apparatus 100. In this embodiment, the position detection control unit 206 determines the absolute moisture content based on the temperature and humidity detection results obtained from the environmental sensor 226, and uses the determined absolute moisture content to determine the voltage value to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25.

[0106] 3. Detection of the electrical resistance value of the secondary transfer roller The detection of the electrical resistance value of the secondary transfer roller 25 by the control unit 200 (resistance value calculation unit 207) in this embodiment will be explained. Figure 14 is a timing chart showing the state of each part during the image forming operation in this embodiment. t400 to t407 in Figure 14 indicate timings, respectively.

[0107] When the preprocessing for the image formation operation begins, the resistance calculation unit 207 starts applying a voltage (first test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 while the fixing roller 51 and the secondary transfer roller 25 are in contact position (t400). The resistance calculation unit 207 also waits until time Tv1 has elapsed for the voltage output to stabilize (t401). After time Tv1 has elapsed, the resistance calculation unit 207 controls the secondary transfer power supply 26 via the voltage control unit 204 in order to converge the current value acquired from the current detection circuit 27 by the current detection control unit 205 to a predetermined target current value Ipre. In other words, if the acquired current value is greater than the target current value Ipre, the voltage output value is lowered, and if the acquired current value is less than the target current value Ipre, the voltage output value is increased (t401~t402). When the acquired current value converges to the target current value Ipre (t402), the resistance value calculation unit 207 calculates the average value (average voltage value) Vavepre of the voltage set by the voltage control unit 204 to the secondary transfer power supply 26 R times at regular intervals Ts (total time Tr) (t403). The position detection control unit 206 stores this average voltage value Vavepre in a predetermined memory area (memory 212 such as RAM). The resistance value calculation unit 207 also determines the electrical resistance value of the secondary transfer roller 25 and the secondary transfer voltage value to be applied to the secondary transfer roller 25 during secondary transfer as follows. Note that this electrical resistance value and secondary transfer voltage value only need to be determined before the application of the secondary transfer voltage begins, as will be described later.

[0108] The resistance value calculation unit 207 calculates the electrical resistance value Rpre of the secondary transfer roller 25 based on the target current value Ipre and the calculated Vavepre using the following formula 3. Rpre=Vavepre÷Ipre (Formula 3)

[0109] Furthermore, the resistance value calculation unit 207 calculates the voltage value Vprint required to supply a predetermined current value Iprint to the secondary transfer roller 25 during the secondary transfer operation, based on the calculated electrical resistance value Rpre, using the following formula 4. Vprint=α×Iprint×Rpre+β (Formula 4)

[0110] As described above, the resistance calculation unit 207 determines the electrical resistance value Rpre of the secondary transfer roller 25 and further determines the secondary transfer voltage value Vprint to be applied to the secondary transfer roller 25 during secondary transfer. The resistance calculation unit 207 stores the determined electrical resistance value Rpre of the secondary transfer roller 25 and the determined secondary transfer voltage value Vprint to be applied to the secondary transfer roller 25 during secondary transfer in a predetermined storage area (memory 212 such as RAM).

[0111] In the secondary transfer operation, a transfer material P is present between the secondary transfer roller 25 and the intermediate transfer belt 13 (or opposing roller 15). Therefore, the electrical resistance value of the secondary transfer section N2 is higher than the electrical resistance value Rpre calculated as described above by the amount of the transfer material P. α and β in equation 4 above are coefficients that take into account the increase in electrical resistance value due to the transfer material P, and are coefficients that can be uniquely determined by environmental conditions such as temperature and humidity, and conditions such as the basis weight of the transfer material P.

[0112] Next, the control unit 200 sets the voltage value to be applied to the secondary transfer roller 25 by the voltage control unit 204 to the voltage value Vprint a certain time Tva before the tip of the transfer material P reaches the secondary transfer section N2 (t404). The control unit 200 also waits until a certain time Tvb has elapsed after the tip of the transfer material P reaches the secondary transfer section N2 (t405). Then, after a certain time Tvb has elapsed, the control unit 200 controls the secondary transfer power supply 26 by the voltage control unit 204 in order to converge the current value obtained from the current detection circuit 27 by the current detection control unit 205 to a predetermined target current value Iprint. In other words, if the obtained current value is greater than the target current value Iprint, the voltage output value is lowered, and if the obtained current value is less than the target current value Iprint, the voltage output value is increased (t405~t406). Furthermore, the control unit 200 sets the voltage value to be applied to the secondary transfer roller 25 by the voltage control unit 204 to the voltage value Vprint a certain time Tvc before the rear end of the transfer material P reaches the secondary transfer section N2 (t406). The control unit 200 also waits until a certain time Tvd has elapsed after the rear end of the transfer material P reaches the secondary transfer section N2, and then stops applying voltage to the secondary transfer roller 25 by the voltage control unit 204 (t407).

[0113] Thus, in this embodiment, the resistance value calculation unit 207 determines the electrical resistance value Rpre of the secondary transfer roller 25 during the preprocessing stage of the image forming operation. This electrical resistance value Rpre is used to determine the secondary transfer voltage value Vprint during the secondary transfer operation, and is also used to determine the voltage value applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25 in the position detection operation, as will be described later.

[0114] 4. Detection of the position of the secondary transfer roller Next, the detection (determination) of the position of the secondary transfer roller 25 by the control unit 200 (position detection control unit 206) in this embodiment will be described.

[0115] Figure 15 is a timing chart diagram of an example of the position detection operation in this embodiment. This figure shows an example in which the position of the secondary transfer roller 25 in state D and state B is detected using the voltage value determined based on the electrical resistance value of the secondary transfer roller 25 obtained during the image formation operation as described above. t500 to t509 in Figure 15 indicate the timings, respectively.

[0116] The position detection control unit 206 moves the fixing roller 51 to the separated position using the movement control unit 203 in order to detect the current value flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. That is, it starts the fixing motor 221 to rotate in the reverse direction (t500), and when it detects that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t501), it waits until time Tf has elapsed. Then, when time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t502). In addition, before executing the position detection operation, the position detection control unit 206 determines the voltage value Vp to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25 based on the electrical resistance value Rpre of the secondary transfer roller 25 obtained during the image forming operation. This voltage value Vp only needs to be determined before the application of voltage to the secondary transfer roller 25 begins in order to detect its position, as will be described later. The calculation may also be performed while the secondary transfer roller 25 is moving.

[0117] Figure 16 is a graph illustrating the relationship between the absolute moisture content detected by the environmental sensor 226 and the voltage value output by the secondary transfer power supply 26, for explaining the method of determining the voltage value Vp in this embodiment. The position detection control unit 206 stores in advance the voltage values ​​Vh1, Vh2, and Vh3 required to flow a current value Ip for each of the absolute moisture content E1, E2, and E3 when the electrical resistance value of the secondary transfer roller 25 is the highest expected electrical resistance value Rh, in a predetermined storage area (memory 212 such as RAM). Similarly, the position detection control unit 206 stores in advance the voltage values ​​Vl1, Vl2, and Vl3 required to flow a current value Ip for each of the absolute moisture content E1, E2, and E3 when the electrical resistance value of the secondary transfer roller 25 is the lowest expected electrical resistance value Rl, in a predetermined storage area (memory 212 such as RAM). Here, as shown in Figure 16, when the absolute moisture content during the position detection operation is Ep and the electrical resistance value of the secondary transfer roller 25 obtained during the image formation operation is Rp, the position detection control unit 206 calculates the voltage value Vp as follows. That is, in the relationship between the absolute moisture content and the voltage value when the electrical resistance value is Rl, the position detection control unit 206 calculates the voltage value Vlp required to flow the current value Ip when the absolute moisture content is Ep by linear interpolation from Vl2 and Vl3. Similarly, in the relationship between the absolute moisture content and the voltage value when the electrical resistance value is Rh, the voltage value Vhp required to flow the current value Ip when the absolute moisture content is Ep is calculated by linear interpolation from Vh2 and Vh3. Then, in the relationship between the absolute moisture content and the voltage value when the electrical resistance value is Rp, the voltage value Vp required to flow the current value Ip is calculated from the relationship between the voltage value Vlp and the voltage value Vhp.

[0118] Then, the position detection control unit 206 starts applying the voltage Vp (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t502). After the time Tv1 for the voltage output to stabilize has elapsed (t503), the position detection control unit 206 acquires the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti) via the current detection control unit 205. Then, the position detection control unit 206 calculates the average value (average current value) Iave1 of the current flowing through the secondary transfer roller 25 at the first position of the secondary transfer roller 25 when the fixing roller 51 is in a separated position (t504). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). At approximately the same time, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t504).

[0119] Next, the position detection control unit 206 uses the movement control unit 203 to move the fixing roller 51 back to the separated position in order to detect the current value flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position. In other words, the position detection control unit 206 waits from the time Tv2 elapsed, after stopping the application of the voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 (t504), until the output of the secondary transfer power supply 26 stops. Then, after time Tv2 has elapsed, the position detection control unit 206 starts the fixing motor 221 to rotate in the reverse direction and starts the movement of the fixing roller 51 so that it goes through the contact position and then back to the separated position (t505). After that, the position detection control unit 206 waits until time Tf elapses, after detecting that the signal from the phase detection sensor 224 has switched from a contact detection signal to a separated detection signal (t506). Then, after time Tf has elapsed, the position detection control unit 206 stops the fixing motor 221 and completes the movement of the fixing roller 51 (t507). Almost simultaneously, the position detection control unit 206 starts applying the voltage Vp (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t507). After time Tv1 has elapsed for the voltage output to stabilize (t508), the position detection control unit 206 uses the current detection control unit 205 to acquire the current value detected by the current detection circuit 27 S times at regular intervals Ts (total time Ti). Then, the position detection control unit 206 calculates the average value (average current value) Iave2 of the current flowing through the secondary transfer roller 25 at the second position of the secondary transfer roller 25 when the fixing roller 51 is in the separated position (t509). The position detection control unit 206 stores this average current value Iave2 in a predetermined memory area (memory 212 such as RAM). At approximately the same time, the position detection control unit 206 stops the application of voltage (second test voltage) from the secondary transfer power supply 26 to the secondary transfer roller 25 via the voltage control unit 204 (t509).

[0120] The position detection control unit 206 compares the average current value Iave1 at the first position of the secondary transfer roller 25 with the average current value Iave2 at the second position of the secondary transfer roller 25. The position detection control unit 206 then determines that the larger current value is state D (the secondary transfer roller 25 is in contact position) and the smaller current value is state B (the secondary transfer roller 25 is in separation position). The position detection control unit 206 also stores information relating the current position of the secondary transfer roller 25 (contact position or separation position) to the phase of the fixing separation cam 222 in a predetermined memory area (memory 212 such as RAM).

[0121] In this embodiment, during the image forming operation, the electrical resistance value Rpre of the secondary transfer roller 25 is determined when the secondary transfer roller 25 is in contact with the intermediate transfer belt 13 (in the contact position). During the position detection operation, the voltage value Vp required to supply a predetermined current value Ip to the secondary transfer roller 25 is determined based on the electrical resistance value Rpre and the absolute moisture content Ep. This voltage value Vp is then determined as the voltage value applied to the secondary transfer roller 25 when detecting its position. Therefore, the average current value Iave1 detected when the secondary transfer roller 25 is in the contact position will be close to the predetermined current value Ip. On the other hand, the average current value Iave2 detected when the secondary transfer roller 25 is in the separated position will be smaller than the average current value Iave1 detected at the contact position.

[0122] Thus, according to this embodiment, the current flowing through the secondary transfer roller 25 while it is in contact with the intermediate transfer belt 13 can be brought closer to a predetermined current value Ip, regardless of the electrical resistance value of the secondary transfer roller 25. Therefore, the position of the secondary transfer roller 25 (whether it is in contact or separated from the belt) can be accurately detected (determined), regardless of the electrical resistance value of the secondary transfer roller 25. Furthermore, by suppressing the flow of excessive current through the secondary transfer roller 25, it is possible to simplify the configuration of the current detection circuit 27 and the secondary transfer roller 25.

[0123] In this embodiment, the electrical resistance value of the secondary transfer roller 25 was determined during the image formation operation, but this is not the only way to do so. For example, the electrical resistance value of the secondary transfer roller 25 may be determined during operations other than the image formation operation that are performed before the position detection operation, such as calibration (image density control or position shift correction control) or power-on processing (during control operations, adjustment operations).

[0124] Furthermore, during position detection, the system may calculate the electrical resistance value Rpre based on the voltage values ​​Vavepre and Ipre.

[0125] 5. Procedure for position detection operation Next, the procedure for position detection in this embodiment will be explained using Figure 17. Figure 17 is a flowchart showing the procedure for position detection in this embodiment.

[0126] The position detection control unit 206 checks whether the fixing roller 51 is in a separated position (S801). If it is not separated (No in S801), the movement control unit 203 performs a separation operation to move the fixing roller 51 to the separated position (S802). The procedure for this separation operation is the same as the procedure shown in Figure 8(b) described in Example 1. Next, the position detection control unit 206 calculates a voltage value Vp based on the electrical resistance value Rpre of the secondary transfer roller 25 obtained during the image formation operation and the absolute moisture content Ep detected by the environmental sensor 226, as explained using Figure 16 (S803). The position detection control unit 206 stores this voltage value Vp in a predetermined memory area (memory 212 such as RAM). In other words, the position detection control unit 206 determines the voltage value Vp to be applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25.

[0127] Next, the position detection control unit 206 has the voltage control unit 204 apply a voltage of the above voltage value Vp (second test voltage) to the secondary transfer roller 25 (S804), and waits until time Tv1 has elapsed (S805 "No"). Then, once time Tv1 has elapsed (S805 "Yes"), the position detection control unit 206 has the current detection control unit 205 acquire the current values ​​flowing through the secondary transfer roller 25 S times at time intervals Ts (S806~S808). Once the position detection control unit 206 has acquired the current values ​​for S times (S807 "Yes"), it calculates the average value (average current value) Iave1 of the acquired current values ​​(S809). The position detection control unit 206 stores this average current value Iave1 in a predetermined memory area (memory 212 such as RAM). Furthermore, the position detection control unit 206 stops the application of voltage (second test voltage) to the secondary transfer roller 25 by the voltage control unit 204 (S810).

[0128] Next, the position detection control unit 206 checks whether the average current value has been calculated twice (S811). If the average current value has not been calculated twice (No in S811), the position detection control unit 206 moves the position of the secondary transfer roller 25 and calculates the second average current value Iave2, similar to the process of calculating the first average current value Iave1 (S802-S810). Once the second average value Iave2 has been calculated (Yes in S811), the position detection control unit 206 compares the absolute difference between the first average current value Iave1 and the second average current value Iave2 with the error threshold Ierr (S812). If the error threshold Ierr is greater than the absolute difference (Yes in S812), the position detection control unit 206 determines that it has failed to detect the position of the secondary transfer roller 25 (S813). Furthermore, if the absolute value of the above difference is greater than or equal to the error threshold Ierr (No in S812), the position detection control unit 206 compares the first average current value Iave1 with the second average current value Iave2 (S814). If the second average current value Iave2 is greater than the first average current value Iave1 (Yes in S814), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the contact position (S815). Also, if the first average current value Iave1 is greater than the second average current value Iave2 (No in S814), the position detection control unit 206 determines that the current position of the secondary transfer roller 25 is the separation position (S816). In S815 and S816, the position detection control unit 206 stores information relating the current position of the secondary transfer roller 25 with the phase of the fixing separation cam 222 in a predetermined storage area (memory 212 such as RAM).

[0129] 6. Effects As described above, in this embodiment, the position detection unit 206 performs a position detection operation in which the transfer member 25 is moved to multiple positions relative to the image carrier 13 by the moving unit 223 and the position of the transfer member 25 is detected. Before performing the position detection operation, a first test voltage is applied to the transfer member 25 and based on the detection result of the detection unit 27 obtained, the second test voltage to be applied to the transfer member 25 in the position detection operation is set. In this embodiment, the position detection unit 206 sets the second test voltage based on the detection result of the detection unit 27 obtained when the first test voltage is applied to the transfer member 25 in a preparatory operation when performing the image forming operation. In this embodiment, the position detection unit 206 sets the second test voltage based on the voltage value detection result of the detection unit 27 obtained when the voltage value of the first test voltage is adjusted so that the current value flowing through the transfer member 25 approaches a predetermined current value, and the predetermined current value. In particular, in this embodiment, the position detection unit 206 determines the electrical resistance value of the transfer member based on the detection result of the voltage value obtained by applying a first test voltage to the transfer member 25 and the predetermined current value, and sets a second test voltage based on the electrical resistance value.

[0130] Furthermore, according to this embodiment, even when the electrical resistance value of the secondary transfer roller 25 changes, the position of the secondary transfer roller 25 can be accurately detected (determined). In addition, it is possible to suppress excessive current flow to the secondary transfer roller 25, thereby simplifying the configuration of the current detection circuit 27 and the secondary transfer roller 25. Moreover, according to this embodiment, the processing time for the position detection operation can be shortened compared to embodiments 1 and 2.

[0131] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0132] In the above-described embodiment, a separation cam was used to move the fixing roller 51 and the secondary transfer roller 25. However, the present invention is not limited to this configuration, and for example, a separation lever may be used to move the fixing roller 51 and the secondary transfer roller 25. The separation lever can be configured to swing, for example, to move the bearing member of the fixing roller 51 and the bearing member of the secondary transfer roller 25 toward or toward the opposing members of the fixing roller 51 and the secondary transfer roller 25, respectively.

[0133] Furthermore, in the above-described embodiment, the current flowing through the secondary transfer roller 25 was detected when the secondary transfer roller 25 was in either a contact position or a separated position, and the position of the secondary transfer roller 25 was detected (determined) based on the difference. This makes it possible to detect (determine) with greater accuracy whether the secondary transfer roller 25 is in a contact position or a separated position. In addition, this also makes it possible to detect with greater accuracy when the detection of the position of the secondary transfer roller 25 has failed. However, the present invention is not limited to this configuration. For example, in the above-described embodiment, the voltage value applied to the secondary transfer roller 25 when detecting the position of the secondary transfer roller 25 was set so that a current of a predetermined value flows when the secondary transfer roller 25 is in a contact position. In such a case, the voltage of the said voltage value may be applied to the secondary transfer roller 25, and it may be detected (determined) that the secondary transfer roller 25 is in a contact position if a current of a predetermined threshold or higher flows, and in a separated position if only a current of less than the threshold flows.

[0134] Furthermore, although the present invention was applied to a tandem-type image forming apparatus (color image forming apparatus) in the above-described embodiments, the present invention can also be applied to, for example, a monochrome image forming apparatus using only black. In this case, for example, the present invention can be applied to a transfer unit that transfers a toner image from an image carrier such as a photosensitive drum to a transfer material.

[0135] Furthermore, although the transfer member was a roller-shaped member in the above-described embodiment, the present invention is not limited to such a configuration. The transfer member may be a fixed or rotatable brush-shaped member having elastic brush fibers, or an elastic (flexible) film-like (sheet-like) member, etc. [Explanation of Symbols]

[0136] 1 Photosensitive drum 13 Intermediate transfer belt 15. Secondary transfer opposing roller 25 Secondary transfer roller 26 Secondary Transfer Power Supply 27 Current detection circuit 50 Fixing device 51 Fixing roller 100 Image forming apparatus 200 Control Unit 222 Fixing Separation Cam 223 Secondary transfer separation cam

Claims

1. an image carrier that carries a toner image; a transfer member that contacts the image carrier and forms a transfer section that transfers a toner image from the image carrier to a transfer material; a moving unit that moves the transfer member to a plurality of positions relative to the image carrier, including a contact position where the transfer member is in contact with the image carrier and a spaced position where the transfer member is spaced from the image carrier; a drive unit that drives the moving unit; an application unit that applies a voltage to the transfer member; a detection unit that detects at least one of a voltage applied to the transfer member by the application unit and a current that flows through the transfer member when the application unit applies a voltage to the transfer member; a position detection unit that detects the position of the transfer member; and The position detection unit sets a second test voltage based on the detection result obtained by the detection unit when the application unit applies a first test voltage to the transfer member, and detects the position of the transfer member based on the detection result of the current value obtained by the detection unit when the application unit applies the second test voltage to the transfer member.

2. The image forming apparatus described in claim 1, characterized in that the position detection unit performs a position detection operation in which the moving unit moves the transfer member to multiple positions relative to the image carrier and detects the position of the transfer member, and sets the second test voltage to be applied to the transfer member in the position detection operation based on the detection result of the detection unit obtained by applying the first test voltage to the transfer member in the position detection operation.

3. The image forming apparatus according to claim 2, characterized in that the position detection unit sets the second test voltage based on the detection result of the voltage value obtained by the detection unit when adjusting the voltage value of the first test voltage so that the current value flowing through the transfer member approaches a predetermined current value.

4. The image forming apparatus according to claim 2, characterized in that the position detection unit sets the second test voltage based on the detection result of the current value obtained by the detection unit when the first test voltage of a predetermined voltage value is applied to the transfer member and the predetermined voltage value.

5. The image forming apparatus according to claim 4, wherein the position detection unit determines the electrical resistance value of the transfer member based on the detection result of the current value obtained by applying the first test voltage to the transfer member and the predetermined voltage value, and sets the second test voltage based on the electrical resistance value.

6. The image forming apparatus described in claim 1, characterized in that the position detection unit performs a position detection operation in which the moving unit moves the transfer member to multiple positions relative to the image carrier and detects the position of the transfer member, and sets the second test voltage to be applied to the transfer member in the position detection operation based on the detection result of the detection unit obtained by applying the first test voltage to the transfer member before performing the position detection operation.

7. The image forming apparatus according to claim 6, characterized in that the position detection unit sets the second test voltage based on the detection result obtained by applying the first test voltage to the transfer member during a preparatory operation when performing an image forming operation.

8. The image forming apparatus of claim 6 or 7, characterized in that the position detection unit sets the second test voltage based on the detection result of the voltage value obtained by the detection unit when adjusting the voltage value of the first test voltage so that the current value flowing through the transfer member approaches a predetermined current value, and the predetermined current value.

9. The image forming apparatus according to claim 8, wherein the position detection unit determines the electrical resistance value of the transfer member based on the detection result of the voltage value obtained by applying the first test voltage to the transfer member and the predetermined current value, and sets the second test voltage based on the electrical resistance value.

10. The position detection unit In the position detection operation, the transfer member is moved to a first position, which is one of the contact position and the separation position, and a second position, which is the other of the contact position and the separation position; when the transfer member is at each of the first position and the second position, the application unit applies the second test voltage to the transfer member, and the detection result of the current value by the detection unit is obtained; When the current value acquired when the transfer member is at the first position is greater than the current value acquired when the transfer member is at the second position, output at least one of information indicating that the first position is the contact position or information indicating that the second position is the separation position, An image forming apparatus as described in any one of claims 2 to 9, characterized in that when the current value acquired when the transfer member is in the first position is smaller than the current value acquired when the transfer member is in the second position, at least one of information indicating that the first position is the separation position or information indicating that the second position is the abutment position is output.

11. The image forming apparatus according to claim 10, wherein the position detection unit outputs information indicating that detection of the position of the transfer member has failed when the difference between the current value acquired when the transfer member is in the first position and the current value acquired when the transfer member is in the second position is smaller than a predetermined value.

12. 12. The image forming apparatus according to claim 1, wherein the position detection unit sets the second test voltage based on a detection result obtained by the detection unit when the transfer member is in the contact position.

13. a driven portion that is driven by the driving portion common to the moving portion and moves to a first predetermined position and a second predetermined position; a sensor that detects the position of the driven part; and When the sensor detects that the driven portion is at the first predetermined position, the transfer member is at the contact position, and when the sensor detects that the driven portion is at the second predetermined position, the transfer member is at the contact position or the separated position, An image forming apparatus according to any one of claims 1 to 11, characterized in that the position detection unit sets the second test voltage based on the detection result of the detection unit obtained when the sensor detects that the driven part is at the first predetermined position, and detects the position of the transfer member based on the detection result of the current value obtained when the sensor detects that the driven part is at the second predetermined position.

14. 14. The image forming apparatus according to claim 13, wherein the driven portion is a member that moves a fixing member that fixes the toner image transferred onto the transfer material onto the transfer material.

15. 15. The image forming apparatus of claim 1, wherein the moving section is capable of moving the transfer member to a first contact position and a second contact position as the contact position, and the contact pressure of the transfer member against the image carrier is greater when the transfer member is at the first contact position than when the transfer member is at the second contact position.

Images