Image-forming device

Abstract

To suitably obtain an electrification potential of a photosensitive drum that is an image carrier in an image-forming device for simultaneously generating an electrification bias and a negative transfer bias.SOLUTION: A first power supply generates a voltage of a first polarity, supplies the voltage of the first polarity to a charging member as a charging bias, and supplies the voltage of the first polarity to a transfer member. A second power supply generates a voltage of a second polarity that is opposite to the first polarity and supplies the voltage of the second polarity to the transfer member. Control means operates the first power supply and the second power supply during a current detection period for detecting a charging current. Accordingly, a superposed voltage generated by superposing the voltage of the first polarity and the voltage of the second polarity is applied to the transfer member. Based on a charging current detected during a current detection period, a surface potential of an image carrier is obtained.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Image forming apparatuses that use electrophotography require multiple power supply circuits to generate various high voltages (e.g., charging bias, developing bias, transfer bias, cleaning bias, etc.). The transfer bias is a positive voltage (positive transfer bias) that is applied to the transfer roller to promote the transfer of the toner image. The cleaning bias is a negative voltage (negative transfer bias) that reversely transfers the toner adhering to the transfer roller to the photosensitive drum. This has required a power supply circuit to generate the positive transfer bias and another power supply circuit to generate the negative transfer bias. Patent Document 1 proposes sharing the power supply circuit that generates the negative charging bias as a power supply circuit to generate the cleaning bias. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-206414 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, accurately determining the surface potential of the photosensitive drum (e.g., charging potential such as dark area potential and light area potential) is necessary to accurately control the density of the toner image transferred to a sheet and to suppress toner fogging. In Patent Document 1, when a charging bias is applied to the charging roller, a negative transfer bias is also applied to the transfer roller. As a result, while discharge occurs between the charging roller and the photosensitive drum, discharge also occurs between the transfer roller and the photosensitive drum, making it difficult to accurately determine the charging potential.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suitably determine the charging potential of a photosensitive drum, which is an image carrier, in an image forming apparatus that simultaneously generates a charging bias and a negative transfer bias. [Means for solving the problem]

[0006] The present invention is, for example, an image carrier; a motor that rotates and drives the image carrier; a charging member that charges the surface of the image carrier that is rotationally driven by the motor; a light source that irradiates the surface of the image carrier with light to form an electrostatic latent image; a developing member for developing the electrostatic latent image with toner charged to a first polarity to form a toner image; a transfer member that transfers the toner image from the image carrier to a transfer material; a first power source that generates a voltage of the first polarity, supplies the voltage of the first polarity to the charging member as a charging bias, and supplies the voltage of the first polarity to the transfer member; a second power supply that generates a voltage of a second polarity opposite to the first polarity and supplies the voltage of the second polarity to the transfer member; a detecting means for detecting a charging current flowing between the charging member and the image carrier; a control unit that controls the first power source and the second power source and determines the surface potential of the image carrier based on the charging current, The control means operates the first power source and the second power source during a current detection period in which the detection means detects the charging current, thereby applying a superimposed voltage generated by superimposing a voltage of the first polarity and a voltage of the second polarity to the transfer member, and is configured to determine the surface potential of the image carrier based on the charging current detected during the current detection period. [Effects of the Invention]

[0007] According to the present invention, in an image forming apparatus that simultaneously generates a charging bias and a negative transfer bias, it is possible to suitably determine the charging potential of a photosensitive drum, which is an image carrier. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Figure 2] Diagram explaining the power supply unit [Figure 3] Diagram explaining the power supply circuit [Figure 4] Diagram explaining how to detect charging current [Figure 5] FIG. 10 is a diagram showing a detection sequence of a comparative example. [Figure 6] FIG. 1 is a diagram showing a detection sequence according to a first embodiment; [Figure 7] 1 is a flowchart showing a control method according to a first embodiment. [Figure 8] Diagram showing the relationship between environmental conditions (moisture content) and load resistance [Figure 9] FIG. 10 is a diagram showing a detection sequence according to a second embodiment; [Figure 10] 10 is a flowchart showing a control method according to a second embodiment. [Figure 11] FIG. 10 is a diagram showing a detection sequence according to a third embodiment. [Figure 12] 10 is a flowchart showing a control method according to a third embodiment. [Figure 13] FIG. 10 is a diagram showing a detection sequence according to a fourth embodiment. [Figure 14] Flowchart showing a control method according to a fourth embodiment DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0010] <First Example> (1) Image forming device 1 shows an electrophotographic image forming apparatus 100. The image forming apparatus 100 is equipped with a detachable process cartridge 22. The process cartridge 22 includes a photosensitive drum 1, a charging roller 2, a developing roller 24, a toner container 7, a cleaning member 41, and a nonvolatile memory 30.

[0011] The engine controller 12 has a CPU 13. The CPU 13 is a processor that controls each component of the image forming apparatus 100. The CPU 13 uses an environmental sensor 26 to detect the temperature and humidity of the environment in which the image forming apparatus 100 is installed, and detects the amount of moisture from the detected temperature and humidity.

[0012] The photosensitive drum 1 is an image carrier that is rotated by a motor M1 shown in FIG. 2. The charging roller 2 contacts the photosensitive drum 1 and rotates in a driven manner relative to the photosensitive drum 1. A charging bias (e.g., a negative high voltage) is applied to the charging roller 2 from a high-voltage power supply 40. This causes a discharge between the photosensitive drum 1 and the charging roller 2, and a discharge current (charging current) flows from the photosensitive drum 1 to the charging roller 2. As a result, the surface of the photosensitive drum 1 is charged. The potential (surface potential) of the surface of the photosensitive drum 1 that passes through the charging nip formed between the photosensitive drum 1 and the charging roller 2 changes to a predetermined charging potential (dark potential VD) (charging process). The core of the photosensitive drum 1 is connected to the frame ground and is at 0 V. The potentials described below are based on the frame ground. An organic photosensitive layer made of resin is provided around the metal core.

[0013] As the photosensitive drum 1 rotates, the charged surface portion is irradiated with light output from a light source 3 of an exposure device 4. As a result, the surface potential of the exposed surface portion changes to a light-area potential VL (exposure process). Furthermore, an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum 1 by exposure. The light source 3 may be a semiconductor laser or a light-emitting diode such as an organic EL. EL is an abbreviation for electroluminescence.

[0014] As the photosensitive drum 1 rotates, the exposed surface portion reaches the developing roller 24. The high-voltage power supply 40 is a power supply device that applies a developing bias to the developing roller 24. As a result, the toner T in the toner container 7 adheres to the surface of the photosensitive drum 1 through the developing roller 24, and a toner image is formed (developing process).

[0015] As the photosensitive drum 1 rotates, the toner image is transported to the transfer nip. The transfer nip is formed between the transfer roller 8 and the photosensitive drum 1. The CPU 13 controls the high-voltage power supply 40 to apply a positive transfer bias (e.g., a bias of positive polarity) to the transfer roller 8. As a result, the toner image carried on the surface of the photosensitive drum 1 is transferred to the sheet P (transfer process). The sheet P is stored in a sheet cassette 5, and is fed and conveyed by a feed roller 10.

[0016] The sheet P is transported to the fixing device 19. The fixing device 19 has a cylindrical pressure roller and a cylindrical heating film. The fixing device 19 applies heat and pressure to the sheet P and the toner image, fixing the toner image onto the sheet P (fixing process). The discharge rollers 11 discharge the sheet P, which has been transported along the transport path, onto the sheet tray 17.

[0017] Incidentally, toner T may remain on the surface of the photosensitive drum 1 without being transferred to the sheet P. When the transfer roller 8 is used for a long period of time, the surface of the transfer roller 8 may become dirty with toner T. Therefore, when the transfer process is completed, the CPU 13 controls the high-voltage power supply 40 to apply a cleaning bias (e.g., a high voltage of negative polarity) to the transfer roller 8. This causes the toner T that has accidentally adhered to the transfer roller 8 to be re-transferred to the photosensitive drum 1. As the photosensitive drum 1 rotates, the surface portion to which the toner T has adhered comes into contact with a cleaning member 41 called a cleaning blade. The cleaning member 41 scrapes the toner T off the photosensitive drum 1 and collects it.

[0018] In this way, a positive transfer bias that promotes the transfer of the toner image and a negative transfer bias that cleans the transfer roller 8 are alternately applied to the transfer roller 8. The high-voltage power supply 40 requires a power supply circuit that generates the charging bias, a power supply circuit that generates the developing bias Vdc, a power supply circuit that generates the positive transfer bias, and a power supply circuit that generates the negative transfer bias. However, as will be described later, by using the power supply circuit that generates the charging bias as the power supply circuit that also generates the negative transfer bias, it is possible to make the high-voltage power supply 40 more compact.

[0019] The image density in the image forming apparatus 100 correlates with the development contrast Vcont. The development contrast Vcont is the potential difference between the light area potential VL of the photosensitive drum 1 and the development bias Vdc. On the other hand, if toner T adheres to the non-exposed area of the surface of the photosensitive drum 1, the white area of the sheet P may become dirty. This phenomenon is called toner fogging. The toner fogging correlates with the development back contrast Vback. In other words, toner fogging can be reduced by appropriately controlling the development back contrast Vback. The development back contrast Vback is the potential difference between the dark area potential VD of the photosensitive drum 1 and the development bias Vdc.

[0020] In this way, the CPU 13 appropriately controls Vcont using the light area potential VL as a reference potential, and appropriately controls Vback using the dark area potential VD as a reference potential. This means that the surface potentials of the photosensitive drum 1 (dark area potential VD and light area potential VL) need to be appropriately calculated.

[0021] (2) High-voltage power supply (2-1) Overview of high-voltage power supplies 2 shows a high-voltage power supply 40. The high-voltage power supply 40 includes a charging power supply 201 that generates a charging bias and a negative transfer bias, a developing power supply 204 that generates a developing bias, and a transfer power supply 205 that generates a positive transfer bias. In the first embodiment, the charging bias and the negative transfer bias are generated as a common bias. In other words, whenever the charging bias is output, the negative transfer bias is also output.

[0022] The charging power supply 201 has a power supply circuit 202 that generates a negative high voltage based on instructions from the CPU 13, and a current detection circuit 203 that detects the current (charging current) flowing through the charging roller 2 and reports it to the CPU 13. The transfer power supply 205 has a power supply circuit 206 that generates a positive high voltage based on instructions from the CPU 13, and a current detection circuit 207 that detects the current (transfer current) flowing through the transfer roller 8 and reports it to the CPU 13.

[0023] The CPU 13 drives the motor M1 through the drive circuit 208. The motor M1 rotates and drives a rotating body such as the photosensitive drum 1. The exposure device 4 irradiates the surface of the photosensitive drum 1 with light based on an image signal supplied from the engine controller 12. The CPU 13 controls the fixing temperature of the fixing device 19 based on the amount of moisture detected by the environment sensor 26. In the second embodiment, the amount of moisture is used for yet another purpose. The CPU 13 executes a control program stored in the ROM area of the memory 15 and controls the image forming apparatus 100 in accordance with the control program. ROM is an abbreviation for read-only memory.

[0024] (2-2) Power supply circuit FIG. 3 is a circuit diagram showing an example of the power supply circuits 202 and 206. Vcc and Vref denote reference voltage sources. The power supply circuit 202 includes a drive circuit 303, a voltage setting circuit 304, a transformer T2, and rectifier circuits 305 and 306. The voltage setting circuit 304 generates a primary voltage corresponding to a voltage setting signal (e.g., a PWM signal) output from the CPU 13 and applies it to one end of the primary winding of the transformer T1. The drive circuit 303 is connected to the other end of the primary winding of the transformer T1. The drive circuit 303 switches the transformer T1 based on an on / off signal (drive signal) output from the CPU 13. This generates a secondary voltage in the secondary winding of the transformer T1. The rectifier circuit 305 rectifies and smooths the secondary voltage to generate an output voltage Vout1. The output voltage Vout1 is a negative high voltage and is applied to the charging roller 2.

[0025] The secondary voltage generated in the secondary winding of transformer T1 is rectified and smoothed by another rectifier circuit 306 to become a negative output voltage Vout2. The negative output voltage Vout2 is applied to transfer roller 8. As is clear from FIG. 2, the negative high voltage generated by power supply circuit 202 is applied to both charging roller 2 and transfer roller 8 simultaneously in parallel.

[0026] The feedback circuit 307 is a circuit for feeding back the output voltage Vout1 to the voltage setting circuit 304. The voltage setting circuit 304 adjusts the primary side voltage of the transformer T1 so that the output voltage Vout1 matches a target voltage set by the CPU 13. In the first embodiment, for the sake of simplicity, the voltage output from the rectifier circuit 305 of the power supply circuit 202 matches the voltage output from the rectifier circuit 306.

[0027] The current detection circuit 203 is configured with, for example, an operational amplifier, a shunt resistor, a bypass capacitor, etc. The current detection circuit 203 detects a current I314, which is the sum of a current I310 flowing through the charging roller 2 and a current I313 flowing from the feedback circuit 307. The current detection circuit 203 reports the detection result of the current I314 to the CPU 13 via a connection point J312.

[0028] The power supply circuit 206 includes a drive circuit 321, a voltage setting circuit 322, a transformer T2, and a rectifier circuit 319. The voltage setting circuit 322 generates a primary voltage corresponding to a voltage setting signal (e.g., a PWM signal) output from the CPU 13 and applies the generated voltage to one end of the primary winding of the transformer T2. The drive circuit 321 is connected to the other end of the primary winding of the transformer T2. The drive circuit 321 switches the transformer T2 based on an on / off signal (drive signal) output from the CPU 13. This generates a secondary voltage in the secondary winding of the transformer T2. The rectifier circuit 319 rectifies and smooths the secondary voltage to generate a positive high voltage (output voltage Vout2).

[0029] When the power supply circuit 202 and the power supply circuit 206 operate simultaneously, the output voltage Vout2 becomes a superimposed voltage (superimposed bias) generated by superimposing the negative voltage output from the power supply circuit 202 and the positive voltage output from the power supply circuit 206.

[0030] The CPU 13 controls the negative voltage generated by the power supply circuit 202 by supplying a voltage setting signal to the voltage setting circuit 304. The CPU 13 controls the positive voltage generated by the power supply circuit 206 by supplying a voltage setting signal to the voltage setting circuit 322. In other words, the CPU 13 can also control the superimposed bias.

[0031] The current detection circuit 207 is composed of, for example, an operational amplifier, a shunt resistor, and a bypass capacitor. The current detection circuit 207 detects the current I316 flowing from the transfer roller 8 to the rectifier circuit 306 and reports the detection result to the CPU 13 via the connection point J323. Similarly, the current detection circuit 207 detects the current I317 flowing from the power supply circuit 206 to the transfer roller 8 and reports the detection result to the CPU 13 via the connection point J323. Here, the current I316 is a current generated when the power supply circuit 202 applies a negative voltage to the transfer roller 8. The current I317 is a current generated when the power supply circuit 206 applies a positive voltage to the transfer roller 8. Note that even when the power supply circuits 202 and 206 operate simultaneously and a superimposed bias is applied to the transfer roller 8, the current detection circuit 207 detects the current flowing through the transfer roller 8 and reports the detection result to the CPU 13. This current is the sum (difference) of the current I317 and the current I316.

[0032] (2-3) Superimposed bias The positive transfer bias generated by the power supply circuit 206 is, for example, +3000 [V]. The negative transfer bias and cleaning bias generated by the power supply circuit 202 are, for example, -1000 [V]. In this case, the superimposed bias applied to the transfer roller 8 during image formation is +2000 [V]. Therefore, during image formation, a charging bias of -1000 [V] is applied to the charging roller 2, and a transfer bias of +2000 [V] is applied to the transfer roller 8. Furthermore, during cleaning, the output of the positive transfer bias is stopped, and a cleaning bias of -1000 [V] is applied to the transfer roller 8.

[0033] (3) Current detection method Fig. 4 shows a method for detecting the current flowing through the charging roller 2 in the first embodiment. Fig. 5 shows a detection sequence for the charging current and charging potential in a comparative example. Note that the exposure device 4 may be constantly performing exposure while current detection is being performed.

[0034] While the photosensitive drum 1 is rotating, the power supply circuit 202 applies a negative DC voltage to the charging roller 2 and the transfer roller 8. The current detection circuit 203 detects the charging current flowing through the charging roller 2.

[0035] As shown in Figure 5, periods A, B, and C are the times required for the photosensitive drum 1 to rotate twice (two cycles). Period A is the first current detection period. During period A, a negative transfer bias Va' is applied to the transfer roller 8, and a charging bias Va is applied to the charging roller 2. Period B is a waiting period for returning the surface potential of the photosensitive drum 1 to 0 [V]. Period C is the second current detection period.

[0036] Here, in period A, Va' = Va = -600 [V]. The charging bias is -600 [V]. In period B, Va' = Va = 0 [V]. In period C, Va' = Va = -1200 [V]. Because the charging bias is -1200 [V], a large-scale discharge occurs. In periods A to C, the positive transfer bias is not output.

[0037] During period A, a surface portion of the photosensitive drum 1 whose surface potential is 0 [V] during the first rotation passes the charging roller 2, causing a small discharge and a charging current Ij to flow. The surface potential of the surface portion where the discharge occurred changes from 0 [V] to a dark potential VD. In this way, during the first rotation of the photosensitive drum 1, the current detection circuit 203 detects the charging current Ij. That is, the charging current Ij is detected in the uncharged surface portion. Note that, as the photosensitive drum 1 rotates, the surface portion with a dark potential VD = -50 [V] moves to the exposure position of the exposure device 4. At the exposure position, the surface portion is irradiated with light, and the surface potential of the surface portion changes from the dark potential VD to a light potential VL = 0 to -50 [V]. Note that development and transfer are not performed.

[0038] During the second rotation of the photosensitive drum 1, the surface portion whose surface potential is the light area potential VL passes through the charging roller 2, causing a small discharge and causing a charging current Ik to flow. The surface potential of the surface portion where the discharge occurred changes from the light area potential VL to the dark area potential VD. Note that the potential difference between the light area potential VL and the charging bias is smaller than the potential difference between 0 V and the charging bias. Therefore, the charging current Ik is smaller than the charging current Ij.

[0039] In this way, the current detection circuit 203 detects the charging current Ik during the second rotation of the photosensitive drum 1. In this example, the discharge current Ij is -3.16 [uA]. The discharge current Ik=-2.18 [uA]. "u" stands for micro.

[0040] During period C, a charging bias Vb of -1200 [V] is applied to the charging roller 2. The negative transfer bias Vb' is also -1200 [V]. Period C is divided into periods i to iii. Period i is the period during which a surface portion with a surface potential of 0 [V] passes over the charging roller 2. When this surface portion passes over the charging roller 2, a discharge occurs, a charging current Ib flows, and the surface potential changes from 0 [V] to the dark potential VD. The current detection circuit 203 detects this charging current Ib. In this example, Ib = -47.39 [uA]. The dark potential VD during period C is -650 [V]. The light potential VL = -90 [V].

[0041] Period ii is the period during which the surface portion that has passed through the transfer roller 8 passes the charging roller 2. This surface portion passes through the transfer roller 8 during period i. A negative transfer bias Vb' of -1200 [V] is also applied to the transfer roller 8, causing discharge. Therefore, the surface potential of this surface portion is offset from 0 [V] to a predetermined negative potential. When the surface portion with a predetermined negative surface potential passes through the charging roller 2 during period ii, discharge occurs again, and the charging current Ic is detected. Because the surface potential of the surface portion that has passed through the transfer roller 8 is offset, the charging current Ic decreases by the offset current I_offset. In this example, Ic = -37.39 [uA]. I_offset is -10 [uA]. Thus, the discharge current Ib detected during period i of the first rotation of the photosensitive drum 1 differs from the discharge current Ic detected during period ii. |Ib| > |Ic|.

[0042] During period iii, the surface portion with a light potential VL of -90 V, where discharge occurred on the transfer roller 8, passes over the charging roller 2. As described above, a discharge occurs when this surface portion passes over the transfer roller 8, and the surface potential of this surface portion changes from -90 V to a predetermined negative potential. When the surface portion with the predetermined negative surface potential passes over the charging roller 2 during period iii, a discharge occurs again, and the charging current Id is detected. |Ic| > |Id|. The charging current Id also decreases by the offset current I_offset. In this example, Id = -31.50 uA.

[0043] In this way, it was found that when the charging bias and the negative transfer bias are shared, an error occurs in the detection result of the charging current. Therefore, in the first embodiment, a method is proposed to reduce the error between the detection result of the charging current of the photosensitive drum 1 on the first rotation and the detection result of the charging current on the second rotation, even when the charging bias and the negative transfer bias are shared.

[0044] FIG. 6 shows the detection sequence for charging current and charging potential in the first embodiment. The difference between FIG. 6 and FIG. 5 is that during period C, the power supply circuit 206 operates to output the positive transfer bias Vc, thereby applying a superimposed bias to the transfer roller 8. In particular, the superimposed bias is adjusted to a voltage that makes it difficult for discharge to occur between the surface of the photosensitive drum 1 and the transfer roller 8. For example, the positive transfer bias Vc is set to +800 [V]. The negative transfer bias Va' is -1200 [V]. Therefore, the superimposed bias is -400 [V]. If the superimposed bias is -400 [V], no discharge occurs on the transfer roller 8 even if the surface potential is 0 [V]. No discharge occurs on the transfer roller 8 even if the surface potential is the dark potential VD. No discharge occurs on the transfer roller 8 even if the surface potential is the light potential VL. In other words, even if a surface portion passes through the transfer roller 8, the surface potential of that surface portion does not change.

[0045] As shown in Figure 6, during periods i and ii, the surface portion of the charging roller 2 with a surface potential of 0 [V] passes through the charging roller 2. Therefore, the charging current Ib detected during period i matches the charging current Ie detected during period ii. The charging current Ib = Ie = -47.39 [uA]. Therefore, it was confirmed that the offset current I_offset became 0 [uA].

[0046] During period iii, the surface portion whose surface potential is the light area potential VL passes through the charging roller 2. Therefore, the charging current If detected during period iii is constant. The discharge current If = -41.50 [uA]. As expected, the offset current I_offset is 0 [uA].

[0047] According to the first embodiment, during the detection period of the discharge current (charging current), a positive transfer bias is superimposed on a negative transfer bias to generate a superimposed bias, which is applied to the transfer roller 8. This reduces the discharge at the transfer roller 8 that causes disturbance, and the charging current flowing through the charging roller 2 can be detected with high accuracy.

[0048] There is also a method of estimating the discharge start voltage by detecting the discharge current flowing through the charging roller 2 while gradually changing the high voltage applied to the charging roller 2. Compared to this method, the first embodiment uses two values (-600V, -1200V) as the charging bias, and therefore can detect the charging current in a short time.

[0049] (4) Calculation method of charged potential The CPU 13 calculates the surface potential (charge potential) of the photosensitive drum 1 using the following formula. The dark potential VD and light potential VL are calculated as the charge potential. These are necessary to accurately control Vcont and Vback.

[0050] i = (|Ib| - |Ij|) ÷ (|Vb| - |Va|) =(47.39[uA]-3.16[uA])÷(1200[V]-600[V]) =0.073463[uA / V]···Eq1 Here, i is the discharge current that flows through the charging roller 2 when the charging bias changes by about 1 V. In other words, i indicates the amount of current change per unit voltage. Ib is the charging current detected in period i within period C. Ij is the charging current detected during the first rotation of the photosensitive drum 1 in period A. Va is the charging bias in period A. Vb is the charging bias in period C.

[0051] Next, the CPU 13 calculates the dark potential VD and the light potential VL using the following formulas.

[0052] VD=|Ib|÷i =47.39[uA]÷0.073463[uA / V] =645.1[V]···Eq2 VL=VD-|If|÷i =645.1[V]-41.50[uA]÷0.073463[uA / V] =80.1[V]···Eq3 where If is the discharge current detected in period iii.

[0053] As described above, the measured value of the dark potential VD is -650 [V]. The calculated value of the dark potential VD is -645.1 [V]. The measured value of the light potential VL is -90 [V]. The calculated value of the light potential VL is -80.1 [V]. Therefore, it can be said that the error is sufficiently small. Therefore, the first embodiment can accurately determine the charging potential in a short time.

[0054] (5) Flowchart FIG. 7 shows a method for detecting the surface potential, which is executed by the CPU 13 according to a control program.

[0055] In S701, the CPU 13 starts the rotation of the photosensitive drum 1 and the exposure of the exposure device 4. For example, the CPU 13 instructs the drive circuit 208 to rotate the motor M1. The drive circuit 208 starts driving the motor M1 in accordance with the rotation instruction. The motor M1 starts driving the rotation of the photosensitive drum 1. Note that the start of rotation of the photosensitive drum 1 and the start of exposure of the exposure device 4 may be different. However, it is preferable that the rotation start timing of the photosensitive drum 1 precedes the exposure start timing of the exposure device 4.

[0056] In S702, the CPU 13 starts the first current detection.

[0057] In S703, the CPU 13 starts the power supply circuit 202 to start outputting the charging bias Va and the negative transfer bias Va'. For example, the charging bias Va and the negative transfer bias Va' are each -600 [V].

[0058] In S704, the CPU 13 detects the charging current Ij and the charging current Ik. For example, the CPU 13 may convert the analog detection result output from the current detection circuit 203 into a digital value and obtain the average value of multiple digital values (sample values) as the charging current Ij. Note that, as in the above-mentioned formula, there may be cases where the charging current Ik is not used to calculate the charging potential. In this case, obtaining the charging current Ik may be omitted.

[0059] In step S705, the CPU 13 ends the first current detection. The CPU 13 stores the charging currents Ij and Ik in the RAM area of the memory 15. RAM is an abbreviation for random access memory. The CPU 13 also stops the power supply circuit 202 and restores the surface potential of the photosensitive drum 1 to 0 V.

[0060] In step S706, the CPU 13 starts the second current detection. The CPU 13 secures a period B between steps S705 and S706, and returns the surface potential of the photosensitive drum 1 to 0 [V].

[0061] In S707, the CPU 13 starts the power supply circuit 202 to start outputting the charging bias Vb and the negative transfer bias Vb'. For example, the charging bias Vb and the negative transfer bias Vb' are each -1200 [V].

[0062] In S708, the CPU 13 activates the power supply circuit 206 to start outputting the positive transfer bias Vc. The positive transfer bias Vc is, for example, +800 V. Note that it is preferable that the output of the positive transfer bias Vc be started before the surface portion to which the charging bias Vb has been applied in S707 reaches the transfer nip. S707 and S708 may be executed simultaneously.

[0063] In S709, the CPU 13 detects the charging currents Ib, Ie, and Ik. For example, the CPU 13 may convert the analog detection result output from the current detection circuit 203 into a digital value and obtain the average value of multiple digital values (sample values) as the charging currents Ib, Ie, and If. Note that there may be cases where the charging current Ie is not used as in the above formula. In such cases, obtaining the charging current Ie may be omitted.

[0064] In S710, the CPU 13 ends the second current detection. The CPU 13 stores the charging currents Ib, Ie, and If in the RAM area of the memory 15. The CPU 13 also stops the power supply circuits 202 and 206, the exposure device 4, and the motor M1.

[0065] In S711, the CPU 13 calculates the dark potential VD and the light potential VL. The CPU 13 may read the detection results from the memory 15 and apply the detection results to the formulas Eq1 to Eq3 to obtain the dark potential VD and the light potential VL.

[0066] According to the first embodiment, in the image forming apparatus 100 that simultaneously generates a charging bias and a negative transfer bias, it is possible to suitably determine the charging potential of the photosensitive drum 1. Specifically, when detecting the discharge current (charging current) flowing through the charging roller 2, a superimposed bias is applied to the transfer roller 8 so that discharge is less likely to occur in the transfer roller 8. Here, the superimposed bias is generated by superimposing a positive transfer bias on a negative transfer bias.

[0067] Although −600 [V] is used as the charging bias Va during period A and −1200 [V] is used as the charging bias Vb during period C, these are merely examples. As long as the detection result of the charging potential can be obtained accurately, the charging biases Va and Vb may be increased or decreased.

[0068] As an example of the superimposed bias, +800 [V] is used. However, this is only one example. Any superimposed bias that does not easily cause discharge between the surface of the photosensitive drum 1 and the transfer roller 8 can be used. Therefore, the superimposed bias may be higher or lower than +800 [V].

[0069] <Second Example> In the first embodiment, a fixed value of the positive transfer bias Vc is output during the period C to generate a superimposed bias that does not cause discharge between the photosensitive drum 1 and the transfer roller 8. However, the positive transfer bias Vc may be variable. The resistance value of the transfer roller 8 varies depending on the environment (e.g., temperature, humidity, and atmospheric moisture content). In other words, the conditions for generating discharge depend on the environment. Therefore, in the second embodiment, the positive transfer bias Vc is adjusted according to the conditions of the environment in which the image forming apparatus 100 is installed. In other words, the superimposed bias is adjusted.

[0070] (1) Characteristics of the transfer roller FIG. 8 shows the environmental characteristics of the transfer roller 8. The vertical axis shows the load resistance [MΩ] of the transfer roller 8. The horizontal axis shows the moisture content [g / m3]. m3 stands for cubic meter. As FIG. 8 shows, the load resistance of the transfer roller 8 changes depending on the moisture content of the atmosphere in which the transfer roller 8 is placed. Therefore, the CPU 13 determines the transfer bias to be supplied to the transfer roller 8 in accordance with the current detected by the current detection circuit 207.

[0071] (2) Control method of the positive transfer bias, which is a part of the superimposed bias 9 shows a method for controlling the superimposed bias applied to the transfer roller 8 in the second embodiment. As an example, the moisture content is assumed to be 9.0 [g / m3]. The resistance value of the transfer roller 8 is assumed to be 400 [MΩ].

[0072] In this example, a current detection period Z (hereinafter abbreviated as "period Z") and period G are added before period A. Period Z is a period during which the transfer current flowing between the transfer roller 8 and the photosensitive drum 1 is detected, and the transfer bias is adjusted so that the detected transfer current becomes the target value Iz. Period G is a waiting period during which the surface potential of the photosensitive drum 1 is returned to 0 [V].

[0073] During period Z, the CPU 13 controls the power supply circuit 202 so that the charging bias Vb and the negative transfer bias Vb' are output simultaneously. Here, the charging bias Vb and the negative transfer bias Vb' output during period Z are the same as the charging bias Vb and the negative transfer bias Vb' during period C. The target value Iz is determined in advance based on the environmental conditions and is stored in the ROM area of the memory 15. Here, Iz is assumed to be -1.0 [uA].

[0074] The CPU 13 compares the transfer current detected by the current detection circuit 207 with the target value Iz and gradually increases the positive transfer bias until they match. For example, when the transfer current reaches the target value Iz, the positive transfer bias Vz is +800 V. The CPU 13 stores the positive transfer bias Vz in the RAM area of the memory 15. During period C, the CPU 13 reads the positive transfer bias Vz from the memory 15 and causes the power supply circuit 206 to output the positive transfer bias Vz. During period C, the CPU 13 also controls the power supply circuit 202 to output the charging bias Vb and the negative transfer bias Vb'. This applies a superimposed bias, which is the sum of the positive transfer bias Vz and the negative transfer bias Vb', to the transfer roller 8. As a result, discharge between the transfer roller 8 and the photosensitive drum 1 is less likely to occur. The length of period Z is, for example, approximately 3 to 5 seconds. After period Z ends, the CPU 13 waits for period G to elapse. When the period G ends, the CPU 13 proceeds to the periods A to C described in the first embodiment.

[0075] (3) How to determine the target value Iz The resistance value of the transfer roller 8 varies depending on the amount of moisture in the atmosphere. Therefore, the target value Iz should be determined or selected according to the amount of moisture in the atmosphere. For example, the ROM area of the memory 15 may store a table holding target values Iz for various amounts of moisture. This table is determined in advance through experiments or simulations. The CPU 13 obtains the target value Iz corresponding to the amount of moisture detected by the environmental sensor 26 from the table. This table may be realized by a mathematical function or program module that takes the amount of moisture as input and outputs the target value Iz. The resistance value of the transfer roller 8 may be used instead of the amount of moisture. The temperature and humidity detected by the environmental sensor 26 may be used instead of the amount of moisture.

[0076] For example, if the moisture content is less than 5.8 [g / m3], the target value Iz is determined to be -0.7 [uA]. If the moisture content is 5.8 [g / m3] or more and less than 18 [g / m3], the target value Iz is determined to be -1.0 [uA]. If the moisture content is 18 [g / m3] or more, the target value Iz is determined to be -4.0 [uA]. The table holds such correspondence relationships between moisture content and target value Iz.

[0077] If the detection result of the environmental sensor 26 is 1.0 [g / m3], the CPU 13 references the table and determines the target value Iz to be -0.7 [uA]. According to Figure 8, the resistance value of the transfer roller 8 is 600 [MΩ]. The forward transfer bias Vz when the transfer current reaches the target value Iz of -0.7 [uA] is calculated using the following equation. Vz = -Vb + (-0.7 [uA] x 600 [MΩ]) =-(-1200[V])-420[V] =+780[V]···Eq4 When the detection result of the environment sensor 26 is 22 [g / m3], the CPU 13 refers to the table and determines the target value Iz to be -4.0 [uA]. According to Figure 8, the resistance value of the transfer roller 8 is 100 [MΩ]. The forward transfer bias Vz when the transfer current reaches the target value Iz of -4.0 [uA] is calculated using the following equation. Vz = -Vb + (-4.0 [uA] x 100 [MΩ]) =-(-1200[V])-400[V] =+800[V]···Eq5 The target value Iz may be stored in the memory 15, or may be stored in a nonvolatile memory 30 provided in the process cartridge 22. In this case, the CPU 13 reads out the target value Iz corresponding to the moisture content from the nonvolatile memory 30.

[0078] (4) Flowchart Fig. 10 shows a control method in the second embodiment. In Fig. 10, steps common to Fig. 7 are given the same reference numerals, and their explanation will be omitted.

[0079] In S1001, the CPU 13 starts the motor M1 through the drive circuit 208 to start the rotation of the photosensitive drum 1. Note that the exposure device 4 remains off.

[0080] In S1002, the CPU 13 activates the current detection circuit 207 to start detecting the transfer current.

[0081] In S1003, the CPU 13 starts the power supply circuit 202 and starts outputting the charging bias Vb and the negative transfer bias Vb'. The charging bias Vb and the negative transfer bias Vb' are, for example, -1200 [V].

[0082] In S1004, the CPU 13 detects the environmental conditions (e.g., moisture content) using the environmental sensor 26 and determines the target value Iz based on the environmental conditions. The environmental conditions may be any parameter capable of detecting the electrical resistance value of the transfer roller 8. The CPU 13 determines the target value Iz corresponding to the environmental conditions based on a table, mathematical function, or program module stored in the memory 15 or the non-volatile memory 30. For example, if the moisture content is 9 [g / m3], the target value Iz is determined to be -1 [uA]. S1004 may be executed before S1003.

[0083] In S1005, the CPU 13 activates the power supply circuit 206 to start outputting the positive transfer bias.

[0084] In S1006, the CPU 13 detects the transfer current using the current detection circuit 207 and determines whether the transfer current exceeds the target value Iz. If the transfer current does not exceed the target value Iz, the CPU 13 proceeds from S1006 to S1007. In S1007, the CPU 13 increases the forward transfer bias by one level (e.g., +100 V). Thereafter, the CPU 13 proceeds from S1007 to S1006. If it is determined in S1006 that the transfer current has reached the target value Iz, the CPU 13 proceeds from S1006 to S1008. Note that in this embodiment, the determination of whether a certain measurement value has exceeded a threshold may be interpreted as a determination of whether a certain measurement value is equal to or greater than the threshold. Similarly, the determination of whether a certain measurement value has fallen below the threshold may be interpreted as a determination of whether a certain measurement value is equal to or less than the threshold. Furthermore, if the i-th current value among multiple measurement values (current values) arranged in chronological order exceeds the threshold, the voltage value corresponding to the i-th current value may be used. Alternatively, a voltage value corresponding to the (i-1)th current value that does not exceed the threshold may be used. A voltage value corresponding to the threshold may be calculated by linearly interpolating the (i-1)th current value and the i-th current value. This concept related to comparison is applicable not only to S1006 but also to other steps.

[0085] In S1008, the CPU 13 stops outputting the biases (positive transfer bias, charging bias Vb, and negative transfer bias Vb').

[0086] In S1009, the CPU 13 starts the exposure apparatus 4 and causes the exposure apparatus 4 to start exposure. Note that S1008 and S1009 may be executed almost simultaneously. Thereafter, the CPU 13 executes S702 to S711. Note that the CPU 13 secures a period G between S1009 and S702.

[0087] The second embodiment can achieve the same effects as the first embodiment. Furthermore, in the second embodiment, the positive transfer bias is adjusted according to the environmental conditions. Therefore, in the method for determining the surface potential of the photosensitive drum 1, the superimposed bias is adjusted with precision so that discharge is less likely to occur at the transfer nip. As a result, the second embodiment can determine the surface potential with greater precision than the first embodiment.

[0088] <Third Example> In the first embodiment, the superimposed bias was a fixed value that prevented discharge from occurring between the photosensitive drum 1 and the transfer roller 8. In the second embodiment, the superimposed bias was a variable value that changed depending on environmental conditions. In the third embodiment, a method is proposed for shortening the time required to determine the superimposed bias and the normal transfer bias.

[0089] (1) Dark decay of charged potential As the photosensitive drum 1 rotates, the surface of the photosensitive drum 1 passes through the contact area (charging nip) with the charging roller 2. When the surface passes through the charging nip, the surface potential (charging potential) of that surface decays. This phenomenon is called dark decay.

[0090] As shown in Figure 9, in period A of the second embodiment, the charging current is detected when the charging potential changes from 0 [V] to the dark potential VD during the first rotation of the photosensitive drum 1. In period Z, a charging bias of -1200 [V] is applied to the charging roller 2. Therefore, period G is required between period Z and period A to return the surface potential to 0 [V] through dark decay. Therefore, if period G is shortened, the overall time required to determine the charging potential is shortened.

[0091] (2) Shortening method FIG. 11 shows a method for detecting the charge potential in the third embodiment. As an example, the moisture content is assumed to be 9.0 g / m3, and the resistance of the transfer roller 8 is assumed to be 400 MΩ. Comparing FIG. 11 with FIG. 9, in FIG. 11, period Z is placed between periods A and B, and period G is omitted. In other words, period B functions as period G.

[0092] During period A, the charging bias Va and negative transfer bias Va' are output simultaneously. During the first rotation of the photosensitive drum 1, the charging current Ij is detected when the charging potential changes from 0 [V] to the dark potential VD. During the second rotation of the photosensitive drum 1, the charging current Ik is detected when the charging potential changes from the light potential VL to the dark potential VD. Then, the CPU 13 transitions from period A to period Z.

[0093] The charging bias Vb during period Z is the same as the charging bias Vb during period C. The negative transfer bias Vb' during period Z is the same as the negative transfer bias Vb' during period C. The target value Iz of the transfer current during period Z is determined based on the table described in the second embodiment. For example, if the moisture content is 9.0 g / m3, the target value Iz is determined to be -1.0 uA. The CPU 13 gradually increases the positive transfer bias until the transfer current detected by the current detection circuit 207 reaches -1.0 uA. The CPU 13 stores the positive transfer bias Vz (e.g., +800 V) when the transfer current reaches the target value Iz in memory 15. During period C, the CPU 13 reads the positive transfer bias Vz from memory 15, superimposes the positive transfer bias Vz on the negative transfer bias to generate a superimposed bias, and applies the superimposed bias to the transfer roller 8. This suppresses discharge from the transfer roller 8 to the photosensitive drum 1 during period C.

[0094] (3) Flowchart 12 shows a method for detecting the charge potential in the third embodiment. Steps that have already been described are given the same reference numerals.

[0095] The CPU 13 executes steps S701 to S705. This completes the first detection of the charging current. After that, the CPU 13 proceeds from step S705 to step S1201.

[0096] In S1201, the CPU 13 controls the exposure apparatus 4 to turn off exposure in order to adjust the superimposed bias. From S1201, the CPU 13 proceeds to S1002 described in the second embodiment.

[0097] The CPU 13 executes steps S1002 to S1009. This determines the positive transfer bias Vz when the transfer current reaches the target value Iz. The CPU 13 then proceeds from step S1009 to step S706, and executes steps S706 to S710 (detection of the second charging current). The CPU 13 ensures a period B between steps S1009 and S706.

[0098] In this way, period Z (adjustment of the superimposed bias) is executed between period A (first detection of the charging current) and period B. This integrates period G and period B, thereby reducing the overall time required for the method of detecting the charging potential. Other effects of the third embodiment are the same as those of the second embodiment.

[0099] <Fourth Example> The fourth embodiment aims to further reduce the processing time compared to the third embodiment. Specifically, in the fourth embodiment, the superimposed bias is adjusted (period Z) within period A. That is, the first detection of the charging current and the adjustment of the superimposed bias are performed simultaneously in parallel.

[0100] (1) How to reduce processing time FIG. 13 shows a method for detecting the charge potential in the fourth embodiment. As already explained in FIG. 9 (second embodiment), during period Z, the output of the charge bias Vb and negative transfer bias Vb' (e.g., -1200 [V]) begins before the output of the positive transfer bias. When the negative transfer bias Vb' is applied and the positive transfer bias is not applied, the transfer current is, for example, -3.0 [uA]. The target value Iz during period Z is, for example, -1.0 [uA]. Therefore, the CPU 13 can gradually increase the positive transfer bias until the transfer current detected by the current detection circuit 207 reaches the target value Iz.

[0101] When the charging bias Va and the negative transfer bias Va' are -600 [V], the dark area potential VD of the photosensitive drum 1 is -50 [V]. In this state, only a small amount of discharge occurs from the transfer roller 8 to the photosensitive drum 1. The charging current Ij flowing through the charging roller 2 is -3.16 [uA]. A difference occurs between the charging current flowing when the surface portion of the transfer roller 8 not affected by the discharge passes through the charging roller 2 and the charging current flowing when the surface portion of the transfer roller 8 affected by the discharge passes through the charging roller 2. However, the amount of discharge is small when the charging bias Va and the negative transfer bias Va' are both -600 [V]. Therefore, the difference Δ in the charging current is only 0.15 [uA]. In the first embodiment, the difference Δ may be converted to the difference ΔVD in the dark area potential VD and the difference ΔVL in the light area potential VL when the charging bias Vb and the negative transfer bias Vb' are both -1200 [V]. In this case, ΔVD is 2 [V] and ΔVL is 0.3 [V]. Therefore, these differences are negligible. In other words, as shown in Figure 13, in the fourth embodiment, period A and period Z are overlapped.

[0102] During period A (period A also serves as period Z), initially, charging bias Va and negative transfer bias Va' are output simultaneously. At this time, the transfer current flowing through transfer roller 8 is -1.5 [uA]. As explained in the second embodiment, the target value Iz when the moisture content is 9.0 [g / m3] is -1.0 [uA]. Therefore, CPU 13 gradually increases the positive transfer bias until the transfer current detected by current detection circuit 207 reaches the target value Iz. When the transfer current reaches the target value Iz, the positive transfer bias Vz is +200 [V]. At this time, the superimposed bias may be calculated based on the following formula:

[0103] Superimposed bias Vx' = Negative transfer bias Va' + Positive transfer bias Vx =-600 + 0 =-600[V]···Eq6 Superimposed bias Vy' = Negative transfer bias Va' + Positive transfer bias Vy =-600 + 100 =-500[V]···Eq7 Superimposed bias Vz' = negative transfer bias Va' + positive transfer bias Vz =-600 +200 =-400[V]···Eq8 The transfer currents Ix, Iy, and Iz corresponding to the superimposed biases Vx', Vy', and Vz' can be calculated using the following formulas. Ix = -600[V]÷400[MΩ] =-1.5[uA]···Eq9 Iy = -500[V]÷400[MΩ] =-1.25[uA]···Eq10 Iz = -400[V]÷400[MΩ] =-1.0[uA]···Eq11 During the period in which the positive transfer bias Vz was determined to be +200 [V], the charging bias Va and negative transfer bias Va' were each -600 [V]. The charging bias Vb and negative transfer bias Vb' applied during period C were each -1200 [V]. Therefore, the positive transfer bias Vz" during period C was determined by adding the difference between Va and Vb to Vz.

[0104] Vz"=Vz+(Va-Vb) =+200+(-600-(-1200)) =+800[V]···Eq12 (2) Flowchart 14 shows a method for detecting the charge potential in the fourth embodiment. Steps that have already been described are given the same reference numerals.

[0105] In S1401, the CPU 13 starts the motor M1 through the drive circuit 208 to start the rotation of the photosensitive drum 1. At this point, the exposure device 4 maintains the exposure off state.

[0106] In S1402, the CPU 13 acquires the environmental conditions using the environmental sensor 26 and determines the target value Iz of the transfer current based on the environmental conditions. For example, the CPU 13 refers to a table based on the environmental conditions (amount of moisture in the atmosphere) and acquires the target value Iz corresponding to the environmental conditions.

[0107] In S1403, the CPU 13 activates the current detection circuit 203 to start the first detection of the charging current.

[0108] In S1404, the CPU 13 activates the current detection circuit 207 to start detecting the transfer current.

[0109] In step S1405, the CPU 13 starts the power supply circuit 202 to start outputting the charging bias Va and the negative transfer bias Va'.

[0110] In S1406, the CPU 13 activates the power supply circuit 206 to start outputting the positive transfer bias.

[0111] In S1407, the CPU 13 detects the transfer current using the current detection circuit 207 and determines whether the transfer current has exceeded the target value Iz. If the transfer current has not exceeded the target value Iz, the CPU 13 proceeds from S1407 to S1408. In S1408, the CPU 13 increases the positive transfer bias by approximately one step. Thereafter, the CPU 13 proceeds from S1408 to S1407. If it is determined in S1407 that the transfer current has exceeded the target value Iz, the CPU 13 proceeds from S1407 to S1409.

[0112] In S1409, the CPU 13 detects the charging current Ij using the current detection circuit 203. In S1410, the CPU 13 ends the first detection of the charging current. Thereafter, the CPU 13 executes S1008, S1009, and S706 to S711 described with reference to FIG. 12. However, the positive transfer bias used in S708 is the positive transfer bias Vz" determined in S1407 and S1408. Also, in S709, the charging currents Ib and If are detected. The CPU 13 secures a period B between S1009 and S706.

[0113] In the fourth embodiment, period A and period Z are overlapped. Therefore, the fourth embodiment can further reduce the processing time than the third embodiment. Other effects of the fourth embodiment are the same as those of the third embodiment.

[0114] In the first to fourth embodiments, the polarity of the toner T is negative, so the charging bias and negative transfer bias are negative, and the positive transfer bias is positive. However, if the polarity of the toner T is positive, the polarities of these biases are opposite.

[0115] <Technical ideas derived from examples> (Item 1) an image carrier; a motor that rotates and drives the image carrier; a charging member that charges the surface of the image carrier that is rotationally driven by the motor; a light source that irradiates the surface of the image carrier with light to form an electrostatic latent image; a developing member for developing the electrostatic latent image with toner charged to a first polarity to form a toner image; a transfer member that transfers the toner image from the image carrier to a transfer material; a first power source that generates a voltage of the first polarity, supplies the voltage of the first polarity to the charging member as a charging bias, and supplies the voltage of the first polarity to the transfer member; a second power supply that generates a voltage of a second polarity opposite to the first polarity and supplies the voltage of the second polarity to the transfer member; a detecting means for detecting a charging current flowing between the charging member and the image carrier; a control unit that controls the first power source and the second power source and determines the surface potential of the image carrier based on the charging current, The image forming apparatus is configured such that, during a current detection period in which the detection means detects the charging current, the control means operates the first power source and the second power source to apply a superimposed voltage generated by superimposing a voltage of the first polarity and a voltage of the second polarity to the transfer member, and determines the surface potential of the image carrier based on the charging current detected during the current detection period.

[0116] The charging roller 2 is an example of a charging member that charges the surface of the photosensitive drum 1, which is driven to rotate by the motor M1. The developing roller 24 is an example of a developing member that forms a toner image by developing an electrostatic latent image using toner T charged to a first polarity (e.g., negative polarity). The transfer roller 8 is an example of a transfer member that transfers the toner image from the photosensitive drum 1 to a transfer material (e.g., sheet P, intermediate transfer body). The power supply circuit 202 is an example of a first power supply that generates a voltage of a first polarity, supplies the voltage of the first polarity to the charging roller 2 as a charging bias, and outputs the voltage of the first polarity to the transfer roller 8. The voltage of the first polarity applied to the transfer roller 8 may be a so-called cleaning bias. The power supply circuit 206 is an example of a second power supply that generates a voltage of a second polarity opposite to the first polarity, and supplies the voltage of the second polarity to the transfer roller 8. The voltage of the second polarity may be, for example, a transfer bias for facilitating the transfer of the toner image from the photosensitive drum 1 to the sheet P. The current detection circuit 203 is an example of a detection unit that detects the charging current flowing between the charging roller 2 and the photosensitive drum 1. The CPU 13 is an example of a control unit that controls the first and second power sources and determines the surface potential (e.g., dark potential VD, light potential VL) of the photosensitive drum 1 based on the charging current. The CPU 13 operates the first and second power sources during the current detection period (e.g., periods i to iii) to apply a superimposed voltage generated by superimposing a voltage of a first polarity and a voltage of a second polarity to the transfer roller 8. Furthermore, the CPU 13 is configured to determine the surface potential of the photosensitive drum 1 based on the charging current detected during the current detection period. This allows the surface potential (charging potential) of the photosensitive drum 1 to be suitably determined in the image forming apparatus 100, which simultaneously generates a charging bias and a negative transfer bias. The polarity of the superimposed voltage may be either the first polarity or the second polarity.

[0117] (Item 2) 2. The image forming apparatus according to item 1, wherein the voltage of the first polarity and the voltage of the second polarity used during the current detection period are adjusted so that discharge occurs between the charging member and the image carrier, but discharge does not occur between the transfer member and the image carrier.

[0118] During the current detection period, a voltage of a first polarity (e.g., positive transfer bias) and a voltage of a second polarity (e.g., charging bias Vb) are used. These are adjusted so that a discharge occurs between the charging roller 2 and the photosensitive drum 1, but a discharge does not occur between the transfer roller 8 and the photosensitive drum 1. In other words, the voltage applied between the transfer roller 8 and the photosensitive drum 1 is adjusted to be less than the discharge start voltage. This further reduces the influence of the transfer roller 8 on the surface potential of the photosensitive drum 1, allowing the charging current to be detected with high accuracy.

[0119] (Item 3) the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the first period, the voltage of the second polarity is not supplied to the transfer member; During the second period, the voltage of the second polarity is supplied to the transfer member; 3. The image forming apparatus according to item 1 or 2, wherein the control unit is configured to determine the surface potential of the image carrier based on the first charging current detected by the detection unit in the first period and the second charging current detected by the detection unit in the second period.

[0120] Period A is an example of a first period. Period C is an example of a second period. The absolute value of the voltage of the first polarity (e.g., −600 V) during the first period may be smaller than the absolute value of the voltage of the first polarity (e.g., −1200 V) during the second period. As shown in FIGS. 6, 9, and 11, during the first period (e.g., period A), the voltage of the second polarity (e.g., positive transfer biases Vc and Vz) is not supplied to the transfer roller 8. During the second period (e.g., period C), the voltage of the second polarity is supplied to the transfer roller 8. The CPU 13 may calculate the surface potential of the photosensitive drum 1 based on the first charging current (e.g., Ij) detected during the first period and the second charging current (e.g., Ib) detected during the second period. During the second period, the absolute value of the voltage of the first polarity is increased. Therefore, if this is left as it is, discharge is more likely to occur on the transfer roller 8. Therefore, by superimposing a voltage of the second polarity on the voltage of the first polarity, discharge is less likely to occur on the transfer roller 8.

[0121] (Item 4) Item 4. The image forming apparatus according to item 3, wherein the control unit is configured to obtain a current change amount per unit voltage by dividing the difference between the first charging current and the second charging current by the difference between the voltage of the first polarity supplied to the transfer member in the second period and the voltage of the first polarity supplied to the transfer member in the first period, and to obtain a dark potential, which is the surface potential of the image carrier, by dividing the second charging current by the current change amount.

[0122] As shown in formula Eq1, the CPU 13 divides the difference between the first charging current Ij and the second charging current Ib by the difference between the voltage of the first polarity supplied to the transfer roller 8 in the second period and the voltage of the first polarity supplied to the transfer roller 8 in the first period. This determines the current change amount i per unit voltage. As shown in formula Eq2, the CPU 13 may determine the dark potential VD, which is the surface potential of the photosensitive drum 1, by dividing the second charging current Ib by the current change amount i. Formulas Eq1 and Eq2 are merely examples, and other formulas may be used to determine the dark potential VD.

[0123] (Item 5) the current detection period includes a first period, a second period, and a third period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period, and an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the third period; During the first period, the voltage of the second polarity is not supplied to the transfer member; the voltage of the second polarity is supplied to the transfer member during the second period and the third period; the first period and the second period are periods during which a surface portion of the image bearing member that is at a dark potential passes through the charging member, the third period is a period during which a surface portion of the image bearing member that is at a light area potential passes through the charging member, 5. The image forming apparatus according to any one of items 1 to 4, wherein the control unit is configured to determine the surface potential of the image carrier based on a first charging current detected by the detection unit in the first period, a second charging current detected by the detection unit in the second period, and a third charging current detected by the detection unit in the third period.

[0124] The current detection period may include a first period (e.g., period A), a second period (e.g., period i), and a third period (e.g., period iii). The absolute value of the voltage of the first polarity (e.g., −600 V) in the first period is smaller than the absolute value of the voltage of the first polarity (e.g., −1200 V) in the second period. The absolute value of the voltage of the first polarity (e.g., −600 V) in the first period is smaller than the absolute value of the voltage of the first polarity (e.g., −1200 V) in the third period. In the first period, a voltage of the second polarity is not supplied to the transfer roller 8. In the second and third periods, a voltage of the second polarity (e.g., positive transfer bias) is supplied to the transfer roller 8. The first and second periods are periods during which a surface portion of the photosensitive drum 1 at the dark potential VD passes over the charging roller 2. In the third period, a surface portion of the photosensitive drum 1 at the light potential VL passes over the charging roller 2. The CPU 13 may determine the surface potential of the photosensitive drum 1 based on the first charging current Ij detected in the first period, the second charging current Ib detected in the second period, and the third charging current If detected in the third period.

[0125] (Item 6) 6. The image forming apparatus according to item 5, wherein the control means is configured to obtain a current change amount per unit voltage by dividing the difference between the first charging current and the second charging current by the difference between the voltage of the first polarity supplied to the transfer member in the second period and the voltage of the first polarity supplied to the transfer member in the first period, obtain a voltage by dividing the third charging current by the current change amount, and obtain a light area potential of the image carrier by subtracting the voltage from the dark area potential of the image carrier.

[0126] As shown in the formula Eq3, the CPU 13 may obtain a voltage by dividing the third charging current If by the current change amount i, and may obtain the light potential VL by subtracting the obtained voltage from the dark potential VD.

[0127] (Item 7) 7. The image forming apparatus according to claim 6, wherein the control unit is configured to obtain the dark potential by dividing the second charging current by the amount of change in current.

[0128] As shown in the formula Eq2, the CPU 13 may obtain the dark potential VD by dividing the second charging current Ib by the current change amount i. (Item 8) 2. The image forming apparatus according to item 1, wherein the control unit adjusts the voltage of the second polarity used during the current detection period in accordance with the resistance value of the transfer member.

[0129] As mentioned in the second embodiment, the CPU 13 may adjust the voltage of the second polarity used during the current detection period in accordance with the resistance value of the transfer roller 8. This allows the influence of the negative transfer bias during the current detection period to be reduced with precision.

[0130] (Item 9) The image forming apparatus further includes an environment detection unit that detects the environmental conditions of the environment in which the image forming apparatus is installed, 9. The image forming apparatus according to any one of items 1 to 8, wherein the control unit adjusts the voltage of the second polarity used during the current detection period in accordance with the environmental conditions.

[0131] The environmental sensor 26 is an example of an environmental detection unit. As described in the second embodiment, the CPU 13 may adjust the voltage of the second polarity (e.g., the positive transfer bias Vz) used during the current detection period according to the environmental conditions. This allows the influence of the negative transfer bias during the current detection period to be reduced with precision.

[0132] (Item 10) Further, a current detecting means for detecting a transfer current flowing through the transfer member is provided, 10. The image forming apparatus according to item 9, wherein the control unit determines a target value of the transfer current based on the environmental conditions, and while changing the output voltage of the second power supply, determines the output voltage when the transfer current reaches the target value as the voltage of the second polarity.

[0133] The current detection circuit 207 is an example of a current detection unit that detects the transfer current flowing through the transfer roller 8. The CPU 13 may determine a target value Iz of the transfer current based on environmental conditions, and while changing the output voltage of the second power supply, determine the output voltage when the transfer current reaches the target value as a voltage of the second polarity (e.g., positive transfer bias Vz).

[0134] (Item 11) The apparatus further includes a storage means for storing a plurality of environmental conditions and a plurality of target values in one-to-one correspondence; Item 11. The image forming apparatus according to item 10, wherein the control unit acquires the target value corresponding to the environmental condition detected by the environmental detection unit from the storage unit.

[0135] The memory 15 and the non-volatile memory 30, or the tables, mathematical functions, or program modules stored therein, function as a storage means for storing a plurality of environmental conditions and a plurality of target values in one-to-one correspondence. The CPU 13 may obtain the target values corresponding to the environmental conditions from the storage means.

[0136] (Item 12) 10. The image forming apparatus according to item 9, wherein an adjustment period in which the voltage of the second polarity used in the current detection period is adjusted precedes the current detection period.

[0137] As illustrated in FIG. 9, the adjustment period (eg, period Z) in which the voltage of the second polarity used in the current detection period is adjusted may precede the current detection period (eg, periods A to C).

[0138] (Item 13) Item 13. The image forming apparatus according to item 12, wherein a waiting period for returning the surface potential of the image carrier to 0 V is provided between the adjustment period and the current detection period.

[0139] As illustrated in FIG. 9, period G is an example of a waiting period that is provided between the adjustment period and the current detection period and is used to return the surface potential of the image carrier to 0V.

[0140] (Item 14) the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the first period, the voltage of the second polarity is not supplied to the transfer member; During the second period, the voltage of the second polarity is supplied to the transfer member; 2. The image forming apparatus according to item 1, wherein an adjustment period in which the voltage of the second polarity used in the second period is adjusted is provided between the first period and the second period.

[0141] 11, the adjustment period (e.g., period Z) may be provided between the first period (e.g., period A) and the second period (e.g., period C). This makes it possible to omit period G, and the surface potential of the photosensitive drum 1 can be obtained in a shorter time.

[0142] (Item 15) Item 15. The image forming apparatus according to item 14, wherein a waiting period for returning the surface potential of the image carrier to 0 V is provided between the adjustment period and the second period.

[0143] As illustrated in FIG. 11, a waiting period (e.g., period B) for returning the surface potential of the photosensitive drum 1 to 0 V may be provided between the adjustment period (e.g., period Z) and the second period (e.g., period C).

[0144] (Item 16) the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the second period, the voltage of the second polarity is supplied to the transfer member; Item 2. The image forming apparatus according to item 1, wherein an adjustment period in which the voltage of the second polarity used in the second period is adjusted is provided within the first period or is provided in parallel with the first period.

[0145] As illustrated in FIG. 13, an adjustment period (e.g., period Z) in which the voltage of the second polarity (positive transfer bias Vz") used in the second period (e.g., period C) is adjusted may be provided within the first period (e.g., period A) or in parallel with the first period. This will enable the surface potential of the photosensitive drum 1 to be obtained in an even shorter time.

[0146] (Item 17) a current detection means for detecting a transfer current flowing through the transfer member; and an environmental detection means for detecting an environmental condition, the control means determines a target value of the transfer current based on the environmental conditions; Item 17. The image forming apparatus according to item 16, wherein the control unit determines the voltage of the second polarity to be output during the second period based on the output voltage when the transfer current reaches the target value while changing the output voltage of the second power supply during the adjustment period.

[0147] As illustrated in FIG. 13, during an adjustment period (e.g., period Z), CPU 13 changes the output voltage of power supply circuit 206 to determine the output voltage (e.g., positive transfer bias Vz) when the transfer current reaches target value Iz. Furthermore, CPU 13 may determine a voltage of a second polarity (e.g., positive transfer bias Vz") based on the positive transfer bias Vz.

[0148] (Item 18) Item 18. The image forming apparatus according to item 17, wherein the control unit determines the voltage of the second polarity by adding a difference between the voltage of the first polarity applied to the transfer member during the adjustment period and the first period and the voltage of the first polarity applied to the transfer member during the second period to the output voltage when the transfer current reaches the target value.

[0149] As shown in Eq12, the CPU 13 calculates the difference between the voltage of the first polarity (e.g., charging bias Va) applied to the transfer roller 8 during the adjustment period and the first period, and the voltage of the first polarity (e.g., charging bias Vb) applied to the transfer roller 8 during the second period. The CPU 13 may determine the voltage of the second polarity (e.g., positive transfer bias Vz") by adding this difference to the output voltage (e.g., positive transfer bias Vz) when the transfer current reaches the target value.

[0150] (Item 19) the light source does not expose the image carrier during the first period and exposes the image carrier during the second period; Item 18. The image forming apparatus according to item 17, wherein the control unit controls the second power supply so as to gradually increase the output voltage during the adjustment period until the transfer current reaches the target value.

[0151] 13, the light source 3 does not expose the photosensitive drum 1 to light during periods A and Z. The light source 3 exposes the photosensitive drum 1 to light during period C. The CPU 13 may gradually increase the output voltage of the power supply circuit 206 until the transfer current reaches the target value Iz.

[0152] (Item 20) 20. The image forming apparatus according to claim 1, wherein the first power source is configured to output a charging voltage having the first polarity to the charging member, while outputting a transfer voltage having the first polarity to the transfer member, the transfer voltage varying in conjunction with the charging voltage.

[0153] (Item 21) 21. The image forming apparatus according to claim 1, wherein the polarity of the superimposed voltage is the first polarity.

[0154] The polarity of the superimposed voltage may be the second polarity.

[0155] (Item 22) 22. The image forming apparatus according to any one of items 1 to 21, wherein the voltage of the second polarity supplied from the second power source to the transfer member is a transfer bias for promoting transfer of the toner image from the image carrier to the transfer material.

[0156] (Item 23) 23. The image forming apparatus according to claim 1, wherein the voltage of the first polarity supplied from the first power source to the transfer member is a cleaning bias for retransferring toner adhering to the transfer member to the image carrier.

[0157] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0158] 1. Photosensitive drum, 2. Charging roller, 8. Transfer roller, 13. CPU, 201. Charging power supply, 205. Transfer power supply

Claims

1. an image carrier; a motor that rotates and drives the image carrier; a charging member for charging the surface of the image carrier that is rotated by the motor; a light source that irradiates the surface of the image carrier with light to form an electrostatic latent image; a developing member for developing the electrostatic latent image with toner charged to a first polarity to form a toner image; a transfer member that transfers the toner image from the image carrier to a transfer material; a first power source that generates a voltage of the first polarity, supplies the voltage of the first polarity to the charging member as a charging bias, and supplies the voltage of the first polarity to the transfer member; a second power supply that generates a voltage of a second polarity opposite to the first polarity and supplies the voltage of the second polarity to the transfer member; a detecting means for detecting a charging current flowing between the charging member and the image carrier; a control unit that controls the first power source and the second power source and determines the surface potential of the image carrier based on the charging current, The image forming apparatus is configured such that, during a current detection period in which the detection means detects the charging current, the control means operates the first power source and the second power source to apply a superimposed voltage generated by superimposing a voltage of the first polarity and a voltage of the second polarity to the transfer member, and determines the surface potential of the image carrier based on the charging current detected during the current detection period.

2. 2. The image forming apparatus according to claim 1, wherein the voltage of the first polarity and the voltage of the second polarity used during the current detection period are adjusted so that a discharge occurs between the charging member and the image carrier, but a discharge does not occur between the transfer member and the image carrier.

3. the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the first period, the voltage of the second polarity is not supplied to the transfer member; During the second period, the voltage of the second polarity is supplied to the transfer member; 2. The image forming apparatus according to claim 1, wherein the control means is configured to determine the surface potential of the image carrier based on the first charging current detected by the detection means during the first period and the second charging current detected by the detection means during the second period.

4. 4. The image forming apparatus according to claim 3, wherein the control means is configured to obtain a current change amount per unit voltage by dividing a difference between the first charging current and the second charging current by a difference between the voltage of the first polarity supplied to the transfer member in the second period and the voltage of the first polarity supplied to the transfer member in the first period, and to obtain a dark potential, which is a surface potential of the image carrier, by dividing the second charging current by the current change amount.

5. the current detection period includes a first period, a second period, and a third period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period, and an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the third period; During the first period, the voltage of the second polarity is not supplied to the transfer member; the voltage of the second polarity is supplied to the transfer member during the second period and the third period; the first period and the second period are periods during which a surface portion of the image bearing member that is at a dark potential passes through the charging member, the third period is a period during which a surface portion of the image bearing member that is at a light area potential passes through the charging member, 2. The image forming apparatus according to claim 1, wherein the control means is configured to determine the surface potential of the image carrier based on a first charging current detected by the detection means in the first period, a second charging current detected by the detection means in the second period, and a third charging current detected by the detection means in the third period.

6. 6. The image forming apparatus according to claim 5, wherein the control means is configured to obtain a current change amount per unit voltage by dividing a difference between the first charging current and the second charging current by a difference between the voltage of the first polarity supplied to the transfer member in the second period and the voltage of the first polarity supplied to the transfer member in the first period, obtain a voltage by dividing the third charging current by the current change amount, and obtain a light area potential of the image carrier by subtracting the obtained voltage from a dark area potential of the image carrier.

7. 7. The image forming apparatus according to claim 6, wherein said control means is configured to obtain said dark potential by dividing said second charging current by said current change amount.

8. 2. The image forming apparatus according to claim 1, wherein the control unit adjusts the voltage of the second polarity used during the current detection period in accordance with a resistance value of the transfer member.

9. The image forming apparatus further includes an environment detection unit that detects the environmental conditions of the environment in which the image forming apparatus is installed, 2. The image forming apparatus according to claim 1, wherein the control unit adjusts the voltage of the second polarity used during the current detection period in accordance with the environmental conditions.

10. Further, a current detecting means for detecting a transfer current flowing through the transfer member is provided, 10. The image forming apparatus according to claim 9, wherein the control unit determines a target value of the transfer current based on the environmental conditions, and while changing the output voltage of the second power supply, determines the output voltage when the transfer current reaches the target value as the voltage of the second polarity.

11. The apparatus further includes a storage means for storing a plurality of environmental conditions and a plurality of target values in one-to-one correspondence; 11. The image forming apparatus according to claim 10, wherein the control unit acquires the target value corresponding to the environmental condition detected by the environment detection unit from the storage unit.

12. The image forming apparatus according to claim 9 , wherein an adjustment period in which the voltage of the second polarity used in the current detection period is adjusted precedes the current detection period.

13. 13. The image forming apparatus according to claim 12, wherein a waiting period for returning the surface potential of the image carrier to 0 V is provided between the adjustment period and the current detection period.

14. the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the first period, the voltage of the second polarity is not supplied to the transfer member; During the second period, the voltage of the second polarity is supplied to the transfer member; 2. The image forming apparatus according to claim 1, wherein an adjustment period in which the voltage of the second polarity used in the second period is adjusted is provided between the first period and the second period.

15. 15. The image forming apparatus according to claim 14, wherein a waiting period for returning the surface potential of the image carrier to 0 V is provided between the adjustment period and the second period.

16. the current detection period includes a first period and a second period; an absolute value of the voltage of the first polarity in the first period is smaller than an absolute value of the voltage of the first polarity in the second period; During the second period, the voltage of the second polarity is supplied to the transfer member; 2. The image forming apparatus according to claim 1, wherein an adjustment period in which the voltage of the second polarity used in the second period is adjusted is provided within the first period or is provided in parallel with the first period.

17. a current detection means for detecting a transfer current flowing through the transfer member; and an environmental detection means for detecting an environmental condition, the control means determines a target value of the transfer current based on the environmental conditions; 17. The image forming apparatus according to claim 16, wherein the control unit determines the voltage of the second polarity to be output during the second period based on the output voltage when the transfer current reaches the target value while changing the output voltage of the second power supply during the adjustment period.

18. 18. The image forming apparatus according to claim 17, wherein the control means determines the voltage of the second polarity by adding a difference between the voltage of the first polarity applied to the transfer member during the adjustment period and the first period and the voltage of the first polarity applied to the transfer member during the second period to the output voltage when the transfer current reaches the target value.

19. the light source does not expose the image carrier during the first period and exposes the image carrier during the second period; 18. The image forming apparatus according to claim 17, wherein the control unit controls the second power supply so as to gradually increase the output voltage during the adjustment period until the transfer current reaches the target value.

20. 2. The image forming apparatus according to claim 1, wherein the first power source is configured to output a charging voltage having the first polarity to the charging member, and to output a transfer voltage having the first polarity to the transfer member, the transfer voltage varying in conjunction with the charging voltage.

21. The image forming apparatus according to claim 1 , wherein the polarity of the superimposed voltage is the first polarity.

22. 2. The image forming apparatus according to claim 1, wherein the voltage of the second polarity supplied from the second power source to the transfer member is a transfer bias for facilitating transfer of the toner image from the image carrier to the transfer material.

23. 2. The image forming apparatus according to claim 1, wherein the voltage of the first polarity supplied from the first power source to the transfer member is a cleaning bias for retransferring toner adhering to the transfer member to the image carrier.

24. an image carrier; a motor that rotates and drives the image carrier; a charging member for charging the surface of the image carrier that is rotated by the motor; a light source that irradiates the surface of the image carrier with light to form an electrostatic latent image; a developing member for developing the electrostatic latent image with toner charged to a first polarity to form a toner image; a transfer member that transfers the toner image from the image carrier to a transfer material; a first power source that generates a voltage of the first polarity and applies the voltage of the first polarity to both the charging member and the transfer member; a second power supply that generates a voltage of a second polarity opposite to the first polarity and applies the voltage of the second polarity to the transfer member; a detecting means for detecting a charging current flowing between the charging member and the image carrier; a control unit that controls the first power source and the second power source and determines the surface potential of the image carrier based on the charging current, the control means controls the first power source to generate a voltage of the first polarity during an image forming period in which the toner image is formed, and to supply the voltage of the first polarity to the charging member as a charging bias and to supply the voltage of the first polarity to the transfer member as a cleaning bias; the control means controls the second power source during the image formation period to generate a voltage of the second polarity and supply the voltage of the second polarity to the transfer member as a transfer bias for facilitating transfer of the toner image from the image carrier to the transfer material; the control means keeps the first power source operating and stops the second power source during a cleaning period for cleaning the transfer member; The image forming apparatus is configured such that, during a current detection period in which the detection means detects the charging current, the control means operates the first power source and the second power source to apply a superimposed voltage generated by superimposing a voltage of the first polarity and a voltage of the second polarity to the transfer member, and determines the surface potential of the image carrier based on the charging current detected during the current detection period.

Images