Image forming apparatus and control method for a motor used therein

Abstract

To obtain, in feedback control of a motor operating while switching between a plurality of rotation speeds, stable following of the motor when a target is switched to a lower speed without using a special speed detector or circuit.SOLUTION: An image forming apparatus comprises: a driving circuit that drives a motor upon reception of a driving signal; a speed detector that outputs a rotation signal at a time interval according to the rotation speed of the motor; and a feedback control unit that obtains the current rotation speed of the motor from the rotation signal output from the speed detector, and sequentially updates and outputs the driving signal to the driving circuit so that the motor rotates at a target speed. When the rotation signal from the speed detector is not output within a period from the previous update of the driving signal until the update this time, the feedback control unit considers that the speed detector has output a rotation speed lower than a target rotation speed and updates the driving signal.SELECTED DRAWING: Figure 6

Description

Technical Field

[0004] ,

[0001] This disclosure relates to an image forming apparatus and a method for controlling a motor used therein.

Background Art

[0002] In an image forming apparatus, in addition to driving mechanisms related to the image forming process, one or more motors are used to convey originals and printing paper related to image formation. Some of these motors apply speed feedback control to their driving. A typical motor to which speed feedback control is applied is a DC motor. Speed feedback control detects the actual rotational speed of a motor using a speed detector such as a tachogenerator that detects the rotational speed of the motor or an encoder that detects the rotational angle, and controls the driving of the motor so that the detected rotational speed approaches a predetermined speed. When the rotational speed is faster than the target speed, the driving is weakened according to the difference in speed from the target to decelerate, and conversely, when the rotational speed is slower than the target speed, the driving is strengthened according to the difference in speed from the target to accelerate. By performing appropriate feedback control, the rotation of the motor can be maintained at a predetermined speed against fluctuations in load.

[0003] By the way, among the motors used in an image forming apparatus, there are some that not only rotate at a constant speed but also need to change the speed or stop at a predetermined timing. For example, the following rotation control method regarding stop control is known. It relates to the control of a registration operation for conveying paper to a transfer position in accordance with the timing of image formation. The registration operation abuts against a registration roller that is stopped on the downstream side in the paper conveyance direction in order to convey the paper to the transfer position in accordance with the timing of image formation. At that time, when a deflection for correcting skewing is formed on the leading end side of the paper, the upstream conveyance roller is temporarily stopped, and the rotation of both the conveyance roller and the registration roller is started in accordance with the timing of image formation.

[0004] When the transport roller is temporarily stopped, the interval between pulse signals from the encoder increases as the rotation speed decreases, resulting in a longer waiting time for pulse input. During this waiting time, feedback control, which determines the relationship between the actual speed and the target speed during deceleration based on the magnitude of the pulse signal interval and performs acceleration and deceleration accordingly, cannot be executed. As a result, the stopping distance may be unstable, and the stopping time may also be long. Therefore, the rotational speed is reduced to a reference speed by repeatedly performing at least one of the following actions from the start of deceleration: braking and acceleration or free running (coasting), and then braking is continued until the vehicle comes to a stop (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2014-139636 [Overview of the project] [Problems that the invention aims to solve]

[0006] The method described in Patent Document 1 aims to suppress variations in the stopping distance (paper transport distance) and stopping time from the time the resist sensor detects the leading edge of the paper until the transport roller stops. However, in the transport control of originals and printing paper, it is sometimes necessary to control the motor not only by stopping it, but also by rapidly switching the target speed. For example, there is a control that reduces the transport speed just before the printing paper is discharged to the output tray. It is preferable to discharge the paper after sufficiently slowing it down so that it is neatly stacked on the output tray. Also, when switching back a printed sheet of paper that has had an image printed on one side to guide it to a double-sided printing path for printing on the other side, the transport speed is reduced before the switchback. If the trailing edge of the printing paper and the leading edge of the following printing paper overlap during these decelerations, a paper jam will occur. To avoid a paper jam, it is necessary to ensure sufficient spacing between the following printing sheets, but if the spacing is too large, the productivity of image formation will decrease. Therefore, when temporarily reducing the transport speed as described above, it is preferable to reduce it as quickly as possible and shorten the deceleration period as much as possible. Thus, feedback control that changes the target speed in a roughly stepped manner and makes the motor rotation follow that change is desirable.

[0007] While I've used printing paper as an example, the same principle applies to controlling the document transport device that handles the transport of original documents. However, if the target speed after the change is lower than the target speed before the change, an undershoot will occur in the motor's rotational speed as it attempts to keep up with the change to the target speed. If the time interval of the pulse signals from the encoder at the target speed is comparable to the motor's response time, there may be moments during the undershoot when the time interval of the pulse signals becomes longer than the motor's response time. Here, the motor's response time is the time expressed, for example, as the motor's mechanical time constant under load.

[0008] Feedback control is preferably performed at time intervals shorter than the motor's response time in order to achieve stable control of the system. However, if the time interval of the pulse signal is longer than the motor's response time, the information that forms the basis of feedback control, which accelerates and decelerates the motor based on the pulse signal, cannot be obtained, and feedback control at predetermined time intervals becomes substantially impossible. By using a high-resolution encoder to shorten the pulse interval, it may be possible to obtain a pulse interval shorter than the motor's response time even when changing to a lower target speed. However, using a high-resolution encoder shortens the pulse time interval not only when the motor is rotating at low speeds but also at high speeds. At high speeds, processing related to the pulse signal will be performed more frequently than necessary.

[0009] The deceleration periods during paper or document ejection, or during switchbacks, as described above, represent only a small fraction of the overall transport control process. Therefore, it could be argued that processing pulse signals more frequently than necessary is unreasonable. This disclosure is made in consideration of the circumstances described above, and provides a method for obtaining stable motor tracking when switching to a lower speed in feedback control of a motor that operates by switching between multiple rotational speeds, without using special speed detectors or circuits. [Means for solving the problem]

[0010] This disclosure provides an image forming apparatus comprising: a drive circuit that drives a motor upon receiving a drive signal; a speed detector that outputs a rotation signal at time intervals corresponding to the rotation speed of the motor; and a feedback control unit that obtains the current rotation speed of the motor from the rotation signal output from the speed detector and sequentially updates and outputs a drive signal to the drive circuit so that the motor rotates at a target speed, wherein the feedback control unit updates the drive signal if no rotation signal is output from the speed detector during the period from the previous update of the drive signal to the current update, by assuming that the speed detector has output a rotation speed lower than the target rotation speed.

[0011] Furthermore, from a different perspective, this disclosure relates to a control unit that controls a motor used in an image forming apparatus, comprising the steps of: acquiring rotation signals at time intervals corresponding to the rotation speed of the motor using a speed detector; obtaining the current rotation speed of the motor from the rotation signals output from the speed detector; sequentially updating and outputting a drive signal to a drive circuit so that the motor rotates at a target speed; and updating the drive signal if, during the period from the previous update of the drive signal to the current update, the speed detector is deemed to have output a rotation speed lower than the target speed. This provides a motor control method for performing the following. [Effects of the Invention]

[0012] In the image forming apparatus according to this disclosure, if the current rotational speed cannot be obtained because no rotational signal is output from the speed detector during the period between the previous update of the drive signal and the current update, the feedback control unit updates the drive signal by assuming that the speed detector has output a rotational speed lower than the target rotational speed. Therefore, stable motor tracking can be obtained when the target speed is switched to a lower speed without using a special speed detector or circuit. The motor control method disclosed in this information also produces similar effects. [Brief explanation of the drawing]

[0013] [Figure 1] This is a perspective view showing the external appearance of a multifunction printer, which is an example of an image processing device as described in this disclosure. [Figure 2] Figure 1 is a cross-sectional view showing the configuration of the multifunction printer. [Figure 3] Figures 1 and 2 are block diagrams showing the configuration related to the control and transport drive of the multifunction printer. [Figure 4] These are explanatory diagrams showing the motors arranged in the image reading and printing sections, as shown in Figures 1 to 3. [Figure 5] These are explanatory diagrams showing the motors arranged in the document transport unit shown in Figures 1 to 3. [Figure 6] This is an explanatory diagram showing a configuration example of a motor and a feedback control unit in this embodiment. [Figure 7] This is a flowchart showing an example of a case where a processor executes the processing of the feedback control unit in this embodiment. [Figure 8] This is an explanatory diagram showing the relationship between an example of the waveform of the rotation signal output from the encoder and the current rotation speed derived by the rotation speed measurement unit based on the rotation signal in this embodiment. [Figure 9] This is a waveform diagram showing an example of the transient response waveform during deceleration operation by a conventional method shown as a comparative example in FIG. 8. [Figure 10] FIG. 10 is a waveform diagram showing an example of the transient response waveform during deceleration operation by the method of this embodiment.

Embodiments for Carrying out the Invention

[0014] Hereinafter, this disclosure will be described in more detail with reference to the drawings. Note that the following description is illustrative in all respects and should not be construed as limiting this disclosure. ≪Configuration Example of Image Forming Apparatus≫ FIG. 1 is a perspective view showing the appearance of a digital multifunction device which is an embodiment of the image forming apparatus of this disclosure. FIG. 2 is a cross-sectional view showing the mechanical configuration of the multifunction device shown in FIG. 1. FIG. 3 is a block diagram showing the configuration related to the control and conveyance drive of the multifunction device shown in FIGS. 1 and 2.

[0015] As shown in FIGS. 1 to 3, the multifunction device 100 includes an image reading unit 111 for reading a document, an operation unit 105 for receiving user operations, a communication circuit 107 (see FIG. 3), and a printing unit 115 for performing image formation, in the main body. The operation unit 105 notifies the operator of the state and settings of the device and receives instructions. The communication circuit 107 enables the control unit 101 to communicate with external devices. Furthermore, paper feed trays 18A, 18B, 18C, and 18D are provided below the printing unit 115. A discharge tray 39A is provided above the printing unit 115 and below the image reading unit 111, and a discharge tray 39B protruding from the right end of the printing unit 115 is provided. The main unit is equipped with a document transport unit 103 that transports the document to the scanning unit. The multifunction printer 100 also includes a control unit 101 that controls its operation (see Figure 3).

[0016] Here, we will describe the internal configuration of the main unit of the multifunction printer 100 shown in Figure 2. The multifunction printer 100 forms toner images of four colors—yellow (Y), magenta (M), cyan (C), and black (BK)—using an electrophotographic process, and prints a color image on a printing sheet by superimposing these images. Alternatively, it prints a monochrome image on a printing sheet using a single color (e.g., black). For this purpose, four development units 12, four photoreceptor drums 13, four chargers 14, and four drum cleaners 15 are provided. The photoreceptor drums 13, four chargers 14, and four drum cleaners 15 are configured as a single, detachable process unit. Below the process unit 30, a light scanning unit 11 is positioned to scan and expose the photoreceptor drums 13 corresponding to each color with a laser beam polarized by a polygon mirror 11M.

[0017] The multifunction printer 100 is equipped with process units 30y, 30m, 30c, and 30k for each color. However, in Figure 2, only the components of the yellow process unit 30y are labeled, while those for the other colors are omitted. Process units may also be referred to as process unit 30 using representative codes. It should be understood that explanations using representative codes apply to each of the colors Y, M, C, and K.

[0018] Above the process unit 30 are toner storage units 27 corresponding to each color. The developing unit 12 and the toner storage units 27 also have units for each color, similar to the process unit 30, but only the yellow unit is labeled, while the other colors are omitted.

[0019] The multifunction printer 100 further includes an image processing circuit 41 that generates an input signal to the optical scanning unit 11 (see Figure 3). The image processing circuit 41 processes the image data of the original document read by the image reading unit 111 to generate exposure data related to the exposure pattern of the photoreceptor drum 13. The exposure data corresponds to the pattern of the electrostatic latent image formed on the surface of the photoreceptor drum 13. Under the control of the control unit 101 shown in Figure 3, the system controls the photoreceptor drum 13 to form a toner image of Y, M, C, or K through an electrophotographic process consisting of cleaning by the drum cleaner 15, charging by the charger 14, exposure by the light scanning unit 11, and development by the development unit 12.

[0020] The toner images formed in Y, M, C, and K on the photoreceptor drum 13 are transferred onto the intermediate transfer belt 21 by the primary transfer roller 16 via the intermediate transfer belt 21. The control unit 101 rotates the intermediate transfer belt 21 in synchronization with the rotation of the photoreceptor drums 13 of each color, and moves the toner images transferred onto the intermediate transfer belt 21 to a position where they contact the secondary transfer unit 23 at the right end. The control unit 101 rotates the transfer belt stretched across the secondary transfer unit 23 in synchronization with the intermediate transfer belt. Then, it transfers the toner images from the intermediate transfer belt 21 onto the print sheets fed from the paper trays 18A to 18D.

[0021] The control unit 101 transports the printed sheet, onto which the toner image has been transferred by the secondary transfer unit 23, to the fixing unit 17. The fixing unit 17 heats and pressurizes the printed sheet as it passes between the opposing heating roller R11 and pressure roller, fixing the toner image transferred to the printed sheet to the sheet.

[0022] The control unit 101 discharges the printed sheet that has passed through the fuser unit 17 to the discharge tray 39A. Alternatively, it switches the sheet back once at the discharge roller R13 and discharges it via the discharge roller R14 to the discharge tray 39B on the right side. Or, it guides the switched-back printed sheet to the double-sided transport path where the double-sided transport rollers R15, R16, and R17 are located, and returns it to the transfer section where the secondary transfer unit 23 is located. Then, it transfers the toner image to the back side of the printed sheet and discharges the printed sheet via the fuser unit 17 to the discharge tray 39A or 39B.

[0023] As shown in Figure 3, the control unit 101 includes hardware resources such as a processor 121, RAM 122, and non-volatile memory 123. The non-volatile memory 123 stores control programs and data in a rewritable format. The processor 121 executes the control program pre-stored in the non-volatile memory 123 and works in cooperation with the hardware resources to realize the functions of the control unit 101. The control unit 101 includes a motor control unit 125 that controls multiple motors, as will be described later. The motor control unit 125 may include a feedback control unit 127. However, the configuration is not limited to this, and the feedback control unit 127 may be configured using different hardware resources than the control unit 101.

[0024] The block of the printing unit 115 shown in Figure 3 primarily shows the motors that serve as drive sources for paper transport and document transport, as well as the loads (rollers, etc.) driven by each motor. The order of the rollers shown in Figure 3 corresponds to the arrangement of the rollers in Figure 2 to some extent. The arrangement of each load is indicated by numerals in Figure 2, and the arrangement of each motor is shown in Figures 4 and 5. As shown in Figure 3, the paper feed motor 50 drives the paper feed roller R01, the pickup roller R02, and the separation roller R03 via the paper feed clutch 52. The pickup roller R02 descends when the paper feed solenoid 53 is turned on and contacts the uppermost print sheet contained in the paper feed tray 18A, feeding the print sheet between the paper feed roller R01 and the separation roller R03.

[0025] The paper feed motor 50 also drives the transport rollers R04, R05, R06, and R07 via the transport clutch 54. The developing motor 58 drives the developing unit 12 and the processing unit 30. The transfer transport motor 60 drives the secondary transfer drive roller R09, the intermediate transfer drive roller R10, and the heating roller R11 and transport roller R12 of the fixing unit 17. The resist motor 56 drives the resist roller R08 to perform resist operations on the printed sheet.

[0026] Furthermore, the discharge reversal motor 64 drives the discharge rollers R13 and R14 in a reversible manner. The discharge roller R13 can be shifted by the shift motor 65 in a direction perpendicular to the transport direction of the printed sheet (towards the front and back relative to the paper surface in Figure 2). By shifting the roller, the position of the printed sheets discharged into the discharge tray 39A can be shifted and stacked on a part-by-part or job-by-job basis. The double-sided conveying motor 62 drives the double-sided conveying rollers R15, R16, and R17.

[0027] In addition, there are several motors for the printing unit 115 that are not shown in Figure 3. For example, there is a lift-up motor that lifts up the printing sheets stored in the paper trays 18A to 18D. There are also motors that rotate the polygon mirror 11M of the optical scanning unit 11, and toner motors that supply toner stored in the toner storage units 27 of each color to the corresponding developing unit 12.

[0028] Furthermore, the image reading unit 111 of the multifunction printer 100 includes a document scanning unit 68 that scans a document placed on a transparent document tray 67 located on the upper surface of the image reading unit 111, as shown in Figure 4. It also includes a scan motor 69 that drives the document scanning unit 68. Furthermore, the document transport unit 103, which is positioned to cover the document tray 67, includes a document feed motor 71, a document registration motor 73, and a document transport motor 75, as shown in Figures 3 and 5.

[0029] The document feed motor 71 drives the document feed roller R31, the document pickup roller R32, and the document separation roller R33 (see Figures 2 and 3). When the document feed solenoid 72 is turned on, the document pickup roller R32 descends and contacts the uppermost document placed in the document feed tray 43, feeding that document between the document feed roller R31 and the document separation roller R33. The document registration motor 73 drives the document registration roller R34 to perform the registration operation on the print sheet. That is, it corrects the skew of the document fed and transported from the document feed tray 43. The document transport motor 75 drives the document transport rollers R35 and R36 and the document ejection roller R37.

[0030] Motor Feedback Control The multifunction printer 100 is equipped with multiple motors of different types, each suited to the required performance. For applications that require switching between different rotational speeds over a wide range, a DC motor with feedback control is used. In this embodiment, the feedback-controlled DC motors are applied to the following: In the printing unit 115, they are applied to the paper feed motor 50, the resist motor 56, the developer motor 58, the transfer transport motor 60, the duplex transport motor 62, and the discharge reversal motor 64. A stepping motor is applied to the shift motor 65. A stepping motor is used for the scan motor 69 provided in the image reading unit 111. The feedback-controlled DC motor is further applied to the document feed motor 71, document registration motor 73, and document transport motor 75 of the document transport unit 103.

[0031] This section describes control methods for decelerating by significantly changing the rotational speed. The discharge reversal motor 64 reduces the transport speed just before the printed sheet finishes passing the discharge roller R13. This allows the printed sheet to be slowly discharged into the discharge tray 39A so as not to fly too far from the position of the discharge roller R13. In the case of double-sided printing, the transport speed of the printed sheet is reduced just before it finishes passing the discharge roller R13 after the first side has been printed, and then it is transported in the reverse direction to guide it to the double-sided transport path. Also, the transport speed is reduced just before the printed sheet finishes passing the discharge roller R14. This allows the printed sheet to be slowly discharged into the discharge tray 39B so as not to fly too far from the position of the discharge roller R14. Furthermore, the document transport motor in the document transport unit 103 reduces its transport speed just before the document finishes passing the document discharge roller R37. In this way, the document is slowly discharged into the document discharge tray 45 so that it does not fly far away from the document discharge tray 45.

[0032] Figure 6 is an explanatory diagram showing an example configuration of the motor and feedback control unit in this embodiment. As shown in Figure 6, the feedback control unit 127 includes a target rotational speed acquisition unit 129, a rotational speed measurement unit 131, a detection lower limit processing unit 133, and a motor drive unit 135. The motor drive unit 135 generates and outputs a drive signal to the driver circuit 137. The driver circuit 137 drives a DC motor 138 equipped with an encoder 139. As described in the explanation of Figure 3, the feedback control unit 127 may be configured using the hardware resources of the control unit 101, or it may be configured using separate hardware resources. That is, the processor 121 of the control unit 101 may perform the processing as the feedback control unit 127, or it may be configured as a separate circuit from the control unit 101.

[0033] Furthermore, in the examples shown in Figures 3 and 6, the feedback control unit 127 is assumed to have its functions implemented using a processor, but this is only one possible configuration. All or part of the functions of the feedback control unit 127 may be implemented using hardware circuits. The feedback control unit 127 basically executes feedback control processing at time intervals shorter than the mechanical time constant of the DC motor 138 when driving the load. The DC motor 138 shown in Figure 6 should be understood as, for example, the ejection reversal motor 64, or the document transport motor 75. However, it is not limited to these, and should not be understood as corresponding to various motors to which a DC motor is applied in this embodiment. Furthermore, the configuration shown in Figure 6 is not limited to a DC motor, but can be any motor to which feedback control is applied.

[0034] The driver circuit 137 shown in Figure 6 receives a drive signal from the feedback control unit 127 and drives the DC motor 138. In other words, the driver circuit 137 corresponds to the drive circuit that drives the motor. Furthermore, the encoder 139 corresponds to a speed detector that outputs a rotation signal at intervals corresponding to the motor's rotation speed. In Figure 6, the target rotational speed acquisition unit 129 acquires the target rotational speed from the processor 121, which acts as a motor control unit 125 that performs higher-level control.

[0035] The rotational speed measurement unit 131 acquires a rotational signal from the encoder 139. It then stores the time at which the rotational signal was acquired. The rotational speed measurement unit 131 stores at least the most recent time when a rotational signal was obtained from the encoder 139 and the time when the previous rotational signal was obtained. The rotation speed measurement unit 131 performs a process to determine the current rotation speed of the DC motor 138 based on the time interval between the time when the most recent rotation signal was obtained and the time when the previous rotation signal was obtained.

[0036] However, as described above, the feedback control unit 127 repeatedly executes the processing related to feedback control. If the rotational speed of the DC motor 138 is low, the time interval between the signals output from the encoder 139 may be too wide, and it may occur that no rotational signals are obtained between the time of the previous processing and the time of the current processing. Alternatively, it may occur that no rotational signals are obtained for a period of time preceding the current processing, corresponding to the response time of the feedback control system, such as the mechanical time constant of the motor in a load-driven state. This can also be described as a state where the most recent rotational signal is not obtained. When such a state occurs, the rotational speed measurement unit 131 cannot derive the current rotational speed of the DC motor 138 based on the most recent rotational signal. In that case, the detection limit processing unit 133 outputs a predetermined rotation speed instead of the current rotation speed that the rotation speed measurement unit 131 should derive. The motor drive unit 135 calculates the difference between the target speed output from the target rotation speed acquisition unit 129 and the current rotation speed of the DC motor 138 output from the rotation speed measurement unit 131 via the detection lower limit processing unit 133, and outputs a drive signal corresponding to the difference to the driver circuit 137.

[0037] Flowchart The procedure for when the processor executes the feedback control process described above will be explained with reference to the flowchart. Figure 7 is a flowchart showing an example in this embodiment where the processor performs the processing as the feedback control unit 127. The processor may be the processor for the control unit 101, or it may be a processor dedicated to the feedback control unit 127, for example.

[0038] As shown in Figure 7, the processor obtains the target rotational speed at that time from the higher-level motor control unit 125 (step S11). The higher-level motor control unit 125 provides the target rotational speed at each point in time according to the progress of the printing job, etc. Once the target rotational speed is obtained, the processor checks whether a rotational signal has been acquired from the encoder 139 (step S13). Here, it is determined whether the most recent rotational signal was acquired within a predetermined period of time retrospectively from the present (step S15). An example of a predetermined period is the period from the time the feedback control unit last updated the drive signal to the present. Alternatively, another example is a period corresponding to the motor's response time, i.e., a period corresponding to the motor's mechanical time constant under load.

[0039] If the processor determines that the most recent rotation signal has been acquired within a predetermined period (Yes in step S15), it calculates the current motor rotation speed based on the time interval between the time the most recent rotation signal was acquired and the time the previous rotation signal was acquired (step S17). The rotation speed is calculated considering the resolution of the encoder 139 (the rotation angle of the motor shaft from the time one rotation signal is output until the next rotation signal is output, which can also be described as the time interval of rotation signals for a given rotation speed). Then, the process proceeds to step S21, which will be described later. On the other hand, if it is determined that no recent rotation signal has been acquired within a predetermined period (No in step S15), the processor determines that it cannot calculate the current rotation speed of the motor and instead outputs a predetermined rotation speed (step S19). Then, the process proceeds to step S21, which will be described later.

[0040] Let's further discuss the problems that arise when the most recent rotation signal is not acquired within a specified period. If a rotation signal obtained at a point further back than the specified period is used as the most recent rotation signal, it will ultimately be the same value as the current rotation speed at the time of the previous update. If the target speed is the same as last time, the same drive signal as the previously updated drive signal will be output. Even if the target speed is not the same as last time, the rotation signal is the same as last time, so the feedback control will not reflect the current motor rotation speed. The problems that become apparent in that case are as already described as challenges. Specifically, in the case of undershoot when the target speed is changed and decelerating occurs, if the time interval of the rotation signal from encoder 139 becomes longer than the motor's response time, stable control of the feedback system cannot be achieved.

[0041] In this embodiment, the processor outputs a predetermined rotational speed to ensure stable motor tracking. The predetermined rotational speed is lower than the target rotational speed. Preferably, this rotational speed is even lower than the rotational speed at which the time interval of the rotational signal from the encoder 139 falls within the period from the previous feedback control process to the current feedback control process. The specific effects will be discussed later, but for now, let's continue explaining the processor's processing. In step S21, the processor calculates the deviation of the current speed by taking the difference between the target speed and the current motor rotation speed. Based on the calculated deviation, it calculates a drive signal to be provided to the driver circuit 137 and outputs it to the driver circuit 137 (step S23). That is, it performs processing as a controller of feedback control and updates the drive signal. The drive signal corresponding to the deviation can be determined by applying, for example, known feedback control theory methods to determine gain characteristics and filter characteristics suitable for the control system. However, it is not limited to this, and methods other than classical feedback control theory may be used.

[0042] After updating the drive signal, the processor determines whether to continue driving the motor (step S25). That is, it determines whether it has received a command to stop driving from the higher-level motor control unit 125. If it has received a command to stop driving (No in step S25), it stops driving the DC motor 138 (step S27) and terminates the feedback control process. If the DC motor 138 is to continue driving (Yes in step S25), wait for a predetermined period of time to elapse (loop in step S29), and then return to step S11. The above is an example of the processing performed by a processor that implements feedback control.

[0043] <<Obtaining the current rotation speed>> The rotational speed measurement unit 131 obtains the current rotational speed of the DC motor 138 based on the rotational signal from the encoder 139. A conventional method (comparative example) and the method according to this embodiment will be described. Figure 8 is an explanatory diagram showing an example of the waveform of the rotation signal output from the encoder 139 and its relationship to the current rotation speed determined by the rotation speed measurement unit 131 based on that rotation signal. As shown in Figure 8, the encoder 139 outputs a binary rotation signal that transitions from low level to high level and from high level to low level when the rotation shaft of the DC motor 138 rotates by a predetermined angle. The horizontal axis in Figure 8 represents time.

[0044] For example, the encoder 139 transitions the rotation signal from a low level to a high level and then from a high level to a low level each time the rotation axis of the DC motor 138 rotates by 3.6 degrees. In other words, during one rotation (360-degree rotation) of the rotation axis, the rotation signal transitions from a low level to a high level 100 times. It also transitions from a high level to a low level 100 times, outputting 100 pulse signals. The rotation signal shown in Figure 8 shows that the interval between transitions is gradually widening. This indicates that the DC motor 138 is gradually decelerating. The multiple dashed lines extending vertically in Figure 8 represent time intervals of 1 millisecond, from t0 to t10. In this embodiment, the feedback control unit 127 performs feedback control processing at time intervals of 1 millisecond. In other words, the "predetermined period" in the processing shown in step S29 of Figure 7 corresponds to the 1-millisecond time interval shown in Figure 8.

[0045] The pulse signal waveforms of the rotation signal (1) to (4) show the time intervals between the low-level to high-level transitions, i.e., the rising edges, of each rotation signal. In Figure 8, the comparative examples below the rotation signal, Cases 1 to 3, show [1] to [4] on the right side, which indicate the current rotation speed determined by the rotation speed measurement unit 131 from the above-mentioned time intervals (1) to (4). For example, [1] is the current rotation speed determined by the rotation speed measurement unit 131 from time interval (1), and the rotation speed V[1] when the encoder 139 outputs a rotation signal with 100 rising edges per revolution, as in the numerical example above, is: V[1] = 1 / (100 × time interval (1)) (rps) =60 / (100 × time interval (1))(rpm) It is calculated as follows. The same applies to [2] through [6]. Furthermore, [C] shown in Figure 8 indicates a predetermined rotation speed that the detection limit processing unit 133 outputs instead of the current rotation speed when the detection limit processing unit 133 determines that the time interval of the rotation signal is too wide to determine the current rotation speed.

[0046] The comparative example shown in Figure 8 is a method for deriving the current rotation speed in feedback control processing at 1-millisecond intervals from time t0 to t10, as follows: Even if the rising edge of the most recent rotation signal is not input between the previous processing and the current processing, that is, even if the most recent rotation signal is from before the previous processing, the current rotation speed is derived based on the time interval between the previous rotation signal and the rotation signal before that. For example, in the processing at time t4, the rising edge of the most recent rotation signal was not input during the period from the current time t4 to the previous time t3. The rising edge of the most recent rotation signal was input before time t3. However, the rotation speed measurement unit 131 derives the current rotation speed [2] based on the time interval (2) between the most recent rotation signal and the previous rotation signal. Consequently, the current rotation speed derived at time t2 and time t3 is both [2]. The same is true for time t4, t6 to t8 and time t10.

[0047] In contrast, "Case 1" shown in Figure 8 is one of the methods (first embodiment) according to this embodiment, in which the current rotation speed cannot be derived unless both the most recent rotation signal and the rotation signal immediately preceding it were input during the period leading up to the previous processing. For example, in the processing at time t2, the rising edge of the most recent rotation signal was not input during the period from the current time t2 to the previous time t1. Therefore, the rotation speed measurement unit 131 determines that it cannot derive the current rotation speed. Based on this determination, the detection lower limit processing unit 133 outputs a predetermined rotation speed [C]. The same applies to processing from time t3 onwards.

[0048] Incidentally, the most recent rotation signal from encoder 139 and the rotation signal immediately preceding it can both be input during the period leading back to the previous processing if the time interval between rotation signals from encoder 139 is equal to or shorter than 1 ms, which is the time it takes to repeat the feedback control processing. At rotational speeds where the time interval between rotation signals exceeds 1 ms, the rotational speed measurement unit 131 cannot determine the current rotational speed in case 1. The lower limit of rotational speed, that is, the rotational speed at which the time interval between rotation signals becomes 1 ms, is 10 ps, ​​or 600 rpm, in this embodiment.

[0049] "Case 2" shown in Figure 8 refers to a method different from "Case 1" in this embodiment (second embodiment). The current rotation speed can be determined if both the most recent and the one immediately preceding it satisfy the following conditions: For the most recent rotation signal, it is a condition that it was input during the period leading up to the previous feedback control process. For the rotation signal immediately preceding the most recent rotation signal, it is a condition that it was input during the period leading up to the feedback control process before that. If both conditions are met, the current rotation speed is determined based on the time interval between the most recent rotation signal and the one immediately preceding it. In "Case 2," for example, in the processing at time t2, the most recent rotation signal was input during the period from the current time t2 back to the previous time t1. Furthermore, the rotation signal immediately preceding the most recent rotation signal was input within the period going back to the time before that, t0. Therefore, the rotation speed measurement unit 131 determines that it can derive the current rotation speed based on the time interval (2). This is a less stringent criterion compared to "Case 1," where it is determined that the current rotation speed cannot be derived in the processing at time t2.

[0050] Case 3 is an example of an even looser standard (third embodiment). The method in Case 3 derives the current rotation speed based on the time interval between the current rotation signal and the previous rotation signal, provided that the most recent rotation signal was input between the time of the current processing and the time of the previous processing. It does not matter when the rotation signal immediately preceding the most recent was input.

[0051] For example, in the processing at time t5, the most recent rotation signal was input during the period from the current time t5 to the previous time t4. Therefore, the rotation speed measurement unit 131 determines that it can derive the current rotation speed based on the time interval (3). The same applies to time t9.

[0052] The criteria for determining whether or not the current rotational speed can be derived based on the rotational signal are not limited to the aforementioned Cases 1, 2, and 3, but can be modified in various ways. For example, as a case that falls between Case 2 and Case 3, which has a looser standard, one possible criterion is that it is sufficient if the rotation signal immediately preceding the most recent rotation signal was input within a period of three or more predetermined numbers prior (for example, three rotations prior) (Fourth Embodiment). Furthermore, a modified version is also conceivable in which the response time of the DC motor 138 under load is used as the reference, rather than the time interval of the feedback control processing shown in 1-millisecond increments in Figure 8 (Fifth Embodiment). The time interval for repeating the feedback control processing is usually set to a shorter interval than the response time of the system described above in order to achieve stable feedback control. In the example shown in Figure 8, the time interval of the feedback control processing is 1 millisecond, but an example of a mechanical time constant representing the response time of the DC motor 138 is 3.8 milliseconds. For example, this method assumes that if both the most recent rotation signal and the rotation signal immediately preceding it are input within a period that goes back to the time represented by this mechanical time constant, the current rotational speed can be derived, but otherwise the current rotational speed cannot be derived.

[0053] <<Effect of the detection limit processing unit on feedback control>> The advantages of the feedback control according to this embodiment will be explained below with reference to the comparative example shown in Figure 8, along with examples of the response waveforms of the feedback control. Figure 9 is a waveform diagram showing an example of a transient response waveform during deceleration operation using the conventional method shown as a comparative example in Figure 8. In contrast, Figure 10 is a waveform diagram showing an example of a transient response waveform during deceleration operation using the method of this embodiment. Note that the feedback control methods in Figures 9 and 10 correspond to the comparative example and Case 1 shown in Figure 8, respectively, but the rotation signal waveform shown in Figure 8 is illustrative and does not directly correspond to the waveforms shown in Figures 9 and 10.

[0054] In Figures 9 and 10, the target rotational speed changes in a stepwise manner from the initial rotational speed of 2446 rpm to 623 rpm. In Figures 9 and 10, this change occurs over a period of 3.5 ms to 4.5 ms on the time axis. The abrupt change in the target rotational speed to a low speed corresponds, for example, to the deceleration control used when the discharge reversal motor 64 is decelerated just before the trailing edge of the printed sheet finishes passing the discharge roller R13. In Figure 9, the actual rotational speed of the DC motor 138 remains constant, approximately equal to the initial rotational speed of 2446 rpm, from 3.5 ms to 6.5 ms on the time axis, when the target rotational speed changes to 623 rpm. This can be attributed to the response delay of the feedback control system. Subsequently, from 6.5 ms on the time axis, deceleration begins towards the target rotational speed of 623 rpm. Each point on the waveform represents a 1 ms interval and corresponds to the point in time when the feedback control process is repeated. At the processing point of 10.5 ms on the time axis, the actual rotational speed of the DC motor 138 has decelerated to 877 rpm. The feedback control unit 127 then performs the next feedback control process at 11.5 ms, 1 ms later.

[0055] At 11.5ms on the time axis, the actual rotational speed of the DC motor 138 has dropped to 500rpm. That is, it is a rotational speed lower than 600rpm, which is the lower limit of rotational speed at which two rising edges can exist during the 1ms time step in which the feedback control process is repeated: the most recent rotational signal and the rotational signal immediately preceding it. At 500rpm, which is lower than the lower limit of 600rpm, the rotational speed measurement unit 131 derives the same rotational speed as the previous feedback control process as the current rotational speed, similar to the cases of times t3, t4, t6 to t8, or t10 in the comparative example shown in Figure 8. As a result, the same drive signal as before is output. In other words, since no new rotational signal is input from the encoder 139 during the time interval in which the feedback control is repeated, the current rotational speed is not updated and the past state is maintained, and the drive signal also maintains the past state.

[0056] In Figure 9, the waveform of the "current rotational speed for control purposes" from 10.5 ms on the time axis onward indicates a state where the rotational speed is not updated and the past rotational speed is maintained. The motor drive unit 135, for example, at 11.5 ms on the time axis, sets the unupdated past rotational speed as the current rotational speed for control purposes. As a result, it determines that the DC motor 138 is rotating at a rotational speed faster than the target rotational speed and outputs a drive signal to further reduce the rotational speed of the DC motor 138. The "Drive Signal DUTY" at 11.5ms on the time axis shown in Figure 9 indicates the state. A drive signal DUTY of zero corresponds to a no-drive state where the DC motor 138 is not driven at all, and 100 corresponds to a state where the DC motor 138 is fully driven. Even after 11.5ms on the time axis, the drive signal DUTY remains approximately zero (no-drive). This is simply because no new rotation signal is input from the encoder 139, and therefore the current rotation speed cannot be determined. As shown in "Actual Current Rotation Speed" in Figure 9, the actual rotation speed of the DC motor 138 decreases below the target rotation speed and eventually the DC motor 138 stops.

[0057] Thus, if the time interval of the rotation signal from the encoder 139 is too wide during the undershoot period of deceleration, and the current rotation speed cannot be updated, the rotation speed of the DC motor 138 cannot be made to follow the target rotation speed. As a result, the DC motor 138 may stop. According to the method of this embodiment, such problems can be avoided with a simple configuration.

[0058] The waveform shown in Figure 10 illustrates an example corresponding to the feedback control method of "Case 1" in Figure 8. At a point in the time axis after 11.5 ms, when the rising edges of the most recent rotation signal and the rotation signal immediately preceding it cannot exist within a 1 ms time interval, the rotation speed measurement unit 131 determines that it cannot derive the current rotation speed. Upon this determination, the detection lower limit processing unit 133 outputs a predetermined rotation speed as the current rotation speed for control purposes. In the example shown in Figure 10, the predetermined value of the rotation speed is zero. Note that the value of the rotation speed output by the detection lower limit processing unit 133 in Figure 10 is just one example. The predetermined rotation speed output by the detection lower limit processing unit 133 when the rotation speed measurement unit 131 determines that it cannot derive the current rotation speed can be set by the designer based on experiments to a value suitable for each individual feedback control system. The preferred range of rotational speeds has a lower limit of zero. The upper limit is the rotational speed at which the two rising edges of the most recent rotational signal and the one immediately preceding it cannot exist within the time interval (1 ms in this embodiment) during which the feedback control process is repeated. In the example shown in Figure 10, the rotational speed at which the rising edges of the most recent rotational signal and the rotational signal immediately preceding it equal the 1ms time interval during which the feedback control process is repeated corresponds to a rotational speed at which 1000 rotational signals are output per second. Since the encoder 139 outputs 100 rotational signals per revolution, its rotational speed corresponds to 10 rps, or 600 rpm. Therefore, the upper limit of the preferred range for a given rotational speed is 600 rpm.

[0059] At 11.5ms and 12.5ms on the time axis shown in Figure 10, the rotational speed measurement unit 131 determines that no rotational signals were input in the preceding 1ms period, and therefore the current rotational speed cannot be determined. Based on this determination, the detection lower limit processing unit 133 outputs zero as the current rotational speed for control purposes. The motor control unit 125 determines that the current rotational speed is low relative to the target rotational speed and outputs a drive signal to accelerate the DC motor 138. In Figure 10, the value of the drive signal DUTY output at 11.5ms on the time axis is 34, and the value of the drive signal DUTY output at 12.5ms on the time axis is 42.

[0060] With such a duty cycle drive signal output, the actual rotational speed of the DC motor 138 changes from deceleration to acceleration at 12.5 ms on the time axis. Then, at 13.5 ms on the time axis, the actual rotational speed increases to 650 rpm. After that, the actual rotational speed overshoots beyond the target rotational speed of 623 rpm before converging to the target rotational speed. As shown in Figure 10, according to this embodiment, even if there is a temporary undershoot during deceleration, the DC motor 138 does not stop and continues to rotate in accordance with the target rotational speed, thereby achieving stable feedback control.

[0061] As stated above, (i) The image forming apparatus according to this disclosure comprises a drive circuit that receives a drive signal and drives a motor; a speed detector that outputs a rotation signal at time intervals corresponding to the rotation speed of the motor; and a feedback control unit that obtains the current rotation speed of the motor from the rotation signal output from the speed detector and sequentially updates and outputs a drive signal to the drive circuit so that the motor rotates at a target speed, wherein the feedback control unit updates the drive signal if no rotation signal is output from the speed detector during the period from the previous update of the drive signal to the current update, by deeming that the speed detector has output a rotation speed lower than the target rotation speed.

[0062] In this disclosure, the motor is used in an image forming apparatus and is subject to feedback control. While a DC motor is a typical example of the type of motor, it is not particularly limited to any motor to which feedback control can be applied, such as an induction motor or a synchronous motor. Furthermore, the speed detector outputs a rotation signal indicating that the motor is rotating at time intervals corresponding to its rotational speed. A specific example of this is an encoder that outputs a signal every time the motor rotates by a predetermined angle. Furthermore, the feedback control unit updates and outputs a drive signal based on the rotation signal so that the motor rotates at a target speed. A specific embodiment of this is a configuration in which the feedback control unit is configured using a processor. However, some or all of the functions may be configured by hardware (circuits).

[0063] The feedback control unit sequentially updates the drive signal. The time interval for this update is basically shorter than the time the motor can respond.

[0064] Furthermore, preferred embodiments of this disclosure will be described. (ii) If at least one rotation signal is output from the speed detector between the previous update of the drive signal and the current update, the feedback control unit may obtain the current rotation speed of the motor based on the time interval between that rotation signal and the rotation signal immediately preceding it. In this embodiment, the current motor rotation speed reflects the most recent state because it is based on at least one rotation signal that was input between the last update and the current state.

[0065] (iii) The feedback control unit may perform a process to update the drive signal based on the current rotational speed of the motor at a preset time interval. In this embodiment, the feedback control unit updates the drive signal at predetermined time intervals, so that the motor's feedback control is performed continuously and substantially continuously.

[0066] (iv) The time interval may be set based on the response characteristics of the motor. According to this embodiment, the drive signal is updated by repeatedly performing feedback control processing at time intervals corresponding to the motor's response characteristics, thus substantially continuous feedback control of the motor is achieved.

[0067] (v) If no rotation signal is output from the speed detector during the period from the previous update of the drive signal to the current update, the feedback control unit may update the drive signal by assuming that the speed detector has output a rotation speed even lower than the rotation speed that falls within the period from the previous update to the current update, based on the time interval of the rotation signal from the speed detector. According to this embodiment, even if the rotational speed is low and no rotational signal is output from the speed detector between the previous update and the current update, stable feedback control can be achieved without stopping the motor by assuming that a rotational signal corresponding to a rotational speed higher than the rotational speed at which the rotational signal from the speed detector would normally be output during that period.

[0068] (vi) The feedback control unit may consider the motor to be stopped and update the drive signal if no rotation signal is output from the speed detector during the period between the previous update of the drive signal and the current update. According to this embodiment, even if the rotational speed is low and no rotational signal is output from the speed detector during the period between the previous update and the current update, stable feedback control can be achieved without stopping the motor by treating the motor as stopped during that period and controlling it accordingly.

[0069] (vii) The motor may be a motor used for transporting paper or originals in an image forming apparatus. According to this embodiment, stable feedback control can be achieved without stopping the motor even when the paper or document transport speed is temporarily reduced.

[0070] (viii) One aspect of this disclosure is a control unit for controlling a motor used in an image forming apparatus, comprising the steps of: acquiring rotation signals at time intervals corresponding to the rotation speed of the motor using a speed detector; obtaining the current rotation speed of the motor from the rotation signals output from the speed detector; sequentially updating and outputting a drive signal to a drive circuit so that the motor rotates at a target speed; and updating the drive signal if, during the period from the previous update of the drive signal to the current update, the speed detector is deemed to have output a rotation speed lower than the target speed. This includes a method for controlling a motor to perform the following actions.

[0071] This form of disclosure includes combinations of any of the forms described above. In addition to the embodiments described above, various modifications of this disclosure are possible. These modifications should not be construed as being outside the scope of this disclosure. This disclosure should include the meaning of equivalents to the claims and all variations within that scope. [Explanation of Symbols]

[0072] 11: Optical scanning unit, 11M: Polygon mirror, 12: Developing unit, 13: Photoconductor drum, 14: Charger, 15: Drum cleaner, 16: Primary transfer roller, 17: Fixing unit, 18A, 18B, 18C, 18D: Paper feed tray, 21: Intermediate transfer belt, 23: Secondary transfer unit, 27: Toner storage unit, 30, 30y, 30m, 30c, 30k: Process unit, 39A, 39B: Output tray, 41: Image processing circuit, 43: Document feed tray, 45: Document output tray, 50: Paper feed motor, 52: Paper feed clutch, 53: Paper feed solenoid, 54: Transport clutch, 56: Resist motor, 58: Developing motor, 60: Transfer transport motor, 62: Duplex transport motor, 64: Output reversal motor, 65: Shift motor, 67: Document tray, 68: Document scanning unit, 69: Scan motor, 71: Document feed motor, 72: Document feed solenoid, 73: Document registration motor, 75: Document transport motor 100: Multifunction printer, 101: Control unit, 103: Document transport unit, 105: Operation unit, 107: Communication circuit, 111: Image reading unit, 115: Printing unit, 121: Processor, 122: RAM, 123: Non-volatile memory, 125: Motor control unit, 127: Feedback control unit, 129: Target rotation speed acquisition unit, 131: Rotation speed measurement unit, 133: Detection lower limit processing unit, 135: Motor drive unit, 137: Driver circuit, 138: DC motor, 139: Encoder R01: Paper feed roller, R02: Pickup roller, R03: Separation roller, R04, R05, R06, R07, R12: Conveyor rollers, R08: Register roller, R09: Secondary transfer drive roller, R10: Intermediate transfer drive roller, R11: Heating roller, R13, R14: Discharge rollers, R15, R16, R17: Double-sided conveyor rollers, R31: Document feed roller, R32: Document pickup roller, R33: Document separation roller, R34: Document register roller, R35, R36: Document transport rollers, R37: Document discharge roller

Claims

1. A drive circuit that receives a drive signal and drives the motor, A speed detector that outputs a rotation signal at time intervals corresponding to the rotation speed of the motor, The system includes a feedback control unit that obtains the current rotational speed of the motor from the rotation signal output from the speed detector and sequentially updates and outputs a drive signal to the drive circuit so that the motor rotates at a target speed, The feedback control unit updates the drive signal if no rotation signal is output from the speed detector during the period between the previous update of the drive signal and the current update of the drive signal, by applying a predetermined rotation speed, excluding zero, as the current rotation speed of the motor.

2. The image forming apparatus according to claim 1, wherein the feedback control unit obtains the current rotational speed of the motor based on the time interval between the rotational signal and the rotational signal immediately preceding it, if at least one rotational signal is output from the speed detector between the previous update of the drive signal and the current update.

3. The image forming apparatus according to claim 1, wherein the feedback control unit performs a process to update the drive signal based on the current rotational speed of the motor at a preset time interval.

4. The image forming apparatus according to claim 3, wherein the time interval is set based on the response characteristics of the motor.

5. The image forming apparatus according to claim 1, wherein if no rotation signal is output from the speed detector during the period from the previous update to the current update of the drive signal, the feedback control unit updates the drive signal by considering that the speed detector has output a rotation speed even lower than the rotation speed that falls within the period from the previous update to the current update, based on the time interval of the rotation signal from the speed detector.

6. The image forming apparatus according to claim 1, wherein the motor is a motor for transporting paper or a document in the image forming apparatus.

7. A control unit that controls the motor used in an image forming apparatus, The steps include: acquiring a rotation signal at time intervals corresponding to the rotation speed of the motor using a speed detector; The steps include obtaining the current rotational speed of the motor from the rotational signal output from the speed detector, The steps include sequentially updating and outputting a drive signal to the drive circuit so that the motor rotates at a target speed, If no rotation signal is output from the speed detector during the period between the previous update of the drive signal and the current update, the drive signal is updated by applying a predetermined rotation speed, excluding zero, as the current rotation speed of the motor. A method for controlling a motor to perform the following action.

Images