Patent Publication Number: US-7581803-B2

Title: Image forming apparatus, method and computer readable medium for executing predetermined error processes in response to a moveable member error

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present document incorporates by reference the entire contents of Japanese priority document, 2003-304442 filed in Japan on Aug. 28, 2003. 
   BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to an image forming apparatus such as a facsimile, a printer, a copying machine, or a multifunction product and, more particularly, to an image forming apparatus which transfers a visual image on an image carrier to a movable object side such as a conveyer belt or an intermediate transfer belt at a counter position between the image carrier and the movable object. 
   2) Description of the Related Art 
   In recent years, a color image forming apparatus that forms a high-quality image has gained popularity both at home and abroad. In particular, a tandem type color image forming apparatus, in which a plurality of image forming units are arranged along a conveyer unit to realize an image forming process at a high speed, is popular. 
   In the tandem type color image forming apparatus, optical beams emitted from a plurality of light sources are irradiated on the plurality of image carriers arranged in the apparatus, to form an electrostatic latent image. Developing agents having different colors (for example, toners of three colors: yellow (Y), magenta (M), and cyan (C), or of the three colors and black (Bk)) are caused to adhere to the electrostatic latent images, to form toner images that serve as real images. Thereafter, a recording material such as recording paper or the like, carried on the movable object such as a conveyer belt, is sequentially conveyed to transfer positions of the image carriers. The toner images are superimposed on the recording material to transfer the image. The transferred toner image is fixed on the recording material to form a multi-color image. 
   In the color image forming apparatus, the toner image is transferred at a high speed, onto the recording material such as a recording paper moving in the conveyance direction. Feedback control may be performed with respect to the moving speed of the movable body, to stabilize positioning accuracy of the respective colors. 
   Japanese Unexamined Patent Publication No. H11-146675 discloses an image forming apparatus that determines an error when a speed error of the conveyer belt drive motor or a positioning error exceeds a tolerance value, and stops photoconductor members and a transfer belt. 
   Japanese Unexamined Patent Publication No. H11-24507 discloses an image forming apparatus that performs drive source control based on results obtained by reading scales formed on the rotating member (transfer belt), and performs feedback control of the speed of the transfer belt, to position images on the transfer member with high accuracy. 
   However, due to the following problems in execution of the feedback control, an erroneous image may be output. 
   As the first problem, when a toothless output pulse is generated from an out-of-order encoder, a decrease in speed or lack of a position forward distance is detected. Thus, when the feedback control is performed in this state, the rotating speed of a motor that rotationally drives a movable member increases excessively. 
   As the second problem, when noise is superimposed on an encoder output, it is detected that a speed increases or that a position forward distance is excessive. Hence, when the feedback control is performed in this state, the rotating speed of the motor that rotationally drives the movable member reduces excessively. 
   As the third problem, when movement of the movable member suddenly varies, the rotating speed of the motor that rotationally drives the movable member excessively increases or reduces. Depending on circumstances, control oscillation can occur. 
   In Japanese Unexamined Patent Publication No. H11-146675 stated above, when a control error is detected by comparing the speed error tolerance with the positioning error tolerance, the photoconductor members and the transfer belt are stopped. 
   However, the problems are unique to when the feedback control is performed. If the feedback control is not performed, though color shift may worsen slightly, a user can obtain an image having practically no problem. Therefore, prohibiting image formation without reason results in downtime of the image forming apparatus, which some users do not like. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to at least solve the problems in the conventional technology. 
   An image forming apparatus according to an aspect of the present invention includes an image forming unit that forms an image; a movable member; a driving unit that rotationally drives the movable member; a moving information detecting unit that detects any one of a moving distance and a moving speed of the movable member; a deviation information calculating unit that calculates any one of a position deviation based on the moving distance detected, and a speed deviation based on the moving speed detected; a drive controlling unit that provides control of any one of the position deviation calculated and the speed deviation calculated, to control the driving unit based on a quantity of control; an error determining unit that determines an occurrence of any one of a detection error and a control error, based on any one of the moving distance detected, the moving speed detected, and the quantity of control; and an error process executing unit that executes a predetermined error process if the error determining unit determines that there is an error. 
   An image forming method according to another aspect of the present invention includes mask period deciding including deciding whether a control error decision mask period selected from among through up, a settling &amp; transfer contact period, a transfer spacing period, and through down, is set; pulse interval deciding including deciding whether an encoder pulse interval falls within a predetermined range, if the control error decision mask period is not selected at the mask period deciding; prohibition deciding including deciding whether an image formation prohibition process is selected as a process after an error decision, if it is decided at the pulse interval deciding that the encoder pulse interval does not fall within the predetermined range; prohibiting the image formation prohibition process, if it is decided at the prohibition deciding that the image formation prohibition process is selected; and stopping feedback control and rotating a drive motor at a predetermined speed, if it is decided at the prohibition deciding that the image formation prohibition process is not selected. 
   A computer-readable recording medium according to still another aspect of the present invention records thereon a computer program that realizes the above image forming method on a computer. 
   The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a configuration of an image forming apparatus according to a first embodiment of the present invention; 
       FIG. 2  is an example of toner mark rows for alignment; 
       FIG. 3  is a timing chart that is obtained when write timing in a sub-scan direction is corrected; 
       FIG. 4  is a timing chart that is obtained when write timing in a main scan direction is corrected; 
       FIG. 5  is a functional block diagram of a process of stabilizing movement properties of a conveyer belt; 
       FIG. 6  illustrates a relationship between encoder output pulses and a control cycle timer; 
       FIG. 7  is a functional block diagram of a control process performed in the image forming apparatus; 
       FIG. 9  is a graph of a running sequence of a drive motor; 
       FIG. 8  is a flow chart of a process procedure of an error decision process and an error process; 
       FIG. 10  illustrates a configuration of various control units in the image forming apparatus; and 
       FIG. 11  illustrates a configuration of an image forming apparatus according to a second embodiment. 
   

   DETAILED DESCRIPTION 
   Exemplary embodiments of an image forming apparatus, an image forming method, and a computer product according to the present invention are explained below, with reference to the accompanying drawings. 
   An operation of an image forming apparatus is described first, with reference to  FIG. 9 . The image forming apparatus according to the present invention decides whether a control error decision mask period selected from among through up, a settling &amp; transfer contact period, a transfer spacing period, and through down is set (step S 1 ). If the control error decision mask period is set, the process is shifted to RETURN. If the control error decision mask period is not set, it is determined whether an encoder pulse interval falls within the range given by 0.95T0≦t≦1.05T0 (Equation 1) (step S 2 ). If the encoder pulse interval falls within the range given by Equation 1, the process is shifted to RETURN. If the encoder pulse interval does not fall within the range given by Equation 1, it is determined whether an image formation prohibition process is selected as a post process (step S 3 ). If the image formation prohibition process is selected, the image formation prohibition process is performed (step S 4 ), and the process is shifted to RETURN. If the image formation prohibition process is not selected, feedback control is stopped, a drive motor (stepping motor) is rotated at a predetermined speed (step S 5 ), and the process is shifted to RETURN. In step S 2 , although error decisions are performed at an encoder pulse interval, the error decisions can also be performed by decisions performed at a drive pulse frequency of a drive motor  22  (stepping motor). 
   A tandem type color image forming apparatus according to the present invention is described below with reference to  FIG. 1 . 
   In the tandem type color image forming apparatus, image forming units  6 Y (yellow),  6 M (magenta),  6 C (cyan), and  6 BK (black) of respective colors are sequentially aligned from the upstream side in the conveying direction of a conveyer belt  5  which conveys a sheet of paper  4  separated and fed from a paper feed tray  1  by a paper feed roller  2  and a separation roller  3 . 
   These image forming units  6 Y,  6 M,  6 C, and  6 BK have identical internal configuration except for colors of toner images to be formed. The image forming unit  6 Y, the image forming unit  6 M, the image forming unit  6 C, and the image forming unit  6 BK form a yellow image, a magenta image, a cyan image, and a black image, respectively. 
   Therefore, in the following explanation, the image forming unit  6 Y will be concretely described. Because the other image forming units  6 M,  6 C, and  6 BK are similar to the image forming unit  6 Y, with respect to the constituent elements of the image forming units,  6 M,  6 C, and  6 BK, only symbols discriminated by M, C, and BK are described in  FIG. 1  in place of Y added to the constituent elements of the image forming unit  6 Y, and a description thereof will be omitted. 
   The conveyer belt  5  is an endless belt wound on a drive roller  7  and a driven roller  8  which are rotationally driven. In image formation, the sheets of paper  4  stored on the paper feed tray  1  are sequentially fed from the top, adsorbed to the conveyer belt  5  by electrostatic adsorption, and conveyed to the image forming unit  6 Y, serving as the first image forming unit, by the rotatably driven conveyer belt  5 . In the image forming unit  6 Y, a yellow toner image is transferred to the sheet of paper. 
   The image forming unit  6 Y includes a photoconductor drum  9 Y serving as a photosensitive object, and an electric charging unit  10 Y, an exposing unit  11 , a developing unit  12 Y, a photoconductor cleaner (not shown), and a neutralizer  13 Y arranged around the photoconductor drum  9 Y. The exposing unit  11  is designed to irradiate laser beams  14 Y,  14 M,  14 C, and  14 BK serving as exposure beams corresponding to image colors formed by the image forming units  6 Y,  6 M,  6 C, and  6 BK, respectively. 
   In image formation, the outer peripheral surface of the image forming unit  6 Y is uniformly electrically charged by the electric charging unit  10 Y in the dark, exposed by the laser beam  14 Y emitted from the exposing unit  11 , and an electrostatic latent image corresponding to a yellow image is formed. This electrostatic latent image is changed into a visible image by yellow toner in the developing unit  12 Y, to form a yellow toner image on the photoconductor drum  9 Y. 
   The toner image is transferred onto the sheet of paper  4  by the operation of a transferring unit  15 Y at a position where the photoconductor drum  9 Y is in contact with the sheet of paper  4  on the conveyer belt  5 , to form a yellow image on the sheet of paper  4 . On completion of transferring the yellow image, unnecessary toner left on the outer peripheral surface of the photoconductor drum  9 Y is wiped out by the photoconductor cleaner. Thereafter, the photoconductor drum  9 Y is neutralized by the neutralizer  13 Y for the next image formation. 
   In this manner, the sheet of paper  4  onto which the yellow toner image is transferred by the image forming unit  6 Y is conveyed to the next image forming unit  6 M along the conveyer belt  5 . In the image forming unit  6 M, by the same process as performed by the image forming unit  6 Y, a magenta toner image is formed on the photoconductor drum  9 M, and the toner image is transferred onto the sheet of paper  4  to superimpose the images. The sheet of paper  4  is conveyed to the next image forming units  6 C and  6 BK, and a cyan toner image formed on a photoconductor drum  9 C and a black toner image formed on a photoconductor drum  9 BK are transferred onto the sheet of paper  4  to superimpose the images. In this manner, a full-color superimposed image is transferred onto the sheet of paper  4 . The sheet of paper  4  on which the full-color superimposed image is formed is removed from the conveyer belt  5 , and the superimposed image is fixed onto the sheet of paper  4  by a fixing unit  16 . Thereafter, the sheet of paper  4  is discharged. 
   In the color image forming apparatus, there may be error in center distances between the photoconductor drums  9 Y,  9 M,  9 C, and  9 BK, error in parallelism of the photoconductor drums  9 Y,  9 M,  9 C, and  9 BK, error in installation of polarizing mirrors (not shown) that polarize laser beams in the exposing unit  11 , error in write timing of electrostatic latent images on the photoconductor drums  9 Y,  9 M,  9 C, and  9 BK, and fluctuation in the conveyance speed of the conveyer belt  5 , and the like. Consequently, the toner images of the respective colors are not superimposed at a predetermined position where the toner images are supposed to be superimposed, and position errors occur. 
   Therefore, in the color image forming apparatus, the position errors of the color toner images to be formed must be corrected. Thus, on the opposite side of the conveyer belt  5 , sensors  17 ,  18 , and  19  are arranged on the downstream side of the image forming unit  6 BK to detect and correct static position errors (DC components). The sensors  17 ,  18 , and  19  are supported and arranged on one substrate in a main scan direction perpendicular to the direction of an arrow (moving direction of the conveyer belt) shown in  FIG. 1 . A rotary encoder  20  is attached to the rotating shaft of the driven roller  8  to detect and correct dynamic position errors (AC components) caused by the fluctuation of conveyance speed of the conveyer belt  5 , so that feedback control is performed to the drive motor  22  serving as a drive source of the drive roller  7 . 
   As components of the position errors of the respective colors, the following are known:
         skew error   resist error in sub-scan direction   resist error in main scan direction   magnification error in main scan direction       

     FIG. 2  depicts an example of alignment toner mark rows formed on the conveyer belt  5 . On the conveyer belt  5 , transverse lines and oblique lines K, C, M, and Y are formed. The transverse and oblique lines are detected by the sensors  17 ,  18 , and  19  arranged in the main scan direction (direction perpendicular to the belt direction), to measure skew errors to a reference color (in this case, BK), resist errors in the sub-scan direction, resistor errors in the main scan direction, and magnification errors in the main scan direction. From the measurements, various shift lengths and quantities of correction are calculated. A CPU (described later) corrects the error components as follows. 
   The skew errors are corrected by changing inclinations of mirrors (not shown) that reflect the laser beams of the respective colors in the exposing unit  11 . As a drive source to bias the mirrors, a stepping motor is used. 
   Correction of a resist error in the sub-scan direction is explained with reference to a timing chart, obtained when write timings in the sub-scan direction are corrected, as shown in  FIG. 3 . In this case, a correction resolution is set at 1 dot. An image region signal (write enable signal) in the sub-scan direction controls writing at a timing of a synchronization detection signal. When it is desired to forward a write position by 1 dot as a result of mark detection and calculation, the write enable signal may be made active one-synchronization signal ahead of the timing of the original write enable signal. 
   Correction of a resist error in the main scan direction is explained with reference to a timing chart, obtained when write timing in the main scan direction is corrected, as shown in  FIG. 4 . In this case, a correction resolution is set at 1 dot. As image write clocks, clocks having accurately equal phases are obtained in each line by a trailing edge of a synchronization detection signal. An image is written in synchronization with the clock signal. However, the image write enable signal in the main scan direction is also formed in synchronization with the clock signal. When it is desired to forward a write position by 1 dot as a result of mark detection and calculation, the write enable signal may be made active one clock ahead of the timing of the original write enable signal. In addition, when a magnification in the main scan direction shifts with respect to a reference color, the magnification can be changed by using a device which can change a frequency in fine steps, e.g., a clock generator which uses a VCO (Voltage Controlled Oscillator) or a PLL (Phase Locked Loop). 
   The correction process is executed in the following cases:
         The correction process is executed in initialization performed immediately after a power supply is turned on.   The correction process is automatically executed when a temperature at a predetermined position, e.g., at a part of the exposing unit  11  in the image forming apparatus, is equal to or larger than a predetermined temperature when an increase in temperature at the position is monitored.   The correction process is automatically executed immediately after a print operation for sheets of paper the number of which is larger than a predetermined number.   The correction process is executed when a user initiates the execution of the correction process through an operation panel or a printer driver.       

     FIG. 5  depicts a functional block diagram of a process of stabilizing movement properties of the conveyer belt  5  shown in  FIG. 1 . Based on a detection result detected by a rotary encoder  20 , a controller  21  calculates a speed deviation with respect to a target speed or a position deviation with respect to a target position to stabilize the conveyer belt. The calculated speed deviation or the position deviation is subjected to a PI control calculation to control the drive motor  22  serving as a drive source of the drive roller  7  that moves the conveyer belt  5 . In this manner, in the image forming apparatus according to the present invention, a feedback loop is constructed for the conveyer belt  5 . 
   The first embodiment of the present invention will be described below. In the first embodiment, it is assumed that a stepping motor is used as the drive motor  22  shown in  FIG. 1 .  FIG. 6  illustrates a relationship between encoder output pulses string output according to movement of the conveyer belt and a control cycle timer that performs feedback control.  FIG. 7  is a block diagram of functions of a control process performed in the image forming apparatus. 
   In the control process performed in the image forming apparatus in this embodiment, a driven roller advance angle [rad] is calculated from the number of encoder output pulses at a leading edge of the control cycle timer shown in  FIG. 6 , and compared with a target angle displacement [rad]. An obtained deviation [E(S)] is subjected to filter operation (low-pass filter operation) to cut a high-frequency component to obtain a value [E′(S)]. Based on the value [E′(S)] and a position controller [G(S)], for example, a PI control calculation is performed to add [F(S)] and a reference frequency [F 0 (S)] to change a drive pulse frequency [F′(S)]. The driven roller  8  and the conveyer belt  5  shown in  FIG. 1  are designed such that the driven roller  8  does not slip on the conveyer belt  5 , and an angular displacement and a moving distance of the conveyer belt  5  are equivalent to each other. 
   A method of detecting a control error on the basis of a detection result detected by the rotary encoder  20  attached to the shaft of the driven roller  8  will be described below. 
   Time (t[s]) of an interval between the leading edges of encoder output pulses shown in  FIG. 6  is measured. It is decided whether the measured time falls within the range given by the equation (1) given below. If an average time interval of t is given by T0[S], it is understood that time (t) of a leading edge interval falls within a fluctuation range of 5% that can be considered in practical use. If the time t satisfies the following condition given by equation (1), there is no problem.
 
0.95T0≦t≦1.05T0  (1)
 
   However, if it is decided that the time (t) does not fall within the range-given by equation (1), an error process (described later) is performed. 
   Because a pulse interval t 1  obtained when a toothless encoder output shown in  FIG. 6  is generated does not fall within the range given by equation (1), the pulse interval t 1  is determined as an error. Because a pulse interval t 2  obtained when noise is superimposed on the encoder output does not fall within the range given by equation (1), the pulse interval t 2  is determined as an error. 
   In  FIG. 6 , a process of calculating a quantity of control of the drive motor  22  (stepping motor) shown in  FIG. 1  and setting the quantity of control is performed every control cycle timer. However, it is assumed that the average frequency of the drive frequency is represented by f 0  [pps], it is understood that the drive frequency (f) falls within a fluctuation range of 5% that can be considered in practical use. When the frequency (f) satisfies the following condition given by equation (2), there is no problem.
 
0.95F0≦f≦1.05f0  (2)
 
   However, if it is decided that the frequency (f) does not fall within the range given by (2), an error process (described later) is performed. 
   When a toothless encoder output shown in  FIG. 6  is generated, and when noise is superimposed on the encoder output, the obtained drive frequency (f) does not fall within the range given by (2). Therefore, an error is decided. 
   Since a control error can be decided by any one of the decision made by the encoder output pulse and the decision made by the drive pulse frequency, either method can be applied without any problem. 
   A running process operation in image formation will be described below with reference to  FIG. 8 . 
     FIG. 8  is a graph of a running sequence of the drive motor  22  (stepping motor) that performs full-color printing during image formation. A conveyer belt of the image forming apparatus is always in contact with the black photoconductor  9 BK. However, the conveyer belt has a contacting/separating mechanism (not shown) that separates the conveyer belt from the C, M, and Y photoconductors  9 C,  9 M, and  9 Y during printing for BK, and brings the conveyer belt into contact with the photoconductors  9 C,  9 M, and  9 Y during full-color printing. 
   Through up is performed for 100 ms, 500 ms is required as a settling &amp; transfer contact period, and then general feedback control is performed. Upon completion of printing, transfer spacing is performed for 500 ms, and through down is performed for 100 ms. 
   It is determined that the error decision is not performed during the through up, in which a process of activating and stopping the drive motor  22  (stepping motor) is performed, and during the transfer spacing. 
   It is understood that movement of the belt is unstable in the settling &amp; transfer contact period, and during the through down. Because the error decision conditions are satisfied in these periods, the error decision is not performed in these periods. 
   The error process is a process of switching a state in which the drive motor  22  (stepping motor) is subjected to feedback control based on the detection result detected by the rotary encoder  20  shown in  FIG. 1  to a state in which the drive motor  22  (stepping motor) is rotated at a constant speed and a predetermined drive pulse frequency without being subjected to the feedback control. In this manner, since the drive motor  22  (stepping motor) is not subjected to the feedback control, a color shift may be slightly deteriorated. However, some user may accept such an image. When such a user is prohibited to form an image without reason, downtime of the image forming apparatus is produced. The downtime is not preferable for the position of a user. 
   The error process may be a process of prohibiting image formation. Since some user desires to output only high-quality images being free from color shift, such a user is prohibited to form an image when the control error is decided. 
   As the error process performed when the control error is decided, a process of stopping the feedback control of the drive motor  22  (stepping motor) and switching the drive motor (stepping motor) to the motor at a constant speed and a predetermined drive pulse frequency, and a process of prohibiting image formation can be selected to cope with the needs of various users. 
   It is assumed that the error process is performed when a predetermined error occurs a predetermined number of times. In general, an error occurs even once, an erroneous image is output. The number of times for the error decision is set at 1. 
   The error process is restored such that normal feedback control is performed by turning on/off the power supply of the image forming apparatus. This is because noise superimposition may not be infrequently eliminated since, for example, noise superimposition or the like may occur due to an influence of another apparatus located around the image forming apparatus by accident, due to a situation of power supply, or the like. 
     FIG. 9  is a program flowchart of a process procedure of an error decision process and an error process. 
   It is decided whether any control error decision mask period selected from among through up, a settling &amp; transfer contact period, a transfer spacing period, and through down shown in  FIG. 8  is set (step S 1 ). 
   If any control error decision mask period is set (Yes at step S 1 ), the process is shifted to RETURN. If any control error decision mask period is not set (No at step S 1 ), it is determined whether an encoder pulse interval falls within the range given by 0.95T0≦t≦1.05T0(step S 2 ). 
   If the encoder pulse interval falls within the range given by 0.95T0≦t≦1.05T0 (Yes at step S 2 ), the process is shifted to RETURN. If the encoder pulse interval does not fall within the range given by 0.95T0≦t≦1.05T0 (NO in step S 2 ), it is determined whether an image formation prohibition process is selected by a user as a process to be performed after the error decision (step S 3 ). 
   If the image formation prohibition process is selected (Yes at step S 3 ), the image formation prohibition process is performed (step S 4 ), and the process is shifted to RETURN. If the image formation prohibition process is not selected (No at step S 3 ), the feedback control is stopped, the drive motor (stepping motor) is rotated at a predetermined speed (step S 5 ), and the process is shifted to RETURN. 
   In step S 2 , the error decision is performed at the encoder pulse interval. However, the error decision can be performed at the drive pulse frequency of the drive motor  22  (stepping motor). 
     FIG. 10  illustrates a configuration of various control units in the image forming apparatus. 
   Each of the sensors  17 ,  18 , and  19  includes a light-emitting element (not shown) and a light-receiving element (not shown) that are controlled by an emission amount controlling unit  23 . The output side of the sensor is connected to an I/O port  30  through an AMP  24 , a filter  25 , an A/D converter  26 , and an FIFO memory  28 . The AMP  24  amplifies detection signals obtained by the sensors  17 ,  18 , and  19 . The amplified signals pass through the filter  25 , and are converted by the A/D converter  26  from analog data into digital data. The sampling controlling unit  27  controls sampling of the data, and the FIFO memory  28  stores the sampling data. 
   The sampling controlling unit  27 , the FIFO memory  28 , and the write control board  29  are connected to the I/O port  30 . The rotary encoder  20  is also connected to the I/O port  30  to control an ON/OFF operation of the light-emitting unit. An output pulse is connected to the I/O port  30 . The drive motor  22 , which drives the drive roller that moves the conveyer belt, is also connected to the I/O port  30  to form a feedback control loop. 
   The I/O port  30 , a CPU  31 , a ROM  32 , and a RAM  33  are interconnected by a data bus  34  and an address bus  35 . 
   In the ROM  32 , various programs such as a program to calculate various positional shift lengths of toner images, and a program to perform feedback control are stored. The address bus  35  designates a ROM address, a RAM address, and various input/output devices. 
   The CPU  31  monitors detection signals from the sensors  17 ,  18 , and  19  at predetermined timings. Emission amounts of the light-emitting elements of the sensors  17 ,  18 , and  19  are controlled by the emission amount control unit  23 , to reliably detect toner images even though deterioration or the like of the conveyer belt  5  shown in  FIG. 1  and the light-emitting elements of the sensors  17 ,  18 , and  19  occurs, so that the output levels of light-emitting signals from the light-emitting elements are always constant. 
   The CPU  31  performs setting for the write control board  29  based on quantities of correction calculated from detection results of position detection toner marks, to change a main resist and a sub-resist and to change an image frequency based on a magnification error. The write control board  29  includes devices, which can very finely set output frequencies, such as clock generators using, e.g., VCOs (Voltage Controlled Oscillators) for the respective colors including the reference color. Outputs from the devices are used as image clocks. The CPU  31  also controls a skew controlling stepping motor (not shown) in the exposing unit  11  shown in  FIG. 1 , based on the quantities of correction calculated from the detection results of the position detection toner marks. 
   In addition, the CPU  31  measures pulses from an encoder output pulse to calculate a current position deviation to a target position, performs, a PI control calculation or the like to the position deviation, and outputs the obtained quantity of control to the drive motor  22  to stabilize the traveling of the conveyer belt. In this feedback control, an error decision and an error process shown in the flow chart of  FIG. 9  are performed. 
   An image forming apparatus according to a second embodiment will be described below with reference to  FIG. 11 . 
   The image forming apparatus according to the second embodiment has the following configuration. That is, an intermediate transfer belt  36  is arranged as an intermediate transfer member in place of the conveyer belt  5  in the image forming apparatus according to the first embodiment. Images formed by image forming units  6 Y,  6 M,  6 C, and  6 BK are temporarily transferred onto the intermediate transfer belt  36 , and the images on the intermediate transfer belt  36  are transferred onto a sheet of paper by a transfer belt  37  serving as a transferring unit. The transfer belt  37  also includes a function of conveying a sheet of paper to a fixing unit  16 . A cleaning unit for the intermediate transfer belt  36  is indicated by reference numeral  38 . 
   A toner mark forming unit according to the invention forms position detection toner marks of respective colors on the intermediate transfer belt  36 . Thus, as in  FIG. 1 , sensors  17 ,  18 , and  19  are aligned in a main scan direction perpendicular to a rotating direction of the intermediate transfer belt  36 . The direction of an arrow shown in  FIG. 11  corresponding to the rotating direction of the intermediate transfer belt  36 , and the direction perpendicular to the direction of the arrow corresponds to the main scan direction in which the sensors  17 ,  18 , and  19  are aligned. All the position detection toner marks are formed at positions detected by the sensors  17 ,  18 , and  19 . By the image forming apparatus including the transfer belt shown in  FIG. 11 , the positions of the position detection toner marks on the intermediate transfer belt  36  are detected, so that the positions of images formed on photoconductor drums  9 Y,  9 M,  9 C, and  9 BK can be corrected. 
   Further, the image forming apparatus includes a drive roller  7  and a driven roller  8  like the image forming apparatus shown in  FIG. 1 . A rotary encoder  20  is attached to the shaft of the driven roller  8  as in  FIG. 1 . A drive motor  22  is controlled based on detection results to constitute a feedback loop, and movement of the intermediate transfer belt  36  is stabilized. 
   The embodiments are preferable embodiments of the present invention. Various modifications of the embodiments can be effected without departing from the spirit and scope of the invention. For example, laser beams are applied as the exposing light sources in the embodiments. However, the light sources are not limited to the laser beams. For example, an LED array or the like can also be used. Furthermore, detection of the speed and position of the belt may be performed by using not only the rotary encoder attached to the shaft of the driven roller, but also by a configuration that detects a scale or toner marks formed on the upper or lower surface of the belt. Although the stepping motor is used as the drive motor, the drive motor is not limited to the stepping motor, and a DC motor, an AC motor, or the like can also be used. In addition, although PI control is applied as a control calculation performed by the controller, the control operation is not limited to the PI control, and P control, PID control, H∞ control, or the like can also be applied. 
   An image forming apparatus according to the present invention performs a detected error decision in detection of a movable member and a control error decision in control of a driving unit on the basis of a moving distance or a moving speed of the movable member, and performs a predetermined error process when an error is determined, to prevent erroneous position control and erroneous speed control from being performed. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.