JP5929011B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a technique for measuring an image forming position shift and correcting the shift in an image forming apparatus.

  2. Description of the Related Art Conventionally, in an image forming apparatus, a technique for measuring a deviation of an image forming position and correcting the deviation in order to ensure image quality is known. According to this technology, for example, a plurality of marks are formed using a plurality of colors on a belt for paper conveyance, light is irradiated on the belt from a light projecting portion of the sensor, and the specularly reflected light is emitted from the sensor. Light is received at the light receiving section. Then, the position of the mark is measured by a light reception signal output according to the amount of light received by the sensor, and the deviation of the image forming position is corrected based on the measurement result.

JP 2008-225192 A

  However, with the above technique, the measurement result is likely to be affected by scratches or dirt on the belt surface, an error in the sensor mounting position, and the like, so there is a possibility that the accuracy of the positional deviation correction cannot be ensured.

  The present invention has been completed based on the above-described circumstances, and an object thereof is to ensure the accuracy of positional deviation correction.

  An image forming apparatus disclosed in the present specification includes an image forming unit that forms a plurality of marks on a carrier at intervals, a light projecting unit that irradiates light to the carrier, and the light projecting unit A mark sensor having a first light-receiving unit that receives diffusely reflected light of the light applied to the carrier and outputs a light reception signal corresponding to the amount of received light, and a movement for moving the carrier relative to the mark sensor And when the portion on which the mark is formed on the carrier crosses the optical path of the light irradiated from the light projecting portion, the portion between the marks on the carrier is turned on. A lighting control unit that temporarily turns off the light projecting unit when crossing the optical path, and a correction that measures the position of the mark based on the light reception signal and corrects the deviation of the image forming position based on the measurement result A section.

  In the image forming apparatus, the mark sensor includes a second light receiving unit that receives the regular reflection light of the light emitted from the light projecting unit to the carrier and outputs a light reception signal corresponding to the amount of received light. The image forming unit forms a density patch on the carrier, and the correction unit receives light received from the first light receiving unit when light is emitted from the light projecting unit to the density patch. When the image forming density is corrected based on the signal and the light receiving signal output from the second light receiving unit, and the lighting control unit emits light from the light projecting unit to the mark, A configuration may be adopted in which the amount of light is larger than when the density patch is irradiated.

  Further, in the image forming apparatus, the lighting control unit is configured so that the assumed positions of some of the marks on the front side in the movement direction of the plurality of marks on the carrier are first than the optical path reaches the optical path. The light projecting unit is turned on before time, the assumed position of the mark behind the moving direction is corrected based on the measurement result, and the assumed position of the corrected mark is the optical path It is good also as a structure which makes the said light projection part light before 2nd time shorter than said 1st time rather than reaching.

  The image forming apparatus includes a temperature sensor that detects a temperature around the mark sensor, and the image forming unit increases the interval between the marks as the temperature detected by the temperature sensor increases. The lighting control unit may be configured such that the longer the interval between the marks, the longer the time for turning off the light projecting unit between the marks.

  Further, in the image forming apparatus, the image forming unit has two black marks spaced apart in the moving direction of the carrier on the first mark of a color other than black and the mark of a color other than black. To form a second mark formed between the two black marks, and the correction unit is based on the measurement result of the position of the first mark and the measurement result of the position of the second mark. Thus, a relative image forming apparatus misalignment between the color other than black and black may be obtained, and the misalignment may be corrected.

  Further, in the image forming apparatus, the lighting control unit may check that the mark on the front side in the moving direction of the carrier reaches the optical path before the second mark reaches the optical path. It is good also as a structure which lights the said light projection part.

  Further, in the image forming apparatus, the lighting control unit moves the light projecting unit before the mark on the rear side in the movement direction of the carrier reaches the optical path before passing through the optical path. It is good also as a structure which makes it light-extinguish.

  According to the present invention, the mark sensor receives the diffuse reflected light of the light irradiated on the carrier, and measures the position of the mark based on the amount of the received light. Less affected. Thereby, it is possible to ensure the accuracy of the positional deviation correction.

1 is a side sectional view showing a schematic configuration of a printer according to an embodiment. Block diagram schematically showing the electrical configuration of the printer Diagram showing circuit configuration of mark sensor Waveform diagram showing an example of the PWM signal of the lighting control signal Plan view showing pattern for concentration measurement Flow chart of misalignment correction processing Graph illustrating the relationship between pulse forward current (IFP) and duty ratio when LED is pulsed The graph which shows the relationship between the PWM value of a lighting control signal, and the electric current value which flows into a light projection element The figure explaining the relationship between the pattern for position shift correction, the lighting control signal, the light reception signal of the light receiving element that receives diffusely reflected light, and the time change of the output signal of the comparator The figure explaining the difference of the distance distance of a mark and the lighting interval of a light projection element in case the ambient temperature is 25 degreeC and 50 degreeC

  Next, an embodiment of the present invention will be described with reference to FIGS.

(Entire printer configuration)
FIG. 1 is a side sectional view showing a schematic configuration of the printer 10. The printer 10 (an example of an image forming apparatus) is a direct transfer tandem type color printer that forms a color image using toners of four colors (black K, yellow Y, magenta M, and cyan C). In the following description, the left side in FIG. 1 is the front side of the printer 10, and the back side of the paper is the left side of the printer 10. Further, in FIG. 1, reference numerals are appropriately omitted for the same components between the respective colors.

  The printer 10 includes a main body casing 11, and a cover 11 </ b> A that can be opened and closed is provided on the upper surface thereof. A supply tray 13 on which a plurality of sheets 12 can be stacked is provided at the bottom of the main casing 11. The sheets 12 stacked on the supply tray 13 are fed out one by one by the supply roller 14 and conveyed onto the belt unit 19 of the image forming unit 18 by the registration roller 15.

  The image forming unit 18 includes a belt unit 19, four exposure units 27, four process units 28, and a fixing device 33.

  The belt unit 19 has a configuration in which an annular belt 21 (an example of a carrier) is stretched between a pair of front and rear belt support rollers 20A and 20B. The belt 21 is made of a resin material such as polycarbonate, and the surface thereof is processed into a mirror surface. Therefore, the belt 21 has a larger regular reflectance than the diffuse reflectance. The rear belt support roller 20B (an example of the moving unit) is rotationally driven by the power of a drive motor (not shown), and the belt 21 circulates and moves in the clockwise direction in the drawing, and is electrostatically adsorbed on the upper surface thereof. 12 is conveyed backward. Inside the belt 21, a transfer roller 23 is provided at a position facing the photosensitive drum 30 of each process unit 28 described later with the belt 21 in between.

  Below the belt 21, a mark sensor 50 used for detecting patterns P1 and P2 formed on the belt 21 is provided as will be described later. Further, a cleaner 25 that collects toner, paper dust, and the like attached to the surface of the belt 21 is provided below the belt unit 19.

  Each of the four exposure units 27 has a plurality of LEDs (not shown) arranged in a line in the left-right direction. The four exposure units 27 emit light from the LEDs based on the supplied print data and emit light on the photosensitive drum 30. Irradiate.

  The four process units 28 are provided corresponding to the respective colors of yellow, magenta, cyan, and black (indicated by 28Y, 28M, 28C, and 28K in order), and each color toner (an example of a developer) is provided. A developing device 29, a photosensitive drum 30, and a charger 31 are accommodated. The outer peripheral surface of the photosensitive drum 30 is uniformly positively charged by the discharge from the charger 31 and then exposed by the exposure unit 27, whereby an electrostatic latent image is formed there. Then, the toner is supplied from the developing unit 29 onto the photosensitive drum 30, whereby the electrostatic latent image is visualized as a toner image.

  The toner image on each photosensitive drum 30 is sequentially superimposed on the sheet 12 by the transfer bias voltage applied to the transfer roller 23 while the sheet 12 on the belt 21 passes between the photosensitive drum 30 and the transfer roller 23. Transcribed. The sheet 12 on which the toner image has been transferred is sent to the fixing device 33 by the belt unit 19, where the toner image is thermally fixed, and then discharged onto the upper surface of the cover 11A.

(Electrical configuration of printer)
Next, the electrical configuration of the printer 10 will be described. FIG. 2 is a block diagram schematically showing the electrical configuration of the printer 10.

  As shown in the figure, the printer 10 includes a control unit 41, ROM 42, RAM 43, NVRAM 44, network interface 45, image forming unit 18, temperature sensor 46, and mark sensor 50. The control unit 41 (an example of a lighting control unit and a correction unit) includes a CPU and an application specific integrated circuit (ASIC). The ROM 42 stores a control program for executing various operations of the printer 10 such as density correction processing and positional deviation correction processing, which will be described later, and the control unit 41 controls each unit according to the program read from the ROM 42.

  The RAM 43 is a volatile memory used as a work area of the control unit 41, and the NVRAM 44 is a nonvolatile memory that stores various setting values and the like. The network interface 45 is connected to a network line such as a LAN, and communicates with a terminal device (not shown) connected on the network line. The temperature sensor 46 detects the temperature around the mark sensor 50.

(Configuration of mark sensor)
FIG. 3 is a diagram illustrating a circuit configuration of the mark sensor 50. The mark sensor 50 includes two light projecting circuits 51A and 51B and three light receiving circuits 52A, 52B and 52C. The light projecting circuits 51A and 51B respectively include light projecting elements 53A and 53B made of LEDs. One light projecting element 53A is disposed near the left end of the belt 21, and the other light projecting element 53B is disposed near the right end of the belt 21, and each irradiates the belt 21 with light obliquely.

  The control unit 41 inputs lighting control signals S1 and S2 for controlling the lighting states of the light projecting elements 53A and 53B to the light projecting circuits 51A and 51B, respectively. The lighting control signals S1 and S2 are signals that repeat a PWM (pulse width modulation) signal at a predetermined interval. The PWM signals of the lighting control signals S1 and S2 are smoothed by RC circuits 55A and 55B included in the light projecting circuits 51A and 51B, respectively, and the smoothed voltage is applied to the drive transistors 54A and 54B included in the light projecting circuits 51A and 51B. The current corresponding to the voltage is supplied to the light projecting elements 53A and 53B.

  FIG. 4 is a waveform diagram showing an example of the PWM signal of the lighting control signals S1 and S2. In this example, a PWM signal having a period of 10 μs and a duty ratio of 50% is output for 20 ms. When the lighting control signals S1 and S2 are input to the light projecting circuits 51A and 51B, the light projecting element 53A is used for 20 ms. , 53B are lit with a predetermined amount of light. The controller 41 adjusts the amount of light by adjusting the current flowing through the light projecting elements 53A and 53B by changing the duty ratio of the PWM signals of the lighting control signals S1 and S2.

  Each of the light receiving circuits 52A, 52B, and 52C includes light receiving elements 56A, 56B, and 56C made of phototransistors. The light receiving element 56A is disposed on the optical axis of the specularly reflected light so as to receive the specularly reflected light emitted to the belt 21 from the light projecting element 53A. The light receiving elements 56B and 56C are positioned away from the optical axis of the regular reflection light of the light irradiated on the belt 21 so as to receive the diffuse reflection light of the light irradiated on the belt 21 from the light projecting elements 53A and 53B, respectively. It is arranged in.

  The light receiving elements 56A and 56B each output a light reception signal corresponding to the amount of received light, and the light reception signals are amplified by the amplification circuits 57A and 57B respectively included in the light reception circuits 52A and 52B and controlled as analog light reception signals S3 and S4. Input to the unit 41. The control unit 41 performs AD conversion on the received light reception signals S3 and S4 to obtain digital values thereof. The light reception signal amplified from the light receiving element 56B in the light receiving circuit 52B is also input to the comparator 58A. The light receiving element 56C outputs a light reception signal corresponding to the amount of light received, and the light reception signal is amplified by the amplification circuit 57C included in the light reception circuit 52C and input to the comparator 58B.

  The control unit 41 outputs a reference level signal S5, which is a PWM signal, to the RC circuit 59. The reference level signal S5 is smoothed by the RC circuit 59 and input to the comparators 58A and 58B, respectively. The comparators 58A and 58B compare the received light signals from the light receiving elements 56B and 56C with the reference level given by the reference level signal S5, respectively, and output the detection signals S6 and S7 binarized to the control unit 41. .

(Density correction)
Next, the density correction operation will be described. FIG. 5 is a plan view showing a pattern P1 for density measurement.

  The control unit 41 executes two types of density correction and positional deviation correction as correction processing for ensuring image quality during printing. For example, the density correction and the positional deviation correction processing are performed immediately after the power is turned on, when attachment / detachment of the process unit 28 is detected, or when a predetermined time elapses from the previous correction processing, or a predetermined number of sheets are printed. This is executed when a predetermined condition is satisfied.

  When the density correction is started, the control unit 41 first sets the density measurement pattern P1 as shown in FIG. 5 on the belt 21 toward the left end, that is, the optical path of the light projecting element 53A when the belt 21 is moved. Form in a position that crosses. This density pattern P1 is obtained by arranging a plurality of density patches 65 along the moving direction (arrow A direction) of the belt 21, and a plurality of colors having different densities in yellow, magenta, cyan, and black. (Indicated by a yellow patch 65Y, a magenta patch 65M, a cyan patch 65C, and a black patch 65K).

  And while the pattern P1 crosses the optical path of the light irradiated from the light projecting element 53A as the belt 21 moves, the control unit 41 continuously lights the light projecting element 53A with a constant light amount by the lighting control signal S1. Let At this time, while each density patch 65 passes on the optical path, the value of the light reception signal S3 or S4 is sampled a plurality of times, and the density of each density patch 65 is obtained based on the average value thereof.

  Here, the density patches 65Y, 65M, and 65C formed of yellow, magenta, and cyan toners have the property that the diffuse reflection light increases as the toner density increases. For this reason, in the light receiving element 56A arranged to receive regular reflection light, the amount of diffuse reflection light received in addition to regular reflection light is increased.

  Therefore, the control unit 41 measures the density of the density patches 65Y, 65M, and 65C of these three colors based on the light reception signal S4 from the light receiving element 56B that receives the diffuse reflected light. Further, since the black density patch 65K has a property that the diffuse reflection light is relatively small, the density is measured based on the light reception signal S3 from the light receiving element 56A that receives the regular reflection light.

  The control unit 41 measures the density of each density patch 65 as described above, and then, based on the measurement result, for example, at each gradation obtained by dividing the density from 0% to 100% into 256 equal parts, the image forming unit 18 performs sheeting. Density correction data such that the density when an image is printed on the image 12 is an ideal density is calculated for each color and stored in the NVRAM 44, and the correction process is completed. At the time of printing, the control unit 41 reads the density correction data stored in the NVRAM 44, and adjusts the density of the printed image by setting the light emission intensity of the LED of the exposure unit 27, the developing bias voltage, and the like based on the data. To do.

(Position correction processing)
Next, the operation of the misalignment correction process will be described. FIG. 6 is a flowchart of misalignment correction processing.
When the positional deviation correction process shown in FIG. 6 is started, the control unit 40 first acquires the temperature around the mark sensor 50 by the temperature sensor 46 (S101).

  Here, the LEDs used as the light projecting elements 53A and 53B generally have a forward current (IF) and a pulse forward current (IFP) as absolute maximum ratings. The forward current is a current allowed when the LED is lit continuously, and the pulse forward current is a current allowed when the LED is lit in pulses. If the current passed through the LED exceeds these rated values, there is a high possibility that the LED will fail or the life will be extremely shortened.

  FIG. 7 is a graph illustrating the relationship between the pulse forward current (IFP) and the duty ratio when the LED is turned on with a pulse width of 20 ms. As shown in the figure, the values of the pulse forward current and the forward current become smaller as the ambient temperature becomes higher. For example, when the LED is turned on for 20 ms at a duty ratio of 0.1, that is, at a cycle of 100 ms, a current of 160 mA can be passed at an ambient temperature of 25 ° C., 136 mA at 40 ° C., and 118.4 mA at 50 ° C. In addition, when the duty ratio is 1, that is, when the LED is continuously lit, a forward current of 50 mA at 25 ° C., 43 mA at 40 ° C., and 37 mA at 50 ° C. can be passed.

  As shown in the figure, the LED can flow a larger current at the time of pulse lighting than at the time of continuous lighting, and can flow a larger current as the duty ratio of the pulse lighting is smaller. And as the current flowing through the LED increases, the amount of light increases.

  After acquiring the temperature around the mark sensor 50 in S101 of FIG. 6, the control unit 41 determines the amount of light when the light projecting elements 53A and 53B are turned on in order to perform measurement with appropriate sensitivity (S102). ). In other words, here, the current value at the time of lighting of the light projecting elements 53A and 53B or the PWM value (duty) given to the light projecting circuits 51A and 51B is determined by the lighting control signals S1 and S2.

  The current value during lighting of the light projecting elements 53A and 53B is set to a value smaller than at least the rated value of the pulse forward current at the current ambient temperature. Also, in order to ensure the amount of light, at least a value larger than the current that flows during continuous lighting during density correction, or a value that is larger than the rated value of forward current during continuous lighting at the current ambient temperature .

  More specifically, the control unit 41 obtains a PWM value such that the maximum output of the light reception signals from the light projecting elements 53A and 53B is 4V. Here, FIG. 8 is a graph showing the relationship between the PWM values of the lighting control signals S1 and S2 and the current values flowing through the light projecting elements 53A and 53B. The relationship between the PWM value and the current value shown in this graph can be estimated based on the relationship between the characteristics of the drive transistors 54A and 54B and the voltage given by the PWM signal, and the like. Formula data is stored in the NVRAM 44 or the like.

  The control unit 41 forms a sufficiently large patch on the belt 21 with one of yellow, magenta, and cyan toners, and sets the reference level to be applied to the comparators 58A and 58B to, for example, 0.5V and 2.0V. In each case, the PWM value when the received light signal reaches the reference level by changing the PWM value of the lighting control signals S1 and S2 stepwise, and the current value flowing through the light projecting elements 53A and 53B at that time Ask.

  For example, when the output of the received light signal is 0.5 V and the PWM value is 29%, the current flowing through the light projecting elements 53A and 53B at that time can be estimated to be approximately 12.9 mA based on the above data, and the output is In the case of 2.0V, if the PWM value is 50%, the current can be estimated to be 47.5 mA. From these results, the PWM value at which the light reception signal output is 4V is 78% by linear approximation calculation, and the current flowing through the light projecting elements 53A and 53B at that time can be calculated as 93.7 mA.

  Subsequently, the control unit 41 determines a duty ratio at the time of pulse lighting based on the ambient temperature acquired in S101, and selects a misalignment correction pattern P2 corresponding to the duty ratio (S103). For example, the ROM 42 stores data of the pattern P2 corresponding to each of when the duty ratio at the time of pulse lighting is 5%, 10%, 20%, 30%,... A pattern P2 corresponding to one duty ratio within an allowable range at the ambient temperature is selected.

  For example, when the current value at the time of lighting is set to 100 mA, from FIG. 7, if the ambient temperature is 25 ° C., a duty ratio of about 40% is allowed, and if the ambient temperature is 50 ° C., about 20%. Allowable up to duty ratio. In this embodiment, with a margin, the duty ratio is determined such that 30% duty is 25 ° C. and 10% duty is 50 ° C., and the corresponding pattern P2 is selected.

  FIG. 9 shows a pattern P2 for correcting misalignment, a lighting control signal S1 (or S2), a light reception signal S4 from the light receiving element 56B (56C) that receives diffusely reflected light, and an output signal S6 of the comparator 58A (58B). It is a figure explaining the relationship with the time change of (S7). FIG. 9 shows a part of the pattern P2. The pattern P2 is formed at a position near the left end and a position near the right end on the belt 21, and the optical paths of the light projecting elements 53A and 53B are respectively moved when the belt 21 is moved. It is arranged to cross.

  The pattern P2 includes a first mark 70C, 70M, 70Y elongated in the left-right direction of cyan, magenta, and yellow and a second mark 71Y described below as a set, and a plurality of sets of marks 70C, 70M, 70Y, 71Y are belted. 21 are arranged at intervals in the moving direction (arrow A direction). In the following description, these marks 70C, 70M, 70Y, 71Y are collectively referred to as marks 70.

  The second mark 71Y is formed by overlapping two black marks 72K on the upper surface of the yellow mark 72Y which is wider than the first marks 70C to 70Y with a gap in the moving direction of the belt 21. This is a yellow mark formed between the marks 72K. The width of the second mark 71Y, that is, the interval between the two black marks 72K is made substantially the same as the width of the first marks 70C to 70Y.

  Further, the pattern P2 includes a first group G1 having a large interval between the marks 70 and a second group G2 having a smaller interval than the first group G1. The first group G1 includes one or a plurality of sets of marks 70 positioned on the front side of the belt 21 in the movement direction, and the second group G2 includes a plurality of sets of marks 70 positioned on the rear side of the movement direction of the belt 21. In each group G1, G2, intervals between adjacent marks 70C, 70M, 70Y, 71Y are made equal.

  The control unit 41 forms the pattern P2 on the moved belt 21 by the image forming unit 18 (S104). Subsequently, when the pattern P2 reaches the position of the mark sensor 50, first, each mark of the first group G1 is displayed. 70 is measured (S105). Here, the control unit 41 uses the lighting control signal S <b> 1 (S <b> 2) to turn on the light projecting element 53 </ b> A at a timing at which the portion where each mark 70 is formed on the belt 21 crosses the light path of the light projecting elements 53 </ b> A and 53 </ b> B. , 53B are turned on, and the light projecting elements 53A, 53B are temporarily turned off while the portion between the marks 70 on the belt 21 crosses the optical path.

  In FIG. 9 and FIG. 10 described later, for convenience, the lighting control signal S1 (S2) is shown to always have an ON value during the lighting time t1 or t3 for lighting the light projecting elements 53A and 53B. Actually, as described above, the control unit 41 outputs a PWM signal (see FIG. 4) that periodically repeats the on and off values during the lighting times t1 and t3.

  The control unit 41 determines a predetermined time before the time at which the assumed position of each mark 70 (the position at which the mark 70 is formed when the positional deviation amount is assumed to be 0) arrives on the optical path of the light projecting elements 53A and 53B. The light projecting elements 53A and 53B are turned on, and after the assumed position passes on the optical path, the light projecting elements 53A and 53B are turned off after a predetermined time. Here, the ratio “t1 / t2” of the lighting time t1 for each mark 70 with respect to the time interval t2 of each mark 70 in the second group G2 is set to the duty ratio determined in S103.

  FIG. 10 is a diagram for explaining the difference between the distance between the marks 70 and the lighting intervals of the light projecting elements 53A and 53B when the ambient temperature is 25 ° C. and 50 ° C. FIG. For example, when the ambient temperature is 25 ° C. and the duty ratio of pulse lighting is set to 30%, if the one lighting time t1 is 20 ms, the time interval t2 between the marks 70 is 66.7 ms. At this time, the distance P between the marks 70 of the second group G2 uses a pattern P2 aimed at 13.3 mm when the moving speed of the belt 21 is 200 mm / s.

  Further, when the ambient temperature is 50 ° C. and the duty ratio of pulse lighting is set to 10%, if the one lighting time t1 is 20 ms, the time interval t2 between the marks 70 is 200 ms. At this time, the distance P between the marks 70 of the second group G2 uses a pattern P2 aimed at 40 mm when the moving speed of the belt 21 is 200 mm / s.

  Thus, the control unit 41 selects a pattern in which the distance between the marks 70 is larger as the ambient temperature of the mark sensor 50 is higher, and accordingly, the time for turning off the light projecting elements 53A and 53B between the marks 70 is set. Lengthen. Thus, the distance or time interval between the marks 70 can be suppressed while suppressing the current flowing through the light projecting elements 53A and 53B within an allowable range according to the ambient temperature.

  Further, as shown in FIG. 9, the control unit 41 sets the lighting time t1 for one time to 20 ms for each mark 70 of the second group G2, whereas the control unit 41 sets each mark 70 of the first group G1. On the other hand, the lighting time t3 is set to a time longer than the lighting time for the second group G2, here 30 ms.

  For example, when the lighting time t3 is set to 30 ms, for example, when the ambient temperature is 25 ° C. and the duty ratio of pulse lighting is set to 30%, the time interval t4 between the marks 70 is 100 ms (corresponding to a distance interval of 20 mm) or so. That's it. When the ambient temperature is 50 ° C. and the duty ratio is set to 10%, the time interval t4 between the marks 70 is set to 300 ms (corresponding to a distance interval of 60 mm) or more.

  When the control unit 41 starts measurement of the first group G1, the light projecting elements 53A and 53B are turned on 10 ms (example of the first time) before the assumed position of each mark 70 reaches the light path of the light projecting elements 53A and 53B. Light up. Then, after a time (10 ms in this example) that the assumed position of the mark 70 passes on the optical path has passed, the light projecting elements 53A and 53B are turned off after 10 ms has passed.

  The output of the light reception signal when measuring the first marks 70C to 70Y of cyan, magenta, and yellow is relatively small immediately after the light emitting elements 53A and 53B are turned on because the diffuse reflectance of the surface of the belt 21 is low. When each first mark 70C to 70Y crosses the optical path, the first mark 70C to 70Y increases by receiving diffusely reflected light and decreases again after passing through each first mark 70C to 70Y. Based on the timing at which the value of the detection signal S6 (S7) output by comparing this output with the reference level Th by the comparators 58A and 58B changes from H to L at the position of the first mark 70, the control unit 41. Can measure the positions of the first marks 70C to 70Y.

  When measuring the second mark 71Y, the light projecting elements 53A and 53B are turned on while the black mark 72K on the front side in the moving direction of the belt 21 is on the optical path, and the second mark 71Y has passed the optical path. Thereafter, when the black mark 72K on the rear side in the movement direction is on the optical path, the light projecting elements 53A and 53B are turned off.

  Since both the black marks 72K have low diffuse reflectance, the light reception signal when measuring the second mark 71Y changes in the same way as when measuring the yellow first mark 70Y. Can measure the position of the second mark 71Y based on the outputs of the comparators 58A and 58B. However, since the positions of the front edge and the rear edge of the second mark 71Y in the moving direction are determined by the position of the black mark 72K, the position of the second mark 71Y reflects the black image forming position.

  The control unit 41 obtains the amount of deviation from the assumed position of each mark 70 from the measurement result of the first group G1. When the first group G1 has a plurality of sets of marks 70, the amount of deviation from the assumed position is obtained for each set of marks 70, and the average value thereof is taken. Then, the control unit 41 corrects the assumed position by adding the shift amount to the assumed position of each mark 70 of the second group G2 (S106).

  Subsequently, the control unit 41 measures the position of each mark 70 in the second group G2 (S107). At this time, the light projecting elements 53A and 53B are turned on, for example, 5 ms (second time example) before the corrected assumed position of each mark 70 of the second group G2 reaches the optical path. Then, after a time (10 ms in this case) that the assumed position of the mark 70 passes on the optical path has passed, the light projecting elements 53A and 53B are turned off after 5 ms has passed.

  That is, for the mark 70 in the first group G1, where the actual position of the mark 70 and the assumed position are likely to be relatively large, the light projecting elements 53A and 53B are moved earlier than the timing at which the assumed position reaches the optical path. By turning on and turning off after the assumed position passes through the optical path, it is possible to avoid the measurement error of the mark 70 due to the deviation of the lighting timing.

  On the other hand, since the assumed position of the mark 70 of the second group G2 is corrected based on the measurement result of the first group G1, the deviation between the actual position of the mark 70 and the assumed position is relatively small. For this reason, before the assumed position of each mark 70 of the second group G2 reaches the optical path, it is lit at a timing later than the first group G1, and after the assumed position passes, at an earlier timing than the first group G1. Can be turned off. As a result, the distance between the marks 70 can be reduced, and a large number of marks 70 can be measured in a short time.

  Thus, after measuring the position of each mark 70 of the first group G1 and the second group G2, the control unit 41 determines the position of each of the first marks 70C to 70Y with respect to the second mark 71Y in each set of marks 70. The amount of misregistration is obtained and averaged over all sets of marks 70 to obtain the amount of misregistration of cyan, magenta, and yellow image forming positions with respect to black. Then, correction values for canceling these misregistrations are calculated, the correction values are stored in the NVRAM 44 as misregistration correction data (S108), and this misregistration correction process is terminated.

  At the time of printing, the control unit 41 reads the positional deviation correction data stored in the NVRAM 44, and adjusts the position of the image to be printed by setting the exposure timing of the LED of the exposure unit 27 based on the data. .

(Effect of this embodiment)
As described above, according to the present embodiment, the mark sensor 50 receives the diffusely reflected light of the light irradiated on the belt 21 and measures the position of the mark 70 based on the amount of the received light. Less susceptible to surface scratches and dirt. That is, as in the conventional case, when the regular reflection light of the light irradiated on the belt is received, when the light is irradiated on the uneven portion formed by scratches on the belt surface or adhesion of foreign matters, the regular reflection light is reflected. Since the angle changes, the amount of received light varies greatly and the measurement result is adversely affected. On the other hand, when the diffuse reflected light of the light applied to the belt 21 is received as in the present embodiment, the amount of received light is less likely to fluctuate even when light is applied to the uneven portion of the belt 21 surface. Measurement results are less affected. Accordingly, it is possible to ensure the accuracy of positional deviation correction.

  In general, since the amount of diffusely reflected light received on the belt is smaller than the amount of regular reflected light of the same light, there is a concern that the measurement accuracy may be reduced. If a light projecting element with high emission intensity or a light receiving element with high light receiving sensitivity is used in order to ensure the accuracy, the cost increases.

  On the other hand, according to the present embodiment, when the mark 70 on the belt 21 crosses the light path of the light projecting elements 53A and 53B, the light projecting elements 53A and 53B are turned on, and when the portion between the marks 70 crosses temporarily. By turning off the light, it is possible to increase the amount of light by causing a larger current to flow through the light projecting elements 53A and 53B than during continuous lighting. Thereby, the measurement accuracy can be ensured. In addition, since the accuracy can be maintained even when a light projecting element having a relatively low light emission intensity or a light receiving element having a low light receiving element is used, the cost can be suppressed.

  Further, when irradiating the mark 70 with light at the time of positional deviation correction, the amount of light is made larger than when irradiating the density patch 65 at the time of density correction. Thereby, it is possible to ensure the accuracy of the positional deviation correction.

  Further, among the plurality of marks 70 included in the pattern P2, the light projecting elements 53A and 53B are set to the first time before the assumed position of the mark 70 of the first group G1 on the front side in the moving direction of the belt 21 is on the optical path. The projected position of the remaining mark 70 is corrected based on the measurement result, and the light projecting elements 53A, 53A, and the projected element 53A, before the second time shorter than the first time before the corrected estimated position of the mark 70 reaches the optical path. 53B is lit.

  Thereby, the mark 70 (first group G1) on the front side in the moving direction can be lighted with sufficient margin before the optical path of the mark sensor 50 reaches the assumed position of the mark 70, thereby suppressing the occurrence of a measurement error. . Further, for the mark 70 on the rear side in the movement direction (second group G2), after correcting the assumed position based on the measurement result of the front mark 70, the assumed position is turned on immediately before reaching the optical path, The lighting time of the light projecting elements 53A and 53B can be shortened.

  Further, as the ambient temperature of the mark sensor 50 detected by the temperature sensor 46 is higher, the distance between the marks 70 is increased, and accordingly, the time for turning off the light projecting elements 53A and 53B between the marks is lengthened. In general, the LED used as the light projecting elements 53A and 53B of the mark sensor 50 has a smaller ratio of time (duty ratio) that can be lit with a constant current as the ambient temperature is higher. Therefore, as the detected temperature is higher, the distance between the marks 70 is increased, and the time during which the light projecting elements 53A and 53B are turned off between the marks 70 is increased accordingly, whereby the characteristics of the light projecting elements 53A and 53B are increased. The pulse lighting interval can be adjusted according to the above.

  Further, as the misregistration correction mark 70, the two black marks 72K are superimposed on the first mark 70C to 70Y of a color other than black and the mark 72Y of a color other than black with an interval therebetween. The second mark 71Y configured between the marks 72K is used. In general, a black mark is difficult to measure because of its low reflectance, but with the above-described configuration, a relative shift of the image forming apparatus between black and a color other than black can be accurately measured and corrected.

  Further, the light projecting elements 53A and 53B are turned on before the second mark 71Y reaches the optical path after the black mark 72K on the front side of the second mark 71Y reaches the optical path of the light projecting elements 53A and 53B. This makes it possible to lengthen the turn-off time as compared with the case where the light projecting elements 53A and 53B are turned on in front of the front black mark 72K, and avoid the output of unnecessary light reception signals.

  Further, after the black mark 72K on the rear side of the second mark 71Y reaches the light path of the light projecting elements 53A and 53B, the light projecting elements 53A and 53B are turned off before passing through the light path. Accordingly, the turn-off time can be shortened compared to the case where the light projecting elements 53A and 53B are turned off after the rear black mark 72K passes through the optical path, and the output of unnecessary light reception signals can be avoided.

<Related invention>
The following inventions can also be implemented as techniques related to the above invention.

1. The invention in which the distance between the marks 70 is increased as the ambient temperature of the mark sensor 50 detected by the temperature sensor 46 is increased, and the time for turning off the light projecting elements 53A and 53B between the marks 70 is increased accordingly. The present invention can also be applied to the case where the position of the mark 70 is measured based on the received light amount of regular reflected light instead of diffuse reflected light at the time of deviation correction. In this case, for example, the mark sensor is provided with two right and left light receiving circuits 52A each having a light receiving element 56A that receives specularly reflected light, and a value obtained by comparing each received light signal with a reference level by a comparator is given to the control unit 41. It is good also as a structure which inputs.

2. In the invention that corrects the assumed position of the subsequent mark based on the measurement result of the mark measured earlier and shortens the lighting time, the position of the mark 70 is corrected based on the received light amount of the regular reflection light instead of the diffuse reflection light at the time of misalignment correction. It can also be applied when measuring the position. In this case, for example, the left and right light receiving circuits 52A having light receiving elements 56A that receive specularly reflected light are provided in the mark sensor, and values obtained by comparing the respective light reception signals with the reference level by the comparator are given to the control unit 41. It is good also as a structure which inputs.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

(1) In the above embodiment, as the ambient temperature of the mark sensor 50 detected by the temperature sensor 46 is higher, the distance between the marks 70 is increased, and the light projecting elements 53A and 53B are turned off between the marks 70 accordingly. Although the example of increasing the time is shown, the present invention can also be applied to a device in which the distance interval between the marks 70 and the time for turning off the light projecting elements 53A and 53B between the marks 70 are not changed depending on the temperature.

(2) In the above-described embodiment, the assumed position of the subsequent mark is corrected based on the measurement result of the previously measured mark to shorten the lighting time. However, in the present invention, each mark included in the pattern It can also be applied to those in which the lighting time for and the distance between the marks 70 are not changed.

(3) In the above embodiment, the one corresponding to the duty ratio determined from the pre-stored misregistration correction pattern P2 has been shown. However, according to the present invention, the predetermined duty ratio is selected. Based on this, the distance between the marks 70 may be determined, and the pattern P2 may be formed dynamically.

(4) In the above embodiment, the present invention is applied to a direct transfer tandem color LED printer. However, the present invention is not limited to this. For example, the present invention may be a laser printer, an intermediate transfer system, or a 4-cycle system. The present invention can be applied to other types of image forming apparatuses.

DESCRIPTION OF SYMBOLS 10 ... Printer, 18 ... Image formation part, 20B ... Belt support roller, 21 ... Belt, 41 ... Control part, 46 ... Temperature sensor, 50 ... Mark sensor, 70C, 70M, 70Y ... 1st mark, 71Y ... 2nd mark , 72Y ... yellow mark, 72K ... black mark, G1 ... first group, G2 ... second group

Claims (6)

An image forming unit for forming a plurality of marks on the carrier at intervals, and forming a density patch on the carrier;
A light projecting unit for irradiating the carrier with light, and a first light receiving unit for receiving diffuse reflection light of the light emitted from the light projecting unit to the carrier and outputting a light reception signal according to the amount of received light A mark sensor having a second light receiving unit that receives specularly reflected light emitted from the light projecting unit to the carrier and outputs a light reception signal corresponding to the amount of light received;
A moving unit that moves the carrier relative to the mark sensor;
The light projecting unit is turned on when the portion where the mark is formed on the carrier crosses the optical path of the light emitted from the light projecting unit, and the portion between the marks on the carrier is configured to pass the optical path. A lighting control unit that temporarily turns off the light projecting unit when crossing,
The position of the mark is measured based on the light reception signal, the deviation of the image forming position is corrected based on the measurement result, and the first light reception is performed when the density patch is irradiated with light from the light projecting unit. A correction unit that corrects the image formation density based on the light reception signal output from the second light reception unit and the light reception signal output from the second light reception unit;
With
The lighting control unit is an image forming apparatus in which when the light is emitted from the light projecting unit to the mark, the amount of light is larger than when the light is applied to the density patch. An image forming unit that forms a plurality of marks on the carrier at intervals, and
A light projecting unit that irradiates the carrier with light, and a first light receiving unit that receives diffuse reflected light of the light emitted from the light projecting unit to the carrier and outputs a light reception signal corresponding to the amount of light received A mark sensor having
A moving unit that moves the carrier relative to the mark sensor;
The light projecting unit is turned on when the portion where the mark is formed on the carrier crosses the optical path of the light emitted from the light projecting unit, and the portion between the marks on the carrier is configured to pass the optical path. A lighting control unit that temporarily turns off the light projecting unit when crossing,
A correction unit that measures the position of the mark based on the light reception signal, and corrects the deviation of the image forming position based on the measurement result,
The lighting control unit sets the light projecting unit at a first time before the assumed positions of some of the marks on the carrier on the front side in the moving direction of the carrier reach the optical path. The assumed position of the mark behind the moving direction is corrected based on the measurement result and the assumed position of the corrected mark is corrected to reach the optical path. An image forming apparatus that turns on the light projecting unit a second time shorter than the time. An image forming unit that forms a plurality of marks on the carrier at intervals, and
A light projecting unit that irradiates the carrier with light, and a first light receiving unit that receives diffuse reflected light of the light emitted from the light projecting unit to the carrier and outputs a light reception signal corresponding to the amount of light received A mark sensor having
A moving unit that moves the carrier relative to the mark sensor;
A temperature sensor for detecting a temperature around the mark sensor;
The light projecting unit is turned on when the portion where the mark is formed on the carrier crosses the optical path of the light emitted from the light projecting unit, and the portion between the marks on the carrier is configured to pass the optical path. A lighting control unit that temporarily turns off the light projecting unit when crossing,
A correction unit that measures the position of the mark based on the light reception signal, and corrects the deviation of the image forming position based on the measurement result,
The image forming unit increases the interval between the marks as the temperature detected by the temperature sensor increases.
The lighting control unit increases the time during which the light projecting unit is turned off between the marks, as the interval between the marks is larger. The image forming apparatus according to any one of claims 1 to 3 ,
The image forming unit overlaps the two black marks by overlapping a first mark of a color other than black and a mark of a color other than black with an interval in the moving direction of the carrier. Forming a second mark formed between the marks,
The correction unit obtains a relative image forming position shift between the color other than black and black based on the measurement result of the position of the first mark and the measurement result of the position of the second mark, and An image forming apparatus that corrects misalignment. The image forming apparatus according to claim 4 .
The lighting control unit turns on the light projecting unit before the second mark reaches the optical path after the mark on the front side in the moving direction of the carrier among the two black marks reaches the optical path. Image forming apparatus. The image forming apparatus according to claim 4 or 5 , wherein:
The lighting control unit turns off the light projecting unit before the mark on the rear side in the moving direction of the carrier reaches the optical path before passing through the optical path.

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