JP7653765B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus .

Among image forming devices, there is one that transports a recording medium on an endless belt and forms a color image on the recording medium using multiple color materials while it is being transported. One type of image forming device known is an inkjet recording device that has multiple print units and ejects ink droplets of different colors from each print unit, causing the ink droplets of each color to land at predetermined positions on the recording medium, forming a color image.

In the inkjet recording device described above, an endless belt is supported by two belt support rollers, one of which acts as a drive roller and the other as a driven roller, to move the belt in a circular motion. When the belt meanders, the landing positions of the ink droplets of each color on the recording medium shift from their original positions. This causes the color of the color image formed on the recording medium to shift from the desired color, resulting in so-called color shift. Belt meandering is a phenomenon in which the belt is displaced in a direction perpendicular to the direction of belt movement (hereinafter also referred to as the "belt width direction").

Patent document 1 describes an invention relating to an inkjet recording device that has a first nozzle and a second nozzle arranged at a predetermined interval in the transport direction of the recording medium, detects the amount of meandering of an endless belt using two detection units, and controls the ejection of ink from the second nozzle according to the detected amount of meandering of the belt.

JP 2016-137635 A

In the inkjet recording device described above, when the belt is rotated by the belt support roller, eccentricity occurs, which is a misalignment between the center of rotation of the belt support roller and the central axis of the roller. When the belt support roller becomes eccentric, the belt expands and contracts slightly, which makes it impossible to accurately detect the amount of belt meandering (hereinafter also referred to as "belt meandering amount").

The invention described in Patent Document 1 employs a configuration in which the distance between the two inkjet heads, each of which has a nozzle, and the distance between the two detection units described above are set to the same distance. However, this configuration does not reduce the detection error of the amount of belt meandering caused by the eccentricity of the belt support roller.

The present invention has been made to solve the above-mentioned problems, and its object is to provide an image forming apparatus that can reduce detection errors in the amount of belt meandering caused by eccentricity of the belt support roller.

The image forming apparatus according to the present invention includes an endless belt for transporting a recording medium, a belt support roller for supporting the belt and moving the belt in a circular motion, a plurality of displacement sensors for detecting the amount of displacement of the belt in a direction perpendicular to the direction of the belt movement, an image forming section for forming an image on the recording medium transported by the belt, a meander amount detection section for determining the amount of meandering of the belt based on the detection results of the amount of displacement of the belt by the plurality of displacement sensors, and a print control section for correcting the print position of the image in the width direction of the belt based on the amount of meandering of the belt determined by the meander amount detection section, the plurality of displacement sensors being spaced apart in the direction of the belt movement and spaced apart at intervals corresponding to an integer multiple of the circumference of the belt support roller. Furthermore, the image forming apparatus according to the present invention includes a plurality of displacement sensors arranged within a thickness of the corresponding image forming section in the direction of transport of the recording medium, and the print control section corrects the print position of the image based on information on the amount of meandering of the belt one revolution before detected by the meander amount detection section.

The present invention can reduce detection errors in the amount of belt meandering caused by eccentricity of the belt support roller.

1 is a schematic diagram of an image forming apparatus 10 according to an embodiment of the present invention. 1A and 1B are a plan view and a side view of a belt portion of an image forming apparatus according to an embodiment of the present invention. 1A and 1B are diagrams showing an example of a displacement sensor according to an embodiment of the present invention, where FIG. 1A is a schematic perspective view and FIG. FIG. 4 is a diagram showing a nozzle surface of a carriage of a print unit. 2 is a block diagram showing an example of the configuration of a control system of the image forming apparatus according to the embodiment of the present invention. FIG. 6A and 6B are diagrams illustrating an example of measurement data measured by a displacement amount measuring unit. FIG. 13 is a diagram showing an example of the amount of belt meandering at a PU1s position relative to a PU4s position. 13 is a diagram showing the positional relationship between the origin detection sensor and displacement sensors PU1s and PU4s, and addresses assigned to each section into which the circumference of the belt is divided. FIG. 11A to 11C are diagrams showing a time series of a process for measuring the amount of meandering. FIG. 10 is a diagram showing the continuation of FIG. FIG. 11 is a diagram showing the continuation of FIG. 10 is a flowchart showing a processing procedure when the image forming apparatus executes printing while calculating the amount of meandering and correcting the print position. FIG. 11 is a plan view showing an example in which displacement sensors are arranged at both ends of a conveyor belt. 11A and 11B are diagrams illustrating how a conveyor belt expands and contracts in a width direction. FIG. 2 is a plan view showing an arrangement of displacement sensors in the image forming apparatus according to the embodiment of the present invention. 13A and 13B are diagrams illustrating an example of the amount of belt meandering obtained when the interval between the displacement sensors is not set to an integral multiple of the roller circumference. 13A and 13B are diagrams illustrating an example of the amount of belt meandering obtained when the interval between the displacement sensors is set to an integer multiple of the roller circumference. 1A and 1B are diagrams showing a first modified example of a displacement sensor, in which A is a schematic perspective view and B is a schematic plan view. 13A and 13B are diagrams showing a second modified example of the displacement sensor, in which A is a schematic perspective view and B is a schematic plan view. FIG. 13 is a schematic perspective view showing another aspect of the second modified example of the displacement sensor. 13A and 13B are diagrams showing a third modified example of the displacement sensor, in which A is a schematic perspective view and B is a schematic plan view.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In this specification and the drawings, components having substantially the same functions or configurations are denoted by the same reference numerals, and duplicated descriptions will be omitted.
In this embodiment, the description will be given in the following order.
1. Schematic configuration of image forming apparatus 2. Control configuration of image forming apparatus 3. Calculation process of meander amount 4. Printing process procedure 5. Interpolation process 6. Influence of eccentricity of belt support roller 7. Displacement sensor arrangement 8. Reason for suppressing the influence of eccentricity of belt support roller 9. Modification of displacement sensor 10. Other embodiment of the present invention

1. General configuration of image forming apparatus
Fig. 1 is a schematic diagram of an image forming apparatus 10 according to an embodiment of the present invention, and Fig. 2 is a plan view and a side view of a belt portion of the image forming apparatus according to the embodiment of the present invention.
1 and 2, the image forming apparatus 10 is an inkjet recording apparatus that forms an image by ejecting ink droplets onto a recording medium 11. The image forming apparatus 10 includes a belt driving device 12 and an image forming section 14.

(Belt drive device)
The belt driving device 12 includes an endless belt 20, and a driving roller 21 and a driven roller 22 that support the belt 20. The belt 20 is looped between the driving roller 21 and the driven roller 22. The belt 20 transports the recording medium 11 in the Y direction. That is, the transport direction of the recording medium 11 is the Y direction. Since the recording medium 11 is transported in accordance with the movement of the belt 20, the moving direction of the belt 20 and the transport direction of the recording medium 11 are substantially the same direction. In the following description, the Y direction is also referred to as the transport direction Y. The transport distance of the recording medium 11 by the belt 20 is also simply referred to as the transport distance of the belt 20. The transport distance of the belt 20 is substantially the same as the moving distance of the belt 20.

An origin mark 17 is provided on the belt 20. The origin mark 17 is a mark that indicates the origin of one revolution of the belt. The origin mark 17 is provided near one end 20a of the belt 20 in the belt width direction. The origin mark 17 is also configured as a small diameter hole that penetrates the belt 20 in the thickness direction. The origin mark 17 is not limited to a hole. An origin detection sensor 18 is also provided downstream in the transport direction Y of the recording medium 11. The origin detection sensor 18 is a sensor for detecting the origin mark 17. The origin detection sensor 18 is disposed opposite the position where the origin mark 17 passes by as the belt 20 moves around. The origin detection sensor 18 is configured, for example, using a reflective optical sensor. The origin detection sensor 18 is disposed downstream of the print unit PU6 that is located furthest downstream in the transport direction Y among the six print units PU.

The driving roller 21 and the driven roller 22 are provided as an example of a belt support roller. The driving roller 21 and the driven roller 22 are arranged parallel to each other. The driving roller 21 is a roller that is rotated in the A direction by the driving of a conveying motor (not shown). The conveying motor is a driving source for conveying the recording medium 11 by the belt driving device 12. The belt 20 moves in a circle according to the rotation of the driving roller 21. The belt 20 moves in the y1 direction on the outward path from the driven roller 22 to the driving roller 21, and moves in the y2 direction on the return path from the driving roller 21 to the driven roller 22. The driven roller 22 is a roller that rotates according to the movement of the belt 20 accompanying the rotation of the driving roller 21. The rotation direction of the driven roller 22 is the same as the rotation direction A of the driving roller 21.

A rotary encoder 25 is attached to the shaft of the drive roller 21. The rotary encoder 25 includes a disk attached concentrically to the drive roller 21 and a detection unit fixedly installed near the outer periphery of the disk, although this is not shown. The disk rotates together with the drive roller 21. A large number of slits are formed on the outer periphery of the disk at equal intervals in the circumferential direction. The detection unit detects the passage of the slits of the rotating disk. The detection unit also outputs a pulse signal (A-phase signal and B-phase signal) each time it detects the passage of a slit. The rotary encoder 25 outputs, for example, 4096 A-phase pulse signals during one rotation of the drive roller 21. Therefore, by counting the number of A-phase pulse signals output by the detection unit, the movement amount of the belt 20 and the conveying distance of the recording medium 11 corresponding to the movement amount can be accurately and in real time. In this embodiment, the rotary encoder 25 is used to grasp the movement amount of the belt 20, but other devices, such as a laser Doppler measurement device, may also be used.

As shown in FIG. 2, the belt drive device 12 is provided with a first belt guide 15 and a second belt guide 16. The first belt guide 15 and the second belt guide 16 are guides for preventing the belt 20 from deviating from the drive roller 21 and the driven roller 22. The first belt guide 15 is disposed near the drive roller 21, and the second belt guide 16 is disposed near the driven roller 22. The first belt guide 15 is disposed upstream of the drive roller 21 in the moving direction y1 of the belt 20, and the second belt guide 16 is disposed upstream of the driven roller 22 in the moving direction y2 of the belt 20. The first belt guide 15 and the second belt guide 16 are, for example, made of a metal plate or the like, and are pressed against both ends of the belt 20 in the width direction from the side. The first belt guide 15 contacts both ends of the belt 20 on the outward path from the driven roller 22 to the drive roller 21, thereby preventing the belt 20 from deviating from the drive roller 21. The second belt guide 16 contacts both ends of the belt 20 on the return path from the drive roller 21 to the driven roller 22, thereby preventing the belt 20 from deviating from the driven roller 22. Note that the shape of the end of the belt 20 changes due to contact with the first belt guide 15 and the second belt guide 16, but does not change in the section from the second belt guide 16 to the first belt guide 15, i.e., the section passing under multiple print units PU.

(Image forming section)
The image forming section 14 includes a plurality of print units PU (PU1 to PU6). In the present embodiment, as an example, the image forming section 14 includes six print units PU1 to PU6. The print units PU1 to PU6 eject ink of different colors onto the recording medium 11 transported by the belt 20, thereby forming an image on the recording surface of the recording medium 11. Each of the print units PU1 to PU6 includes a carriage 31 (see FIG. 4). The six print units PU1 to PU6 are arranged between the drive roller 21 and the driven roller 22 along the longitudinal direction of the belt 20. The six print units PU1 to PU6 are arranged in order from the upstream side to the downstream side in the transport direction Y of the recording medium 11.

In FIG. 1, the image forming section 14 is configured with six print units PU (PU1 to PU6), but as shown in FIG. 2, the image forming device 10 can be equipped with up to eight print units PU. The number of print units PU can be any number and is not limited to the above example.

The recording medium 11 on which the image is formed may be, for example, a cloth or paper wound in a roll or folded in a Z-shape. The term "image formation" can be replaced with "printing". The recording medium 11 is conveyed in the conveying direction Y by the belt 20. The recording medium 11 is fed from a feed section (not shown) and supplied to the upstream end of the belt 20. A pressure roller 13a is provided at the upstream end of the belt 20. The pressure roller 13a is a roller that presses the recording medium 11 against the surface of the belt 20. The recording medium 11 is pressed against the belt surface of the belt 20 by the pressure roller 13a, so that it adheres to the belt surface, and is conveyed by moving in the Y direction together with the belt 20 in this state. The recording medium 11 is separated from the belt 20 at a position past the first belt guide 15 and taken up by a take-up section (not shown).

The belt drive device 12 included in the image forming device 10 is a large device in which the circumference of the belt 20 is approximately 28 m. At the installation site of the image forming device 10, the belt 20 is first wound around the drive roller 21 and driven roller 22 to adjust the length of the belt 20. Next, the longitudinal ends of the belt 20 are welded together to form an endless belt. After that, both ends of the belt 20 in the width direction are cut so that the width dimension of the belt 20 has a predetermined dimension. As a result of this on-site cutting, the width direction ends of the belt 20 are rough rather than smooth.

(Displacement Sensor)
The image forming apparatus 10 includes a plurality of displacement sensors 23. Each of the displacement sensors 23 detects the amount of displacement of the belt 20 in a direction perpendicular to the moving direction y1 of the belt 20. The plurality of displacement sensors 23 are disposed at different positions in the moving direction y1 of the belt 20. The plurality of displacement sensors 23 are disposed at intervals in the moving direction y1 of the belt 20. The number of the plurality of displacement sensors 23 is the same as the number of print units PU. One displacement sensor 23 is disposed at each of a plurality of measuring points set in the moving direction y1 of the belt 20. After the image forming apparatus 10 is installed, the positions of the respective measuring points and the distances between the measuring points are measured and input to the image forming apparatus 10, so that the positions and distances become known information in the image forming apparatus 10.

The displacement sensors 23 are arranged at positions corresponding to the print units PU1 to PU6, one at a time. Specifically, the displacement sensors 23 are installed at positions along the end 20a of the belt 20 and at positions that are the center of the corresponding print unit PU in the transport direction Y. Therefore, the displacement sensors 23 and the print units PU are arranged at the same intervals from each other in the transport direction Y. In the following description, the displacement sensor 23 arranged at a position corresponding to the print unit PU1 is referred to as the displacement sensor PU1s by adding the letter "s" to the end of the print unit code PU1, and similarly, the displacement sensor 23 arranged at a position corresponding to the print unit PU2 is referred to as the displacement sensor PU2s. The displacement sensor 23 arranged at a position corresponding to the print unit PU3 is referred to as the displacement sensor PU3s, and the displacement sensor 23 arranged at a position corresponding to the print unit PU4 is referred to as the displacement sensor PU4s. Additionally, the displacement sensor 23 arranged at a position corresponding to print unit PU5 is referred to as displacement sensor PU5s, and the displacement sensor 23 arranged at a position corresponding to print unit PU6 is referred to as displacement sensor PU6s.

The print units PU (PU1 to PU6) are arranged at equal intervals in the transport direction Y, and the displacement sensors 23 (PU1s to PU6s) are also arranged at equal intervals in the transport direction Y. Of the multiple measurement points described above, the measurement point corresponding to any one of the print units PU is set as the reference point. In this embodiment, as an example, the measurement point corresponding to print unit PU4 is set as the reference point.

FIG. 3A is a diagram illustrating an example of a displacement sensor according to an embodiment of the present invention.
In this embodiment, a non-contact type displacement sensor, more specifically, a transmission type laser displacement sensor is used as the displacement sensor 23. The displacement sensor 23 is attached on a side frame 27. The side frame 27 is disposed outside the end 20a of the belt 20 in the belt width direction, and is disposed along the end 20a of the belt 20.

The displacement sensor 23, which is a transmissive laser displacement sensor, includes a light emitting section 23a that emits laser light and a light receiving section 23b that receives the laser light. The light emitting section 23a emits a belt-shaped laser light of a predetermined width that is collimated by a lens (not shown). The light receiving section 23b receives the laser light emitted by the light emitting section 23a on the light receiving surface of a line type CCD sensor (not shown). The light receiving section 23b is disposed facing the light emitting section 23a at a predetermined distance from the light emitting section 23a. The displacement sensor 23 is installed on the optical path between the light emitting section 23a and the light receiving section 23b so that the belt 20 blocks part of the belt-shaped laser light traveling from the light emitting section 23a to the light receiving section 23b. The displacement sensor 23 detects the position of the shadow generated by the belt 20 blocking part of the laser light based on the light receiving result of the light receiving section 23b, thereby detecting the position of the end 20a of the belt 20 in the belt width direction. The sensing position of the displacement sensor 23 is position P1 on the laser axis that is set by the light-emitting portion 23a and the light-receiving portion 23b of the displacement sensor 23, as shown in the plan view of FIG. 3B.

FIG. 4 is a front view showing the carriage of the print unit.
4, the carriage 31 is a frame member for holding the multiple recording heads 32 in the correct positional relationship, and is made of a highly rigid metal plate or the like. The carriage 31 is fitted with multiple recording heads 32 each covering a printing range for one line in the main scanning direction X. The multiple recording heads 32 are arranged in a staggered pattern in the main scanning direction X. The main scanning direction X is a direction perpendicular to the transport direction Y, i.e., a direction parallel to the belt width direction. Each recording head 32 ejects ink droplets from an ink ejection port 33 in response to an input drive signal. The print unit PU is a unit that incorporates the carriage 31 to which the multiple recording heads 32 are attached, and a head drive circuit 43, which will be described later.

When the displacement sensor 23 is disposed in correspondence with the print unit PU as described above, the position at which the displacement sensor 23 is disposed should be within the thickness of the print unit PU in the transport direction Y, preferably at the center of the thickness of the print unit PU. As shown in FIG. 4, when multiple nozzle rows are present on one carriage 31, it is desirable to set the sensing position P1 (see FIG. 3B) of the displacement sensor 23 to the center position Pc of the multiple nozzle rows. Note that, by performing an interpolation process described later, the meandering amount of the belt 20 at the ideal center position Pc can be estimated even if the displacement sensor 23 is disposed at a position shifted from the ideal center position Pc. In the following description, the meandering amount of the belt 20 is also referred to as the "belt meandering amount."

2. Control configuration of image forming apparatus
FIG. 5 is a block diagram showing an example of the configuration of a control system of the image forming apparatus according to the embodiment of the present invention.
5, the image forming apparatus 10 includes a transport control unit 41, a print data generating unit 42, a head driving circuit 43, a meandering amount detecting unit 50, etc. The head driving circuit 43 is a circuit provided in the print unit PU. Of these, elements other than those related to the printing of images can be provided in the belt driving device 12. The elements related to the printing of images are the print data generating unit 42 and the print unit PU.

The transport control unit 41 controls the transport of the recording medium 11 by controlling the drive of the transport motor that rotates the drive roller 21. The print data generation unit 42 generates print data corresponding to the image to be printed by executing RIP (Raster Image Processor) processing and the like based on the print job received from an external device. The print data generation unit 42 outputs the generated print data to the head drive circuit 43 in the print unit PU. The head drive circuit 43 generates a drive signal based on the print data input from the print data generation unit 42, and drives each recording head 32 according to this drive signal.

The meandering amount detection unit 50 determines the meandering amount of the belt 20 based on the detection results of the displacement amount of the belt 20 by the multiple displacement sensors 23. More specifically, the meandering amount detection unit 50 determines the meandering amount of the belt 20 based on the difference between the displacement amount of the belt 20 detected by the displacement sensor 23 arranged at one measurement point and the displacement amount of the belt 20 detected by the displacement sensor 23 arranged at the other measurement point at the same location as the one measurement point. Note that the term "same location" in this specification is not limited to the same location in the strict sense, but also means the same location in the case where various error factors are included, such as dimensional errors of the belt 20, meandering of the belt 20, dimensional changes due to expansion and contraction of the belt 20, dimensional errors and dimensional changes of each part of the belt drive device 12, and even the detection accuracy of the rotary encoder 25. The meandering amount detection unit 50 performs the function of controlling the measurement for detecting the meandering amount of the belt 20 and executing arithmetic processing using the measurement results. The meandering amount detection unit 50 also functions to output meandering information regarding the detected meandering amount of the belt 20 to the head drive circuit 43 of each print unit PU.

The meandering amount detection unit 50 includes a CPU (Central Processing Unit) 51, a ROM (Read Only Memory) 52, a RAM (Random Access Memory) 53, a meandering information output unit 54, an I/O input unit 55, an encoder input unit 56, and an ADC (Analog-to-digital converter) 57, and these elements are interconnected via a bus.

The detection signal of the origin detection sensor 18 is input to the I/O input unit 55. The output signal of the rotary encoder 25 is input to the encoder input unit 56. The output of each displacement sensor 23 is input to the ADC 57.

The CPU 51 executes the program stored in the ROM 52, and the RAM 53 temporarily stores various data when the CPU 51 executes the program. By executing the program stored in the ROM 52, the CPU 51 functions as a movement amount measuring unit 61, a displacement amount measuring unit 62, and a meander amount calculating unit 63. The program executed by the CPU 51 can be provided by being recorded on a computer-readable recording medium (such as a CD-ROM), and this program or the recording medium can also be extracted as an invention. In addition to being provided by being recorded on a recording medium, the program can also be provided via a wired or wireless network.

The movement amount measuring unit 61 continuously measures the movement amount of the belt 20 in the y1 direction and the y2 direction shown in FIG. 1 and FIG. 2. The movement amount measuring unit 61 counts the pulse signal output by the rotary encoder 25 based on the time when the origin detection sensor 18 detects the origin mark 17. The movement amount measuring unit 36 then measures the movement amount of the belt 20 and the transport distance of the recording medium 11 based on the movement amount in real time based on the time when the origin mark 17 reaches the detection position of the origin detection sensor 18. With regard to the pulse signal output by the rotary encoder 25, the movement amount of the belt 20 per pulse is a constant amount and is known due to the configuration of the image forming apparatus 10. Therefore, the movement amount measuring unit 61 counts the pulse signal output by the rotary encoder 25, and can accurately measure the movement amount of the belt 20 and the transport distance of the recording medium 11 from the count value in real time.

The displacement amount measuring unit 62 measures the amount of displacement of the belt 20 in the belt width direction with the displacement sensor 23 at one measurement point. The displacement amount measuring unit 62 also measures the amount of displacement at the same location where the displacement amount was measured by the displacement sensor 23 at one measurement point with the displacement sensor 23 at another measurement point, based on the amount of movement of the belt 20 measured by the movement amount measuring unit 61 and the known distance between the measurement points. The displacement amount measuring unit 62 also records the measurement data obtained by the above measurements in the RAM 53.

For example, each time the rotary encoder 25 outputs a pulse signal, the displacement amount measuring unit 62 measures the displacement amount of the belt 20 at the time the pulse signal is output using each of the displacement sensors 23. Furthermore, the displacement amount measuring unit 62 records in the RAM 53 the displacement amount of the belt 20 measured by each of the displacement sensors 23 at the time the pulse signal is output and the movement amount of the belt 20 measured by the movement amount measuring unit 61 at the time of output in association with each other.

In this embodiment, the distance between the measurement points of adjacent displacement sensors 23 in the moving direction y1 of the belt 20 is set to be an integer multiple of the conveying distance in the measurement period of the displacement amount of the belt 20 (in this embodiment, the conveying distance per pulse of the rotary encoder 25). As a result, the same location of the belt 20 in the conveying direction Y is measured by the displacement sensors 23 at each measurement point.

The measurement period for the displacement amount described above may be any value as long as an integer multiple of the transport distance in the measurement period is the distance between the measurement points. For example, the measurement period may be the transport distance corresponding to the pixel pitch in the transport direction.

The meandering amount calculation unit 63 calculates the amount of meandering of the belt 20 at one measurement point relative to another measurement point by taking the difference between the amount of displacement at the same location measured and recorded by the displacement amount measurement unit 62 at one measurement point and another measurement point.

Figure 6 is a diagram showing an example of measurement data measured by the displacement amount measurement unit 62. Graph A on the upper side of Figure 6 is a graph of measurement data recorded by linking the displacement amount of the belt 20 measured by the displacement sensor 23 (PU1s) arranged at a position corresponding to the print unit PU1 with the transport distance of the belt 20 after the origin detection sensor 18 detects the origin mark 17. On the other hand, graph B on the lower side of Figure 6 is a graph corresponding to measurement data recorded by linking the displacement amount of the belt 20 measured by the displacement sensor 23 (PU4s) arranged at a position corresponding to the print unit PU4 with the transport distance of the belt 20 after the origin detection sensor 18 detects the origin mark 17. In graphs A and B, the vertical axis indicates the displacement amount of the belt 20, and the horizontal axis indicates the transport distance of the belt 20 after the origin mark 17 is detected.

For example, as shown in FIG. 2, if the reference point Pr is the position of PU4s and the measurement point Ps is the position of PU1s, then the measurement point Ps is located at a distance La (m) upstream in the transport direction Y (movement direction y1) from the reference point Pr. In this case, the measurement data measured by PU4s is offset by La (m) (graph B is offset overall to the left by La (m)) and the measurement data of PU1s is subtracted from this offset measurement data of PU4s to determine the amount of meandering of the belt 20 at the position of PU1s relative to the position of PU4s. In this embodiment, the position of PU4s in the transport direction Y is the position of print unit PU4, and the position of PU1s is the position of print unit PU1.

The output signal of the displacement sensor 23 contains a DC component. Therefore, by removing the DC component from the output signal of the displacement sensor 23, it is possible to extract the component of the displacement amount of the belt 20 regardless of the absolute position of the displacement sensor 23. Therefore, it is not necessary to accurately install the relative mounting positions of the multiple displacement sensors 23 on a straight line.

FIG. 7 is a diagram showing an example of the amount of belt meandering at the PU1s position relative to the PU4s position.
7 shows graph C, which is the difference between graph B, which is offset to the left by La(m), and graph A. Graph C shows the relationship between the transport distance of belt 20 from the point in time when origin mark 17 is detected by origin detection sensor 18 and the amount of meandering of belt 20 at the position of displacement sensor PU1s relative to the position of displacement sensor PU4s.

The meander amount detection unit 50 applies a low-pass filter to the difference value data obtained by subtracting the above-mentioned graph B from graph A in order to remove spike-like noise caused by fuzz on the end of the belt 20 or vertical shaking or vibration of the belt 20. The data from which noise has been removed by the low-pass filter is used as meander information of the belt 20, and this meander information is output to the head drive circuit 43 of the corresponding print unit PU. Here, the meander amount is rounded to about the pixel pitch in the main scanning direction by applying a low-pass filter to the above-mentioned difference value data. The reason for rounding to about the pixel pitch is that the print position correction by the print control unit 43a described later cannot be finer than the pixel pitch in the main scanning direction.

Returning to FIG. 5, the head drive circuit 43 of each print unit PU has a print control unit 43a. Based on the meandering information output from the meandering information output unit 54 of the meandering amount detection unit 50, the print control unit 43a changes (corrects) the print position of the image in the width direction (main scanning direction) of the belt 20 so that the meandering amount (meandering amount relative to a reference point) at the position of that print unit PU is offset. For example, if the position of print unit PU4 is the reference point and the meandering amount at the position of print unit PU1 relative to the reference point at a predetermined point of the belt 20 is a distance of three pixels to the left end side of the belt 20, the image of the line to be printed when the predetermined point of the belt 20 arrives at the position of print unit PU1 is shifted three pixels to the left end side of the belt 20 and printed.

The amount of meandering of the belt 20 repeats approximately the same change with one revolution of the belt 20 being one cycle. The amount of meandering of the belt 20 also changes gradually due to temperature fluctuations and the end 20a of the belt 20 coming into contact with the first belt guide 15 and the second belt guide 16, but since this change occurs slowly, there is almost no change compared to the previous revolution.

On the other hand, it is difficult to measure the amount of displacement of the belt 20 in the width direction using a displacement sensor 23 installed at the position of the print unit PU, calculate the amount of meandering, and shift the printing position based on this in real time.

Therefore, in this embodiment, the meander amount detection unit 50 outputs the meander information obtained by the measurement of one revolution before to the print control unit 43a of the head drive circuit 43, and the print control unit 43a corrects the print position based on the meander information of one revolution before. By using the meander information of one revolution before in this way, it is possible to calculate the meander amount and control the shifting of the print position with ample time.

In addition, by using the meandering information from one revolution ago, the reference point can be set to any position, such as the center position in the transport direction Y, such as the position corresponding to print unit PU4. For example, even if the meandering of belt 20 occurs monotonically to one side from the upstream side to the downstream side in the transport direction Y, correction of the print position can be distributed to both the positive and negative directions by setting the center position in the transport direction Y, such as the position of print unit PU4, as the reference point. Therefore, compared to setting the reference point at the position of the most upstream print unit PU or the most downstream print unit PU, correction can be made to offset the amount of meandering with a small amount of correction regardless of which end side belt 20 meanders.

3. Calculation of the amount of meandering
Next, the meander amount calculation process performed by the meander amount calculation unit 63 will be specifically described.
In this embodiment, as shown in Fig. 2, the position of the displacement sensor PU4s is set as a reference point Pr, the position of the displacement sensor PU1s is set as a measurement point Ps, and the amount of meandering of the belt 20 at the position of the displacement sensor PU1s relative to the position of the displacement sensor PU4s is calculated. The displacement sensor PU1s is located at the position of the print unit PU1, and the displacement sensor PU4s is located at the position of the print unit PU4.

In this embodiment, as an example, the circumference of the belt 20 is divided into 14 sections, and the displacement sensors 23 (PU1s, PU4s) and the origin detection sensor 18 are arranged in the positional relationship shown in FIG. 8. In FIG. 8, the starting positions of each of the 14 sections of the belt 20, which are divided starting from the position of the origin mark 17, are numbered from 0 to 13. Meanwhile, FIG. 9 to FIG. 11 show the measurement status in chronological order.

First, the timing when the origin detection sensor 18 detects the origin mark 17 is set as the reference time for starting measurement by each displacement sensor 23 (PU1s, PU4s) and measurement is started (Q1 in FIG. 9). Each graph in FIG. 9 to FIG. 11 shows the amount of displacement of the belt end measured by the displacement sensor 23 (PU1s), the amount of displacement of the belt end measured by the displacement sensor 23 (PU4s), and the amount of belt meandering at the PU1s position relative to the PU4s position. The vertical axis is the amount of displacement of the belt end (amount of belt meandering), and the horizontal axis is the position on the belt 20, with the position of the origin mark 17 being 0. Each value on the horizontal axis indicates the address number assigned when the circumference of the belt 20 is divided into 14 parts.

As mentioned above, the positional relationship between the origin detection sensor 18 and each displacement sensor 23 (PU1s, PU4s) is known. Therefore, based on the timing at which the origin detection sensor 18 detects the origin mark 17, it is possible to know which position (address) on the belt 20 the displacement sensor PU1s or the displacement sensor PU4s is measuring. For example, at the start of measurement shown in Q1 of FIG. 9, the displacement sensor PU1s is located at address 5 on the belt 20, and the displacement sensor PU4s is located at address 3 on the belt 20.

Q2 in FIG. 9 shows the measurement status when the belt 20 has been moved one section from the start of measurement. The displacement sensor PU1s measures the amount of displacement of the belt end of the belt 20 from address 5 to address 6, and the displacement sensor PU4s measures the amount of displacement of the belt end of the belt 20 from address 3 to address 4. Q3 in FIG. 9 shows the measurement status when the belt 20 has been moved two sections from the start of measurement. The displacement sensor PU1s measures the amount of displacement of the belt end of the belt 20 from address 5 to address 7, and the displacement sensor PU4s measures the amount of displacement of the belt end of the belt 20 from address 3 to address 5.

After this, the measurement data from each displacement sensor PU1s, PU4s for the same location on the belt 20 (the portion from address 5 onwards) is collected. Therefore, the meandering amount calculation unit 63 starts a calculation to find the meandering amount by taking the difference between the displacement amounts of the belt ends at the same location. Q4 in Figure 10 shows the measurement situation and the meandering amount calculated at the timing when the belt 20 has been moved three sections from the start of measurement. At the timing when the belt 20 has been moved three sections from the start of measurement, the measurement data for the same location is collected for the range from address 5 to 6 on the belt 20. Therefore, the meandering amount calculation unit 63 calculates the meandering amount by taking the difference between the measurement data from the displacement sensor PU1s and the measurement data from the displacement sensor PU4s.

Q5 in FIG. 10 shows the measurement status and the calculated meander amount at the timing when the belt 20 has been moved 13 sections from the start of measurement. Q6 in FIG. 10 shows the measurement status and the calculated meander amount at the timing when the belt 20 has been moved 14 sections from the start of measurement, that is, when the belt 20 has made exactly one revolution from the start of measurement. When the meander amount detection unit 50 recognizes that the origin mark 17 has been detected again by the origin detection sensor 18 and that the belt 20 has made one revolution, it starts feedback (output) of meander information to the head drive circuit 43. That is, since meander amount data at the PU1s position relative to the PU4s position exists from the position of address 5, the meander amount detection unit 50 starts outputting meander information to the head drive circuit 43 of the print unit PU1 in accordance with the timing when the position of address 5 of the belt 20 reaches the position of the print unit PU1, or slightly before that, taking into account the processing time in the head drive circuit 43.

Based on the meandering information received from the meandering amount detection unit 50, the head drive circuit 43 of the print unit PU1 shifts the image printing position in the main scanning direction so as to offset the meandering amount of the belt 20, and outputs a drive signal to each recording head 32 in accordance with the shifted printing position.

After that, as shown in Q7 and Q8 in Figure 11, the displacement and meandering amounts of the belt ends are rewritten and updated with the latest data, and the same process is repeated from the second revolution of the belt onwards.

In addition, in Q7 of FIG. 11, in the calculation to obtain the amount of meandering by taking the difference in the amount of displacement of the belt end of belt 20 from address 3 to address 5, the measurement data of displacement sensor PU4s is data from the second revolution, while the measurement data of displacement sensor PU1s is data measured in the previous revolution, i.e., one revolution ago. Between second belt guide 16 and first belt guide 15, the end of belt 20 does not contact belt guides 15, 16. Therefore, if the data is from the same revolution, it is considered that the shape of the belt end will not change, but if the revolution is different, the shape of the belt end may have changed due to contact with second belt guide 16, etc. For this reason, when taking the difference between the measurement data of one displacement sensor and the measurement data of another displacement sensor, it is desirable that those measurement data are from the same revolution.

Therefore, when the measurement data from which the difference is to be taken is the measurement data from the same revolution, for example, data for two revolutions is held (stored) and successively updated to the latest data, and the amount of meandering is calculated by taking the difference between the measurement data from the same revolution. Also, before printing starts, the belt 20 is moved around two or more revolutions so that the amount of meandering can be calculated from the measurement data from the same revolution at all belt positions, and then printing can be started.

4. Printing process procedure
12 is a flowchart showing a processing procedure when the image forming apparatus 10 executes printing while obtaining the amount of meandering and correcting the print position. In this embodiment, for simplicity, as in FIGS. 9 to 11, a case will be described in which the amount of meandering of the belt 20 at the position of the displacement sensor PU1s relative to the position of the displacement sensor PU4s is obtained and the print position of the image is corrected. Also, it is assumed that the displacement sensor PU1s is located at the position of the print unit PU1, and the displacement sensor PU4s is located at the position of the print unit PU4. Note that in reality, the same processing is performed for all print units PU1 to PU6 corresponding to the multiple displacement sensors 23 (PU1s to PU6s). However, if the position of the displacement sensor PU4s is used as the reference point, the same processing may be performed for the print units PU1 to PU3, PU5, and PU6 corresponding to the displacement sensors other than the displacement sensor PU4s.

The counter that counts the number of origin detections is reset to 0 in the initial state. First, the belt drive device 12 drives the belt 20 to move in a circular motion, and the transport of the recording medium 11 and printing on the recording medium 11 are started (step S101). Next, it is determined whether the transport (printing) of the recording medium 11 has been completed (step S102), and if the transport (printing) has not been completed, it is confirmed whether the origin detection sensor 18 has detected the origin mark 17 (step S103). If the transport (printing) has been completed (step S102; Yes), this process is terminated.

If the origin detection sensor 18 detects the origin mark 17 (step S103; Yes), the position on the belt 20 measured by the displacement sensors PU1s and PU4s (the position in the conveying direction Y relative to the origin mark 17) is reset based on the known positional relationship between the origin detection sensor 18 and the displacement sensors PU1s and PU4s (step S104), and then the number of origin detections is incremented by 1 (step S105), and the process proceeds to step S107.

If the origin detection sensor 18 does not detect the origin mark 17 (step S103; No), the position on the belt 20 measured by the displacement sensors PU1s and PU4s (the position in the conveying direction Y relative to the origin mark 17) is updated by adding only the conveying distance since the origin detection sensor 18 detected the origin mark 17 (the distance obtained by counting the output pulses of the rotary encoder 25 from the time the origin mark 17 was detected) (step S106), and then the process proceeds to step S107.

In step S107, the amount of displacement of the belt end detected by the displacement sensor PU1s is stored in association with the position of the displacement sensor PU1s on the belt 20, and the amount of displacement of the belt end detected by the displacement sensor PU4s is stored in association with the position of the displacement sensor PU4s on the belt.

Next, it is checked whether the number of origin detections is equal to or greater than a predetermined value (e.g., 2), and if the number of origin detections is less than the predetermined value (step S108; No), the process returns to step S102 and continues.

If the number of origin detections is equal to or greater than a predetermined value (step S108; Yes), the difference between the measurements (displacement of the belt end) of the displacement sensors PU1s and PU4s from one revolution ago, which corresponds to the current belt position of the displacement sensor PU1s, is taken to calculate the amount of belt meandering at the position of the displacement sensor PU1s relative to the position of the displacement sensor PU4s (step S109). Furthermore, the amount of belt meandering is applied to a low-pass filter (step S110), and the amount of belt meandering thus obtained is output to the head drive circuit 43 of the print unit PU1 for feedback (step S111).

Next, it is confirmed whether printing is being performed (step S112). If printing is being performed (step S112; Yes), the head drive circuit 43 shifts the image data in the main scanning direction (belt width direction) so as to offset the amount of meandering (step S113), and ejects ink from each recording head 32 according to the shifted image data to print an image on the recording medium 11 (step S114). After that, the process returns to step S102 to continue.

If printing is not in progress (step S112; No), the process returns to step S102 and continues without executing steps S113 and S114.

In this way, by measuring the conveying distance of the belt 20 in real time based on the output pulses of the rotary encoder 25, it is possible to know at which position on the belt 20 in the conveying direction Y the displacement sensor 23 at each measurement point is measuring the displacement of the belt end. Therefore, even if the conveying speed of the belt 20 fluctuates, it is hardly affected and the difference between the measured values measured at each measurement point for the same location on the belt 20 can be taken. As a result, regardless of the conveying speed of the belt 20, the amount of meandering of the belt 20 can be accurately determined and the printing position can be corrected to offset the amount of meandering.

<5. Interpolation Processing>
Next, the interpolation process based on the amount of meandering of the belt 20 will be described.
In the design of the image forming apparatus 10, for example, due to mechanical constraints, the displacement sensor 23 may not be installed at the center position (ideal position) of each print unit PU in the transport direction Y. In that case, the displacement sensor 23 is installed at a location different from the ideal position (preferably as close to the ideal position as possible). In such a situation, the meandering amount calculation unit 63 estimates the meandering amount at a position other than the measurement point where the displacement sensor 23 is installed by an interpolation process based on the meandering amounts at multiple measurement points. In other words, the meandering amount at the ideal position is obtained by an interpolation process based on the meandering amounts measured by the multiple displacement sensors 23. Note that, when the displacement sensor 23 is installed at a position shifted from the ideal position, it is desirable to set the deviation amount of each displacement sensor 23 from the ideal position to be the same. This allows the same interpolation process to be performed, making the interpolation process easier overall.

For example, the distance from the reference point to the first measurement point is L1, the distance from the reference point to the second measurement point is L2, the distance from the reference point to a predetermined print unit PU (ideal position) is L3 (where L1<L3<L2), the meandering amount at the first measurement point for the same location on belt 20 is D1, and the meandering amount at the second measurement point is D2. In this case, the meandering amount Dx at that location on belt 20 at the ideal position relative to the reference point can be calculated by the following formula (1).
Dx=D1+(D2-D1)×(L3-L1)/(L2-L1) (1)

The calculation of the above formula (1) is an interpolation, but if extrapolation is used, the meandering amount at a position downstream of the measurement point can be estimated. Therefore, the print position may be corrected based on the estimated meandering amount of the current revolution, without using the meandering amount of the previous revolution. For example, when L1<L2<L3, the meandering amount Dx at the ideal position can be calculated using the following formula (2).
Dx=D2+(D2-D1)×(L3-L1)/(L2-L1) (2)

(Measure the amount of displacement at both the left and right ends of the belt)
In the above description, the amount of meandering is obtained by measuring the amount of displacement of one end of the belt 20 with the displacement sensor 23. However, as shown in FIG. 13, the displacement sensors 23 may be disposed at both ends of the belt 20 in the width direction.

When the circumference of the belt 20 is long, not only does the belt 20 meander, but the belt 20 also expands and contracts in the width direction as shown in FIG. 14. This can cause the ink landing position in the width direction (main scanning direction) of the belt 20 to deviate from the target position. By arranging displacement sensors 23 at both the left and right ends of the belt 20, it becomes possible to detect the change in the expansion and contraction of the belt 20 in the width direction using the pair of left and right displacement sensors 23, and to correct the printing position using the detection results.

For example, assume that the amount of meandering at the position of print unit PU1 relative to the position of print unit PU4 is +0.05 mm at the right end of belt 20 and +0.03 mm at the left end of belt 20. In this case, by detecting each amount of meandering with a pair of displacement sensors 23, the average of these meandering amounts, +0.04, can be regarded as the amount of meandering and the print position can be corrected. This makes it possible to appropriately correct the print position regardless of the expansion and contraction of belt 20 in the width direction.

6. Effect of eccentricity of belt support rollers
Next, the effect of the eccentricity of the belt support roller on the amount of meandering of the belt will be described.
When the drive roller 21, which corresponds to a belt support roller, becomes eccentric, the belt 20 that moves around the drive roller 21 expands and contracts slightly due to the eccentricity of the drive roller 21. As a result, the displacement amount of the belt 20 detected by the displacement sensor 23 includes an error due to the eccentricity of the drive roller 21. This error is superimposed on the displacement amount of the belt 20 as an amplitude component corresponding to the rotation period of the drive roller 21. For this reason, if the meandering amount of the belt 20 is calculated based on the displacement amount of the belt 20 detected by the displacement sensor 23, the meandering amount also includes an error, which may cause the correction of the printing position of the image to be performed inappropriately. Therefore, in order to more appropriately correct the printing position of the image, it is effective to reduce the detection error of the belt meandering amount due to the eccentricity of the drive roller 21.

7. Displacement Sensor Arrangement
Therefore, in this embodiment, in order to reduce detection errors of the belt meandering amount due to eccentricity of the drive roller 21, a configuration is adopted in which a plurality of displacement sensors 23 are arranged at predetermined intervals based on the circumferential length of the drive roller 21. The intervals between the displacement sensors 23 in the moving direction y1 of the belt 20 are defined based on the sensing positions of the displacement sensors 23. As a result, for example, the interval between the displacement sensor PU4s and the displacement sensor pu1s is defined by the distance between the sensing positions of the displacement sensors PU4s and PU1s.

Here, the predetermined interval based on the circumference of the drive roller 21 is preferably an interval that corresponds to an integer multiple of the circumference of the drive roller 21. This point will be explained in more detail using Figure 15.

(Spacing of displacement sensors)
15, if the circumference (m) of the drive roller 21 is "L" and a reference point Pr is the position of the displacement sensor PU4s of the print unit PU4, the distance between the displacement sensor PU4s at this reference point Pr and the displacement sensor PU3s located upstream of it is set to L, which corresponds to one time the circumference L of the drive roller 21. The distance between the displacement sensors PU4s and PU2s is set to 2L, which corresponds to twice the circumference L of the drive roller 21, and the distance between the displacement sensors PU4s and PU1s is set to 3L, which corresponds to three times the circumference L of the drive roller 21.

On the other hand, the distance between the displacement sensors PU4s and PU5s is set to L, which is equivalent to one time the circumferential length L of the drive roller 21, and the distance between the displacement sensors PU4s and PU6s is set to 2L, which is equivalent to twice the circumferential length L of the drive roller 21. When two print units PU7 and PU8 are provided downstream of the print unit PU6, the distance between the displacement sensors PU4s and PU7s is set to 3L, which is equivalent to three times the circumferential length L of the drive roller 21, and the distance between the displacement sensors PU4s and PU8s is set to 4L, which is equivalent to four times the circumferential length L of the drive roller 21. The distance between the adjacent displacement sensors 23 in the moving direction y1 of the belt 20 is set to the same dimension as the circumferential length L of the drive roller 21.

(Roller circumference)
The circumferential length L of the drive roller 21 may be determined based on the formula L = πD, where D (m) is the diameter of the drive roller 21, but more preferably, the circumferential length L of the drive roller 21 is determined taking into account the thickness of the belt 20. Specifically, the circumferential length L of the drive roller 21 is determined based on the formula L = π(D + 2t), where t (m) is the thickness of the belt 20, and a plurality of displacement sensors 23 (PUs1 to PUs8) are arranged at predetermined intervals based on this circumferential length L, that is, at intervals that are an integer multiple of the roller circumferential length L. In this embodiment, six displacement sensors 23 (PUs1 to PUs6) are arranged in the moving direction y1 of the belt 20.

(Installation error)
When manufacturing and installing the belt driving device 12 and the image forming device 10, it is practically impossible to completely eliminate processing errors and installation errors of the components. For this reason, it is difficult to precisely arrange the multiple displacement sensors 23 (PUs1 to PUs6) at a predetermined interval based on the circumferential length L of the driving roller 21. Therefore, the interval between the multiple displacement sensors 23 (PUs1 to PUs6) may include a predetermined error. Specifically, the error in the arrangement interval in the installed state of the multiple displacement sensors 23 (PUs1 to PUs6) with respect to the predetermined interval based on the circumferential length L of the driving roller 21 is preferably within ±5% of the circumferential length L of the driving roller 21, and more preferably within ±3% of the circumferential length L of the driving roller 21.

8. Why the effect of eccentricity of the belt support roller is suppressed
As described above, by arranging the multiple displacement sensors 23 (PUs1 to PUs6) at predetermined intervals based on the circumferential length L of the drive roller 21, it is possible to suppress the influence of eccentricity of the drive roller 21. The reason for this is as follows.

First, the displacement of the belt 20 detected by each displacement sensor 23 (PUs1 to PUs6) includes an error due to the eccentricity of the drive roller 21. This error appears in the detection data of the belt displacement amount as an amplitude component according to the rotation period of the drive roller 21. In addition, when the belt 20 is transported by the rotation of the drive roller 21 by a distance corresponding to the circumferential length L of the drive roller 21, the amplitude component of the error due to the eccentricity of the drive roller 21 also appears at the destination of the transport. Therefore, if the interval between one measurement point and another measurement point is set to an integer multiple of the circumferential length L of the drive roller 21, the error can be canceled by taking the difference between the belt displacement amount at the same location measured at the one measurement point and the other measurement point. Therefore, the detection error of the belt meandering amount can be reduced. This point will be explained more specifically by taking, for example, a case where the reference point is the position of the displacement sensor PU4s and the measurement point is the position of the displacement sensor PU1s.

Figure 16 is a diagram showing an example of the amount of belt meandering obtained when the spacing between the displacement sensors is not set to an integer multiple of the roller circumference. In Figure 16, the vertical axis shows the amount of belt meandering in pixel (px) size, and the horizontal axis shows the belt transport distance (m). The print resolution of the image forming device 10 of this embodiment is 720 dpi, so the size of one pixel is 35 μm.

The belt meandering amount shown in FIG. 16 is the belt meandering amount at the position of the displacement sensor PU1s relative to the position of the displacement sensor PU4s. The data of the belt meandering amount is data for one revolution of the belt obtained by measuring the difference between the belt displacement amount measured by the displacement sensor 23 (PU4s) at the measurement point and the belt displacement amount measured by the displacement sensor 23 (PU4s) at the reference point, with the same point in the circumferential direction of the belt 20 as the measurement target of the displacement sensor 23. The belt meandering amount was obtained under the conditions that the circumference of the belt 20 is about 28 m, the circumference of the drive roller 21 is 1.1 m, and the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is 3.41 m. Under these conditions, the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is not an integer multiple of the circumference of the drive roller 21. Specifically, the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is 3.1 times the circumference of the drive roller 21. In this case, a wave (error) W with an amplitude of about 0.5 pixels is superimposed on the amount of meandering during the rotation period C of the drive roller 21.

Figure 17 shows an example of the amount of belt meandering obtained when the spacing between the displacement sensors is set to an integer multiple of the roller circumference. In Figure 17, the vertical axis shows the amount of belt meandering in pixel (px) size, and the horizontal axis shows the belt transport distance (m). The size of one pixel is 35 μm.

The belt meandering amount shown in FIG. 17 was obtained under the conditions that the circumference of the belt 20 is about 28 m, the circumference of the drive roller 21 is 1.1 m, and the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is 3.30 m. Under these conditions, the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is an integer multiple of the circumference of the drive roller 21. Specifically, the distance between the displacement sensor 23 (PU4s) and the displacement sensor 23 (PU1s) is three times the circumference of the drive roller 21. In this case, the wave (error) W that appears in the rotation period C of the drive roller 21 is canceled, and the detection result of the original belt meandering amount with the error removed is obtained. This makes it possible to suppress the effect of the eccentricity of the drive roller 21. Therefore, the printing position of the image can be corrected more appropriately. As a result, it is possible to suppress the color shift of the image to a small extent and improve the image quality.

In addition, if the error in the placement spacing of the multiple displacement sensors 23 (PUs1 to PUs6) when they are installed is within ±5% of the circumference of the drive roller 21, relative to a predetermined spacing based on the circumference of the drive roller 21, the wave (error) W that appears in the rotation period C of the drive roller 21 can be suppressed to approximately 0.25 (px), or about half the size of one pixel. Therefore, the effect of the eccentricity of the drive roller 21 on the correction accuracy of the print position can be sufficiently suppressed.

In the above embodiment, the multiple displacement sensors 23 are arranged at a predetermined interval based on the circumference of the drive roller 21 among the drive roller 21 and driven roller 22 that constitute the belt support roller, but the present invention is not limited to this, and multiple displacement sensors 23 may be arranged at a predetermined interval based on the circumference of the driven roller 22. When the outer diameter of the drive roller 21 and the outer diameter of the driven roller 22 are the same, the degree of influence of the eccentricity of the belt support roller on the detection result of the belt meandering amount is greater for the drive roller 21 than for the driven roller 22. In that case, it is preferable to arrange multiple displacement sensors 23 at a predetermined interval based on the circumference of the drive roller 21.

In addition, when the outer diameter of the drive roller 21 and the outer diameter of the driven roller 22 are different, taking into consideration the degree of influence that the eccentricity of the belt support roller has on the detection result of the amount of belt meandering, it is preferable to arrange multiple displacement sensors 23 at a predetermined distance based on the circumference of the roller with the larger outer diameter.

For example, when the position of the displacement sensor PU4s is used as a reference point as described above, it is preferable to arrange multiple displacement sensors 23 at a predetermined distance based on the circumference of the roller that is closer to this reference point.

9. Modified Examples of Displacement Sensor
In the above embodiment, the displacement sensor 23 is a non-contact type displacement sensor, and a transmission type laser displacement sensor is given as an example of the non-contact type displacement sensor, but the present invention is not limited to this, and various types of displacement sensors can be adopted. Below, several modified examples of the displacement sensor will be described.

(First Modification of Displacement Sensor)
FIG. 18A is a schematic perspective view showing a first modified example of the displacement sensor, and FIG. 18B is a schematic plan view showing the first modified example of the displacement sensor.
18A and 18B, the displacement sensor 231 is a non-contact type displacement sensor, more specifically, a reflective laser displacement sensor. The displacement sensor 231 includes a light emitting unit and a light receiving unit, not shown. The displacement sensor 231 emits a laser beam 261 from the light emitting unit toward the end 20a of the belt 20, and the laser beam 261 is reflected by the end 20a of the belt 20. The laser beam 261 reflected by the end 20a of the belt 20 is received by the light receiving unit of the displacement sensor 231. At this time, when the belt 20 is displaced in a direction perpendicular to the belt moving direction y1, that is, in the belt width direction, the position of the laser beam 261 received by the light receiving unit of the displacement sensor 231 changes according to the displacement direction and the displacement amount. The output signal of the displacement sensor 231 changes depending on the position where the light receiving unit receives the laser beam 261. Therefore, the displacement amount of the belt 20 can be detected by the displacement sensor 231. In this case, the sensing position of the displacement sensor 231 is a position P2 on the laser axis along the optical path of the laser light 261.

(Second Modification of Displacement Sensor)
FIG. 19A is a schematic perspective view showing a second modified example of the displacement sensor, and FIG. 19B is a schematic plan view showing the second modified example of the displacement sensor.
In FIG. 19A and FIG. 19B, the displacement sensor 232 is a non-contact type displacement sensor, and more specifically, is a camera that reads a reference line 234 through a lens 233. The reference line 234 is provided in a straight line on the belt 20. The reference line 234 is formed along the end 20a of the belt 20. The reference line 234 may be any line that can be read by the displacement sensor 232 made of a camera. For example, the reference line 234 may be formed by printing or the like, or may be formed by a concave groove. The reading area 235 of the displacement sensor 232 is set to be larger than the line width of the reference line 234, with the position of the reference line 234 as the reference (center). A light source for reading the reference line 234 may be provided as necessary. The light source for reading may be built into the displacement sensor 232, or may be provided separately from the displacement sensor 232.

When detecting the amount of displacement of the belt 20 using the displacement sensor 232, the displacement sensor 232 reads a reference line 234 provided on the belt 20. At this time, when the belt 20 is displaced in the belt width direction, the position of the reference line 234 read by the displacement sensor 232 changes in the belt width direction depending on the displacement direction and amount. Therefore, the displacement amount of the belt 20 can be detected by the displacement sensor 232. In this case, the sensing position of the displacement sensor 232 is position P3 on the central axis of the lens 233 that is perpendicular to the belt movement direction y1.

In the second modified example described above, when the displacement sensor 232 is configured by a camera, the reference line 234 provided on the belt 20 is the object to be read, but the present invention is not limited to this, and for example, as shown in FIG. 20, a plurality of reference marks 236 provided on the belt 20 may be the object to be read. The reference marks 236 are formed at regular intervals in the belt movement direction y1. The reference marks 236 are also formed along the end 20a of the belt 20. The reading area 235 of the displacement sensor 232 is set larger than the size of the reference mark 236, with the passing position of the reference mark 236 as the reference (center).

When detecting the amount of displacement of the belt 20 using the displacement sensor 232, the displacement sensor 232 reads a reference mark 236 provided on the belt 20. At this time, when the belt 20 is displaced in the belt width direction, the position of the reference mark 236 read by the displacement sensor 232 changes in the belt width direction depending on the direction and amount of displacement. Therefore, the displacement sensor 232 can detect the amount of displacement of the belt 20. The sensing position of the displacement sensor 232 is as described above.

(Third Modification of Displacement Sensor)
FIG. 21A is a schematic perspective view showing a third modified example of the displacement sensor, and FIG. 21B is a schematic plan view showing the third modified example of the displacement sensor.
21A and 21B, the displacement sensor 237 is a contact type displacement sensor, more specifically, a contact type displacement sensor having a movable roller 238. The movable roller 238 serves as a contactor that comes into contact with the end 20a of the belt 20. In addition to the movable roller 238, the displacement sensor 237 includes a roller support member 239, a slide rod 240, and a sensor body 241. The roller support member 239 is attached to the tip of the slide rod 240. The roller support member 239 supports the movable roller 238 so as to be rotatable. The slide rod 240 is supported by the sensor body 241 so as to be slidable in the X direction (main scanning direction).

The movable roller 238 and roller support member 239 are provided so as to be movable in the X direction integrally with the slide rod 240. The sensor body 241 has a built-in position detection element (not shown) that detects the position of the slide rod 240 in the X direction. The movable roller 238 is kept in constant contact with the end 20a of the belt 20 by the biasing force of a biasing member (not shown). For example, a spring member can be used as the biasing member.

When detecting the displacement of the belt 20 by the displacement sensor 237, the movable roller 238 is brought into contact with the end 20a of the belt 20, and the belt 20 is moved in the y1 direction while maintaining this contact state. At this time, when the belt 20 is displaced in the belt width direction, the position of the movable roller 230 in the X direction changes according to the displacement direction and displacement amount, and similarly, the position of the slide rod 240 in the X direction also changes. Therefore, the displacement amount of the belt 20 can be detected by the displacement sensor 237 by detecting the change in the position of the slide rod 240 in the X direction with the position detection element described above. In this case, the sensing position of the displacement sensor 237 is position P4 on the central axis of the slide rod 240, which passes through the contact point between the end 20a of the belt 20 and the movable roller 238. Note that the contact type displacement sensor used as the displacement sensor may have any configuration as long as it has a contactor that contacts the end 20a of the belt 20 and a detection unit that detects the movement (position change) of the contactor.

10. Other embodiments of the present invention
The technical scope of the present invention is not limited to the above-described embodiments, but also includes forms in which various modifications and improvements are made within the scope that can derive specific effects obtained by the constituent elements of the invention and their combinations.

For example, in the above embodiment, an inkjet recording device has been described as an example of an image forming device, but the present invention is not limited to this, and can be applied to any type of image forming device, such as an LED printer, so long as it uses a belt 20 to transport a recording medium 11. Furthermore, the present invention is not limited to the belt drive device 12 used in an image forming device, and can be applied to belt drive devices for any purpose.

In addition, although terms such as "parallel" and "orthogonal" are used in this specification, these terms do not mean only "parallel" and "orthogonal" in the strict sense, but also include the meanings of "approximately parallel" and "approximately orthogonal" within the scope in which they can perform their functions.

10...image forming apparatus, 12...belt driving device, 14...image forming unit, 20...belt, 21...driving roller (belt support roller), 22...follower roller (belt support roller), 23, 231, 232, 237, PU1s to Pu8s...displacement sensor, 43a...printing control unit, 50...meander amount detection unit, 61...movement amount measurement unit, 62...displacement amount measurement unit, 63...meander amount calculation unit, 234...reference line, 236...reference mark, Y...conveyance direction, y1, y2...movement direction

Claims (10)

An endless belt for transporting a recording medium;
A belt support roller that supports the belt and moves the belt in a circulating manner;
a plurality of displacement sensors for detecting the amount of displacement of the belt in a direction perpendicular to the direction of movement of the belt;
an image forming section for forming an image on the recording medium conveyed by the belt;
a meandering amount detection unit that determines a meandering amount of the belt based on detection results of the displacement amounts of the belt by the plurality of displacement sensors;
a print control unit that corrects a print position of an image in a width direction of the belt based on the meandering amount of the belt determined by the meandering amount detection unit;
Equipped with
the plurality of displacement sensors are disposed at intervals in a moving direction of the belt and at intervals corresponding to an integer multiple of a circumferential length of the belt support roller,
the plurality of displacement sensors are arranged within a range within a thickness of the corresponding image forming unit in a conveying direction of the recording medium,
The print control unit corrects the print position of the image based on information on the meandering amount of the belt one revolution before, which is detected by the meandering amount detection unit.
Image forming device. the plurality of displacement sensors are disposed at a plurality of measurement points set in a moving direction of the belt,
The meandering amount detection unit determines the amount of meandering of the belt based on a difference between an amount of displacement of the belt detected by a displacement sensor arranged at one measurement point and an amount of displacement of the belt detected by a displacement sensor arranged at another measurement point at the same location as the one measurement point.
The image forming apparatus according to claim 1 . The meandering amount detection unit is
A movement amount measuring unit that measures the movement amount of the belt;
a displacement amount measuring unit that measures the displacement amount of the belt at the same location on the belt using a displacement sensor at one measurement point and a displacement sensor at another measurement point based on the amount of movement measured by the movement amount measuring unit and a known distance between the measurement points;
a meandering amount calculation unit that calculates a meandering amount of the belt at the one measurement point relative to the other measurement point by calculating a difference between the displacement amounts of the same location measured by the displacement amount measurement unit at the one measurement point and the other measurement point;
The image forming apparatus according to claim 2 , further comprising: The image forming apparatus according to claim 1 , wherein the displacement sensor is a non-contact type displacement sensor. The belt is provided with a reference line or mark;
The non-contact displacement sensor is a camera that reads the reference line or the reference mark.
5. The image forming apparatus according to claim 4 . The image forming apparatus according to claim 1 , wherein the displacement sensor is a contact type displacement sensor. The belt support roller is composed of a drive roller and a driven roller,
The image forming apparatus according to claim 1 , wherein the plurality of displacement sensors are arranged at intervals corresponding to an integer multiple of a circumferential length of the drive roller. The belt support roller is composed of a drive roller and a driven roller,
7. The image forming apparatus according to claim 1 , wherein the plurality of displacement sensors are arranged at intervals corresponding to an integer multiple of a circumferential length of one of the drive roller and the driven roller, which has a larger outer diameter. the plurality of displacement sensors are disposed at a plurality of measurement points set in a moving direction of the belt, and one of the plurality of measurement points is set as a reference point, and the plurality of displacement sensors include a first displacement sensor disposed at the reference point and a second displacement sensor disposed at the measurement point,
The belt support roller is composed of a drive roller and a driven roller,
The image forming apparatus according to claim 1 , wherein the plurality of displacement sensors are arranged at intervals corresponding to an integer multiple of a circumferential length of one of the drive roller and the driven roller which is closer to the reference point. An image forming apparatus as described in any one of claims 1 to 9 , wherein the error in the placement spacing of the multiple displacement sensors in the installation state is within ±5% of the circumference of the belt support roller, with respect to a spacing equivalent to an integer multiple of the circumference of the belt support roller.

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