Belt meandering preventing device and image forming apparatus including the same

A belt meandering preventing device includes a belt displacement detection unit detecting the amount of displacement in a belt width direction of an endless belt rotatably stretched over support parts; and a belt meandering correction unit correcting the displacement in the belt width direction of the endless belt based on the amount of displacement detected by the belt displacement detection unit. The belt displacement detection unit includes a moving part moving in association with the displacement of the endless belt or an edge of the endless belt in the belt width direction and optical sensors outputting signals with output levels corresponding to the proportions of the moving part in optical paths of the optical sensors. The optical sensors are arranged such that the output levels of the optical sensors change as the endless belt is displaced in the belt width direction in a predetermined high-resolution detection range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A certain aspect of the present invention relates to a belt meandering preventing device and an image forming apparatus including the belt meandering preventing device.

2. Description of the Related Art

Certain types of image forming apparatuses, e.g., copiers and printers, use an endless belt such as an intermediate transfer belt, a photosensitive belt, or a paper conveying belt to form an image. Such an endless belt is normally stretched over two or more rollers including a drive roller. While the endless belt is run (driven or rotated), the endless belt is often displaced in a direction (hereafter called a belt width direction) orthogonal to its running direction. This displacement may be hereafter called belt meandering. When an image is formed on the outer surface of the endless belt or on a recording medium placed on the outer surface of the endless belt, the belt meandering causes distortion of the image. Also, when a multi-color image is formed by sequentially forming single-color images on the endless belt such that they overlap each other, the belt meandering causes misalignment of the single-color images in the belt width direction and thereby causes problems such as a color shift and color shading. Since such a color shift and color shading are easily noticed by the user, it is important to properly prevent the belt meandering when forming a color image.

In a known method, the tilt of one or more support rollers (hereafter called steering rollers) for supporting an endless belt is controlled to prevent the meandering of the endless belt (this method is hereafter called a steering method). Compared with a method where a rib or a guide provided at one end in the belt width direction of the inner surface of an endless belt is hooked to an end face of a support roller to prevent the belt meandeting, the steering method makes it possible to reduce the external force applied to the endless belt. Therefore, the steering method improves the running stability and durability of an endless belt and provides higher reliability.

When the steering method is employed, it is necessary to detect the amount of displacement of the endless belt in the belt width direction and thereby to determine the controlled variable (the amount of tilt) of the steering roller. Also, to properly prevent the belt meandering by controlling the steering roller, it is important to detect the amount of displacement (hereafter called the amount of meandering) in the belt width direction at a high resolution. However, for the reasons described below, it is difficult to achieve both a required detection range (a detectable range of the amount of meandering) and a required detection resolution.

Just after an endless belt is installed or replaced by an assembly worker or a service person, there is normally a positional error of ±2-3 mm from the correct position in the belt width direction. When this positional error is taken into account, a detection range of ±2-3 mm is necessary to detect the amount of displacement of the endless belt in the belt width direction. Meanwhile, to keep the amount of meandering of the endless belt within a certain range and thereby to effectively prevent a color shift and color shading in a multi-color image, a. detection resolution of about 0.005 mm is necessary. That is, for a required detection range of ±2-3 mm, a detection resolution of more than 1000× (0.005 mm) is necessary. Needless to say, it is possible to achieve both a wide detection range and a high detection resolution as described above by using very expensive sensors. Practically, however, it is necessary to achieve both a wide detection range and a high detection resolution by using a simple sensor configuration including inexpensive analog-output optical sensors with an output voltage range of 0-5 V. However, to obtain a 1000× resolution for the above detection range using inexpensive analog-output optical sensors with an output voltage range of 0-5 V, it is necessary to detect a voltage (a sensor output) in units of 5 mV. Considering the noise in a device and the capability of an analog-to-digital conversion circuit of a controller, it is difficult to properly and reliably detect a voltage in units of 5 mV.

Japanese Patent Application Publication No. 2008-275800 and Japanese Patent Application Publication No. 2005-338522 propose belt meandering preventing devices in trying to achieve both a wide detection range and a high detection resolution using multiple inexpensive sensors.

The belt meandering preventing device disclosed in JP2008-275800 includes a first detection unit for detecting the amount of displacement of an endless belt in the belt width direction in a range of ±1 mm from a normal position of the endless belt; and a second detection unit for detecting an overrun that is a displacement of the endless belt of more than ±5 mm from the normal position of the endless belt. The belt meandering preventing device corrects the displacement of the endless belt in the belt width direction according to the amount of displacement detected by the first detection unit, or stops the endless belt and reports an error if an overrun is detected by the second detection unit. The first detection unit is a displacement sensor positioned to face a swinging direction of a swing arm that swings about a spindle in accordance with the displacement of the endless belt in the belt width direction. With the belt meandering preventing device of JP2008-275800, the displacement of the endless belt within the detection range (±1 mm from the normal position of the endless belt) of the first detection unit can be corrected, and also damage to the endless belt caused by an overrun can be prevented by detecting the overrun with the second detection unit provided separately from the first detection unit. The belt meandering preventing device disclosed in JP2005-338522 includes a swing arm that swings about a spindle in accordance with the displacement of an endless belt in the belt width direction and first and second displacement sensors facing the swinging direction of the swing arm and placed at different. distances from the spindle. The first displacement sensor closer to the spindle has a wider detection range and a lower resolution; and the second displacement sensor further from the spindle has a narrower detection range and a higher resolution. The belt meandering device of JP2005-338522 corrects the displacement of the endless belt in the belt width direction within the detection range (±1 mm from a normal position of the endless belt) of the second displacement sensor based on a signal from the second displacement sensor with a higher detection resolution, and corrects the displacement of the endless belt in the belt width direction beyond the detection range of the second displacement sensor based on a signal from the first displacement sensor with a lower detection resolution.

Both of the belt meandering preventing devices of JP2008-275800 and JP2005-338522 use two sensors to detect the displacement of the endless belt in the belt width direction. However, with the configurations of JP2008-275800 and JP2005-338522, the width of a high-resolution detection range where the displacement of the endless belt can be detected at a high resolution is substantially the same as the width of a high-resolution detection range that is achievable by one sensor. Thus, with the configurations of JP2008-275800 and JP2005-338522, it is difficult to detect the displacement of an endless belt in the belt width direction at a high detection resolution in a wide detection range.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a belt meandering preventing device includes a belt displacement detection unit detecting the amount of displacement in a belt width direction of an endless belt rotatably stretched over support parts; and a belt meandering correction unit correcting the displacement in the belt width direction of the endless belt based on the amount of displacement detected by the belt displacement detection unit. The belt displacement detection unit includes a moving part moving in association with the displacement of the endless belt or an edge of the endless belt in the belt width direction and optical sensors outputting signals with output levels corresponding to the proportions of the moving part in optical paths of the optical sensors. The optical sensors are arranged such that the output levels of the optical sensors change as the endless belt is displaced in the belt width direction in a predetermined high-resolution detection range. The belt displacement detection unit is configured to combine the output signals of the optical sensors such that the rate of change of an output level of the combined signal with respect to the amount of displacement in the belt width direction of the endless belt within the high-resolution detection range becomes greater than the rates of change of the output levels of the respective optical sensors and to detect the amount of displacement based on the combined signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A configuration of a printer as an example of an electrophotographic image forming apparatus according to an embodiment of the present invention is described below.

FIG. 1is schematic diagram of a printer of this embodiment.

The printer includes two optical scanning units1YM and1CK and four process units2Y,2M,2C, and2K that, respectively, form yellow (Y), magenta (M), cyan (C), and black (K) toner images. The printer also includes a paper-feed path30, a before-transfer conveyance path31, a manual-paper-feed path32, a manual-feed tray33, a resist roller pair34, a conveyor belt unit35, a fusing unit40, a conveyance path switching unit50, a paper-ejection path51, a paper-ejection roller pair52, a paper-catch tray53, a first paper-feed cassette101, a second paper-feed cassette102, and a re-feeding unit.

Each of the first paper-feed cassette101and the second paper-feed cassette102contains sheets of recording paper P as recording media. With the rotation of a paper-feed roller101aor102a, the uppermost sheet of the recording paper P is fed from the paper-feed cassette101or102to the paper-feed path30. The paper-feed path30is followed by the before-transfer conveyance path31for conveying the recording paper P in a section leading to a secondary transfer nip described later. The recording paper P fed from the paper-feed cassette101or102goes through the paper-feed path30and enters the before-transfer conveyance path31.

The manual-feed tray33is attached to a side of the case of the printer such that it can be opened and closed with respect to the case. With the manual-feed tray33opened, sheets of recording paper P can be manually placed on the upper surface of the manual-feed tray33. The uppermost sheet of the recording paper P on the manual-feed tray33is fed by a feeding roller of the manual feed tray33toward the before-transfer conveyance path31.

Each of the optical scanning units1YM and1CK includes laser diodes, a polygon mirror, and lenses (not shown) and drives the laser diodes according to image information obtained by a separate scanner or sent from a personal computer to optically scan photoconductors3Y,3M,3C, and3K of the process units2Y,2M,2C, and2K. The photoconductors3Y,3M,3C, and3K of the process units2Y,2M,2C, and2K are rotated counterclockwise inFIG. 1by a driving unit (not shown). The optical scanning unit1YM deflects laser beams in the rotational axis directions of the photoconductors3Y and3M and thereby optically scans the photoconductors3Y and3M. As a result, electrostatic latent images corresponding to Y image information and M image information are formed on the corresponding photoconductors3Y and3M. Similarly, the optical scanning unit1CK deflects laser beams in the rotational axis directions of the photoconductors3C and3K and thereby optically scans the photoconductors3C and3K. As a result, electrostatic latent images corresponding to C image information and K image information are formed on the corresponding photoconductors3C and3K.

The process units2Y,2M,2C, and2K, respectively, include drum-shaped photoconductors3Y,3M,3C, and3K used as latent image carriers. In addition, each of the process units2Y,2M,2C, and2K includes various components that are disposed around the photoconductor3Y/3M/3C/3K and supported on the same support. The process units2Y,2M,2C, and2K are removably attached to the printer body. The process units2Y,2M,2C, and2K have substantially the same configuration except that they use toners of different colors. Take, for example, the process unit2Y. The process unit2Y includes the photoconductor3Y and a developing unit4Y that develops an electrostatic latent image formed on the surface of the photoconductor3Y to form a Y-toner image. The process unit2Y also includes a charging unit5Y for uniformly charging the surface of the photoconductor3Y being rotated, and a drum cleaning unit6Y for removing post-transfer residual toner adhering to the surface of the photoconductor3Y after it passes through a primary transfer nip for the Y component described later.

The printer shown byFIG. 1is configured as a tandem image forming apparatus where four process units2Y,2M,2C, and2K are arranged along the rotational direction of an intermediate transfer belt61described later.

In this embodiment, the photoconductor3Y is shaped like a drum and is made by forming a photosensitive layer on a base tube made of, for example, aluminum by applying an organic photosensitive material. Alternatively, a photoconductor shaped like an endless belt may be used as the photoconductor3Y.

The developing unit4Y uses a two-component developer (hereafter, simply called a developer) including magnetic carriers and nonmagnetic yellow toner to develop a latent image. Alternatively, the developing unit4Y may be configured to use a one-component developer including no magnetic carrier instead of a two-component developer. A Y-toner supplying unit (not shown) supplies Y toner from a Y-toner bottle103Y to the developing unit4Y.

The drum cleaning unit6Y includes a cleaning blade made of polyurethane rubber that is brought into contact with the photoconductor3Y. A drum cleaning unit with a different configuration may also be used. In this embodiment, a rotatable fur brush that is brought into contact with the photoconductor3Y is also provided to improve the cleaning performance. The fur brush also scrapes lubricant from a solid lubricant (not shown), reduces the scraped lubricant to a fine powder, and applies the fine powder to the surface of the photoconductor3Y.

A discharge lamp (not shown) is also provided above the photoconductor3Y as a component of the process unit2Y. The discharge lamp illuminates and thereby discharges the surface of the photoconductor3Y after it passes through the drum cleaning unit6Y. The discharged surface of the photoconductor3Y is uniformly charged by the charging unit5Y and is then optically scanned by the optical scanning unit1YM. The charging unit5Y is rotated while being supplied with a charging bias from a power supply (not shown). Instead of the charging unit5Y, a scorotron charger may be used. A scorotron charger charges a photoconductor without contact.

The process units2M,2C, and2K have configurations similar to the configuration of the process unit2Y described above.

A transfer unit60is disposed below the process units2Y,2M,2C, and2K. The transfer unit60includes an endless intermediate transfer belt61stretched over multiple support rollers. The intermediate transfer belt61is in contact with the photoconductors3Y,3M,3C, and3K and is rotated (or run) clockwise inFIG. 1by the rotation of one of the support rollers. With this configuration, primary transfer nips for the C, M, Y, K components are formed between the photoconductors3C,3M,3Y, and3K and the intermediate transfer belt61.

Near the primary transfer nips for the Y, M, C, K components, the intermediate transfer belt61is pressed against the photoconductors3Y,3M,3C, and3K by primary transfer rollers62Y,62M,62C, and62K disposed inside of the belt loop, i.e., inside of a space surrounded by the inner surface of the intermediate transfer belt61. A primary transfer bias is applied to the primary transfer rollers62Y,62M,62C, and62K from a power supply (not shown). As a result, primary transfer electric fields are formed at the primary transfer nips for the Y, M, C, K components, and the primary transfer electric fields cause toner images on the photoconductors3Y,3M,3C, and3K to be electrostatically transferred onto the intermediate transfer belt61.

When the intermediate transfer belt61is rotated clockwise, the outer surface of the intermediate transfer belt61passes through the primary transfer nips for the Y, M, C, K components in sequence. As a result, the toner images are sequentially transferred to the outer surface of the intermediate transfer belt61and are superposed on the outer surface (primary transfer process). As a result of the primary transfer process, a superposed toner image with four colors (hereafter called a four-color toner image) is formed on the outer surface of the intermediate transfer belt61.

A secondary transfer roller72used as a secondary transfer part is provided below the intermediate transfer belt61. Also, a secondary transfer backup roller68is disposed inside of the loop of the intermediate transfer belt61so as to contact the inner surface of the intermediate transfer belt61. The secondary transfer roller72is in contact with the outer surface of the intermediate transfer belt61at a position corresponding to the secondary transfer backup roller68. With this configuration, a secondary transfer nip is formed between the outer surface of the intermediate transfer belt61and the secondary transfer roller72.

A secondary transfer bias is applied to the secondary transfer roller72by a power supply (not shown). Meanwhile, the secondary transfer backup roller68in the belt loop is grounded. With this configuration, a secondary transfer electric field is formed in the secondary transfer nip.

The resist roller pair34is disposed to the right of the secondary transfer nip. Two rollers of the resist roller pair34feed the recording paper P to the secondary transfer nip in synchronization with the movement of the four-color toner image on the intermediate transfer belt61. At the secondary transfer nip, the four-color toner image on the intermediate transfer belt61is caused to be transferred onto the recording paper P by the secondary transfer electric field and the nip pressure, and forms a full color image in combination with the white color of the recording paper P.

After the intermediate transfer belt61passes through the secondary transfer nip, toner (post-transfer residual toner) that has not been transferred onto the recording paper P remains on the outer surface of the intermediate transfer belt61. The post-transfer residual toner is removed by a belt cleaning unit75that is in contact with the intermediate transfer belt61.

Meanwhile, the recording paper P that has passed through the secondary transfer nip is separated from the intermediate transfer belt61and is passed to the conveyor belt unit35. The conveyor belt unit35includes a drive roller37, a driven roller38, and an endless conveyor belt36stretched over the drive roller37and the driven roller38. The conveyor belt36is rotated counterclockwise inFIG. 1by the rotation of the drive roller37. The recording paper P passed from the intermediate transfer belt61is held on the outer surface of the conveyor belt36and is conveyed to the fusing unit40by the rotation of the conveyor belt36.

A re-feeding unit is formed by the conveyance path switching unit50, a re-feeding path54, a switchback path55, and an after-switchback conveyance path56. The conveyance path switching unit50switches destinations of the recording paper P received from the fusing unit40between the paper-ejection path51and the re-feeding path54. When simplex printing where an image is formed only on one side of the recording paper P is performed, the conveyance path switching unit50selects the paper-ejection path51as the destination of the recording paper P. As a result, the recording paper P only on one side of which an image has been formed is conveyed via the paper-ejection path51to the paper ejection-roller pair52and ejected onto the paper-catch tray53. Also, when the recording paper P on both sides of which images have been formed is received from the fusing unit40in a duplex printing mode, the conveyance path switching unit50selects the paper-ejection path51as the destination of the recording paper P. As a result, the recording paper P on both sides of which images have been formed is ejected onto the paper-catch tray53. Meanwhile, when the recording paper P only on a first side of which an image has been formed is received from the fusing unit40in a duplex printing mode, the conveyance path switching unit50selects the re-feeding path54as the destination of the recording paper P.

The recording paper P conveyed into the re-feeding path54enters the switchback path55connected to the re-feeding path54. When the entire recording paper P enters the switchback path55, the conveying direction of the recording paper P is reversed. The switchback path55is also connected to the after-switchback conveyance path56. The recording paper P being conveyed in the reverse direction enters the after-switchback conveyance path56and as a result, the recording paper P is turned over. The turned-over recording paper P is conveyed via the after-switchback conveyance path56and the paper-feed path30to the second transfer nip again. After a toner image is transferred onto a second side of the recording paper P at the secondary transfer nip and the toner image is fused onto the second side by the fusing unit40, the recording paper P is ejected via the conveyance path switching unit50, the paper-ejection path51, and the paper ejection-roller pair52onto the paper-catch tray53.

Next, a belt drive unit for driving the intermediate transfer belt61is described.

FIG. 2is a schematic diagram of a belt drive unit according to an embodiment of the present invention.

The belt drive unit of this embodiment includes support rollers63,67,68,69, and71; the intermediate transfer belt61that is an endless belt stretched over the support rollers63,67,68,69, and71; a tilting mechanism that is driven by a steering motor23used as a driving source and tilts the support roller (steering roller)63; an edge sensor24used as a belt displacement detection unit for detecting the amount of displacement (amount of meandering) of the intermediate transfer belt61in the belt width direction (hereafter may be called the amount of belt displacement); and a steering control unit21that determines the amount of tilt of the steering roller63based on the amount of belt displacement detected by the edge sensor24, and controls the steering motor23to control the tilting mechanism such that the amount of tilt of the steering roller63matches the determined amount of tilt. Thus, the belt drive unit is configured to prevent the meandering of the intermediate transfer belt61by changing the amount of tilt of the steering roller63. In this embodiment, the tilting mechanism and the steering control unit21constitute a belt meandering correction unit. Although the support roller67is used as the drive roller in this embodiment, a different one of the support rollers may be used as the drive roller.

The steering control unit21may be implemented by a separate microcomputer or may be implemented by a controller provided in the printer of this embodiment. The steering control unit21adjusts the amount of tilt of the steering roller63based on the amount of belt displacement detected by the edge sensor24and thereby performs a feedback control to keep the intermediate transfer belt61in a normal (target) position in the belt width direction. As long as these functions are achieved, the steering control unit21may have any configuration or may be implemented by any device.

FIG. 3is a drawing illustrating an exemplary configuration of the edge sensor24.

FIG. 4is a drawing illustrating an exemplary configuration of transmissive optical sensors24eand24fof the edge sensor24.

As shown inFIG. 3, an L-shaped arm part used as a moving part and rotatably supported on a spindle24cis disposed at one side (edge) of the intermediate transfer belt61. The arm part is biased (or pulled) by a spring24aso that a contact part24bof the arm part is always in contact with the side of the intermediate transfer belt61. The contacting pressure of the contact part24bcaused by the spring24ais set at an appropriate level such that the side of the intermediate transfer belt61is not deformed. The arm part also includes a light-shielding part24d. As shown inFIG. 4, each of the transmissive optical sensors24eand24fincludes a light-emitting part24gand a light-receiving part24hthat face each other with the light-shielding part24dpositioned between them. As shown inFIG. 3, the optical sensors24eand24fare arranged along the direction in which the light-shielding part24dmoves when the arm part rotates about the spindle24c.

With the edge sensor24configured as described above, the movement (meandering) of the intermediate transfer belt61in the belt width direction (indicated by arrow B inFIG. 3) is converted via the contact part24bcontacting the side of the intermediate transfer belt61into the rotational movement of the arm part about the spindle24c. While the leading edge or the rear edge of the light-shielding part24din the rotational direction of the arm part is in the sensor ranges of the optical sensors24eand24f, the output levels of the optical sensors24eand24fchange according to the rotational movement of the arm part. Accordingly, the sensor outputs of the optical sensors24eand24findicate the amount of belt displacement (meandering) of the intermediate transfer belt61. In this embodiment, as shown inFIG. 2, the edge sensor24is disposed between the drive roller67and the secondary transfer backup roller68in the belt movement direction (rotational direction).

Each of the optical sensors24eand24foutputs an analog voltage corresponding to the intensity of light received by the light-receiving part24hand may be implemented by an inexpensive optical sensor such as an analog-output transmissive photointerrupter.

The configuration of the edge sensor24is not limited to that described above as long as the edge sensor24includes multiple optical sensors each of which outputs a signal with an output level corresponding to the proportion of a moving part, which moves in association with the displacement in the belt width direction of the intermediate transfer belt61, in the optical path, and the optical sensors are disposed such that output levels of all the optical sensors change when the intermediate transfer belt61is displaced in the belt width direction in a predetermined range. For example, although the movement of the intermediate transfer belt61in the belt width direction is converted into the rotational movement of the moving part (arm part), the edge sensor24may be configured such that the movement of the intermediate transfer belt61in the belt width direction is converted into the linear movement of the moving part. Also, the edge sensor24may be configured to directly detect an edge of the intermediate transfer belt61in the belt width direction without using the moving part and thereby to detect the amount of displacement of the intermediate transfer belt61in the belt width direction.

FIG. 5is a perspective view, seen from above at an oblique angle, of the tilting mechanism provided at one end (drive end) of the steering roller63.

FIG. 6is a perspective view, seen from below at an oblique angle, of the tilting mechanism.

In this embodiment, as shown inFIG. 2, the tilting mechanism for tilting the steering roller63employs a cantilevered wire method. The tilting mechanism is described below in more detail.

A drive pulley86is provided on the output shaft of the steering motor23. A timing belt88is stretched over the drive pulley86and a wind-up pulley87. The wind-up pulley87includes a belt pulley part around which the timing belt88is wound and a wire pulley part to which one end (hereafter called a drive end) of a wire80is fixed. The belt pulley part and the wire pulley part are coaxial and are formed as a monolithic structure. When the steering motor23is driven and the drive pulley86is rotated, the wind-up pulley87is rotated via the timing belt88and the drive end portion of the wire80is wound around the wire pulley part. In this embodiment, the diameter of the wire pulley part is smaller than the diameter of the belt pulley part. Accordingly, the wind-up pulley87is configured as a deceleration unit.

Also in this embodiment, the drive end of the wire80is fixed to the wind-up pulley87. Meanwhile, the other end of the wire80is wound around a moving pulley83and fixed to a wire holding part84. The moving pulley83is rotatably supported by one end of a long roller holder81. The drive end of the steering roller63is rotatably supported by the other end of the roller holder81. The roller holder81is rotatably supported by a spindle82at a point in its length direction. Also, the roller holder81is biased by a tension spring85in the clockwise direction around the spindle82inFIG. 2. In other words, the tension spring85biases the moving pulley83upward and thereby applies tension to the wire80wound around the moving pulley83. Thus, the tension spring85functions as a tension applying unit that constantly applies an appropriate amount of tension to the wire80.

A wire part80ais being pulled by a tension spring89and as a result, a bias causing counterclockwise rotation inFIG. 2is applied to the wind-up pulley87. The wire part80aand the tension spring89are provided to reduce the drive torque of the steering motor23. When the steering motor23is driven (rotated) in a direction against the bias provided by the tension spring85, the load of the steering motor23is increased by the bias applied by the tension spring85. However, the bias applied by the tension spring89in the rotational direction reduces the load of the steering motor23.

In the tilting mechanism configured as described above, when the steering motor23is driven, the wire80is wound around or unwound from the wind-up pulley87. As a result, the moving pulley83moves and causes the roller holder81to rotate about the spindle82. The rotation of the roller holder81in turn causes the drive end of the steering roller63to move with respect to the other end and the steering roller63is tilted. With the tilting mechanism of this embodiment where the wire80is wound around the wind-up pulley87, the maximum amount of movement of the wire80is large. This in turn makes it possible to increase the tilting range within which the steering roller63can be tilted. On the other hand, if the tilting range of the steering roller63is too wide and the roller holder81is likely to interfere with surrounding parts, a limiting part for limiting the rotation range of the roller holder81may be provided. In this embodiment, a stopper95is provided as the limiting part as shown inFIG. 5.

Also, since the maximum amount of movement of the wire80is large, it is possible to achieve a sufficient tilting range of the steering roller63even when a deceleration unit is employed. This in turn makes it possible to accurately control the amount of tilt of the steering roller63by using the deceleration unit. In this embodiment, the rotational motion of the steering motor23is decelerated by the ratio between the diameters of the belt pulley part and the wire pulley part of the wind-up pulley87, the moving pulley83, and the ratio between the distances from the spindle82to the respective ends of the roller holder81; and the reduced rotational motion is transmitted to the roller holder81. This configuration makes it possible to increase the resolution of the amount of tilt of the steering roller63and thereby makes it possible to accurately control the amount of tilt of the steering roller63.

Unlike a cam method where a cam is used instead of a wire, the wire method employed in this embodiment makes it possible to place the steering motor23in a position distant from the steering roller63. This in turn makes it possible to more flexibly lay out the components around the steering roller63. Also, compared with a method where a loop of wire is used as disclosed in JP2008-275800, the cantilevered wire method employed in this embodiment makes it possible to reduce the space necessary for the wire and makes it easier to handle the wire.

FIG. 7is a block diagram illustrating a control mechanism of a belt meandering preventing device of the belt drive unit.

The steering control unit21outputs a motor control signal (motor drive signal) for controlling the steering motor23. The steering motor23is implemented, for example, by a stepping motor or a linear motor, the rotational angle and the rotational speed of which can be accurately controlled. In this embodiment, a stepping motor is used as the steering motor23. The steering control unit21is connected to the edge sensor24and receives belt position information from the edge sensor24. The steering control unit21is also connected to a photointerrupter25described later and receives reference tilted position information from the photointerrupter25. The steering control unit21is further connected to a storage unit22. The storage unit22stores the amount of movement (rotational angle) of the steering motor23as a reference rotational angle (movement reference value) when the reference tilted position information is input from the photointerrupter25.

Whether the tilted position of the steering roller63matches the reference tilted position is determined by detecting the position of a position-shifting part that moves together with the steering roller63according to the amount of tilt of the steering roller63. In this embodiment, a filler91fixed to the roller holder81, which rotates along with the tilting of the steering roller63, is used as the position-shifting part. A light-emitting part and a light-receiving part of the photointerrupter25are disposed on the corresponding sides of the moving path of the filler91. The photointerrupter25is placed in a position that corresponds to a position where the filler91is present when the steering roller63is in the reference tilted position. When the steering roller63is in the reference tilted position, the filler91interrupts the optical path of the photointerrupter25and the output level of the light-receiving part becomes less than or equal to a predetermined value. When the output level of the photointerrupter becomes less than or equal to the predetermined value, the reference tilted position information is input to the steering control unit21. The steering control unit21determines that the steering roller63is in the reference tilted position if the reference tilted position information is received.

The steering control unit21stores, in the storage unit22, the amount of movement (rotational angle) of the steering motor23that is detected when the reference tilted position information is input from the photointerrupter25as a reference rotational angle (movement reference value). The reference rotational angle stored in the storage unit22is updated at every predetermined adjustment timing. In this embodiment, the adjustment timing is defined as the timing when the printer is powered on. Therefore, the reference rotational angle is updated each time when the printer is turned on. With this configuration, even if the wire80, which is a component of the tilting mechanism, is stretched due to some reason, a control error caused by the stretched wire80is corrected each time the printer is powered on.

FIG. 8is a drawing illustrating a control mechanism of the edge sensor24.

In this embodiment, the edge sensor24is configured such that when the arm part rotates clockwise inFIG. 8, the rear edge of the light-shielding part24denters the sensor range of the first optical sensor24eand the leading edge of the light-shielding part24denters the sensor range of the second optical sensor24fsubstantially at the same timing; and also the rear edge of the light-shielding part24dexits from the sensor range of the first optical sensor24eand the leading edge of the light-shielding part24dexits from the sensor range of the second optical sensor24fsubstantially at the same timing. With this configuration, the sensor outputs Va and Vb of the optical sensors24eand24fshow waveforms as shown inFIG. 9Awhere the horizontal axis indicates the amount of belt displacement (displacement in the clockwise direction inFIG. 8is represented by a change in the plus direction and displacement in the counterclockwise direction inFIG. 8is represented by a change in the minus direction).

In this embodiment, the detection ranges (the ranges within which the output levels vary according to the displacement of the intermediate transfer belt61in the belt width direction) of the optical sensors24eand24foverlap each other. The overlapping detection range of the optical sensors24eand24fwhere the amount of belt displacement can be detected at a high detection resolution may be used as a detection range (high-resolution detection range) C of the edge sensor24. More particularly, in this embodiment, output levels exceeding a threshold Vth are used for detection to remove noise and the detection range (high-resolution detection range) C of the edge sensor24is defined as shown inFIG. 9A.

Although the detection ranges of the optical sensors24eand24fare substantially the same in the example shown inFIG. 9A, the edge sensor24may be configured such that the detection ranges of the optical sensors24eand24fpartly overlap each other as long as the overlapping detection range has a width that is sufficient as the high-resolution detection range C.

Also in this embodiment, the arm part is adjusted such that, in terms of the clockwise rotational direction of the arm part, the rear edge of the light-shielding part24dis substantially at the center of the sensor range of the first optical sensor24eand the leading edge of the light-shielding part24dis substantially at the center of the sensor range of the second optical sensor24fwhen the intermediate transfer belt61is in the normal position (where the amount of belt displacement=0) in the belt width direction. Therefore, in the graph ofFIG. 9A, the intermediate transfer belt61is in the normal position (the amount of belt displacement=0) in the belt width direction when the sensor output Va of the first optical sensor24eequals the sensor output Vb of the second optical sensor24f(Vb−Va=0).

As shown inFIG. 8, in the edge sensor24of this embodiment, a differential signal (Vb−Va) between the sensor outputs Va and Vb of the optical sensors24eand24fis obtained by an analog circuit27, the differential signal (combined signal) is converted by an A/D conversion circuit28into a digital signal, and the digital signal is output as the belt position information to the steering control unit21. Alternatively, the differential signal (Vb−Va) between the sensor outputs of the optical sensors24eand24fmay be obtained by software processing using a microcomputer.FIG. 9Bshows the differential signal (Vb−Va) between the sensor outputs of the optical sensors24eand24f. As shown inFIG. 9B, the gradient of the differential signal (Vb−Va) in the high-resolution detection range C is greater than the gradient of the respective sensor outputs Va and Vb of the optical sensors24eand24f. The gradient indicates the detection resolution in the high-resolution detection range C. Thus, the detection resolution in the high-resolution detection range C of the edge sensor24of this embodiment is greater than the detection resolutions of the respective optical sensors24eand24f.

FIG. 10is a flowchart showing a control process for preventing the belt meandering.

When a print job is entered (S1), rotation of the intermediate transfer belt61is started (S2), and an image forming process is performed according to the print job (S3). During the image forming process, the edge sensor24detects the displacement (meandering) of the intermediate transfer belt61in the belt width direction (S4); and the steering control unit21calculates a controlled variable (a target rotational angle) of the steering motor23based on the detected amount of displacement and controls the rotational angle of the steering motor23to Match the calculated target rotational angle (this process is hereafter called a belt meandering preventing process).

In a belt meandering preventing method of this embodiment, a threshold Vth is used to appropriately switch steering control steps (or modes). As shown inFIG. 9A, the threshold Vth is set at a value that is lower than the voltage at a point where lines indicating the sensor outputs Va and Vb of the optical sensors24eand24fintersect with each other. The steering control unit21obtains the sensor outputs Va and Vb of the optical sensors24eand24fand switches steering control steps (modes) as described below by comparing the sensor outputs Va and Vb with the threshold Vth.

When both of the sensor outputs Va and Vb of the optical sensors24eand24fare greater than the threshold Vth (YES in S5), the steering control unit21controls the steering motor23such that a differential signal (Vb−Va) becomes zero (i.e., so that the amount of belt displacement becomes zero) (S6) and thereby corrects the meandering of the intermediate transfer belt61. More specifically, when the output shaft of the steering motor23is rotated counterclockwise inFIG. 2while the steering roller63is in a horizontal position, the wire80is wound around the wind-up pulley87and the roller holder81is rotated in the θ1direction inFIG. 2. As a result, the drive end of the steering roller63is lifted by the roller holder81and is tilted according to the amount of lift. The tilt in turn causes the intermediate transfer belt61wound around the steering roller63to shift away from the drive end of the steering roller63in the belt width direction. Meanwhile, when the output shaft of the steering motor23is rotated clockwise inFIG. 2while the steering roller63is in a horizontal position, the wire80is unwound from the wind-up pulley87and the roller holder81is rotated in the θ2direction inFIG. 2. As a result, the drive end of the steering roller63is pressed down by the roller holder81and is tilted according to the amount of pressing down. The tilt in turn causes the intermediate transfer belt61wound around the steering roller63to shift toward the drive end of the steering roller63in the belt width direction. Thus, in this embodiment, the displacement (positional change) of the intermediate transfer belt61is detected by the edge sensor24and the tilt of the steering roller63is appropriately controlled by driving the steering motor23based on the detected amount of belt displacement to correct the meandering of the intermediate transfer belt61.

Referring back toFIG. 10, if the sensor output Va of the first optical sensor24eis greater than the threshold Vth but the sensor output Vb of the second optical sensor24fis less than or equal to the threshold Vth (YES in S7), it can be assumed that the position in the width direction of the intermediate transfer belt61is in a range D that is beyond the high-resolution detection range C of the edge sensor24in the plus direction. Therefore, in this case, the steering control unit21switches to a control step (mode) where the steering motor23is controlled by a predetermined control amount so that the intermediate transfer belt61shifts in the minus direction of belt displacement (to the left inFIG. 9A) (S8). As a result, the intermediate transfer belt61is returned to a position in the belt width direction where the steering control step of S6can be performed. After the position of the intermediate transfer belt61in the belt width direction is returned to the high-resolution detection range C by the above control step (S8), the steering control step of S6using the differential signal (Vb−Va) can be performed to correct the meandering of the intermediate transfer belt61.

If the sensor output Vb of the second optical sensor24fis greater than the threshold Vth but the sensor output Va of the first optical sensor24eis less than or equal to the threshold Vth (YES in S9), it can be assumed that the position in the width direction of the intermediate transfer belt61is in a range E that is beyond the high-resolution detection range C of the edge sensor24in the minus direction. Therefore, in this case, the steering control unit21switches to a control step (mode) where the steering motor23is controlled by a predetermined control amount so that the intermediate transfer belt61shifts in the plus direction of belt displacement (to the right inFIG. 9A) (S10). As a result, the intermediate transfer belt61is returned to a position in the belt width direction where the steering control step of S6can be performed. After the position of the intermediate transfer belt61in the belt width direction is returned to the high-resolution detection range C by the above control step (S10), the steering control step of S6using the differential signal (Vb−Va) can be performed to correct the meandering of the intermediate transfer belt61.

If the intermediate transfer belt61is displaced beyond the high-resolution detection range C, it is not possible to determine the accurate position of the intermediate transfer belt61in the belt width direction based on a detection result (the amount of belt displacement) of the edge sensor24and to perform a steering control process based on the detection result of the edge sensor24. In this embodiment, however, even if the intermediate transfer belt61is displaced greatly, the direction of displacement of the intermediate transfer belt61can be determined based on the sensor outputs Va and Vb of the optical sensors24eand24fwithout using an additional sensor. Therefore, even if the intermediate transfer belt61is displaced beyond the high-resolution detection range C, it is not necessary to immediately stop the intermediate transfer belt61and to perform maintenance. Thus, this embodiment makes it possible to reduce the frequency of maintenance.

If both of the sensor outputs Va and Vb of the optical sensors24eand24fare less than or equal to the threshold Vth (NO in S9), error information indicating a sensor output error is reported to an upper controller and a sensor error process is performed to stop the intermediate transfer belt61(S11). This is because a situation where both of the sensor outputs Va and Vb are less than or equal to the threshold Vth does not normally occur in this embodiment. A sensor output error is, for example, caused by a break in the harness of the optical sensor24eor24f, failure of the light-emitting part24gor the light-receiving part24h, or a smear on the light-emitting part24gor the light-receiving part24h. When the error information is reported, maintenance is performed to correct the sensor output error.

The control process including steps S4through S11is repeated until the image forming process is completed (S12).

According to this embodiment, the edge sensor24can be implemented by two inexpensive analog-output optical sensors24eand24f. The edge sensor24outputs belt position information represented by a difference between sensor outputs in the detection ranges of the optical sensors24eand24f(the ranges within which the output levels vary according to the displacement of the intermediate transfer belt61in the belt width direction, i.e., the ranges where the amount of belt displacement is detectable). This configuration of the edge sensor24makes it possible to achieve a detection resolution higher than the detection resolutions of the respective optical sensors24eand24f. In other words, in this embodiment, an overlapping detection range of the optical sensors24eand24fwith a relatively low detection resolution and a relatively wide detection range is used as the detection range of the edge sensor24to achieve a wide high-resolution detection range C. This configuration makes it possible to achieve a high detection resolution that is not achievable by using a single optical sensor having a detection range with the same width as the high-resolution detection range C.

If a single optical sensor is used to detect the amount of belt displacement and if the intensity of received light in the entire detection range of the optical sensor is reduced due to, for example, a toner smear on the light emitting part24gor the light-receiving part24h, the output level of the optical sensor corresponding to the normal position in the belt width direction of the intermediate transfer belt61also decreases and the correspondence between the normal position and the output level of the optical sensor becomes inaccurate. As a result, it becomes difficult to keep the intermediate transfer belt61in the normal position in the belt width direction by performing a steering control process and to properly prevent the meandering of the intermediate transfer belt61. This in turn makes it necessary to frequently perform maintenance to adjust the output level of the optical sensor or to clean the smear. Meanwhile, in this embodiment, since the amount of belt displacement is detected based on a differential signal (Vb−Va) between the sensor outputs Va and Vb of the optical sensors24eand24f, the output level of the differential signal (Vb−Va) corresponding to the normal position in the belt width direction of the intermediate transfer belt61remains zero even if the output levels of the optical sensors24eand24fare reduced due to a smear. Thus, with this embodiment, the correspondence between the normal position and the output level of the differential signal is maintained even if the optical sensors24eand24fare smeared over time. This in turn makes it possible to reduce the frequency of maintenance for adjusting the output levels of the optical sensors or cleaning the smear.

Next, a first variation of the belt meandering preventing method of the above embodiment is described.

In the first variation, descriptions of components and processes that are the same as the above embodiment are omitted.

FIG. 11is a drawing illustrating a configuration of two slits24iand24jprovided in a light-shielding part24dof an arm part of the first variation in comparison with the positions of the light-receiving parts24hof the optical sensors24eand24f.

The first variation is different from the above embodiment in that the slits24iand24jare formed in the light-shielding part24d. In the first variation, a distance d2between the slits24iand24jis less than a distance d1between the light-receiving parts24hof the optical sensors24eand24f. However, as long as the detection ranges (the ranges within which the output levels vary according to the displacement of the intermediate transfer belt61in the belt width direction) of the optical sensors24eand24foverlap each other and the overlapping detection range has a desired width as the high-resolution detection range, the distance d2between the slits24iand24jmay not be less than the distance d1between the light-receiving parts24hof the optical sensors24eand24f.

Also, a width Ws (the length in the rotational direction of the arm part) of the slits24iand24jis greater than the width (the length in the rotational direction of the arm part) of the light receiving parts24hof the optical sensors24eand24f.

FIG. 12Ais a graph showing the sensor outputs Va and Vb of the first and second optical sensors24eand24f. InFIG. 12A, the horizontal axis indicates the amount of belt displacement (displacement in the clockwise direction inFIG. 8is represented by a change in the plus direction and displacement in the counterclockwise direction inFIG. 8is represented by a change in the minus direction); and the vertical axis indicates the output levels of the sensor outputs Va and Vb.

FIG. 12Bis a graph showing the difference (Va−Vb) between the sensor outputs Va and Vb of the optical sensors24eand24f.

FIG. 12Cis a graph showing the sum (Va+Vb) of the sensor outputs Va and Vb of the optical sensors24eand24f.

FIG. 13Ais a drawing illustrating the position of the light-shielding part24drelative to the light-receiving parts24hof the optical sensors24eand24fwhen the position of the intermediate transfer belt61in the belt width direction is in the high-resolution detection range C of the edge sensor24.

FIG. 13Bis a drawing illustrating the position of the light-shielding part24drelative to the light-receiving parts24hof the optical sensors24eand24fwhen the position of the intermediate transfer belt61in the belt width direction is in the range D that is beyond the high-resolution detection range C of the edge sensor24in the plus direction.

FIG. 13Cis a drawing illustrating the position of the light-shielding part24drelative to the light-receiving parts24hof the optical sensors24eand24fwhen the position of the intermediate transfer belt61in the belt width direction is in an error range F that is beyond the range D in the plus direction.

In the first variation, as shown inFIGS. 12A through 12C, five ranges are defined in association with the positional relationships between the slits24iand24jand the light-receiving parts24hof the optical sensors24eand24f. The high-resolution detection range C corresponds to the overlapping detection range of the optical sensors24eand24fwhere the sensor outputs Va and Vb are both greater than a threshold Vth. In the range D, the sensor output Vb is greater than the threshold Vth but the sensor output Va is less than or equal to the threshold Vth. In the range E, the sensor output Va is greater than the threshold Vth but the sensor output Vb is less than or equal to the threshold Vth. In error ranges F and G, both of the sensor outputs Va and Vb are less than or equal to the threshold Vth. When the sensor outputs are in the high-resolution detection range C, portions of the light-receiving parts24hof the optical sensors24eand24fare in the corresponding slits24iand24j. When the sensor outputs are in the range D, the entire light-receiving part24hof the optical sensor24fis in the slit24j. When the sensor outputs are in the range E, the entire light-receiving part24hof the optical sensor24eis in the slit24i. When the sensor outputs are in the range F or G, neither of the light-receiving parts24hof the optical sensors24eand24fare in the slits24iand24j.

In the first variation, when the intermediate transfer belt61is in the normal position (the amount of belt displacement=0) in the belt width direction, the light-shielding part24dis positioned as shown inFIG. 13Awith respect to the light-receiving parts24hof the optical sensors24eand24f. If the intermediate transfer belt61is displaced in the plus direction of belt displacement from the normal position, a part of the sensor range of the second optical sensor24fenters the slit24jof the light-shielding part24d; and if the intermediate transfer belt61further shifts in the plus direction, the entire sensor range (the entire light-receiving part) of the second optical sensor24fenters the slit24jas shown inFIG. 13B. Then, if the intermediate transfer belt61is displaced in the plus direction furthermore, the sensor range of the second optical sensor24fexits from the slit24jand is shielded by the light-shielding part24das shown inFIG. 13C. The position of the light-shielding part24dwith respect to the light-receiving parts24hof the optical sensors24eand24falso changes in a similar manner when the intermediate transfer belt61moves from the normal position in the minus direction of belt displacement.

As shown byFIG. 12C, the first variation makes it possible to determine that the sensor outputs are in the error range F or G based on the sum (Va+Vb) of the sensor outputs Va and Vb of the optical sensors24eand24f. Even if the intermediate transfer belt61is displaced beyond the high-resolution detection range C of the edge sensor24, the intermediate transfer belt61can be returned to the high-resolution detection range C by tilting the steering roller63as long as it is in the range D or E and the belt meandering can be prevented without stopping the intermediate transfer belt61. However, if an overrun occurs and the intermediate transfer belt61is displaced to the error range F or G where the intermediate transfer belt61may be damaged or come off the support rollers, it is preferable to stop the intermediate transfer belt61and perform maintenance. The first variation makes it possible to correctly detect the overrun of the intermediate transfer belt61without being influenced by noise by monitoring an event where the sum (Va+Vb) of the sensor outputs Va and Vb becomes lower than or equal to the threshold Vth.

FIG. 14is a flowchart showing a control process for preventing the belt meandering according to the first variation.

Details of steps that are the same as those inFIG. 10are omitted in the descriptions below.

In the belt meandering preventing method of the first variation, if the sum (Va+Vb) of the sensor outputs Va and Vb of the optical sensors24eand24fis less than or equal to the threshold Vth (YES in S21), it is assumed that the position of the intermediate transfer belt61in the belt width direction is in the error range F or G (an overrun has occurred). In this case, error information indicating the overrun is reported to an upper controller and an overrun error process is performed to stop the intermediate transfer belt61(S22). This configuration makes it possible to perform maintenance to correct the overrun.

Next, a second variation of the belt meandering preventing method of the above embodiment is described.

Descriptions of components and processes that are the same as those of the above embodiment and the first variation are omitted here.

FIG. 15is an elevational view of an edge sensor124according to the second variation.

FIG. 16is a side view of the edge sensor124according to the second variation.

The edge sensor124of the second variation is different from the edge sensor24described above in that a contact part in contact with the side (edge) of the intermediate transfer belt61is implemented by a contact pin124kextending from one end of an L-shaped arm part in the axis direction of a spindle124c. Let us assume that the intermediate transfer belt61is a resin film having a thickness of 0.05-0.1 mm and made of a high-strength material such as polyimide. In this case, if the contact pin124kis made of a normal resin material, the contact pin124kmay be abraded over time by friction with the side of the intermediate transfer belt61and it becomes difficult to correctly detect the belt displacement. For this reason, the contact pin124kis preferably made of metal that is hardly abraded by friction with the side of the intermediate transfer belt61. Also, if the contact pin124kis configured to rotate to prevent the abrasion, it is difficult to obtain accurate detection results. Therefore, the contact pin124kis preferably fixed so as not to rotate.

In the edge sensor124of the second variation, two optical sensors are implemented by one light-emitting part124hand a bipartite light-receiving element including light-receiving areas124eand124f. A slit124iis formed in a light-shielding part124d. The width (or the length in the direction of rotation of the arm part around the spindle124c) of the slit124iis substantially the same as the width of each of the light-receiving areas124eand124f. In the second variation, sensor outputs similar to the sensor outputs Va and Vb in the first variation are output from the light-receiving areas124eand124fof the bipartite light-receiving element. Therefore, it is possible to perform a belt meandering preventing process and an overrun error process in a manner similar to the first variation.

Also, since two optical sensors of the edge sensor124of the second variation are implemented by one light-emitting part and two light-receiving areas, it is possible to reduce the costs of the edge sensor124.

To perform the overrun error process, it is necessary to configure the edge sensor124to be able to determine whether the position of the intermediate transfer belt61in the belt width direction is in the error range F or G based on the sum (Va+Vb) of the sensor outputs Va and Vb and the threshold Vth. In the second variation, the width of the slit124iand the total width of the light-receiving areas124eand124fare adjusted such that it is possible to determine whether the position of the intermediate transfer belt61in the belt width direction is in the error range F or G based on the sum (Va+Vb) of the sensor outputs Va and Vb and the threshold Vth.

Also, the width of the high-resolution detection range C of the edge sensor124can be adjusted by adjusting the width of the slit124i.

Next, a third variation of the belt meandering preventing method of the above embodiment is described.

In the third variation, a light-shielding part of an edge sensor224includes one slit224ias in the second variation. However, the edge sensor224of the third variation includes three optical sensors, and light-receiving parts of the three optical sensors are implemented by light-receiving areas224a,224b, and224cof a tripartite light-receiving element. Other configurations of the third variation are substantially the same as those of the second variation, and descriptions of those configurations are omitted here.

FIGS. 17A through 17Care drawings illustrating a tripartite light-receiving element of the edge sensor224of the third variation.

FIG. 18Ais a graph showing a sensor output Va from a first light-receiving area224a, a sensor output Vb from a second light-receiving area224b, and a sensor output Vc from a third light-receiving area224c. InFIG. 12A, the horizontal axis indicates the amount of belt displacement (displacement in the clockwise direction inFIG. 8is represented by a change in the plus direction and displacement in the counterclockwise direction inFIG. 8is represented by a change in the minus direction); and the vertical axis indicates the output levels of the sensor outputs Va, Vb, and Vc.

FIG. 18Bis a graph showing a difference (Va−Vb) between the sensor output Va of the first light-receiving area224aand the sensor output Vb of the second light-receiving area224band a difference (Vb−Vc) between the sensor output Vb of the second light-receiving area224band the sensor output Vc of the third light-receiving area224c.

InFIGS. 17A through 17C, an area surrounded by a dotted line indicates the position of the slit224i.

In the third variation, as shown inFIGS. 18A and 18B, five ranges are defined in association with the positional relationships between the slit224iand the light-receiving areas224a,224b, and224c. The five ranges include a high-resolution detection range C1where the detection ranges of the first light-receiving area224aand the second light-receiving area224boverlap each other; a high-resolution detection range C2where the detection ranges of the second light-receiving area224band the third light-receiving area224coverlap each other; a control unnecessary range H between the two high-resolution detection ranges C1and C2; a range D corresponding to a case where the intermediate transfer belt61shifts in the plus direction of belt displacement beyond the high-resolution detection range C2of the edge sensor224; and a range E corresponding to a case where the intermediate transfer belt61shifts in the minus direction of belt displacement beyond the high-resolution detection range C1of the edge sensor224.

When the position of the intermediate transfer belt61in the belt width direction is in the high-resolution detection range C1or C2, the edge sensor224of the third variation outputs a differential signal (Va−Vb) or a differential signal (Vb−Vc). Thus, in the high-resolution detection areas C1and C2, it is possible to detect the amount of belt displacement at a high detection resolution as in the second variation.

When the position of the intermediate transfer belt61in the belt width direction is in the control unnecessary range H, it is assumed that the intermediate transfer belt61is near the normal position in the belt width direction. Therefore, in the third variation, no steering control process is performed when the position of the intermediate transfer belt61in the belt width direction is in the control unnecessary range H, i.e., when the amount of displacement is allowable. Alternatively, the steering control unit21may be configured to perform a steering control process based on the sensor output Va of the first light-receiving area224aand/or the sensor output Vc of the light-receiving area224ceven when the position of the intermediate transfer belt61in the belt width direction is in the control unnecessary range H. In this case, it is not possible to achieve a detection resolution as high as that in the high-resolution detection ranges C1and C2. Still, however, it is possible to perform an effective steering control process since the amount of belt displacement is small.

When the position of the intermediate transfer belt61in the belt width direction is in the range D or E, the belt meandering preventing process is performed in a manner similar to the second variation.

Next, a fourth variation of the belt meandering preventing method of the above embodiment is described.

In the fourth variation, a width Ds (or the length in the direction of rotation of the arm part around the spindle124c) of the slit124iand a width Dp of each of the light-receiving areas124eand124fare optimized. Other configurations are substantially the same as those of the second variation. In the fourth variation, it is assumed that the width Dp the light-receiving area124eand the width Dp of the light-receiving area124fare the same.

FIG. 19is a drawing used to describe the width Ds (the length in the vertical direction) of the slit1241and the width Dp (the length in the vertical direction) of the light-receiving areas124eand124fin comparison with each other.

In the fourth variation, the width Ds of the slit124iand the width Dp of the light-receiving areas124eand124fare determined to satisfy preferably formula (1), more preferably formula (2), and still more preferably formula (3) below.
Dp≦Ds≦2×Dp(1)
1.5×Dp<Ds<1.8×Dp(2)
Ds≈1.7×Dp(3)

FIG. 20A through 20Dare graphs showing approximate output levels of the sensor outputs Va and Vb under conditions (A) through (D) indicating different relationships between the slit width Ds of the slit124iand the width Dp of the light-receiving areas124eand124f.

The condition (A) is Ds<Dp; the condition (B) is Ds=Dp; the condition (C) is Ds=2×Dp; and the condition (D) is Ds>2×Dp.

Under the condition (A) (Ds<Dp), the waveforms of both of the sensor outputs Va and Vb have trapezoidal shapes as shown inFIG. 20A.

After the leading edge of the slit124iin the moving direction reaches the first light-receiving area124e, the received light intensity of the first light-receiving area124egradually increases and therefore the sensor output Va gradually increases. Under the condition (A), the width Ds of the slit124iis less than the width Dp of the first light-receiving area124e. Therefore, even when the entirety of the slit124ioverlaps the first light-receiving area124e, the sensor output Va does not reach a maximum received light intensity Vmax that is output when light is received by the entire first light-receiving area124e. Instead, the sensor output Va remains at a constant voltage less than Vmax until the leading edge of the slit124imoves outside of the first light-receiving area124e. After the leading edge of the slit124iin the moving direction moves outside of the first light-receiving area124e, the received light intensity of the first light-receiving area124egradually decreases and therefore the sensor output Va gradually decreases.

Meanwhile, after the leading edge of the slit124iin the moving direction reaches the second light-receiving area124f, the received light intensity of the second light-receiving area124fgradually increases and therefore the sensor output Vb gradually increases. Thus, the sensor output Vb shows the same waveform as that of the sensor output Va.

Under the condition (A), the distance that the slit124imoves from the start of the sensor output Va until the end of the sensor output Vb is represented by 2×Dp+Ds.

Under the condition (B) (Ds=Dp), the waveforms of both of the sensor outputs Va and Vb have triangular shapes as shown inFIG. 20B.

After the leading edge of the slit124iin the moving direction reaches the first light-receiving area124e, the received light intensity of the first light-receiving area124egradually increases and therefore the sensor output Va gradually increases. Under the condition (B), since the width Ds of the slit124iis the same as the width Dp of the first light-receiving area124e, the entirety of the first light-receiving area124ecan fit in the slit124i. Therefore, light can be received by the entire first light-receiving area124and the sensor output Va can reach the maximum received light intensity Vmax. However, since the width Ds of the slit124iis the same as the width Dp of the first light-receiving area124e, a portion of the light-shielding part124dfollowing the rear edge of the slit124ireaches the first light-receiving area124eimmediately after the entire first light-receiving area124eenters the slit124i. Therefore, the sensor output Va starts to decrease immediately after it reaches the maximum received light intensity Vmax.

Meanwhile, after the leading edge of the slit124iin the moving direction reaches the second light-receiving area124f, the received light intensity of the second light-receiving area124fgradually increases and therefore the sensor output Vb gradually increases. Thus, the sensor output Vb shows the same waveform as that of the sensor output Va. Under the condition (B), the distance that the slit124imoves from the start of the sensor output Va until the end of the sensor output Vb is represented by 3×Dp (=3×Ds).

Under the condition (C) (Ds=2×Dp), similarly to the condition (A), the waveforms of both of the sensor outputs Va and Vb have trapezoidal shapes as shown inFIG. 20C. However, under the condition (C), the height of the waveforms (maximum sensor output level) is greater than that in the condition (A).

After the leading edge of the slit124iin the moving direction reaches the first light-receiving area124e, the received light intensity of the first light-receiving area124egradually increases and therefore the sensor output Va gradually increases. Under the condition (B), since the width Ds of the slit124iis greater than the width Dp of the first light-receiving area124e, light can be received by the entire first light-receiving area124while the first light-receiving area124eis in the slit124i. Therefore, the sensor output Va can reach the maximum received light intensity Vmax. Also under the condition (C), since the width Ds of the slit1241is two times greater than the width Dp of the first light-receiving area124e, the entire first light-receiving area124eremains in the slit124ifrom when the first light-receiving area124eenters the slit124iuntil the slit124imoves a distance corresponding to the width Dp of the first light-receiving area124e. Therefore, the sensor output Va remains at Vmax until a portion of the light-shielding part124dfollowing the rear edge of the slit124ireaches the first light-receiving area124eand gradually decreases thereafter.

Meanwhile, after the leading edge of the slit124iin the moving direction reaches the second light-receiving area124f, the received light intensity of the second light-receiving area124fgradually increases and therefore the sensor output Vb gradually increases. Thus, the sensor output Vb shows the same waveform as that of the sensor output Va.

Under the condition (C), when the center of the slit124iin the moving direction is at the boundary between the light-receiving areas124eand124f, both of the light-receiving areas124eand124fenter the slit124iand both of the sensor outputs Va and Vb reach the maximum received light intensity Vmax.

Also, under the condition (B), the distance that the slit124imoves from the start of the sensor output Va until the end of the sensor output Vb is represented by 4×Dp (=2×Ds).

Under the condition (D) (Ds>2×Dp), similarly to the condition (C), the waveforms of both of the sensor outputs Va and Vb have trapezoidal shapes as shown inFIG. 20D. However, under the condition (D), the sensor outputs Va and Vb remain at the maximum received light intensity Vmax concurrently for a certain period of time.

Under the condition (D), since the width Ds of the slit124iis greater than two-fold of the width Dp of the first light-receiving area124e, the entirety of both of the first light-receiving area124eand the second light-receiving area124fcan fit in the slit124ifor a period of time. Therefore, both of the sensor outputs Va and Vb remain at the maximum received light intensity Vmax for a period of time.

Under the condition (D), the distance that the slit124imoves from the start of the sensor output Va until the end of the sensor output Vb becomes greater than 4×Dp (=2×Ds).

In the fourth variation, similarly to the above embodiment, a difference (Vb−Va) between the sensor outputs Va and Vb is obtained and the differential signal (combined signal) is output to the steering control unit21to perform the belt meandering preventing process. The fourth variation makes it possible to improve the detection resolution near the center of the entire detection range and thereby to provide a high-resolution detection range and also makes it possible to detect the amount of belt displacement in adjacent ranges beyond the high-resolution range.

Under the condition (A) where Ds<Dp, the maximum levels of the sensor outputs Va and Vb are lower than the maximum received light intensity Vmax. Therefore, under the condition (A), the detection resolution based on the differential signal (Vb−Va) is lower than the detection resolutions under the conditions (B), (C), and (D) where the sensor outputs Va and Vb reach the maximum received light intensity Vmax. Also under the condition (A), the width Ds of the slit124iis smaller than the width Dp of the light-receiving areas124eand124f. Therefore, a detection possible range (a range from the start point of the sensor output Va to the end point of the sensor output Vb) under the condition (A) is narrower than the detection possible ranges under the conditions (B), (C), and (D) where the width Ds of the slit124iis greater than or equal to the width Dp of the light-receiving areas124eand124f.

Under the condition (D) where Ds>2×Dp, since the sensor outputs Va and Vb reach the maximum received light intensity Vmax as shown inFIG. 20D, it is possible to achieve a high detection resolution based on the differential signal (Vb−Va). However, under the condition (D), an output matching range (with a certain width) where both the sensor outputs Va and Vb remain at Vmax exists near the center of the detection possible range (i.e., near the center of the high-resolution detection range). In the output matching range, since the gradient of the differential signal (Vb−Va) (i.e., the detection resolution) becomes zero, it is not possible to detect the position of the intermediate transfer belt61in the belt width direction. That is, under the condition (D), a range where the position of the intermediate transfer belt61is not detectable exists in the high-resolution detection range.FIG. 21is a graph showing actual sensor outputs Va and Vb under the condition (D); andFIG. 22is a graph showing a differential signal (Vb−Va) between the actual sensor outputs Va and Vb shown inFIG. 21. FromFIG. 22, it is apparent that the position of the intermediate transfer belt61is not detectable near the center of the total detection range (i.e., near the center of the high-resolution detection range).

Meanwhile, under the condition (B) where Ds=Dp and the condition (C) where Ds=2×Dp, since the sensor outputs Va and Vb reach the maximum received light intensity Vmax as shown inFIGS. 20B and 20C, it is possible to achieve a high detection resolution based on the differential signal (Vb−Va). Also under the conditions (B) and (C), the output matching range where both the sensor outputs Va and Vb remain at Vmax does not exist near the center of the detection possible range and therefore the gradient of the differential signal (Vb−Va) (i.e., the detection resolution) does not become zero in the high-resolution detection range. Thus, with the conditions (B) and (C), it is possible to detect the position of the intermediate transfer belt61at a high detection resolution within the entire high-resolution detection range.

For the above reasons, in the fourth variation, the width Ds of the slit124iand the width Dp of the light-receiving areas124eand124fare determined to satisfy formula (1) Dp≦Ds≦2×Dp described above.

FIG. 23Ais a graph showing approximate output levels of sensor outputs Va and Vb when formula (3) which is more preferable than formula (1) is true.

FIG. 23Bis a graph showing a differential signal (Vb−Va) between the sensor outputs Va and Vb shown inFIG. 23A.

When the condition (B) Ds=Dp and the condition (C) Ds=2×Dp are compared, the detection resolution in the high-resolution detection range C obtained based on the differential signal (Vb−Va) under the condition (B) is greater than that under the condition (C), but the width of the high-resolution detection range C under the condition (B) is narrower than that under the condition (C). That is, under the condition of formula (1) (Dp≦Ds≦2×Dp), the detection resolution becomes higher and the width of the high-resolution detection range C becomes narrower as the relationship between the width Ds of the slit124iand the width Dp of the light-receiving areas124eand124fbecomes closer to the condition (B); and the detection resolution becomes lower and the width of the high-resolution detection range C becomes wider as the relationship becomes closer to the condition (C). Meanwhile, under the condition of formula (1) (Dp≦Ds≦2×Dp), normal-resolution detection ranges I1and I2providing a detection resolution corresponding to the detection resolution of the respective sensor outputs Va and Vb are present adjacent to the high-resolution detection range C. The normal-resolution detection ranges I1and I2do not provide a detection resolution as high as that in the high-resolution detection range C, but still provide a detection resolution that corresponds to the detection resolution of the respective sensor outputs Va and Vb. Also, under the condition of formula (1) (Dp≦Ds≦2×Dp), the normal-resolution detection ranges I1and I2become wider as the high-resolution detection range C becomes narrower. Therefore, it is possible to adjust the balance between the width of the high-resolution detection range C and the width of the detection possible range by adjusting the relationship between the width Ds of the slit124iand the width Dp of the light-receiving areas124eand124fwithin the range of formula (1) (Dp≦Ds≦2×Dp). In the fourth variation, the optimum relationship between the width of the high-resolution detection range C and the width of the detection possible range is achieved when formula (3) (Ds≈1.7×Dp) is satisfied.

Next, a fifth variation of the belt meandering preventing method of the above embodiment is described.

In the fifth variation, instead of the differential signal (Vb−Va) between the sensor outputs Va and Vb, a signal representing a ratio ((Va−Vb)/(Va+Vb)) of the difference (Va−Vb) between the sensor outputs Va and Vb to the sum (Va+Vb) of the sensor outputs Va and Vb is used by the steering control unit21for the belt meandering preventing process. Other configurations are substantially the same as those of the fourth variation.

FIG. 24Ais a graph showing exemplary output levels of the sensor outputs Va and Vb.

FIG. 24Bis a graph showing a differential signal (Va−Vb) and a sum signal (Va+Vb) of the sensor outputs Va and Vb shown inFIG. 24A.

FIG. 24Cis a graph showing a ratio (Va−Vb)/(Va+Vb) of the differential signal (Va−Vb) to the sum signal (Va+Vb) shown inFIG. 24B.

As described above, when the belt meandering preventing process is performed using a differential signal under the condition of formula (1), the normal-resolution detection ranges I1and I2providing a detection resolution lower than the detection resolution of the high-resolution detection range C are present adjacent to the high-resolution detection range C. Therefore, the detection resolution changes at the boundary between the high-resolution detection range C and the normal-resolution detection ranges I1and I2and as a result, the linearity of the detection resolution in the detection possible range becomes poor. In an actual case, even if the detection resolution is high in a part of the detection possible range but the linearity of the detection resolution in the detection possible range is poor, it is difficult to stably perform the belt meandering preventing process.

For this reason, in the fifth variation, a ratio signal ((Va−Vb)/(Va+Vb)) indicating a ratio of the differential signal (Va−Vb) to the sum signal (Va+Vb) is used instead of the differential signal (Vb−Va) to perform the belt meandering preventing process. As shown inFIG. 24B, the level of the sum signal (Va+Vb) is high at its center and the gradient of the sum signal is small near the highest point. Because of the small gradient of the sum signal (Va+Vb), the detection resolution near the center of the ratio signal ((Va−Vb)/(Va+Vb)) is lower than the detection resolution in the high-resolution detection range C of the differential signal (Va−Vb). Meanwhile, the detection resolution in other parts of the detection possible range of the ratio signal ((Va−Vb)/(Va+Vb)) is higher than the detection resolution in the normal-resolution detection ranges I1and I2(i.e., a detection resolution corresponding to the detection resolution of the respective sensor outputs Va and Vb). Therefore, it is possible to obtain a signal providing a high detection resolution and good linearity in the entire detection possible range by adjusting the range near the highest point of the sum signal (Va+Vb) where the gradient is small to match the high-resolution detection range C at the center of the differential signal (Va−Vb).

Next, a sixth variation of the belt meandering preventing method of the above embodiment is described. In the sixth variation, the belt meandering preventing method of the fourth variation or the fifth variation is performed by using the light-receiving parts24hof the optical sensors24eand24fdescribed in the first variation instead of the bipartite light-receiving element. The relationship among the distance d1between the light-receiving parts24hof the optical sensors24eand24f, the distance d2between the slits24iand24j, and the width Dp of the light-receiving areas124eand124fare determined to satisfy formula (4) below.
d2−d1=Dp(4)

As a result, the positional relationships between the slits24iand24jand the light-receiving parts24hof the optical sensors24eand24fbecome as shown inFIG. 25, and the sensor outputs of the optical sensors24eand24fbecome similar to the sensor outputs in the fourth variation or the fifth variation.

Next, a seventh variation of the belt meandering preventing method of the above embodiment is described.

In the seventh variation, in addition to a threshold Vth (hereafter called an overrun threshold Vth) for detecting an overrun, a second threshold Vthsens (hereafter called a sensor failure threshold Vthsens) for detecting failure of sensors is used in the belt meandering preventing process performed by the steering control unit21. Other configurations are substantially the same as those of the first variation.

FIG. 26Ais a graph where the horizontal axis indicates the amount of belt displacement and the vertical axis indicates output levels of the sensor outputs Va and Vb of the optical sensors24eand24f.

FIG. 26Bis a graph showing the sum (Va+Vb) of the sensor outputs Va and Vb and the thresholds Vth and Vthsens.

In the seventh variation, as shown inFIGS. 26A and 26B, two thresholds Vth and Vthsens are used and seven ranges are defined in association with the positional relationships between the slits24iand24jand the light-receiving parts24hof the optical sensors24eand24f. In other words, two ranges are added to the five ranges defined in the first variation. The seven ranges include a high-resolution detection range C that corresponds to the overlapping detection range of the optical sensors24eand24fwhere the sensor outputs Va and Vb are both greater than the overrun threshold Vth; a range D where the sensor output Vb is greater than the overrun threshold Vth but the sensor output Va is less than or equal to the overrun threshold Vth; a range E where the sensor output Va is greater than the overrun threshold Vth but the sensor output Vb is less than or equal to the overrun threshold Vth; overrun ranges F′ and G′ where both of the sensor outputs Va and Vb are greater than the sensor failure threshold Vthsens but are less than or equal to the overrun threshold Vth; and failure ranges J and K where both of the sensor outputs Va and Vb are less than or equal to the sensor failure threshold Vthsens.

Thus, in the seventh variation, the error ranges in the first variation are further divided into the overrun ranges F′ and G′ and the sensor failure ranges J and K so that the cause of an error can be narrowed down based on sensor outputs.

FIG. 27is a flowchart showing a control process for preventing the belt meandering according to the seventh variation.

In the seventh variation, after the amount of belt displacement is detected (S4), the steering control unit21determines whether the sum (Va+Vb) of the sensor outputs Va and Vb of the optical sensors24eand24fis greater than the sensor failure threshold Vthsens (S31). If the sum (Va+Vb) is less than or equal to the sensor failure threshold Vthsens (NO in S31), the steering control unit21performs a sensor error process to output failure information indicating sensor failure (S32) and thereby to allow the user, for example, to replace the optical sensors24eand24f. If the sum (Va+Vb) is greater than the sensor failure threshold Vthsens (YES in S31), the steering control unit21determines whether the sum (Va+Vb) is greater than the overrun threshold Vth (S33). If the sum (Va+Vb) is less than or equal to the overrun threshold Vth (NO in S33), the steering control unit21performs the overrun error process described in the first variation (S34). If the sum (Va+Vb) is greater than the overrun threshold Vth (YES in S33), the steering control unit21controls the steering motor23as described in the above embodiment.

The printer according to the embodiment and its variations described above includes the intermediate transfer belt61that is an endless belt stretched over the support rollers63,67,68,69, and71. An image formed on the outer surface of the intermediate transfer belt61is transferred onto the recording paper P. The printer also includes a belt meandering preventing device including the edge sensor24(124,224) used as a belt displacement detection unit for detecting the amount of belt displacement of the intermediate transfer belt61in the belt width direction and the steering control unit21used as a belt meandering correction unit for correcting the displacement of the intermediate transfer belt61in the belt width direction based on the detected amount of belt displacement. The edge sensor24(124,224) includes the arm part used as a moving part that moves in association with the displacement of the intermediate transfer belt61in the belt width direction; and the optical sensors24eand24f(124eand124f;224a,224b, and224c) that output signals with output levels corresponding to the proportions of the light-shielding part24d(124d) of the arm part in the optical paths of the optical sensors. The optical sensors are arranged such that their output levels change as the intermediate transfer belt61is displaced in the belt width direction within the high-resolution detection range C (C1, C2). The output signals of the optical sensors are combined such that the rate of change of the output level of the combined signal Vb−Va (Va−Vb, Vb−Vc) with respect to the amount of belt displacement of the intermediate transfer belt61in the belt width direction within the high-resolution detection range C (C1, C2) becomes greater than the rates of change of the output levels of the respective optical sensors. The combined signal is output as the amount of belt displacement (belt position information). This configuration makes it possible to achieve a wide high-resolution detection range C (C1, C2) with a high detection resolution using the edge sensor24(124,224) implemented by inexpensive optical sensors.

Also, according to the embodiment and its variations described above, each of the optical sensors24eand24f(124eand124f;224a,224b, and224c) of the edge sensor24(124,224) is configured to output a signal with the maximum output level when the position of the intermediate transfer belt61in the belt width direction is in a range that is beyond one end of the high-resolution detection range C (C1, C2) and to output a signal with the minimum output level when the position of the intermediate transfer belt61in the belt width direction is in a range that is beyond the other end of the high-resolution detection range C (C1, C2). This configuration makes it possible to determine the direction of displacement of the intermediate transfer belt61based on the sensor outputs of the optical sensors without using additional sensors even when the intermediate transfer belt61is displaced beyond the high-resolution detection range C (C1, C2). Accordingly, even if the intermediate transfer belt61is displaced beyond the high-resolution detection range C (C1, C2), this configuration makes it possible to correct the displacement of the intermediate transfer belt61to return the intermediate transfer belt61to the high-resolution detection range C (C1, C2), instead of stopping the intermediate transfer belt61and performing maintenance. Thus, this configuration makes it possible to reduce the frequency of maintenance.

According to the second and third variations, the optical sensors of the edge sensor124(224) are implemented by an optical sensor unit including one light-emitting part124hand the light-receiving parts124eand124f(224a,224b, and224c) that output signals with output levels corresponding to the proportions of the light-shielding part124dof the arm part in the optical paths of light emitted from the light-emitting part124h. This configuration makes it possible to reduce the costs of the edge sensor124(224).

In the first and second variations, the light-shielding part24d(124d) may have the light-passing slits24iand24j(or the light-passing slit124i); and the optical sensors24eand24f(124eand124f) of the edge sensor24(124) may be implemented by transmissive optical sensors that output signals with output levels corresponding to the proportions of the light-shielding part24d(124d) shielding the optical paths. The transmissive optical sensors may be arranged such that when the rear edge of one of the light-passing slits, in terms of the moving direction of the light-shielding part24d(124d) when the intermediate transfer belt61is displaced in one belt width direction in the high-resolution detection range C, is substantially at the center of one of the transmissive optical sensors, the leading edge of the same light-passing slit or the leading edge of the other light-passing slit is located substantially at the center of the other one of the transmissive optical sensors. This configuration makes it possible to maximize the width of the high-resolution detection range C.

If the output levels of both of the transmissive optical sensors24eand24f(124eand124f) are greater than the threshold Vth, a combined signal, which is a differential signal (Vb−Va, Va−Vb) between the output signals of the transmissive optical sensors24eand24f(124eand124f), is used to determine the amount of belt displacement (belt position information). Meanwhile, if one of the output levels of the transmissive optical sensors24eand24f(124eand124f) is less than or equal to the threshold Vth, a signal with the maximum output level of one of the transmissive optical sensors24eand24f(124eand124f) having the higher output level is used to determine the amount of belt displacement (belt position information). Using the threshold Vth makes it possible to remove noise from sensor outputs and thereby to stably perform a belt meandering preventing process.

Also, if both of the output levels of the transmissive optical sensors24eand24f(124eand124f) are less than or equal to the threshold Vth, the steering control unit21outputs an error signal indicating a sensor error, i.e., functions as an error signal outputting unit. The error signal allows the user to perform maintenance to correct the sensor error.

Also in the first and second variations, when both of the output levels of the transmissive optical sensors24eand24f(124eand124f) are greater than the threshold Vth, an adjustment signal, which is the sum signal (Va+Vb) of the outputs signals of the transmissive optical sensors24eand24f(124eand124f), may be generated and the light intensity of the light-emitting parts24g(or the light-emitting part124h) may be adjusted based on the adjustment signal. In this case, the steering control unit21also functions as a light intensity adjusting unit.

In the fourth variation, the edge sensor124includes the light-receiving parts124eand124fthat output signals with output levels corresponding to the proportions of the light-shielding part124dhaving the light-passing slit124iin the optical paths of light emitted from the light-emitting part124h. The light-receiving parts124eand124fare arranged next to each other along the moving direction of the light-shielding part124d(or the slit124i). The edge sensor124is configured such that the length Dp of the light-receiving parts124eand124fin the moving direction of the slit124i(or the light-shielding part) and the length Ds of the slit124iin the moving direction of the slit124isatisfy a condition Dp≦Ds≦2×Dp and more preferably a condition 1.5×Dp<Ds<1.8×Dp. With this configuration, the high-resolution detection range has a detection resolution that is sufficient but lower than the maximum detection resolution. This in turn makes it possible to increase the detection possible range and thereby makes it possible to achieve both a sufficiently high detection resolution and a wide detection possible range.

In the sixth variation, the edge sensor24includes the light-receiving parts24hthat output signals with output levels corresponding to the proportions of the light-shielding part24dhaving the light-passing slits24iand24jin the optical paths of light emitted from the light-emitting parts24g. The light-receiving parts24hare arranged apart from each other in the moving direction of the slits24iand24j(or the light-shielding part24d). The edge sensor124is configured such that the length Dp of the light-receiving parts24hin the moving direction of the slits24iand24jand the length Ds of the slits24iand24jin their moving direction satisfy a condition Dp≦Ds≦2×Dp and more preferably a condition 1.5×Dp<Ds<1.8×Dp. Also, the center distance d1between the light-receiving parts24hin the moving direction of the slits24iand24j(or the light-shielding part), the center distance d2between the slits24iand24jin the moving direction of the slits24iand24j, and the width Dp of the light-receiving parts24hare configured to satisfy a condition d2−d1=Dp. Also with this configuration, the high-resolution detection range has a detection resolution that is sufficient but lower than the maximum detection resolution. This in turn makes it possible to increase the detection possible range and thereby makes it possible to achieve both a sufficiently high detection resolution and a wide detection possible range. In the fifth variation, a combined signal (Va−Vb)/(Va+Vb) of the output signals Va and Vb from the light-receiving parts124eand124fis used to determine the amount of belt displacement. This configuration makes it possible to improve the linearity of the detection resolution in the detection possible range and thereby makes it easier to stably perform the belt meandering preventing process.

In the seventh variation, the steering control unit21generates a sum signal (Va+Vb) of the output signals from the light-receiving parts24h, compares the level of the sum signal (Va+Vb) with a threshold Vth and a sensor failure threshold Vthsens lower than the threshold Vth, and outputs failure information indicating sensor failure if the sum signal (Va+Vb) is less than the sensor failure threshold Vthsens. Thus, the steering control unit21also functions as a failure information outputting unit. This configuration makes it possible to quickly deal with sensor failure.

An aspect of the present invention makes it possible to provide a belt-meandering preventing device that can detect displacement of an endless belt in the belt width direction in a wide detection range and at a high detection resolution using multiple inexpensive sensors, and an image forming apparatus including the belt meandering preventing device.

In an embodiment of the present invention, a belt displacement detection unit includes multiple optical sensors that output signals with output levels corresponding to the proportions of a moving part in their optical paths. The moving part moves in association with the displacement of an endless belt or an edge of the endless belt in the belt width direction. The optical sensors may be implemented by inexpensive transmissive or reflective optical sensors.

In an embodiment of the present invention, output signals from the optical sensors in their detection ranges (the ranges within which the output levels vary according to the displacement of the endless belt in the belt width direction) are combined such that the rate of change of the output level of the combined signal with respect to the amount of belt displacement of the endless belt in the belt width direction (i.e., the detection resolution of the combined signal) becomes greater than the rate of change of the output levels of the respective optical sensors (i.e., the detection resolution of each output signal or each optical sensor). This configuration makes it possible to achieve a detection resolution that is higher than that provided by each optical sensor in an overlapping detection range (high-resolution detection range) where the detection ranges of the optical sensors overlap each other. In other words, an overlapping detection range of the optical sensors24eand24fwith a relatively low detection resolution and a relatively wide detection range is used to provide a wide high-resolution detection range. This configuration makes it possible to achieve a high detection resolution that is not achievable by a single optical sensor whose detection range has the same width as that of the high-resolution detection range.

At least one of the optical sensors may be configured to output a signal with the maximum output level when the position of the endless belt in the belt width direction is beyond one end of the high-resolution detection range and to output a signal with the minimum output level when the position of the endless belt in the belt width direction is beyond the other end of the high-resolution detection range. This configuration is beneficial because of the reasons described below.

Belt meandering preventing devices disclosed in JP2008-275800 and JP2005-338522 use displacement sensors to detect the displacement of an endless belt in the belt width direction. Generally, an inexpensive displacement sensor outputs a signal with the same output level (0 V) regardless of whether an object (swing arm) moves beyond the detection range in one swinging direction or the other swinging direction. Therefore, with the related-art belt meandering preventing devices, it is not possible to determine the direction of displacement of an endless belt based on sensor outputs if the endless belt moves out of a detection range where the amount of displacement of the endless belt in the belt width direction can be detected. Therefore, with the related-art belt meandering preventing devices, when an endless belt is displaced in the belt width direction beyond the detection range, it is not possible to correct the position of the endless belt and it is necessary to stop the rotation of the endless belt and perform maintenance to manually correct the position of the endless belt in the belt width direction. Particularly, when a narrow detection range is set to obtain a necessary detection resolution, the above problem increases the frequency of maintenance.

According to the above embodiment of the present invention, even if an endless belt is displaced beyond the high-resolution detection range, it is possible to determine the direction of displacement based on the output signal of at least one of the optical sensors provided to detect the amount of displacement of the endless belt in the belt width direction in the high-resolution detection range. This in turn makes it possible to correct the position of an endless belt in the belt width direction to return the endless belt to the high-resolution detection range, instead of immediately stopping the endless belt and performing maintenance, even if the endless belt is displaced beyond the high-resolution detection range. Thus, the above embodiment makes it possible to reduce the frequency of maintenance.

The present application is based on Japanese Priority Application No. 2009-264549 filed on Nov. 20, 2009 and Japanese Priority Application No. 2010-131386 filed on Jun. 8, 2010, the entire contents of which are hereby incorporated herein by reference.