Image forming apparatus for forming image on sheet

An image forming apparatus may comprise the following elements. A stacking unit in which a sheet is stacked. A conveyance unit conveys the sheet. A time-counting unit counts a conveyance time from when the conveyance unit starts to convey the sheet until the sheet arrives at a predetermined position on a conveyance path. A determination unit determines that over stacking has occurred, if the conveyance time of the sheet exceeds a first over stacking threshold value.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus for forming an image on a sheet.

Description of the Related Art

An image forming apparatus has a stacking unit for stacking sheets. As the stacking unit, a feed cassette and a manual bypass tray provided within the image forming apparatus are used. If the number of sheets that exceeds the number expected in design is stacked in these stacking units, a feeding failure may occur. Japanese Patent Laid-Open No. 05-278896 proposes an image forming apparatus that detects over stacking by measuring the height of a sheet bundle stacked in a cassette, using a sensor.

However, if the height of a bundle of envelope type sheets is detected using a sensor, over stacking is erroneously detected in some cases. Since air is likely to accumulate in an envelope type sheet, the envelope type sheet can be easily pressed down. A feed cassette is provided with a locking claw for regulating the number of stacked sheets, but a large number of envelope type sheets are forcibly over-stacked by pressing down the large number of envelope type sheets in some cases. Thus, since the height of the sheet bundle is visually low, over stacking of the sheets cannot be accurately detected even with the sensor for detecting the height of the sheet bundle.

SUMMARY OF THE INVENTION

The present invention detects over stacking of sheets more accurately than with conventional techniques. The present invention provides an image forming apparatus comprising the following elements. A stacking unit in which a sheet is stacked. A conveyance unit is configured to convey the sheet. A time-counting unit is configured to count a conveyance time from when the conveyance unit starts to convey the sheet until the sheet arrives at a predetermined position on a conveyance path. A determination unit is configured to determine that over stacking has occurred, on the conveyance time of the sheet exceeding a first over stacking threshold value.

DESCRIPTION OF THE EMBODIMENTS

[Configuration of Image Forming Apparatus]

An image forming apparatus100will be described usingFIG. 1. Although the image forming apparatus100according to this embodiment is an electrophotographic printer, an image forming apparatus to which the present invention is applicable may employ other image forming methods, such as an inkjet method and a thermal transfer method. The image forming apparatus100has four image forming units (stations), and forms toner images of yellow (Y), magenta (M), cyan (C), and black (K). InFIG. 1, referenced signs Y, M, C, and K, which are associated with the respective colors, are assigned to the four image forming units. A photosensitive drum1is a photosensitive member and an image carrier, and rotates clockwise at a predetermined circumferential speed (process speed). A charging roller2uniformly charges the surface of the photosensitive drum1. An optical scanning device9outputs a light beam in accordance with an image signal. The surface of the photosensitive drum1is irradiated with a light beam, and an electrostatic latent image is formed. Toner is attached to a development roller6to develop an electrostatic latent image, and a toner image is formed. YMCK toner images are transferred to an intermediate transfer belt12in an overlapping manner by a primary transfer roller11, and a multi-color image is obtained.

A feed cassette23is an exemplary stacking unit in which sheets are stacked. Sheets S contained in the feed cassette23are picked up by a pickup roller35, and are sent out to a conveyance path by a feed roller24. The pickup roller35and the feed roller24are each an exemplary conveyance unit for conveying a sheet. Skew correction is executed when a leading end of a sheet S abuts against a registration roller17. The sheet S is conveyed to a secondary transfer unit by the registration roller17. The toner image conveyed by the intermediate transfer belt12is subjected to secondary transfer onto the sheet by a secondary transfer roller16. A fixing device18fixes the toner image to the sheet S and discharges the sheet S to the outside of the image forming apparatus100.

A manual bypass tray38is an exemplary stacking unit in which sheets are stacked. The manual bypass tray38pivots around a fulcrum37, thereby being switched between a housed state of being housed in the image forming apparatus100and a usage state where the sheets S can be stacked therein. The sheets S stacked in the manual bypass tray38are picked up by a paper feed roller36, sent out to the conveyance path by a conveyance roller39, and moved toward the registration roller17. The paper feed roller36and the conveyance roller39are each an exemplary conveyance unit for conveying a sheet.

A controller50is a control unit for comprehensively controlling the overall image forming apparatus100. An operation unit59has a display device and an input device. The controller50detects whether or not the sheets S are stacked in the manual bypass tray38using a sheet sensor53. Furthermore, the controller50determines whether or not a conveyance delay or a jam has occurred using a sheet sensor52. A sheet sensor for detecting whether or not the sheets S are stacked may also be provided in the feed cassette23. The sheet sensor53may be called a tray sensor, and the sheet sensor52may be called a registration sensor. The sheet sensor53is for detecting the presence of the sheets S, whereas the sheet sensor52is used for detecting a leading end and a trailing end of each sheet S and detecting the conveyance time of each sheet S.

Functions of the controller50will be described usingFIG. 2. The overall image forming apparatus100is comprehensively controlled by executing control programs stored in a CPU51or a storage device55. The storage device55has a memory such as a ROM or a RAM. The controller50sets image forming conditions in accordance with an image formation mode designated through an operation unit59. The image forming conditions are, for example, the conveyance speed of sheets S, the fixing temperature of the fixing device18, and the like. For example, the image formation modes may include a plain paper mode for forming an image on plain paper, a cardboard mode for forming an image on cardboard, an envelope mode for forming characters on an envelope, and the like. The controller50stores the image forming conditions for the respective image formation modes in the storage device55, and reads out the image forming conditions corresponding to the designated image formation mode.

The CPU51detects whether or not sheets S are stacked in the manual bypass tray38using the sheet sensor53. Furthermore, the CPU51determines whether or not a conveyance delay or a jam has occurred using the sheet sensor52. If a sheet S has caused a conveyance delay or a jam on the conveyance path, the CPU51causes a message indicating the occurrence of the conveyance delay or the jam to be displayed on the operation unit59. If over stacking of sheets S in the feed cassette23or the manual bypass tray38is detected, the CPU51causes a message indicating the occurrence of the over stacking to be displayed on the operation unit59. A conveyance delay is a phenomenon in which the conveyance time of a sheet S becomes too long to be able to ensure the accuracy of an image forming position or the like, and may also be called a conveyance error or a failure in conveyance. A jam refers to, in the narrow sense, a phenomenon in which a sheet S is stuck and jammed on the conveyance path. A phenomenon in which sheets S cannot be fed from the manual bypass tray38due to over stacking is also a kind of a jam. Thus, over stacking may cause the case where a sheet S has successfully been fed from the manual bypass tray38but the conveyance time is too long, or the case where the feeding fails.

The CPU51uses an environment sensor54to acquire environmental parameters such as the absolute moisture amount, environmental temperature, and environmental humidity of the environment where the image forming apparatus100is installed. Image forming conditions for respective combinations of the image formation modes and the environmental parameters are stored in the storage device55in the controller50. The controller50reads out, from the storage device55, the image forming conditions corresponding to a combination of the designated image formation mode and the environmental parameters acquired from environment sensor54.

The CPU51outputs a feed start signal to a drive circuit56for driving a motor57. The drive circuit56, upon receiving the feed start signal, starts to drive the motor57. The CPU51sets the conveyance speed in advance corresponding to the image formation mode in the drive circuit56. The motor57rotates at a rotation speed corresponding to the set conveyance speed. The CPU51may control a solenoid or the like for driving a pickup roller35.

As shown inFIG. 3, when the leading end of the sheet S pulls down a flag46provided near the registration roller17, the sheet sensor52outputs a detection signal indicating that the leading end of the sheet S has been detected to the controller50. The flag46may be called as a flapper. The CPU51counts, using a timer or a counter, the conveyance time from the timing at which conveyance of a sheet S is started until the timing at which the sheet sensor52detects the leading end of the sheet S.

FIG. 4is a diagram showing an exemplary detection signal of the sheet sensor52. The horizontal axis indicates the time, and the vertical axis indicates the level of the detection signal. At time t1, conveyance of a first sheet S is started. At time t2, the leading end of the sheet S arrives at the sheet sensor52, and the detection signal changes from OFF to ON. The CPU51determines the period from the time t1to the time t2to be conveyance time T1of the first sheet S. At time t3, the trailing end of the first sheet S passes the sheet sensor52, and the level of the detection signal is switched from ON to OFF. At time t4, conveyance of a second sheet S is started. At time t5, the leading end of the second sheet S arrives at the sheet sensor52, and the detection signal changes from OFF to ON. The CPU51determines the period from the time t4to the time t5to be a conveyance time T2of the second sheet S. At time t6, the trailing end of the second sheet S passes the sheet sensor52, and the level of the detection signal is switched from ON to OFF.

FIG. 5Ashows exemplary conveyance time at the time when a plurality of sheets S are continuously fed. The conveyance time T may vary more or less depending on a friction state of the pickup roller35and the paper feed roller36, and the type of the sheets S (thickness, basis weight, envelope type or not, presence of surface coating etc.). However, if the plurality of sheets S are correctly stacked in the manual bypass tray38, and a jam does not occur, the conveyance time of each sheet S stays within a tolerance X.

FIG. 5Bshows an exemplary conveyance time at the time when a plurality of sheets S are continuously fed. In particular, a conveyance delay (conveyance error) has occurred on the fifth sheet S. As shown inFIG. 5B, there are cases where a conveyance delay occurs on the second and subsequent sheets S after a job has started. The CPU51monitors the conveyance time T of each sheet S, and determines whether or not the conveyance time T exceeds a threshold value Tm. If the conveyance time T exceeds the threshold value Tm, the CPU51determines that a conveyance delay has occurred, discharges all sheets S that remain on the conveyance path, and stops the motor57. The threshold value Tm for determining a conveyance delay is set to a value that deviates from the tolerance X and is larger than the upper limit value of the tolerance X. If the conveyance time exceeds the threshold value Tm, the accuracy of image formation on the sheets S is not ensured, and accordingly, the CPU51stops image formation.

The CPU51may preferentially feed the first sheet, and cause the first sheet S to wait at the registration roller17until the image forming unit is ready. In this case, the CPU51may not determine that a conveyance delay has occurred regarding the first sheet S, even if the conveyance time T exceeds the threshold value Tm. The period from when a job has started started until the image forming unit is ready is longer than a normal conveyance time T. Therefore, conveyance delay of the first sheet S is largely permissible. Thus, in this embodiment, the conveyance delay determination is applied to the second and subsequent sheets S.

However, there are cases where the first sheet S does not arrive at the flag46even though the conveyance time T when the first feeding operation for the first sheet S was performed, greatly exceeds the threshold value Tm. That is to say, there are cases where, even if the count value of the timer of the CPU51exceeds a retry threshold value Tr, the sheet sensor52cannot detect the leading end of the sheet S (the threshold value Tm may be employed as the retry threshold value Tr). In this case, the CPU51instructs the drive circuit56to perform a second feeding operation (retry). Note that the CPU51may continue, also during the retry, counting the conveyance time T starting from the first feeding operation. If the first sheet S does not arrive at the flag46even after the conveyance time T has exceeded a jam threshold value Tj, the CPU51determines that a jam has occurred. If the CPU51detects a jam, the CPU51causes a message indicating the occurrence of the jam to be displayed on the operation unit59, and transmits this message to an email address of a maintenance person via the communication device58. The message may be delivered in the form of an email, for example.

[Configuration of Manual Bypass Tray]

A configuration of the manual bypass tray38will be described in detail with reference toFIGS. 6 to 8. As already shown inFIG. 1, the manual bypass tray38is arranged in a side surface of a housing of the image forming apparatus100. In an unused state, the manual bypass tray38is housed within the housing of the image forming apparatus100. When in use, the manual bypass tray38is opened pivoting around the fulcrum37. The manual bypass tray38stops pivoting upon forming a certain angle relative to the housing side surface. As shown inFIG. 6, the manual bypass tray38has two side regulating plates43and a tray unit40. The two side regulating plates43can move on the tray unit40in a direction perpendicular to the conveyance direction, and causes both ends (left end and right end) of a sheet bundle in the direction perpendicular to the conveyance direction to be aligned. The tray unit40pivots around a fulcrum41. The tray unit40is biased in a direction approaching the paper feed roller36by a biasing mechanism (not shown). The leading ends of sheets S are regulated as a result of the sheets S falling down due to gravity and abutting against an abutting wall42.

As shown inFIG. 7A, the tray unit40is pushed down in a direction of moving away from the paper feed roller36, against the biasing mechanism by a tray control mechanism (not shown) when paper is not passed through. Thus, the sheets S stacked in the tray unit40move to a position where they are not in contact with the paper feed roller36.

As shown inFIG. 7B, when paper passes through, the tray unit40is lifted up by the biasing mechanism as the tray control mechanism retracts. Thus, the uppermost sheet S stacked in the tray unit40comes into contact with the paper feed roller36. Note that when a plurality of sheets S are continuously passed through, lifting up and lowering of the tray unit40is repeated every time one sheet S passes through. The bundle of sheets S slides down in a downward direction under its own weight due to vibration caused by lifting up and lowering of the tray unit40, and the sheets S readily abut against the abutting wall42. The tray control mechanism may be driven by a driving source (motor, solenoid, etc.) controlled by the CPU51.

As shown inFIG. 8, the manual bypass tray38may have guiding members45and locking claws44. The guiding members45are provided near the paper feed roller36. The sheets S placed on the tray unit40slide down toward the abutting wall42. At this time, the guiding members45guide the sheets S such that the leading end of the sheets S can readily go under the paper feed roller36without resting on the paper feed roller36. Furthermore, the guiding members45guide the sheets S such that the leading ends of the fed sheets S do not rise and get caught on the paper feed roller36.

The flag48of the sheet sensor53is installed upstream to the abutting wall42in the conveyance direction. Upon a sheet S being stacked in the tray unit40, the leading end of the sheet S pushes the flag48before abutting against the abutting wall42. Thus, the sheet sensor53detects that the sheet S is stacked on the tray unit40, and outputs a detection signal to the CPU51. When a sheet S is not stacked in the tray unit40, the flag48is restored to its initial position, and therefore, the sheet sensor53does not detect a sheet S. Upon the level of the detection signal from the sheet sensor53changing from a high level to a low level, the CPU51determines that the sheet S is not stacked in the tray unit40.

The locking claws44are provided respectively in the two side regulating plates43. The locking claws44regulate the sheets S so as not to ride up onto the side regulating plates43. Thus, shifting of the sheet S in the direction perpendicular to the conveyance direction is suppressed. If sheets S are over-stacked, the bundle of the sheets S comes into contact with the locking claws44. Conveyance resistance of the sheets S increases due to the friction force exerted between the sheets S and the locking claws44, and a conveyance delay of the sheets S may occur. Whether or not the uppermost sheet S of the bundle of stacked sheets S comes into contact with the locking claws44is a guide for the amount of stacked sheets that ensures correct conveyance operation.

Here, envelopes are employed as exemplary envelope type sheets. Here, a state where the envelopes are over-stacked in the manual bypass tray38will be described in detail.

Case where a Bundle of Envelopes is Stacked Under Locking Claws44

As shown inFIG. 9, the envelopes E have a bag-like shape. That is to say, each of the envelopes E has a layer of air. When a plurality of envelopes E are stacked, a bundle of envelopes E with elastic force that can be easily pressed down is formed.

As shown inFIG. 10A, since the bundle of envelopes E can be easily pressed down, even a bundle of the envelopes E whose number exceeds the upper limit number can be inserted under the locking claws44. However, the resistance force Fa generated by pressing down the bundle is exerted onto the bundle of the envelopes E from the locking claws44. The bundle enters a state of being strongly held down by the locking claws44, and a friction force between the envelopes E and the locking claws44increases.

Case where Envelopes are Stacked on Locking Claws44

As shown inFIG. 10B, if a bundle of envelopes E whose number is much larger than the upper limit number is stacked on the tray unit40, a part of the bundle is stacked so as to ride up onto the locking claws44. This part of the bundle comes into contact with the guiding member45. Since the bundle of envelopes E can be easily pressed down, the bundle can also be pushed in under the guiding members45. However, the resistance force Fb generated by pressing down the bundle of envelopes E is exerted onto the bundle from the guiding members45. The bundle enters a state of being strongly held down by the guiding member45, and a friction force is generated between the envelopes E and the guiding members45.

[Feeding of Envelopes in Over-Stacked State]

Next, the behavior of the conveyance mechanism when the envelopes E are over-stacked will be described in detail.

Over-Stacked State of Envelopes by Certain Number of Envelopes of Less

In the case where the envelopes E are over-stacked and the number of envelopes E is only slightly larger than the upper limit number determined in the design of the image forming apparatus100, the resistance force Fa generated by the locking claws44and the resistance force Fb generated by the guiding members45is small.FIG. 11shows a relationship between the number of envelopes to be passed through and the conveyance time at the time when the number of stacked envelopes E is larger than the upper limit number by two. Although a first envelope E and a second envelope E correspond to over-stacked envelopes E, the respective conveyance times of those envelopes are within the tolerance X. Accordingly, no conveyance delay occurs.

Over-Stacked State of Envelopes by Certain Number of Envelopes or More

If the number of over-stacked envelopes E is larger than or equal to the upper limit number by a certain number, the resistance force Fa and the resistance force Fb increase. As shown inFIG. 12, if the envelopes E are over-stacked under the locking claws44, the envelopes E continues to receive the resistance force Fa from the locking claws44all the way from when feeding is started until the leading end of an envelope E pulls down the flag46. If a large conveyance resistance is generated, the paper feed roller36and the conveyance roller39slip and the conveyance speed of the envelopes E decreases. As shown inFIG. 13A, the conveyance time T counted by the CPU51is longer than the conveyance time T at the time when the envelopes E whose number is smaller than or equal to the upper limit number are stacked, and deviates from the tolerance X. If a plurality of envelopes E continue to be passed through in this state, the amount of over-stacked envelopes decreases, and the conveyance time converges into the tolerance X.

As shown inFIG. 13B, if the conveyance time T deviates from the tolerance X and also exceeds the threshold value Tm, the CPU51determines that a conveyance delay (conveyance error) has occurred, and stops the motor57after discharging all envelopes E on the conveyance path.

Over-Stacked State where Many More Envelopes are Stacked

If many more envelopes E are stacked on the tray unit40, the resistance forces Fa and Fb further increase. Since the amount of slipping of the rollers also increases with an increase in the resistance forces Fa and Fb, the conveyance speed further decreases, or not even a single envelope E can be fed. In this case, the envelopes E do not reach the flag46before the conveyance time exceeds the jam threshold value Tj, and therefore, the CPU51determines that a jam has occurred, and stops the motor57.

However, in the case where preferential feeding is employed, the CPU51executes second feeding (retry) even if the first feeding of a first envelope of a job has failed. That is to say, if the envelope E does not reach the flag46for the first feeding before the conveyance time T exceeds the retry threshold value Tr, a retry is executed. If the envelope E reaches the flag46before the accumulated conveyance time T of the first envelope E exceeds the jam threshold value Tj, the CPU51continues the job. On the other hand, if the envelope E does not reach the flag46even after the conveyance time T exceeds the jam threshold value Tj, the CPU51determines that a jam has occurred on the envelope E, and stops the motor57. Thus, in the case where the conveyance time T is continuously counted from when the first feeding is started, if the envelope E does not reach the flag46even after the conveyance time T has exceeded the retry threshold value Tr, the retry is executed. Furthermore, if the envelope E does not reach the flag46even after the conveyance time T exceeds the jam threshold value Tj that is larger than the retry threshold value Tr, it is declared that a jam has occurred. Note that the retry threshold value Tr may be the same as the threshold value Tm. That is to say, the threshold value Tm may be used as the retry threshold value Tr for the first envelope E, and regarding the second and subsequent envelopes E, the threshold value Tm may be used as a threshold value for determining a conveyance delay.

A procedure for determining over stacking of the sheets S on the manual bypass tray38will be described in detail. If the bundle of sheets S receives the resistance force Fa and the resistance force Fb due to over stacking, the conveyance time T increases. Then, if the conveyance time T exceeds a threshold value Tk, which is equal to or larger than the upper limit value of the tolerance X, the CPU51determines that over stacking has occurred.

As shown inFIG. 14A, the threshold value Tk for determining over stacking is set to a value smaller than the threshold value Tm for determining a conveyance delay. Thus, the occurrence of a negligible conveyance delay that cannot be considered to be a conveyance delay is a condition for determining over stacking. The over stacking threshold value Tk is set to a value that is the same as or larger than the upper limit value of the tolerance X, and is stored in the storage device55.

As shown inFIG. 14B, at time t1, the CPU51starts to feed the first sheet S in a print job, and the CPU51starts to count the conveyance time T. If the feeding is performed normally, the leading end of the sheet S arrives at the flag46at time t2. Also, at time t5, the trailing end of the sheet S passes the flag46.

On the other hand, if the conveyance time T exceeds the over stacking threshold value Tk (i.e., if the leading end of the sheet S does not arrive at the flag46even after the time t3), the CPU51determines that over stacking has occurred. If the conveyance time T exceeds the conveyance error threshold value Tm (i.e., if the leading end of the sheet S does not arrive at the flag46even after the time t4), the CPU51may determine that a conveyance error has occurred. Note that detection of a conveyance error using the threshold value Tm is not applied to the first sheet S, but is applied to the second and subsequent sheets S. If the conveyance time T exceeds the retry threshold value Tk (i.e., if the leading end of the sheet S does not arrive at the flag46even after time t6), the CPU51determines that a feeding error has occurred. If the first feeding has failed, at time t7, the CPU51retries the feeding. If the retry is successful, at time t8, the leading end of the first sheet S arrives at the flag46. Note that if the leading end of the sheet S does not arrive at the flag46even after the conveyance time T has exceeded the jam threshold value Tj, the CPU51determines that a jam has occurred. As shown inFIG. 14B, the value of the conveyance time T at the time when a retry occurs is much larger than the normal conveyance time T at the time when a retry does not occur, and exceeds the over stacking threshold value Tk. Accordingly, even in the case where a retry has occurred due to over stacking, the CPU51can detect over stacking based on the conveyance time T.

If the conveyance time T of the second or subsequent sheet S exceeds the threshold value Tk and the threshold value Tm, the CPU51determines that a conveyance delay has occurred, and stops image formation. Also, since the conveyance time T exceeds the threshold value Tk, the CPU51determines that over stacking has occurred.

Although this embodiment has mainly described the case where envelopes E are stacked in the manual bypass tray38, this is only an example. For example, the present invention is also applicable to a case where plain paper is stacked in the feed cassette23. That is to say, as a result of over-stacking envelopes E in the feed cassette23, a conveyance delay may occur due to the locking claws, the guiding members, or the like provided in the feed cassette23. Accordingly, the CPU51can also apply over stacking determination similar to that for the manual bypass tray38to the feed cassette23as well. Over stacking determination is applicable not only to the envelopes E but also to types of media with which a conveyance delay may occur due to the locking claws, the guiding members, or the like as a result of over stacking.

Conditions for Executing Over Stacking Determination

Over stacking determination does not need to be always executed. This is because there are situations where over stacking is likely to occur and is unlikely to occur. Therefore, conditions for executing over stacking determination will be described. A conveyance delay is likely to occur particularly due to over stacking of envelopes E. For this reason, the CPU51may consider the type of sheets S designated through the operation unit59being envelope type sheets such as envelopes E to be a condition for executing over stacking determination. Thus, the accuracy of over stacking determination can be improved based on the conveyance time. Furthermore, the CPU51will not erroneously determine that a conveyance delay caused by other factors is due to over stacking.

The CPU51may determine that the type of sheets S is the envelopes E in accordance with the image formation mode (e.g., envelope mode) that is set through the operation unit59. The CPU51may also determine that the type of sheets S corresponds to envelopes E when the size of the sheets S that is set through the operation unit59is a typical size for envelopes E. The size of the envelopes E is stored in the storage device55, and is read out and used by the CPU51. The CPU51may specify the sheet type using a media sensor for identifying the sheet type.

Incidentally, for a sheet S such as an envelope E, a large curl occurs as the amount of vapor in the air increases following a rise in temperature and humidity. Furthermore, an envelope E may largely swell up in some cases. As shown inFIG. 15, even when envelopes E are not over-stacked, the bundle of envelopes E becomes thick, and a large resistance force Fa is applied thereto from the locking claws44. For this reason, even though the envelopes E are not over-stacked, the conveyance speed decreases, the conveyance time T exceeds the over stacking threshold value Tk, and over stacking is detected. Under the environmental conditions that the absolute amount of vapor is 15 g/m^3 or less, the temperature is 35° C. or less, and the humidity is 70% or less, an end portion of an envelope E will not largely curl or largely swell. Therefore, the CPU51may determine whether or not to execute over stacking determination in accordance with the environmental conditions acquired by the environment sensor54. For example, it is assumed that execution conditions are that the amount of vapor is 15 g/m^3 or less, the temperature is 35° C. or less, and the humidity is 70% or less. Under such conditions, a curling or a swelling is not likely to occur, and accordingly, erroneous detection of over stacking decreases.

The CPU51can detect over stacking at the time of the first sheet S in a situation where a fixed number of sheets S or more are over-stacked exceeding the upper limit value. This is because, in general, the conveyance resistance exerted on the first sheet S is larger than the conveyance resistance exerted on the second sheet S. Even if the conveyance delay due to conveyance resistance varies, the CPU51can detect over stacking at least by the fifth sheet S. That is to say, there will be almost no cases where over stacking is detected for the first time at the fifth or subsequent sheet S. This is because, every time a sheet S is successfully conveyed, the height of the bundle of the sheets S lowers, and the conveyance resistance also decreases. Therefore, the condition for executing the over stacking determination may be that over stacking determination is executed from the first sheet S up to an nth(e.g., fifth) sheet S in a job. n is determined by experiments or simulation. This will improve the accuracy of over stacking determination. Upon a job being started, the CPU51counts the number of conveyed sheets S, and executes over stacking determination if the count value is smaller than or equal to a sheet number threshold value. Also, the CPU51stops over stacking determination if the count value exceeds the sheet number threshold value. Although the exemplary sheet number threshold value is five, this number may be determined in accordance with the shape or the conveyance resistance of the locking claws44and the guiding members45.

FIG. 16is a flowchart showing over stacking determination. Upon an instruction to form an image being input through the operation unit59or a host computer, the CPU51executes the following processing.

In step S1, the CPU51starts to feed the sheet S. For example, the CPU51outputs a control signal to cause the drive circuit56to start to drive the motor57. The drive circuit56starts to drive the motor57based on the control signal. Note that the sheet S is fed from a feeding port (the feed cassette23or the manual bypass tray38) designated by the job.

In step S2, the CPU51starts to count the conveyance time T using a timer or a counter.

In step S3, the CPU51determines whether or not to execute over stacking determination based on whether or not the conditions for executing over stacking determination are satisfied. Although several conditions have been listed as the execution conditions, the CPU51determines to execute over stacking determination when all of the above conditions are satisfied. Alternatively, the CPU51may determine to execute over stacking determination when one or a plurality of the conditions are satisfied. If the CPU51determines not to execute over stacking determination, the CPU51causes the image forming unit to form the image unless a conveyance delay or a jam has been detected based on the delay threshold value Tm or the jam threshold value Tj. If the CPU51detects a conveyance delay, the CPU51stops image formation after all sheets S on the conveyance path are discharged from the image forming apparatus100. Also, if the CPU51detects a jam, the CPU51stops the image formation. The CPU51may also output a message regarding the conveyance delay or the jam to the operation unit59. The CPU51may output a message for giving advice about an upper limit stacking amount and the correct stacking manner of sheets S, to the operation unit59. In step S3, if the conditions for executing over stacking determination are not satisfied, the CPU51ends this processing. On the other hand, if, in step S3, the conditions for executing over stacking determination are satisfied, the CPU51proceeds to step S4. In step S4, the CPU51determines whether or not the sheets S are over-stacked in the manual bypass tray based on whether or not the conveyance time T exceeds the over stacking threshold value Tk.

In step S4, if over stacking is not detected, the CPU51ends this processing. On the other hand, if the CPU51determines that the conveyance time T exceeds the over stacking threshold value Tk, the CPU51proceeds to step S5.

In step S5, the CPU51outputs an over stacking message.

Over Stacking Information

Although the message regarding over stacking may be output to the operation unit59, it may alternatively be transmitted to a computer (e.g., a server in a maintenance company) on a network via the communication device58. For example, the CPU51may transmit, as an email, the over stacking message indicating the occurrence of over stacking to an email address of a maintenance person (maintenance company) with whom a maintenance contract has been concluded regarding the image forming apparatus100. Note that the over stacking message may be transmitted to the server of the maintenance company using a communication protocol other than the email. Note that in the case where the message regarding over stacking is transmitted to the maintenance company, the message does not have to be output to the operation unit59. The maintenance company may inform a user of the image forming apparatus100of the occurrence of over stacking by email or orally, as part of the maintenance contract. Furthermore, the maintenance company may give advice about the upper limit stacking amount and the correct stacking manner of sheets S, points that the user needs to be careful of in envelope printing, or the like.

Thus, in the case where the conveyance time T exceeds the over stacking threshold value Tk but a conveyance delay or a jam has not been detected, the CPU51can give a warning about over stacking. Over stacking may cause a conveyance delay or a jam. Therefore, by giving over stacking notification, an operator will recognize the correct stacking amount, and the occurrence of a conveyance delay or a jam will decrease.

By transmitting an over stacking message to the maintenance company, the operator is spared the time and effort to contact the maintenance company. The maintenance company can inform the operator of appropriate advice based on the over stacking message received from the image forming apparatus100.

Erasing of Over Stacking Message

After the over-stacked state is resolved, the CPU51stops the output of the over stacking message, or erases the over stacking message. A condition for stopping the output of the over stacking message or erasing the over stacking message will be called a resolution condition. The resolution condition may be that the sheet sensor53no longer detects a sheet S (i.e., no sheet S is present on the tray unit40anymore). This is because, if there are no more sheets S, the over stacking situation has been absolutely resolved.

In the case where the type of sheets S being envelopes is the condition for executing over stacking determination, the type of the sheets S having been changed to something other than envelopes may alternatively be the resolution condition. For example, if the image formation mode is changed to a mode other than an envelope mode (e.g., a plain paper mode) or the like, the CPU51stops the output of the over stacking message. Also, if the size of the sheets S is changed to a size unique to the envelope, the CPU51may stop the output of the over stacking message.

As shown inFIG. 17, if the conveyance time T of the sheets S becomes smaller than or equal to the over stacking threshold value Tk in a state where the over stacking message (over stacking information) is output, the CPU51starts to count the number of sheets S. The CPU51may stop the output of the over stacking message when the conveyance time T of each of m successive sheets S is smaller than the threshold value Tk. The degree of over stacking decreases every time a sheet S is conveyed. Accordingly, if the conveyance time T of each of the m (e.g., five) successive sheets S is smaller than the threshold value Tk, it is highly likely that the over-stacked state has been resolved. Thus, the conveyance time T of each of the m successive sheets S being smaller than the threshold value Tk may be employed as the resolution condition.

In Embodiment 1, the conveyance time T of each sheet is counted with the first feeding operation as a reference. That is to say, the conveyance time T is not reset even if a retry occurs. In Embodiment 2, the conveyance time T is reset to 0 if a retry occurs. This means that the conveyance time T is recounted from the timing at which the second feeding operation is started.

Another exemplary procedure for determining over stacking will be described in detail. Note that descriptions of items that are common to already-described items will be omitted. As described above, there are cases where, in the first feeding operation, a sheet S does not reach the flag46by the time the jam threshold value Tj elapses, due to a conveyance delay caused by the slipping or the rollers. However, there are cases where a sheet S reaches the flag46by executing the second feeding operation (retry). As shown inFIG. 18, there are cases where the leading end of the envelope E reaches the vicinity of the flag46as a result of the first feeding operation. Accordingly, if the conveyance time T is counted from the timing of starting the second feeding operation, the value of the conveyance time T becomes smaller than the lower limit value of the tolerance X, as shown inFIG. 19A. Even though the conveyance time T thus becomes smaller than or equal to the threshold value Tk, it cannot be said that over stacking has been resolved. Therefore, in this embodiment, even in the case of counting the conveyance time T from the timing of starting a retry, over stacking that may cause slipping of the rollers can be detected.

As shown inFIG. 19B, when a retry occurs on the first sheet S due to over stacking, the CPU51compares the conveyance time T with a threshold value Tk2. The threshold value Tk2is set to a value that is smaller than or equal to the lower limit value of the tolerance X. If the conveyance time T of a sheet S for which the retry has been performed is smaller than the threshold value Tk2, the CPU51determines that over stacking has occurred.

Thus, the CPU51determines that over stacking has occurred when the first conveyance time T of a certain sheet S exceeds the threshold value Tk and the second conveyance time T of this sheet S is below the threshold value Tk2. As a result, even in the case of counting the conveyance time T from the timing of starting a retry, over stacking that may cause slipping of the rollers can be detected.

According to Embodiments 1 and 2, if the conveyance time T deviates from the tolerance X, the CPU51determines that over stacking has occurred. However, if the resistance forces Fa and Fb generated by over stacking become too large, even the first sheet S cannot be fed. The counting of the conveyance time T can also be designed to be completed only after the leading end of a sheet S has reached the flag46. In this case, in a state where not even a single sheet S can be fed, the conveyance time T cannot be counted, and the CPU51cannot determine over stacking. In this embodiment, the following over stacking determination method is introduced.

An over stacking determination method according to Embodiment 3 will be described using a flowchart inFIG. 20. Compared withFIG. 16, inFIG. 20, steps S10and S11are inserted between steps S2and S3.

In step S10, the CPU51determines whether or not a jam has occurred. For example, the CPU51determines that a jam has occurred when the leading end of a sheet S cannot be detected by the sheet sensor52, even if a retry has been executed N times. If a jam has not occurred, the CPU51proceeds to step S3. On the other hand, if a jam is detected, the CPU51proceeds to step S11.

In step S11, the CPU51forcibly substitutes a predetermined value Tk3with the conveyance time T. As shown inFIG. 21, the predetermined value Tk3is a value that is smaller than the conveyance error threshold value Tm and larger than the over stacking threshold value Tk. Thus, the conveyance time T is ascertained even if a sheet S does not reach the flag46. Moreover, since the predetermined value Tk3that is larger than the over stacking threshold value Tk is substituted with the conveyance time T, over stacking is detected in step S4. Thus, in Embodiment 3, over stacking can be detected even if a jam occurs on the first sheet S.

Functions of the CPU51will be described usingFIG. 22. The functions of the CPU51are achieved by the CPU51executing programs stored in the ROM. The functions of the CPU51may be achieved by hardware such as an FPGA (field programmable gate array) or an ASIC (application specific IC). A configuration may also be employed in which some functions are achieved by the CPU51and software, and the other functions are achieved by hardware. As described usingFIG. 4and regarding step S2, a time-counting unit61counts the conveyance time T from when the motor57starts to convey the sheet S until the sheet S arrives at a predetermined position on the conveyance path. As described regarding step S4, a determination unit62determines whether or not the conveyance time T of the sheet S exceeds the threshold value Tk, which is a first over stacking threshold value. In particular, the determination unit62determines that over stacking has occurred if the conveyance time T of a sheet S exceeds the threshold value Tk that is the first over stacking threshold value. By thus paying attention to the conveyance time of a sheet without using a sensor for estimating/measureing the height of a bundle of sheets S, over stacking of the sheets can be more accurately detected than with conventional techniques. Note that an image formation control unit63controls the image forming unit to form an image on the sheet S if the conveyance time T does not exceed the threshold value Tk. If the conveyance time T of a sheet S exceeds the threshold value Tk, the image formation control unit63controls the image forming unit not to form an image on the sheet S.

As described usingFIG. 14B, a jam detection unit64may detect that a conveyance delay has occurred on a sheet S based on whether or not the conveyance time T of the sheet S exceeds the threshold value Tm, which is a conveyance delay threshold value. The jam detection unit64may detect that a jam has occurred on a sheet S based on whether or not the conveyance time T of the sheet S exceeds the jam threshold value Tj. Note that the threshold value Tk for detecting over stacking is smaller than the threshold value Tm and the jam threshold value Tj. As described usingFIG. 14Aand the like, if over stacking occurs, a negligible conveyance delay occurs. Note that excessive over stacking may cause a conveyance delay or a jam. Therefore, a conveyance delay and a jam can be prevented in advance by setting the threshold value Tk to be smaller than the threshold value Tm and the jam threshold value Tj.

As described regarding Embodiment 3, the time-counting unit61starts to count a predetermined time upon a conveyance unit such as the motor57starting to convey a sheet. If the sheet does not arrive at a predetermined position on the conveyance path within this predetermined time, a substituting unit65substitutes a value that exceeds the threshold value Tk with the conveyance time T. The feeding may be retried several times within the predetermined time. As a result, if sheets S are not successfully conveyed even once since the sheets S are stacked in the manual bypass tray38, the jam detection unit64determines that a feeding jam has occurred. In this case, the conveyance time T is not defined in some cases. Therefore, the substituting unit65may cause the determination unit62to determine that over stacking has occurred by substituting a value that exceeds the threshold value Tk with the conveyance time T.

The determination unit62may determine whether or not to execute over stacking determination in accordance with the number of sheets S that have been conveyed since a conveyance job was started. For example, the determination unit62may execute over stacking determination until a predetermined number of sheets S has been conveyed after the sheets S are stacked in the manual bypass tray38, and thereafter stop the determination. If sheets S are over-stacked in the manual bypass tray38, the conveyance time T from the first sheet until an nth sheet is likely to be long. This is because, the higher the bundle of the sheets S is, the larger the resistance forces Fa and Fb are, which increases conveyance resistance. Accordingly, erroneous detection of over stacking decreases by executing the over stacking determination only in a period from when an image forming job is started until the nthsheet S is conveyed.

A measuring unit66may measure environmental conditions of the environment where the image forming apparatus100is set, using the environment sensor54or the like. The determination unit62executes over stacking determination if the environmental conditions measured by the measuring unit66are predetermined environmental conditions. Erroneous detection of over stacking is likely to occur under certain environmental conditions. Accordingly, when erroneous detection of over stacking is likely to occur, over stacking determination is skipped. That is to say, over stacking determination may be executed only under environmental conditions under which the accuracy of the over stacking determination is high. For example, over stacking determination is executed when the absolute amount of vapor is smaller than or equal to a predetermined amount of vapor, the environmental temperature is lower than or equal to a predetermined temperature, and the environmental humidity is lower than or equal to a predetermined humidity.

As described inFIG. 8, the manual bypass tray38may have the locking claws44as regulating members for regulating the height of the sheets S stacked in the manual bypass tray38. As described inFIG. 10A, since over-stacked sheets S receive the resistance force Fa from the locking claws44, the conveyance time T is likely to become long. Accordingly, as a result of paying attention to the conveyance time T, over stacking can be accurately detected.

Upon an instruction to convey the sheets S being given to the motor57and the drive circuit56, the time-counting unit61starts to count the conveyance time T. However, as described regarding Embodiment 2, the time-counting unit61may be configured to re-count the conveyance time T if an instruction to retry conveyance of sheets S is given to the motor57and the drive circuit56. In the latter case, the determination unit62may determine whether or not the conveyance time T of a sheet S is smaller than the threshold value Tk2(i.e., smaller than a second over stacking threshold value), which is smaller than the threshold value Tk. For example, the determination unit62may determine that over stacking has occurred if the conveyance time T of a sheet S is smaller than the threshold value Tk2(i.e., smaller than the second over stacking threshold value) that is smaller than the threshold value Tk. As described usingFIG. 18, if a sheet S has approached the flag46as a result of the first feeding operation, the value of the conveyance time T counted with the retry as a reference is smaller than the tolerance X. Then, over stacking can be accurately detected using the threshold value Tk2that is smaller than the threshold value Tk.

The image formation control unit63controls the image forming unit to form an image on a sheet S if the conveyance time T of the sheet S does not exceed the threshold value Tk and the conveyance time T of the sheet S is not smaller than the threshold value Tk2. On the other hand, the image formation control unit63may be configured not to form an image on a sheet S if the conveyance time T of the sheet S exceeds the threshold value Tk or the conveyance time T of the sheet S is smaller than the threshold value Tk. When over stacking is thus detected, the image forming unit may be controlled so as to give priority to resolving over stacking and not to form an image. As a result, the operator can easily recognize over stacking.

If the determination unit62determines that sheets S are over-stacked in the manual bypass tray38, the output unit67may output over stacking information indicating that the sheets S are over-stacked. As a result, the operator and the maintenance company more easily recognizes over stacking. A sheet number counter70may function as a count unit for counting the number of sheets S conveyed from the manual bypass tray38after the determination unit62determines that the sheets S are over-stacked in the manual bypass tray38. That is to say, the sheet number counter70performs counting so as to check how many successive sheets a phenomenon in which the conveyance time of the sheets conveyed from the manual bypass tray38did not exceed the first over stacking threshold value has occurred. The output unit67may be configured to stop the output of the over stacking information if the count value of the sheet number counter70reaches a stop threshold value. The degree of over stacking decreases every time a sheet S is conveyed. Accordingly, at the point when several sheets S are completely conveyed, it is likely that an over-stacked state has been resolved. Accordingly, over stacking notification may be stopped based on the number of conveyed sheets S.

The detection unit68may detect whether or not sheets S are stacked in the manual bypass tray38. For example, the detection unit68detects the presence of sheets S using the aforementioned sheet sensor53. After the determination unit62determines that sheets S are over-stacked in the manual bypass tray38, if it is detected by the detection unit68that no sheet S is stacked in the manual bypass tray38, the output unit67may stop the output of the over stacking information. This is because, if not even a single sheet S is present in the manual bypass tray38, over stacking has been absolutely resolved.

If the type of the sheets S stacked in the manual bypass tray38is changed after the determination unit62determines that the sheets S are over-stacked in the manual bypass tray38, the output unit67may stop the output of the over stacking information. In the case of envelope type sheets such as envelopes, over stacking is often an issue. Therefore, when the type of sheets S is changed, the output of the over stacking information may be temporarily stopped. For example, if the type of sheets S stacked in the manual bypass tray38is changed from envelopes to plain paper, over stacking of envelopes has been resolved.

The output unit67may display the over stacking information on the display device of the operation unit59. The operator can thereby be visually notified of the over stacking. Also, the output unit67may use the communication device58as a transmission unit for transmitting a message including the over stacking information. The over stacking message may be transmitted to an email address of the maintenance person (maintenance company) of the image forming apparatus100. As a result, the maintenance company can notify a customer of a method for resolving over stacking as part of maintenance service.

The image forming apparatus100may further be provided with an identifying unit69for identifying the type of sheets S. The determination unit62may determine whether or not the conveyance time T of a sheet S, the type of which has been identified as the envelope type sheet, exceeds the threshold value Tk. In the case of envelope type sheets such as envelopes, over stacking is often an issue. Accordingly, the over stacking determination may be executed only when the envelope type sheets are used. This will improve the accuracy of the over stacking determination. The identifying unit69may identify the type of sheets S as the envelope type sheet if an envelope mode is designated from a plurality of control modes provided in the image forming apparatus100. The operation unit59may also function as a size designation unit for designating the size of the sheets S. The identifying unit69may identify the type of sheets S as the envelope type sheet based on the size of the sheets S. Thus, the type of sheets S may be specified from indirect information.

The aforementioned Japanese Patent Laid-Open No. 05-278896 proposes an image forming apparatus that detects, using a sensor, over stacking by estimating, in an analog manner, the height of a sheet bundle stacked in a cassette.

However, with the technique described in Japanese Patent Laid-Open No. 05-278896, if, for example, a sheet is bent, the height of a sheet bundle is incorrectly measured, and therefore, over stacking may be erroneously determined. This embodiment detects over stacking of sheets more accurately than with conventional techniques.

Functions of the controller50will be described usingFIG. 23. The overall image forming apparatus100is comprehensively controlled by executing control programs stored in the CPU51or the storage device55. The storage device55has a memory such as a ROM or a RAM. The controller50sets image forming conditions in accordance with an image formation mode designated through the operation unit59. The image forming conditions are, for example, the conveyance speed of sheets S, the fixing temperature of the fixing device18, and the like. For example, the image formation modes may include a plain paper mode for forming an image on plain paper, a cardboard mode for forming an image on cardboard, an envelope mode for forming characters on an envelope, and the like. The controller50stores the image forming conditions for the respective image formation modes in the storage device55, and reads out the image forming conditions corresponding to the designated image formation mode.

The CPU51counts the conveyance time T using the sheet sensor52for determining whether or not a conveyance delay or a jam has occurred. If a sheet S causes a conveyance delay or a jam on the conveyance path, the CPU51causes a message indicating the occurrence of the conveyance delay or the jam to be displayed on the display device of the operation unit59. If the CPU51detects over stacking of sheets S in the feed cassette23or the manual bypass tray38, the CPU51causes a message indicating the occurrence of the over stacking to be displayed on the display device of the operation unit59. A conveyance delay is a phenomenon in which the conveyance time T of a sheet S becomes too long to be able to ensure the accuracy of an image forming position or the like, and may also be called a conveyance error or a failure in conveyance. A jam refers to, in the narrow sense, a phenomenon in which a sheet S is stuck or clogs on the conveyance path. A phenomenon in which large a conveyance resistance is applied to sheets S due to over stacking in the feed cassette23and not even a single sheet S can be conveyed is also a kind of jam. Thus, over stacking may cause the case where a sheet S has been successfully fed from the feed cassette23but the conveyance time T is too long, or the case where the feeding fails.

The CPU51detects, using a surface sensor153, whether or not the surface of a sheet S located uppermost in the plurality of sheets S stacked in the feed cassette23has been lifted up (raised) to a predetermined height H. That is to say, the surface sensor153is an exemplary surface detection unit for detecting whether or not the surface of a sheet S stacked on the intermediate plate143has been lifted up to the predetermined height by the motor160. The CPU51detects, using a position sensor154, the position of a rear end regulating plate141(FIG. 25) for regulating the position of the rear end of sheets S stacked in the feed cassette23. The position sensor154is an exemplary position detection unit for detecting the position of the rear end regulating plate141.

The motor57is a drive source for driving the conveyance rollers such as the pickup roller35and the feed roller24. The CPU51outputs a feed start signal to the drive circuit56for driving the motor57. The drive circuit56, upon receiving the feed start signal, starts to drive the motor57. The CPU51sets the conveyance speed in advance corresponding to the image formation mode in the drive circuit56. The motor57rotates at a rotation speed corresponding to the set conveyance speed. The motor160is a so-called lift-up motor, and is a motor for lifting up the intermediate plate on which sheets S are placed in the feed cassette23. The motor160is an exemplary lift-up unit for lifting up the intermediate plate such that a sheet S stacked on the intermediate plate, which is a plate member, comes into contact with the pickup roller35. The CPU51drives the motor160such that the surface sensor153detects an uppermost sheet Sa being located at the height H. A cassette sensor161is an exemplary pull-out/push-in detection unit for detecting that the feed cassette23has been pulled out from, and pushed into, the housing101of the image forming apparatus100. The feed cassette23is a drawer-like cassette, for example. When the operator stores sheets S, the feed cassette23is drawn out from the housing101. Upon storing the sheets S being completed, the feed cassette23is inserted in the housing101. The CPU51detects, using the feed cassette23, that the feed cassette23has been pulled out and pushed in.

As shown inFIG. 24, when the leading end of a sheet S pulls down the flag46provided near the registration roller17, the sheet sensor52outputs a detection signal indicating that the leading end of the sheet S has been detected, to the controller50. The CPU51counts, using a timer or a counter, the conveyance time from the timing at which conveyance of a sheet S is started until the timing at which the sheet sensor52detects the leading end of the sheet S. The CPU51may obtain a difference in time data acquired from a real time-clock (RTC) in order to count the conveyance time. Note that the example of the detection signal of the sheet sensor52is as shown inFIG. 4.

As mentioned above,FIG. 5Ashows exemplary conveyance time at the time when a plurality of sheets S are continuously fed. The conveyance time T may more or less vary depending on a friction state of the pickup roller35and the type of sheets S (thickness, basis weight, envelope type or not, presence of surface coating etc.). However, if the plurality of sheets S are correctly stacked in the feed cassette23, and a jam does not occur, the conveyance time of each sheet S stays within the tolerance X.

FIG. 5Bshows exemplary conveyance time at the time when a plurality of sheets S are continuously fed. In particular, a conveyance delay (conveyance error) has occurred on a fifth sheet S. As shown inFIG. 5B, there may be cases where a conveyance delay occurs for the second and subsequent sheets after a job is started. The CPU51monitors the conveyance time T of each sheet S, and determines whether or not the conveyance time T exceeds a delay threshold value Tm. If the conveyance time T exceeds the delay threshold value Tm, the CPU51determines that a conveyance delay has occurred, discharges all sheets S that remain on the conveyance path, and stops the motor57. The delay threshold value Tm for determining a conveyance delay is set to a value that deviates from the tolerance X and is larger than an upper limit value of the tolerance X. If the conveyance time exceeds the delay threshold value Tm, the accuracy of image formation on a sheet S is not ensured, and accordingly, the CPU51stops image formation.

The CPU51may preferentially feed the first sheet S, and cause the first sheet S to wait at the registration roller17until the image forming unit is ready. In this case, the CPU51may not determine that a conveyance delay has occurred regarding a first sheet S, even if the conveyance time T of the first sheet S exceeds the delay threshold value Tm. That is to say, the conveyance delay determination processing for the first sheet S may be skipped. The time from when a job is started until the image forming unit is ready is longer than a normal conveyance time T, which is a conveyance time obtained by dividing a conveyance distance from the position of the leading end of a sheet S contained in the feed cassette23to the sheet sensor52by the conveyance speed. Therefore, the conveyance delay of the first sheet S is highly permissible. Thus, in this embodiment, the conveyance delay determination is applied to the second and subsequent sheets S.

However, there are cases where the first sheet S does not arrive at the flag46even though the conveyance time T at the time of performing a first feeding operation for the first sheet S greatly exceeds the threshold value Tm. That is to say, even if the count value of the timer of the CPU51exceeds the retry threshold value Tr, the sheet sensor52cannot detect the leading end of the sheet S (the threshold value Tm may be employed as the retry threshold value Tr). In this case, the CPU51instructs the drive circuit56to perform a second feeding operation (retry). Note that the CPU51may continue, also during the retry, counting of the conveyance time T starting from the first feeding operation. If the first sheet S does not arrive at the flag46even after the conveyance time T exceeds the jam threshold value Tj, the CPU51determines that a jam has occurred. If the CPU51detects a jam, the CPU51causes a message indicating the occurrence of the jam to be displayed on the operation unit59, and transmits this message to an email address of a maintenance person via the communication device58. The message may be delivered in the form of an email, for example.

[Configuration of Feed Cassette]

A configuration of the feed cassette23will be described usingFIGS. 25 to 27. As described above, the feed cassette23can be pulled out from, and pushed into, the housing101of the image forming apparatus100. As shown inFIG. 25, the feed cassette23has a cassette tank140. An inner bottom face of the cassette tank140is provided with the rear end regulating plate141capable of moving in both the conveyance direction of sheets S and the opposite direction (that may be called front and rear directions). The operator moves the rear end regulating plate141in accordance with the size of the sheets S. The rear end regulating plate rear141is an exemplary regulating unit for regulating and aligning the rear end position of sheets S. Thus, conveyance of a plurality of sheets S is started from roughly the same position. The position of the rear end regulating plate141is detected by the aforementioned position sensor154. The bottom face of the cassette tank140is provided with two side regulating plates142capable of moving in directions perpendicular to the conveyance direction of sheets S (that may be called left and right directions or width directions). The two side regulating plates142regulate and align the positions of both ends of sheets S. The intermediate plate143is a plate member that is provided in the feed cassette23and on which sheets S are stacked, and pivots around the fulcrum144.

As shown inFIG. 26A, when the feed cassette23is drawn out of the housing101, the intermediate plate143is located near the bottom face of the cassette tank140. The operator stacks a bundle of sheets S in the feed cassette23, and manually moves the rear end regulating plate141such that the rear end regulating plate141abuts against the rear end of the sheets S stacked on the intermediate plate143. Similarly, the operator manually moves the side regulating plates142such that the side regulating plates142abut against the left end and the right end of the sheets S stacked on the intermediate plate143. Thus, the bundle of the sheets S stacked on the intermediate plate143is aligned in both the front and rear directions and the left and right directions.

As shown inFIG. 26B, upon detecting that the feed cassette23has been inserted into the housing101with the cassette sensor161, the CPU51drives the lift-up motor160to lift up the intermediate plate143. As a result, the intermediate plate143pivots around the fulcrum144. When the uppermost sheet S pushes up a surface flag146, the surface sensor153outputs a detection signal indicating that the sheet S has been detected, to the CPU51. For example, upon the surface of a sheet S reaching the height H as shown inFIG. 26B, the surface sensor153outputs the detection signal. The CPU51recognizes, based on the detection signal, that the surface of the sheet S has reached the height H, and stops the motor160. Note that the smaller the number of sheets S stacked on the intermediate plate143is, the larger the amount of lifting up of the intermediate plate143is, and the longer the operating time of the motor160is. On the contrary, the larger the number of stacked sheets S is, the smaller the amount of lifting up of the intermediate plate143is, and the shorter the operating time of the motor160is. That is to say, the CPU51can estimate the number of sheets stacked on the intermediate plate143by estimating/measuring the operating time of the motor160using a timer or a counter. Every time a sheet S is fed to the conveyance path, the position of the surface of the uppermost sheet S stacked on the intermediate plate143lowers. When the surface sensor153no longer detects the surface of a sheet S, the CPU51again drives the motor160and lifts up the sheets S.

Thus, by providing the rear end regulating plate141and the like, sheets S stacked in the feed cassette23are always fed from the same position in the conveyance direction, without depending on the number of stacked sheets S. Furthermore, by lifting up the intermediate plate143, the pressure applied to the sheets S by the pickup roller35is always kept at a constant, without depending on the number of stacked sheets.

The feed cassette23is provided with an upper limit value of the height of a bundle of sheets S (upper limit sheet number). In terms of design, it is ensured that the image forming apparatus100can normally form an image if the height of the bundle of sheets S is smaller than or equal to the upper limit value. As shown inFIG. 27, a side surface of each side regulating plate142is provided with a mark72indicating the upper limit value. The mark72may be adhered, or may be a groove.

As shown inFIG. 27, two locking claws147are provided in the rear end regulating plate141. The locking claws147suppress sheets S riding up onto the side regulating plate141. If sheets S ride up onto the rear end regulating plate141, the sheets S that have ridden up shift to the upstream side (rearward) in the conveyance direction. As a result, the conveyance time T estimated/measured by the sheet sensor52becomes longer than the normal conveyance time of sheets S. This may cause an error when detecting over stacking based on the conveyance time T or estimating the length of sheets S in the conveyance direction based on the conveyance time T. Accordingly, the locking claws147are provided in the rear end regulating plate141. Note that the height of the surfaces of the locking claws147that face the surface of sheets S is roughly the same as the upper limit value of the height of a bundle of the sheets S.

As described above, the position sensor154is provided to detect the position at which the rear end regulating plate141is located corresponding to the size of the sheets S. The CPU51identifies the size of the sheets S stacked in the feed cassette23based on the position of the rear end regulating plate141detected by the position sensor154. However, although the accuracy of sheet size identification is high if the rear end regulating plate141is correctly positioned in accordance with the sheet size, the identification result will be incorrect if the rear end regulating plate141is positioned so as to be separate from the rear end of the sheets S. Accordingly, the CPU51may identify the sheet size by also using sheet size information that is input through the input device of the operation unit59, sheet size information received from a host computer through the communication device58, sheet size information obtained based on the conveyance time T, or the like.

[Over Stacking in Feed Cassette]

A state where sheets S are over-stacked in the feed cassette23will be described in detail. Here, a description will be given of the case of over stacking where sheets S are stacked on the locking claws147and the case of over stacking where the sheets S are forcibly stacked below the locking claws147.

Case where Sheets S are Stacked on the Locking Claws147

As shown inFIG. 28, if a large number of sheets S are stacked to a height that exceeds the upper value in design, some sheets ride up onto the locking claws147. Note that the upper limit sheet number corresponding to the upper limit value varies depending on the thickness of the sheets S. Since the positions in the conveyance direction of some sheets S that have ridden up onto the locking claws147are not regulated by the rear end regulating plate141, these sheets S shift to the upstream side in the conveyance direction (i.e., in the direction opposite to the conveyance direction).

FIGS. 29A to 29Dshow the behavior of the sheets S when a small amount of sheets S are over-stacked on the locking claws147. As shown inFIG. 29A, the leading ends of sheets Sa that are over-stacked so as to ride up onto the locking claws147are located while shifting in the direction opposite to the conveyance direction. Note that the pickup roller35is not in contact with the sheet Sa at a point before conveyance is started. As shown inFIG. 29B, the CPU51brings the pickup roller35into contact with the over-stacked sheets Sa by lifting up the intermediate plate143or lowering the pickup roller35. The pickup roller35is lifted up and lowered by a drive source such as a motor or a solenoid that is driven by the CPU51. Note that, as shown inFIG. 29B, the amount of shift of the over-stacked sheets S in the conveyance direction is small, and therefore, the pickup roller35can come into contact with the uppermost sheet Sa in the bundle of the sheets S. Accordingly, as shown inFIG. 29C, the sheet S to be fed by the pickup roller35is the uppermost sheet Sa. As shown inFIG. 29D, the leading end of the fed sheet Sa pushes down the flag46, and further abuts against a nip portion of the registration roller17.

Note that at the timing at which the CPU51outputs a control signal for instructing the drive circuit56to feed sheets, the leading end position of the sheet Sa is upstream of the leading end position of normally stacked sheets S. That is to say, the conveyance distance from the leading end position of the sheets Sa to the flag46is longer than the conveyance distance from the leading end position of the normally stacked sheets S to the flag46. That is to say, the conveyance time T of the sheets Sa is longer than the conveyance time of the normally stacked sheets S. The normally stacked sheets S are sheets S that constitute a sheet bundle whose height is lower than or equal to the upper limit value. That is to say, the normally stacked sheets S are sheets S that are stacked at an expected position in both the conveyance direction and the height direction in terms of design.

As shown inFIG. 30A, although the conveyance time T of five over-stacked sheets Sa exceeds the upper limit value of the tolerance X, this conveyance time T does not exceed the delay threshold value Tm for determining the conveyance delay. Since the sixth and subsequent sheets S are normally stacked, the conveyance time T of these sheets stays within the tolerance X. However, if the conveyance time T deviates from the tolerance X and further exceeds the delay threshold value Tm as shown inFIG. 30B, the CPU51determines that a conveyance delay (conveyance error) has occurred, and stops image formation after discharging all sheets S existing on the conveyance path.

On the other hand, as shown inFIG. 31A, there are cases where the amount of over-stacked sheets S further increases, and the over-stacked sheets Sa are stacked while further shifting to the downstream side. As shown inFIG. 31B, the pickup roller35cannot come into contact with the sheets Sa even after having been lowered, and comes into contact with a sheet Sb that is stacked below the sheets Sa. The sheet Sb is stacked below the locking claws147, and therefore stacked at a normal position in the conveyance direction. For this reason, the sheet to be picked up when the pickup roller35starts to rotate is the sheet Sb. Upon the sheet Sb starting to move, an over-stacked sheet Sa stacked on the sheet Sb is conveyed together with the sheet Sb still on the sheet Sb.

As shown inFIG. 31C, if the over-stacked sheet Sa comes into contact with the pickup roller35, the sheet Sb loses contact with the pickup roller even though it has not reached the feed roller24, and stops at this place. As shown inFIG. 31D, only the sheet Sa is conveyed by the pickup roller35, and reaches the flag46. Note that second and subsequent over-stacked sheets Sa are conveyed to a position where these sheets Sa can come into contact with the pickup roller35, as shown inFIGS. 31C and 31D. That is to say, the behavior thereafter is the same as the behavior of the sheet Sa described usingFIGS. 29A to 29D.

As shown inFIG. 31A, the leading end position of the uppermost sheet Sa in over-stacked sheets Sa is shifted to the upstream side in the conveyance direction relative to the leading end position of the normally stacked sheet Sb. Accordingly, the conveyance time T of the sheets Sa is longer than the conveyance time T of the sheet Sb. Accordingly, the CPU51can determine whether or not over stacking has occurred based on the conveyance time T of the sheets Sa.

A procedure for determining over stacking in the feed cassette23will be described in detail. As mentioned above, the conveyance time T of the over-stacked sheets Sa is longer than the conveyance time T of the normally stacked sheets S, but does not exceed the delay threshold value Tm. Therefore, an over stacking threshold value Tk is defined as shown inFIG. 32. The over stacking threshold value Tk is larger than or equal to the upper limit value of the tolerance X, and is smaller than the delay threshold value Tm. The CPU51determines whether or not over stacking has occurred by comparing the conveyance time T of a sheet S detected using the sheet sensor52with the over stacking threshold value Tk. That is to say, if the conveyance time T of a sheet S exceeds the over stacking threshold value Tk, the CPU51determines that this sheet S is an over-stacked sheet. Furthermore, if the conveyance time T of a sheet S does not exceed the over stacking threshold value Tk, the CPU51determines that this sheet S is a normally stacked sheet. Thus, the CPU51can detect, as an over-stacked sheet, a sheet that is not delayed to the extent that it is determined that a conveyance delay has occurred. Note that if the conveyance time T of a sheet S that is conveyed secondly or subsequently after an image forming job is started exceeds the delay threshold value Tm, the CPU51determines that a conveyance delay has occurred, and also determines that over stacking has occurred.

Conditions for Executing Over Stacking Determination

(1) The CPU51may not always execute over stacking determination, and may execute over stacking determination when execution conditions are satisfied. Several execution conditions are conceivable. In order to stack a sheet S in the feed cassette23, the operator needs to pull out and push in the feed cassette23from/to the housing of the image forming apparatus100. Accordingly, the CPU51executes over stacking determination for the first fed sheet S after the cassette sensor161detects the pulling out or pushing in of the feed cassette23. That is to say, the execution condition is that the sheet S is the first sheet S that is fed after the pulling out and pushing in of the feed cassette23has been detected. This is because, if sheets S are over-stacked, over stacking is always detected for the first sheet S, and over stacking will not be detected for the first time at the time of a second or subsequent sheet S even though over stacking did not detected for the first sheet S. Thus, the accuracy of the over stacking determination is improved by applying the over stacking determination only on the first sheet S after the feed cassette23is inserted. For example, in the case of control for always executing the over stacking determination, over stacking may be erroneously detected even though a conveyance delay has been caused by other factors. Accordingly, if the over stacking determination is executed only when the execution condition is satisfied, over stacking can be more accurately detected.

(2) An execution condition may be employed as such that the difference between the height of sheets S estimated from the operating time of the lift-up motor160and the upper limit value for assuring the operation is smaller than or equal to a predetermined threshold value. When over stacking has occurred, the height of the sheet S is close to the upper limit value. Accordingly, in a case where the height of sheets S is greatly lower than the upper limit value, it is unlikely that over stacking has occurred. Also, if over stacking determination is executed in such a situation, it may be determined that over stacking has occurred even though a conveyance delay has been caused by other factors. Accordingly, over stacking can be more accurately detected with the execution condition that the height of sheets S is close to the upper limit value.

(3) As shown inFIG. 33A, if the rear end regulating plate141is located at a position separate from the rear end of sheets S, the sheets S will be located at a position that is shifted in the direction opposite to the conveyance direction. As a result, even the conveyance time T of a sheet S that is not over-stacked will exceed the over stacking threshold value Tk. Therefore, an execution condition that the rear end regulating plate141is located in a correct position may be employed. This execution condition can reduce the likelihood of the conveyance time T becoming long due to the rear end regulating plate141being arranged in an incorrect position and results in erroneous detection of over stacking. The following method is conceivable as a method for determining whether or not the rear end regulating plate141is located in a correct position.

As shown inFIG. 33B, the CPU51converts time Tp taken from when the leading end of a sheet S reaches the flag46until the rear end of the sheet S passes the flag46into the size (length in the conveyance direction) of the sheet S. The length L in the conveyance direction is calculated by multiplying the conveyance speed v of the sheet S by the time Tp. On the other hand, the CPU51detects the position of the rear end regulating plate141using the position sensor154, and converts this position into the size of the sheet S. The CPU51compares the size acquired using the sheet sensor52with the size acquired using the position sensor154. If the size acquired using the sheet sensor52is smaller than the size acquired using the position sensor154, the CPU51determines that the rear end regulating plate141is arranged in an incorrect position. Alternately, the CPU51may compare the size designated by the operation unit59or the host computer with the size acquired using the position sensor154. If the size designated by operation unit59or the host computer is smaller than the size acquired using the position sensor154, the CPU51determines that the rear end regulating plate141is arranged in an incorrect position.

(4) As the number of sheets S on which images are formed increases, roller surfaces of the pickup roller35and the feed roller24are worn down, resulting in a decrease of the roller diameter and a decrese of frictional resistance with respect to the sheets S. As the rollers are further worn down, the conveyance distance of each sheet S that is conveyed by rotating the rollers once decreases. Accordingly, the conveyance time T of sheets S of the same size gradually becomes longer. That is to say, the smaller the degree of wear of a contact member that forms the surface of each roller is, the higher the accuracy of the over stacking determination based on the conveyance time T is. Therefore, the degree of wear of the rollers being small may be employed as the execution condition. The accuracy of the over stacking determination is improved by executing the over stacking determination only when the rollers are not too worn down.

In the image forming apparatus according to this embodiment, if the rollers (contact members) are worn to the extent that a conveyance delay is caused, a phenomenon in which the conveyance time of a sheet S exceeds the over stacking threshold value Tk at least once in 500 sheets S, which corresponds to one cassette in this embodiment, even in the case where sheets are not over-stacked, occurs five consecutive times or more.

Therefore, in this embodiment, if the phenomenon in which the conveyance time of a sheet S exceeds the over stacking threshold value Tk at least once among M sheets (e.g., 500 sheets) occurs N (e.g., five) consecutive times or more, the CPU51determines that the degree of wear of the pickup roller35and/or the feed roller24exceeds the tolerance thereof. The tolerance of the degree of wear is determined from the viewpoint of the accuracy of the over stacking determination accuracy. Thus, if the degree of wear exceeds the tolerance, the CPU51interrupts the over stacking determination. N and M are determined in advance by simulation or experiments.

Note that the CPU51may resume over stacking determination if the pickup roller35and/or the feed roller24is replaced. If the pickup roller35and/or the feed roller24is not in a state of being too worn to cause a conveyance delay, erroneous detection of over stacking does not occur on even a single sheet S in K (e.g.,4000) sheets S. That is to say, the conveyance time of none of the K sheets S will exceed the over stacking threshold value Tk. Therefore, if the CPU51interrupts over stacking determination as a result of the determination that wear has occurred, the CPU51may determine whether or not the pickup roller35and/or the feed roller24has been replaced with a new one based on whether or not the conveyance time of each of the K consecutive sheets S does not exceed the over stacking threshold value Tk. If the conveyance time of each of the K consecutive sheets S does not exceed the over stacking threshold value Tk, the CPU51resumes over stacking determination. Note that if information indicating that the pickup roller35and/or the feed roller24has been replaced with a new one is input through the operation unit59, the CPU51may resume over stacking determination. K is determined in advance by simulations or experiments.

As mentioned above,FIG. 16is a flowchart showing over stacking determination. Upon an instruction to form an image being input through the operation unit59or a host computer, the CPU51executes the following processing.

In step S1, the CPU51starts to feed a sheet S. For example, the CPU51outputs a control signal (feed start signal) to the drive circuit56to start driving the motor57. The drive circuit56starts to drive the motor57based on the control signal. Note that the sheet S is fed from a feeding port (the feed cassette23or the manual bypass tray38) designated by a job.

In step S2, the CPU51starts to count the conveyance time T using a timer or a counter.

In step S3, the CPU51determines whether or not to execute over stacking determination based on whether or not the conditions for executing over stacking determination are satisfied. Although several conditions have been listed as the execution conditions, the CPU51determines to execute over stacking determination when all of the above conditions are satisfied. Alternatively, the CPU51may determine to execute over stacking determination when one or a plurality of conditions are satisfied. Upon determining not to execute over stacking determination, the CPU51causes the image forming unit to form an image unless the CPU51detects a conveyance delay based on the delay threshold value Tm or detects a jam based on the jam threshold value Tj. If the CPU51detects a conveyance delay, the CPU51stops image formation after all sheets on the conveyance path are discharged from the image forming apparatus100. Also, if the CPU51detects a jam, the CPU51stops image formation. The CPU51may also output a message regarding the conveyance delay or the jam to the operation unit59. The CPU51may output a message for giving advice about an upper limit stacking amount and a correct stacking manner of the sheets S to the operation unit59. In step S3, if the conditions for executing the over stacking determination are not satisfied, the CPU51ends this processing. On the other hand, if, in step S3, the conditions for determining the over stacking determination are satisfied, the CPU51proceeds to step S4.

In step S4, the CPU51determines whether or not sheets S are over-stacked in the feed cassette23based on whether or not the conveyance time T exceeds the over stacking threshold value Tk. In step S4, if over stacking is not detected, the CPU51ends this processing. That is to say, the CPU51continues image formation. On the other hand, if the CPU51determines that the conveyance time T exceeds the over stacking threshold value Tk, the CPU51proceeds to step S5.

In step S5, the CPU51outputs an over stacking message. The CPU51stops image formation after all sheets S that exist on the conveyance path are discharged. The CPU51can detect that all sheets S existing on the conveyance path have been discharged using a sheet sensor (not shown) installed in a discharge portion on the conveyance path, for example.

Erasing of Over Stacking Message

After the over-stacked state is resolved, the CPU51stops the output of the over stacking message, or erases the over stacking message. A condition for stopping the output of the over stacking message or erasing the over stacking message will be called a resolution condition. Alternatively, the CPU51displays a resolution message indicating that the over-stacked state has been resolved on the operation unit59, or transmits the resolution message via the communication device58. The resolution condition may be that the cassette sensor161detects the execution of the pulling out and pushing in of the feed cassette23. Also, as shown inFIG. 34, if the conveyance time T of a sheet S becomes smaller than or equal to the over stacking threshold value Tk in a state where the over stacking message (over stacking information) is output, the CPU51may stop the output of the over stacking message. The degree of over stacking decreases every time a sheet S is conveyed. Accordingly, if the conveyance time T of a sheet S falls below the over stacking threshold value Tk, it is likely that the over-stacked state has been resolved. Thus, the conveyance time T of a sheet S falling below the over stacking threshold value Tk may be employed as the resolution condition.

According to Embodiment 1, if the conveyance time T exceeds the over stacking threshold value Tk, the CPU51determines that over stacking has occurred. Incidentally, over stacking occurs even if sheets S are stacked below the locking claws147in some cases. As shown inFIG. 35A, if sheets S, the number of which exceeds the upper limit sheet number, are forcibly pushed in below the locking claws147, the rear end portion of the sheets S is strongly held down by the locking claws147. For this reason, the conveyance resistance exerted onto the sheets S is larger than the conveyance resistance applied to sheets S that are not over-stacked. In particular, there are cases where the conveyance resistance becomes so large that the pickup roller35cannot convey sheets S. If the leading end of a sheet S is not detected by the sheet sensor52even though the elapsed time since paper feeding was started exceeds the jam threshold value Tj, the CPU51determines that a jam has occurred.

The counting of the conveyance time T can also be designed to be completed only after the leading end of a sheet S reaches the flag46. In this case, in a state where no sheet S can be fed, the conveyance time T cannot be counted, and the CPU51cannot determine over stacking. In this embodiment, the following over stacking determination method is introduced.

An over stacking determination method according to Embodiment 5 will be described using the flowchart inFIG. 20. Compared withFIG. 16, inFIG. 20, steps S10and S11are inserted between steps S2and S3.

In step S10, the CPU51determines whether or not a jam has occurred. For example, the CPU51determines that a jam has occurred when the leading end of a sheet S cannot be detected by the sheet sensor52even if a retry has been executed J times. If a jam has not occurred, the CPU51proceeds to step S3. On the other hand, if a jam is detected, the CPU51proceeds to step S11.

In step S11, the CPU51forcibly substitutes a predetermined value Tk3with the conveyance time T. As shown inFIG. 35B, the predetermined value Tk3is a value that is smaller than a delay threshold value Tm for detecting a conveyance error and larger than the over stacking threshold value Tk. Thus, the conveyance time T is ascertained even if a sheet S does not reach the flag46. Moreover, since the predetermined value Tk3that is larger than the over stacking threshold value Tk is substituted with the conveyance time T, over stacking is detected in step S4. Thus, in Embodiment 5, over stacking can be detected even if a jam occurs on the first sheet S.

As shown inFIG. 36A, at least some of the over-stacked sheets S ride up onto the locking claws147and are significantly shifted in the direction opposite to the conveyance direction in some cases. As shown inFIG. 36B, since the pickup roller35is not in contact with an uppermost sheet Sa, a sheet Sb is fed by the pickup roller35. As shown inFIG. 36C, the sheet Sa is also conveyed together with the sheet Sb, and the Sa reaches the feeding position of the pickup roller35. At this point, the sheet Sb has already reached the nip portion of the feed roller24. Accordingly, as shown inFIG. 36D, the sheet Sa and the sheet Sb continue to be conveyed thereafter. This phenomenon is called double feeding. The conveyance time T counted by the CPU51is a period from the timing at which feeding is started until the timing at which the sheet Sb reaches the flag46. Accordingly, this conveyance time T is within the tolerance X in some cases. This is because the sheet Sb is conveyed from the regular position. Accordingly, in the case where double feeding occurs, the over stacking cannot be correctly determined based on the conveyance time T.

The CPU51can identify that double feeding has occurred when the size that is designated in advance is different from the size that is measured using the sheet sensor52. Therefore, the CPU51determines that over stacking has occurred when the size of a sheet S acquired using the sheet sensor52is larger than the size corresponding to the position of the rear end regulating plate141detected by the position sensor154.

As mentioned above, the CPU51can detect the size in the conveyance direction based on the elapsed time from the timing at which the sheet sensor52detects the leading end of a sheet S until the timing at which the sheet sensor52detects the rear end thereof. As shown inFIG. 37A, the CPU51starts to count the time for obtaining the sheet size at the timing at which the leading end of the sheet Sb reaches the flag46. As shown inFIG. 37B, the counting of the time is stopped at the timing at which the sheet Sa passes the flag46, and the counted elapsed time Tp is converted into the sheet size. A conversion formula, a conversion table, or the like may be stored in the storage device55.

As shown inFIG. 38A, the sheet size obtained by the CPU51using the sheet sensor52is La. The sheet size obtained by the CPU51using the position sensor154is Lb. InFIG. 38A, La>Lb holds, and therefore, the CPU51determines that over stacking has occurred. Thus, if the sheet sizes disagree (La>Lb), the CPU51stops image formation after discharging all sheets S existing on the conveyance path. Furthermore, the CPU51outputs information indicating the disagreement of the sheet sizes and information indicating the occurrence of over stacking to the operation unit59and the communication device58.

Conditions for Executing Over Stacking Determination

As shown inFIG. 38B, if the rear end regulating plate141is positioned while being shifted rearward of the rear end of sheets S, a sheet size La obtained using the sheet sensor52agrees with a sheet size Lb obtained using the position sensor154in some cases. In such cases, the CPU51cannot detect over stacking from the sheet sizes La and Lb.

Therefore, the rear end regulating plate141being located in a correct position may be employed as a condition for executing the over stacking determination. As mentioned above, the CPU51acquires the information about the sheet size designated by the operator via the operation unit59or the communication device58. Accordingly, the CPU51executes over stacking determination if the sheet size designated by the operator agrees with the sheet size Lb obtained using the position sensor154. On the other hand, the CPU51skips the over stacking determination if the sheet size designated by the operator disagrees with the sheet size Lb obtained using the position sensor154. Thus, over stacking will be accurately detected even if double feeding and over stacking simultaneously occur.

Functions of the CPU51will be described usingFIG. 39. As described usingFIG. 4and regarding step S2, a time-counting unit170counts the conveyance time T from when the motor57and the pickup roller35start to convey a sheet S until the sheet S arrives at a predetermined position on the conveyance path. As described regarding step S4or the like, the determination unit62may determine whether or not the conveyance time T of the sheet S, which is conveyed first after the cassette sensor161has detected that the feed cassette23has been pulled out and pushed in, exceeds the over stacking threshold value Tk. The determination unit62determines whether or not sheets S are over-stacked in the feed cassette23based on whether or not the conveyance time T exceeds the over stacking threshold value Tk. An image formation control unit63controls the image forming unit to form an image on the sheet if the conveyance time T does not exceed the over stacking threshold value Tk. If the conveyance time T of a sheet S exceeds the over stacking threshold value Tk, the image formation control unit63controls the image forming unit not to form an image on the sheet S. By thus paying attention to the conveyance time of a sheet without using a sensor for estimating the height of a bundle of sheet S, over stacking of the sheets can be more accurately detected than with conventional techniques.

The jam detection unit64may detect the aforementioned conveyance delay. That is to say, the jam detection unit64is an exemplary detection unit for detecting that a conveyance delay has occurred on a sheet S based on whether or not the conveyance time of the sheet S exceeds the conveyance delay threshold value. Note that the over stacking threshold value may be the same as the conveyance delay threshold value.

In the design of the image forming apparatus, the maximum stacking height or the maximum number of stacked sheets that ensures normal operation may be defined. Furthermore, the CPU51may function as a stacking degree determination unit for determining the degree of stacking of sheets on the feed cassette23. If the CPU51determines that the stacking degree corresponds to a stacking volume within a predetermined range with respect to the stacking height or the number of stacked sheets, the CPU51may cause the determination unit62to execute determination of whether or not over stacking has occurred. Note that the predetermined range with respect to the stacking height or the number of stacked sheets refers to a range of variation in accuracy with which the stacking degree determination unit can determine the stacking height or the number of stacked sheets, for example.

As described regarding the condition for executing the over stacking determination, the lift-up time required for lifting up of the intermediate plate143may be employed as the execution condition. The intermediate plate143is a plate member that is lowered to the lowermost portion upon the feed cassette23being pulled out of the image forming apparatus. A lift-up control unit168controls the motor160through the drive circuit56and lifts up the intermediate plate143such that the sheets S stacked on the intermediate plate143come into contact with the pickup roller35. The measuring unit166measures the lift-up time necessary for lifting up the intermediate plate143. When the lift-up time is smaller than a lift-up threshold value, the CPU51determines that the stacking degree corresponds to a stacking volume within the predetermined range with respect to the stacking height or the number of stacked sheets. Thus, when the lift-up time is smaller than the lift-up threshold value, the determination unit62determines whether or not sheets S are over-stacked in the feed cassette23. Over stacking may occur when the number of stacked sheets S is close to the upper limit number. Accordingly, the accuracy of the over stacking determination is improved by activating the over stacking determination in a situation where over stacking is likely to occur. Also, when the lift-up time is not smaller than the lift-up threshold value, the determination unit62does not determine whether or not sheets S are over-stacked in the feed cassette23. Thus, erroneous detection of over stacking in a situation where it is unlikely that over stacking has occurred will be suppressed.

The image forming apparatus100may have the surface sensor153for detecting whether or not the surface of a sheet S stacked on the intermediate plate143has been lifted up to a predetermined height H by the motor160. Upon the sheet S stacked on the intermediate plate143having been lifted up to the predetermined height H, the lift-up control unit168stops the motor160through the drive circuit56. Thus, the lifting up of the intermediate plate143stops. Accordingly, it is then possible to always maintain the position of the leading end of the sheet a S at the same position, and the accuracy of the measurement of the conveyance time T improves. That is to say, the accuracy of the over stacking determination based on the conveyance time T also improves. Furthermore, it is also possible to maintain the pressure to be applied to the sheets S by the pickup roller35at a fixed level.

As described regarding the conditions for executing the over stacking determination, the position of the rear end regulating plate141may be employed as the execution condition. The determination unit62determines whether or not the rear end regulating plate141is correctly positioned with respect to the size of the sheets S stacked in the feed cassette23. For example, the determination unit62determines whether or not the position of the rear end regulating plate141detected by the position sensor154corresponds to the size of the sheets S stacked in the feed cassette23. The determination unit62executes over stacking determination if the detected position of the rear end regulating plate141corresponds to the size of the sheets S. On the other hand, the determination unit62does not execute over stacking determination if the detected position of the rear end regulating plate141does not correspond to the size of the sheets S. The determination unit62may determine that the position of the rear end regulating plate141corresponds to the size of the sheets S stacked in the feed cassette23when the size of conveyed sheets S agrees with the size of the sheets S obtained based on the position of the rear end regulating plate141. Also, the determination unit62may determine that the position of the rear end regulating plate141does not correspond to the size of the sheets S stacked in the feed cassette23when the size of the conveyed sheet S does not agree with the size of the sheets S obtained based on the position of the rear end regulating plate141. More specifically, the size of the sheets S conveyed by the pickup roller35can be estimated/measured by a size estimation unit171. The size estimation unit171may be called as a size measuring unit. Furthermore, a conversion unit169converts the position of the rear end regulating plate141detected by the position sensor154into the size of the sheets S (e.g., the length in the conveyance direction). The determination unit62determines whether or not sheets S are over-stacked in the feed cassette23when the size of the sheets S conveyed from the pickup roller35agrees with the size of the sheets S obtained based on the position of the rear end regulating plate141. On the other hand, the determination unit62may not determine whether or not sheets S are over-stacked in the feed cassette23if the size of the sheets S conveyed by the pickup roller35does not agree with the size of the sheets S obtained based on the position of the rear end regulating plate141. Thus, since the over stacking determination is executed in a situation where over stacking is likely to occur, the accuracy of the over stacking determination will improve. The image forming apparatus100may further have the size estimation unit171for estimating/measuring the size of the sheets S conveyed by the pickup roller35. The determination unit62may determine whether or not the execution condition is satisfied by comparing the size of the sheets S estimated/measured by the size estimation unit171with the size of the sheets S obtained based on the position of the rear end regulating plate141. The operation unit59and the communication device58each may function as an input unit for inputting the size of the sheets S conveyed by the pickup roller35. The determination unit62may determine whether or not the execution condition is satisfied by comparing the input size of the sheets S with the size of the sheets S obtained based on the position of the rear end regulating plate141.

The image forming apparatus100may further have the jam detection unit64for determining whether or not a sheet S has jammed based on the elapsed time since the pickup roller35started to convey the sheet S. As described usingFIG. 20and the like, the determination unit62may determine that sheets S are over-stacked in the feed cassette23if a sheet S has jammed. Also, if a sheet does not arrive at a predetermined position on the conveyance path in the period from when conveyance of the sheet is started until the time-counting unit170ends the counting of a predetermined time, the determination unit62may determine that sheets S are over-stacked. As described usingFIG. 20and the like, if a sheet S has jammed, the substituting unit65may cause the determination unit62to determine that sheets S are over-stacked in the feed cassette23by substituting the conveyance time Y with the predetermined value Tk3that is larger than the over stacking threshold value Tk. Thus, over stacking determination can be performed even in a situation where the conveyance time T cannot be determined as a result of the occurrence of a jam. Similarly, if a sheet does not arrive at the predetermined position on the conveyance path in a period from when conveyance of the sheet is started until the time-counting unit170ends the counting of the predetermined time, the substituting unit65may substitute the conveyance time T with the predetermined value Tk3.

As described regarding Embodiment 3, if double feeding occurs, the size of the sheets S conveyed by the pickup roller35does not agree with the size of the sheets S obtained based on the position of the rear end regulating plate141in some cases. The determination unit62determines whether or not the size of the sheets S estimated/measured by the position sensor154and the conversion unit169agrees with the size of the sheets S that is input through the operation unit59or the host computer. If the rear end regulating plate141is correctly positioned with respect to sheets S, the estimated/measured size is to agree with the input size. Accordingly, the determination unit62may determine, based on the result of this determination, whether or not the rear end regulating plate141is correctly positioned with respect to the sheets S. The determination unit62determines whether or not the rear end regulating plate141is correctly positioned, and whether or not the size of the sheets S estimated/measured by the size estimation unit171is larger than the size obtained by the position sensor154or the size designated through the operation unit59or the like. When the rear end regulating plate141is correctly positioned and the size estimated/measured by the size estimation unit171is larger than the size estimated/measured by the position sensor154and the conversion unit169or the designated size, the determination unit62may determine that sheets S are over-stacked in the feed cassette23even if the conveyance time T does not exceed the over stacking threshold value Tk. For example, as described usingFIG. 38A, disagreement of the sheet sizes is likely to occur if sheets S are over-stacked on the locking claws147. Therefore, the determination unit62may determine whether or not over stacking has occurred based on agreement/disagreement of the sheet sizes.

As described regarding the execution conditions, an identification unit173for identifying whether or not the pickup roller35has deteriorated may further be provided. If the pickup roller35has not deteriorated, the determination unit62determines whether or not sheets S are over-stacked in the feed cassette23. On the other hand, if the pickup roller35has deteriorated, the determination unit62does not determine whether or not sheets S are over-stacked in the feed cassette23. If the pickup roller35and/or the feed roller24have deteriorated, the conveyance time T becomes long. Accordingly, the accuracy of the over stacking determination based on the conveyance time T may decrease. Therefore, the determination unit62may increase the determination accuracy by executing the over stacking determination only when the pickup roller35or the like has not deteriorated. The identification unit173may determine that the pickup roller35has deteriorated if a phenomenon in which the conveyance time of sheets S exceeds the over stacking threshold value Tk at least once among M sheets (e.g., 500 sheets) has occurred N (e.g., five) consecutive times. That is to say, it is determined whether or not a phenomenon in which the conveyance time exceeds the over stacking threshold value at least once in a first predetermined number of sheets has consecutively occurred a predetermined number of times. Also, if the conveyance time of none of the predetermined number of consecutive sheets S exceeds the over stacking threshold value Tk, the identifying unit173may determine that the pickup roller35has not deteriorated. If the conveyance time of none of the consecutive sheets S whose number is larger than a predetermined number (e.g., 4000) exceeds the over stacking threshold value Tk, the identifying unit173may resume the determination of whether or not sheets S are over-stacked in the feed cassette23. That is to say, the over stacking determination may be resumed if the conveyance time of a second predetermined number of consecutive sheets or more does not consecutively exceed the over stacking threshold value after it is determined that the pickup roller35has deteriorated. Thus, if it is presumed that the pickup roller35or the like has been replaced with a new one, the over stacking determination may be resumed.

Note that the execution conditions may be that the rollers have not been worn down, the sheet to be conveyed is the first sheet after the feed cassette23is pulled out and pushed in, the estimated/measured amount of stacked sheets S is close to the upper limit stacking amount, and the position of the rear end regulating plate141is correct.

If the determination unit62determines that the sheets S are over-stacked in the feed cassette23, the output unit67may output over stacking information indicating that the sheets S are over-stacked. As a result, the operator and the maintenance company more easily recognizes over stacking. Note that if it is determined that the over stacking of sheets S in the feed cassette23has been resolved after the determination unit62determines that the sheets S are over-stacked in the feed cassette23, the output unit67may stop the output of the over stacking information. Thus, the operator or the maintenance company can easily recognize that over stacking has been resolved. Also, if the cassette sensor161detects pulling out and pushing in of the feed cassette23after the determination unit62determines that sheets S are over-stacked in the feed cassette23, the output unit67may stop the output of the over stacking information. This is because, if the feed cassette23is pulled out and pushed in, there is a possibility that the operator has removed over-stacked sheets S. If the conveyance time no longer exceeds the over stacking threshold value after the determination unit62determines that sheets are over-stacked, the output unit67may stop the output of the over stacking information.

The output unit67may display the over stacking information on the display device of the operation unit59. The operator can thereby be visually notified of over stacking. Also, the output unit67may use the communication device58as a transmission unit for transmitting a message including the over stacking information. The over stacking message may be transmitted to an email address of the maintenance person (maintenance company) of the image forming apparatus100. As a result, the maintenance company can notify a customer of a method for resolving over stacking as part of maintenance service.

Note that, according to the embodiments, the manual bypass tray38and the feed cassette23each function as a stacking unit in which sheets are stacked. The registration roller17and the like function as a conveyance unit for conveying sheets. The CPU51and the time-counting unit61each function as a time-counting unit for counting the conveyance time from when the conveyance unit starts to convey a sheet until the sheet arrives at a predetermined position on the conveyance path. The output unit67, the display device of the operation unit59, the communication device58, and the like each function as an output unit for outputting information regarding over stacking. The CPU51, the image formation control unit63, and the like each function as a control unit for causing the output unit to output information regarding over stacking and causing sheet conveyance by the conveyance unit to be continued, if the conveyance time of a sheet exceeds a first threshold value (the first over stacking threshold value etc.). Furthermore, the CPU51, the image formation control unit63, and the like each function as a control unit for stopping image formation and discharging sheets, or stopping sheet conveyance, if the conveyance time of a sheet exceeds a second threshold value (the conveyance delay threshold value and the jam threshold value etc.) that is larger than the first threshold value. Accordingly, if over stacking is detected, a message informing of over stacking is output, but sheet conveyance and image formation are continued. Furthermore, if a conveyance delay or an erroneous print is detected, image formation is stopped or interrupted and all sheets existing on the conveyance path are discharged. If a jam is detected, sheet conveyance is stopped. The output of the message for giving over stacking notification may be continued unless the over stacking is resolved.

This application claims the benefit of Japanese Patent Application No. 2015-129204, filed Jun. 26, 2015, and Japanese Patent Application No. 2015-129205, filed Jun. 26, 2015, which are hereby incorporated by reference wherein in their entirety.