Image processing apparatus with preheating control

An image forming apparatus has a fuser that fuses a toner image onto a printing medium by heat and pressure. The fuser temperature is sensed, and a heater in the fuser is controlled to keep the fuser at a constant temperature appropriate for fusing. In addition to constant-temperature control, the fuser is preheated to prevent an anticipated drop in its temperature when the printing medium encounters the fuser. The preheating process is carried out selectively and is omitted when not required, as determined from, for example, the temperature of the fuser when the printing medium enters the image forming unit of the image forming apparatus, the interval between the transport of different sheets of the printing medium, or the average amount of heat generated by the heater during a preceding interval.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus having a fuser that thermally fuses a recording agent onto printing media, more particularly to the temperature control of the fuser.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. H7-248701 describes a method of controlling the temperature of this type of fuser by using a temperature sensor to sense the temperature of the fuser and a comparator to compare the sensed temperature with a threshold. A heater in the fuser is turned on and off according to the comparison result, using different turn-on and turn-off thresholds.

When this method is practiced, the temperature sensor generally senses the surface temperature of a fusing roller. This temperature tends to drop when the surface of the printing medium makes contact with the fusing roller. Although the temperature sensor may detect the drop and turn on the heater, it takes time for heat to reach the surface from the heater, which is located at the center of the fusing roller. The low temperature is therefore not corrected immediately, causing a so-called cold offset that can produce gloss irregularities and possible inadequate fusing.

This problem is aggravated by the relatively low fusing temperatures that are now used to reduce power consumption and increase printing speed in much image forming apparatus. To obtain adequate heating of printing media at these low fusing temperatures, the area of contact between the fusing rollers and the printing medium must be increased. The diameter of the fusing roller or the thickness of the layer of rubber on its surface has therefore been increased, so that it takes even longer for heat to reach the surface from the heater, lengthening the duration of the cold offset.

The cold offset could be mitigated by temporarily raising the turn-on threshold before each sheet of printing media entered the fuser, but this scheme would invite the opposite problem: a hot offset could occur.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image forming apparatus with a fuser that can maintain a stable fusing temperature throughout the fusing process, including the initial part of the fusing process.

The invented image forming apparatus has an image forming unit and a fuser. A toner image is formed on a printing medium as the printing medium is transported through the image forming unit, then fused onto the printing medium by heat and pressure as the printing medium passes through the fuser. The fuser is heated by a heater. A temperature sensor senses the temperature of the fuser. A control unit controls the heater according to the temperature sensed by the temperature sensor to hold the fuser at a predetermined target temperature during the fusing process. The control unit also has the heater preheat the fuser before the fusing process to prevent an anticipated drop in the temperature of the fuser when the printing medium encounters the fuser. The preheating process is carried out selectively according to, for example, the temperature of the fuser sensed as the printing medium enters the image forming unit, the interval between the transport of different sheets of the printing medium, or the average amount of heat generated by the heater during an interval preceding the time when the printing medium enters the image forming unit.

The heater may be disposed inside a fusing roller in the fuser. The preheating process provides the fusing roller with a store of internal heat that is conducted to the surface of the roller in time to compensate for the initial loss of heat to the printing medium, thereby anticipating the temperature drop that occurs when the printing medium encounters the fusing roller and giving the constant-temperature control process a compensating head start so that the surface of the fusing roller can be kept at an appropriate temperature throughout the fusing process.

The fuser may have two fusing rollers with respective heaters and temperature sensors, the preheating of each fuser roller being controlled independently.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters.

First Embodiment

Referring toFIG. 1, the image processing apparatus1in the first embodiment is a two-sided color printer with a printing process unit2having black (K), yellow (Y), magenta (M), and cyan (C) developing units2a,2b,2c,2dand corresponding transfer rollers2f. Printing media can be fed into the printing process unit2from either a cassette3or a double-sided paper feeding unit4. Before entering the printing process unit2, the printing media are aligned by a registration roller5and detected by a printing media sensor6. Within the printing process unit2, the printing media are transported by a transport belt8, which carries the printing media between a photosensitive drum in each developing unit2a,2b,2c,2dand the corresponding transfer roller2f. The printing process unit2functions as the image forming unit by forming black, yellow, magenta, and cyan toner images on the photosensitive drums in the developing units2a,2b,2c,2dand transferring the toner images to the printing media as the printing media travel through the printing process unit2. After leaving the printing process unit2, the printing media pass through a fuser9in which the toner images are fused onto the printing media.

In the following description, the printing media will be assumed to be sheets of printing paper15stored in the cassette3. The sheets of printing paper15are picked up one by one by a hopping roller10and fed into the first part11aof the transport path11, which takes them to the registration roller5. After the leading edge of the printing paper15has been aligned against the registration roller5to correct transport skew, the registration roller begins to turn, sending the printing paper15onto the transport belt8, to which the printing paper is attracted by electrostatic force. Upon leaving the fuser9, if printing has been completed, the printing paper15is directed by a path selection guide12to a third part11cof the transport path11, which takes the printing paper through a delivery unit13and delivers the printed pages into a delivery tray at the top of the image forming apparatus1. In this part11cof the transport path, the printing paper15is driven by a delivery roller14, and is detected by a delivery sensor16. In double-sided printing, after an image has been fused onto one side of the printing paper15, the path selection guide12sends the printing paper15to the double-sided paper feeding unit4, which turns the printing paper over, then returns the printing paper to the first part11aof the transport path so that another image can be formed on the other side.

The transport mechanism that transports the printing media thus includes the registration roller5, transport belt8, hopping roller10, and delivery roller14. The path selection guide12is normally set in the position indicated by the solid lines, but is moved to the position indicated by the dashed lines to guide the printing paper15into the double-sided paper feeding unit4for double-sided printing. In the XYZ coordinate system shown in the drawing, the positive X axis indicates the direction in which the printing paper15travels through the printing process unit2, the Y axis is parallel to the axes of rotation of the transfer rollers2f, and the Z axis is orthogonal to the X and Y axes.

The printing media sensor6is, for example, a photoelectric sensor disposed near the transport path11between the registration roller5and the printing process unit1. The printing media sensor6outputs a media detection signal at, for example, the high logic level (H) while printing paper15is traveling past the printing media sensor6and at the low logic level (L) at other times, i.e. when no printing paper15is present facing the printing media sensor6. Similarly, the delivery sensor16outputs a media exit signal that is high (H), for example, when the printing paper15is traveling past the delivery sensor16and low (L) at other times.

The fuser9includes an upper fusing roller51and a lower fusing roller52, the axes of rotation of which are parallel to the Y axis in the coordinate system shown in the drawing. The upper fusing roller51turns in direction A; the lower fusing roller52turns in direction B. The upper fusing roller51is driven by a motor; the lower fuser roller is urged by springs (not shown) against the upper fusing roller51and turns in compliance with the upper fusing roller51.

The surface temperatures of the fusing rollers51,52are detected by respective fusing roller temperature sensors64,65of the contact type, comprising thermistors. Each temperature sensor includes a plate spring that presses the thermistor end against the fusing roller surface with a predetermined force. The thermistor end of the lower fusing roller temperature sensor65follows the slight upward and downward motion of the lower fusing roller52caused by the passage of printing media between the fusing rollers.

The fusing rollers51,52are heated by respective internal heaters66,67. Both fusing rollers51,52have surfaces made of a resilient material such as rubber.

Referring toFIG. 2, the control system of the image forming apparatus1includes an apparatus controller100that manages and controls the entire image forming process. A separate fusing control unit101, connected to the apparatus controller100by a serial communication interface, controls the operations related to fusing. The fusing control unit101includes a computing device such as a microprocessor with functions for executing commands received from the apparatus controller100, returning data to the apparatus controller100, sequencing the fusing operations, and synchronizing these operations with other image forming operations.

In particular, the fusing control unit101is connected to a fusing motor driving circuit102that receives a drive pulse signal DRPS from the fusing control unit101, increments an internal phase counter according to the received pulses, and switches the excitation phase of a fusing motor103according to the phase count, thereby rotationally driving the fusing motor103, which is a stepping motor.

The fusing control unit101is also connected to the printing media sensor6, from which it receives the media detection signal MEDT; the delivery sensor16, from which it receives the media exit signal MEEX; the fusing roller temperature sensors64,65, from which it receives roller surface temperature information; and a timer A104to which the fusing control unit101supplies a set-time signal SETT to set a timer value and from which it receives a time-up signal TMUP when the set time has elapsed. The roller surface temperature information is received from the temperature sensors64,65in analog signal form; the fusing control unit101converts the analog signals to digital values.

An upper fusing roller heater driving circuit105receives an on/off switching signal SWIU from the fusing control unit101and drives the upper fusing roller heater66, turning the upper fusing roller heater66on when the switching signal SWIU is at the high logic level (H) and off when the switching signal SWIU is at the low logic level (L), for example. A lower fusing roller heater driving circuit106receives a switching signal SWIB from the fusing control unit101and similarly drives the lower fusing roller heater67. The fusing control unit101outputs the switching signals SWIU and SWIB according to the fuser roller surface temperatures sensed by the fusing roller temperature sensors64,65so as to bring the fusing rollers51,52to desired target temperatures, as described below.

The operations of the image forming apparatus that are relevant to fusing temperature control will now be described with reference toFIGS. 1–5.

The fusing control unit101obtains the target temperatures tmu, tmb of the upper and lower fusing rollers51,52from the apparatus controller100, and performs constant-temperature control so as to bring the surface temperatures of the fusing rollers51,52to the target temperatures. In constant-temperature control, at regular intervals of, for example, 0.1 second, the fusing control unit101determines the present surface temperatures tru, trb of the upper and lower fusing rollers51,52from the temperature information sensed by the fusing roller temperature sensors64,65, calculates temperature error values indicating the differences between the surface temperatures and the corresponding target temperatures, calculates fuser roller temperature gradient values, and sets the switching signals SWIU and SWIB to the high or low logic level according to the results of these calculations. The switching signals SWIU and SWIB remain at the set levels until the next interval of, for example, 0.1 second.

The fuser roller temperature gradient values are the differences between the present surface temperature values and the surface temperatures a certain number of intervals before. To calculate these gradient values, the fusing control unit101stores the past temperature values detected over that number of intervals. The fusing control unit101uses a weighted sum of the temperature error and temperature gradient values to decide how to set the switching signals SWIU and SWIB. For example, switching signal SWIU is set to turn the upper fusing roller heater66on if the surface temperature tru of the upper fusing roller51is declining and is already lower than the target temperature tmu, or if the surface temperature tru of the upper fusing roller51is still somewhat above the target temperature tmu but is declining rapidly.

When the printing process begins, the apparatus controller100sends the fusing control unit101a printing start command. The fusing control unit101then monitors the media detection signal MEDT from the printing media sensor6. When MEDT goes high, indicating that the sheet of printing paper15representing the first page of the printing job is about to enter the printing process unit2, the fusing control unit101begins output of the driving pulse signal DRPS to the fusing motor driving circuit102to start turning the fusing motor103, and detects the surface temperatures tru, trb of the upper and lower fusing rollers51,52sensed by the upper and lower fusing roller temperature sensors64,65.

If the surface temperature tru of the upper fusing roller51is less than a preset preheating threshold tss, the fusing control unit101ceases constant-temperature control of this roller and executes a preheating process to raise the roller temperature. First, the fusing control unit101outputs a set-time signal SETT to set a timer value equivalent to a certain preheating time Tp in timer A104, starts timer A104, and sets switching signal SWIU to turn on the upper fusing roller heater66. The fusing roller heater66remains in the on state until the fusing control unit101receives the time-up signal TMUP from timer A104, indicating that the preheating time Tp has elapsed, at which point the fusing control unit101reverts to constant-temperature control.

The preheating threshold temperature tss may be higher than the target temperature tmu. The preheating time Tp may be a fixed value substantially equal, for example, to the time taken for the leading edge of the printing paper15to arrive at the fuser9after passing the printing media sensor6. Alternatively, the preheating time Tp may be varied according to, for example, the type of printing media and the printing speed. When the apparatus controller100sends the fusing control unit101the printing start command, it may also send information on the basis of which the fusing control unit101can select an appropriate preheating time Tp.

If the surface temperature tru of the upper fusing roller51is greater the preheating threshold tss when the media detection signal MEDT goes high, the fusing control unit101does not execute the preheating process for the upper fusing roller51, but continues constant-temperature control.

A similar preheating process is also executed for the lower fusing roller52if its surface temperature trb is less than the preheating threshold tss when the media detection signal MEDT goes high.

The fusing control unit101continues constant-temperature control while the first page is fed through the fusing unit9. When the media exit signal MEEX indicates (by going low, for example) that the first printed page has left the fuser9and passed the delivery sensor16, the fusing control unit101temporarily stops the rotation of the fusing motor103. Then if further pages are printed in succession, each time a new page arrives at the printing media sensor6, processing similar to the processing described above is repeated.

FIG. 3shows an example of the timing of the preheating process described above, taking the upper fusing roller as an example. Before time t1, the fusing control unit101carries out constant-temperature control, using switching signal SWIU to turn the upper fuser roller heater66on and off to keep the surface temperature tru of the upper fusing roller51close to its target temperature tmu. At time t1, the fusing control unit101receives a printing start command and begins driving the motor103that turns the upper fuser roller51. When the leading edge of the printing paper15is recognized by the printing media sensor6at time t2, the fusing control unit101compares the surface temperature tru of the upper fusing roller51with the preheating threshold tss. As the surface temperature tru is lower than the preheating threshold tss at this time, the fusing control unit101holds switching signal SWIU at the high logic level for a preheating interval of length Tp from time t2to time t3, turning the upper fusing roller heater66on for this interval. After the elapse of the preheating time Tp, normal constant-temperature control resumes. Driving of the motor103that turns the upper fuser roller51stops when the trailing edge of the printing paper15passes the delivery sensor16and the media exit signal MEEX goes low at time t4.

FIG. 4shows a similar example in which the preheating process is not executed. Constant-temperature control is carried out as inFIG. 3, but for one reason or another, the surface temperature tru of the upper fusing roller51rises above the threshold value tss. The printing start command is received at time t5and the fusing rollers begin to turn. The temperature tru of the upper fusing roller51is still above the threshold tss when the leading edge of the printing paper15is recognized by the printing media sensor6at time t6. The fusing control unit101accordingly does not execute the preheating process but continues normal constant-temperature control, allowing the roller temperature tru to return to a level near its target value tmu. In this example, the upper fusing roller heater6remains off during the interval of length Tp from time t6to time t7, and is turned on slightly after time t7, when the fusing control unit101recognizes the rapid drop in the surface temperature tru that occurs when the upper fusing roller51begins to lose heat to the printing paper15. The fusing rollers stop turning when the media exit signal MEEX goes low at time t8.

Temperature control of the lower fusing roller52is carried out by similar timing, but is independent of the temperature control of the upper fusing roller51, assuring that each fusing roller stays at or near its target temperature during the fusing process.

In a variation of the first embodiment, the fuser9has a single fusing roller that turns in compliance with a heat-resistant belt. The heat-resistant belt transports the printing medium and presses it against the fusing roller, which has an internal heater. Alternatively, the belt may be heated. Constant-temperature control and preheating are carried out as described above.

In another variation of the first embodiment, the preheating time Tp is varied according to the difference (tmu−tru) between the surface temperature tru and the target temperature tmu as illustrated inFIG. 5, for example. In this example, as the difference (tmu−tru) varies over an interval from a first difference d1to a second difference d2(where d2>d1), the preheating time Tp varies linearly from a minimum value of zero to a maximum value of Tpm. Preheating is not carried out if the difference (tmu−tru) is equal to or less than d1, and the maximum preheating time Tpm is used if the difference (tmu−tru) is equal to or greater than d2. This scheme enables the fusing rollers to receive adequate preheating without being overheated. The preheating time Tp can also be varied according to a weighted sum of the type used in constant-temperature control, taking both the temperature difference and its rate of change or gradient into consideration.

By preheating the fusing rollers before fusing begins, the first embodiment enables the fuser rollers to store enough internal heat near their surfaces to reduce the surface temperature drop that occurs when printing media come into contact with the fusing rollers, despite the thermal resistance and heat capacity of the fusing rollers. When a plurality of pages are printed in succession, the preheating operation before the second and subsequent pages can be skipped if the roller surface temperature remains high, avoiding the supply of excessive heat to the fusing rollers that could occur if preheating were to be carried out unconditionally.

Second Embodiment

The second embodiment is a color printer having the mechanical configuration shown inFIG. 1and the control system shown inFIG. 6. The second embodiment differs from the first embodiment in the addition of a second timer B114to the control system. The fusing control unit101inFIG. 6uses timer B to decide whether or not to execute the preheating process, instead of making this decision by comparing the fusing roller surface temperatures with a threshold temperature as in the first embodiment.

Like timer A104, timer B114receives a set-time signal SETT from the fusing control unit101, measures a corresponding interval of time, and returns a time-up signal TMUP to the fusing control unit101at the end of the interval. The fusing control unit101uses timer B to measure a post-fusing interval Td starting from the time at which the delivery sensor16recognizes the trailing edge of the printing paper15. The fusing control unit101executes the preheating process if the printing media sensor6does not detect the next sheet of printing paper (or other printing media) within the post-fusing interval Td. If the printing media sensor6detects the next sheet of printing paper within the post-fusing interval Td, the preheating process is omitted.

The description of the operation of the control system inFIG. 6will be preceded by an explanation of how the internal and surface temperatures of the upper and lower fusing rollers51,52of the fuser9vary under different conditions. Table 1 summarizes these variations.

In case 1) in Table 1, fusing begins after the fusing rollers51,52have been left in the standby state and the preheating process has not been performed. In the standby state, the rotation of the fusing rollers51,52is halted but constant-temperature control is continued. The standby state in case 1) has continued long enough for both the internal temperatures and the surface temperatures tru, trb of the fusing rollers51,52to settle into a steady state at appropriate levels (OK) near the target temperatures tmu, tmb. When fusing starts, however, the surface temperatures of the fusing rollers51,52abruptly drop as heat is lost to the printing paper15, causing a cold offset. The cold offset occurs because the poor thermal conductivity of the rubber surface material of the fusing rollers prevents heat from being rapidly replenished from the interiors of the fusing rollers to compensate for the heat lost from the surfaces.

In case 2) in Table 1, fusing begins after a standby interval followed by preheating. The preheating process raises the internal temperature of the fusing rollers from the appropriate surface level (OK) to a somewhat higher level (hot), so heat begins to flow from the interiors of the fusing rollers toward their surfaces. When fusing starts, this heat reaches the surfaces of the rollers, offsetting the loss of heat to the printing media. The surface temperatures of the fusing rollers accordingly remain at the appropriate level (OK).

In case 3) in Table 1, pages are printed in succession, and the fusing of each page begins without a preceding standby interval, or with only a brief standby interval. Even though no preheating process is executed, the internal temperatures of the fusing rollers51,52are still hot when fusing begins. The reason is that due to constant-temperature control during continuous printing, heat is continually flowing from the interiors of the fusing rollers51,52to the surfaces of the fusing rollers to replace the heat lost to the printing media, so the control process must keep the interiors of the fusing rollers hotter than their surfaces. Even though the heaters66,67may be turned off during the brief interval between the fusing of one page and the next, during this interval little heat is lost from the surfaces of the fusing rollers51,52, so the flow of heat from their interiors is slowed and the interiors of the fusing rollers retain enough heat to compensate for the surface temperature drop at the start of the fusing of the next page. The surface temperatures of the fusing rollers51,52therefore remain at suitable levels as in case 2).

In case 4) in Table 1, pages are printed in continuous succession but the preheating process is executed unconditionally. As a result of the preheating process the internal temperatures of the fusing rollers51,52, which were higher than the target temperatures to begin with, become even higher, and the surface temperatures of the fusing rollers also rise above the appropriate levels. A hot offset therefore occurs.

Both the cold offset in case 1) and the hot offset in case 4) in Table 1 should be avoided. For consistent fusing, the preheating process should be executed if the standby state has continued for a comparatively long time, but should not be executed if the standby state lasts for only a short time, or for zero time as during continuous printing.

The operation of the second embodiment will now be described with reference toFIG. 7, taking the temperature control of the upper fusing roller51as an example.

InFIG. 7, following an initial standby interval, a single page is printed during a first printing cycle T1; then after an intervening period, two pages are printed successively during printing cycles T2and T3. Printing cycles T1and T2are separated by a time longer than the post-fusing interval Td; printing cycles T2and T3are separated by a time shorter than the post-fusing interval Td. The fusing rollers51,52are driven rotationally during the three printing cycles T1, T2, T3.

During the initial standby interval (not shown), the fusing control unit101carries out constant-temperature control, bringing both the surface temperature tru of the upper fusing roller51and its internal temperature, which is not observable, close to the target temperature tmu. When the media detection signal MEDT input from the printing media sensor6goes high at time t11to indicate that the first page is about to enter the printing process unit2, the fusing control unit101sets switching signal SWIU to the high level for the preheating time Tp, as measured by timer A104, to execute the preheating process. When the media delivery signal MEEX goes low at time t12, indicating that the trailing edge of the first page has passed the delivery sensor16, the fusing control unit101sends timer B114a set-time signal SETT, thereby writing a value equivalent to the post-fusing interval Td in timer B114.

The post-fusing interval Td times out at time t13, before the next page is detected, and timer B114returns a time-up signal TMUP to the fusing control unit101. During the post-fusing interval Td, the internal temperature of the upper fusing roller51, which was somewhat high during the printing cycle T1, returns to the standby level. When the media detection signal MEDT goes high at time t14to indicate that the leading edge of the next page has passed the printing media sensor6, since the fusing control unit101has already received a time-up signal from timer B, it sets switching signal SWIU to the high level to turn on the fusing roller heater66in the upper fusing roller51for the preheating time Tp and execute the preheating process again.

When the second page printing cycle T2ends at time t15, the fusing control unit101sends another set-time signal SETT to timer B114to begin counting the next post-fusing interval Td. At time t16, before the post-fusing interval Td ends, the media detection signal MEDT goes high, indicating that the printing media sensor6has detected the next page. The internal temperature of the upper fusing roller51is still somewhat high at this time, as is the surface temperature tru. Since the fusing control unit101has not received a time-up signal yet, it does not execute the preheating process at the start of the printing cycle T3of the next page, which begins with detection of the printing paper15at time t16. The fusing control unit101turns the upper fusing roller heater66on when the surface temperature tru of the upper fusing roller falls below the target value tmu, ignores the time-up signal at time t17, and leaves the heater66on until the end of the printing cycle at time t18.

In printing cycles T1and T2, when the fusing control unit101executes the preheating process because the printing operation for the next page starts after the elapse of the post-fusing interval Td, the preheating process raises the internal temperature of the fusing roller as in case 2) in Table 1, so that the surface temperature remains normal even after fusing begins. In printing cycle T3, although the fusing control unit101does not execute the preheating process because the printing operation for printing the next page starts during the post-fusing interval Td, the internal temperature of the fusing roller is still hotter than normal, as in case 3) in Table 1, so once again the surface temperature of the fusing roller remains normal after fusing begins.

Similar temperature control is also carried out for the lower fusing roller52.

In a variation of the second embodiment, the length of the preheating interval Tp is varied according to the length of the standby time from the end of one printing cycle to the start of the next printing cycle. For example, the fusing control unit101may use timer B to count the elapse of a post-fusing interval Td repeatedly until the printing of the next page begins. The preheating time Tp may then be varied according to the number of times the count is repeated so that the preheating time Tp increases stepwise with the length of the standby time, as shown inFIG. 8. The preheating time thus becomes responsive to the presumed degree of internal cooling of the fuser rollers and cooling of the roller heaters during the lapse of time after the end of printing.

In another variation of the second embodiment, the fusing control unit101receives information from the apparatus controller100indicating when pages will be printed continuously, and omits the preheating process on the basis of that information.

Like the first embodiment, the second embodiment avoids the temperature drop at the start of fusing due to the heat transfer delay in the fusing rollers. In addition, the second embodiment infers the internal temperature of the fusing rollers from the length of the standby interval between pages, and avoids unnecessary preheating when the fusing rollers are still internally hot, even though their surface temperature may be lower than normal because of heat absorption by the printing media, thereby avoiding the occurrence of hot offset.

Third Embodiment

The third embodiment is a color printer having the mechanical configuration shown inFIG. 1and the control system shown inFIG. 9. The third embodiment differs from the first embodiment in the addition of an average heater-on time calculator107to the control system. The fusing control unit101inFIG. 9uses the average heater-on time calculator107to decide whether or not to execute the preheating process, instead of making this decision by comparing the fusing roller surface temperatures with a temperature threshold.

The average heater-on time calculator107receives the switching signals SWIU and SWIB output from the fusing control unit101to the roller heater driving circuits. At regular averaging intervals, the average heater-on time calculator107calculates the average percentage of time during which each of the roller heaters has been turned on during the preceding averaging interval. The calculated average heater-on times of the upper and lower fusing roller heaters will be denoted Tavu and Tavb, respectively. An average on-time of 100% indicates that the fusing roller heater66or67has been on continuously during the most recent averaging interval; an average on-time of 0% indicates that the fusing roller heater has been off continuously during that interval.

FIG. 10shows a conceptual example of the changes over time of the surface temperature tru of the upper fusing roller51and its average heater-on time Tavu when a plurality of pages are printed continuously under constant-temperature control, without preheating. Constant-temperature control is carried out by the same method for both the upper and lower fusing rollers51,52, so the same concept applies to the lower fusing roller52.

At the first stage of continuous printing, because of the large drop in the fusing roller surface temperature due to the internal heat transfer delay in the fusing roller, the surface temperature tru falls below the target value tmu. The heater is therefore turned on continuously for an extended time, and the average heater-on time Tavu takes on a value near 100%.

Thereafter, as printing continues and the surface temperature tru gradually converges on the target temperature tmu, the average heater-on time Tavu also converges on a constant value. This value is the value at which the amount of heat generated by the heater is equal to the amount of heat lost to the printing paper15, and is about 50% in the present embodiment.

If continuous printing ends at time t22, when the amount of heat generated by the heater66in the upper fusing roller51is approximately equal to the amount of heat lost to the printing paper15, it will be necessary to execute the preheating process to prevent a temperature drop due to heat transfer delay when printing resumes, even if this occurs only shortly afterward. If continuous printing were to end at time t21, however, when the amount of heat generated in the upper fusing roller51was greater than the amount of heat being lost to the printing paper15, the preheating process should not be executed if printing resumes shortly thereafter, because there is still enough heat in the interior of fusing roller to raise the surface temperature in preparation for the temperature drop, and preheating would cause the fusing roller to overheat.

The operation of the control system in the third embodiment will now be described on the basis of the flowchart inFIG. 11. For simplicity, this flowchart illustrates only the preheating control of the upper fusing roller51, omitting the preheating control of the lower fusing roller52and the constant-temperature control process that the fusing control unit101executes when it is not executing the preheating process.

At the beginning of the operation flow inFIG. 11, the fusing control unit101is waiting for the arrival of a printing start command from the apparatus controller100(step S1). After receiving this command, the fusing control unit101waits for the MEDT signal from the printing media sensor6to indicate that the printing paper15is about to enter the printing process unit2(step S2). When the printing paper15is detected by the printing media sensor6, the fusing control unit101starts supplying drive pulses that rotationally drive the fusing rollers51,52at a predetermined speed (step S3), and decides whether or not the average heater-on time Tavu is currently greater than a heater-on time threshold Tcom such as, for example, 50% (step S4).

If the average heater-on time Tavu is less than the heater-on time threshold Tcom, the fusing control unit101sets switching signal SWIU to the high logic level for the preheating interval Tp, turning on the upper fusing roller heater66for this interval and executing the preheating process (step S5). Conversely, if the average heater-on time Tavu is equal to or greater than the heater-on time threshold Tcom, processing proceeds to the next step without execution of the preheating process.

Next, the fusing control unit101waits for the media exit MEEX signal that indicates that the fused printing paper15has emerged from the fuser9and passed the delivery sensor16(step S6). When this signal is received, the fusing control unit101stops rotationally driving the fusing rollers51,52(step S7), and returns to step S1to await the arrival of another printing start command from the apparatus controller100, after which the same processing is repeated.

The third embodiment avoids excessive heating by deciding whether or not to execute the preheating process according to an estimate of the heater temperature and the temperature of the interior of the roller.

In a variation of the third embodiment, the preheating time Tp is varied according to the difference (Tcom−Tavu) between the average heater-on time Tavu and the heater-on time threshold Tcom. A scheme similar to the one illustrated inFIG. 5can be employed, for example.

The third embodiment provides a way to compensate for the internal heat transfer delay of the fusing rollers without the risk of overheating when the difference between the surface temperature and the internal temperature of the fusing rollers is large, as it may be immediately after the printing of a succession of pages ends, for example. Furthermore, since differences in heat absorption due to differences in the thickness of printing media and differences in the rate of heat absorption per unit time due to differences in printing speed appear as differences in the average heater-on time, the preheating process is also responsive to the type of printing media and type of printing, enabling the roller temperature to be kept consistently near the target temperature even under conditions of fluctuating thermal load.

The three different ways of deciding whether or not to execute the preheating process illustrated in the preceding embodiments can be combined. For example, it is possible to decide whether or not to execute preheating by carrying out all three decision processes, weighting the individual decision results, and comparing the weighted sum with a decision threshold.

The invention can also be practiced in an image forming apparatus having a fuser of the belt type instead of a fuser of the roller type.

Those skilled in the art will recognize that further variations are possible within the scope of invention, which is defined by the appended claims.