Image Forming Apparatus Having a Controlled Fixing Means

1. Field of the invention

The present invention relates to an image forming apparatus such as an electrophotographic copying machine, laser printer, or the like, and particularly to a fixing unit incorporated into the image forming apparatus for fixing a toner image on a sheet.

2. Description of the Related Art

In a fixing unit provided in an image forming apparatus for fixing a toner image on a sheet, there is included a heating roller in the form of a cylinder which is internally provided with a heater. The heater is on-off controlled while a surface temperature of the heating roller being detected so that the surface temperature of the heating roller (fixing temperature) can be maintained at a substantially fixed level.

However, the heat released from the heater is sequentially propagated from the air within the heating roller to an inner circumferential surface of the roller, and to an outer circumferential surface of the roller. Accordingly, there is some delay in change in the surface temperature of the heating roller relative to the on-off control of the heater. As a consequence, the surface temperature continues to rise (overshoot) even in the case where the heater is turned off at a fixed temperature. On the other hand, the surface temperature continues to fall (undershoot) even in the case where the heater is turned on at a fixed temperature.

The overshoot and undershoot not only vary in individual fixing units in some degree, but also depend on the ambient temperature and the size of the sheet to be used. Normally, a target temperature based on which the heater is on-off controlled is determined assuming a fixed ambient temperature and a standard size of sheet. However, in the case where the ambient temperature rises or a small-sized sheet is used, the overshoot becomes larger, whereby the sheet is displaced relative to the heating roller due to warping thereof and melting of toner. On the other hand, in the case where the ambient temperature falls or a large-sized sheet is used, the undershoot becomes larger, whereby the following problems will occur: toner is fixed to the sheet in an unsatisfactory manner; toner which is not fixed to the sheet deposits on the heating roller and stains a next sheet.

It is an object of the invention to provide an image forming apparatus capable of maintaining a fixing temperature at a specified level regardless of changes in ambient temperature.

It is another object of the invention to provide an image forming apparatus capable of maintaining a fixing temperature at a specified level regardless of the type of a sheet to be used.

It is still another object of the invention to provide an image forming apparatus which demonstrates a satisfactory fixing performance by maximally suppressing an overshoot and an undershoot.

An image forming apparatus in accordance with the invention measures the surface temperature of a heating roller of a fixing unit having a heater provided therein and maintains the fixing temperature at a specified level by controlling an on-time which starts upon the surface temperature reaching a specified temperature and lasts until the heater is turned on, and an off-time which starts upon the surface temperature reaching the specified temperature and lasts until the heater is turned off. Data concerning the on-time and off-time are stored in a memory beforehand according to use conditions such as the ambient temperature and the size of sheet to be used. The fixing operation is initially carried out in accordance with the data prestored in the memory. Transition of the surface temperature of the heating roller accompanied by the fixing operation is detected, and new data concerning the on-time and off-time are calculated based on the detection result. The calculation result is fed back to the next fixing operation.

The above and further objects and features of the invention will be more fully apparent from the following detailed description with accompanying drawings.

FIG. 1 is a schematic diagram showing a laser beam printer;

FIG. 2 is a block diagram showing a control circuit for a fixing unit;

FIG. 3 is a chart showing a difference in fixing temperature change due to a difference in ambient temperature;

FIG. 4 is a flow chart showing summarily a temperature control;

FIG. 5 is a chart showing a control timing and a change in the fixing temperature in an initial preheating control;

FIG. 6 is a chart showing an on-off control timing and a change in the fixing temperature of the heater in a standby state;

FIG. 7 is a chart showing a change in the fixing temperature before a temperature control pattern is corrected;

FIG. 8 is a chart showing a change in the fixing temperature after the temperature control pattern is corrected;

FIG. 9 is a chart representing the overshoot value as a function of a heater off-timing .DELTA.t;

FIG. 10 is a chart showing a control in the case where the peak of the overshoot does not reach a target temperature;

FIG. 11 is a chart showing a control in the case where the bottom of the undershoot does not reach the target temperature;

FIG. 12 is a chart showing a control in the case where the peak of the overshoot does not reach the target temperature;

FIG. 13 is a chart showing a change in the fixing temperature, on- and off-timings of the heater, and timer start timings in a fixing temperature control;

FIG. 14 is a chart showing a change in the fixing temperature, on- and off-timings of the heater, and timer start timings in a fixing temperature control;

FIGS. 15(a), 15(b), and 15(c) are charts respectively showing a change in the fixing temperature, on- and off-timings of the heater, and timer start timings in a fixing temperature control;

FIGS. 16(a), 16(b), and 16(c) are charts respectively showing a change in the fixing temperature, on- and off-timings of the heater, and timer start timings in a fixing temperature control;

FIG. 17 is a flow chart showing a main routine of a CPU;

FIG. 18 is a flow chart showing a subroutine for controlling the fixing temperature;

FIG. 19 is a flow chart showing a subroutine for controlling an initial preheating of the fixing unit;

FIG. 20 is a flow chart showing a subroutine for estimating the ambient temperature;

FIGS. 21, 22 are flow charts showing a subroutine for controlling the fixing temperature in the standby state;

FIG. 23 is a flow chart showing a subroutine for estimating the ambient temperature;

FIGS. 24, 25 are flow charts showing a subroutine for a control at the time of overshoot;

FIGS. 26, 27 are flow charts showing a subroutine for a control at the time of undershoot;

FIG. 28 is a flow chart showing a subroutine for a control at the target temperature;

FIGS. 29, 30 are flow charts showing a subroutine for executing a correction calculation;

FIG. 31 is a chart showing a response of the fixing temperature relative to an on-state and an off-state of the heater;

FIG. 32 is a flow chart showing a subroutine for a control at the time of overshoot;

FIG. 33 is a flow chart showing a subroutine for a control at the time of undershoot;

FIG. 34 is a flow chart showing a subroutine for a control at the target temperature;

FIGS. 35, 36 are flow charts showing a subroutine for executing a correction calculation.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIRST EMBODIMENT

A first embodiment will be described with reference to FIGS. 1 to 30. FIG. 1 is a diagram schematically showing a construction of a laser beam printer provided with a fixing unit 5 representing a first embodiment of the invention. This printer comprises a laser beam scanning/optical unit 1, an image forming unit 2 including a photosensitive drum 3, a feeding unit 4 including a cassette removably attachable to a main body of the printer, a toner fixing unit 5, a discharge tray 6 and a central processing unit (CPU) 7. Individual elements and an image forming process are identical to the existing image forming apparatus, and thus description thereof will be omitted.

The fixing unit 5 comprises a heating roller 10 and a pressing roller 11. The heating roller 10 is provided internally with a heat lamp (hereinafter referred to as a heater) 12 as heat source. The pressing roller 11 is disposed below the heating roller 10 for pressing the roller 10 from below. A thermister 13 is disposed in a specified position in the vicinity of an outer circumferential surface of the heating roller 10 for detecting the temperature.

As shown in FIG. 2, to the CPU 7 are connected a memory 20 for storing control data to be described below and a size detector 21 for detecting the size of a sheet to be used. A fixing temperature signal from the thermistor 13 is inputted to the CPU 7, which in turn outputs a heater remote signal to a switch 14 through which the heater 12 is on-off controlled in a manner to be described below.

With a temperature control by on-off controlling the heater 12, a surface temperature of the heating roller 10 temporarily rises and then falls after the heater 12 is turned off (overshoot), and the surface temperature temporarily falls and then rises after the heater 12 is turned on (undershoot) as shown in FIG. 3 according to a thermal capacity of a heat transmission path from the heater 12 to the heating roller 10. FIG. 3 shows the result of a control for maintaining the surface temperature of the heating roller 10 at 185.degree. C., wherein a one-dot chain line A represents a state where the ambient temperature is high, and a solid line B represents a state where the ambient temperature is low. As will be apparent from FIG. 3, the overshoot becomes large when the ambient temperature is high. On the other hand, the undershoot becomes large when the ambient temperature is low. The undershoot becomes even larger when the sheet is passed through.

In the first embodiment, in order to maintain the surface temperature at a target temperature by maximally suppressing the overshoot and undershoot, the heater 12 is turned on or off before the surface temperature reaches the target temperature. In order to suppress the overshoot, the heater 12 is turned off earlier when the ambient temperature is high than when the ambient temperature is low. In order to suppress the undershoot, the heater 12 is turned on earlier when the ambient temperature is low than when the ambient temperature is high.

As a parameter for determining fixing temperature characteristics, there exist a variation in thermal capacity inherent in the fixing unit 5, a variation in output of the heater 12, and differences in size of sheets besides the ambient temperature. The fixing temperature characteristics determined by these parameters will be known when a printing operation is repeated several times. Accordingly, in case of a multi-printing operation in which more than several prints are to be made, the temperature can be controlled by means of a feedback control in consideration of variation factors. However, since a feedback control cannot be effected for the first several prints, one temperature control pattern is selected from a plurality of those prestored in the memory 20. The printing operation for first several prints is executed according to the selected temperature control pattern. In the case where the printing operation is executed more than several times, the temperature control pattern is corrected based on data obtained from the preceding printing operation, and the printing operation for the subsequent prints is executed according to the corrected temperature control pattern. In this way, the control data is renewed each temperature control pattern, thereby sufficiently responding to factors which are not to change for a short period of time, such as changes over time which are inherent in the fixing unit 5 and output of the heater 12.

However, the prior fixing control data is useless for factors which are to change greatly each printing operation, such as the sheet size and the ambient temperature (e.g., early morning and daytime). Accordingly, it is necessary to store a plurality of control data classified according to these factors.

FIG. 4 is a flow chart conceptually showing the foregoing control. More specifically, when a main switch of the printer is turned on, the temperature of the fixing unit 5 is adjusted in Step S1, i.e., preheating of the heating roller 10 is started so as to rise the surface temperature of the heating roller 10 up to a predetermined level. Upon completing adjustment of the temperature of the heating roller 10 in Step S2, the external ambient temperature is estimated in Step S3 based on the time required for the preheating of the roller 10. In the case where a printing operation has not started yet (NO in Step S4), i.e., the printer is in a standby state, the external ambient temperature is estimated based on an undershoot time while the surface temperature of the heating roller 10 is changing in Step S5. Estimation of the ambient temperature is carried out by the use of TABLE 1 prestored in the memory 20.

When the printing operation is started (YES in Step S4), timings at which the heater 12 is turned on or off are determined from TABLE 2 based on the external ambient temperature and a sheet size to be used in Step S6.

The next heater on-, off-timings are calculated based on the maximum values of the overshoot and undershoot according to the actual temperature control characteristics during the printing operation in Step S7, and TABLE 2 is rewritten and stored in the memory 20. Calculation in Step S7 is executed in accordance with TABLE 3. While the printing operation is being continued (NO in Step S8), the printing operation is executed with the sequentially corrected control patterns. When the printing operation is completed (YES in Step S8), the main routine returns to Step S4.

Parameters (variation factors) of the fixing temperature will be described here.

AMBIENT TEMPERATURE:

Generally, printers and electrophotographic copying machines are not provided with a device for measuring an external ambient temperature. It is not preferable to provide such a measuring device specially for embodying the present invention since it results in complicated construction of the printers and the copying machines. In view of this, a method has been developed which estimates the external ambient temperature without providing the measuring device. One method is, as shown in Step S3 of FIG. 4, to measure the external ambient temperature on the basis of the time required from the power-on to completion of preheating of the fixing unit 5. Another method is, as shown in Step 5 of FIG. 4, to measure the external ambient temperature on the basis of the undershoot time during the standby state.

FIG. 5 shows a change in the fixing temperature (the surface temperature of the heating roller 10) immediately after the main is turned on and thereby the heater remote signal is changed to on-level. The ambient temperature changes with a time t1 which starts upon the rise of the heater remote signal to the on-level and lasts until completion of the preheating. Therefore, the ambient temperature can be estimated according to the value of the preheating time t1.

FIG. 6 shows a change in the fixing temperature in a standby state. A time t2 which starts upon the rise of the heater remote signal to the on-level and lasts until the fall thereof to the off-level depends on the ambient temperature. Therefore, the ambient temperature can be estimated according to the value of the on-off time t2.

In the first embodiment, as shown in TABLE 1, the ambient temperature is estimated into four stages: high temperature; normal temperature; low temperature 1; and low temperature 2.

SHEET SIZE:

The degree at which the sheet takes the heat from the heating roller 10 when passing through the fixing unit 5 differs depending on the size thereof. The larger the sheet, especially with respect to a feeding direction thereof, the more heat is taken from the heating roller 10. In the first embodiment, the size of sheets is classified into three groups, namely "large," "medium," and "small" as shown in TABLE 2. "Large" sheets include those whose length in the feeding direction is 300 mm or longer (for example, A3, A4). "Medium" sheets include those whose length in the feeding direction lies between 251 mm and 299 mm (for example, A4 when longitudinally fed, B5). "Small" sheets include those whose length in the feeding direction is 250 mm or shorter (for example, A5, B6).

In view of four stages of ambient temperature and three groups of sheet size, initial temperature control patterns, twelve in total (see TABLE 2), are prepared in advance and stored in the memory 20.

Next, there will be described correction of the temperature control pattern.

As shown in FIG. 7, a moment at which the fixing temperature becomes lower than the target temperature of 185.degree. C. is used as a starting point. It is assumed that the overshoot of .DELTA.c1 occurs when the heater 12 is turned off a time .DELTA.t1 after the starting point of the overshoot. Further, a moment at which the fixing temperature becomes in excess of the target temperature of 185.degree. C. is used as a starting point of the undershoot. It is assumed that the undershoot of .DELTA.c2 occurs when the heater 12 is turned on .DELTA.t2 after the starting point.

Regarding the overshoot, the overshoot .DELTA.cn (n=1, 3, 5, . . . ) can be represented as a function of .DELTA.tn (n=1, 3, 5, . . . ) which starts from the starting point thereof and lasts until the heater 12 is turned off. This function .DELTA.cn can be approximated to the following linear function (1) of .DELTA.tn:

The purpose here is to obtain .DELTA.tn which makes .DELTA.cn approximately 0. Firstly, .DELTA.c(n+2)=a(n+2).DELTA.t(n+2)+b(n+2) can be obtained from the equation (1). Constants an, bn are respectively equal to a(n+2), b(n+2), i.e., an=a(n+2), bn=b(n+2). From these expressions, the constants an, bn are obtained as follows:

By using .DELTA.t1 obtained from TABLE 2, .DELTA.cn can be measured as shown in FIG. 7. Here, in order to apply the equations (2), (3), two pairs of data are required. Accordingly, .DELTA.c3 is measured tentatively on the assumption that .DELTA.t3 =2/3(.DELTA.t1). By using two pairs of data (.DELTA.t1, .DELTA.c1), (.DELTA.t3, .DELTA.c3) thus obtained, constants a1, b1 are obtained based on the equations (2), (3).

By using the constants a1, b1, the following equation (4) can be obtained based on the equation (2).

wherein .DELTA.c5=0.

Thereby, the time .DELTA.t5 determining a next timing at which the heater 12 is turned off (see FIG. 8) can be obtained. Similarly, the time .DELTA.t7 determining a further next timing at which the heater 12 is turned off can be obtained by calculating a3, b3 from another two pairs of data (.DELTA.t3, .DELTA.c3), (.DELTA.t5, .DELTA.c5) .DELTA.t7=-b3/a3

Hereafter, these calculations are made repeatedly. FIG. 9 is a graph showing the calculation results.

What should be taken note of here is:

(1) In the case where .DELTA.tn is calculated to be negative, it is assumed that .DELTA.tn=0 since the negative control cannot be executed.

(2) In the case where a peak temperature of the overshoot is below the target temperature of 185.degree. C. as shown in FIG. 10, i.e., .DELTA.cn is negative, the heater 12 is turned on at that moment, which is in turn used as a starting point of the time .DELTA.t(n+2) for the next heater off-timing.

(3) In the case where the heater on-timing is too early, whereby a bottom temperature of the undershoot is above the target temperature of 185.degree. C. as shown in FIG. 11, the heater 12 is turned off at that moment. However, since the heater off-timing is irregular, the time .DELTA.t(n+2) which is determined by [.DELTA.tn, .DELTA.cn], [.DELTA.t(n-2), .DELTA.c(n-2)] is again adopted as a time .DELTA.t(n+4) for a next off-timing.

The undershoot is determined similarly to the overshoot. Upon lapse of the time .DELTA.t2, the heater 12 is turned on, whereby .DELTA.c2 is obtained. Upon lapse of the time .DELTA.t4=(2/3)t2, the heater 12 is turned on, whereby .DELTA.c4 is obtained. The constants a2, b2 are obtained based on the data (.DELTA.t2, .DELTA.c2), (.DELTA.t4, .DELTA.c4), and a time .DELTA.t6 is obtained for a next on-timing. Thereafter, calculation to obtain data for .DELTA.t(n+4) based on [tn, cn], [t(n+2), c(n+2)] is made repeatedly.

Similar to the overshoot, what should be taken note of in terms of the undershoot is:

(1) In the case where .DELTA.tn is calculated to be negative, it is assumed that .DELTA.tn=0.

(2) In the case where a bottom temperature of the undershoot is above 185.degree. C., the heater 12 is turned off at that moment. The heater on-timing is used as a starting point of the time .DELTA.tn for a next on-timing.

(3) In the case where the heater off-timing is too early, whereby a peak temperature of the overshoot is below 185.degree. C. as shown in FIG. 12, the time .DELTA.t(n+2) which is determined by [.DELTA.tn, .DELTA.cn], [.DELTA.t(n-2), .DELTA.c(n-2)] is again adopted as a time .DELTA.t(n+4) for a next off-timing.

Next, a control procedure by the CPU 7 will be described with reference to flow charts shown in FIGS. 17 to

FIG. 17 shows a main routine of the CPU 7. When the program starts upon the printer being powered on, initialization is executed such as reset of RAM in Step S11 and an internal timer starts in Step S12. In Step S13, keyed inputs from an unillustrated console panel and inputs from various sensors within the printer are processed. In Step S14, it is discriminated whether a printing operation is being performed. If the printing operation is being performed (YES in Step S14), the print operation such as feeding and transporting of the sheet, exposure, and development is carried out in Step S15, and the main routine proceeds to Step S16. If the result of discrimination in Step S14 is NO, the main routine proceeds directly to Step S16. In Step S16, a subroutine is executed in which the fixing temperature is controlled as described in detail below. Next, in Step S17, display on the console panel, a trouble processing, or other processing is executed. When completion of the internal timer is confirmed, the main routine returns to Step S12. The internal timer is adapted to determine a duration (10 msec. in this embodiment) of one execution of the main routine, and serves as a reference for a timer and a counter used in the respective subroutines.

FIG. 18 shows a subroutine of a fixing temperature control executed in Step S16.

Firstly, it is discriminated whether the printing operation is being performed in Step S21. If the printing operation is not being performed (NO in Step S21), it is discriminated whether the initial transient control is being effected in Step S22, i.e., the main switch is turned on and thereby the printer is brought into the initial transient state. The printer enters the initial transient control phase in Step S23 immediately after the main switch being turned on. More specifically, a time tA required to complete application of preheating power supply shown in FIG. 13 is measured and the external ambient temperature is estimated. Then, the fixing temperature control is effected in the printing operation immediately after the initial transient control. When the printer is standby, the standby control is executed in Step S24. In other words, the undershoot time tB in FIG. 13 is measured and the external ambient temperature is estimated. The estimated external ambient temperature is used as temperature control data in the printing operation.

On the other hand, if it is discriminated that the printing operation is being performed in Step S21, this subroutine can directly proceed to the fixing temperature control for the printing operation even in the case where the printing operation is started in a region C during the overshoot. This is because the timer used for turning on or off the heater 12 is started. The fixing temperature controls include a control at the time of overshoot executed in Step S28 (a region E in FIG. 13), a control at the time of undershoot executed in Step S30 (a region D in FIG. 13), and a control at the time when the fixing temperature is equal to the target temperature executed in Step S31.

For the above temperature controls, the current fixing temperature c1 is measured by the thermistor 13 in Step S25. In Step S26, the current fixing temperature c1 is compared with the fixing temperature c2 measured in the preceding routine. Since the duration of one routine is set at such a short period of 10 msec., the subroutine immediately returns to the main routine if c1=c2. If c1.noteq.c2, it is discriminated whether or not the current fixing temperature c1 is higher than the target temperature of 185.degree. C. in Step S27. If the current fixing temperature c1 is higher than the target temperature (YES in Step S27), the control at the time of overshoot is effected in Step S28. If, on the contrary, the current fixing temperature c1 is not higher than the target temperature (NO in Step S27), it is discriminated whether the fixing temperature c1 is lower than the target temperature in Step S29. If the current fixing temperature c1 is lower than the target temperature (YES in Step S29), the control at the time of undershoot is executed in Step S30. If the fixing temperature is equal to the target temperature, the control at the target temperature is executed.

FIG. 19 shows a subroutine of the initial transient control executed in Step S23.

Firstly, the heater 12 is turned on in Step S41. In Step S42, the current fixing temperature c1 is measured. Next, in Step S43, it is discriminated whether application of preheating power supply has been completed. If the application of preheating power supply has not yet completed (NO in Step S43), a preheating counter is set in Step S48 so as to count down the time required for completion of preheating power application, i.e., the fixing temperature reaches the target temperature. In Step S49, the current fixing temperature c1 is stored as temperature c2 in Step S49. Upon completion of preheating power application, a subroutine of estimating the ambient temperature is executed in Step S44 (see FIG. 20). Thereafter, the heater 12 is turned off in Step S45, and the preheating counter is reset in Step S46. Consequently, the current fixing temperature c1 is stored as temperature c2 in Step S47.

FIG. 20 shows the subroutine of ambient temperature estimation executed in Step S44.

In this subroutine, the count value of the preheating counter is discriminated to be smaller than 7000, 6000, and 8000 respectively in Steps S61, S62, and S65. The count value is indicative of the duration of the application of preheating power supply (1 count=10 msec.), and corresponds to TABLE 1. Accordingly, if the count value is smaller than 6000, "high temperature" is stored as ambient temperature in the memory 20 in Step S63. If the count value is between 6000 and 6999, "normal temperature" is stored as ambient temperature in the memory 20 in Step S64. If the count value is between 7000 and 7999, "low temperature 1" is stored as ambient temperature in the memory 20 in Step S66. If the count value is 8000 or greater, "low temperature 2" is stored as ambient temperature in the memory 20 in Step S67.

FIGS. 21, 22 show a subroutine of a control in the standby state executed in Step S24.

The purpose of this subroutine is to on-off control the heater 12 so as to maintain the fixing temperature at 185.degree. C. in the standby state, measure the undershoot time, and estimate the ambient temperature based on the measured undershoot time, and determine on-, and off-timings of the heater during the copying operation. The on-, and off-timings are determined by timers Ton, and Tof respectively. As shown in FIG. 14, the timer Ton is started from an overshoot starting point E1, and the timer Tof is started from an undershoot starting point D1.

Firstly, the current fixing temperature c1 is measured in Step S71. In Step S72, it is discriminated whether the temperature c1 is equal to 185.degree. C. If the temperature c1 is equal to 185.degree. C. (YES in Step S72), it indicates either the starting point D1 or E1. Accordingly, the timer Ton, or Tof is reset in Step S73. If the temperature c1 is not equal to 185.degree. C. (NO in Step S72), it is discriminated whether the temperature c1 is higher than 185.degree. C. in Step S74. If the temperature c1 is higher than 185.degree. C. (YES in Step S74), the heater 12 is turned off in Step S75, and the timer Ton is caused to start counting in Step S76. Subsequently, the current fixing temperature c1 is compared with the fixing temperature c2 measured in the preceding routine in Step S77. If c1>c2, it means the fixing temperature has passed a lower limit of the undershoot, and therefore a peak flag is set in Step S78. The peak flag is adapted to determine where the current temperature lies in the fixing temperature curve.

After the processing in Step S78 or if c2.gtoreq.c1 (YES in Step 77), it is discriminated whether the count value of the undershoot counter is 0 in Step S79 (counting is executed in Step 86 in FIG. 22). If the count value is not 0 (NO in Step S79), the ambient temperature is estimated based on the count value in Step S81. Subsequently, the undershoot counter is reset in Step S82, and the current fixing temperature c1 is stored as temperature c2 in Step S83. If the count value is 0 (YES in Step S79), the current fixing temperature c1 is stored as temperature c2 in Step S80 similarly to Step S83.

On the other hand, if the fixing temperature c1 is not higher than 185.degree. C. (NO in Step S74), the heater 12 is turned on in Step S84. In Step S85, the timer Tof is caused to start counting, and in Step S86 the undershoot counter is caused to start counting. More specifically, the time during which the fixing temperature is not higher than 185.degree. C. is measured in Step S86. Subsequently, the current fixing temperature c1 is compared with the temperature c2 measured in the preceding routine in Step S87. If c1<c2, the peak flag is set in Step S88. If c1.gtoreq.c2, the subroutine proceeds directly to Step S89 in which the current fixing temperature c1 is stored as temperature c2.

FIG. 23 shows a subroutine of the ambient temperature estimation executed in Step S81.

In this subroutine, the count value of the undershoot counter is discriminated to be smaller than 200, 500, and 1000 respectively in Steps S91, S92, and S95. The count value is indicative of the undershoot time (1 count=10 msec.), and corresponds to TABLE 1. Accordingly, if the count value is smaller than 200, "high temperature" is stored as ambient temperature in the memory 20 in Step S93. If the count value is between 200 and 499, "normal temperature" is stored as ambient temperature in the memory 20 in Step S94. If the count value is between 500 and 999, "low temperature 1" is stored as ambient temperature in the memory 20 in Step S96. If the count value is 1000 or more, "low temperature 2" is stored as ambient temperature in the memory 20 in Step S97.

FIGS. 24, 25 show a subroutine of a control at the time of overshoot which is executed in Step S28. This subroutine will be described with reference to a graph showing temperature changes in FIG. 15.

Firstly, a flag indicative of completion of calculations (calculation completion flag) is reset in Step S101, and it is discriminated whether the heater 12 is on in Step S103. Initially, the result of discrimination in Step S103 is NO since the heater 12 is off, and the subroutine proceeds to Step S104 in which the timer Ton is caused to start counting. In Step S105, the data .DELTA.tn for a proper heater on-timing is read out from the memory 20. Subsequently, the count value ton of the timer Ton is compared with the data .DELTA.tn in Step S106. If the count value ton has not reached the data .DELTA.tn, the subroutine proceeds to Step S109 in which it is discriminated whether the peak flag is set. In the case where the peak of the overshoot has been reached, a processing in Steps S110 to S116 is executed. In the case where the peak of the overshoot has not been reached, a processing in Steps S117 to S122 is executed. The peak of the overshoot has not been reached yet initially, and accordingly the current fixing temperature c1 is compared with the fixing temperature c2 measured in the preceding routine. During the time when the temperature is still on the increase, the temperature c1 is stored as temperature c2 in Step S122. When the count value ton of the timer Ton becomes equal to the data .DELTA.tn (YES in Step S106) while the these Steps are repeated, the heater 12 is turned on in Step S107 (see FIG. 15(a)), and the timer Ton is reset in Step S108.

On the other hand, in the case where the current fixing temperature c1 reaches c2, i.e., the peak of the overshoot is reached (YES in Step S117), before the count value ton of the timer Ton becomes equal to the data .DELTA.tn, a peak value .DELTA.cof is calculated by subtracting 185 from the fixing temperature c1 in Step S118. The calculated peak value .DELTA.cof is stored in the memory 20 in Step S119. Simultaneously, the peak flag is set in Step S120, and the current fixing temperature c1 is stored as temperature c2 in Step S121. Thereafter, if the result of discrimination is YES in Step S109, the fixing temperature c1 is stored as temperature c2 in Step S116 while the temperature is still on the decrease. When the count value ton of the timer Ton becomes equal to the data .DELTA.tn (YES in Step S106) during the time when these Steps are being repeated in this way, the heater 12 is turned on in Step S107 (see FIG. 15(b)), and the timer Ton is reset in Step S108.

Upon the heater 12 being turned on, a processing of Steps S103, S109, S110, and S116 is repeated hereafter until the fixing temperature falls below 185.degree. C.

As shown in FIG. 15(c), in the case where the fixing temperature does not fall to 185.degree. C., i.e., the heater 12 is turned on, the fixing temperature reaches the peak of the overshoot and starts increasing (YES in Step S110), the heater 12 is turned off in Step S111. In Step S112, the current fixing temperature c1 is stored as temperature c2, and the value of .DELTA.tn is calculated so as to determine the next heater on-timing and the temperature control pattern is corrected in Step S113. A subroutine of this correction calculation will be described in detail later with reference to FIGS. 29, 30. Thereafter, in Step S114, the peak flag is reset, and in Step S115 the timer Ton is reset, whereby causing it to start counting from 1.

FIGS. 26, 27 are flow charts showing a subroutine of a control at the time of undershoot executed in Step S30. This subroutine will be described with reference to a graph showing temperature variations in FIG. 16.

Firstly, the calculation completion flag is reset in Step S121, and it is discriminated whether the heater 12 is turned off in Step S123. Initially, the result of discrimination in Step S123 is NO since the heater 12 is on. Accordingly, the subroutine proceeds to Step S124 in which the timer Tof is caused to start counting. In Step S125, data .DELTA.tf for a proper heater off-timing is read out from the memory 20. Subsequently, in Step S126, a count value tof of the timer Tof is compared with the data .DELTA.tf. If the count value tof has not reached the data .DELTA.tf, i.e., tof<.DELTA.tf, the subroutine proceeds to Step S129. In Step S129, it is discriminated whether the peak flag is set. If the bottom of the undershoot has been passed, a processing of Steps S130 to S136 is executed. If the bottom of the undershoot has not yet been passed, a processing of Steps S137 to S142 is executed. Initially, the bottom of the undershoot has not been reached, and accordingly the current fixing temperature c1 is compared with the fixing temperature c2 measured in the preceding routine in Step S137. The current fixing temperature c1 is stored as temperature c2 in Step S142 while the fixing temperature is on the decrease. When the count value tof of the timer Tof becomes equal to the data .DELTA.tf (YES in Step S126) during the time when these Steps are repeated, the heater 12 is turned off in Step S127 (see FIG. 16(a)). In Step S128, the timer Tof is reset.

On the other hand, in the case where the current fixing temperature c1 reaches c2, i.e., the bottom of the undershoot is reached (YES in Step S137) before the count value tof of the timer Tof becomes equal to .DELTA.tf, the bottom value .DELTA.con is calculated by subtracting the fixing temperature c1 from 185 in Step S138. The calculated bottom value .DELTA.con is stored in the memory 20 in Step S139. Simultaneously, the peak flag is set in Step S140, and the current fixing temperature c1 is stored as temperature c2 in Step S141. Thereafter, the result of discrimination in Step S129 is YES, and accordingly the fixing temperature c1 is stored as temperature c2 in Step S136 while the fixing temperature is on the increase. When the count value tof of the timer Tof becomes equal to the data .DELTA.tf (YES in Step S126) during the time when these Steps are repeated, the heater 12 is turned off in Step S127 (see FIG. 16(b)). In Step S128, the timer Tof is reset.

After the heater 12 is turned off, it is waited until the fixing temperature reaches 185.degree. C. by repeating Steps S123, S129, S130, S136.

As shown in FIG. 16(c), in the case where the fixing temperature does not reach 185.degree. C., i.e., the heater 12 is turned off and the fixing temperature passes the bottom of the undershoot and starts decreasing (YES in Step S130), the heater 12 is turned on in Step S131. Further, the current fixing temperature c1 is stored as temperature c2 in Step S132, and the value .DELTA.tf is calculated so as to determine the next heater off-timing and the temperature control pattern is corrected based on the calculated value .DELTA.tf in step S133. A subroutine of this correction calculation will be described in detail with reference to FIGS. 29, 30. Thereafter, the peak flag is reset in Step S134, and, in step S135, the timer Tof is reset, whereby causing it to start counting from 1.

FIG. 28 is a flow chart showing a subroutine of a control executed in Step S31 at the target temperature.

This subroutine is called upon the fixing temperature reaching the target temperature. Firstly, the peak flag is reset in Step S151. Subsequently, it is discriminated whether count values ton, tof of the timers Ton, Tof are 0 in Steps S152, S155 respectively. If both count values ton, tof are 0, this subroutine proceeds directly to Step S158. If the count value ton is not 0, the timer Ton is reset in Step S153. If the count value ton is 0, but the count value tof is not 0, the timer Tof is reset in Step S156. During the time when the timer Ton is counting, the heater 12 is off despite the fact that the fixing temperature is required to decrease below 185.degree. C. Accordingly, the heater 12 is turned on in Step S154, and the subroutine proceeds to Step S158. During the time when the timer Tof is counting, the heater 12 is off despite the fact that the fixing temperature is required to increase above 185.degree. C. Accordingly, the heater 12 is turned off in Step S157, and the subroutine proceeds to Step S158. In Step S158, the correction calculation to be described hereinafter is processed, FIGS. 29, 30 are flow charts showing the subroutine of correction calculation control executed in Steps S113, S133, S158.

Firstly, it is discriminated whether the calculation completion flag is reset in Step S161. In Step S162, there is discriminated the presence or absence of the data to be calculated. In the case where the calculation has been completed or the absence of the data to be calculated is discriminated, this subroutine is completed immediately.

In the case where the presence of the data to be calculated is discriminated, a control mode is checked in Step S163. In the case of an overshoot control mode, the next heater off-time is calculated in Steps S164 to S169. In the case of an undershoot control mode, the heater on-time is calculated in Steps S170 to S175.

In the case of the overshoot control mode, since only one pair of data is available initially, the data flag has already been reset (NO in Step S164). In Step S168, the data .DELTA.tf is tentatively set at (2/3).DELTA.tf. Simultaneously, .DELTA.cof1, .DELTA.tf1 are respectively set at .DELTA.cof, .DELTA.tf. Subsequently, the data flag is set in Step S169. Next time this subroutine is called, the result of discrimination in Step S164 is YES, and the next off-time .DELTA.tf' is calculated from the latest data (.DELTA.tf, .DELTA.cof) and the previous data (.DELTA.tf1, .DELTA.cof1) with the use of equations (2), (3), (4), and the calculated off-time .DELTA.tf' is stored in the memory 20 in Step S165. Thereafter, the latest data (.DELTA.tf, .DELTA.cof) is set as data (.DELTA.tf1, .DELTA.cof1), the calculation result .DELTA.tf' is set as .DELTA.tf and stored in the memory 20 in Step S166. Further, in Step S167, the calculation completion flag is set, whereby this subroutine is completed.

In the case of the overshoot control mode as well, the processing is executed which is basically similar to that executed in the overshoot control mode so as to calculate the next heater on-time. More specifically, .DELTA.tn is set as (2/3).DELTA.tn in Step S173 in the first routine, and the next on-time .DELTA.tn' is calculated from the data (.DELTA.tn, .DELTA.con), (.DELTA.tn1, .DELTA.con1) and calculated on-time is stored in the memory 20 in Step S171 in the second and subsequent routines.

It should be understood that the on-time .DELTA.tn' and off-time .DELTA.tf' are assumed to be 0 in the case where they are negative in any calculation processing.

SECOND EMBODIMENT

A second embodiment will be described with reference to FIGS. 31 to 36. The first embodiment is directed to a temperature control which is effected so as to maintain the fixing temperature at a fixed level as much as possible by approximating the amplitude of overshoot and undershoot of the fixing temperature to 0. On the contrary, the second embodiment is directed to a temperature control which is effected so as to maintain the fixing temperature at a fixed level as much as possible by approximating the response of the fixing temperature, i.e., the overshoot time and undershoot time, to 0. The apparatus and device shown in FIGS. 1, 2 are employed in this embodiment. A concept of this control is similar to the one shown with reference to the flow chart in FIG. 4 except that next heater on-, or off-timing is calculated on the basis of the overshoot or undershoot time in Step S6.

Response of the fixing temperature is obtained by measuring the time during which the fixing temperature changes in a specified manner after the heater 12 is turned on or off. In the second embodiment, as shown in FIG. 31, when the heater 12 is turned on, a response time is defined as time .DELTA.trn which starts when the fixing temperature falls below the target temperature of 185.degree. C. and ends when it rises above the target temperature again. Further, when the heater 12 is turned off, a response time is defined as time .DELTA.trf which starts when the fixing temperature rises above the target temperature of 185.degree. C. and ends when it falls below the target temperature again.

The heater on-timing .DELTA.tn is determined by starting a timer upon the fixing temperature reaching 185.degree. C. The response time .DELTA.trn is influenced by the ambient temperature, sheet size, or the like as described above. The response time .DELTA.trn is a function of .DELTA.tn, and defined as a linear function: .DELTA.trn=a.DELTA.tn+b.

If two pairs of data (.DELTA.tn, .DELTA.trn), (.DELTA.tn1, .DELTA.trn1) are obtained, constants a, b can be calculated using the equations (5), (6).

Since the purpose of this control is to approximate .DELTA.trn to 0, the next heater on-time is defined as follows.

Similarly, the heater off-time is defined as follows.

Next there will be described a control procedure by the CPU.

The control procedure is similar to the one described with reference to FIGS. 17 to 30 in the first embodiment except for the following points.

It does not particularly matter the peak of the overshoot and the bottom of the undershoot in the temperature control in the standby state shown in FIGS. 21, 22 in the second embodiment. Accordingly, Steps S77, S78, S87, S88 are omitted.

In the control at the time of overshoot and undershoot, the subroutine shown in FIG. 32 is executed instead of the one shown in FIGS. 24, 25, and the subroutine shown in FIG. 33 is executed instead of the one shown in FIGS. 26, 27. In the control at the time of overshoot, each time the subroutine shown in FIG. 32 is called, a response counter Trf is caused to start counting in Step S102. In the control at the time of undershoot, each time the subroutine shown in FIG. 33 is called, a response counter Trn is caused to start counting in Step S122. The processing executed in other Steps S103 to S108, S123 to S128 in the second embodiment is similar to the one executed in the same Steps in the first embodiment.

Further, in the control at the target temperature, the subroutine shown in FIG. 34 is executed instead of the one shown in FIG. 28.

In this embodiment, upon the fixing temperature reaching the target temperature, the temperature control is changed to the undershoot control if the overshoot control has been executed. On the other hand, the temperature control is changed to the overshoot control if the undershoot control has been executed.

Firstly, in Step S201, it is discriminated whether the temperature control has been the overshoot control or undershoot control. If the temperature control has been the overshoot control, a count value trf of the response counter Trf is stored in Step S202. In Step S203, it is discriminated whether the count value ton of the timer Ton is 0. If the count value ton is 0 (YES in Step S203), this subroutine proceeds to Step S210. If the count value ton is not 0 (NO in Step S203), the timer Ton is reset in Step S204 and the heater 12 is turned on in Step S205 because the fixing temperature is decreasing below the target temperature, then proceeding to Step S210.

On the contrary, if the temperature control has been the undershoot control, the count value trn of the response counter Trn is stored in Step S206. It is discriminated whether the count value tof of the timer Tof is 0 in Step S207. If the count value tof is 0 (YES in Step S207), the subroutine proceeds to Step S210. If the count value tof is not 0 (NO in Step S207), the timer Tof is reset in Step S208 and the heater 12 is turned off in Step S209 because the fixing temperature is increasing above the target temperature, then proceeding to Step S210.

In Step S210, the correction calculation is executed. Subsequently, the response counters Trn, Trf are reset in Step S211, whereby completing this subroutine.

FIGS. 35, 36 shows a subroutine of a correction calculation control executed in Step S210.

This subroutine is basically similar to the one shown in FIGS. 29, 30. The data to be calculated are (.DELTA.tf, .DELTA.trf), (.DELTA.tf1, .DELTA.trf1) at the time of overshoot, and (.DELTA.tr, .DELTA.trn), (.DELTA.tn1, .DELTA.trn1) at the time of undershoot. The next off-time .DELTA.tf' and the next on-time .DELTA.tn' are calculated in the respective cases. Step numbers used in FIGS. 35, 36 are same as those used in FIGS. 29, 30.

It will be understood that a fixing unit in accordance with the present invention is not limited to the foregoing two embodiments, and can be modified in various manners within the scope of the invention.

For example, the invention may be applied to an electrophotographic copying machine of the analog type which uses visible light besides a laser beam printer.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within meets and bounds of the claims, or equivalence of such meets and bounds thereof are therefore intended to be embraced by the claims.