Abstract:
An image fixing apparatus controls temperature of a rotating member for fixing with compensation for delay by using a Smith predictor with a controlled object model to reduce temperature ripple effectively. In one example, the image fixing apparatus includes a pair of rotating members to form a nip portion, a heater to heat the rotating member, a temperature sensor to detect temperature of the rotating member, a calculator to calculate a heat value to attain a target temperature of the rotating member, a controller to control the heater based on a calculation result of the heat value with compensation for delay using a model according to the calculation result, and a selector to select a control in which an output of the compensation is input into the calculator or a control in which the output of the compensation is not input into the calculator.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an image fixing apparatus and an image forming apparatus equipped with the fixing apparatus, and more particularly to an image fixing apparatus capable of fixing a toner image onto a recording medium by effectively controlling an image fixing temperature in an electrophotographic process. 
   2. Discussion of the Background 
   A background image forming apparatus such as a printer, a copying machine, and a facsimile using an electrophotographic method is generally provided with an image fixing apparatus that melts and fixes a toner image onto a recording medium with heat. This heating type image fixing apparatus commonly includes a heater, a fixing member, and a rotating member. The heater is energized with power to produce heat. The fixing member is heated up to a target temperature by the heater. The rotating member is arranged in contact with the fixing member to form a nip region therebetween. Since the heater keeps the nip region at the target temperature, a recording medium carrying a toner layer thereon undergoes an image fixing process as it passes through the nip region. 
   Stably maintaining the target temperature is a key element of good quality image formation. If the temperature is higher or lower than the target, a poor image, referred to as an offset image, may be formed or poor fixing may occur. Furthermore, reducing a warm-up period of the image fixing apparatus is also needed for shortening a waiting time. 
   One exemplary attempt to shorten the warm-up time is to reduce a heat capacity of the fixing member. This attempt produces a high heat nip between the fixing member, in a form of a thin film having a relatively low heat capacity, and the rotating member, while achieving a reduction of warm-up time. This attempt also achieves a successful use of a belt-shaped fixing member. In this case, the fixing belt is extended between two or more rollers including a first roller having a relatively low thermal conductivity and a second roller serving as a heat source. The first roller closely faces a third roller via the fixing belt to form a nip with the fixing belt. 
   In addition to these low heat capacity examples, the fixing member having a low heat capacity may be provided with an alternative heat source, that is, an induction heater for directly heating the fixing member. 
   The above techniques, however, require a high accuracy in controlling temperature of the fixing member due to its low heat capacity. 
   Controlling electric power to the heater controls temperature of the fixing member. For controlling electric power, temperature sensors are provided in the fixing apparatus such as a thermo pile and a thermo sensitive register to detect a temperature of the fixing member. When the detected temperature is lower than the target temperature, the heater is turned on. When the detected temperature is higher than the target temperature, the heater is turned off. This is called an ON-OFF controlling method. 
   Although the ON-OFF controlling is used in this method, carrying out precise temperature control may be difficult due to a temperature ripple. 
   A temperature ripple induces to use a compensation in which a calculator calculates an amount of operations and outputs the amount to a controlled object based on the desired temperature value and the amount of feedbacks. For example, the amount of proportional integral (PI) or proportional integral differential (PID) operations is calculated as the compensation. 
   Even if the control method with the compensation is used, an exothermic delay in the heater, a heat transmission delay from the heater to a fixing member surface, a detecting delay from a slow response of a temperature sensor, and a delay from a heater driver to the temperature sensor may occur. They may cause a temperature ripple. Furthermore, the temperature ripple may easily occur in a belt fixing apparatus, because the heat delay in the belt is greater. 
   For reducing the temperature ripple, two or more output patterns of a specific pulse corresponding to a detected temperature may be stored and one of the control patterns may be selected according to the detected temperature to carry out the temperature control. As another method, calculating the change rate of difference between the last control timing temperature and the present detected temperature is used for predicting the next control timing temperature to control the temperature. The number of the specific pulses or the width of the specific pulse is used for the control. 
   SUMMARY OF THE INVENTION 
   The present invention provides a novel image fixing apparatus that controls temperature of a rotating member for fixing with compensation for delay by using a Smith predictor with a controlled object model to reduce temperature ripple effectively. In one example, the image fixing apparatus includes a pair of rotating members to form a nip region, a heater to heat the rotating member, a temperature sensor to detect temperature of the rotating member, a calculator to calculate heat value to attain a target temperature of the rotating member, a controller to control the heater based on a calculated result of the heat value with compensation for delay using a model according to the calculated result, and a selector to select a control such that an output of the compensation is input into the calculator or a control such that the output of the compensation is not input into the calculator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is an illustration illustrating an exemplary configuration of an image fixing apparatus according to an exemplary embodiment of the present invention; 
       FIG. 2  is an illustration of a circuit diagram illustrating a PWM drive circuit connected to a controller of the image fixing apparatus of  FIG. 1 ; 
       FIG. 3  is a block diagram illustrating a control system of the controller of  FIG. 2 ; 
       FIG. 4  is a block diagram illustrating a Smith predictor added to the control system; 
       FIG. 5  is a graph showing variations of a rising temperature of a rotating member under control by the control system of  FIG. 3 ; 
       FIG. 6  is a block diagram of another control system; 
       FIG. 7  illustrates a flowchart of a temperature control procedure performed by the control system of  FIG. 6 ; 
       FIG. 8  is a graph showing variations of a rising temperature of the rotating member through the temperature control procedure of  FIG. 7 ; 
       FIG. 9  illustrates a flowchart of another temperature control procedure; 
       FIG. 10  is a graph showing different variations of a rising temperature of the rotating member through the temperature control procedure of  FIG. 9 ; 
       FIG. 11  illustrates a flowchart of a temperature control procedure performed by the control system of  FIG. 6 ; 
       FIG. 12  is an illustration illustrating another image fixing apparatus according to an exemplary embodiment of the present invention; 
       FIG. 13  is a block diagram of a control system of the image fixing apparatus illustrated in  FIG. 12 ; and 
       FIG. 14  is a block diagram illustrating an exemplary configuration of an image forming apparatus having the image fixing apparatus of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image fixing apparatus  1  according to an exemplary embodiment of the present invention is explained. 
   The image fixing apparatus  1  of  FIG. 1  is for use in an electrophotographic image forming apparatus such as a laser printer, a digital copier, a facsimile machine, a printer-fax-copy multifunction machine, etc. This image fixing apparatus  1  uses a roller fixing system. As illustrated in  FIG. 1 , the fixing apparatus  1  includes rotating members  11  and  12 , a heater  13 , a controller  14 , and a temperature sensor  15 . The rotating member  11  includes the heater  13  such as a halogen lamp heater, IH heater, etc. A toner image unfixedly held on a recording medium (e.g., a paper sheet) is pressed and heated in a nip press region formed between the rotating members  11  and  12 , and is fixed on the recording medium. The temperature sensor  15  such as a thermo pile or a thermo sensitive register detects the temperature of the rotating member  11 . The controller  14  control the temperature, driving the heater  13  based on the detected temperature. 
     FIG. 2  is an illustration of a circuit diagram illustrating a pulse width modulation (PWM) drive circuit  21  as a controller to perform a temperature control of the heater  13 . The controller  14  communicates with an image forming apparatus  101  (see  FIG. 14 ) through an interface  22 . The controller  14  also receives a detection signal from the temperature sensor  15 , and outputs a PWM pulse signal into the PWM drive circuit  21 . The PWM drive circuit  21  includes a PWM drive signal generating circuit  23 , a power switching circuit  24 , and a zero crossing detection circuit  25 . The PWM drive circuit  21  PWM-drives the heater  13 . 
     FIG. 3  is a block diagram of the control system of  FIG. 2  in which a feedback control is carried out based on a temperature detected by the temperature sensor  15 . As illustrated in  FIG. 3 , this control system utilizes two portions to cause a time delay considered to be a wasting time period. One time delay referred to as a delay d 1  is generated in the PWM drive circuit  21  and the other time delay referred to as a delay d 2  is generated in a section between the heater  13  and the sensor  15 . The delay d 1  is caused due to an ON-OFF controlling of the heater  13 , and delay d 2  is caused due to a heat transmission from the driver of the heater  13  to the detection by the temperature sensor  15 . Because of these delays d 1  and d 2 , this control system may produce a temperature ripple. When the output of the sensor  15  becomes larger than a target temperature after heating the heater  13 , the heating is stopped. The output of the sensor  15  does not, however, decline by the period of the delay even if the heating is stopped. This may cause the temperature ripple. If a gain of the control system is lowered so that the temperature ripple by the phase delay may not occur, a control error will increase. This may cause another problem. For example, although a steady-state error is small with a PID compensation, a response may become slow. After all, when disturbances and errors occur in the control system, it takes time to reduce them. 
   To solve the above problem, a Smith predictor is used.  FIG. 4  is a block diagram of the control system in which the Smith predictor is added. The Smith predictor  31  outputs a delay compensation using a model based on the result of the calculation of the amount of heating required for setting a temperature of the rotating members  11  and  12 . The Smith compensating method used with the Smith predictor  31  makes the control possible assuming a controlled object without delay. The Smith predictor  31  includes a controlled object model  34  predicted according to a delay d. The Smith predictor  31  also includes a controlled object model  35 . By using the Smith predictor and a calculator  32  for operating the amount of heating to set the temperature of the rotating members  11  and  12 , the delay is reduced in a feedback loop of this predicting model. Thus, a parameter design of the calculator  32  may be performed to the controlled object without delay. As a result, since the control is performed to the controlled object model  34  predicted according to the delay d, the temperature ripple by the above-mentioned delay may be controlled. 
     FIG. 5  is a graph showing a relation of the temperature of the rotating member  11  and time. When the above control method is applied to the fixing apparatus  1 , the temperature ripple is reduced at the time of continuous feeding of the paper to the image forming apparatus  101  as shown in  FIG. 5 . However, the temperature curve becomes gentle near a target temperature at the time of increasing temperature. This increases a waiting time for using the fixing apparatus  1 . 
     FIG. 6  is a block diagram of another example of the control system in which the Smith predictor is added. This control system includes a switch  33  that selects whether to perform the compensation or not. When at least the recording medium is fed continuously in the nip part of the rotating members  11  and  12 , the switch  33  selects the compensation and the delay compensation output is applied to an input side of the calculator  32  and the control is carried out. 
   At the time of continuous feeding of the recording medium, the temperature ripple may easily occur. The temperature ripple is caused by a transmission delay of heating from turning on the heater  13  to the transmission to the surface, a detection delay by the slow response (large time constant) of the temperature sensor  15 , and the delay from the driver of the heater  13  to the temperature sensor  15 . 
   At the time of continuous feeding of the recording medium, the switch  33  selects the compensation and the delay compensation is carried out to reduce the temperature ripple. Thereby, the image may be fixed on the recording medium with stable quality in continuous feeding of the recording medium. At the time of starting to set a temperature of the rotating members  11  and  12 , the switch  33  selects no compensation and the delay compensation output is not applied to the input side of the calculator  32  and the delay compensation is not carried out. 
     FIG. 7  illustrates a flowchart of the control system of  FIG. 6 . The temperature of the rotating member  11  is detected with the temperature sensor  15  (Step S 1 ). When not reloaded, No of Step S 2  is selected. When the detected temperature of the temperature sensor  15  is not satisfied for feeding to the fixing apparatus  1 , No of Step S 3  is selected. In steps S 4  and S 5 , the delay compensation output is not input into the calculator  32 . In steps S 6  and S 7 , a predetermined standby time is waited for, and then the operation returns to step S 3 . When the detected temperature of the temperature sensor  15  is satisfied for feeding to the fixing apparatus  1 , Yes of Step S 3  is selected, and the delay compensation output is input into the calculator  32  (Step S 8 ), and the recording medium is fed (Step S 9 ). 
   A temperature rise time is a time from being in the so-called standby state (Steps S 6  and S 7 ) to reaching the target temperature that paper can be fed. 
   That is, since paper is not fed at the temperature rise, even if some temperature ripples arise, it does not effect the image quality after fixing. On the other hand, the temperature ripple will be reduced but the temperature rise time will increase if control with a delay compensation is performed. 
   So, a quick rise to the target temperature at the time of a temperature rise without a control in which the delay compensation output is input into the input side of the calculator can be realized.  FIG. 8  is a graph showing a relation of the temperature of the rotating member  11  and time verifying the effect of this invention. As shown from  FIG. 8 , the temperature rises quickly, and by the delay compensation after attaining the target temperature, the temperature rise time and the temperature ripple may be reduced. Moreover, when the target temperature is changed, the delay compensation output is not input into the input side of the calculator  32  of  FIG. 6 . This is applied when the target temperature is changed during the delay compensation being performed after attaining the target temperature. 
     FIG. 9  illustrates a flowchart of the control system of  FIG. 6  when the above compensation method is applied. When the delay compensation output is input into the input side of the calculator  32  (Step S 11 ), the temperature sensor  15  detects temperature (Step S 12 ). When the target temperature is changed (Yes of Step S 13 ), the delay compensation output is not input into the input side of the calculator  32  (Step S 14 ). When the detected temperature with the temperature sensor  15  is high enough to feed a recording medium into the fixing apparatus  1  (Yes of Step S 15 ), the delay compensation output is input into the input side of the calculator  32  (Step S 16 ), and the recording medium is fed (Step  21 ). 
   When the target temperature is not changed (No of Step S 13 ), the delay compensation output is input into the input side of the calculator  32  (Step S 17 ), and after a predetermined waiting in a standby state (Step S 18 ), the control returns to Step S 13 . 
   When the detected temperature with the temperature sensor  15  is not high enough to feed a recording medium into the fixing apparatus  1  (No of Step S 15 ), the delay compensation output is not input into the input side of the calculator  32  (Step S 19 ), and after predetermined waiting in a standby state (Step S 20 ), the control returns to Step S 15 . 
   Thereby, the temperature may quickly rise and fall to the target temperature.  FIG. 10  is a graph showing a relation of the temperature of the rotating member  11  and time verifying the effect of this invention. As shown from  FIG. 10 , the target temperature is quickly attained and the temperature ripple may be reduced. Furthermore, when the speed of the recording medium through the nip between the rotating members  11  and  12  is changed, the delay compensation output may not be input into the input side of the calculator  32 . 
     FIG. 11  illustrates a flowchart of the control system of  FIG. 6  when the above compensation method is applied. The differences between  FIG. 9  and  FIG. 11  are Steps S 31  and S 32 . Instead of detecting the temperature with the temperature sensor  15  (instead of Step S 12 ), the rotating speed of the rotating members  11  and  12  is detected with a rotating sensor (not shown) (Step S 31 ). Instead of judging whether the target temperature is changed (instead of Step S 13 ), whether the rotating speed of the rotating members  11  and  12  is changed is judged (Step S 32 ). Except for these operations in Steps S 31 , S 32 , the other controls are the same as those of  FIG. 9 . When changing the rotating speed of the rotating members  11  and  12 , an overshoot or an undershoot of temperature may occur easily. With using the control of  FIG. 1 , the target temperature may be quickly attained for a rising or falling of temperature. 
     FIG. 12  is an illustration illustrating a configuration of another embodiment of a fixing apparatus according to the present invention. The fixing apparatus uses a method of thermal belt fixing. A belt  16  transmits heat from a heater  13  to a rotating member  11 . The same reference element numbers as in  FIG. 1  indicate the same elements as in  FIG. 1 . 
     FIG. 13  is a block diagram of the control system of  FIG. 12 . Due to the heat transmission from the heater  13  to the belt  16 , a delay db from the heater  13  to the sensor  15  occurs. That delay is longer than that of  FIG. 1 . In this system, a larger temperature ripple may occur, so it is difficult to keep temperature control accuracy. Therefore, the above mentioned control is applied to the fixing apparatus of  FIG. 12 . 
     FIG. 14  is a block diagram illustrating a configuration of an embodiment of an image forming apparatus according to the present invention. This image forming apparatus  101  is a digital copier. The image forming apparatus  101  includes a scanner  102  that reads the image of an original, a printer engine  103  that forms an image on a recording medium by an electronic photograph system based on the read image data, and a controller  104  that controls the whole image forming apparatus  101  intensively. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   This patent specification is based on Japanese patent applications, No. JPAP2005-262455 filed on Sep. 9, 2005, and No. JPAP2006-124682 filed on Apr. 28, 2006 in the Japan Patent Office, the entire contents of each of which are hereby incorporated by reference herein.