Patent Publication Number: US-7711280-B2

Title: Image forming apparatus, fixation device and heat control method for a fixation device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, a fixation device and a heat control method for a fixation device. 
   2. Description of the Related Art 
   In an image forming apparatus such as a copying machine, a printer, and a facsimile machine, an image is fixed on a recording medium by transcribing a toner image formed on a photoconductor drum onto a transcription paper as a recording medium and, subsequently applying a heating treatment on it by a fixation roller of a fixation device which roller is referred to as a heating roller. In the fixation device, a fixation roller heated by a heat generation member such as a halogen heater and a member heated by means of electromagnetic induction (a roller with heating means is referred to as a fixation roller, below.) and a pressurizing roller arranged to oppose the fixation roller are pressurized by and contacted to each other so as to form a mutual pressurizing and contacting part referred to as a nip part, and the recording medium on which a toner image has been transcribed passes through and heated at the nip part. 
   A thermistor as a contact-type temperature sensor is configured to contact a fixation roller and fixation failure caused by temperature non-uniformity on the fixation roller is prevented by measuring the temperature of the fixation roller by using the thermistor and controlling the temperature of a heat generation member so that the surface temperature of the fixation roller is constant. However, in the first printing after turning on a power supply, when printing is performed on the condition that a transcription paper and toner have been cooled, the heat of the fixation roller transmits to the transcription paper at the time of fixation and, therefore, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. Particularly, the lowering of the temperature of the nip part is significant in the case of performing printing under a low-temperature environment and non-uniformity of fixation easily occurs. 
   Commonly, in order to prevent such temperature lowering, it is possible to prevent the temperature lowering through paper passage by increasing the thickness of a fixation roller thereby increasing the heat capacity thereof. 
   However, if the thickness of the fixation roller is increased, it takes starting time period until it reaches a certain temperature at the time of heat generation and a time period for providing a printable condition is required from turning on a power supply, whereby there is a problem of degrading the use convenience of a user. As a method for reducing the starting time period, there is provided a method for providing a pressurizing roller with an auxiliary heater so as to compensate the temperature lowering caused by paper passage, but, in this case, a certain cost of a heater, a certain capacity of a power supply, and further a driver for driving an auxiliary heater are needed thereby increasing the cost and there is a problem of consuming extra energy. 
   Then, for example, it is suggested to prevent a temporary lowering of the surface temperature of a fixation roller by heating the fixation roller and a transcription paper using infrared rays generated from a halogen electric bulb in Japanese Laid-Open Patent Application No. 09-054519. Also, it is suggested to prevent a temporary lowering of the surface temperature by heating an intermediate transcriber and toner using a heat pipe in Japanese Laid-Open Patent Application No. 11-065330. 
   On the other hand, commonly, it is possible to prevent temperature lowering caused by paper passage by increasing the thickness of a fixation roller and increasing the heat capacity thereof in order to prevent the temperature lowering. However, if the thickness of a fixation roller, it takes starting time period until it reaches a certain temperature at the time of heat generation and a time period for providing a printable condition is required from turning on a power supply, whereby the use convenience of a user is degraded. Then, as a method for reducing the starting time period, there is provided a method for providing a pressurizing roller with an auxiliary heater so as to compensate the temperature lowering caused by paper passage. However, in this case, a certain cost of a heater, a certain capacity of a power supply, and further a driver for driving an auxiliary heater are needed thereby increasing the cost and there is a problem of consuming extra energy. 
   Therefore, it is necessary to reduce starting time period until it reaches a certain temperature at the time of heat generation by reducing the thickness of a fixation roller, but, as described above, particularly in the first printing under a low-temperature environment, when printing is performed on the condition that a transcription paper and toner have been cooled, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. In this case, since the temperature is changed like a transient response, even if the temperature is detected by a thermistor after the temperature is changed, some time period is practically required until the temperature reaches a target temperature by raising heating temperature and it cannot be followed in a response time. 
   There are problems of increasing the cost as described above in the prevention of temporary temperature lowering at the time of fixation by previously heating a transcription paper, toner or an intermediate transcriber by a heat pipe or other means as the conventional techniques described above, and of increasing a apparatus surface area for configuring them. 
   Therefore, it is necessary to reduce starting time period required for reaching a certain temperature at the time of heat generation by reducing the thickness of a fixation roller, but, as described above, if printing is performed at the condition that a transcription paper or toner have been cooled, the temperature of a nip part is lowered, whereby there is a problem such that non-uniformity of fixation occurs. In this case, since the temperature is changed like a transient response, the following control is hardly conducted in the transient time by the temperature control using a thermistor. 
   As notice of the problem of such a conventional technique is taken, it is desired to suppress degradation of image quality caused by temporal lowering of surface temperature of a fixation roller or degradation of image quality under a lower temperature environment. 
   Meanwhile, as such a technique, for example, a conventional technique disclosed in Japanese Laid-Open Patent Application No. 2002-237377 is known. The conventional technique disclosed in Japanese Laid-Open Patent Application No. 2002-237377 aims at providing a fixation device with a high fixation performance which satisfies the reduction of reactive current provided on commercial alternating current, the prevention of audible frequency noise, constant heating output with an inexpensive structure and an image forming apparatus with a high fixation performance and little image quality degradation. Then, it is an electrically inductive heating method which chops and applies direct current obtained by rectification of alternating current to a resonant circuit including an electric coil arranged in vicinity to an object to be heated and a capacitor for resonance connected thereto by the repetition of switching on and off of a switching element, characterized by detecting the variation of a voltage between the resonant circuit and the switching element so as to switch on the switching element in synchronization with it and detecting current through the switching element so that it switches off the switching element after an on-time based on the time required for reaching an instantaneous value of an envelop level proportional to a voltage wave pattern obtained by the rectification of the alternating current. 
   Meanwhile, an inverter is used in the conventional technique which includes a fixation device disclosed in Japanese Laid-Open Patent Application No. 2002-237377. In such an inverter, a constant OFF time period (P) is necessarily retained and then the duration of an ON time period is adjusted to conduct the electrically inductive heating of a load, herein a heated part of a fixation roller, as shown in  FIGS. 13  ( a ) and ( b ). 
   However, when driving pulses as shown in FIG.  13  ( b ) is turned off, a resonance voltage is generated between both terminals of a switching element. Commonly, a control to retain an OFF time period (P) which is longer than the duration time of the resonance voltage is conducted. The relationship between the resonance voltage and the switching off in this case is shown in  FIGS. 14  ( a ) and ( b ). Thus, when the control to retain the OFF time period (P) is conducted and a control part encounters a difficulty, and further, when the switch is turned on at the time of generation of a resonance voltage as shown in  FIG. 14  ( b ), the resonance is consequently enhanced and a lot of stress is generated in the switching element, which may lead to the breaking thereof if the worst happens. 
   As notice of the problem of such a conventional technique is taken, it is desired to prevent a switching element from breaking. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided an image forming apparatus comprising a fixation device having a heating part configured to heat and fix a toner image formed on a recording medium, which comprises a rotational velocity detecting part configured to detect a rotational velocity of a conveyance roller configured to convey the recording medium and a control part configured to estimate a surface temperature of the recording medium based on the rotational velocity of the conveyance roller detected by the rotational velocity detecting part, to estimate a temperature change of the heating part at a time of fixation based on the estimated surface temperature, and to determine a heating control variable of the heating part based on the temperature change. 
   According to another aspect of the present invention, there is provided a method of controlling heating of a fixation device having a heating part configured to heat and fix a toner image formed on a recording medium, which comprises detecting a rotational velocity of a conveyance roller configured to convey the recording medium, estimating a surface temperature of the recording medium based on the detected rotational velocity of the conveyance roller, estimating a temperature change of the heating part at a time of fixation based on the estimated surface temperature, and determining a heating control variable of the heating part based on the estimated temperature change. 
   According to another aspect of the present invention, there is provided a fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, which comprises a part configured to retain a certain or longer off-time period of a driving pulse for the switching element, in the control section. 
   According to another aspect of the present invention, there is provided a fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, which comprises a part configured to retain a certain or shorter on-time period of a driving pulse for the switching element, in the control section. 
   According to another aspect of the present invention, there is provided a fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, which comprises a part configured to imperatively stop an output of a driving pulse when the control part does not function. 
   According to another aspect of the present invention, there is provided a fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, which comprises a part configured to imperatively stop an output of a driving pulse when the control part is abnormal. 
   According to another aspect of the present invention, there is provided a fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, which comprises a part configured to detect a resonance voltage wave pattern in the control part by using the signal transmitting part having an insulating function. 
   According to another aspect of the present invention, there is provided an image forming apparatus comprising the fixation device according to one aspect of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing the entire system configuration of an image forming apparatus according to an embodiment of the present invention. 
       FIG. 2  is a schematic diagram showing the configuration of a fixation device. 
       FIG. 3  is a graph showing the temperature change of a nip part between a fixation roller and a pressurizing roller dependent on the temperature of a transcription paper, wherein (a) shows the temperature change under an environment of ordinary temperature and (b) shows an temperature change under the environment of lower temperature. 
       FIG. 4  is a graph showing the relationship between the diameter and temperature of a conveyance roller. 
       FIG. 5  is a graph showing the relationship between the angular velocity and temperature of a driven roller. 
       FIG. 6  is a perspective view that schematically shows one example of a driven roller. 
       FIG. 7  is a block diagram showing a temperature control circuit of a fixation device. 
       FIG. 8  is a diagram showing a conversion table between the rotational velocity of a driven roller and the control variable of a fixation heater. 
       FIG. 9  is a graph showing one example of control results, wherein (a) shows the result of conventional control and (b) shows the result of control when an embodiment of the present invention is used. 
       FIG. 10  is a perspective view that schematically shows another example of a driven roller. 
       FIG. 11  is a schematic diagram showing a typical configuration of a fixation device according to an embodiment of the present invention. 
       FIG. 12  is a diagram showing the conditions of driving pulses, a resonance voltage, and coil current in the embodiment. 
       FIG. 13  is a diagram showing the condition of a driving wave pattern applied to an inverter. 
       FIG. 14  is a diagram showing the relationship between a resonance voltage and a switching operation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention are described with reference to the drawings below. 
   First Embodiment 
   In the first embodiment of the present invention, a transcription paper  207 , a heater  203 , a fixation device  121 , a resist roller  149  and driven rollers  150  (and  151 ), the combination of a timer  704  and a sensor  302  or  401 , a CPU  701 , a timer  704 , a flag  302   a , the combination of a light-emitting element  302   b  and a light-receiving element  302   c , the combination of a light-emitting element  401   b  and a light-receiving element  401   c , and a ROM  702  are used as a recording medium, a heating part, a fixation device, conveyance rollers, a rotational velocity detecting part, a control part, a counting part, a light blocking member, a detecting part, a slit detecting part, and a storage part, respectively. 
     FIG. 1  is schematic diagram showing the entire system configuration of an image forming apparatus according to the first embodiment of the present invention. The image forming apparatus according to the first embodiment of the present invention is an example of a multiple function processing machine having a copying function and another function such as a printer function and a facsimile function. The coping function, printer function and facsimile function of the multiple function processing machine can be appropriately switched and selected by using an application switching key of an operation part which is not shown in the figure. That is, a copying mode, a printer mode, and a facsimile mode are conducted in the cases of selection of the copying function, the printer function, and the facsimile function, respectively. 
   The image forming apparatus is provided for forming a monochrome image and basically composed of a body  100 , a writing unit  118  mounted on the top of the body  100 , an image reading device  106  mounted on the writing unit  118 , and further, an automatic document feeding device  101  (referred to as an “ADF”, below) mounted thereon. 
   The copying mode is conducted as follows. A stack of papers are put on a document table  102  of the ADF  101  such that an image surface is directed upward, and when a start key on the operation part (which is not shown in the figure) is pushed down, a bottom document is fed to a predetermined location on a document table  105  composed of a contact glass by using a feeding roller  103  and a feeding belt  104 . The ADF  101  has a counting function for counting up the number of documents every time when the feeding of a piece of document is completed. After image information of a document on the contact glass  105  is read by the image reading device  106  as an image input part, it is ejected onto a paper ejection table  108  by using the feeding belt  104  and an ejection roller  107 . 
   When a next document lying on the document table  102  is sensed by a document set sensor  109 , a bottom document on the document table  102  is similarly fed to the predetermined location on the contact glass  105  by using the feeding roller  103  and the feeding belt  104 . The feeding roller  103 , the feeding belt  104 , and the ejection roller  107  are driven by a conveyance motor (which is not shown in the figure). 
   While the image reading device  106  illuminates a document on the contact glass  105  by using two lamps  128  for illuminating it and moves in a sub-scanning direction, the document is read by line-scanning it, reflecting reflected light therefrom to a predetermined direction via a first mirror  129 , a second mirror  130 , and a third mirror  131 , and imaging an reduced image onto CCD image sensor  133  as a photoelectric converter via a lens unit  132 . 
   While image data are read from the document by using the image reading device  106 , light-writing is conducted based on a document image by using an image processing part which is not shown in the figure and the writing unit  118  as a writing part and a latent image is formed on a photoconductor drum  117 . The writing unit  118  is composed of a laser light-emitting device  134 , an f-θ lens  135 , a reflecting mirror  136 , etc. Additionally, an exposure light source is laser light but not limited to it, and may be, for example, an LED array, etc. 
   The body  100  is composed of the photoconductor drum  117 , a development device  119 , the fixation device  121 , a paper ejection unit  122 , first through thirds paper feeding device  110 - 112 , a vertical conveyance unit  116 , etc. The photoconductor drum  117  is uniformly charged by an electrical charger which is not shown in the figure and subsequently light-exposed to light with information from the writing unit  118  so as to form a latent image. The latent image on the photoconductor drum  117  is developed is developed by the development device  119  so as to provide a toner image. 
   A conveyance belt  120  is provided below the photoconductor drum  117 . The conveyance belt  120  is used as both a conveyance part for a transcription paper as a recording medium and a transcription part, on which a transcription bias voltage is applied from a power supply (which is not shown in the figure), and while the transcription paper  207  conveyed from the vertical conveyance belt  116  and sent by the resist roller  149  is conveyed at the same speed as the rotational velocity of the photoconductor drum  117 , the toner image on the photoconductor drum  117  is transcribed on the transcription paper  207 . A toner image  206  on the transcription paper  207  (in  FIG. 2 ) is fixed by the fixation device  121 , which is ejected from the ejection paper unit  122  onto an ejection paper tray  123 . The photoconductor drum  117  is cleaned by a cleaning device which is not shown in the figure after the transcription of the toner image. Herein, the photoconductor drum  117 , the electrical charger, the writing unit  118 , the development device  119 , and the transcription device constitute an image forming apparatus for forming an image on a transcription paper according to image data. The photoconductor drum  117  is rotationally driven at a certain speed by a main motor. Also, a conveyance roller pair including the resist roller  149  is provided on a route of sending the transcription paper  207  to the photoconductor drum  117 . One of the conveyance roller pair is a driving roller and the other is a driven roller. In  FIG. 1 , a driven roller associated with the resist roller  149  is denoted by a reference numeral of  150  and the driven roller of the conveyance roller pair is denoted by a reference numeral of  151 . 
   The paper ejection unit  122  is provided with a double-face conveyance route. That is, there is provided a reversal conveyance route  125  to which a transcription paper is sent by a conveyance roller pair  124  from the middle of the paper ejection unit  122 , an image forming conveyance route  126  for conveying the transcription paper reversed on the reversal conveyance route  125  to the side of the vertical conveyance unit  116  again, and a paper ejection conveyance route  127  for returning the reversed transcription paper to the side of the paper ejection unit  122  again. An image is formed on both faces of the transcription paper on the double face conveyance route, and paper ejection onto the paper ejection tray  123  can be conducted while the face on which an image is formed is faced down. 
   A first paper feeding device  110 , a second paper feeding device  111 , and a third paper feeding device  112  as paper feeding devices feed a transcription paper stacked on a first tray  113 , a second tray  114 , and a third tray  115 , respectively, when selection thereof is conducted, and the transcription paper is conveyed to a location at which the photoconductor  117  contacts, by the vertical conveyance unit  116 . 
   In the embodiment of the present invention, at least one of driven rollers in the paper feeding parts on a conveyance route of the transcription paper  207  (for example, a driven roller  150  or  151  described below) is provided with a part for detecting the rotational velocity of the driven roller, so that the rotational velocity of the driven roller is detected during the conveyance of the transcription paper  207 . The detection part will be described in detail below. 
   In addition, on a printing mode, image data from the exterior instead of the image data from an image processing device are input into the writing unit  118  and an image is formed on a transcription paper by an image forming part. Also, on a facsimile mode, image data from the image reading device  106  are transmitted to a partner by a facsimile transceiver which is not shown in the figure, and image data from the partner are received by the facsimile transceiver and input into the writing unit  118  instead of image data from the image processing device, whereby an image is formed on a transcription paper by the image forming device. 
     FIG. 2  is a schematic diagram showing the configuration of the fixation device  121 . In the fixation device  121 , the fixation roller  201  as a fixing member is pressed by the pressurizing roller  202  as a pressurizing member at a certain pressure by using a pressurizing pat which is not shown in the figure. The pressurizing roller  202  is composed of an elastic member whose surface is silicone rubber, etc., and has a predetermined elasticity in this embodiment. Since the fixing member and the pressurizing member are commonly and frequently rollers, the rollers are drawn in  FIG. 2  but one or both of them may be composed of an endless belt(s). The fixation device  121  has a heater  203  such as a halogen heater and an electrically inductive heating device as a heat source which receives a voltage from an AC power supply and generates heat, and the heater  203  is provided at an arbitrary position at which the fixation roller  201  can be heated whereby, for example, the heater  203  is arranged inside the fixation roller  201  as shown in the figure and the fixation roller  201  is heated from the inside thereof. 
   The fixation roller  201  and the pressurizing roller  202  are rotationally driven by a driving mechanism which is shown in the figure. A temperature sensor  205  is provided to contact the surface of the fixation roller  201  and sense the surface temperature (fixation temperature) of the fixation roller  201 . When the transcription paper  207  as a recording medium which carries toner  206  passes through a nip part between the fixation roller  201  and the pressurizing roller  202 , the toner  206  is molten by heating of the fixation roller  201  and a pressure of the pressurizing roller  202  and fixed on the recording medium. In the embodiment of the present invention, the temperature of the heater  203  is controlled depending on the detected value of the temperature sensor  205  so that the surface temperature of the fixation roller  201  is constant as described below, and thus fixation failure caused by temperature ununiformity of the fixation roller  201  is prevented. 
     FIG. 3  is a graph showing the temperature change of a nip part between the fixation roller  201  and the pressurizing roller  202  dependent on the temperature of a transcription paper, wherein (a) shows the temperature change under an environment of ordinary temperature and (b) shows the temperature change under an environment of lower temperature. That is, after the power supply of the body is turned on, the heating of the heater  203  in controlled such that the surface temperature of the fixation roller  201  reaches a target temperature, and after the surface temperature reaches the target temperature, control of printing is started. When paper printing is conducted with the control of printing, it can be seen that the variation of the temperature which is caused by paper printing is small under an environment of ordinary temperature as shown in  FIG. 3  ( a ) whereas the variation of the temperature is large under an environment of lower temperature as shown in  FIG. 3(   b ). This is why the heat of the fixation roller  201  transfers to the transcription paper  207  at the time of fixation where paper printing is conducted under a condition such that the transcription paper  207  and the toner  206  have been cooled. 
   This depends on the relationship between the thickness and heat capacity of the fixation roller, as described above. Therefore, in the embodiment of the present invention, the processes of: 
   1) the conveyance velocity of the transcription paper  207  is detected on the conveyance route for a transcription paper as provided by the vertical conveyance unit  116  described above; 
   2) the temperature change of the transcription paper  207  is estimated in real time based on the detected conveyance velocity; and 
   3) temperature lowering at the time of fixation is further estimated based on the temperature of a transcription paper to preliminarily correct the temperature of the fixation roller  201  (to control it to be higher) 
   are conducted, so that the temporal lowering of the temperature of the nip part is prevented even though the thickness of the fixation roller  201  is small. 
     FIG. 4  is a graph showing the relationship between the diameter and temperature of a conveyance roller. That is, the conveyance roller is commonly made of a metal with a high heat-conductivity such as aluminum, and, in the case of a metal roller, as shown in  FIG. 4 , it has characteristics such that the radius r of the roller decreases if the temperature is low, whereas the radius r of the roller increases if the temperature is high. 
   On the other hand, the following relationship:
 
ω= V/r  
 
is satisfied among the conveyance velocity V of the transcription paper  207 , the radius r and rotational angular velocity ω of the driven roller.
 
   Herein, when the conveyance velocity of the transcription paper  207  is constant, the rotational angular velocity ω of the driven roller increases if the roller radius r of the driven roller decreases, whereas the rotational angular velocity ω of the driven roller increases decreases if the roller radius r of the driven roller increases. 
   In other words, the rotational velocity of the driven roller increases if the temperature is lowered, whereas the rotational velocity of the driven roller decreases if the temperature is raised, as shown in  FIG. 5 .  FIG. 5  is a graph showing the relationship between the angular velocity and temperature of a driven roller. 
   For this reason, it can be seen that the temperature of the transcription paper  207  can be estimated by monitoring the rotational velocity of the conveyance roller on the conveyance route for the transcription paper  207 . Furthermore, since it can be empirically estimated how much temperature lowering is caused based on the temperature of the transcription paper  207 , it is possible to prevent the temperature lowering at the time of paper printing if the temperature of the heater  203  is controlled depending on the rotational velocity of the driven roller. 
     FIG. 6  is a perspective view that schematically shows one example of a driven roller. A sensor  302  for sensing one conveyance roller pair is provided to the driven roller  150  in one conveyance roller pair (herein, including the resist roller  149 ) among the plural conveyance roller pairs in the vertical conveyance unit  116  which provides one conveyance route for the transcription paper  207 . The sensor  302  is composed of a flag  302   a  for sensing each rotation of the driven roller  150  attached to a shaft  150   a  of the driven roller  150 , a light-emitting element  302   b  arranged to irradiate the flag  302   a  with light, and a light-receiving element  302   c  arranged to receive light from the light-emitting element  302   b , and is a so-called photo-interrupter-type sensor for sensing the rotation of the flag  302   a  by blocking light from the light-emitting element  302   b  by using the rotating flag  302   a . As the rotation is detected by the flag  302   a , a pulse signal is output from the sensor  302 . 
     FIG. 7  is a block diagram showing a temperature control circuit of a fixation device. The temperature control circuit is composed of a CPU  701  for conducting the control of the entire of the image forming apparatus as well as the control of the receipt of image data signal input from an exterior device which is not shown in the figure and transmission and receipt of a control command signal, a ROM  702  in which a program is stored, a RAM  703  used as a work memory, a timer  704  for conducting a timing measurement, a PWM controller  705  for generating a heater control signal and a heater driving circuit  706 . The CPU  701 , the ROM  702 , the RAM  703 , the timer  704 , and the PWM controller  705  are interconnected via a bus interface  609 , which enables the read-write processing of data and the performance of control according to an instruction from the CPU  701 . 
   An output from the sensor  302  attached to the driven roller  150  in the vertical conveyance unit  116  is input into the CPU  701  as interruption. Also, the timer  704  has a configuration to repeat a count-up operation for each of certain time period, to conduct zero clear according to an instruction of the CPU  701 , and to be readable of a count value in the CPU  701 . An interruption signal from the sensor  302  is input into the CPU  701  for each rotational time period of the driven roller  150 , a count value is acquired from the timer  704  for each interruption input, and an operation for zero-clear is conducted after the acquisition. Then, the count value acquired from the timer  704  indicates one time period of the driven roller  150 . 
   Also, the driven roller  150  has a characteristic of increasing or decreasing the roller radius depending on the temperature at the time of conveying the transcription paper  207  as described above, and it is empirically possible to estimate the temperature of the transcription paper  207  based on the rotational velocity of the driven roller  150 , as seen in  FIG. 5 . Furthermore, it is empirically possible to estimate the variation of fixation temperature of the driven roller  150  from the temperature of the transcription paper  207 . Then, based on them, a conversion table between the rotational velocity of the driven roller and a control variable of a fixation heater as shown in  FIG. 8  is created and stored in the ROM  702 . A control is conducted such that the control variable of the fixation heater is varied based on the conversion table between the rotational velocity of the driven roller and the control variable of the timer  704  and a time period of one rotation of the driven roller which is acquired by the timer  704  before the transcription paper  207  enters the fixation roller  201 . 
   When the transcription paper  207 , itself, is at lower temperature, the temperature of the nip part is lowered at the time of approaching the fixation roller  201 . Hence, as described above, when the rotational velocity of the driven roller  150  is large,  FIG. 5  shows that the temperature is low, and therefore, control is made such that the temperature of the nip part is not lowered and the duty of the PWM is high. Thus, when the transcription paper  207  approaches the fixation roller  201 , fixation failure is prevented by preliminarily raising the fixation temperature. 
   On the other hand, when the transcription paper  207 , itself, is at higher temperature or ordinary temperature, the temperature of the nip part is not lowered even if it approaches the fixation roller  201 . Therefore, as described above, when the rotational velocity of the driven roller  150  is small, the duty of the PWM is controlled by a conventional value since  FIG. 6  shows that the temperature is high. 
   Also, the PWM controller  705  is connected to the bus interface of the CPU  701  and a driving signal for the fixation heater is generated according to the ON/OFF time period and duty set by the CPU  701 . Since the driving signal generated by the PWM controller  705  is at a DC level, the heater  203  is finally AC-controlled by the heater driving circuit  706 . 
     FIG. 9  is a graph showing one example of control results, wherein (a) shows the result of conventional control and (b) shows the result of control when the embodiment of the present invention is used. Each shows the output of the PWM controller  705  under an environment of lower temperature and the temperature change of the nip part. At the time of turning on of the power supply of the body, a control is made while the duty of the PWM is improved, in order to quickly reach the target temperature, and after reaching the target temperature, a printing operation is started. When the printing operation is started and the transcription paper  207  approaches the fixation roller, the temperature of the nip part temporally lowers as shown in (a) in the case of a conventional control. 
   However, when the embodiment of the present invention is used, since the radius of the driven roller decreases under an environment of lower temperature as described above, the rotational velocity of the driven roller increases at a time when the transcription paper  207  is conveyed in the vertical conveyance unit  116 , and, therefore, the duty of the PWM is controlled to be high such that the temperature of the fixation heater is high as shown in  FIG. 8 . Accordingly, as shown in  FIG. 9(   b ), when the transcription paper  207  is conveyed in the vertical conveyance unit  116 , the temperature of the fixation roller  201  is preliminarily raised, and, even though the transcription paper  207  approaches the fixation roller  201  so as to lower the temperature of the nip part, it is not lowered below the target temperature and the ununiformity of fixation can be prevented. 
   As described above, the degradation of image quality caused by the temporal lowering of temperature can be prevented by controlling the surface temperature of the fixation roller  201  depending on the temperature of the transcription paper  207  before performing the fixation. 
   In the embodiment described above, although the process for detecting the rotational velocity of the driven roller  150  on the conveyance route for the transcription paper  207  before the transcription has been described, a driven roller on a transcription belt is applicable for an image forming apparatus such that a toner image on a photoconductor is transcribed on the transcription belt and the toner image on the transcription belt is transcribed on a transcription paper. 
   Also, although the process for detecting the rotational velocity of the driven roller by measuring a time period of one rotation of the roller has been described, the number of an output pulse(s) of a sensor  401  in a predetermined time period may be measured by attaching a disk  401   a  on which plural slits are formed to a shaft  150   a  of a driven roller  150 , arranging a light-emitting element  401   b  and a light-receiving element  401   c  so as to sandwich the disk  401   a , and providing an encoder-shaped sensor  401  for generating a light pulse by using the slits, as shown in  FIG. 10 . 
   The first embodiment of the present invention can be applied to a fixation device for fixing a toner image on a recording medium by means of heating and pressurizing, an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a digital multiple-function-processing machine, with such a fixation device, and a method of controlling heating of such a fixation device. 
   Second Embodiment 
   In the following second embodiment of the present invention, numerals  1200 ,  1100 ,  1104 , and  1203  denote a control section, a driving section, a switching element, a safety device, respectively, and a pulse transformer  1204 , the combination of a first and second photocouplers  1205  and  1206 , and the combination of a heating control part including the driving section  1100  and the control section  1200  and a fixation part  1300  are used as a signal transmitting part, a part for detecting a resonance voltage waveform, and a fixation device, respectively. 
     FIG. 11  is a schematic diagram showing a typical configuration of a fixation device with an electrically inductive heating device according to the second embodiment of the present invention. In the figure, the electrically inductive heating device is composed of a driving section (primary section)  1100  and a control section (secondary section)  1200 . The driving section  1100  is mainly composed of an alternating current power supply  1101 , a rectification part  1102 , a filtering part  1103 , a switching element  1104 , a resonance capacitor  1105  and a heating coil  1106 , wherein the switching element  1104  is composed of a switch  1107  and a diode  1108  and the diode  1108  is connected to the switch  1107  in parallel. The control section  1200  is composed of a control part  1201 , an AND circuit  1202  and a safety device  1203 , wherein the output of the AND circuit  1202  is input into the pulse transformer  1204  and the safety device  1203 . Additionally, a signal (driving pulse) from the control section  1200  is transmitted to the driving section  1100  in an electrically-insulated condition by the pulse transformer  1204 . The resonance voltage waveform is transmitted from the driving section  1100  to the control section  1200  by the first and second photocouplers  1205  and  1206  and is detected in the control section  1201 . 
   In such an electrically inductive heating device, a driving pulse is provided from the control part  1201  to the AND circuit  1202  and the output of the AND circuit  1202  is provided to the switching element  1104  through the pulse transformer  1204 , whereby the switch  1107  is opened or closed. Then, alternating current is provided to or blocked from the heating coil  1106  by closing or opening the switch  1107 . The diode  1108  is provided for blocking an inverse voltage applied between both ends of the switch  1107 . Thus, the alternating current provided from the alternating current power supply  1101  is provided to the heating coil  1106  through the rectification part  1102  and the filtering part  1103 . Then, an alternating magnetic field generating at the heating coil  1106  acts on a heating part  1300  (as a fixation part drawn in the figure) of a fixation roller, so as to generate eddy current in the heating part (fixation part)  1300 , and, thereby, it is heated. Additionally, the fixation roller is described herein but a fixation belt including a heated object may be used. 
   Thus, a pulse for driving the switching element  1104  is generated by the control part  1201  of the control section  1200  so as to control the switching of the switching element  1104  via the pulse transformer  1204 . In the control part  1201 , driving pulses as shown in  FIG. 12(   a ) are controlled by the pulse transformer  1204 , whereby coil current as shown in  FIG. 12(   c ) is generated in the heating coil  1106  and eddy current is generated in a heater of the fixation part  1300  by an alternating magnetic field generating from the heating coil  1106 , so as to heat it. The longer the on-time period of the driving pulse is, heating energy becomes higher. As described above, a resonance voltage is generated between both ends of the switching element  1104  at the time of turning off the driving pulses as shown in  FIG. 12(   b ), and the duration thereof is determined by the values of the heating coil  1106  and resonance capacitor  1105 . Then, the control part  1201  is controlled to provide an off-time period longer than the duration of the resonance voltage as shown in  FIG. 14(   a ) and not to turn on the switching element  1104  during the generation of a resonance voltage thereon. 
   However, as described above, where an abnormal event occurs in the control part  1201  and the off-time period decreases, the switching element  1104  is turned on during the generation of a resonance voltage as shown in  FIG. 14(   b ), whereby significant stress is applied on the switching element  1104 , which may be led to breaking thereof. 
   Then, in the embodiment, timing of turning off the switching element  1104  is detected and the safety device  1203  has a function such that the switching element  1104  is not turned on during a predetermined time period after turning off thereof. As the safety device  1203  has such a function, turning on of the switching element  1104  during the generation of the resonance voltage can be avoided and the degradation and breaking of the switching element  1104  can be prevented. 
   An abnormal event in the control part  1201  is sensed by input of a watchdog timer overflow (WDTOVF) signal into the safety device  1203 . That is, in the embodiment, as the watchdog timer overflow (WDTOVF) signal is input into the safety device  1203 , the safety device  1203  outputs a low level signal into the AND circuit  1202 . Thus, since the output of the AND circuit  1202  becomes a low level and no driving signal is output to the pulse transformer  1204 , the switching element  1104  is not driven (turned on). Therefore, when the resonance voltage is output as shown in  FIG. 14(   b ), the switching element  1104  is not driven and the abnormality is not enhanced. 
   Also, the output from the AND circuit  1202  is input into the safety device  1203  as seen in  FIG. 11 . Accordingly, when the output of the AND circuit is at a high level, in other words, when the safety device  1203  monitors a time period of turning on the switching element  1104  and a time period of turning on over a predetermined certain time period is sensed, an abnormal signal is output to the control part  1201 . Accordingly, the control part  1201  lowers the level of a driving pulse into the pulse transformer  1204  to a low level. As a result, the switching element  1104  is turned off. Thus, as the switching element  1104  is once turned off, the safety device  1203  subsequently continues to output the abnormal signal over a certain time period. Accordingly, an OFF state is retained over the certain time period, during which no return to an abnormal state is conducted. 
   Thus, even when an abnormal event occurs in the control part  1201 , no switching operation affects the resonance voltage by the safety device  1203 , and in the embodiment, the first and second photocouplers  1205  and  1206  are further provided such that the resonance voltage waveform is monitored by the first and second photocouplers  1205  and  1206  and the safety is further maintained. 
   That is, an output of the first photocoupler  1205  connected to an end of the switching element  1104  and an output of the second photocoupler  1206  connected to the first photocoupler  1205  in parallel and an inverter  1207  in series are input into the control part  1201 . The first photocoupler  1205  conducts sensing of the voltage waveform of a resonance voltage applied on the switching element  1104 , and when the resonance voltage disappears, the output becomes HIGH and the HIGH signal is input into the control part  1201 . Accordingly, the control part  1201  stops the output of a driving pulse to the pulse transformer  1204 . 
   On the other hand, the second photocoupler  1206  is connected to the inverter  1207  in series, and therefore, senses a voltage waveform inverse to that of the first photocoupler  1205 . That is, as a resonance voltage is applied on the switching element  1104 , the output becomes HIGH and the HIGH signal is input into the control circuit  1201 . While the output is HIGH, the control part  1201  outputs a driving pulse to the pulse transformer  1204  and a switching operation of the switching element  1104  is conducted. 
   Thus, if a part for transmitting the occurrence of an abnormal event in the control part  1201 , in the embodiment, the safety device  1203 , is included, the control part  1201  can detect the occurrence of an abnormal event. 
   Also, the safety device  1203  includes a function to detect timing of turning on the switching element  1104  and not to turn on the switching element  1104  over a predetermined time period (which is composed of the AND circuit and the safety circuit  1203  in the embodiment) and the timing of turning on the switching element  1104  is prevented from overlapping the timing of generating the resonance voltage, whereby the deterioration or breaking of the switching element  1104  can be prevented. Also, since the occurrence of an abnormal event in the control part  1201  is transmitted from the safety device  1203  to the control part  1201  by an abnormal signal, the control part  1201  can detect the occurrence of an abnormal event. 
   Further, since the safety device  1203  has a function to detect an abnormal signal of the control part  1201 , for example, a watchdog timer overflow WDTOVF signal, a malfunction of the electrically inductive heating fixation device can be prevented when an abnormal event occurs in the control part  1201 . 
   In addition, since the lowering of a resonance voltage can be transmitted to the control part  1201  by the first photocoupler  1205 , the control part  1201  can make a control such that the switching element  1104  is not turned on during the generation of the resonance voltage. 
   On the other hand, when the generation of a resonance voltage is transmitted to the control part  1201  by using the photocoupler and a time period of generation of the resonance voltage is detected, the precision thereof is deteriorated due to the temperature change over an off-time period of the photocoupler, etc. In order to address the problem, the two photocouplers  1204  and  1206  are used and utilized in reversed polarities in the embodiment, and signals at the time of turning on the photocouplers  1204  and  1205  are utilized for the rising and dropping of a resonance voltage so that the precision of detection of a generating time period of the resonance voltage can be improved. 
   According to the configuration of the embodiment, exerted are some effects such that 
   1) since the control section includes a part for retaining a certain or longer off-time period of pulses, the deterioration or breaking of a switching element can be prevented; 
   2) since the control section includes a part for retaining a certain or shorter on-time period of a pulse, the deterioration or breaking of a switching element can be prevented; 
   3) since a part for imperatively stopping the output of a pulse is included when the control part does not function, a malfunction of an electrically inductive heating fixation device can be prevented when an abnormal event occurs in the control part; 
   4) since a resonance voltage waveform is detected in the control section by using a signal transmitting part having an electrically insulating function, the state of the resonance voltage can be added in control factors; and 
   5) since two photocouplers are used in reversed polarities, the influence of distortion such as a temperature change of an off-time period of the photocoupler can be reduced. 
   The second embodiment of the present invention can be applied to a fixation device in which an electrically inductive heating is used for generating electrically inductive current in an electrical conductor in immediate proximity to an electric coil (electrically conductive coil) by applying alternating current thereon, particularly, a fixation device which uses the electrically inductive heating control of a PWM control for supplying an electrical power to a resonance circuit including an electrically inductive heating coil and a resonance capacitor connected thereto by chopping direct current for which alternating current is rectified, using an inverter, and an image forming apparatus such as a printer, a copying machine, a facsimile machine and a digital multiple function processing machine with a composite function thereof, which uses the fixation device. 
   [Appendix] 
   Typical embodiments (1-1) to (1-12) of the present invention are described below. 
   (1-1) An image forming apparatus comprising a fixation device having a heating part configured to heat and fix a toner image formed on a recording medium, characterized by comprising a rotational velocity detecting part configured to detect a rotational velocity of a conveyance roller configured to convey the recording medium and a control part configured to estimate a surface temperature of the recording medium based on the rotational velocity of the conveyance roller detected by the rotational velocity detecting part, to estimate a temperature change of the heating part at a time of fixation based on the estimated surface temperature, and to determine a heating control variable of the heating part based on the temperature change. 
   (1-2) The image forming apparatus as described in (1-1) above, characterized in that the rotational velocity detecting part comprises a sensor configured to output a signal for each rotation of the conveyance roller and a counting part configured to count an output of the sensor in a predetermined time period. 
   (1-3) The image forming apparatus as described in (1-2) above, characterized in that the sensor is provided on a driven roller as a conveyance roller. 
   (1-4) The image forming apparatus as described in (1-3) above, characterized in that the sensor comprises a light blocking member provided on a shaft of the driven roller and a detecting part configured to detect a light blocking condition of the light blocking member for each rotation thereof. 
   (1-5) The image forming apparatus as described in (1-1) above, characterized in that the rotational velocity detecting part comprises a disk having plural slits through which light passes and co-axially provided on a shaft of the conveyance roller, a slit detecting part configured to output a signal every time when a slit on the disk is detected, and a counting part configured to count an output of the slit detecting part in a predetermined time period. 
   (1-6) The image forming apparatus as described in any of (1-1) to (1-5) above, characterized in that the control part determines a surface temperature of a sheet and an estimated temperature change and heating control variable of the heating part by an operation based on information of the rotational velocity of the conveyance roller. 
   (1-7) The image forming apparatus as described in any of (1-1) to (1-6) above, characterized by further comprising a storage part which preliminarily stores a heating control variable determined by information of a rotational velocity of the conveyance roller, wherein the control part determines the heating control variable based on a table value in the storage part which is based on the information of the rotational velocity of the conveyance roller. 
   (1-8) The image forming apparatus as described in any of (1-1) to (1-7) above, characterized in that the conveyance roller is made of a metal with a high heat-conductivity. 
   (1-9) The image forming apparatus as described in any of (1-1) to (1-7) above, characterized in that the heating part comprises a halogen heater. 
   (1-10) The image forming apparatus as described in any of (1-1) to (1-7) above, characterized in that the heating part comprises an electrically inductive heating part. 
   (1-11) A method of controlling heating of a fixation device having a heating part configured to heat and fix a toner image formed on a recording medium, characterized by comprising detecting a rotational velocity of a conveyance roller configured to convey the recording medium, estimating a surface temperature of the recording medium based on the detected rotational velocity of the conveyance roller, estimating a temperature change of the heating part at a time of fixation based on the estimated surface temperature, and determining a heating control variable of the heating part based on the estimated temperature change. 
   (1-12) The method of controlling heating of a fixation device as described in (1-11) above, characterized in that the conveyance roller whose rotational velocity is detected is a driven roller. 
   According to any of typical embodiments (1-1) to (1-12) above, it may be possible to prevent degradation of image quality caused by temporal lowering of surface temperature of a fixation roller or degradation of image quality under a lower temperature environment. 
   Other typical embodiments (2-1) to (2-12) of the present invention are described below. 
   (2-1) A fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, characterized by comprising a part configured to retain a certain or longer off-time period of a driving pulse for the switching element, in the control section. 
   (2-2) The fixation device as described in (2-1) above, characterized in that the part configured to retain a certain or longer off-time period comprises a safety device configured to detect a timing of turning off the switching element and not to turn on the switching element during a predetermined time period after turning off thereof. 
   (2-3) A fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, characterized by comprising a part configured to retain a certain or shorter on-time period of a driving pulse for the switching element, in the control section. 
   (2-4) The fixation device as described in (2-3) above, characterized in that the part configured to retain a certain or shorter on-time period comprises an AND circuit to which a driving pulse from the control part is input and which outputs it to the driving section and a safety device configured to monitor an on-output of the driving pulse from the AND circuit and output an abnormal signal to said control part when the on-output continues during a preliminarily set time period or longer. 
   (2-5) A fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, characterized by comprising a part configured to imperatively stop an output of a driving pulse when the control part does not function. 
   (2-6) The fixation device as described in (2-5) above, characterized in that the part configured to imperatively stop an output of a driving pulse is a safety device configured to output an abnormal signal to the control part. 
   (2-7) A fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, characterized by comprising a part configured to imperatively stop an output of a driving pulse when the control part is abnormal. 
   (2-8) The fixation device as described in (2-7) above, characterized in that the part configured to imperatively stop an output of a driving pulse is a safety device configured to stop an output of a driving pulse to the driving section when a watchdog timer overflow signal is received from the control part. 
   (2-9) A fixation device comprising an electrically inductive heating part configured to generate a driving wave pattern for a switching element by a control part in a control section thereof and to transmit the driving wave pattern to a driving section thereof by a signal transmitting part having an insulating function provided between the driving section and the control section, characterized by comprising a part configured to detect a resonance voltage wave pattern in the control part by using the signal transmitting part having an insulating function. 
   (2-10) The fixation device as described in (2-9) above, characterized in that the part configured to detect a resonance voltage wave pattern is a photocoupler. 
   (2-11) The fixation device as described in (2-10) above, characterized by comprising two of the photocouplers, both of which are used in reversed polarities. 
   (2-12) An image forming apparatus characterized by comprising the fixation device as described in any of (2-1) to (2-11) above. 
   According to any of typical embodiments (2-1) to (2-12) above, it may be possible to prevent a switching element from breaking since the switching element may be turned on when a resonance voltage is generated. 
   The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese priority application No. 2005-310103 filed on Oct. 25, 2005 and Japanese priority application No. 2005-316996 filed on Oct. 31, 2005, the entire contents of which are hereby incorporated by reference.