JP2007226150A - Fixing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing apparatus capable of highly accurately performing the temperature control of an object to be measured, and to provide an image forming apparatus. <P>SOLUTION: A voltage Vcut1, a voltage Vcut2 and a voltage Vcut 3 by using a compensation temperature signal Vd outputted from a second thermistor 512 as a parameter are generated, based on a plurality of straight lines (linear function) approximating a graph presenting showing the relation between a compensation temperature T and the signal level Vb of a differential amplitude signal, and the appropriate voltage is selected from among the voltages, and the selected one is compared with the signal level Vb, and the power supply to the heater 42 is cut off according to the comparison result. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fixing device that performs a fixing process for fixing a toner image transferred onto a transfer material in an image forming apparatus, and an image forming apparatus including the fixing device.

  2. Description of the Related Art Conventionally, a fixing device mounted on an image forming apparatus such as a copying machine is installed on the downstream side of an image forming unit, and a toner image transferred on a sheet (transfer material) in the image forming unit is heated on the sheet. A fixing process for fixing is performed. Such a fixing device is provided with a fixing roller heated by a heat source (heater) and a pressure roller pressed against the fixing roller.

  Then, the transfer-finished paper is supplied toward the nip formed between the pressure roller and the fixing roller rotating around the axis, and the fixing roller heats the paper passing through the nip. The toner is melted by supplying the toner, and the fixing process is performed by fixing the molten toner to the paper surface.

  In such a fixing device, for example, if an abnormal temperature rise occurs in the fixing roller, the fixing device may be damaged or broken, and therefore it is necessary to appropriately control the temperature of the fixing roller. Therefore, conventionally, a temperature sensor is disposed at a position opposite to a predetermined position on the peripheral surface of the fixing roller, and when an abnormal temperature rise occurs in the fixing roller, a process of stopping power supply to the fixing roller is performed.

  By the way, a non-contact type infrared temperature sensor may be employed for detecting the temperature of the fixing roller. This infrared temperature sensor includes two thermistors and an infrared film, and one thermistor detects the temperature of a measurement object such as a fixing roller (hereinafter referred to as a detection temperature) via the infrared film. On the other hand, the other thermistor detects, for example, the temperature of the casing of the infrared temperature sensor (hereinafter referred to as compensation temperature), and outputs the temperature derived based on these detection outputs as the temperature of the measurement object. It is configured.

  However, in the infrared temperature sensor, there may be a plurality of combinations of detection temperature and compensation temperature corresponding to the output of the same infrared temperature sensor, so that the actual temperature of the measurement object and the predetermined reference value When the method of determining the stop of the power supply to the fixing roller based on the comparison result is employed, it is assumed that a certain reference value is set as the predetermined reference value. Even if the output of the temperature sensor is obtained, the combination of the detected temperature and the compensation temperature corresponding to this output is not uniquely derived, and it is considered that the power supply to the measurement object cannot be stopped accurately. It is done.

In Patent Document 1 below, for such a problem, in a temperature detection circuit including an infrared detection thermistor and a temperature compensation thermistor, a resistance element connected in series to the temperature compensation thermistor is added, and the temperature of the object is predetermined. A method has been proposed in which the output value of the temperature detection circuit is constant when the temperature is reached.
Japanese Patent Laid-Open No. 2003-302288

  However, since the output signal of the thermistor has a very low signal level, when a resistance element is added to the temperature detection portion, the signal level is further reduced and the S / N is deteriorated. As a result, the temperature of the fixing roller may not be properly managed.

  SUMMARY An advantage of some aspects of the invention is that it provides a fixing device and an image forming apparatus that can perform temperature management of a measurement object with high accuracy.

  According to the first aspect of the present invention, by supplying heat to at least one of a pair of rollers that are in contact with each other on a cylindrical peripheral surface, toner adhering to the sheet conveyed between the pair of rollers is transferred to the sheet. A heating means for fixing the heating means, a power supply means for generating heat by supplying power to the heating means, and a temperature of a predetermined measurement object to which heat is supplied by the heating means And a second sensor for detecting a temperature of an object different from the measurement object as a compensation temperature, and the temperature based on the outputs of the first and second sensors is A temperature detection means that outputs the temperature of the object to be measured, and a detected temperature derived using the output of the first sensor of the temperature detection means and a predetermined reference value are compared, and based on the comparison result Heating A power shut-off means for shutting off power supply to the stage, and one predetermined value is selected from a plurality of default values related to the predetermined reference value, and the predetermined value is used as the predetermined reference value for the power shut-off means. And a reference value providing means for providing the fixing device.

  According to the present invention, one predetermined value is selected from a plurality of predetermined values related to the predetermined reference value used in the comparison by the power cut-off means, and the predetermined value is provided to the power cut-off means as the predetermined reference value. Thus, the predetermined reference value to be compared with the detected temperature can be set according to the outputs of the first and second sensors.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the reference value providing unit sets the predetermined reference value in accordance with an output of the second sensor. It is.

  According to this invention, since the predetermined reference value is set according to the output of the second sensor, even if the output value of the first sensor is the same, according to the output of the second sensor. Therefore, it is possible to set a reference value corresponding to the output value of the temperature detecting means that changes, and the power supply to the heating means can be cut off accurately.

  According to a third aspect of the present invention, in the fixing device according to the second aspect, the reference value providing unit is a plurality of units that approximately represent a relationship between the detected temperature having an extreme value and the output of the second sensor. The detected temperature corresponding to the obtained output of the second sensor is derived using a linear function, and the detected temperature is provided as the predetermined reference value to the power cut-off means. It is.

  According to this invention, a plurality of linear functions that approximately represent the relationship between the detected temperature having an extreme value and the output of the second sensor are used to correspond to the obtained output of the second sensor. Since the detected temperature is derived, the reference value providing means can be configured with a relatively simple circuit.

  According to a fourth aspect of the present invention, in the fixing device according to any one of the first to third aspects, the predetermined measurement object whose temperature is detected by the first sensor is one of the pair of rollers. The temperature of the roller, and the object whose temperature is detected by the second sensor is the temperature of the casing of the temperature detecting means.

  According to this invention, the first sensor detects the temperature of one of the pair of rollers, and the second sensor detects the temperature of the casing of the temperature detecting means. The effect | action by the invention in any one of claim | item 1 thru | or 3 is acquired.

  According to a fifth aspect of the present invention, there is provided an image forming unit that performs an image forming operation on a sheet based on predetermined image data, and a toner attached to the sheet with respect to the sheet after the image forming operation by the image forming unit An image forming apparatus comprising: the fixing device according to claim 1, wherein the fixing device is fixed to the paper.

  According to the present invention, in the image forming apparatus, the action according to any one of the first to fourth aspects can be obtained.

  According to the present invention, the predetermined reference value to be compared with the detected temperature can be set according to the outputs of the first and second sensors. Therefore, the power supply to the heating means can be properly interrupted, and the temperature of the measurement object can be controlled with high accuracy.

  The image forming apparatus according to the present invention will be described below. FIG. 1 is a diagram illustrating an internal configuration of the first embodiment of the image forming apparatus.

  As shown in FIG. 1, the image forming apparatus 1 includes a sheet supply unit 2 positioned below the apparatus 1, an image forming unit 3 positioned above the sheet supply unit 2, and a downstream side of the image forming unit 3. And a discharge unit 5 positioned above the image forming unit 3.

  The paper supply unit 2 includes paper cassettes 6 to 8 configured to be able to replenish paper P by being pulled out from a main body frame (not shown) of the image forming apparatus 1. 8 are sent to the outlet side (right side in FIG. 1) of the paper feed cassettes 6-8 by the rotation operation of the paper feed rollers 9-11, and the uppermost by the retard rollers 12-14. The paper P at the position is fed to the first paper transport unit 15 one by one.

  The first paper transport unit 15 transports the paper P fed from the paper supply unit 2 toward the image forming unit 3. The first paper transport unit 15 includes a registration sensor 16 in which a pair of reflective photosensors are disposed at both ends in a direction (main scanning direction) orthogonal to the paper surface of FIG. 1, and the paper P is the registration sensor 16. When the toner image is conveyed to the position of the sheet P, a registration roller 17 disposed on the downstream side of the registration sensor 16 aligns a toner image formed on the surface of the photosensitive drum 18 described later with the leading end position of the sheet P. In this way, the sheet is conveyed between the photosensitive drum 18 and the transfer roller 22 after the conveyance timing is taken.

  The image forming unit 3 includes a photoconductive drum 18 rotatably supported on a photoconductive drum, and a charger 19, a laser scanning unit 20, and a developing unit around the photoconductive drum 18 along the rotation direction. 21, a transfer roller 22 and a cleaning unit 23, and a predetermined toner image is formed on the photosensitive drum 18 by an electrophotographic process, and the toner image is transferred to the paper P.

  The charger 19 charges the peripheral surface of the photosensitive drum 18 with a certain polarity (in this embodiment, positive polarity).

  Although not shown in detail, the laser scanning unit 20 includes a laser emitter, a polygon mirror, and a reflecting mirror, and responds to image data of an original obtained from an image input device (not shown) such as an externally connected personal computer or scanner. The laser beam with the intensity is output from a laser emitter and irradiated to the exposure area of the positively charged photosensitive drum 18 through a polygon mirror and a reflecting mirror. By this irradiated laser light, the surface potential of the photosensitive drum 18 is decreased according to the image, and an electrostatic latent image corresponding to the document image data is formed on the surface of the photosensitive drum 18. This light irradiation is repeatedly scanned in the width direction of the rotating photosensitive drum 18 (the direction perpendicular to the paper surface in FIG. 1) to the exposure area between the charger 19 and the developing unit 21 by the rotation of the polygon mirror. The

  The developing unit 21 includes a developing roller 24 disposed opposite to the photosensitive drum 18 and a toner container 25 storing toner, and the positive charge on the photosensitive drum 18 is reduced (the potential is reduced) by the laser beam. The toner having the same polarity in the developing region, that is, positively charged toner is attached to the portion of the electrostatic latent image by using the developing roller 24. As a result, the electrostatic latent image formed on the photosensitive drum 18 is developed with toner, and a toner image is formed on the surface of the photosensitive drum 18 as a visible image.

  The transfer roller 22 is disposed to face the photoconductive drum 18 in a non-contact state, and is formed on the photoconductive drum 18 by performing negative corona discharge from the back surface of the paper P in the transfer area of the photoconductive drum 18. The transferred toner image is electrostatically transferred onto the paper P.

  The cleaning unit 23 has a cleaning blade (not shown) formed to have a length substantially the same as the width direction dimension of the photosensitive drum 18, and the front end of the cleaning blade is exposed to the biasing force of a biasing means (not shown). By urging the surface of the body drum 18, the residual toner adhering to the surface of the photoreceptor drum 18 after the transfer is scraped off.

  The fixing unit 4 performs a fixing process for fixing the toner transferred onto the paper by pressurizing and heating the paper on which the toner image is transferred. The heat shielding box 26 and the heat shielding box 26 A fixing roller 27 disposed in the upper part of the heat shield box 26; and a pressure roller 28 disposed in pressure contact with the fixing roller 27 in the lower part of the heat shielding box 26.

  The fixing roller 27 includes a heater 42 (see FIG. 2), and is normally set at a constant temperature. When the sheet comes into contact with the fixing roller 27, the sheet is heated.

  The discharge unit 5 is formed, for example, on the upper surface of the main body frame of the image forming apparatus 1, and sequentially accumulates the sheets after the fixing process conveyed from the fixing unit 4 through the second sheet conveying unit 29. .

  The operation input unit 30 is used to input operations such as an instruction to start an image forming operation by the image forming apparatus 1 and designation of the type of paper P. The control unit 31 includes a microcomputer having a built-in storage unit including, for example, a ROM that stores a control program and a RAM that temporarily stores data, and associates driving of each member in the image forming apparatus 1 described above with each other. It is something to control.

  FIG. 2 is a block diagram illustrating a circuit configuration of the fixing unit 4. As shown in FIG. 2, the fixing unit 4 includes a temperature detection unit 41, a heater 42, and a heater control circuit 43. The heater control circuit 43 includes a differential amplifier circuit 44, an AD conversion circuit 45, A Vcut1 generation circuit 46, a Vcut2 generation circuit 47, a Vcut3 generation circuit 48, a selection circuit 49, and a hard cut circuit 50 are provided. In addition, the control part 31 in a figure is corresponded to the above-mentioned control part 31. FIG.

  In the fixing unit 4, the temperature detection unit 41 generates two temperature signals Vs and Vd (hereinafter also referred to as voltages Vs and Vd), and the temperature signals Vs and Vd are supplied to the differential amplifier circuit 44. The differential amplifier circuit 44 amplifies the signal level difference (Vd−Vs) of these signals by a predetermined coefficient γ. The amplified signal Vb (= γ · (Vd−Vs)) is input to the AD conversion circuit 45, and the amplified signal Vb is AD converted by the AD conversion circuit 45. Is input. Note that one of the temperature signals generated by the temperature detection unit 41 is also input to the AD conversion circuit 45.

  On the other hand, the temperature signal Vd of the temperature signals Vs and Vd generated by the temperature detection unit 41 is input to the Vcut1 generation circuit 46 and the Vcut2 generation circuit 47, respectively. Comparison signals Vcut1 and Vcut2 (hereinafter also referred to as voltages Vcut1 and Vcut2) may be generated. Further, the Vcut3 generation circuit 48 generates a comparison signal Vcut3 (hereinafter also referred to as voltage Vcut3).

  The comparison signals Vcut1, Vcut2, and Vcut3 generated by the Vcut1 generation circuit 46, the Vcut2 generation circuit 47, and the Vcut3 generation circuit 48, respectively, are input to the selection circuit 49, and one of the comparison signals Vcut1, Vcut2, and Vcut3 is input to the selection circuit 49. The comparison signal is selected. The selected comparison signal is input to the hard cut circuit 50 as the selection signal Vcut.

  The hard cut circuit 50 is supplied with an instruction signal for supplying power to the heater 42 from the control unit 31 and normally supplies power to the heater 42 (when no abnormal temperature rise of the measurement object has occurred). . The hard cut circuit 50 compares the selection signal Vcut input from the selection circuit 49 with the amplified signal Vb input from the differential amplifier circuit 44, and cuts off the power supply to the heater 42 according to the comparison result ( Stop.

  FIG. 3 is a diagram illustrating a circuit configuration of the temperature detection unit 41 and the differential amplifier circuit 44.

As shown in FIG. 3, the temperature detection unit 41 includes an infrared temperature sensor 51, operational amplifiers 411 and 412, and resistance elements r ₃ and r ₄ . The infrared temperature sensor 51 includes a first thermistor 511, a second thermistor 512, and an infrared film (not shown). The first and second thermistors 511 and 512 have negative resistance values that decrease in resistance as the temperature increases. NTC (Negative Temperature Coefficient Thermistor) having the following temperature coefficient.

  The first thermistor 511 detects the temperature of the infrared film that receives and absorbs the infrared radiation radiated from the measurement object (the fixing roller 27 in the present embodiment) (hereinafter referred to as a detection temperature), while the second thermistor 512 is The temperature of another infrared film isolated from the infrared ray radiated from the measurement object (the fixing roller 27 in this embodiment) (hereinafter referred to as compensation temperature) is detected.

  The compensation temperature detected by the second thermistor 512 corresponds to, for example, the temperature of the casing of the infrared temperature sensor 51 or the ambient temperature, and the resistance value of the first thermistor 511 is the temperature of the casing of the infrared temperature sensor 51. Therefore, the second thermistor 512 detects, for example, the temperature of the casing of the infrared temperature sensor 51 and the atmospheric temperature in order to detect the degree of the influence and eliminate the influence. Note that Td and Tc shown in FIG. 3 indicate the resistance values of the first and second thermistors 511 and 512, and whether a resistance element having the resistance values Td and Tc exists in the first and second thermistors 511 and 512. This is a pseudo representation.

Infrared temperature sensor 51, with the first thermistor 511 and the resistance element r ₁ are connected in series, the second thermistor 512 and the resistance element r ₂ are connected in series, these series circuits supply Vp and the ground Are connected in parallel.

The non-inverting input terminal of the operational amplifier 411, while a resistance element r ₁ is connected to the connection point a between the first thermistor 511, connected inverting input terminal, an output terminal of the operational amplifier 411 via a resistor r ₃ Has been. The non-inverting input terminal of the operational amplifier 412, while a resistive element r ₂ is connected to a connection point b between the second thermistor 512, connected inverting input terminal, an output terminal of the operational amplifier 412 via a resistor r ₄ Has been.

  The temperature detector 41 outputs the outputs of the operational amplifiers 411 and 412 to the differential amplifier circuit 44 and the like as temperature signals Vs and Vd. Hereinafter, the temperature signal Vs is referred to as a detection temperature signal Vs, and the temperature signal Vd is referred to as a compensation temperature signal Vd.

The differential amplifier circuit 44 is for amplifying a signal level difference between the detection temperature signal Vs and the compensation temperature signal Vd output from the temperature detecting section 41, an operational amplifier 441, a resistor _r 5 ~r _8, the capacitor _{C 1} Is provided.

Inverting input terminal of the operational amplifier 441, while the temperature signal Vs be connected to the output terminal of the operational amplifier 411 in the temperature detecting portion 41 via the resistance element r ₅ is input, a non-inverting input terminal, the resistor element r ₆ is connected to the output terminal of the operational amplifier 412 in the temperature detection unit 41, and the temperature signal Vd is input thereto.

In addition, a parallel circuit of the resistor element r ₇ and the capacitor C ₁ is connected between the inverting input terminal and the output terminal of the operational amplifier 441, and the non-inverting input terminal of the operational amplifier 411 is connected to the resistive element r _8. Is connected to the ground via

  The output terminals c and d shown in FIG. 3 correspond to the terminals c and d shown in FIG. 2, and the output of the operational amplifier 441 is converted from the output terminal c to the AD conversion circuit 45 and the hard cut as the differential amplification signal Vb. The output from the operational amplifier 412 is output from the output terminal d to the Vcut1 generation circuit 46, the Vcut2 generation circuit 47, the Vcut3 generation circuit 48, and the AD conversion circuit 45 as the compensation temperature signal Vs.

  Returning to FIG. 2, the AD conversion circuit 45 AD-converts the output value of the differential amplifier circuit 44 and outputs the output value after the AD conversion to the control unit 31. The control unit 31 instructs the hard cut circuit 50 to supply power to the heater 42. The heater 42 is installed in the fixing roller 27 and heats the fixing roller 27.

  The Vcut1 generation circuit 46 shown in FIG. 2 has the circuit configuration shown in FIG. As shown in FIG. 4, the Vcut1 generation circuit 46 uses the compensation temperature signal Vd and compares the comparison target with the differential amplification signal Vb output from the differential amplification circuit 44 in the hard cut circuit 50. This circuit generates a signal Vcut1, and includes operational amplifiers 461 and 462 and resistance elements R1 to R9.

  The inverting input terminal of the operational amplifier 461 is connected to the input terminal d to which the compensation temperature signal Vd is input via the resistance element R1, and is connected to the output terminal via the resistance element R2. The non-inverting input terminal is connected to the ground via the resistor element R3, and the connection point e1 between the inverting input terminal of the operational amplifier 461 and the resistor element R1 is connected to the ground.

  The output terminal of the operational amplifier 461 and the inverting input terminal of the operational amplifier 462 are connected via a resistance element R4, and the connection point e2 between the inverting input terminal of the operational amplifier 462 and the resistance element R4 is connected to the ground via a resistance element R5. Has been.

  A series circuit of a resistance element R6 and a resistance element R7 is formed between the power supply Vp and the ground, and the non-operational amplifier 462 is connected to a connection point e3 between the resistance element R6 and the resistance element R7 via the resistance element R8. The inverting input terminal is connected. Further, the non-inverting input terminal and the output terminal of the operational amplifier 462 are connected via a resistance element R9.

  The Vcut1 generation circuit 46 having the above configuration generates a comparison signal Vcut1 represented by the following formula (1) based on the compensation temperature signal Vd. This will be described later.

Vcut1 = α ₁・ Vd + β ₁・ ・ (1)
The comparison signal Vcut1 is output from the output terminal e4 of the Vcut1 generation circuit 46 to the selection circuit 49 described later.

  The Vcut2 generation circuit 47 shown in FIG. 2 has the circuit configuration shown in FIG. As shown in FIG. 5, the Vcut2 generation circuit 47 uses the compensation temperature signal Vd in the same manner as the Vcut1 generation circuit 46, and the differential amplification signal Vb output from the differential amplification circuit 44 in the hard cut circuit 50. Is a circuit that generates a comparison signal Vcut2 to be compared, and includes operational amplifiers 471 and 472 and resistance elements R11 to R19.

  The inverting input terminal of the operational amplifier 471 is connected to the input terminal d to which the compensation temperature signal Vd is input via the resistance element R11, and is connected to the output terminal via the resistance element R12. The non-inverting input terminal is connected to the ground via the resistor element R13, and the connection point f1 between the inverting input terminal of the operational amplifier 471 and the resistor element R11 is connected to the ground.

  The output terminal of the operational amplifier 471 and the non-inverting input terminal of the operational amplifier 472 are connected via a resistance element R14, and the connection point f2 between the non-inverting input terminal of the operational amplifier 472 and the resistance element R14 is grounded via a resistance element R15. It is connected to the.

  A series circuit of a resistance element R16 and a resistance element R17 is formed between the power supply Vp and the ground, and an inversion of the operational amplifier 472 is connected to a connection point f3 between the resistance element R16 and the resistance element R17 via the resistance element R18. The input terminal is connected. Further, the inverting input terminal and the output terminal of the operational amplifier 472 are connected via a resistance element R19.

  The Vcut2 generation circuit 47 having the above configuration generates a comparison signal Vcut2 represented by the following equation (2) based on the compensation temperature signal Vd. This will be described later.

Vcut2 = α ₂ · Vd + β ₂ (2)
The comparison signal Vcut2 is output from the output terminal f4 of the Vcut2 generation circuit 47 to the selection circuit 49.

  FIG. 6 is a diagram showing a circuit configuration of the Vcut3 generation circuit 48 and the selection circuit 49. As shown in FIG. 6, the Vcut3 generation circuit 48 includes a Zener diode (constant voltage diode), and has an anode connected to the ground and a cathode connected to the output terminal of the operational amplifier 494 in the selection circuit 49. The Vcut3 generation circuit 48 generates the Zener diode offset voltage Ve as Vcut3.

  The selection circuit 49 selects a comparison target to be compared with the differential amplification signal Vb output from the differential amplification circuit 44 in the hard cut circuit 50 from the voltages Vcut1, Vcut2, and Vcut3. This is output to the hard cut circuit 50, and includes operational amplifiers 491 to 494, logic circuits 495 and 496, diodes D1 and D2, and resistance elements R21 to R26.

  The operational amplifier 491 constitutes a voltage follower, and its non-inverting input terminal is connected to the input terminal e4 of the selection circuit 49 to which the comparison signal Vcut1 output from the Vcut1 generation circuit 46 is input. Further, the inverting input terminal and the output terminal of the operational amplifier 491 are connected by a conducting wire. The terminal e4 is the same as the terminal e4 shown in FIG.

  The operational amplifier 492 constitutes a voltage follower, similar to the operational amplifier 491. Its non-inverting input terminal is connected to the input terminal f4 of the selection circuit 49 to which the comparison signal Vcut2 output from the Vcut2 generation circuit 47 is input. It is connected. Further, the inverting input terminal and the output terminal of the operational amplifier 492 are connected by a conducting wire. The terminal f4 is the same as the terminal f4 shown in FIG.

  The operational amplifier 493 functions as a comparator that compares the comparison signals Vcut1 and Vcut2 input to the input terminals e4 and f4, respectively, and an inverting input terminal thereof is connected to the input terminal e4 via the resistor element R21. On the other hand, the non-inverting input terminal is connected to the input terminal f4 via the resistance element R22.

  The output terminal of the operational amplifier 491 is connected to one terminal of the resistor element R23 at the connection point g1, and the resistor element R23 is connected to the resistor element R25 at the connection point g2. The output terminal of the operational amplifier 492 is connected to one terminal of the resistor element R24 at a connection point g3. The resistor element R24 is connected to the inverting input terminal of the operational amplifier 494 functioning as an adder circuit at the connection point g4. Moreover, the connection point g2 and the connection point g4 are connected by the conducting wire.

  The output terminal of the operational amplifier 493 and the input terminal of the logic circuit 495 are connected at a connection point g5, and the input terminal of the logic circuit 496 is connected to the connection point g5. The logic circuit 495 is a circuit obtained by inverting the input of the NOT circuit (negative circuit), and the logic circuit 496 is a NOT circuit.

  The anode of the diode D1 is connected to the connection point g1, while the cathode is connected to the output terminal of the logic circuit 495. The anode of the diode D2 is connected to the connection point g3, while the cathode is connected to the output terminal of the logic circuit 496.

  The non-inverting input terminal of the operational amplifier 494 is connected to the ground via the resistor element R26, and the output terminal is connected to the other terminal of the resistor element R25 at the connection point g6. Further, the cathode of the Zener diode (Vcut3 generation circuit 48) is connected to the connection point g6.

  The function of the selection circuit 49 having the above configuration will be described.

  FIG. 8 is a graph showing the relationship between the compensation temperature T and the signal level (voltage) Vb of the differential amplification signal at different detection temperatures.

  As shown in FIG. 8, each graph has a relationship in which the signal level Vb is maximized at a certain compensation temperature. For example, when attention is paid to the graph (1) for a certain detected temperature, the signal level Vb is The compensation temperature T at which the voltage Vb1 is obtained includes two temperatures, that is, a temperature T1 and a temperature T2.

  Here, when the temperature T3 between the temperature T1 and the temperature T2 (T1 <T3 <T2) is set as a boundary value that changes the suitability of power supply to the heater 42, the state Q1 shown in FIG. The corresponding state) is a state in which power supply to the heater 42 should be continued, while the state (state corresponding to the temperature T2) Q2 is a state in which power supply to the heater 42 should be stopped.

  However, when a certain compensation temperature is set as a boundary value that changes the suitability of power supply to the heater 42 as described above, there are two compensation temperatures T of the temperature T1 and the temperature T2 for the same signal level Vb. Therefore, it is impossible to distinguish between the state Q1 and the state Q2. That is, it is unclear whether the power supply to the heater 42 should be continued or stopped at the obtained one signal level Vb. Therefore, it is impossible to control the power supply to the heater 42 with the reference value to be compared with the signal level Vb being constant.

  Therefore, in this embodiment, the threshold value (reference value) to be compared with the signal level Vb is changed according to the compensation temperature T. Hereinafter, this point will be described.

  FIG. 9 is a diagram showing the relationship between the compensation temperature T and the signal level Vd of the compensation temperature signal output from the second thermistor 512. As shown in FIG. 9, the signal level Vd of the compensation temperature signal decreases as the compensation temperature T increases.

  FIG. 10 pays attention to the graph (1) of FIG. 8, and the relationship between the compensation temperature T and the signal level Vb shown in the graph (1) is the relationship between the compensation temperature T and the compensation temperature signal Vd shown in FIG. It is the graph converted into the relationship between the signal level Vd of a compensation temperature signal, and the signal level Vb using the relationship.

  As shown in FIG. 10, in the graph (2) converted into the relationship between the signal level Vd of the compensation temperature signal and the signal level Vb of the differential amplification signal, the signal level Vd of the compensation temperature signal is a certain value (FIG. 10). Then, the graph becomes a maximum (maximum value Ve (V)) at 2.5 (V). This graph (2) can be approximately expressed by the following plurality of linear functions (3) to (5).

Vb = α ₁ · Vd + β ₁ (Vd ≦ Vd1) (3)
Vb = Ve (Vd1 <Vd <Vd2) (4)
Vb = α ₂ · Vd + β ₂ (Vd ≧ Vd2) (5)
The voltage Vd1 is a compensation voltage at the intersection T1 between the graph (3) expressed by the equation (3) and the graph (4) expressed by the equation (4), and the voltage Vd2 is expressed by the equation (4). Is the compensation voltage at the intersection T2 between the graph (4) represented by (5) and the graph (5) represented by the equation (5).

In this embodiment, as shown in the equation (1), the Vcut1 generation circuit 46 that generates the voltage (α ₁ · Vd + β ₁ ) in the equation (3) as the voltage Vcut1, and the voltage Ve in the equation (4) A Vcut3 circuit 48 that generates (constant value) as the voltage Vcut3, and a Vcut2 generation circuit 47 that generates the voltage (α ₂ · Vd + β ₂ ) in the equation (5) as the voltage Vcut2, as shown in the equation ( ₂ ). Is provided.

  Further, the reference that the selection circuit 49 selects from the voltage Vcut1, the voltage Vcut2, and the voltage Vcut3 is made to correspond to the range of the signal level Vd expressed by the above equations (3) to (5). That is, the range of Vd ≦ Vd1 expressed by the formula (3) corresponds to the range expressed by Vcut1 ≦ Vcut2 and Vcut1 <Ve, and the range of Vd1 <Vd <Vd2 expressed by the formula (4). Corresponds to the range expressed by Vcut1> Ve (Vcut3) and Vcut2> Ve (Vcut3), and the range of Vd ≧ Vd2 expressed by the equation (5) is expressed by Vcut1 ≧ Vcut2 and Vcut2 <Ve. Corresponds to the range.

  Therefore, the selection circuit 49 is configured such that the magnitude relationship between the voltages Vcut1, Vcut2, and Vcut3 generated by the Vcut1 generation circuit 46, the Vcut2 generation circuit 47, and the Vcut3 generation circuit 48 is Vcut1 ≦ Vcut2 and Vcut1 <Ve (Vcut3). The voltage Vcut1 is selected. When Vcut1> Ve (Vcut3) and Vcut2> Ve (Vcut3), the voltage Ve (Vcut3) is selected. When Vcut1 ≧ Vcut2 and Vcut2 <Ve (Vcut3), the voltage Vcut2 is selected. select.

  As a result, the signal level Vd of the compensation temperature signal and the signal to be compared with the signal level Vd have a one-to-one relationship, and the object to be compared with the signal level Vd is based on the signal level Vd of the compensation temperature signal. Only one voltage will be derived.

  In this way, a voltage is selected from the voltages Vcut1, Vcut2, and Vcut3 according to the range of the signal level Vd (and thus the compensation temperature T) of the compensation temperature signal, and the signal related to the selected voltage is the hardware level. From the output terminal g7 of the selection circuit 49 to be output to the cut circuit 50, the voltage Vcut is output to the hard cut circuit 50.

  FIG. 7 is a diagram illustrating a circuit configuration of the hard cut circuit 50. As shown in FIG. 7, the hard cut circuit 50 includes an operational amplifier 501, resistance elements R31 to R33, a logic circuit 502, and a transistor Tr1.

  The operational amplifier 501 functions as a comparator that compares the level of the selection signal Vcut, which is the output of the selection circuit 49, with the differential amplification signal Vb of the differential amplification circuit 44. The inverting input terminal receives the resistance element R31. Is connected to the input terminal g7 of the hard cut circuit 50 to which the selection signal Vcut is inputted, while the non-inverting input terminal is inputted with the differential amplification signal Vb through a resistance element R32. The hard cut circuit 50 is connected to the input terminal h1.

  The logic circuit 502 functions as an AND circuit (logical product circuit), one input terminal is connected to the output terminal of the operational amplifier 501, and the other input terminal is the input terminal h2 of the hard cut circuit 50. It is connected to the. An instruction signal (“H (high) signal”) for supplying power from the control unit 31 to the heater 42 is input to the input terminal h2.

  A connection point h3 between one terminal of the logic circuit 502 and the output terminal of the operational amplifier 501 is connected to the power supply Vp through the resistance element R33. The transistor Tr1 functions as a switching element, and the base terminal is connected to the output terminal of the logic circuit 502. The collector terminal of the transistor Tr1 is connected to the heater 42, and the emitter terminal is connected to the ground.

  The hard cut circuit 50 having the above configuration uses the selection signal Vcut selected by the selection circuit 49 as a threshold (reference value), compares the threshold with the differential amplification signal Vb, and performs differential processing. When the signal level of the amplified signal Vb is higher than the signal level of the selection signal Vcut (when the “H (high)” signal is output from the operational amplifier 501), the power supply to the heater 42 is cut off (stopped).

  As described above, it is output from the second thermistor 512 on the basis of a plurality of straight lines (primary number) representing the relationship between the compensation temperature T and the signal level Vb of the differential amplification signal. Generating a voltage Vcut1, a voltage Vcut2, and a voltage Vcut3 using the compensation temperature signal Vd as a parameter, selecting an appropriate one from the voltages, and comparing the selected one with the signal level Vb, Since the power supply to the heater 42 is cut off according to the comparison result, the signal level Vd of the compensation temperature signal and the signal to be compared with the signal level Vd have a one-to-one relationship, and the compensation temperature Based on the signal level Vd of the signal, the only voltage to be compared with the signal level Vd can be derived.

  As a result, the power supply to the heater 42 can be appropriately cut off, and the temperature of the measurement object (fixing roller 27) can be controlled with high accuracy. Further, since the resistance is not added to the temperature detection unit as in the prior art, the temperature detection accuracy is not lowered.

  Further, even if the detected temperature signal Vs is the same, the voltage Vcut (threshold value) to be compared with the signal level Vb is changed according to the compensation temperature signal Vd which is an element that changes the signal level Vb of the differential amplification signal. As a result, the threshold value (reference value) corresponding to the compensation temperature signal Vd can be set, and the power supply to the heater 42 can be accurately cut off.

  Further, since the graph showing the relationship between the compensation temperature T and the signal level Vb of the differential amplification signal is approximately expressed by a plurality of straight lines (the number of first order), the Vcut1 generation circuit 46, the Vcut2 generation circuit 47, and the Vcut3 The generation circuit 48 and the selection circuit 49 can be realized with a relatively simple configuration.

1 is a diagram illustrating an internal configuration of a first embodiment of an image forming apparatus. 2 is a block diagram illustrating a circuit configuration of a fixing unit. FIG. It is a figure which shows the circuit structure of a temperature detection part and a differential amplifier circuit. It is a figure which shows the circuit structure of a Vcut1 production | generation circuit. It is a figure which shows the circuit structure of a Vcut2 production | generation circuit. It is a figure which shows the circuit structure of a Vcut3 production | generation circuit and a selection circuit. It is a figure which shows the circuit structure of a hard cut circuit. It is a graph which shows the relationship between the compensation temperature T and the signal level (voltage) Vb of a differential amplification signal in different detection temperature. It is a figure which shows the relationship between the said compensation temperature T and the signal level Vd of the compensation temperature signal output from a 2nd thermistor. Paying attention to the graph (1) in FIG. 8, the relationship between the compensation temperature T and the signal level Vb shown in the graph (1) is expressed using the relationship between the compensation temperature T and the compensation temperature signal Vd shown in FIG. 4 is a graph converted into the relationship between the signal level Vd and the signal level Vb of the compensation temperature signal.

Explanation of symbols

4 fixing unit 27 fixing roller 28 pressure roller 31 control unit 41 temperature detection unit 42 heater 43 heater control circuit 44 differential amplification circuit 46 Vcut1 generation circuit 47 Vcut2 generation circuit 48 Vcut3 circuit 49 selection circuit 50 hard cut circuit 51 infrared temperature sensor 511, 512 thermistor

Claims (5)

Heating means for fixing toner adhering to the paper conveyed between the pair of rollers by supplying heat to at least one of the pair of rollers in contact with each other on the cylindrical peripheral surface;
Power supply means for generating heat by supplying power to the heating means; and
A first sensor for detecting a temperature of a predetermined measurement object to which heat is supplied by the heating unit; and a second sensor for detecting a temperature of an object different from the measurement object as a compensation temperature. Temperature detection means for outputting a temperature based on the outputs of the first and second sensors as the temperature of the measurement object;
A power cutoff means for comparing a detected temperature derived from the output of the first sensor of the temperature detection means with a predetermined reference value and shutting off the power supply to the heating means based on the comparison result When,
And a reference value providing means for selecting one predetermined value from a plurality of predetermined values related to the predetermined reference value and providing the predetermined value as the predetermined reference value to the power cut-off means. Fixing device to do.   The fixing device according to claim 1, wherein the reference value providing unit sets the predetermined reference value in accordance with an output of the second sensor.   The reference value providing means corresponds to the obtained output of the second sensor using a plurality of linear functions that approximately represent the relationship between the detected temperature having an extreme value and the output of the second sensor. The fixing device according to claim 2, wherein the detected temperature is derived, and the detected temperature is provided to the power cut-off unit as the predetermined reference value.   The predetermined measurement object whose temperature is detected by the first sensor is the temperature of one of the pair of rollers, and the object whose temperature is detected by the second sensor is the temperature detection The fixing device according to claim 1, wherein the fixing device is a temperature of a housing of the means. Image forming means for performing an image forming operation on a sheet based on predetermined image data;
An image comprising: the fixing device according to claim 1, wherein a toner attached to the sheet is fixed to the sheet after the image forming operation by the image forming unit. Forming equipment.

Images