JP4773751B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic system.

  Conventionally, the following techniques are used in a fixing device of an image forming apparatus.

(1) Technology Using Current Detection Unit In the image forming apparatus, in order to perform printing after power is turned on, the temperature of the fixing device must be set to a predetermined temperature or higher. In order to rapidly increase the temperature of the fixing device, it is necessary to increase the amount of current supplied to the fixing device.

  However, commercial AC power supplies are regulated by the current rating of the outlet, and only currents below the rated current can be used.

  In view of this, as disclosed in Patent Document 1 and the like, a means for detecting a current flowing in the fixing device is provided, and control is performed to supply more current to the fixing device below the rated current.

(2) Technology using overcurrent detection means The fixing device is provided with temperature detection means for detecting the temperature of the heating element in the fixing device, and adjusts the amount of supplied current using the current control means according to the result, The fixing device is controlled to a predetermined temperature.

  In such a fixing device, if either the temperature detecting means or the control means does not function normally, the fixing device may be overheated, resulting in a failure of the device.

  Therefore, as disclosed in Patent Document 2 and the like, an overcurrent detection unit that detects a state in which an excessive current flows in the fixing device is provided, and when an excessive current flows in the fixing device, an energization cutoff device is provided. And forcibly stopping the current supply to the fixing device.

JP-A-10-274901 Japanese Patent Laid-Open No. 06-202512 JP 04-44075 A JP 04-44076 A JP 04-44077 A JP 04-44078 A JP 04-44079 A Japanese Patent Laid-Open No. 04-44080 JP 04-44081 A JP 04-44082 A JP 04-44083 A Japanese Patent Laid-Open No. 04-204980 Japanese Patent Laid-Open No. 04-204981 Japanese Patent Laid-Open No. 04-204982 Japanese Patent Laid-Open No. 04-204983 Japanese Patent Laid-Open No. 04-204984

  However, the image forming apparatus having both the current detection unit and the overcurrent detection unit described above has the following problems.

  (1) Since errors occur in the detection characteristics of the current detection means and the overcurrent detection means, it is impossible to perform overcurrent detection and current detection with high accuracy.

  For this problem, there is a method for improving the detection accuracy by adjusting the characteristics of each detection means, but a dedicated adjustment means is required for the two detection means, and this is a cost-effective measure. I couldn't.

  (2) There is a problem that the image forming apparatus malfunctions due to a relative error in detection characteristics generated between the current detection unit and the overcurrent detection unit. As described above, the conventional image forming apparatus controls the amount of current supplied to the fixing device in accordance with the result of the current detection unit.

  In such control, control is performed so that the level of the supply current is equal to or lower than the current at which the overcurrent detection unit operates, so that the image forming apparatus is prevented from malfunctioning.

  However, when the relative accuracy between the detection characteristics of the current detection means and the characteristics of the overcurrent detection means is poor, there is a problem in that the current at the level at which the overcurrent detection means operates is erroneously supplied and the overcurrent detection means malfunctions. It was.

  To solve this problem, there is a method for improving the detection accuracy by adjusting the characteristics of the respective detection means. However, there is a limit to reducing the relative error between the two detection means due to an error generated during the adjustment.

  Accordingly, an object of the present invention is to provide an image forming apparatus that can solve such problems and can further increase the detection accuracy of the current detection unit and can further increase the detection accuracy of the overcurrent detection unit. There is to do.

  The invention of claim 1 is an image in which a toner image formed on an image carrier using electrophotographic process technology is transferred onto a recording medium, and the toner image transferred onto the recording medium is heated and fixed by an electric heating means. In the forming apparatus, the current detection means for detecting the current flowing in the electric heating means, and the voltage value obtained by converting the current detected by the current detection means to the current voltage is compared with the reference voltage of the reference voltage generation means. Overcurrent detection means for detecting an overcurrent, and the electric current based on a ratio between a voltage value obtained by current-voltage conversion of the current detected by the current detection means and a reference voltage value of the reference voltage generation means. And a calculating means for calculating a current flowing through the heating means.

The invention of claim 1 includes a heater, a fixing film whose inner surface slides in contact with the heater, a pressure roller that presses the heater through the fixing film to form a fixing nip portion, and a temperature of the heater A temperature detecting means for detecting the temperature, and a fixing means for fixing the toner image on the recording material by heating while conveying the recording material at the fixing nip, and a detected temperature detected by the temperature detecting means. Power control means for controlling the power supplied to the heater so as to reach the target temperature, current level detection means for outputting a voltage at a level corresponding to the current flowing through the heater, reference voltage generation means for outputting a reference voltage, The voltage output from the current level detection means is compared with the reference voltage output from the reference voltage generation means to detect whether the current flowing through the heater is in an overcurrent state. In the image forming apparatus, the power control unit shuts off the power supply to the heater when the overcurrent detection unit determines that an overcurrent is detected. And a sensitivity adjusting means for adjusting the sensitivity of the current level detecting means so that the voltage output from the current level detecting means at that time is equal to the reference voltage. The power control means sets the ratio of the voltage output from the current level detection means to the ratio of the reference voltage output from the reference voltage generation means when the fixing means is started up so that the detected temperature reaches the target temperature. The power supplied to the heater is controlled so that a current obtained by multiplying the adjustment current becomes a predetermined target current smaller than the adjustment current .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<First Embodiment>
(1) Laser Beam Printer FIG. 1 is a diagram showing the structure of a laser beam printer 100 according to this embodiment. The laser printer 100 includes a deck 101 that stores recording paper P, a deck paper presence sensor that detects the presence or absence of the recording paper P in the deck 101, and a paper size detection sensor 103 that detects the size of the recording paper P in the deck 101. The pickup roller 104 that feeds the recording paper P from the deck 101, the deck paper feeding roller 105 that conveys the recording paper P fed by the pickup roller 104, and the deck paper feeding roller 105 are paired to prevent double feeding of the recording paper P. A retard roller 106 is provided.

  Downstream of the deck paper feed roller 105, the deck 101, a paper feed sensor 107 that detects a paper feed state from a double-side reversing unit described later, and a paper feed transport roller 108 for transporting the recording paper P further downstream. A registration roller pair 109 that conveys the recording paper P in synchronization with the printing timing, and a pre-registration sensor 110 that detects the conveyance state of the recording paper P to the registration roller pair 109 are provided.

  Downstream of the registration roller pair, a process cartridge 112 that forms a toner image on the photosensitive drum 1 based on laser light from the laser scanner unit 111, and a toner image formed on the photosensitive drum 1 on the recording paper P A roller member 113 for transferring and a discharge member (hereinafter referred to as “static elimination needle”) 114 for removing charges on the recording paper P and promoting separation from the photosensitive drum 1 are provided.

  Downstream from the static elimination needle 114 are a conveyance guide 115, a fixing device 116 that thermally fixes the toner image transferred onto the recording paper P, a fixing paper discharge sensor 119 that detects a conveyance state from the fixing device 116, and a fixing device 116. A double-sided flapper 120 is provided for switching the destination of the transported recording paper P from the paper discharge unit to the double-side reversing unit, and a paper discharge state for detecting the paper conveyance state of the paper discharge unit is provided downstream of the paper discharge unit. A paper sensor 121 and a paper discharge roller pair 122 for discharging the recording paper are provided. On the other hand, the recording paper P after one-sided printing is reversed in order to print on both sides of the recording paper P, and the recording paper P is fed to the double-side reversing part side for feeding again to the image forming part by forward / reverse rotation. The recording paper P is transported from a reverse roller pair 123 to be switched back, a reversing sensor 124 for detecting a paper transport state to the reverse roller pair 123, and a lateral registration portion (not shown) for aligning the lateral position of the recording paper P. A D-cut roller 125 for detecting the recording paper P in the double-side reversing unit, and a double-sided conveying roller pair 127 for conveying the recording paper P from the double-side reversing unit to the paper feeding unit. Yes.

(2) Fixing Device FIG. 2 is a schematic diagram showing the structure of the fixing device. This fixing device is a film heating type device disclosed in Patent Documents 3 to 16 and the like. Reference numeral 204 denotes a heat-resistant, heat-insulating, and rigid body stay for fixing the ceramic heater and guiding the inner surface of the film, and is a horizontally long member having a longitudinal direction in the direction crossing the conveyance path of the recording paper 210 (perpendicular to the drawing). 205 is a ceramic heater, which will be described later, and is a horizontally long member having a longitudinal direction in the direction crossing the transfer material conveyance path, which is fitted into a groove formed along the longitudinal direction on the lower surface of the stay and fixed and supported by a heat resistant adhesive. is there.

  Reference numeral 201 denotes a cylindrical heat-resistant film material (hereinafter referred to as “fixing film”), which is loosely fitted to a stay 204 to which a ceramic heater 205 is attached. For example, a cylindrical single-layer film such as PTFE, PFA, FEP, etc. having a heat resistance, releasability, strength, durability, etc. having a thickness of about 40 to 100 μm, or a cylinder such as polyimide, polyamide, PEEK, PES, PPS, etc. It is a composite layer film in which PTFE, PFA, FEP or the like is coated on the outer peripheral surface of the film.

  A pressure roller 202 is a pressure roller in which a heat-resistant elastic layer 207 such as silicone rubber is provided concentrically and integrally on the outer periphery of the core metal 203. The pressure roller 202 and the ceramic heater 205 on the stay 204 side are pressed against the elasticity of the pressure roller 202 with the fixing film 201 interposed therebetween. A range indicated by an arrow N is a fixing nip portion formed by the pressure contact.

  The pressure roller 202 is rotationally driven at a predetermined peripheral speed in the direction of arrow B by a fixing drive motor M2 (118). A rotational force acts directly on the fixing film 201 by the frictional force between the pressure roller 202 and the outer surface of the fixing film 201 in the fixing nip portion N by the rotational driving of the pressure roller 202 (the recording paper 210 is in the direction of arrow A). ), The rotational force indirectly acts on the fixing film 201 via the recording paper 210), and the fixing film 201 slides in pressure contact with the lower surface of the ceramic heater 205 in the clockwise direction. C is rotationally driven. The stay 204 also functions as a film inner surface guide member to facilitate the rotation of the fixing film 201. In order to reduce the sliding resistance between the inner surface of the fixing film 201 and the lower surface of the ceramic heater 205, a small amount of a lubricant such as heat-resistant grease can be interposed between them.

  The fixing nip formed by the ceramic heater 205 and the pressure roller 202 with the fixing film 201 sandwiched in a state where the rotation of the fixing film 201 by the rotation of the pressure roller 202 becomes steady and the temperature of the ceramic heater 205 rises to a predetermined level. The recording paper 210 to be image-fixed is introduced between the fixing film 201 of the part N and the pressure roller 202, and the fixing nip part N is nipped and conveyed together with the fixing film 201. Is applied to the recording paper 210 and the unfixed image through the fixing film 201, and the unfixed image on the recording paper 210 is heated and fixed on the surface of the recording paper 210. The recording paper 210 that has passed through the fixing nip N is separated from the surface of the fixing film 201 and conveyed. 2 indicates the conveyance direction of the recording paper 210.

(3) Ceramic heater 205
FIG. 3 is a view showing the structure of the ceramic heater 205. The ceramic heater 205 is long disposed in a direction orthogonal to the recording paper conveyance direction. Alumina (Al ₂ O ₃ ) is used as the substrate 301, and two heat generation patterns 302a and 302b are formed on one side by printing. The heat generation patterns 302a and 302b are covered with a glass protective film as an electrical insulating layer. Hereinafter, the heater part formed with the heat generation pattern 302a is referred to as “main heater 302a”, and the heater part formed with the heat generation pattern 302b is referred to as “sub-heater 302b”.

  Reference numerals 303a, 303b, and 303c denote power supply electrodes, which are formed so that a voltage can be applied to both ends of the heat generation pattern. The main heater 302a and the sub heater 302b are greatly different in heat distribution. FIG. 4 shows the heat distribution of the main heater 302a and the sub heater 302b. The main heater 302a is formed to increase the amount of heat generated at the center of the ceramic heater 205, and the sub heater 301b is formed to increase the amount of heat generated at the end.

(4) Thermistor The fixing device according to the present embodiment has three thermistors for measuring the temperature of the ceramic heater 205. Each thermistor is pressed onto the ceramic heater 205 with a predetermined pressure. FIG. 4 is a diagram showing the arrangement relationship of the thermistor, and the arrangement of the thermistor in the longitudinal direction of the ceramic heater 205 is indicated by arrows D, E, F, and G. The thermistor 1 is disposed at the center of the ceramic heater 205. On the other hand, the thermistors 2 and 3 are arranged at the ends. Each thermistor is connected to a temperature detection circuit (not shown), and the temperature detection result of each thermistor is input to the CPU 501. FIG. 8 shows an internal circuit of the temperature detection circuit. The thermistors 1, 2 and 3 are connected in series with resistors 804, 806 and 807, respectively. S10, S11, and S12 are detection signals, which vary according to the resistance value of the thermistor that varies with temperature. The detection signals S10, S11, and S12 are connected to the CPU 501 and a safety circuit described later.

(5) Power Control Circuit Next, a power control circuit that supplies power to the ceramic heater 205 will be described. The power control is controlled independently by the main heater 302a and the sub heater 302b. FIG. 5 is a connection diagram of the power supply control circuit. 501 is a CPU, 502 and 503 are first and second triacs, 504 is an AC power source, 505 is a relay, and 507 is a current detection circuit. The first triac 502 and the main heater 302a are connected in series, the second triac 503 and the sub-heater 302b are connected in series, and they are connected to the AC power source 504 in parallel. The first and second triacs 502 and 503 are ON / OFF controlled by ON / OFF of the first and second heater drive signals S1 and S2 from the CPU 501, respectively.

  The relay 505 is inserted between the first and second triacs 502 and 503 and the AC power source 504, and is configured to cut off the energization to the main heater 302a and the sub heater 302b by driving the relay 505. A control signal of the relay 505 is connected to a safety circuit described later. The current detection circuit 507 is inserted between the relay 505 and the AC power source 504. The operation of the current detection circuit 507 will be described later.

  The adjustment of the amount of power supplied to the ceramic heater 205 is realized by phase control that controls the power supplied to each heater by turning on / off the current at the phase angle within one half wave of the AC power source. The CPU 501 performs control using a zero cross signal output from a zero cross detection circuit that detects zero cross timing of an AC power source (not shown).

FIG. 7A shows the drive timing of the zero cross signal and the heater drive signal S1. After a predetermined time t1, t2 from the falling timing of the zero cross signal, the heater drive signal S1 is turned on to control energization to the ceramic heater 205. FIG. 7B is a table showing the relationship between the timings t 1 and t 2 and the power supplied to the ceramic heater 205. The table is for the case where the frequency of the AC power supply is 50 Hz, and the power supplied when energized in all phases is represented as 100%.
A method for controlling the power supply amount during the printing operation will be described later.

(6) Current detection circuit 507
Detecting a total current value of the current flowing to the current detection circuit 507 in the main heater 302a and sub heater 302b, a current level detection signal S 6 indicating a level corresponding to the total current value, an overcurrent indicating the magnitude relationship between the predetermined current value outputs two signals of the detection signal S 5. The operation of the current detection circuit 507 will be described.

(6-1) Current Level Detection Unit FIG. 6 is a circuit diagram of the current detection circuit 507. FIG. 10 is a waveform diagram of each part in the circuit. FIG. 10A shows a waveform when a full-wave current flows through the main heater 302a and the sub heater 302b, and FIG. 10B shows a phase within one half wave. The waveform in the case of performing phase control for controlling the power supplied to the main heater 302a and the sub heater 302b by turning on / off the energization at the corners is shown.

  Terminals 646 and 647 are connected between relay 505 and AC power source 504. Reference numeral 645 denotes a current transformer. A current I601 energized to the main heater 302a and the sub heater 302b is input to the first terminal and the second terminal, and a resistor 640 is output to the output terminals 3 and 4 of the current transformer 645. 643 and diodes 642 and 644 are connected, and a half-wave AC voltage Vt corresponding to the level of the current I601 is generated at the output of the rectifier circuit. The output part of the rectifier circuit is connected to an integrating circuit 660 which is composed of an operational amplifier 633, FET 636, capacitor 634, resistors 635 and 638, and variable resistor 653. The variable resistor 653 is for adjusting the sensitivity of the integrating circuit. The adjustment method will be described later.

  In the integration circuit, an AC voltage is integrated in one cycle, and a voltage corresponding to the integration value is generated between the terminals of the capacitor 634. The voltage across the capacitor 634 is cleared to 0V every cycle by the operation of the FET 636 driven by the RST signal. The RST signal is a signal output from the CPU 501 at a predetermined timing with respect to the zero cross point of the AC power source 504. The voltage Vint at the output terminal of the operational amplifier 633 can be expressed by the following equation.

  Here, ΣVt is an integral value of the output voltage Vt of the rectifier circuit.

  An output of the operational amplifier 633 is input to a differential circuit including an operational amplifier 628, resistors 631, 632, 630, and 629, and a diode 627. As for the resistors in the differential circuit, the resistors 630 and 631, and the resistors 629 and 632 are set to the same resistance value. The differential circuit receives the output voltage Vint of the integrating circuit and the output voltage Vt of the rectifying circuit. The output voltage Vcs of the differential circuit is expressed as follows.

  Here, R 631 is the resistance value of the resistor R 631, and R 632 is the resistance value of the resistor 632.

  The output of the differential circuit is peak-held by the capacitor 626, but is cleared to 0V every cycle by the operation of the FET 626 driven by the RST signal. The voltage Vcs is at a level corresponding to the average value of the half cycle section of the current flowing through the heater 302. The output voltage Vcs of the differential circuit is output as the current level detection signal S6 via the operational amplifier 622 and input to the analog input port of the CPU 501.

(6-2) Overcurrent Detection Unit The output voltage Vcs of the differential circuit is also connected to the operational amplifier 604 and is compared with a predetermined reference voltage Vfd to determine whether the current flowing through the heater 302 is in an overcurrent state. To do. When the output voltage Vcs of the differential circuit is higher than the reference voltage Vfd, the output of the operational amplifier 604 becomes HIGH level. The output of the operational amplifier 604 is a diode 602 and a capacitor 601. The resistor 603 is connected, and the HIGH level state is peak-held for a predetermined period and is output as the overcurrent detection signal S5.

  Here, the reference voltage Vfd will be described. In the present embodiment, the output of the frequency detector 655 that generates a voltage corresponding to the frequency of the AC power supply is used as the reference voltage. The transistor 612 and the resistors 613, 615, 618, 619, and 614 are constant current generation circuits. A constant current is supplied to the capacitor 610, and the voltage Vsq increases. An FET 611 driven by an RST signal is connected to both ends of the capacitor 610, and the voltage Vsq is cleared to 0V by the RST signal. A waveform diagram of the Vsq and RST signals is shown in FIG. The voltage Vsq can be expressed by the following equation.

  Here, R613, R618, and R619 are resistance values of the resistors R613, R618, and R619, respectively, and T1 indicates a time period during which the RST signal is LOW.

  The voltage Vsq is input to the negative input terminal of the operational amplifier 604 as a reference voltage Vfd after the peak value of the voltage Vsq is peaked by a peak hold circuit including an operational amplifier 607, a diode 606, a capacitor 608, and a resistor 605. Thus, by using the frequency detection signal as the reference voltage of the overcurrent detection unit, it is possible to prevent a decrease in overcurrent detection accuracy due to fluctuations in the AC power supply frequency.

(6-3) Adjustment method The current detection circuit is provided with a circuit adjustment function. The purpose of circuit adjustment is to improve the accuracy of current detection and overcurrent detection. The adjustment is performed by adjusting the variable resistor 653 that changes the characteristics of the integration circuit 660 and adjusting the operating current of the overcurrent detection unit to the target value. Specifically, the adjustment current I (ovc) is supplied between the terminal 646 and the terminal 647, and the variable resistor 653 is adjusted so that the peak value of the output voltage Vcs of the differential circuit matches the voltage level of the reference voltage Vfd. . At this time, a signal corresponding to the frequency of the adjustment current I (ovc) is input as the RST signal. By performing circuit adjustment in this way, the operating current for overcurrent detection coincides with the adjustment current I (ovc), and the overcurrent state can be detected with high accuracy.

  The magnitude of the adjustment current I (ovc) is set to be equal to or less than the heater current at which the apparatus is damaged due to excessive heating of the heater. In the present embodiment, the adjustment current I (ovc) is 13A.

(7) Heater Current Calculation Unit The current level detection signal S6 output from the current detection circuit is input to the analog input port of the CPU 501 and performs calculation processing for calculating the absolute value of the current. FIG. 17 is a flowchart of the current value calculation process. First, in S1702, the current level detection signal S6 is sampled. Sampling is performed within the S section of FIGS. 10A and 10B, so that a level signal corresponding to the heater current can be sampled. Next, after sampling the frequency detection signal S7 in S1703, the process proceeds to the current calculation process in S1704. As described in the above circuit adjustment method, the current detection circuit is adjusted so that the peak value of the output voltage Vcs of the differential circuit and the level of the reference voltage Vfd coincide with each other when the adjustment current I (ovc) is applied. The absolute value of the heater current can be calculated by the following equation.

Here, V (S6) the voltage level of the current level detection signal S6, V (S7) is a voltage level of the current level detection signal S 7.

  Thus, by calculating the absolute amount of the current value from the ratio between the output voltage Vcs of the differential circuit as the current detection result and the level of Vfd as the reference value of the overcurrent detection signal, the current amount can be obtained with high accuracy. Can be detected.

  In step S1705, current correction processing is performed. The current value calculated in S1704 is the average current value of the AC waveform supplied to the ceramic heater 205. On the other hand, since the amount of power to the ceramic heater 205 is determined by the effective current value, a process of converting the average current value Is calculated in S1704 into an effective current value Is ′ is performed. This conversion uses a coefficient RMSCONV determined by the table shown in FIG. The coefficient RMSCONV indicates the ratio between the effective current value and the average current value. When phase control is performed, the value changes depending on the phase angle at which energization is started. The table of FIG. 18 shows the relationship of the coefficient RMSCONV with respect to the power supplied during phase control. Correction is performed using the following equation.

Is ′ = Is × RMSCONV (Formula 5)

  The effective current value calculated by Equation 5 is used as a detection result and used for control described later.

(8) Power Control Sequence A power control method in this laser beam printer will be described. In the present embodiment, the main heater 302a and the sub heater 302b are turned on / off at a phase angle within one half wave, thereby performing phase control for controlling the power supplied to the main heater 302a and the sub heater 302b. ing. The control method includes a start-up control in which the heater is driven from the print stop state and a steady temperature control to be performed when the ceramic heater 205 is at a predetermined temperature or higher.

(8-1) Start-up Control At the time of start-up, control is performed to supply power so that the current flowing through the ceramic heater 205 becomes a predetermined target current I (target). FIG. 12 is a flowchart of start-up control. When the CPU 501 receives a print start signal from a controller (not shown), it executes an image forming sequence. At the same time, the first and second heater drive signals S 1 and S 2 are controlled to turn on the first and second triacs 503 and 504, and the temperature of the ceramic heater 205 is started. After the initial supply power Po is input in S1201, the temperature state of the thermistor 1 is detected and recognized, and it is determined whether the temperature reaches 160 ° C. (S1203).

  Here, when the temperature is 160 ° C. or higher, the routine proceeds to steady temperature control. When the temperature is 160 ° C. or lower, the current detection signal S5 is sampled in S1204, and the current value is calculated by the above-described method in S1205. In step S1206, the control amount ΔP of the supplied power is determined according to the difference ΔI between the detection result of the heater current value and the target current I (target). FIG. 13 is a table showing the relationship between the difference ΔI from the target current and the control amount ΔP of the supplied power. The larger the difference ΔI from the target current, the larger the control amount ΔP is set so that the target current can be reached in a short time. After the supply power is updated in S1207, the process returns to S1203 and the same process is repeated, whereby the current flowing through the ceramic heater 205 can be controlled to a predetermined target current I (target).

  The target current value I (target) is the value shown in FIG. The target current value I (target) is set smaller than the adjustment current I (ovc) described above, thereby avoiding malfunction of the overcurrent circuit.

  Thus, by controlling the power supply so that the current flowing to the ceramic heater 205 at the start-up becomes a predetermined target current I (target), the temperature of the ceramic heater 205 can be shortened without damaging the equipment. It becomes possible to start up in time.

(8-2) Steady Temperature Control When the temperature detected by the thermistor 1 reaches a predetermined 160 ° C. in the startup control described above, the control method is switched to steady temperature control. In the steady temperature control, power is supplied so that the temperature detected by the thermistor 1 becomes a predetermined target temperature T (target). FIG. 15 is a flowchart for steady temperature control. After detecting and recognizing the temperature state of the thermistor 1 in S1501, a difference ΔT between the sampling result of the thermistor 1 and the target temperature T (target) is calculated (S1502), and the control amount ΔP of the supplied power is determined according to the magnitude of ΔT. Is determined (S1503). FIG. 16 is a table showing the relationship between the difference ΔT from the target temperature and the control amount ΔP of the supplied power. The larger the difference ΔT from the target temperature is, the larger the control amount ΔP is set, so that the target temperature can be reached in a short time. After the supply power is updated in S1504, the process returns to S1501 and the same process is repeated, whereby the temperature of the thermistor 1 can be controlled to the target temperature T (target).

(9) Safety circuit In the laser beam printer according to the present embodiment, a safety device is provided to avoid overheating of the ceramic heater 205 during energization runaway.

  The safety device 509 is provided with a circuit that cuts off power to the ceramic heater 205 with the following two types of configurations.

(9-1) Energization interruption by abnormal temperature detection FIG. 9 is a circuit diagram of a safety circuit that controls the interruption of energization by thermistor detection. The detection signals S10, S11, and S12 of the thermistors 1, 2, and 3 are input to the negative input terminals of the comparators 901, 902, and 903, respectively, and compared with the reference voltage Vref input to the positive input terminal. Determine abnormal overheating. The reference voltage Vref (the value obtained by dividing the voltage Vcc by the resistors 904 and 905) with respect to the detection signal of the thermistor 1 is compared. When the comparator is turned on according to the result of the voltage comparison, a base current flows through the resistor 912 through the transistor 915 and is turned on. Thereby, relay control signal S4 will be in a LOW state, electricity supply to a relay will be stopped, and relay 505 will be in an interception state. On the other hand, the reference voltage Vref with respect to the detection signal of the thermistor 2 is compared with the value determined by the resistors 906 and 908, and driving the transistor 916 stops the energization of the relay and puts the relay 505 in the cut-off state. Further, the reference voltage Vref with respect to the detection signal of the thermistor 2 is compared with a value determined by the resistors 909 and 910, and the transistor 917 is driven to stop energization of the relay, and the relay 505 is cut off.

(9-2) Energization interruption due to overcurrent detection The overcurrent detection detection signal S5 output from the current detection circuit 507 is input to the safety circuit 509. When the overcurrent detection signal S5 is in a HIGH state due to an overcurrent state, the transistor 925 is turned on, the relay control signal S4 is in a LOW state, energization of the relay is stopped, and the relay 505 is cut off.

  As described above, when the current to the ceramic heater 205 is cut off in accordance with the state of the overcurrent detection signal S5 of the current detection circuit 507, excessive current flows to the ceramic heater 205 due to an abnormality in the apparatus. However, it is possible to prevent damage to the device.

  As described above, in the laser beam printer according to the present embodiment, the operating point of the overcurrent detection circuit is adjusted by the sensitivity adjustment unit of the current detection circuit, thereby realizing a highly accurate overcurrent detection unit.

  In addition, by calculating the absolute value of the current from the relative ratio between the detection value of the current detection circuit and the reference voltage of the overcurrent detection circuit, a highly accurate current detection means is realized.

Furthermore, by calculating the absolute value of the current from the relative ratio between the detection value of the current detection circuit and the reference voltage of the overcurrent detection circuit, a current detection unit and an overcurrent detection unit with high relative accuracy are realized.
<Second Embodiment>
This embodiment is different from the first embodiment only in the configuration of the circuit adjustment unit of the current detection circuit 507. That is, in the first embodiment, the integrating circuit 660 is provided with a variable resistor. However, in this embodiment, as shown in FIG. 19, the resistor 1901 in the frequency detector 655 is used as a variable resistor, and the resistance value is variable. And the current detection circuit was adjusted.

  The voltage Vsq can be expressed by the following equation.

  Here, R1901 is the resistance value of the variable resistor 1901.

  As apparent from Equation 6, the voltage Vsq is changed by making R1901 variable. The voltage Vsq is peak-holed by the peak hold circuit including the operational amplifier 607, the diode 606, the capacitor 608, and the resistor 605, and is input to the negative input terminal of the operational amplifier 604 as the reference voltage Vfd. That is, the level of the reference voltage Vfd can be adjusted by adjusting the resistance value of the variable resistor 1901. The adjustment of the current detection circuit is performed by the same method as in the first embodiment, the variable resistor 653 is adjusted, and the operating current of the overcurrent detection unit is adjusted to the target value.

  By performing circuit adjustment in this way, the operating current for overcurrent detection coincides with the adjustment current I (ovc), and the overcurrent state can be detected with high accuracy.

  As described above, in the laser beam printer according to the present embodiment, high-precision overcurrent detection means is realized by adjusting the operating point of the overcurrent detection circuit by the reference voltage level adjustment means of the overcurrent detection circuit. did.

  In addition, by calculating the absolute value of the current from the relative ratio between the detection value of the current detection circuit and the reference voltage of the overcurrent detection circuit, a highly accurate current detection means is realized.

  Furthermore, by calculating the absolute value of the current from the relative ratio between the detection value of the current detection circuit and the reference voltage of the overcurrent detection circuit, a current detection unit and an overcurrent detection unit with high relative accuracy are realized.

It is sectional drawing which shows the structure of the laser beam printer in 1st Embodiment. 1 is a configuration diagram of a fixing device according to a first embodiment. FIG. It is a figure which shows the structure of the ceramic heater 205 of FIG. It is a figure which shows the example of arrangement | positioning of the thermistor with respect to the ceramic heater 205 of FIG. FIG. 2 is a block diagram illustrating a configuration of a fixing control unit in the first embodiment. FIG. 3 is a circuit diagram of a current detection circuit in the first embodiment. It is explanatory drawing explaining the electric power control method in 1st Embodiment. It is a figure which shows the connection of the thermistor in 1st Embodiment. It is a figure which shows the structure of the safety circuit in 1st Embodiment. FIG. 6 is a waveform diagram of the current detection circuit 507 in FIG. 5. It is a wave form diagram of the frequency detection part 655 of FIG. It is a control flow figure at the time of start-up control in a 1st embodiment. It is a figure which shows the power supply at the time of start-up control in 1st Embodiment as a table. It is explanatory drawing of the target current setting in 1st Embodiment. It is a control flow figure at the time of steady temperature control in a 1st embodiment. It is a supply power table at the time of steady temperature control in a 1st embodiment. It is a flowchart at the time of the heater current calculation in 1st Embodiment. It is a figure which shows the coefficient at the time of the heater current calculation in 1st Embodiment as a table | surface. FIG. 6 is a circuit diagram of a current detection circuit in a second embodiment.

Explanation of symbols

101 Laser beam printer 116 Fixing device 202 Pressure roller 301 Ceramic heater 502, 503 Triac 501 CPU
507 Current detection circuit 509 Safety circuit 642 Diode 625 FET
633 Operational amplifier 634 Capacitor 645 Current transformer 653 Variable resistor 655 Frequency detection unit 660 Integration circuit

Claims (1)

A heater, a fixing film whose inner surface slides while being in contact with the heater, a pressure roller that presses the heater through the fixing film to form a fixing nip portion, and a temperature detection means that detects the temperature of the heater And fixing means for fixing the toner image on the recording material by heating while conveying the recording material at the fixing nip,
Power control means for controlling the power supplied to the heater so that the detected temperature detected by the temperature detection means becomes a target temperature;
Current level detection means for outputting a voltage of a level corresponding to the current flowing through the heater;
A reference voltage generating means for outputting a reference voltage;
An overcurrent detection means for comparing the voltage output from the current level detection means with the reference voltage output from the reference voltage generation means to detect whether the current flowing through the heater is in an overcurrent state; ,
Have
In the image forming apparatus, wherein the power control unit cuts off the power supply to the heater when the overcurrent detection unit determines that the current is an overcurrent.
Sensitivity adjusting means for supplying a predetermined adjustment current to the heater and adjusting the sensitivity of the current level detecting means so that the voltage output from the current level detecting means at that time is equal to the reference voltage. Have
The power control means is a ratio of the voltage output from the current level detection means to the reference voltage output from the reference voltage generation means when starting up the fixing means so that the detected temperature reaches a target temperature. An image forming apparatus , wherein power supplied to the heater is controlled so that a current obtained by multiplying the adjustment current by a predetermined target current smaller than the adjustment current .

Images