JP2009042594A - Temperature control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a temperature control device with an inexpensive second safety circuit for detecting the abnormal state of a CPU and keeps a fixing heater turned off in addition to a safety circuit by means of hardware without the CPU. <P>SOLUTION: The temperature control device 9 includes: a temperature detecting section 23 which detects a fixing temperature; a CPU 41 which outputs a heater control signal for controlling the drive of a fixing heater 21 so that the fixing temperature reaches a target temperature; and a safety circuit 304 which forcibly stops supply of power to the fixing heater 21 when the fixing temperature reaches an abnormal overheat temperature. This temperature control device 9 is provided with a pulse generating circuit 45 which generates an edge signal when the level of the heater control signal is switched; a watchdog circuit 49 receiving an edge signal as a watchdog clear signal; and gate circuits 42 and 44 which turn off the fixing heater 21 in response to an output signal from the watchdog circuit 49 or the heater control signal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a temperature control device incorporated in an image forming apparatus having a thermal fixing device such as a copying machine or a printer, and relates to a temperature detection unit that detects a fixing temperature, and the fixing temperature detected by the temperature detection unit is a target temperature. A temperature that includes a CPU that outputs a heater control signal for driving and controlling the fixing heater, and a safety circuit that forcibly stops power supply to the fixing heater when the fixing temperature reaches an abnormal overheat temperature. The present invention relates to a control device.

  An image forming apparatus adopting an electrophotographic system includes a fixing heater that is driven and controlled by a heater control signal output from a control unit having a CPU, and a fixing heater in order to melt and fix the toner image on a sheet. A fixing device including an internal fixing roller and a pressure roller pressed against the fixing roller is incorporated.

  The fixing roller is controlled to a predetermined target temperature by a fixing heater that is energized and controlled by the control unit. However, if the energization state of the fixing heater is maintained due to an abnormality of the control unit, the fixing roller is overheated to an abnormally high temperature. Since there is a possibility of causing damage to the apparatus, conventionally, a technique for avoiding an abnormal overheating state by a hardware circuit not including a control unit has been proposed.

  For example, there is a safety circuit that connects a thermo switch for monitoring the fixing temperature in series with the energization line to the fixing heater, and forcibly cuts off the power supply to the fixing heater when an abnormal overheat temperature is detected by the thermo switch. Yes.

  However, when the above-described safety circuit is employed, since the thermoswitch is closed and the fixing heater is energized again when the fixing temperature becomes equal to or lower than the abnormal overheating temperature, the fixing temperature is abnormally overheated as shown in FIG. There is a possibility that the fixing roller, the pressure roller, and the devices and resin members installed around the fixing device may be damaged by high heat, or may be distorted or deformed.

  Therefore, in Patent Document 1, the temperature of the fixing heater is monitored, the energization control of the fixing heater and the power on / off control of the power system are performed, and when an abnormality in the fixing heater temperature is detected when the fixing heater is energized, the fixing heater is detected. A single MPU that prohibits energization of the power supply and a power system power supply that is turned off, and an excessive temperature rise that detects an excessive increase in the fixing heater temperature and forcibly turns off the power system power according to the temperature overheating condition Even if the temperature monitoring means of the MPU does not operate normally even if the temperature of the fixing heater rises due to abnormality of the fixing heater control means, etc., the power system is detected by the overtemperature detection / control means. An image forming apparatus characterized by forcibly turning off a power supply has been proposed.

  Specifically, the overtemperature detection / control means includes a watchdog timer circuit that operates when a predetermined time elapses when the temperature of the fixing heater detected by the thermistor is abnormally high, a D-type flip-flop that latches the output signal, and a flip-flop And a relay circuit that cuts off the power supply line to the fixing heater in response to the output signal.

  However, in the above-described over-temperature detection / control means, the flip-flop is reset by the reset signal output from the MPU after the abnormality is detected, so that if an abnormality occurs in the MPU and an incorrect reset signal is output, the fixing is performed. There was a possibility that energization to the heater could not be prevented. Further, in order to cope with a malfunction of the flip-flop due to noise or the like, it is necessary to determine whether or not the malfunction occurs and to output a reset signal, so that there is a problem that the control load on the MPU increases.

Furthermore, due to cost constraints, it may be difficult to assign one of the limited number of MPU output ports as a reset signal or the like for the watchdog circuit.
Japanese Patent Laid-Open No. 10-307514

  In view of the above-described problems, the object of the present invention is to provide an inexpensive second safety circuit that detects an abnormality of the CPU and maintains the fixing heater in an OFF state in addition to a hardware safety circuit that does not involve a CPU. It is in providing a temperature control device.

  In order to achieve the above-mentioned object, a first characteristic configuration of the temperature control device according to the present invention includes a temperature detection unit for detecting a fixing temperature and the temperature detection as described in claim 1 of the document of the claims. A CPU that outputs a heater control signal for driving and controlling the fixing heater so that the fixing temperature detected by the unit reaches a target temperature, and forcibly stops power supply to the fixing heater when the fixing temperature reaches an abnormal overheat temperature A temperature control device configured with a safety circuit that generates an edge signal when the level of the heater control signal is switched, and the edge signal generated by the pulse generation circuit is a watchdog clear signal An input watchdog circuit and a gate circuit for turning off the fixing heater by an output signal of the watchdog circuit or the heater control signal There in that it includes.

  According to the above configuration, since the clear signal of the watchdog circuit is also used as the heater control signal, it is not necessary to set a dedicated port for the clear signal. Also, since the edge signal having a double cycle is generated as a clear signal by the pulse generation circuit in response to the heater control signal having a relatively long cycle, the monitoring cycle can be increased. Furthermore, since the energization of the fixing heater is blocked by the abnormality detection signal output from the watchdog circuit when an abnormality such as CPU runaway occurs via the gate circuit, fixing is performed before and after the abnormal overheating temperature detected by the safety circuit. The disadvantage of maintaining the roller temperature is also eliminated. Such a circuit has a small number of parts and can be constructed very inexpensively.

  In addition to the first feature configuration described above, the second feature configuration includes a zero-cross circuit that generates a zero-cross signal from an AC voltage, and the zero-cross signal output from the zero-cross circuit as described in claim 2 Is input as a clock signal of the watchdog circuit.

  The heater control signal cycle is relatively long, and a zero-cross signal with a relatively long cycle is used as the clock signal for the watchdog circuit in response to the clear signal generated by the pulse generation circuit based on such a long cycle signal. In this case, the number of digits of the counter circuit that counts based on the clock signal can be set small, and the circuit can be configured at low cost. For example, if the frequency of the commercial power supply is 50 Hz, the zero cross signal as the clock signal is 100 Hz, and a maximum of 2.56 seconds can be counted by an 8-bit counter circuit. Therefore, when the heater control signal is switched at a cycle of about 2 seconds, the counter circuit is cleared in about 1 second, and the heater control signal and the zero cross signal are suitably combined as a watchdog circuit clear signal and a clock signal. Can do it.

  In the third feature configuration, in addition to the second feature configuration described above, when the fixing temperature detected by the temperature detection unit is equal to or lower than the target temperature, A gate circuit for blocking input to the watchdog circuit is provided.

  When the CPU is in a normal state and the fixing temperature is lower than the target temperature, the watchdog circuit may malfunction if the heater control signal level is fixed so that the fixing heater is kept on. According to the above-described configuration, since the clock signal is not input to the watchdog circuit in such a case, malfunction can be reliably avoided.

  As described above, according to the present invention, in addition to a hardware safety circuit that does not involve a CPU, a temperature provided with an inexpensive second safety circuit that detects an abnormality of the CPU and maintains an OFF state of the fixing heater. A control device can be provided.

  Hereinafter, a color printer which is an example of an image forming apparatus including the temperature control device of the present invention will be described.

  As shown in FIG. 2, a color printer 100 that employs an electrophotographic method is input from an operation unit 110 on which operation keys for inputting a liquid crystal screen, printing conditions, and the like are arranged, and a personal computer or the like via a network or the like. The image forming unit 120 forms a toner image based on the received image data and the sheet conveyed from either the standard cassette 141 or the two-stage option cassettes 142 and 143 constituting the sheet storage unit 140. Functional blocks such as a transfer unit 130 that transfers the formed toner image, a fixing unit 200 that melts and fixes the toner image on the paper, and a control unit 400 that controls these functional blocks and executes a predetermined image forming process. And a power supply unit 300 that supplies necessary power to the control unit 400 and the like.

  The image forming unit 120 forms an electrostatic latent image by exposing the surface of the photoconductor by scanning a laser beam based on the input image data, and a charging device that uniformly charges the surface of the photoconductor. A photoconductor unit including an exposure device and a developing device that visualizes the formed electrostatic latent image as a toner image is provided for each of the M, C, Y, and K toner colors.

  The transfer unit 130 includes an intermediate transfer belt 132 that superimposes and supports toner images formed by the photosensitive units of the image forming unit 120, a support roller 131 that rotatably supports the intermediate transfer belt 132, and an intermediate transfer belt 132. The image forming apparatus includes a transfer roller 133 that transfers a carried toner image onto a sheet conveyed from the sheet storage unit 140, a blade 134 that removes residual toner on the intermediate transfer belt 132, and the like.

  As shown in FIG. 3, the fixing unit 200 includes a fixing roller 20 in which a fixing heater 21 is housed, and a pressure roller 22 pressed against the fixing roller 20, and serves as a temperature detection unit that detects a fixing temperature on the fixing roller 20. Thermistors 23 are arranged in contact with each other.

  As shown in FIG. 1, the thermistor 23 is connected in series with the resistor R 24, and the divided voltage value of the DC voltage Vdc by the thermistor 23 and the resistor R 24 is input to the control unit 400. In this embodiment, an NTC thermistor is used, and the partial pressure value increases as the temperature increases.

  The power supply unit 300 includes a noise filter 301 provided on the input side of a commercial power supply line, a relay switch 302 as a power switch, and steps down an input AC voltage, and AC / DC converts the stepped-down AC voltage into a DC voltage Vdc. Thus, a power supply circuit 303 that supplies the control unit 400 is provided. A power supply line for supplying power to the fixing heater 21 of the fixing unit 200 is branched immediately after the relay switch 302.

  As shown in FIG. 4A, the power supply circuit 303 is a power supply transformer that receives AC 100V AC voltage input from a commercial power source on the primary side and outputs stepped-down AC voltage AC 26V and AC voltage AC 12V from the secondary side. T30, first DC voltage output unit 31 that AC / DC converts AC voltage AC26V and outputs DC voltage DC24V, and second DC voltage output that AC / DC converts AC voltage AC12V and outputs DC voltage DC5V A unit 35, a zero cross circuit 39 that detects a zero voltage of the AC voltage AC12V and outputs a zero cross signal, and the like are arranged on the substrate.

  The first DC voltage output unit 31 includes a rectifier 32 configured by a diode bridge, a smoothing capacitor C33, a DC regulator 34, and the like. The AC voltage AC26V is full-wave rectified by the rectifier 32 and smoothed by the smoothing capacitor C33. The DC regulator 34 performs DC / DC conversion and outputs a DC voltage DC24V.

  The second DC voltage output unit 35 includes a rectifier 36 configured by a diode bridge, a smoothing capacitor C37, a DC regulator 38, and the like. The AC voltage AC12V is full-wave rectified by the rectifier 36 and smoothed by the smoothing capacitor C37. The DC regulator 38 performs DC / DC conversion and outputs a DC voltage DC5V.

  The zero cross circuit 39 includes a rectifier 390 constituted by a diode bridge, a PNP transistor Q393, an NPN transistor Q396, and the like. As shown in FIG. 4B, when the voltage that has been full-wave rectified by the rectifier 390 is input to the base of the transistor Q393, a pulse signal that is at a low level near the zero cross point is output from the emitter of the transistor Q393. It is input to the base of Q396.

  A rectangular pulse signal whose logic is inverted is output as a zero cross signal from the collector terminal of the transistor Q396. When the frequency of the commercial power supply is 50 Hz (60 Hz), the frequency of the zero cross signal is 100 Hz (120 Hz), and the period T0 is 10 ms (8.3 ms).

  As shown in FIG. 1, the control unit 400 includes a microcomputer 40 mounted on a substrate, peripheral circuits, and the like, and is supplied with power from a DC regulator 38. The microcomputer 40 incorporates a CPU 41, a ROM storing a control program, a RAM used as a work area for the CPU 41, an input / output circuit, and the like. The microcomputer 40 controls each functional block based on a control program executed by the CPU 41 and executes a predetermined image forming process. For example, the microcomputer 40 detects the fixing temperature based on the divided voltage value of the DC voltage Vdc generated by the thermistor 23 and the resistor R24.

  As shown in FIG. 1, the temperature control device 9 according to the present invention for controlling the fixing temperature of the fixing unit 200 includes a thermistor 23 for detecting the fixing temperature, and fixing so that the fixing temperature detected by the thermistor 23 becomes a target temperature. A CPU 41 that outputs a heater control signal for driving and controlling the heater 21 and a thermo switch 304 as a safety circuit that forcibly stops power supply to the fixing heater 21 when the fixing temperature reaches an abnormal overheat temperature are provided.

  The heater control signal output from the CPU 41 is input to the base of the NPN transistor Q42, and the triac 305 is turned on or off by the collector potential. When the fixing heater 21 is turned on to raise the fixing temperature, a low level heater control signal is output, and when the fixing heater 21 is turned off to lower the fixing temperature, a high level heater control signal is output.

  Furthermore, the temperature control device 9 includes a pulse generation circuit 45 that generates an edge signal when the level of the heater control signal is switched, and a watchdog circuit 49 that receives the edge signal generated by the pulse generation circuit 45 as a watchdog clear signal; A gate circuit for turning off the fixing heater 21 by an output signal of the watch dog circuit 49 or a heater control signal is provided.

  As shown in FIG. 5A, the pulse generation circuit 45 includes a heater control signal, a delay circuit composed of a resistor R46 and a capacitor C47 for delaying the heater control signal, and a delay delayed by a predetermined time Δt by the delay circuit. An XNOR circuit 48 to which a signal and a heater control signal are input is provided.

  Since the heater control signal output from the normally operating CPU 41 repeats a high level and a low level, the edge signal of the pulse generation circuit 45 is output at a low level for a predetermined time Δt after the level of the heater control signal is switched. However, when the CPU 41 enters a state such as runaway, the heater control signal is maintained at a high level or a low level, and the edge signal of the pulse generation circuit 45 is maintained at a high level.

  The watchdog circuit 49 receives a zero cross signal output from the zero cross circuit 39 of the power supply circuit 303 as a clock signal, and receives an edge signal output from the pulse generation circuit 45 as a watch dog clear signal.

  The watchdog circuit 49 is composed of, for example, an 8-bit counter circuit, and as shown in FIG. 5B, counts the pulses of the input clock signal, and outputs a high level signal when the number of pulses reaches 255, It is configured to reset the number of pulses counted at the falling edge of the watchdog clear signal.

  The output signal Q of the watchdog circuit 49 is input to the base of an NPN transistor Q44, and the transistor Q42 and the transistor Q44 constitute a wired OR circuit. When a low level signal is input from the watchdog circuit 49 to the transistor Q44, the triac 305 is driven based on the output signal level of the transistor Q42, but a high level signal is input from the watchdog circuit 49 to the transistor Q44. Then, the triac 305 is forcibly turned off, and the power supply to the fixing heater 21 is stopped even if the heater control signal output from the CPU 41 is at a low level. That is, the wired OR circuit composed of the transistor Q42 and the transistor Q44 serves as a gate circuit for turning off the fixing heater 21 by the output signal of the watch dog circuit 49 or the heater control signal.

  If the level of the heater control signal is not switched by the time of just over 2 seconds until the count value of the counter circuit reaches 255, a high level signal is output from the watchdog circuit 49.

  Therefore, as shown in FIG. 6, when the heater control signal is maintained at a low level due to an abnormality such as a runaway of the CPU 41 and does not change for more than 2 seconds, a high level signal is output from the watchdog circuit 49. Thus, the power supply to the fixing heater 21 is forcibly stopped, so that the fixing temperature does not rise to the abnormal overheating temperature or is maintained around the abnormal overheating temperature.

  As described above, since the clear signal of the watchdog circuit 49 is also used as the heater control signal, it is not necessary to set a dedicated port for the clear signal. Further, since the edge signal having a double cycle is generated as a clear signal by the pulse generation circuit 45 in response to the heater control signal having a relatively long cycle, the monitoring cycle can be increased. Further, since the energization to the fixing heater 21 is blocked by an abnormality detection signal output from the watchdog circuit 49 when an abnormality such as a runaway of the CPU 41 occurs through the gate circuit, the abnormal overheat temperature detected by the safety circuit 304 is prevented. The disadvantage that the temperature of the fixing roller 20 is maintained before and after is also eliminated. Such a circuit has a small number of parts and can be constructed very inexpensively.

  Note that if the number of counter digits of the watchdog circuit 49 is adjusted in accordance with the approximate cycle of the heater control signal that varies according to the specifications of the fixing unit 200, a high level signal is output from the watchdog circuit 49. The time until can be adjusted. For example, when the watchdog circuit 49 is configured by a 10-digit counter circuit, the time until the above-described appearance can be set to about 10 seconds.

  Another embodiment will be described below.

  In the above-described embodiment, it has been described on the assumption that the heater control signal level is switched before the counter circuit overflows when the CPU is operating normally. When the temperature of the heater control signal continues to be lower than the target temperature, the level of the heater control signal is maintained at a low level for a long time, and a high level signal may be erroneously output from the watchdog circuit 49.

  Therefore, as shown in FIG. 7, the temperature control device 9 includes a gate circuit 50 that prevents the zero cross signal from being input to the watchdog circuit 49 when the fixing temperature detected by the thermistor 23 is equal to or lower than the target temperature. It is possible to prevent erroneous output.

  In the gate circuit 50, the divided voltage value of the DC voltage Vdc by the thermistor 23 and the resistor R24 is input to the non-inverting input, and the reference voltage Vref set to the divided voltage value when the fixing temperature is lower than the target temperature is inverted. The comparator 51 is input to the input terminal, and the AND circuit 50 is input with the output signal of the comparator 51 and the zero cross signal.

  The comparator 51 outputs a low level signal when the divided voltage corresponding to the fixing temperature is lower than the reference voltage Vref, and outputs a high level signal when the divided voltage corresponding to the fixing temperature is equal to or higher than the reference voltage Vref.

  Accordingly, when the divided voltage corresponding to the fixing temperature is lower than the reference voltage Vref, the signal output from the AND circuit 50 is always at a low level regardless of the signal level of the zero cross signal, so that the watch dog circuit 49 does not operate. When the divided voltage corresponding to the fixing temperature is equal to or higher than the reference voltage Vref, it always becomes high level, and the watchdog circuit 49 operates based on the signal level of the zero cross signal.

  Note that the clock signal input to the watchdog circuit 49 is not limited to the zero cross signal output from the zero cross circuit 39, and as indicated by a broken line in FIG. 1 or FIG. Any signal may be output. In this case, one output port of the microcomputer 40 is used, but the cost for providing the zero cross circuit 39 in the power supply circuit 303 can be reduced.

  Further, without providing the gate circuit 50, as shown in FIG. 8, the heater control signal output from the CPU 41 is configured to switch the level for a short time at an arbitrary cycle shorter than the time when the counter circuit overflows. For example, a low-level edge signal can be output.

  Further, as shown in FIG. 9, when the zero cross signal output from the zero cross circuit 39 is input to the microcomputer 40 and the CPU 41 is configured to control the phase of the fixing heater 21 based on the zero cross signal, one AC current is generated. Since the heater control signal is switched and output once every cycle or half cycle, it is not necessary to provide the gate circuit 50 in the temperature control device 9.

  In the above-described embodiment, the temperature control device 9 is configured by a wired OR circuit in which the gate circuit that turns off the fixing heater 21 by the output signal of the watchdog circuit 49 or the heater control signal is configured by the transistor Q42 and the transistor Q44. However, the present invention is not limited to this. For example, as shown in FIG. 10A, the three-state buffer 6 to which the heater control signal is input and the signal at the inverted logic level of the output signal of the three-state buffer 6 are provided. When the three-state buffer 6 receives a high level signal from the watchdog circuit 49, the output becomes high impedance regardless of the signal level of the heater control signal, and the transistor Q42 outputs the transistor Q42. Is configured to be turned on As shown in FIG. 10B, a gate circuit is formed by the NOR circuit 7 to which the heater control signal and the output signal Q from the watchdog circuit 49 are input, and a high level signal is input from the watchdog circuit 49. In this case, the low-level signal may be output from the NOR circuit 7 regardless of the signal level of the heater control signal.

  In the above-described embodiment, the temperature control device 9 has been described as including the thermo switch 304 as a safety circuit. However, the present invention is not limited to this, and the safety circuit can be used when the fixing temperature reaches an abnormal overheat temperature. What is necessary is just to forcibly stop the power supply to 21. For example, a thermistor 21, a fixing temperature detected by the thermistor 21 and an abnormal overheating temperature are input, and a high level or low level signal is output when the fixing temperature becomes equal to or higher than the abnormal overheating temperature. Based on this, a relay switch that forcibly stops power supply to the fixing heater 21 may be used.

  In the above-described embodiment, the NTC thermistor is used as the temperature detection unit. However, a PTC thermistor may be used. In this case, the logic level of the gate circuit, the reference voltage value input to the comparator, etc. may be appropriately adjusted according to the characteristics of the temperature detection unit.

  In the above embodiment, the case where the temperature control device 9 of the present invention is applied to the color printer 100 has been described. However, the temperature control device 9 according to the present invention is an arbitrary image forming apparatus provided with a fixing heater such as a copying machine. It goes without saying that it is possible to apply to.

  Each of the above-described embodiments is merely an example of the present invention, and the scope of the present invention is not limited by the description. The specific configuration of each part is appropriately selected within the scope of the effects of the present invention. It goes without saying that changes can be designed.

Explanatory drawing of temperature control device Illustration of printer Illustration of fixing unit (A) is explanatory drawing of a power supply part, (b) is explanatory drawing of a zero cross signal. (A) is explanatory drawing of a heater control signal and an edge signal, (b) is explanatory drawing of the output signal of an edge signal and a counter circuit. Explanatory diagram of fixing temperature controlled by heater control signal Explanatory drawing of the temperature control apparatus in another embodiment Explanatory diagram of edge signal and output signal of watchdog circuit in another embodiment Explanatory drawing of the temperature control apparatus in another embodiment (A) is explanatory drawing of the gate circuit in another embodiment, (b) is explanatory drawing of the gate circuit in another embodiment. Explanatory diagram of fixing temperature controlled by heater control signal in the prior art

Explanation of symbols

9: Temperature controller 21: Fixing heater 23: Temperature detector (NTC thermistor)
39: Zero-cross circuit 40: Microcomputer 41: CPU
45: Pulse generation circuit 49: Watchdog circuit 200: Fixing unit 300: Power supply unit 303: Power supply circuit 304: Safety circuit (thermo switch)
305: Triac Q42: Gate circuit (transistor constituting a wired OR circuit)
Q44: Gate circuit (transistor constituting a wired OR circuit)

Claims (3)

A temperature detecting unit for detecting a fixing temperature; a CPU for outputting a heater control signal for driving and controlling the fixing heater so that the fixing temperature detected by the temperature detecting unit becomes a target temperature; and the fixing temperature becomes an abnormal overheating temperature. A temperature control device configured to include a safety circuit that forcibly stops power supply to the fixing heater when the temperature reaches,
A pulse generation circuit that generates an edge signal when the level of the heater control signal is switched; a watchdog circuit to which the edge signal generated by the pulse generation circuit is input as a watchdog clear signal; and an output signal of the watchdog circuit or A temperature control device comprising a gate circuit for turning off the fixing heater by the heater control signal.   The temperature control device according to claim 1, further comprising: a zero cross circuit that generates a zero cross signal from an alternating voltage, wherein the zero cross signal output from the zero cross circuit is input as a clock signal of the watchdog circuit.   The temperature control device according to claim 2, further comprising a gate circuit that prevents the zero cross signal from being input to the watchdog circuit when the fixing temperature detected by the temperature detection unit is equal to or lower than the target temperature.

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