Fixing apparatus and image forming apparatus

Abstract

While it is possible to detect relay failures using a zero-crossing circuit, there was still room for further improvement in the method of detecting relay failures. [Solution] In a first state in which the control means commands the first relay to be turned off and the second relay to be turned off, it acquires the first temperature detected by the detection means. In a second state in which the control means commands the first relay to be turned on and the second relay to be turned off, it acquires the second temperature detected by the detection means. The control means compares the first difference between the second temperature and the first temperature with a threshold value to determine whether or not welding has occurred on the second relay.

Description

Technical Field

[0001] The present invention relates to a fixing device, particularly a fixing device used in an image forming apparatus such as a copying machine or a laser printer.

Background Art

[0002] Conventionally, a relay has been used in the fixing circuit of an image forming apparatus to cut off the power supply to a heater. When a problem occurs in the relay where the contacts are non-conductive, the current to the heater is cut off, so the temperature of the heater does not rise and a low-temperature failure occurs. Therefore, even without detecting a failure of the relay alone, the image forming apparatus can detect an abnormality. On the other hand, for a contact welding failure where the relay contacts are non-powered but conductive, it is necessary to use a detection circuit for detecting a failure of the relay. Patent Document 1 discloses means for detecting the welding of a relay using a zero-cross detection circuit.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Although it is possible to detect a failure of a relay using a zero-cross circuit, there is room for further improvement in the method of detecting a failure of the relay. Therefore, a fixing device that detects a failure of a relay by a method different from the zero-cross circuit is provided.

Means for Solving the Problems

[0005] To achieve the above objective, the device comprises a heater, a first relay connected between a power source and the heater at one end of the heater, a second relay connected between a power source and the heater at the other end of the heater, a detection means for detecting the temperature of the heater, and a control means, wherein the control means acquires a first temperature detected by the detection means in a first state in which the first relay is commanded to be off and the second relay is also turned off, and acquires a second temperature detected by the detection means in a second state in which the first relay is commanded to be on and the second relay is also turned off, and the control means compares a first difference value between the second temperature and the first temperature with a threshold value to determine whether or not welding has occurred on the second relay. [Effects of the Invention]

[0006] According to this disclosure, relay failures can be detected in a manner different from that of zero-crossing circuits. [Brief explanation of the drawing]

[0007] [Figure 1] Schematic diagram of an image forming apparatus [Figure 2] Control circuit of the fixing device [Figure 3] Relay welding detection timing chart [Figure 4] Flowchart for detecting relay welding [Figure 5] Print operation flowchart [Figure 6] Print operation flowchart [Figure 7] Control circuit of the fixing device [Figure 8] Relay welding detection timing chart [Modes for carrying out the invention]

[0008] <Example 1> Figure 1 is a schematic diagram of the image forming apparatus 100. The video controller of the image forming apparatus 100 performs bitmap conversion of character codes and half-toning processing such as dithering of midtone images based on information received from an external device (not shown), such as a host computer. It then transmits a print signal and image information to the engine control unit. When the engine control unit receives image information from the video controller, it irradiates laser light from the scanner unit 106 according to the image information and scans the photosensitive drum 104, which is a photoreceptor charged to a predetermined polarity by a charging roller. This forms an electrostatic latent image on the photosensitive drum 104. Toner is supplied from the developer unit 103 to the formed electrostatic latent image, and an image is formed on the photosensitive drum 104.

[0009] Sheets 108, for example, made of paper, loaded into paper cassettes 102 and 110 are separated and fed one by one by the paper feed roller 107. The fed sheets 108 are transported by a transport section consisting of a transport roller, a registration roller, etc. The sheets 108 are transported from the registration roller to the transfer section in time with the timing when the image formed on the photosensitive drum 104 reaches the transfer section formed by the photosensitive drum 104 and the transfer roller 109. As the sheet 108 passes through the transfer section, a transfer bias is applied to the transfer roller 109, and the image on the photosensitive drum 104 is transferred to the sheet 108. The sheet 108 with the transferred image is heated and fixed by the fixing device 105, thereby fixing the image to the sheet 108. The sheet 108 with the fixed image is discharged onto the paper output tray by the paper output roller.

[0010] The power supply unit 120 is a switching power supply that drives the image forming apparatus 100, is located inside the image forming apparatus 100, and is connected to the AC power supply 201. The operation unit 116 has a display unit (not shown) and operation buttons, and can display messages about jams or malfunctions in the image forming apparatus 100, and allow the user to set print conditions.

[0011] First, the conventional circuit configuration will be explained using Figures 7 and 8. A zero-cross detection circuit 204 is located on the output side of relays 202 and 203. The zero-cross detection circuit 204 outputs High when the output of the AC power supply 201 is at a positive potential, and Low when the output of the AC power supply 201 is at a negative potential.

[0012] When relays 203 and 204 are functioning normally, if relays 202 and 203 are in the off state, the output of the zero-cross detection circuit 204 will also be off. In other words, the output value will be fixed at L. When relays 202 and 203 are in the off state, a pulse will appear at the output of the zero-cross detection circuit 204 if both relays 202 and 203 have experienced a welding failure simultaneously. It is rare for both relays to experience a welding failure at the same time, so the circuit is configured to check for welding failures in one relay at a time.

[0013] Of the relays positioned on both sides of the heater 206, only relay 202 is turned on. If an output pulse appears in the zero-cross detection circuit 204 at this time, it can be detected that a welding fault has occurred in relay 203, which should be in the off state. In this way, if one of the relays positioned on both sides of the heater 206 is energized and a pulse appears in the output of the zero-cross detection circuit 204, it can be detected that a welding fault has occurred in the other relay, which was not energized.

[0014] Figure 8 is a timing chart showing the on / off states of the relays and the pulse signals of the zero-cross detection circuit 204. By turning on relays 202 and 203 at different timings and detecting the change in the output pulse of the zero-cross detection circuit 204, it is possible to detect whether a welding failure has occurred in the relay.

[0015] In Figure 7, a relay welding failure was detected by detecting a change in the output pulse using a zero-crossing detection circuit. However, if the zero-crossing detection circuit is not present, it is not possible to detect a relay welding failure.

[0016] There are two main types of heater control in the fixing device: phase control and frequency control. Phase control controls the power of the heater through the conduction phase, and frequency control controls the power of the heater by the conduction frequency of the half-wave of the heater (that is, when the AC power supply 201 is 50 Hz, it is half of the 20 ms cycle, which is 10 ms).

[0017] In addition, the image forming apparatus needs to satisfy the harmonic current regulation of IEC61000-3-2 and the flicker regulation of IEC61000-3-3. Generally, when performing phase control, the flicker is small and the harmonic current is large. When performing frequency control, the flicker is large and the harmonic current is small. Therefore, the power control of the fixing device is often implemented by combining phase control and frequency control.

[0018] In a fixing device that performs phase control, control synchronized with the phase of the AC power supply 201 is required. For this reason, a zero-cross detection circuit of the AC power supply 201 that detects the timing of the phase is required. On the other hand, in a fixing device that performs frequency control, there is no need for a heater control circuit that is zero-cross synchronized, and some do not have a zero-cross detection circuit 204. In a fixing device that does not have a zero-cross detection circuit 204, a method for detecting the welding of a relay that does not use the zero-cross detection circuit 204 is desired.

[0019] Next, the control circuit of the heater in this embodiment will be described using FIG. 2.

[0020] The zero-cross detection circuit 204 is not arranged in the control circuit of FIG. 2. The control circuit of this heater adopts a frequency control method for the power control of fixing. Or, a phase control method is adopted for the power control of fixing, and the zero-cross detection circuit for phase control is arranged at a position other than the output side of the relays 202 and 203.

[0021] As shown in Figure 2, relays 202 and 203 are located at both ends of the output of the AC power supply 201, one end being the hot terminal and the other the neutral terminal. Relays 202 and 203 are controlled by the control unit 200 of the image forming apparatus 100 using relay drive signals RL1 and RL2. The control unit 200 contains a CPU (Central Processing Unit) 210, which controls the main operations of the image forming apparatus 100, such as paper feeding, transport, image formation, fixing, and paper ejection. A ROM (Read Only Memory) 211 and a RAM (Random Access Memory) 212 are connected to the CPU 210 and function as the memory of the image forming apparatus 100.

[0022] Furthermore, a semiconductor switch 205 is located at the output of relay 203. In Figure 2, the semiconductor switch 205 is located on the neutral side, but it may also be located on the hot side. A triac is used for the semiconductor switch 205, and it is controlled on / off by an FSRD signal from the control unit. A heater 206 is connected to the output of semiconductor switch 205. An insulated thermistor 207 is located on the heater 206, and a thermistor signal TH, which is obtained by resistance division by the thermistor 207 and a resistor (not shown), is input to the control unit 200. The control unit 200 detects the temperature of the heater 206 using the thermistor signal TH and performs heater temperature control and relay welding detection.

[0023] Generally, relays 202 and 203 can fail in two main ways: contact failure and contact welding. Strictly speaking, other malfunctions such as coil breakage can also occur, but the ultimate failure is contact failure. Contact failure often occurs when tiny pieces of resin that have gotten into the relay casing adhere to the relay contacts due to static electricity, and these foreign objects get trapped between the contacts when the relay is opened or closed. On the other hand, contact welding is a phenomenon in which sparks from arc discharge are generated between the contacts when the contacts are opened or closed, gradually roughening the contact surfaces, and causing the contacts to become welded together and stuck to each other. In this embodiment, welding of the relay is detected.

[0024] Figure 3 shows the timing chart for relay welding detection in this embodiment. Relays 202 and 203 are located on both poles of the AC power supply 201, and the system detects which relay is welded.

[0025] Figure 3(a) shows the case where welding has occurred on relay 202, and Figure 3(b) shows the case where welding has not occurred on relay 202. First, in the first state, the control unit 200 obtains the temperature of the thermistor after issuing a command to de-energize relays 202 and 203. Then, in order to detect welding on relay 202, the control unit 200 de-energizes relay 202, i.e., sets the RL1 signal to L. Then, it energizes relay 203, i.e., sets the RL2 signal to H. Next, it sets the FSRD signal to H to turn on semiconductor switch 205 for a predetermined time (for example, 3 seconds). In this second state, after issuing commands by the control unit 200, the control unit 200 detects the temperature rise of heater 206. △TH in the timing chart shows the temperature change of the thermistor after semiconductor switch 205 is turned on.

[0026] The control unit 200 determines whether welding has occurred by comparing the difference value ΔTH = T2 - T1, which represents the temperature difference between the initial temperature of the heater 206 immediately before the FSRD signal is turned on, with the threshold temperature Tc, using T1 as the initial temperature of the heater 206 and T2 as the temperature after the FSRD signal has been turned on for 3 seconds. The threshold temperature Tc is set to, for example, 10°C. If welding has occurred in the relay 202, the contacts will remain connected even when the relay 202 is not energized, and current will flow to the heater 206. As a result, ΔTH will gradually rise and exceed the threshold temperature Tc, so it can be determined that contact welding has occurred in the relay 202. On the other hand, if welding has not occurred in the relay 202, the relay 202 will be disconnected, and no current will flow to the heater 206. Therefore, ΔTH will not exceed the threshold temperature Tc, so it can be determined that contact welding has not occurred in the relay 202.

[0027] Figure 3(c) shows the case where no welding has occurred on relay 203, and Figure 3(d) shows the case where welding has occurred on relay 203. First, in the first state, the control unit 200 commands relays 202 and 203 to be de-energized and acquires the temperature of the thermistor. Then, in order to detect welding on relay 203, it de-energizes relay 203, i.e., sets the RL2 signal to L. Then, it energizes relay 202, i.e., sets the RL1 signal to H. Next, it sets the FSRD signal to H to turn on semiconductor switch 205 for a predetermined time (for example, 3 seconds). In this third state, after the control unit 200 has issued commands, the control unit 200 detects the temperature rise of heater 206. △TH in the timing chart shows the temperature change of the thermistor after semiconductor switch 205 is turned on.

[0028] The control unit 200 determines whether welding has occurred by comparing the difference value ΔTH = T2 - T1, which represents the temperature difference between the initial temperature of the heater 206 immediately before the FSRD signal is turned on, with the threshold temperature Tc, using T1 as the initial temperature of the heater 206 and T2 as the temperature after the FSRD signal has been turned on for 3 seconds. The threshold temperature Tc is set to, for example, 10°C. If welding has occurred in the relay 203, the contacts remain connected even when the relay 203 is not energized, and current flows to the heater 206. As a result, ΔTH gradually rises and exceeds the threshold temperature Tc, so it can be determined that contact welding has occurred in the relay 203. On the other hand, if welding has not occurred in the relay 203, the relay 203 remains disconnected, and no current flows to the heater 206. Therefore, ΔTH does not exceed the threshold temperature Tc, so it can be determined that contact welding has not occurred in the relay 203.

[0029] In this way, the control unit 200 can detect relay welding by detecting the temperature change of the thermistor after issuing an instruction to disconnect one relay and connect the other relay. Even if relays 202 and 203 are welded together at the same time, the detection of △TH for each relay 202 and 203 will exceed the threshold temperature Tc, so it can be detected that both relays are welded together.

[0030] In addition, after image formation is complete, relay welding detection may be performed while residual heat remains in the fixing device 105. In this case, if relays 202 and 203 are not welded, T2-T1 may be a negative value (for example, -5°C). This is because the residual heat temperature of the heater 206 gradually decreases. Thus, although T2-T1 may be a negative value when there is residual heat in the fixing device 105 and no fault in the relays, if welding has occurred in the relays, T2-T1 > Tc, so welding detection can be performed.

[0031] Figure 4 is a flowchart showing the relay welding detection sequence. Here, we will explain the welding detection of relay 202. As explained earlier, the same detection method can be used for relay 203, so its explanation in the flowchart is omitted.

[0032] When relay welding detection is initiated, in S101 the control unit 200 detects the initial temperature T1 of the thermistor 207. In S102 the control unit 200 turns off relay 202, which is the target of detection, and turns on relay 203, which is not the target of detection.

[0033] In S103, the control unit 200 waits as a relay on timer, for example, until 100ms have elapsed. Generally, a relay has an operating time from when the coil is energized until the contact turns on, and a bounce time after the contact turns on until it springs back. The total of these periods is, for example, 30ms, so a margin is included and the relay on timer is set to 100ms. After 100ms has elapsed, in S104, the control unit 200 turns on the semiconductor switch 205. In S105, the control unit 200 waits for 3 seconds to determine whether or not the heater temperature has risen.

[0034] In S106, after 3 seconds have elapsed, the control unit 200 detects the temperature T2 of the thermistor 207. Then, in S107, the control unit 200 turns off the semiconductor switch 205.

[0035] In S108, the control unit 200 determines whether the temperature rise of the thermistor, T2-T1, and the threshold temperature Tc are equal to T2-T1 > Tc. Since the amount of temperature rise T2-T1 varies depending on the wattage of the fixing device 105, the threshold temperature Tc can be set appropriately according to the characteristics of the fixing device 105, but here it is set to 10°C as an example. If T2-T1 > Tc is not true in S108, in S109 the control unit 200 determines that welding has not occurred on the relay 202. Then, in S110 the control unit 200 turns off the relay 203.

[0036] If T2-T1>Tc in S108, then in S111 the control unit 200 determines that welding has occurred in relay 202. Then, in S112, the control unit 200 turns off relay 203. In S113, since the control unit 200 has detected welding in relay 202, it displays a message on the control unit indicating that a malfunction has occurred in relay 202.

[0037] Figure 5 is a flowchart illustrating the image formation operation. Upon receiving a print job, in S201, the control unit 200 executes the relay welding detection sequence for relays 202 and 203, as described in Figure 4. If no welding has occurred on the relays, in S202, the control unit 200 executes the image formation operation based on the print job. Here, as an example, the relay welding detection sequence is described as being performed before the execution of the print job, but it is also possible to detect one relay before the print job and the other relay after the print job.

[0038] Figure 6 is a flowchart showing an image formation operation with a different welding detection sequence timing than that of Figure 5. Upon receiving a print job, in S301, the control unit 200 determines which relay performed welding detection in the previous print job. The type of relay that performed welding detection in the previous print job is stored in RAM 212. If relay 202 did not perform welding detection in the previous print job, in S302, the control unit 200 executes the welding detection sequence for relay 202. If relay 202 did perform welding detection in the previous print job, in S303, the control unit 200 executes the welding detection sequence for relay 203. In S304, the control unit 200 stores the type of relay that performed welding detection in RAM 212. In S305, the control unit 200 executes the image formation operation based on the print job.

[0039] In this way, welding detection of relays 202 and 203 can be performed without using the output value of the zero-cross detection circuit. Since welding detection of relays 202 and 203 can be performed using the triac and thermistor, which are components of the fixing circuit, welding detection can be performed while suppressing cost increases. [Explanation of symbols]

[0040] 200 Control Unit 202, 203 Relay 206 Heater

Claims

1. Heater and, At one end of the heater, a first relay is connected between the power supply and the heater, On the other end of the heater, a second relay is connected between the power supply and the heater, A detection means for detecting the temperature of the heater, Equipped with control means, The control means acquires a first temperature detected by the detection means in a first state in which the first relay is commanded to turn off and the second relay is turned off, and acquires a second temperature detected by the detection means in a second state in which the first relay is commanded to turn on and the second relay is turned off. The fixing device is characterized in that the control means compares a first difference value between the second temperature and the first temperature with a threshold value to determine whether or not welding has occurred on the second relay.

2. In the third state in which the control means commands the first relay to be turned off and the second relay to be turned on, the control means acquires the third temperature detected by the detection means, The fixing device according to claim 1, characterized in that the control means compares the second difference value between the third temperature and the first temperature with the threshold value to determine whether or not welding has occurred on the first relay.

3. The fixing device according to claim 2, characterized in that the control means determines whether welding has occurred on the first relay and whether welding has occurred on the second relay before forming an image on the sheet.

4. The fixing device according to claim 1, characterized in that the detection means detects the second temperature after a predetermined time has elapsed since the second state was reached.

5. The fixing device according to claim 4, characterized in that the control means turns off the first relay after detecting the second temperature with the detection means.

6. The fixing device according to claim 2, characterized in that the detection means detects the third temperature after a predetermined time has elapsed since the third state was reached.

7. The fixing device according to claim 6, characterized in that the control means turns off the second relay after detecting the third temperature with the detection means.

8. Image forming means for forming an image on a sheet, An image forming apparatus comprising a fixing device according to any one of claims 1 to 7 for fixing an image formed on the sheet.

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