JPH11146690A - Overcurrent protection circuit in scanner control of image forming apparatus - Google Patents

Abstract

(57) [Summary] [PROBLEMS] An overcurrent detection operation is performed without the involvement of a microcomputer, and an overcurrent in a scanner of an image forming apparatus capable of reliably performing a drive element protection operation by detecting an overcurrent exceeding a predetermined time. Obtain a current protection circuit. An image forming apparatus that reciprocates a scanner with a motor. An H-type bridge circuit that drives the motor forward and reverse, a microcomputer that detects the rotation speed of the motor based on the detection signal of the encoder, a microcomputer that performs speed calculation and speed control, a motor current value detection unit, and two preset protection current values On the other hand, two comparison means IC51 for comparing the motor current value,
It has an IC 52. When it is determined by the first comparing means IC51 that an overcurrent is flowing through the motor, overcurrent limiting is performed, and when the overcurrent is detected by the first comparing means and the overcurrent is flowing by the second comparing means IC52. Is stopped for a predetermined time, the motor control is stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit in the scanner control of an image forming apparatus having a scanner which travels along a document image and reads image data from the document image, and which reciprocates the scanner by a motor. For example, the present invention can be applied to all techniques for performing speed control, position control, and the like using a DC servomotor.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine, an original placed on a contact glass is irradiated with illumination light from a light source while scanning a scanner, and reflected light from an original image is read as image data. An exposure process is performed by forming an image on a photoconductor in accordance with the read image data, and an image of a document is formed on the photoconductor. FIG. 7 schematically shows the image forming apparatus. 7, a first scanner in which a light source 3 and a first mirror 4 are integrally mounted, and a second mirror 5 and a third mirror 6 are integrally mounted below a contact glass 1 on which a document 2 is placed. A second scanner provided on the optical path reflected by the third mirror 6;
A fixed fourth mirror 8, fifth mirror 9, and sixth mirror 10 are provided in this order, and a protective glass 11 and a photosensitive drum 12 are provided in this order on the optical path reflected by the sixth mirror 10.

In the first scanner, the light source 3 illuminates the document 2 in a slit shape while moving in parallel with the surface of the document 2 from left to right in FIG. 7 along the document 2 on the contact glass 1 at a constant speed V. Then, the first mirror 4 reflects the reflected light in the horizontal direction. The reflected light from the first mirror 4 is turned in the horizontal direction by the second mirror 5 and the third mirror 6 of the second scanner. The returned reflected light is reflected by the imaging lens 7, the fourth, fifth, and sixth mirrors 8, 9, and 10,
The light converges on the photosensitive drum 12, and the image of the document 2 is formed on the photosensitive drum 12. As described above, the first scanner moves along the document 2 at a constant speed V, and in synchronization with this, the second scanner moves from left to right at a speed of V / 2, and The optical path length to the drum surface is always kept constant. By rotating the photosensitive drum 12 in synchronization with the movement of the first and second scanners, image data is read from the original image and the same image as the original image is formed on the photosensitive drum 12. The first and second scanners are driven by a motor, and are returned to their original home positions after one scan is completed.

In order to read such image data, a scanner is driven in a forward / reverse direction using a DC servomotor. FIG. 8 shows an example of a servo control device in a scanner of a conventional image forming apparatus. In FIG. 8, the DC servo motor M21 has four MOS transistors.
FETs Q21 to Q24 (hereinafter simply referred to as "Q21""Q2
2) are connected to the middle point of the H-type bridge circuit. More specifically, a series circuit including Q21 and Q23 and Q2 are connected between the power supply VMM and the ground.
2, Q24 are connected in series, and Q21, Q2
Motor M2 between the connection point 3 and the connection point of Q22 and Q24.
1 is connected. Q21, Q22, Q23, Q2
4 includes a diode D21, which flows a current in a direction opposite to the direction of the current flowing through these Q21, Q22, Q23, Q24.
D22, D23, D24 (hereinafter simply referred to as "D21""D2
2) are connected in parallel. Each Q
On / off control of 21, Q22, Q23, and Q24 is performed by a command from a microcontroller (hereinafter referred to as "microcomputer") 20 described later, and each control of forward / reverse rotation control, speed control, and stop is performed.

The microcomputer 20 has two output ports P0 and P1 for setting the rotation direction of the motor M21,
It has a pulse width modulation (hereinafter referred to as "PWM") signal output port for performing speed control. Output ports P0, P1
Output on / off controls the above Q21 and Q22, respectively. Outputs from the output ports P0 and P1 are also applied to the above Q through inverters 21 and 22 and a buffer, respectively.
22 and Q24 are input to the gates of the gates Q22 and Q2.
4 is turned on / off.

An encoder EC21 is attached to the rotation output shaft of the motor M21.
1 outputs a pulse signal corresponding to the rotation speed of the motor M21, and this pulse signal is input to the counter input port of the microcomputer 20 after being frequency-divided by the frequency dividing circuit 25 as a rotation speed signal. The pulse signal from the encoder EC21 is composed of a plurality of phase signals having a phase shift corresponding to the forward and reverse rotation directions, and the rotation direction is detected by the output of the flip-flop circuit 26 which is inverted depending on the direction of the phase shift. , The rotation direction signal is input to the input port P2 of the microcomputer 20.

As described above, Q21, Q22, Q23,
The H-type bridge circuit consisting of Q24 constitutes a motor drive circuit for switching the current of the motor M21, and the microcomputer 20 for controlling the entire operation receives a PWM signal for controlling the speed of the motor M21 and a setting of the rotation direction. Are output, and the motor M21 is driven by these signals. The motor drive circuit creates a forward rotation direction, a reverse rotation direction, a stop state, and a setting prohibited state as described in the truth table in FIG. 8 by four combinations of the two signals P0 and P1. be able to. The forward rotation direction is a document reading direction (a direction from left to right in FIG. 7) of the scanner described with reference to FIG. 7, and the reverse rotation direction is a return operation for returning the scanner to an original position.

A motor current detecting resistor R21 is added between Q23 and Q24 and the ground. This resistance R2
The current flowing in 1 is converted into a voltage, a voltage value corresponding to the motor current value is detected, and the duty ratio of the PWM signal is changed according to this voltage value to control the motor speed. More specifically, a motor current detecting resistor R is connected between the H-type bridge circuit including Q21, Q22, Q23, and Q24 and ground.
21 is added, the motor current flowing through the resistor R21 is converted into a voltage by the resistor R21, and a voltage value corresponding to the motor current value is detected. This detection voltage is used for the amplifier IC.
The signal is amplified at 21 and input to the differential amplifier IC23. The microcomputer 20 calculates a control target value as a current value based on a deviation between a preset motor speed and the speed of the motor M21 detected by the encoder EC21, and converts the current value as the control target value into a digital signal by D / A conversion. Is converted into an analog signal to obtain a speed control value. The difference between the speed control value and the detected motor current value is calculated by the differential amplifier IC23, and the difference is converted into a PWM signal by the comparator IC24 using the triangular wave output from the triangular wave generation circuit 29. The motor is driven in accordance with the duty ratio of the PWM signal to control the speed.

In the example shown in FIG. 8, when Q24 is on during normal rotation and Q21 is on by a PWM signal, current flows from the power supply VMM through Q21 to the motor M21, and D27,
It flows to the motor current detecting resistor R21 through Q24.
Therefore, the current can be converted to a voltage by the resistor R21 and a voltage value corresponding to the motor current value can be detected. When Q21 is turned off by the PWM signal, D26 is turned on by the back electromotive force of the motor M21, and the regenerative current becomes D27 and Q2.
4. Flow through R21 to ground, then return to motor M21 through D26 from ground.

The direction of rotation is selected by the output ports P0 and P1 of the microcomputer 20. As shown in FIG.
The direction of forward rotation is from the start of the scanner to the end of document reading and from the reverse brake at the time of return deceleration to the stop of the scanner. It is decided. As described above, the control target value is calculated as the current value based on the deviation between the preset motor speed and the speed of the motor M21 detected by the encoder EC21, and the current value as the control target value is digitally converted by D / A conversion. The signal is converted into an analog signal to obtain a speed control target value. The difference between the current value, which is the control target value, and the detected motor current value is calculated by the differential amplifier IC23, and a PWM signal is generated by the comparator IC24 using a triangular wave from the difference, and the duty ratio of the PWM signal is calculated. Corresponding motor M
21 to control the speed.

In the speed control, a control target value is calculated as a current value based on a deviation between a predetermined motor speed and a speed of the motor M21 detected by an encoder EC21 connected to the motor M21, and the control target value is calculated by D / A conversion. The current value, which is a value, is converted into a digital signal and used as a control value. The difference between the current value, which is the control target value, and the detected motor current value is calculated by the differential amplifier IC23, and this difference is used to generate a PWM signal by the comparator IC24 using a triangular wave. The motor is driven accordingly to control the speed. At the time of reverse rotation, Q23 is turned on and Q22 is turned on by the PWM signal.
Is on, current flows from VMM through Q22 to motor M
21 and flows through D25 and Q23 to the motor current detecting resistor R21. Therefore, the current can be converted to a voltage by the resistor R21 and a voltage value corresponding to the motor current value can be detected. When Q22 is turned off by the PWM signal,
D28 is turned on by the back electromotive force of the motor M21, and the regenerative current flows through D25, Q23 and R21 to the ground,
The motor returns from the ground through D28 to the motor M21.

In the scanner control circuit of the image forming apparatus as described above, the motor is overloaded when the motor is locked for some reason or when sudden acceleration or deceleration is performed. At this time, an overcurrent flows through the motor. In particular, when the motor lock state continues for a certain period of time or more, the motor is heated by an overcurrent. Therefore, when an overcurrent flows through the motor, it is necessary to detect the overcurrent and take a protective measure such as stopping the control of the motor.

Although it is necessary to detect overcurrent in order to take protective measures against overcurrent, various overcurrent detection methods have been considered. One of them is a method of judging an overcurrent when the duty ratio of the PWM signal becomes a specified value or more. However, in a current feedback type control device such as the above-described conventional servo control device, the PWM signal is varied at a very high speed by hardware, and is not managed by a microcomputer. It is difficult to determine whether an overcurrent occurs. It is possible to measure the duty ratio with a logic circuit outside the microcomputer, but
Then, the circuit scale becomes large, which causes high cost.

As another method of overcurrent detection, a microcomputer constantly monitors a motor current value, and when a current value exceeding a specified value is detected, the microcomputer performs motor stop control at the time of occurrence or after a lapse of a predetermined time from the occurrence. There is a way.
However, according to this method, since monitoring and control are performed intensively by the microcomputer, the load on the microcomputer is large, and there is a problem that it is not possible to detect overcurrent when the microcomputer is abnormal.

As still another method of overcurrent detection, a PI operation is performed based on a deviation between a motor speed and a target speed.
There is a method of judging a speed abnormality from the calculated value. Although this is originally speed abnormality detection, overcurrent detection is also possible, so that it is also used for overcurrent detection. But,
Also in this method, since the microcomputer performs intensive monitoring and control, the load on the microcomputer is large, and there is a problem that it is impossible to detect an overcurrent when the microcomputer is abnormal.

As another method of detecting an overcurrent, there is an overcurrent protection circuit described in Japanese Patent Application Laid-Open No. 6-351296. This is, in the drive device of the induction motor by the inverter, a current sensor that detects the current value of the inverter, a comparator that compares the current value at startup detected by the current sensor with a preset protection current value, An overcurrent duration monitoring circuit that monitors overcurrent for a predetermined time from when the comparator detects an overcurrent, and a protection circuit that protects a switching element of the inverter based on an output of the overcurrent duration monitoring circuit. Then, the protection circuit is activated when a predetermined time has elapsed while an overcurrent is flowing. According to this circuit, when an overcurrent is flowing, constant current control is performed at the protection current value, so that the inverter current can quickly increase near the protection current.
It goes down and the output of the comparator changes at high speed. Although the overcurrent duration monitoring circuit has a monostable multivibrator and a flip-flop circuit, the output of the comparator is high-speed as described above with respect to an edge of a signal after a lapse of a predetermined time by the multivibrator. , It is difficult to latch a certain state in the next-stage flip-flop circuit, so that satisfactory overcurrent protection cannot be achieved.

[0017]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention performs an overcurrent detection operation without involving a microcomputer, and sets a protection current value for a short-time overcurrent. If the overcurrent is detected for more than a certain period of time, the motor drive is stopped to protect the drive element, and this overcurrent protection operation is performed reliably. It is an object of the present invention to provide an overcurrent protection circuit in a scanner of an image forming apparatus that can be used.

[0018]

According to the first aspect of the present invention,
In an image forming apparatus having a scanner that travels along a document image and reads image data from the document image, and reciprocates the scanner with a motor, the motor is positively controlled by a pulse width modulation signal that determines a motor speed and a signal that determines a rotation direction. An H-type bridge circuit driven in reverse rotation, an encoder attached to the motor shaft, a microcontroller for detecting the rotation speed of the motor, calculating the speed and controlling the speed based on the detection signal of the encoder, and detecting a current value flowing through the motor. A detection unit, and two comparing means for comparing a current value flowing through the motor with two preset protection current values, and based on a comparison result by the first comparing means, an overcurrent flows through the motor. If it is determined that there is an overcurrent, the overcurrent is limited, and the overcurrent is limited by the first comparing means based on the comparison result by the second comparing means. When is the state in which an overcurrent flows from being detected is judged to be continued for a predetermined time to stop the motor control, characterized by protecting the driving element.

According to a second aspect of the present invention, in the first aspect of the present invention, a retriggerable monostable is used as a predetermined time setting means for determining whether or not an overcurrent is flowing for a predetermined time. Using a multivibrator, a gate to which a predetermined clock signal is
The output of the gate is used as a clock input of the retriggerable monostable multivibrator, and when no clock is input for a period longer than a predetermined pulse width, the retriggerable output is output. The state of the monostable multivibrator transits.

According to a third aspect of the present invention, in the second aspect of the present invention, there is provided means for fixing a single state transition by the overcurrent detection circuit, and the supply of current to the motor is stopped by the fixing means. Features.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a servo control device in a scanner of an image forming apparatus according to the present invention will be described below with reference to FIGS. 1 and 2 showing the embodiment of the present invention are the same as those of the conventional example shown in FIG. 8, and thus the same components are denoted by the same reference numerals. In FIG. 1, an H-type bridge circuit composed of four MOSFETs Q21, Q22, Q23 and Q24 (hereinafter simply referred to as "Q21" and "Q22") drives a scanner of an image forming apparatus such as a copying machine. The motor M21 is connected in the middle of the H-type bridge circuit. More specifically, the power supply V
A series circuit consisting of Q21 and Q23 and a series circuit consisting of Q22 and Q24 are connected between MM and the ground.
The motor M21 is connected between the connection point of Q1, Q23 and the connection point of Q22, Q24. Each Q21, Q22, Q2
3, Q24 include these Q21, Q22, Q23, Q2
Diodes D21, D22, D23, D24 (hereinafter simply referred to as "D21"
(Displayed as “D22”) are connected in parallel. Each of Q21, Q22, Q23 and Q24 is a microcomputer 20
On / off control is performed according to a command from the controller, and each control of forward / reverse rotation control, speed control and stop is performed.

The microcomputer 20 has two output ports P0 and P1 for setting the rotation direction of the motor M21,
It has a pulse width modulation (hereinafter referred to as "PWM") signal output port for performing speed control. Output ports P0, P1
Output on / off controls the above Q21 and Q22, respectively. Outputs from the output ports P0 and P1 are also applied to the above Q through inverters 21 and 22 and a buffer, respectively.
22 and Q24 are input to the gates of these Q22 and Q24.
Is turned on and off.

An encoder EC21 is attached to the rotation output shaft of the motor M21.
1 outputs a pulse signal corresponding to the rotation speed of the motor M21, and this pulse signal is input to the counter input port of the microcomputer 20 after being frequency-divided by the frequency dividing circuit 25 as a rotation speed signal. The pulse signal from the encoder EC21 is composed of a plurality of phase signals having a phase shift corresponding to the forward and reverse rotation directions, and the rotation direction is detected by the output of the flip-flop circuit 26 which is inverted depending on the direction of the phase shift. , The rotation direction signal is input to the input port P2 of the microcomputer 20.

As described above, Q21, Q22, Q23,
The H-type bridge circuit consisting of Q24 constitutes a motor drive circuit for switching the current of the motor M21, and the microcomputer 20 for controlling the entire operation receives a PWM signal for controlling the speed of the motor M21 and a setting of the rotation direction. Are output, and the motor M21 is driven by these signals. The motor drive circuit creates a forward rotation direction, a reverse rotation direction, a stop state, and a setting prohibited state as described in the truth table in FIG. 1 by four combinations of the two signals P0 and P1. be able to. The forward rotation direction is the direction in which the scanner moves when reading the document described in FIG. 7 (the direction from left to right in FIG. 7), and the reverse rotation direction is a return operation for returning the scanner to the original position. .

A motor current detecting resistor R21 is added between Q23 and Q24 and the ground. This resistance R2
The current flowing in 1 is converted into a voltage, a voltage value corresponding to the motor current value is detected, and the duty ratio of the PWM signal is changed according to this voltage value to control the motor speed. More specifically, a motor current detecting resistor R is connected between the H-type bridge circuit including Q21, Q22, Q23, and Q24 and ground.
21 is added, the motor current flowing through the resistor R21 is converted into a voltage by the resistor R21, and a voltage value corresponding to the motor current value is detected. This detection voltage is amplified by the amplifier IC21 and input to the differential amplifier IC23. The microcomputer 20 calculates a control target value as a current value based on a deviation between a preset motor speed and the speed of the motor M21 detected by the encoder EC21, and converts the current value as the control target value into a digital signal by D / A conversion. Is converted into an analog signal to obtain a speed control value. The difference between the speed control value and the detected motor current value is calculated by a differential amplifier IC23.
The difference is converted into a PWM signal by the comparator IC 24 using the triangular wave output from the triangular wave generation circuit 29, and the motor is driven in accordance with the duty ratio of the PWM signal to perform speed control. It has become.

During normal rotation, when Q24 is on and Q21 is on by the PWM signal, current flows from the power supply VMM to motor M21 through Q21, and to motor current detecting resistor R21 through D27 and Q24. Therefore, the current can be converted to a voltage by the resistor R21 and a voltage value corresponding to the motor current value can be detected. When Q21 is turned off by the PWM signal, D26 is turned on by the back electromotive force of the motor M21, and the regenerative current flows through D27, Q24, and R21 to the ground, and returns from the ground to the motor M21 through D26.

The direction of rotation is selected by the output ports P0 and P1 of the microcomputer 20. As described above, the control target value is calculated as the current value based on the deviation between the preset motor speed and the speed of the motor M21 detected by the encoder EC21, and the current value as the control target value is digitally converted by D / A conversion. The signal is converted into an analog signal to obtain a speed control target value. The difference between the current value, which is the control target value, and the detected motor current value is calculated by the differential amplifier IC23.
The M signal is generated, and the motor M21 is driven according to the duty ratio of the PWM signal to control the speed.

In the speed control, a control target value is calculated as a current value by a deviation between a predetermined motor speed and a speed of the motor M21 detected by an encoder EC21 connected to the motor M21, and the control target value is calculated by D / A conversion. The current value, which is a value, is converted into a digital signal and used as a control value. The difference between the current value, which is the control target value, and the detected motor current value is calculated by the differential amplifier IC23, and this difference is used to generate a PWM signal by the comparator IC24 using a triangular wave. The motor is driven accordingly to control the speed. At the time of reverse rotation, Q23 is turned on and Q22 is turned on by the PWM signal.
Is on, current flows from VMM through Q22 to motor M
21 and flows through D25 and Q23 to the motor current detecting resistor R21. Therefore, the current can be converted to a voltage by the resistor R21 and a voltage value corresponding to the motor current value can be detected. When Q22 is turned off by the PWM signal,
D28 is turned on by the back electromotive force of the motor M21, and the regenerative current flows through D25, Q23 and R21 to the ground,
The motor returns from the ground through D28 to the motor M21.

The description so far is the same as the conventional example described with reference to FIG. The embodiment shown in FIG. 1 is different from the conventional example shown in FIG. 8 in that a comparator IC 41 is added and the output voltage value of the amplifier IC 21 after converting the motor current into a voltage and an output voltage value set in advance to a predetermined value. The comparator IC41 compares a value obtained by converting the current protection current value into a voltage value, and adds the comparison result to an output of the comparator IC24 that generates the PWM signal. The overcurrent protection current value is a reference voltage ref0 input to the comparator IC41.
Is divided by the resistor R42 and input to the comparator IC41.
The comparators IC24 and IC41 have open collector outputs, and both output terminals are directly wired.

The operation of the above embodiment, in particular, the comparator I
The operation of C24 and IC 41 will be described with reference to FIG. The voltage value of the motor current indicated by A in FIGS.
When the reference voltage ref0 of the comparator IC41 is lower than the protection value, which is the normal operation shown on the left side of FIG. 2, the output of the comparator IC41 is in the open state, and the PWM signal from the comparator IC24 is Are supplied to a driving circuit including Q21 to Q24 and the like. If an overcurrent flows for some reason, the output of the comparator IC41 becomes L level (GND level), and the current supply to the motor is forcibly turned off. When the current supply to the motor is turned off, the motor current decreases, so that the output of the comparator IC41 becomes H level, and a normal PWM signal is supplied again. However, since the overcurrent flows again and exceeds the protection value of the reference voltage ref0, the power supply is immediately turned off. The operation at the time of this overcurrent is shown in the center of FIG. In this way, when an overcurrent occurs, the motor current is limited to the protection current value, and the operation continues as if it were a constant current operation.

Even if there is no abnormality in driving the motor, the motor current may exceed the protection current value when the motor is rapidly accelerated or decelerated. The time during which the motor current equal to or greater than the protection current value flows is short, and in this case, it is not necessary to operate the protection circuit to stop driving the motor. Therefore, when it is determined that the state in which the overcurrent is flowing continues for a predetermined time, the protection circuit is operated. FIGS. 3, 5, and 6 show circuit examples for that purpose. Many parts of the circuit example shown in FIG. 6 are the same as the conventional circuit example described with reference to FIG. 8 or the circuit example according to the embodiment of the present invention shown in FIG. 1, and the circuit example shown in FIG. 5 is added, and the circuit example shown in FIG. 5 is added.

The circuit shown in FIG. 3 includes a first comparator IC 51
, A second comparator IC52, and the second comparator IC5
AND gate I receiving the output of C.2 and a clock signal
C53, a retriggerable monostable multivibrator (hereinafter referred to as "RMM") IC54 having an output of the AND gate IC53 as an A terminal input, and an RM
A flip-flop circuit (hereinafter referred to as “FF”) IC 55 having an output of the MIC 54 as an input, and an open-collector buffer I connected to an inverted output terminal of the FFIC 55
C56.

The first and second comparators IC51 and IC5
The output of the amplifier IC 21 after converting the motor current shown in FIG. 1 into a voltage is input to each of the minus input terminals 2. The reference voltage r is applied to the plus input of the first comparator IC51.
The reference voltage ref2 is input to the plus input of the second comparator IC52 as ef1. Each of the above reference voltages ref1,
ref2 is a voltage dividing resistor R52, R53, R54.
Generated by partial pressure at Reference voltage ref1
Is set to a voltage value corresponding to the protection current value, and the reference voltage ref2 is set to a value lower than the lower limit value of the voltage that fluctuates in a ripple shape at the time of the seemingly constant current driving by the reference voltage ref1. The output terminal of the first comparator IC51 is connected to the output terminal of the comparator IC24 in the circuit shown in FIG. Accordingly, the first comparator IC51 in the circuit example of FIG. 3 is different from the comparator IC4 in the circuit example of FIG.
Same as 1. In the circuit of FIG. 6, Q23 and Q24
Are connected to the anodes of diodes D51 and D52, the cathodes of these diodes D51 and D52 are commonly connected, and the commonly connected diode D5
1. The output terminal of the buffer IC 56 is connected to the cathode of D52.

The signal source of the clock signal input to one of the AND gate ICs 53 may be an appropriate signal source. The circuit shown in FIG. 6 includes a triangular wave generating circuit 29, which generates a triangular wave. Since the rectangular wave is generated before the generation, the rectangular wave can be used as a clock signal. In this case, it is not necessary to newly add a clock generation circuit. FIG. 5 shows an example of a clock generation circuit using the above triangular wave generation circuit. This triangular wave generating circuit is a well-known circuit and includes a comparator IC5.
9. A reference voltage is applied to a plus input terminal of the comparator IC59, and a comparator I is connected to a minus input terminal.
The integrated value of the integrating circuit including the resistor r15 and the capacitor C11 output from C59 is input. The output of the comparator IC 59 is a rectangular wave, which is input as a clock signal to the AND gate IC 53 of FIG. An integration signal of the rectangular wave signal by the capacitor C11 can be used as a triangular wave signal.

Next, the operation of the circuit example described above will be described with reference to FIG. When an overcurrent occurs, the comparator IC51 operates in the same manner as the comparator IC41 of the circuit example shown in FIG. 1, and the comparator IC52 in FIG. 3 operates as follows. The output of the comparator IC52 is fixed at the L level, otherwise, it is fixed at the H level.
An AND of the output signal of the comparator IC 52 and a certain clock signal is taken by the gate IC 53, and the output of the AND gate is input to the A terminal of the RMMIC 54. RMMIC
54 outputs a pulse having a width corresponding to the time constant determined by the resistor R55 and the capacitor C55, triggered by the negative edge of the signal input to the terminal A, where the terminal A has a shorter period than the above time constant. When the negative edge of the signal is input to the controller, a pulse having a width corresponding to the time constant is output again from that point. Therefore, when a pulse equal to or less than the above time constant is continuously input to the A terminal, the RMMIC 54 keeps its output level at the H level.

When the input of the pulse signal having a shorter cycle than the above time constant to the A terminal of the RMMIC 54 is interrupted, as shown in FIG. 4, when the time corresponding to the above time constant elapses from the last input pulse. For the first time, the output of the RMMIC 54 changes to the L level. The state transition of the output signal of the RMMIC 54 is latched by the FFMIC 55 in the next stage,
The protection circuit is operated via the buffer IC 56 by the latch signal of the FIC 55. The operation of the protection circuit is as follows. If the overcurrent is abnormal, that is, if the overcurrent flows for more than a predetermined time, the FFIC 5
5 and the output of the buffer IC 56 become L level,
The diodes D51 and D52 shown in FIG.
The levels of the gates of 23 and Q24 are close to the GND level, Q23 and Q24 are turned off, and no current flows to the motor M21. By the way, during normal operation, FFI
Since the outputs of C55 and the buffer IC 56 are open, the diodes D51 and D52 are turned off, Q23 and Q24 operate normally, and the motor M21 is driven normally.

As is apparent from the above description, according to the embodiment shown in FIGS. 3 to 6, the current value flowing through the motor M21 is changed with respect to two preset protection current values ref1 and ref2. Two comparing means IC5 for comparing
1 and IC52, when it is determined from the comparison result by the first comparing means IC51 that an overcurrent is flowing to the motor M21, the overcurrent is limited, and the second comparing means IC5
If it is determined from the result of comparison that the overcurrent has been detected by the first comparing means IC51 for a predetermined time, the motor control is stopped, and the drive elements Q21, Since Q22, Q23 and Q24 are protected, depending on whether the overcurrent is flowing for a predetermined time or not, there is no problem with an overcurrent within a short time such as at the time of startup, or an original abnormal current. The driving elements Q21,
The protection of Q22, Q23 and Q24 can be performed accurately. Also, as described above, the two comparison means IC51 and IC51
52, it is determined whether there is no problem with the overcurrent or the original abnormal current. Since the microcomputer does not intervene in this determination, no burden is imposed on the microcomputer, and if there is an abnormality in the microcomputer. Even if there is, there is an advantage that the overcurrent protection circuit can be operated normally.

Further, the RMMIC 54 is used as a means for setting a predetermined time for judging whether or not the state in which the overcurrent is flowing has continued for a predetermined time. Means IC52
The output of the gate IC 53 is used as a clock input to the RMMIC 54, and if no clock is input for a period longer than a predetermined pulse width, the state of the RMMIC 54 transits. As a result, the motor control is stopped to protect the drive element, so that the time during which the overcurrent is flowing can be accurately measured, thereby preventing the careless execution of the abnormality detection processing. Further, the above RMMI
The state transition of C54 is fixed by FFIC55 and the driving element Q
Since the motor control by 23 and Q24 is stopped to protect the drive element, the state in which the overcurrent is flowing continues for a predetermined time and this abnormality detection state is maintained once the original abnormality is detected. Thus, the protection of the driving element can be surely performed.

[0039]

According to the first aspect of the present invention, there are provided two comparing means for comparing a current value flowing to the motor with two preset protective current values, and the first comparing means is provided. When it is determined from the result of the comparison that an overcurrent is flowing through the motor, overcurrent limiting is performed. Based on the result of the comparison by the second comparing means, the overcurrent is detected after the overcurrent is detected by the first comparing means. When it is determined that the flowing state has continued for a predetermined time, the motor control is stopped to protect the drive element. It is possible to determine whether there is a problem-free overcurrent within a short period of time or the like, or whether the current is an original abnormal current, and thereby the drive element can be properly protected. Further, as described above, the two comparing means determine whether there is no problem with the overcurrent or the original abnormal current.Since the microcomputer does not intervene in this determination, the load on the microcomputer is reduced. In addition, there is an advantage that the overcurrent protection circuit can be normally operated even if there is an abnormality in the microcomputer.

According to the second aspect of the present invention, the retriggerable monostable multivibrator is used as a predetermined time setting means for determining whether or not the state in which the overcurrent is flowing continues for a predetermined time. The gate to which the clock signal is inputted is opened and closed according to the comparison result by the second comparing means, and the output of the gate is used as the clock input of the retriggerable monostable multivibrator, and the output is set to a certain period longer than a predetermined pulse width. When the clock is not input, the state of the retriggerable monostable multivibrator changes, and the state change stops the motor control to protect the drive element, so that the time during which the overcurrent is flowing can be accurately determined. Measurement can be performed quickly, thereby preventing careless execution of abnormality detection processing. Door can be.

According to the third aspect of the present invention, there is provided a means for fixing a single state transition by the overcurrent detection circuit, and the current supply to the motor is stopped by this fixing means to protect the drive element. When the state in which the overcurrent is flowing continues for a predetermined time and the original abnormality is detected once, the abnormality detection state is maintained, and the protection of the drive element can be surely performed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating an embodiment of an overcurrent protection circuit in scanner control of an image forming apparatus according to the present invention.

FIG. 2 is a timing chart showing an overcurrent protection operation of the embodiment.

FIG. 3 is a block diagram illustrating an example of an overcurrent detection circuit that can be used in the present invention.

FIG. 4 is a timing chart showing an operation of an overcurrent protection circuit in scanner control of the image forming apparatus according to the present invention using the overcurrent detection circuit.

FIG. 5 is a circuit diagram showing an example of a clock signal generation circuit that can be used in the present invention.

6 is a circuit diagram showing an embodiment of an overcurrent protection circuit in scanner control of the image forming apparatus according to the present invention using the overcurrent detection circuit shown in FIG. 3 and the clock signal generation circuit shown in FIG. 5;

FIG. 7 is a front view schematically illustrating an example of a scanner of the image forming apparatus.

FIG. 8 is a circuit diagram illustrating an example of a servo control device in a scanner of a conventional image forming apparatus.

FIG. 9 is a timing chart illustrating an operation of the scanner of the image forming apparatus.

[Explanation of symbols]

M21 Motor Q21 MOS-FET forming an H-type bridge circuit Q22 MOS-FET forming an H-type bridge circuit Q23 MOS-FET forming an H-type bridge circuit Q24 MOS-FET forming an H-type bridge circuit EC21 Encoder IC26 Comparison 20 Microcontroller 23 Triangular wave generation circuit IC51 First comparing means IC52 Second comparing means IC54 Retriggerable monostable multivibrator IC53 Gate IC55 Flip-flop circuit as fixing means

Claims (3)

[Claims] An image forming apparatus which runs along a document image and reads image data from the document image, and which reciprocates the scanner by a motor, determines a pulse width modulation signal for determining a motor speed and a rotation direction. An H-type bridge circuit that drives the motor forward and reverse by a signal, an encoder mounted on the motor shaft, a microcontroller that detects the rotation speed of the motor, calculates the speed, and controls the speed based on the detection signal of the encoder; A detection unit for detecting a current value; and two comparison means for comparing a current value flowing through the motor with two preset protection current values. When it is determined that an overcurrent is flowing through the second comparator, overcurrent limiting is performed. An image forming apparatus comprising: stopping a motor control to protect a driving element when it is determined that a state in which an overcurrent is flowing has been continued for a predetermined time since an overcurrent is detected by the comparing means. Overcurrent protection circuit in scanner control. 2. A gate to which a predetermined clock signal is inputted, using a retriggerable monostable multivibrator as a means for setting a predetermined time for determining whether or not an overcurrent is flowing for a predetermined time. Is opened and closed according to the result of comparison by the second comparing means, the output of the gate is used as the clock input of the retriggerable monostable multivibrator, and when no clock is input for a certain period longer than a preset pulse width, 2. The overcurrent protection circuit according to claim 1, wherein the state of the retriggerable monostable multivibrator changes. 3. The scanner control of an image forming apparatus according to claim 2, further comprising means for fixing a single state transition by the overcurrent detection circuit, wherein the current supply to the motor is stopped by the fixing means. Overcurrent protection circuit.