JP2001042452A - Servo controller, scanner having the servo controller, and image processing device having the scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a servo controller which compensates the fluctuation in reference voltage used for AD and DA conversion which is the cause for an error in set target instruction voltage by embodying the prevention of a malfunc tion at the start of a DC servo motor induced by the offset of a current detector with a simple circuitry. SOLUTION: A malfunction by the offset of the current detector is prevented by setting a specified current value set according to a rotating direction as a target value at the time of driving start of a motor at a value at which the inversion of the rotating direction does not occur by allowing for the predicted offset value. The compensation of the fluctuation in the reference voltage applies correction to the target instruction current value of servo control according to the output characteristic value of the current detector converted in accordance with the value obtained. by AD conversion of the output value of the current detector when the driving current of the motor is zero.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a servo control technique for performing speed control, position control, and the like using a DC servo motor, and more particularly, to an offset of a detector, an AD converter, and a DA converter in a feedback control system. For preventing malfunctions caused by factors such as a steady-state deviation of the reference voltage of the scanner, and controlling movement of a scanner provided in an image processing apparatus (for example, an image reading apparatus, a facsimile or an image forming apparatus such as a copying machine) for a document. The present invention relates to a servo control technique suitable for a computer.

[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 scanned by a scanner having a light source and a transmission optical system and reflected from an original image surface irradiated by light from a light source. The light is captured as image data via a transmission optical system, and an image is formed on a photoconductor to perform an exposure process. Scanning by such a scanner is performed by driving the scanner using a DC servomotor. The DC servo motor driver has four MOS
-It has an H bridge circuit composed of FETs, and selectively operates four MOS FETs by a pulse modulation (PWM) signal and a +/- signal in the rotation direction to perform forward / reverse rotation operation. A feedback system that servo-controls the drive of the DC servo motor includes an encoder attached to the motor shaft, a microcomputer that processes the motor rotation speed detection, speed calculation and speed control signal based on the encoder detection signal, and a current that flows through the motor. A current detection unit for detecting a value and a current direction thereof, a pulse modulation signal for determining a motor speed and a signal for determining a rotation direction (that is, a control target corresponding to a detected current value from the current detection unit and a target speed instructed by the microcomputer). A selection circuit for selectively operating the four MOS-FETs of the H-bridge circuit of the motor driver by a PWM signal and a +/- signal generated based on a comparison value of the current values.

In this feedback system, a rotation speed is detected in a microcomputer by a rotation synchronization pulse signal from an encoder attached to a motor shaft, and a PI between a detected rotation speed and a preset target rotation speed is calculated.
A target instruction current value according to the control operation is set, and the servo control operation is performed so that the operating current of the motor follows the changing target current value so that the rotation speed of the motor reaches the target speed. This control method is generally called software servo. The control is performed at a high speed by an interrupt process by an encoder signal or a time interval interrupt process of about several msec. In addition, in order to make the drive current of the motor follow the target instruction current value and perform the servo operation, a Hall current detector is used as a current detection unit for detecting the current value and the current direction flowing through the motor. It is compared with the target instruction current value. The Hall current detector detects the magnetic flux generated in proportion to the current in a non-contact manner by a combination of a magnetic core and a Hall element, and converts the current value into a voltage, and has a bipolar output type and a unipolar output type. . In the case of the unipolar output type, when a current does not flow through the motor, that is, when the motor is stopped, a voltage of +2.5 V is output. + 2.5V when the voltage is negative current (motor reverse direction)
The following voltage is generated, and the current value has a detection characteristic in which a current-to-output voltage has a linear relationship from the negative side to the positive side with respect to zero. In addition, the bipolar output type has the same detection characteristics that the current vs. output voltage has a linear relationship.
The relationship between the current value and the output voltage value has the same polarity, and extends from the negative side to the positive side with 0 interposed therebetween.

The unipolar output type will be described below as an example. The target instruction current value at the time of stop is a voltage value which is the same as the output voltage of the current detection unit (Hall current detector) when the current value is 0. Set to a voltage higher than the output voltage of the current detector when the current value is 0 to drive in the normal direction, and a voltage lower than the output voltage when the current value of the current detector is 0 to drive in the reverse direction. To be set. The difference between the target command current value specified by the microcomputer and the motor current value from the current detection unit is calculated, the direction of the current flowing to the motor is determined based on the result of the difference +/-, and the control is performed based on the difference. The value is converted into an absolute value by an absolute value circuit, compared with a triangular wave to generate a PWM signal, and the motor is driven via an H-bridge circuit. When actually performing the speed control, a target operation current value at that time is set as a reference value according to a predetermined time chart of the rotation speed of the motor, and a control operation for following the value is performed. However, at the start of driving of the motor, a fixed target instruction current corresponding to a predetermined rotation direction (forward rotation, reverse rotation) is output for a certain period of time. Thus, an appropriate target instruction current value is determined by PI control.

Incidentally, an offset occurs in the output value of the motor current detector. If an offset occurs in the output value of the current detector, the offset also affects the motor current value and the target instruction current value. For example, when the motor starts to move from a stop, in the worst case, the target instruction current in the normal rotation direction Is set and output (depending on the target value set without offset), a phenomenon occurs in which the motor rotates in the reverse direction. In order to improve this problem, there has been proposed a servo control device for correcting an offset. As means for correcting the offset, the offset amount is detected from the detection value of the current detector at the time of the motor current 0, the target instruction current value is uniformly corrected by the detected offset amount, and the control operation is performed based on the corrected value. To do. One method is that the microcomputer recognizes the detected offset value, digitally corrects the target instruction current value with the microcomputer, converts it to analog, and outputs it. The other method is to convert the detected offset amount to analog operation. This is a method in which a target instruction current value output from a microcomputer is held in a peak hold circuit and converted into an analog signal, and then an analog correction process is performed using the detected offset value held in the peak hold circuit.

[0006]

However, according to the offset correction method using the peak hold circuit, it can be realized using inexpensive components. However, there is a concern about an increase in the number of components and the complexity of the circuit. This leads to higher costs. Further, in these exemplified methods, the offset amount is uniformly reflected as a correction value in the target instruction current value, and only substantially corresponds to the offset amount generated in the current detector. In many cases, this offset amount is considerably smaller than the current at the start of driving of the motor, and is absorbed within the set current at the start of driving of the motor, so that the motor does not rotate in the direction opposite to the indicated direction. Because it is rare, the cost performance of using the offset correction method is low. Further, the error factors of the target command voltage (current detector output voltage corresponding to the target command current value of the motor) include not only the offset generated in the current detector, but also the steady state of the reference voltage used in the AD converter and the DA converter. The influence of deviation is large.
When the reference voltage used for the AD and DA converters is used in common with a power supply for driving other logic circuits without using a dedicated IC, an error occurs in the allowable specification of the power supply voltage, and furthermore, for each machine. Affected by variations in power supply.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art in a servo control device that performs speed control by a DC servomotor, and an object thereof is to firstly provide a motor which is caused by an offset of a current detector. An object of the present invention is to provide a servo control device having a means for preventing malfunction at the time of driving start with a simple circuit configuration without increasing the cost and ensuring the same control performance as in the related art. Second, a target instruction capable of performing a correct control operation by compensating for an error due to a steady-state deviation of a reference voltage referred to in AD conversion and DA conversion that causes an error in a target instruction voltage set by a microcomputer. An object of the present invention is to provide a servo control device capable of setting a current.

[0008]

According to a first aspect of the present invention, there is provided a rotating speed detecting means for detecting a rotating speed including a rotating direction of a motor, and a deviation between the rotating speed detected by the detecting means and a preset rotating target speed. Means for calculating a target command current value from the value, and a current detector for detecting a current value flowing through the motor, and a servo operation by causing the motor drive current detected by the current detector to follow the target command current value. In the servo control device that performs, when setting a constant target instruction current to the motor in a stopped state and starting operation, the target instruction current value is set to a value equal to or more than an offset value predicted by the current detector. The servo controller is set so that the DC servo motor does not malfunction due to the influence of the offset value.

A second aspect of the present invention is a rotation speed detecting means for detecting a rotation speed including a rotation direction of a motor, and a target indicated current value is calculated from a deviation value between the rotation speed detected by the detection means and a preset rotation target speed. And a servo controller for performing a servo operation by causing a motor drive current detected by the current detector to follow the target instruction current value, comprising: a current detector that detects a current value flowing through the motor. The current-to-output voltage characteristic of a DA converter for converting the target instruction current value to a voltage corresponding to the target instruction current value,
The DC servo motor is prevented from malfunctioning under the influence of the reference voltage by matching the current-to-output voltage characteristics of the current detector obtained through an AD converter operating at a common reference voltage with the converter. Servo control device.

According to a third aspect of the present invention, in the servo control device according to the second aspect, the means for calculating the target indicated current value from a deviation value between the rotation speed detected by the detection means and a preset rotation target speed is provided. A microcomputer, wherein the AD
Each of the converter and the DA converter has a function built in the microcomputer, and is a servo control device.

According to a fourth aspect of the present invention, in the servo controller according to any one of the first to third aspects, the current detector outputs a fixed voltage when a current does not flow through the motor. When a forward or reverse current flows through the motor, a linear voltage is output around the fixed voltage in accordance with the direction of the current, thereby detecting the value and direction of the current flowing through the motor. A servo control device comprising:

According to a fifth aspect of the present invention, in the servo control device according to the third or fourth aspect, the microcomputer is provided with:
C The output value of the current detector when the servomotor is stopped
When the value obtained by the conversion exceeds a predetermined value, the operation of the device is stopped.

According to a sixth aspect of the present invention, there is provided the servo control device according to any one of the first to fifth aspects, wherein the DC servo motor is rotated forward and reverse, and the movement of the original reading and scanning means is controlled by the rotational force. It is a scanner that is characterized.

According to a seventh aspect of the present invention, there is provided an image processing apparatus including the scanner according to the sixth aspect.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A servo control device according to the present invention and a scanner equipped with the servo control device will be described below with reference to the accompanying drawings. First, an outline of the configuration and operation of a scanner equipped with a servo control device will be described. FIG. 1 is a schematic view of a scanner of an image forming apparatus such as a copying machine. In FIG. 1, the scanner comprises a first scanner having a light source 3 and a first mirror 4 integrally attached thereto, and a second mirror 5 and a third mirror 6 below a contact glass 1 on which a document 2 is placed. And a second scanner attached to the scanner. An imaging lens 7, a fixed fourth mirror 8, a fifth mirror 9, and a sixth mirror 10 are provided in this order on the reflected light path reflected by the third mirror 6, and the reflected light reflected by the sixth mirror 10 is provided. On the optical path, a protective glass 11 and a photosensitive drum 12 are provided in this order.

The first scanner illuminates the original 2 in a slit shape while the light source 3 moves in the direction shown in FIG. 1 along the original 2 on the contact glass 1 at a constant speed V. The mirror 4 reflects in the horizontal direction. The second scanner moves the reflected light from the first mirror 4 to the second scanner while moving in a direction shown in FIG.
The mirror 5 and the third mirror 6 turn back in the horizontal direction. After that, the reflected light passes through the imaging lens 7 and is returned by the fixed fourth mirror 8, fifth mirror 9, and sixth mirror 10.
The image converges on the photoconductor 12 and the image of the document 2 is formed on the photoconductor drum 12. The first scanner moves along the document 2 at a constant speed V, and in synchronism therewith, the second scanner moves parallel to the first scanner at a constant speed V / 2, and The optical path length up to the 12th surface is always kept constant. In synchronization with the movement of the first scanner and the second scanner,
By rotating the photosensitive drum 12, an image is read from the document surface and the same image as the document image is formed on the photosensitive drum 12. As described above, the first scanner and the second scanner are operated at the time of reading scanning, and are returned to the original home position after one reading scanning is completed.

The movement of the scanner from the start of the movement from the home position to the scanning of the image and the return to the home position is performed by the rotation of the forward and reverse directions and the driving of a DC servo motor capable of controlling the speed thereof. . FIG. 2 is a diagram showing an example of a temporal change (time chart) of the rotation speed of the DC servo motor necessary for driving the scanner that performs such scanning. A series of operations of the motor at the time of reading will be described with reference to FIG. First, for reading, when the motor is rotated forward to start the movement of the scanner, the speed immediately rises to the document reading speed, and the document is read while moving at a constant speed. When the reading of the document is completed, the motor is driven in the reverse direction to return the scanner to the home position. In this return operation, in order to return the scanner as quickly as possible, after reading the original, the original is immediately accelerated in the reverse direction to several times the speed at the time of reading the original, and after acceleration, returned at a constant speed while maintaining the high speed, The motor is decelerated by passing a current in the normal rotation direction from a predetermined position near the home position to the motor, so that the power supply to the motor is stopped at the home position to stop the scanner.

Next, a description will be given of a control device of a DC servomotor which needs to control the rotation direction and speed as illustrated in FIG. 2 for driving the scanner. FIG. 3 is a diagram showing a circuit configuration of one embodiment of the servo control device according to the present invention. With reference to FIG. 3, an overall configuration of the apparatus of this embodiment and an outline of the operation of the apparatus will be described below. As shown in FIG. 3, the servo control device includes a DC servo motor M
An apparatus for controlling the driving of the DC servo motor M31
Has an H-bridge circuit composed of four MOS-FETs Q31 to Q34, and performs pulse width modulation (PWM).
Signals and H / L signals that determine the direction of rotation
By selectively operating the S-FETs Q31 to Q34, the motor M3
By changing the drive current supplied to 1, forward and reverse rotation is performed. The feedback system for servo-controlling the DC servomotor M31 includes an encoder EC31 attached to the motor shaft, a microcomputer 30 for processing a rotational speed detection speed of the motor M31 based on a detection signal of the encoder EC31 and processing a speed control signal, and a motor M31. Current detector 40 for detecting the value of the current flowing through
A PWM signal that determines the motor speed and a signal that determines the rotation direction (that is, a PWM signal that is generated based on a comparison value between a detected current value from the current detector 40 and a control target current value according to a target speed instructed by the microcomputer 30). Signal and H / L signal), the four MOSFETs Q
And selection circuits 34 and 35 for selecting and operating 31 to 34.

The operation of the feedback system will now be described in more detail. Here, a rotation synchronizing pulse signal from an encoder EC31 attached to the rotating shaft of the motor M31,
Further, the motor M31 is provided in the microcomputer 30 in accordance with the rotation direction signal detected by the flip-flop 32 which is inverted according to the direction of the phase shift of the multi-phase rotation synchronization pulse signal.
The rotation speed including the rotation direction of the motor M31 is detected, and the detected rotation speed and a preset target rotation speed (for example, the motor M31) are detected.
The target value is set according to a time chart that gives a change in the rotation speed from the state where the motor stops to the time when the engine starts and the operation ends.) Microcomputer 30
Is the motor (drive) to be supplied to the motor M31 as the target value by the PI control from the deviation value of the rotation speed obtained above.
A value corresponding to the current is obtained by calculation. Further, since the obtained target value is a digital value, the target value is further DA converted into an analog value having a reference voltage Vref (+5 V), and the analog control target instruction current value Iref of 0 to Vref (+5 V) is differentially converted. Input to the plus terminal of the amplifier IC33. On the other hand, since the motor drive current Imot of the motor M31 (actually, the voltage value detected by the current detector 40) is input to the minus terminal of the differential amplifier IC33, the difference value (Iref−Imot) from the target value Iref is input. ) Is the differential amplifier IC3
3 is output.

The difference value (Iref-Imot) is calculated by the differential amplifier IC
At 35, the difference from the motor current (0V) when the motor is stopped is taken, and this difference output is used for generating a pulse width modulation (PWM) signal for determining the motor speed and a signal for determining the rotation direction. The PWM signal for determining the motor speed is obtained by comparing the output of the differential amplifier IC 35 through the absolute value circuit 38 with the triangular wave output from the triangular wave generation circuit 33 by the comparator IC 34 and obtaining the output. Note that the absolute value circuit 38 is configured by a full-wave rectifier circuit having operational amplifiers 36 and 37. The signal for determining the rotation direction of the motor is obtained by comparing the output of the differential amplifier IC35 with the comparator IC36.
To determine whether the signal is positive or negative, and obtain the binary signal output.

Four MOS-FETs in an H-bridge circuit
NAND as a selection circuit for selecting and operating Q31 to Q34
Circuits 34 and 35, the input side of one of the NAND circuits 35 receives the PWM signal of the output of the comparator IC 34 and the rotation direction signal of the output of the comparator IC 36, and the input side of the other NAND circuit 34 outputs PWM of comparator IC34 output
A signal and a rotation direction signal obtained by inverting the output of the comparator IC 36 by the inverter IC 37 are inputted, and the signal of the H bridge circuit 4 is inputted.
MOS · FETs Q31 to Q34 are selected and the motor M3
The operation is performed by determining the forward and reverse rotation directions and rotation speeds of 1. During this operation, the motor current of the motor M31 is
0, and is fed back to the input side of the differential amplifier IC35, so that the difference value (Iref−Imot) becomes 0, ie,
The motor current is controlled so that Iref = Imot.

As described above with reference to FIG. 2, the servo control operation of the rotation speed of the motor M31 is performed by changing the drive current of the motor M31 based on the change in the target instruction current value Iref given based on the time chart showing the change in speed with respect to time. Imot
And the operation is performed so that the rotation speed of the motor becomes the target speed. The control operation is performed by the microcomputer 30 as in the circuit of this embodiment.
A software servo method is used in which a deviation value of the rotation speed of the motor 31 from the target speed is obtained, and a value corresponding to a drive current to be supplied to the motor M31 is obtained by calculation as a control target value and output as an analog control target instruction current value Iref. I'm calling According to this control method, high-speed processing is performed by interrupt processing by an encoder signal or time interval interrupt processing of about several msec. In this embodiment, the A / D converter and the D / A converter use a function built in the microcomputer 30 for cost reduction and simplification of the circuit, and the reference voltage Vref is the power supply voltage used in other circuits. VCC (+ 5V) is used.

In order to make the drive current of the motor M31 follow the target instruction current value and perform a servo operation, a Hall current detector is used as the current detector 40 for detecting the current value and the current direction actually flowing through the motor. The detected value is compared with the target indicated current value. The Hall current detector detects a magnetic flux generated in proportion to the current in a non-contact manner by a combination of a magnetic iron core and a Hall element, and converts the current value into a voltage, and has a bipolar output type and a unipolar output type. .
In this embodiment, a unipolar type hole current detector 40 is employed. Note that a detector that outputs a bipolar signal can also be used. FIG. 4 is a diagram showing the current-to-output voltage detection characteristics of the unipolar type Hall current detector used in this embodiment. As shown in the figure, in the case of the unipolar output type, at the time of 0 amps when no current flows in the detection portion, that is, +2.5 in the motor stop state.
Outputs a voltage of V, and generates a voltage of +2.5 V or more for a positive current (temporarily in the forward direction of the motor) and a voltage of +2.5 V or less for a negative current (temporarily in the reverse direction of the motor). Then, the current value has a detection characteristic in which the current-output voltage has a linear relationship from the minus side to the plus side with respect to 0 from the minus side.

When the unipolar type hole current detector 40 is used, the target instruction current value at the time of stop is set by a voltage value which is the same as the output voltage of the Hall current detector 40 when the current value is 0 ampere. In order to drive in the forward direction, the output voltage of the current detector 40 at the time of 0 amperage is +2.5 V or more. To drive in the reverse direction, the output of the current detector 40 is 0 amperage. Voltage +2.5
What is necessary is just to set the voltage below V. The digital value of the microcomputer 30 is that if the reference voltage of the DA and AD converter is +5 V and the resolution is 8 bits, 127 digits is the value of +2.5 V when the motor is stopped, and 127 digits is required to drive in the normal rotation direction. In order to drive in the reverse direction, the above value, a value of 127 digits or less may be set in the DA converter.

When the motor is driven, a target instruction current value Iref at that time is given as a reference value according to a predetermined time chart (see the example of FIG. 2) showing a change in the rotation speed of the motor M31 with respect to time. Then, a control operation for following the value is performed by the PI control. However, at the start of driving the motor M31, the PI
Without entering the control operation, a certain target instruction current corresponding to the forward direction and the reverse direction is set for a certain period of time, and the control operation is performed based on the set value. According to the chart, an appropriate target instruction current value Iref is determined by PI control. Incidentally, an offset occurs in the output value of the current detector 40 of the motor M31. When an offset occurs in the output value of the current detector 40, the offset affects the current value supplied to the motor by the control operation. For example, as described above, a fixed target instruction is given to the stopped motor M31. In the worst case (when the offset value is larger than the target value) when starting by setting the current, the phenomenon that the motor M31 rotates in the reverse direction despite the output of the target instruction current in the forward direction. Will happen.

In the present invention, a means for improving this problem is defined as a constant target instruction current value Iref at the start of driving.
By anticipating the offset value predicted by the current detector 40, by setting a value that does not cause the reversal of the rotation direction, that is, a value that is large enough to cover the offset amount, such a malfunction does not occur. Using a unipolar type Hall current detector 40,
The case where the target instruction current value Iref in the normal rotation direction is set will be specifically described. FIG. 4 shows the characteristics when the unipolar type hole current detector 40 has no offset. According to the output voltage when the motor is stopped (when the current value is 0 ampere) is +2.5 V,
When it is offset to either + or-, the offset value is equal to or greater than the offset value (that is, if the offset value is Δf, then +
In the case of a current in the negative direction, it is equal to or more than (2.5+ | Δf |) V, and in the case of a negative current, it is a voltage of (2.5− | Δf |) V or less. (A value exceeding the range) is set as the target instruction current value Iref. That is, although Imot detected when the motor is stopped fluctuates depending on the offset, if the set target instruction current value Iref is a value equal to or greater than the offset value regardless of the offset in either the + or-direction, (Iref- There is no change in the setting condition of the designated rotation direction without inversion of + and-of Imot). In this case, the motor M3 is moved in the direction opposite to the set direction.
The rotation of the motor 1 does not cause an erroneous operation, which does not adversely affect the control accuracy, and a proper control operation is ensured.

In the above embodiment, when the motor M31 in the stopped state is set to a fixed target instruction current to start the operation, in the above-described embodiment, a malfunction occurs due to the offset of the current detector 40 (the target instruction current in the normal rotation direction is output). Nevertheless, the motor M3
1 rotates in the opposite direction).
Similarly, an element causing a malfunction includes an error due to a steady-state deviation of a reference voltage to be referred to in DA conversion and AD conversion in the microcomputer 30. Since this error is considerably larger than the error value due to the offset of the current detector 40, it is difficult to prevent the malfunction due to the error of the reference voltage by the method described in the above embodiment. In this case, the target target current value must be set to a large value, the power at the start of driving will increase, and vibration or overshoot at the start of movement may cause problems in image reading. Preventing is not a good idea. In the following embodiment, without using the method of the above embodiment,
By correcting the error due to the steady-state deviation of the reference voltage Vref, the constant current value at the start of driving the motor can be set small, and the motor can be driven with a small current to avoid malfunction.

Here, the error due to the reference voltage Vref to be corrected is caused by the D
A, reference voltage Vref used for AD converter function
However, this is caused by the use of the power supply voltage VCC (+5 V) used in other circuits, and is caused by variations and fluctuations of the power supply voltage. VCC (5V) used as a reference voltage is generally ± 5% error (steady-state error)
Referring to the example of FIG. 4, when the reference voltage is 5 V, the output of the current detector is 2.5 V when the motor current is 0 ampere.
Although it is t, when the error is + 5%, it is 121 digits, and when the error is -5%, it is 134 digits, and an error of about ± 6 digits occurs.
When this value is converted to a current value, it corresponds to about 1.18 A. A similar error occurs in DA conversion using a common reference voltage.

The error in the reference voltage Vref affects the target instruction current value Iref set by the microcomputer 30. That is, the microcomputer 30 performs PI control based on a deviation between the rotation speed detected from the rotation synchronization pulse signal from the encoder EC31 and a preset target rotation speed.
A target instruction current value Iref is obtained by digital operation as a set value for that purpose, and thereafter, a voltage corresponding to the target instruction current value obtained by digital operation is DA-converted based on the reference voltage Vref and output (0 to Vref). Because it is. Here, the target instruction current value Iref
Since the digital value obtained as follows has a value corresponding to the current-to-output voltage characteristic in a state where the reference voltage Vref of the DA converter has no error and the current detector 40 has no error, If VCC used as the reference voltage Vref fluctuates from 5 V, the converted value will include an error.

The error that occurs during DA conversion is basically
The current vs. output voltage characteristics of the DA converter that outputs the target instruction current value Iref are matched with the current vs. output voltage characteristics of the current detector 40 obtained through an AD converter that operates with a reference voltage common to this DA converter. That is, a corrected output can be obtained based on the output characteristic of the current detector converted based on the value obtained by AD-converting the output value of the current detector 40. This is because when the output voltage of the current detector 40 is A / D converted, the output voltage of the A / D
This is because the converter is also affected by the fluctuation of the reference voltage. FIG. 5 shows how the fluctuation of the reference voltage in the AD converter affects the current-output voltage characteristics. In FIG. 5, the straight line indicates that there is no error when the reference voltage VCC is 5 V (255 Digit),
The state when there is no error in the current detector 40 is shown,
The read value at (A) is 127 Digit since the output of the current detector 40 is 2.5 V. In the figure, the reference voltage VCC is ± α for the same current-output voltage characteristic.
The current-to-output voltage characteristic after AD conversion when the voltage fluctuates is shown as a straight line and a straight line with the straight line as the center, and since the straight line becomes higher (5 + α) Vcc, the current detector 40 The value is output as a small value. In addition, since the straight line has a low VCC of (5-α) V, the read value of the current detector 40 is output as a large value.

Next, the target instruction current value Iref is corrected in accordance with the current-to-output voltage characteristic after the AD conversion (read value) of the current detector 40 showing the above-mentioned change in accordance with the change of the reference voltage VCC. The method for this will be described below. FIG. 6 is a diagram for explaining this correction method. The horizontal axis represents a target instruction current value (motor current), and the vertical axis represents a digital value corresponding to an output voltage (control value) for setting the target instruction current value. It is. In FIG. 6, a straight line (solid line) indicates a control value set on the assumption that there is no error such as an offset in the motor current detector 40 and the reference voltage VCC is in an ideal state of +5 V. In the case of the resolution, +5 V and 255 digits are made to correspond to each other. When the current value is 0, the current detector 40 has a slope of +2.5 V (127 digits) according to the ideal current-output voltage characteristic of the current detector 40 as shown in the figure. The straight line shown by the straight line (broken line) in the figure shows the current-to-output voltage characteristic (read value) after the AD conversion of the current detector 40 when the reference voltage VCC is higher than (5 + α) V and + 5V. (Corresponding to the straight line in FIG. 5), the output voltage in which the error of the reference voltage VCC is corrected by adjusting the current vs. output voltage characteristic of the DA converter that outputs the target instruction current value Iref in a straight line. Can be obtained as

In the present invention, the current-output voltage characteristic value (read value) of the current detector 40 is obtained based on the voltage value actually detected when the motor drive current is zero. Here, a method of obtaining a current-output voltage characteristic value (read value) of the current detector 40 indicated by a straight line will be described with an example. As a procedure, first, the current detector 40 is set to 0 (A).
The detected output voltage value at the time of is converted by the AD converter of the microcomputer 30, the digital value is read, and the value is set to 0.
The target instruction current value at (A) is assumed. Also, the straight line 0
(A) From the value at the time (127 digits),
A difference β is obtained by subtracting a digital value indicating the target instruction current value at the time (A). Further, in this example, as shown in FIG. 6, since a difference of β is generated with respect to a change of 40 A, the target instruction current value Iref given in advance by a straight line is used.
It is necessary to correct β / 40 with respect to the current vs. output voltage characteristic of the DA converter that outputs. For example, the value at +20 A is obtained by subtracting β from the value at +20 A of a straight line prepared in advance by the operation of the microcomputer 30 (FIG. 6).
Medium, on a straight line), and by subtracting 20 × β / 40. Generally, the digital value of a straight line = (digital value of a straight line) −β− (target indicated current value Iref × β /
40), and the current-output voltage characteristic value (read value) of the current detector 40, that is, the target indicated current value Iref (−40A in this example) obtained by the calculation of the microcomputer 30.
(Indicated by +40 A).

Actually, the microcomputer 30 prepares a current vs. output voltage characteristic straight line shown by a straight line in FIG. 6 in a table or the like, and sets a target instruction based on a speed deviation between the current rotational speed of the motor M31 and the target speed. When the current value is determined, a digital value corresponding to the current value is obtained by referring to the table, and the obtained digital value is DA-converted and output as a voltage value indicating the target instruction current value. Since the output voltage value is a value in which the error of the reference voltage VCC has been corrected, a normal control operation can be performed by calculating a difference from the detected value of the motor current detector 40. in this way,
It is obtained by digital operation by matching the current-output voltage characteristic of the DA converter that outputs the target instruction current value Iref with the current-output voltage characteristic of the current detector 40 after the AD conversion that changes due to the fluctuation of the reference voltage VCC. In the process of converting the voltage corresponding to the target instruction current value to DA and outputting it, the error of the reference voltage Vref can be corrected and a correct value can be output, and the error due to the reference voltage Vref can be corrected. Since the constant current value set as the target value at the start of driving can be set small and the motor can be driven with a small current, there is no occurrence of start-up vibration and overshoot.

According to the above-described embodiment, the constant current value at the start of motor driving can be set small. However, if the fluctuation of the offset of the motor current detector 40 causing the malfunction becomes abnormally large, the motor driving starts. The erroneous operation described above is likely to occur simply because the constant current value at the time is set small. As a countermeasure, in the present embodiment, when the value obtained by AD-converting the output value of the motor current detector 40 when the motor is stopped exceeds a predetermined difference β, the abnormality of the current detector or the abnormality of the power supply voltage occurs. Is determined, and the system is abnormally stopped. This criterion is determined in consideration of the variation of the power supply voltage and the variation of the output of the current detector. The timing of performing the abnormality detection is determined by correcting the error of the reference voltage VCC before the scanner operates when the system power is turned on, and
Since it is considered appropriate to perform the operation when the scanner is stopped between jobs of the system, the abnormality is detected in accordance with this timing.

[0035]

(1) A servo operation is performed by a current detector that detects a motor drive current that can detect a current value that varies from minus to 0 across plus to 0 and that has a linear detection characteristic of current versus output voltage. In the control of the DC servo motor, when starting the motor by setting a constant current,
The malfunction (reversal of the set rotation direction) caused by the offset of the current detector can be prevented by a simple circuit configuration in the present invention. Conventionally, the offset of the current detector is detected and the target indicated current value is detected. In this case, an increase in the number of circuit points and an increase in cost caused by performing the correction are reduced, and an appropriate control operation is ensured. (2) In the control of the DC servomotor, the D / A conversion of the microcomputer that sets the target indicated current value by matching the current-output voltage characteristic of the DA converter that outputs the target indicated current value with the output characteristic value of the current detector. The correction of the error due to the variation of the reference voltage (common to AD conversion and DA conversion) from the reference value that occurs in step (1) is performed, so that the constant current value set as the target value at the start of motor driving is set to a minimum. As a result, the motor can be driven with a small current, so that proper control operation can be performed without causing vibration or overshoot of the movement. Also, when applied to a scanner, by suppressing the current at the start of motor drive, there is no need to set an excessively large target instruction current value at the start of motor drive. By providing the scanner in the image processing apparatus, the jitter at the leading end of the image can be improved. further,
When the error due to the fluctuation of the reference voltage from the reference value is corrected, a malfunction (inversion of the set rotation direction) due to the offset of the current detector increases, and thus the output value of the current detector is AD-converted. The system monitors abnormal values and determines that the current detector is abnormal or the power supply voltage is abnormal when it exceeds the specified value, and executes an abnormal stop of the system, thereby preventing runaway due to circuit malfunction in advance. , Can prevent machine damage.

[Brief description of the drawings]

FIG. 1 is a schematic view of a scanner of an image forming apparatus such as a copying machine to which the present invention is applied.

FIG. 2 shows a temporal change in a rotation speed of a DC servo motor required for driving a scanner.

FIG. 3 is a diagram showing a circuit configuration of an embodiment of a servo control device according to the present invention.

FIG. 4 shows a current-to-output voltage detection characteristic of a unipolar type Hall current detector.

FIG. 5 shows the influence on the current-to-output voltage characteristic due to the fluctuation of the reference voltage in the AD converter.

FIG. 6 shows the current versus output voltage characteristics of the current detector.
FIG. 7 is a diagram illustrating a method for correcting a target instruction current value.

[Explanation of symbols]

30 microcomputer, M31 DC servo motor, EC31 encoder, 40
Current detector, IC33, IC35 ... Differential amplifier, IC
34, IC 36: comparator, 38: absolute value circuit,
33: Triangular wave generating circuit, Q31-34: MOS-FETs constituting H-type bridge circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2H108 AA01 CB05 FB01 FB04 FB05 FB17 FB36 5C072 AA01 BA13 LA02 MB02 MB04 UA06 UA13 XA01 5H571 AA12 AA13 BB04 BB07 EE02 FF01 FF06 FF09 GG02 JJ03 GG02 JJ03 LL28 LL43 MM01 MM08

Claims (7)

[Claims] 1. A rotation speed detection means for detecting a rotation speed including a rotation direction of a motor, a means for calculating a target instruction current value from a deviation value between the rotation speed detected by the detection means and a preset rotation target speed, A servo control device that includes a current detector that detects a current value flowing through the motor, and performs a servo operation by causing a motor driving current detected by the current detector to follow a target instruction current value; When the motor starts operation by setting a constant target instruction current, the target instruction current value is set to a value equal to or larger than an offset value predicted by the current detector, and the DC servo motor sets the target value of the offset value. A servo control device characterized by preventing malfunction due to influence. 2. A rotation speed detection means for detecting a rotation speed including a rotation direction of a motor, a means for calculating a target indicated current value from a deviation value between the rotation speed detected by the detection means and a preset rotation target speed, A servo control device that includes a current detector that detects a current value flowing through the motor, and performs a servo operation by causing a drive current of the motor detected by the current detector to follow the target instruction current value; D to convert the current value to a voltage corresponding to it
By matching the current-to-output voltage characteristic of the A-converter with the current-to-output-voltage characteristic of the current detector obtained through an AD converter operating at a common reference voltage with the DA converter, the DC servo motor can A servo control device characterized in that it does not malfunction due to the influence of voltage. 3. The servo control device according to claim 2, wherein the microcomputer calculates the target instruction current value from a deviation value between the rotation speed detected by the detection unit and a preset rotation target speed, The servo control device according to claim 1, wherein each of the AD converter and the DA converter is based on a function built in the microcomputer. 4. The servo control device according to claim 1, wherein the current detector outputs a fixed voltage when a current does not flow through the motor, and outputs a fixed voltage to the motor in a normal rotation direction. Or, when a current in the reverse direction is flowing, a motor current detector that detects a current value and a direction flowing through the motor by outputting a linear voltage around the fixed voltage according to the direction of the current. A servo control device. 5. The servo control device according to claim 3, wherein the microcomputer obtains a value obtained by performing A / D conversion on an output value of the current detector when the DC servomotor is stopped, exceeding a predetermined value. A servo control device for stopping the operation of the device when the operation is stopped. 6. A scanner according to claim 1, wherein the servo control device according to claim 1 rotates a DC servo motor forward and backward, and controls the movement of a reading and scanning means of the original by the rotation force. 7. An image processing apparatus comprising the scanner according to claim 6.