JP4136212B2 - Servo control device, scanner having the servo control device, and image processing apparatus including the scanner - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a servo control technique for performing speed control, position control, etc. by a DC servo motor, and more specifically, in a feedback control system. , A Means for preventing malfunction caused by factors such as steady deviation of the reference voltage of the D converter or DA converter is provided in an image processing apparatus for an original (for example, an image forming apparatus such as an image reading apparatus, a fax machine, or a copying machine). The present invention relates to a servo control technique suitable for movement control of an equipped scanner.
[0002]
[Prior art]
In an image forming apparatus such as a copying machine, a transmission optical system transmits reflected light from a document image surface irradiated by light source light while scanning a scanner having a light source and transmission optical system on a document placed on a contact glass. Then, the image data is taken in via the image and imaged on the photoconductor to perform exposure processing.
Such scanning of the scanner is performed by driving the scanner using a DC servo motor. The driver of the DC servo motor has an H-bridge circuit composed of four MOS • FETs, and selectively operates the four MOS • FETs by pulse modulation (PWM) signals and +/− signals in the rotation direction. The reverse rotation is performed. The feedback system for servo-controlling the drive of the DC servo motor includes an encoder attached to the motor shaft, a microcomputer for processing the motor rotation speed detection, speed calculation and speed control signal based on the encoder detection signal, and the current flowing through the motor. A current detection unit for detecting the value and its current direction, a pulse modulation signal for determining the motor speed and a signal for determining the rotation direction (that is, a control target corresponding to the detected current value from the current detection unit and the target speed instructed by the microcomputer) And a selection circuit that selectively operates the four MOS • FETs of the H bridge circuit of the motor driver by a PWM signal and a +/− signal generated based on the comparison value of the current value.
[0003]
In this feedback system, the rotational speed is detected in the microcomputer based on the rotational synchronization pulse signal from the encoder attached to the motor shaft, and the target according to the PI control operation is determined based on the deviation between the detected rotational speed and a preset target rotational speed. An instruction current value is set, and the servo control operation is performed so that the motor operating current follows the changing target current value so that the rotational speed of the motor becomes the target speed. This control method is generally called software servo. Control is performed at high speed by interrupt processing by an encoder signal or time interval interrupt processing of about several milliseconds.
In addition, in order to make the motor drive current follow the target command current value and perform servo operation, a Hall current detector is used as a current detection unit that detects the current value and current direction that actually flows through the motor, and the detected value is It is compared with the target command current value. The Hall current detector detects 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. There are bipolar output and unipolar output types. . In the case of the unipolar output type, when no current flows through the motor, that is, when the motor is stopped, a voltage of +2.5 V is output, and when the current is positive (assuming that the motor is in the forward rotation direction), it is +2.5 V or more When the voltage is a negative current (in the reverse direction of the motor), a voltage of +2.5 V or less is generated, and the current value is in a linear relationship between the current and the output voltage from the negative side to the positive side with the 0 in between. have. In addition, the bipolar output type has the same detection characteristic in which the current vs. output voltage has a linear relationship, the relationship between the current value and the output voltage value is the same polarity, and extends from the minus side to the plus side with 0 interposed therebetween.
[0004]
Hereinafter, the unipolar output type will be described as an example. The target instruction current value at the time of stop is set to a voltage value that is the same as the output voltage at the current value 0 of the current detector (Hall current detector), To drive in the forward direction, set a voltage equal to or higher than the output voltage when the current value of the current detector is 0. To drive in the reverse direction, set a voltage equal to or lower than the output voltage when the current value of the current detector is 0. To be made.
The difference between the target command current value instructed 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 control is performed from the above difference. The absolute value circuit converts it into an absolute value as a quantity, compares it with a triangular wave, generates a PWM signal, and drives the motor via the H bridge circuit.
When speed control is actually performed, according to a predetermined time chart of the rotation speed of the motor, the target instruction current value at that time is set as a reference value, and a control operation for following the value is performed. However, at the start of driving of the motor, a constant target instruction current corresponding to a predetermined rotation direction (forward rotation, reverse rotation) is output for a certain period of time, and after the motor actually starts to operate, the above time chart Thus, an appropriate target command current value is determined by PI control.
[0005]
By the way, an offset occurs in the output value of the motor current detector. When an offset occurs in the output value of the current detector, the offset also affects the motor current value and the target command current value.For example, when the motor starts to move from the stop, the target command current in the forward rotation direction is worst. However, a phenomenon occurs in which the motor rotates in the reverse direction even though is set and output (depending on the target value set with no offset). In order to improve this problem, a servo control device that corrects an offset has been proposed.
As an offset correction means, the offset amount is detected from the detected value of the current detector when the motor current is zero, the target instruction current value is uniformly corrected based on the detected offset amount, and the control operation is performed based on the corrected value. Like to do. One is a method in which the microcomputer recognizes the detected offset value, the target instruction current value is digitally corrected by the microcomputer and then analog-converted and output, and the other is the method in which the detected offset amount is converted into an analog operation. This is a method of performing analog correction processing with the detected offset value held in the peak hold circuit after analog conversion of the target command current value output from the microcomputer held in the peak hold circuit.
[0006]
[Problems to be solved by the invention]
However, The eyes As an error factor of the target instruction voltage (current detector output voltage corresponding to the target instruction current value of the motor), in addition to the offset generated in the current detector, it depends on the steady deviation of the reference voltage used in the AD converter and DA converter. A large impact. When the reference voltage used for AD and DA converters is used in common with the power supply that drives other logic circuits, etc., without using a dedicated IC, an error occurs in the allowable specification of the power supply voltage, and further, for each machine It is affected by power supply variations.
[0007]
The present invention has been made in view of the above-described problems of the prior art in a servo control device that performs speed control by a DC servo motor. The Compensating for errors due to steady deviation of reference voltage referenced in AD conversion and DA conversion that cause errors in target instruction voltage set by the icon, it is possible to set a target instruction current capable of correct control operation An object of the present invention is to provide a servo controller.
[0009]
[Means for Solving the Problems]
Claim 1 The invention includes a rotation speed detection means for detecting a rotation speed including a rotation direction of the 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, and A servo control device comprising a current detector for detecting a current value flowing through the motor, and performing a servo operation by causing the motor drive current detected by the current detector to follow the target command current value; A current-to-output voltage characteristic of a DA converter that converts a value into a voltage corresponding to the value is matched with a current-to-output voltage characteristic of the current detector obtained through an AD converter that operates with a common reference voltage with the DA converter. Therefore, the DC servo motor Re The servo control device is characterized in that it does not malfunction due to the influence of the reference voltage.
[0010]
Claim 2 The invention of claim 1 In the servo control device described in the above, the means for calculating the target instruction current value from the deviation value between the rotation speed detected by the detection means and the preset rotation target speed is a microcomputer, and the AD converter and the DA converter Are servo controls characterized by the functions built into the microcomputer.
[0011]
Claim 3 The invention of claim 1 Or 2 In the servo control device described above, the current detector outputs a fixed voltage when no current flows through the motor, and this current when a current in the forward or reverse direction flows through the motor. The servo control device is a motor current detector that detects a current value and a direction flowing through the motor by outputting a linear voltage centered on the fixed voltage according to the direction of the motor.
[0012]
Claim 4 The invention of claim 2 Or 3 The microcomputer stops the operation of the apparatus when the value obtained by AD conversion of the output value of the current detector when the DC servo motor is stopped exceeds a predetermined value. This is a servo control device.
[0013]
Claim 5 The invention of claim 1 to claim 1 4 In this scanner, the DC servo motor is rotated forward and backward by the servo control device according to any one of the above, and the movement of the document reading / scanning means is controlled by the rotational force.
[0014]
Claim 6 The invention of claim 5 The image processing apparatus provided with the scanner described in 1 above.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A servo control device according to the present invention and a scanner equipped with the servo control device will be described based on the following embodiments shown in the accompanying drawings.
First, the configuration and operation outline of a scanner equipped with a servo control device will be described.
FIG. 1 is a schematic diagram of a scanner of an image forming apparatus such as a copying machine.
In FIG. 1, the scanner has a first scanner in which a light source 3 and a first mirror 4 are integrally attached, a second mirror 5 and a third mirror 6 under a contact glass 1 on which a document 2 is placed. The second scanner is attached to. 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 that is turned back by the third mirror 6, and the reflection that is turned back by the sixth mirror 10. A protective glass 11 and a photosensitive drum 12 are provided in this order on the optical path.
[0016]
In the first scanner, the light source 3 illuminates the document 2 in a slit shape while moving in the direction shown in the figure along the document 2 on the contact glass 1 at a constant speed V, and the first mirror 4 reflects the reflected light from the document 2. Reflects horizontally. While the second scanner moves in the direction shown in the drawing in parallel with the first scanner at a constant speed V / 2, the second mirror 5 and the third mirror 6 fold the reflected light from the first mirror 4 in the horizontal direction. Thereafter, the reflected light passes through the imaging lens 7 and is folded by the fixed fourth mirror 8, fifth mirror 9, and sixth mirror 10, and then converges on the photosensitive member 12, so that the image of the document 2 becomes a photosensitive drum. 12 tied on top.
The first scanner moves along the document 2 at a constant speed V, and in synchronization therewith, the second scanner moves parallel to the first scanner at a constant speed V / 2, and the photosensitive drum starts from the surface of the document 2. The optical path length up to 12 planes is always kept constant. By rotating the photosensitive drum 12 in synchronization with the movement of the first scanner and the second scanner, the image is read from the original surface and the same image as the original image is formed on the photosensitive drum 12.
As described above, the first scanner and the second scanner are operated at the time of scanning and are returned to the original home position when one scanning is completed.
[0017]
The scanner starts moving from the home position, scans and scans the image, and returns to the home position by driving a DC servo motor capable of rotating in the forward and reverse directions and controlling its speed.
FIG. 2 is a diagram showing an example of a temporal change (time chart) of the rotational 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 and the movement of the scanner is started, it immediately rises to the original reading speed, and the original is read while moving at a constant speed. When the document reading 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, the document is immediately accelerated in the reverse direction up to several times the speed at the time of document reading after the completion of document reading, and after acceleration, it is returned at a constant speed while maintaining high speed. The motor is decelerated by passing a current in the forward direction from a predetermined position near the home position, and the scanner is stopped by stopping energization of the motor at the home position.
[0018]
Next, a control apparatus for a DC servo motor that requires control of the rotation direction and speed as illustrated in FIG. 2 for driving the scanner will be described.
FIG. 3 is a diagram showing a circuit configuration of an embodiment of the servo control apparatus according to the present invention.
With reference to FIG. 3, the outline of the overall configuration and operation of the apparatus of this embodiment will be described below.
As shown in FIG. 3, this servo control device controls the drive of the DC servo motor M31, and the driver of the DC servo motor M31 has an H bridge circuit composed of four MOS-FETs Q31-34. The four MOS • FETs Q31 to 34 are selectively operated by the pulse width modulation (PWM) signal and the H / L signal for determining the rotation direction, and the drive current supplied to the motor M31 is changed to drive forward and reverse rotation. .
The feedback system for servo-controlling the DC servo motor M31 includes an encoder EC31 attached to the motor shaft, a microcomputer 30 for processing the rotational speed detection speed calculation and speed control signal of the motor M31 based on the detection signal of the encoder EC31, and the motor M31. A current detector 40 for detecting the current value flowing in the current and its current direction, a PWM signal for determining the motor speed and a signal for determining the rotation direction (that is, the detected current value from the current detector 40 and the target speed indicated by the microcomputer 30) Selection circuits 34 and 35 for selectively operating the four MOS • FETs Q31 to 34 of the H bridge circuit by a PWM signal and an H / L signal generated based on a comparison value of control target current values according to I have.
[0019]
The operation of the feedback system will be described in more detail. Here, a flip-flop that is inverted depending on the direction of the phase shift of the rotation synchronization pulse signal from the encoder EC31 attached to the rotation shaft of the motor M31 and the rotation synchronization pulse signal of a plurality of phases. The rotational speed including the rotational direction of the motor M31 is detected in the microcomputer 30 based on the rotational direction signal detected by the computer 32, and the detected rotational speed and a preset target rotational speed (for example, from the state where the motor M31 is stopped). PI control is performed based on a deviation from a target value that is set according to a time chart that gives a change in rotational speed from the start to the end of the operation.
The microcomputer 30 obtains a value corresponding to the motor (drive) current to be supplied to the motor M31 by calculation as a target value by PI control from the deviation value of the rotation speed obtained above. Further, since the obtained target value is a digital value, this is further DA-converted to an analog value for a reference voltage Vref (+5 V), and an analog control target command 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 driving current Imot (actually, the voltage value detected by the current detector 40) of the motor M31 is input to the minus terminal of the differential amplifier IC33, the difference value (Iref−Imot) from the target value Iref. ) Is output from the differential amplifier IC33.
[0020]
The difference value (Iref−Imot) is taken by the differential amplifier IC35 to take a difference from the motor current (0V) when the motor is stopped. This difference output is obtained by a pulse width modulation (PWM) signal that determines the motor speed and the rotation direction. Used to generate a signal to be determined.
The PWM signal that determines the motor speed is obtained by comparing the output of the differential amplifier IC35 through the absolute value circuit 38 with the triangular wave output from the triangular wave generating circuit 33 by the comparator IC34. The absolute value circuit 38 is constituted by a full-wave rectifier circuit having operational amplifiers 36 and 37.
A signal for determining the rotation direction of the motor is obtained as a binary signal output by inputting the output of the differential amplifier IC35 to the comparator IC36 and determining whether the signal is positive or negative.
[0021]
NAND circuits 34 and 35 are provided as selection circuits for selectively operating the four MOS-FETs Q31 to 34 of the H-bridge circuit. On the input side of one NAND circuit 35, a PWM signal output from the comparator IC34 and a comparator are provided. The rotation direction signal of the IC 36 output is input to the input side of the other NAND circuit 34 output, and the PWM signal of the comparator IC 34 output and the rotation direction signal obtained by inverting the output of the comparator IC 36 by the inverter IC 37 are input. Four MOS • FETs Q31 to 34 are selected, and the motor M31 is operated by determining the forward and reverse rotational directions and rotational speed. The motor current of the motor M31 during this operation is detected by the current detector 40 and fed back to the input side of the differential amplifier IC35, so that the difference value (Iref−Imot) is set to 0, that is, Iref = Imot. The motor current is controlled.
[0022]
As described above with reference to FIG. 2, the servo control operation of the rotational speed of the motor M31 follows the drive current Imot of the motor M31 to the change of the target command current value Iref given based on the time chart showing the speed change with respect to time. The operation is executed so that the rotation speed of the motor becomes the target speed.
As in the circuit of this embodiment, the control operation is performed by calculating the deviation value of the rotational speed of the motor M31 with respect to the target speed by the microcomputer 30, and further calculating the value corresponding to the drive current to be supplied to the motor M31 as the control target value. The method for obtaining and outputting the analog control target command current value Iref is called software servo. According to this control method, high-speed processing is performed by interrupt processing using an encoder signal or time interval interrupt processing of about several milliseconds.
In this embodiment, for cost reduction and circuit simplification, the AD converter and the DA converter use functions built in the microcomputer 30, and the reference voltage Vref is the power supply voltage used in other circuits. VCC (+ 5V) is used.
[0023]
In order to cause the drive current of the motor M31 to follow the target command current value and perform the servo operation, a Hall current detector is used as the current detector 40 that detects the current value and current direction that actually flows through the motor, and the detected value is It is compared with the target command current value. Hall current detectors detect magnetic flux generated in proportion to current in a non-contact manner by a combination of a magnetic core and a Hall element, and convert the current value into voltage. There are bipolar output and unipolar output types. .
In this embodiment, a unipolar Hall current detector 40 is employed. Note that a detector of a bipolar output type can also be used.
FIG. 4 is a diagram showing a detection characteristic of current versus output voltage of the unipolar Hall current detector used in this embodiment. As shown in the figure, in the case of the unipolar output type, a voltage of +2.5 V is output at 0 ampere when no current flows in the detection portion, that is, when the motor is stopped. ), A voltage of + 2.5V or higher is generated, and when a negative current (motor reverse direction) is generated, a voltage of + 2.5V or lower is generated. It has a detection characteristic in which the output voltage has a linear relationship.
[0024]
When the unipolar type hall current detector 40 is used, the target indication current value at the time of stop is set to a voltage value that is the same as the output voltage at the time of 0 ampere current value of the hall current detector 40. In order to drive in the reverse direction, the output voltage of the current detector 40 when the current value is 0 ampere + 2.5V or more, and to drive in the reverse direction, the output voltage of the current detector 40 when the current value is 0 ampere +2. What is necessary is just to set the voltage below 5V.
If the reference voltage of the DA / AD converter is + 5V and 8-bit resolution, 127 digits is the value of + 2.5V when the motor is stopped, and 127 digits to drive in the forward direction. In order to drive the above values in the reverse direction, a value of 127 digits or less may be set in the DA converter.
[0025]
When driving the motor, the target instruction current value Iref at that time is given as a reference value according to a predetermined time chart (see the illustration and reference in FIG. 2) showing a change in the rotational speed of the motor M31 with respect to time, and PI By the control, a control operation that follows the value is performed.
However, at the start of driving of the motor M31, the PI control operation is not immediately started, and a constant target instruction current corresponding to the forward rotation direction and the reverse rotation direction is set for a certain period of time, and the control operation is performed according to the set value. After the motor actually starts to operate, an appropriate target command current value Iref is determined by PI control along the time chart.
By the way, 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 input to the motor by the control operation. For example, as described above, a constant 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 with the current set, the phenomenon that the motor M31 rotates in the reverse direction even though the target command current in the forward rotation direction is output. Will happen.
[0026]
To improve this bug The As a fixed target command current value Iref at the start of driving, by setting an offset value predicted by the current detector 40, a value that does not cause reversal of the rotation direction, that is, a large value that can cover the offset amount is set. Therefore, this malfunction is not caused.
This will be specifically described with reference to the case where the unipolar type hall current detector 40 is used to set the target instruction current value Iref in the forward rotation direction. FIG. 4 shows the unipolar type hall current detector 40. Shows the characteristics when there is no offset. According to this, the output voltage when the motor is stopped (current value 0 ampere) is + 2.5V, but it is offset to either + or-. If the offset is + or-, a value equal to or greater than the offset value (ie, if the offset value is Δf, the current in the + direction is (2.5+ | Δf |) V or more, When the current is in the negative direction, the target instruction current value Iref is set to a value of (2.5− | Δf |) V or less and a value exceeding the range of the offset generated around + 2.5V. That is, although Imot detected when the motor is stopped varies depending on the offset, if the set target command current value Iref is a value equal to or larger than the offset value regardless of the offset in either the + or − direction, (Iref− Imot) + and-are not reversed, and there is no change in the setting condition of the designated rotation direction.
In this way, the malfunction that the motor M31 rotates in the direction opposite to the set direction is prevented from adversely affecting the control accuracy, and an appropriate control operation is ensured.
[0027]
When a fixed target instruction current is set to the stopped motor M31 and the operation is started, the above Explanation Then, it is assumed that a malfunction occurs due to the offset of the current detector 40 (an operation in which the motor M31 rotates in the reverse direction even though the target instruction current in the forward rotation direction is output). As an element, there is an error due to a steady deviation of a reference voltage referred to in DA conversion and AD conversion in the microcomputer 30.
This error is considerably larger than the error value due to the offset of the current detector 40. Notation If you try to prevent this, the constant target command current value at the start of motor drive will be set to a large value, so the power at the start of drive will increase, causing problems with image reading due to vibration and overshoot. It is not a good idea to prevent malfunctions with this method.
The following examples are Notation By correcting the error due to the steady deviation of the reference voltage Vref without adopting the method, the constant current value at the start of motor driving can be set small, and the motor can be driven with a small current to avoid malfunction. Is.
[0028]
For the error caused by the reference voltage Vref to be corrected here, the reference voltage Vref used for the DA and AD converter functions built in the microcomputer 30 uses the power supply voltage VCC (+5 V) used in other circuits. This is due to variations and fluctuations in the power supply voltage. VCC (5V) used as a reference voltage generally allows an error (steady deviation) of ± 5%. Referring to the example of FIG. 4, when the reference voltage is 5V, the motor current is 0 ampere. Since the output of the current detector at that time is 2.5 V, it is 127 digits after AD conversion. However, when the error is + 5%, it is 121 digits, and when it is -5%, it is 134 digits, resulting in an error of about ± 6 digits. When this value is converted into a current value, it corresponds to about 1.18 A. The same error occurs also in DA conversion using a common reference voltage.
[0029]
The error of the reference voltage Vref affects the target command current value Iref set by the microcomputer 30. In other words, the microcomputer 30 performs PI control based on the deviation between the rotation speed detected from the rotation synchronization pulse signal from the encoder EC31 and a preset target rotation speed, and the target instruction current value Iref is digitally calculated as a setting value for this purpose. This is because the voltage corresponding to the target indicated current value obtained by the digital calculation is DA-converted based on the reference voltage Vref and output (0 to Vref). Here, the digital value obtained as the target instruction current value Iref is adjusted to the current-to-output voltage characteristic in which there is no error in the reference voltage Vref of the DA converter and there is no error in the current detector 40. Since the value is taken, if VCC used as the reference voltage Vref varies from 5V, an error is included in the converted value.
[0030]
The error that occurs at the time of DA conversion is basically the current obtained through the AD converter operating at the same reference voltage as that of the DA converter. A corrected output can be obtained by the output characteristic of the current detector converted according to the value obtained by AD conversion of the output value of the current detector 40 according to the current vs. output voltage characteristic of the detector 40. This is because, when the output voltage of the current detector 40 is AD-converted, the DA converter is also affected by fluctuations in the reference voltage, as is the case with the output voltage of the AD converter.
FIG. 5 shows the influence of the change in the reference voltage on the AD converter on the current vs. output voltage characteristics.
In FIG. 5, a straight line (1) shows a state in which the reference voltage VCC is 5V (255 Digit) and there is no error and the current detector 40 has no error. The reading at 0 (A) is the current detector. Since the output of 40 is 2.5V, it becomes 127 Digit. In the same figure, the current-to-output voltage characteristics after AD conversion when the reference voltage VCC fluctuates by ± α with respect to the same current-to-output voltage characteristics are represented by a straight line (2) and a straight line (3) with the straight line (1) as the center. The straight line {circle over (2)} outputs the reading value of the current detector 40 as a small value because Vcc becomes as high as (5 + α) V. Further, since the line (3) has a low VCC (5-α) V, the reading value of the current detector 40 is output as a large value.
[0031]
Next, a method for correcting the target command current value Iref in accordance with the current-to-output voltage characteristic after AD conversion (reading value) of the current detector 40 showing the above-described change according to the change of the reference voltage VCC. Is described below.
FIG. 6 is a diagram for explaining this correction method, where the horizontal axis is the target command current value (motor current), and the vertical axis is the digital value corresponding to the output voltage (control value) for setting the target command current value. It is.
In FIG. 6, a straight line (1) (solid line) indicates a control value set under the assumption that the motor current detector 40 has no error such as an offset and the reference voltage VCC is in an ideal state of + 5V. In the case of 8-bit resolution, + 5V corresponds to 255 digits, and when the current value is 0, it is + 2.5V (127 digits) and has a slope according to the ideal current-to-output voltage characteristics of the current detector 40 as shown in the figure. .
The straight line indicated by the straight line (2) (broken line) in the figure shows the current-to-output voltage characteristic (reading value) after the AD conversion of the current detector 40 in a state where the reference voltage VCC is higher than (5 + α) V and + 5V. (Corresponding to the straight line (2) in FIG. 5), and by matching the current vs. output voltage characteristics of the DA converter that outputs the target command current value Iref to the straight line (2), the error of the reference voltage VCC Can be obtained as a control value.
[0032]
In the present invention, the current-to-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, an example is given and demonstrated about how to obtain | require the electric current versus output voltage characteristic value (reading value) of the current detector 40 shown by the straight line (2).
As a procedure, first, the detected output voltage value when the current detector 40 is 0 (A) is converted by the AD converter of the microcomputer 30, the digital value is read, and the value is indicated as a target instruction at 0 (A). The current value. Further, the difference β is obtained by subtracting the digital value indicating the target instruction current value at 0 (A) detected earlier from the value (127 digits) at 0 (A) of the straight line (1).
Furthermore, in this example, as shown in FIG. 6, since a difference of β occurs with respect to the change of 40A, DA conversion for outputting the target command current value Iref given in advance by the straight line (1) It is necessary to correct β / 40 for the current vs. output voltage characteristics of the device. For example, the value at +20 A is obtained by subtracting β from the value at +20 A of the straight line {circle around (1)} prepared in advance by the calculation of the microcomputer 30 (value on the straight line {circle around (3)} in FIG. 6). By subtracting 20 × β / 40. In general, the digital value of the straight line (2) = (digital value of the straight line (1)) − β− (target indicated current value Iref × β / 40). A digital value representing the output voltage corresponding to the value (reading value), that is, the target instruction current value Iref (indicated in this example by −40 A to +40 A) obtained by the calculation of the microcomputer 30 is obtained.
[0033]
Actually, the microcomputer 30 prepares a current vs. output voltage characteristic straight line indicated by a straight line (2) in FIG. 6 in a table or the like, and indicates a target instruction by 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 is corrected, a normal control operation can be performed by taking a difference from the detected value of the motor current detector 40.
Thus, by matching the current-to-output voltage characteristic of the DA converter that outputs the target indicated current value Iref to the current-to-output voltage characteristic of the current detector 40 after AD conversion that changes due to the fluctuation of the reference voltage VCC, the digital An error of the reference voltage Vref can be corrected in the course of the conversion process in which the voltage corresponding to the target indicated current value obtained by the calculation is DA-converted and output, and a correct value can be output, and the error due to the reference voltage Vref is corrected. As a result, the constant current value set as the target value at the start of motor driving can be set small, and the motor can be driven with a small current, so there is no occurrence of vibration or overshoot.
[0034]
According to the above embodiment, the constant current value at the start of motor driving can be set small. However, when the fluctuation of the offset of the motor current detector 40 that causes the malfunction becomes abnormally large, the constant current value at the start of motor driving is set. The above-mentioned malfunction is likely to occur only by setting the current value small.
As a countermeasure, in this embodiment, when the value obtained by AD conversion of the output value of the motor current detector 40 when the motor is stopped exceeds a predetermined difference β, the current detector is abnormal or the power supply voltage is abnormal. It is determined that the system is stopped abnormally. This criterion is determined in consideration of variations in power supply voltage and output variations of the current detector.
The timing for detecting the abnormality is appropriate to correct the error of the reference voltage VCC before the scanner is activated when the system power is turned on and when the scanner is stopped between system jobs. Since it can be considered, abnormality detection is performed at this timing.
[0035]
【The invention's effect】
D In the control of the C servo motor, a reference generated in the DA conversion of the microcomputer that sets the target indicated current value by matching the current-to-output voltage characteristic of the DA converter that outputs the target indicated current value with the output characteristic value of the current detector. Since the error due to fluctuations from the reference value of voltage (common to AD conversion and DA conversion) is corrected, it is set as the target value at the start of motor drive so Since the constant current value can be set to a minimum and the motor can be driven with a small current, an appropriate control operation can be performed without causing vibration or overshoot. In addition, when applied to a scanner, by suppressing the current at the start of motor drive, it is not necessary to set an excessive target instruction current value at the start of motor drive, and an impact generated when a large setting is made is given to the scanner unit. By avoiding this problem and providing the scanner in the image processing apparatus, it is possible to improve the jitter at the leading edge of the image.
Furthermore, when the error due to fluctuations from the reference value of the reference voltage is corrected, the possibility of malfunction due to the offset of the current detector (reversal of the set rotation direction) increases, so the output value of the current detector The AD converted value is monitored, and when it exceeds a pre-defined value, it is judged that the current detector is abnormal or the power supply voltage is abnormal, and the system is stopped abnormally. Can prevent damage to the machine.
[Brief description of the drawings]
FIG. 1 is a schematic diagram 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 the rotational speed of a DC servo motor necessary for driving the 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 current vs. output voltage detection characteristics of a unipolar Hall current detector.
FIG. 5 shows an influence on current vs. output voltage characteristics due to a change in reference voltage in an AD converter.
FIG. 6 is a diagram illustrating a method for correcting a target command current value in accordance with a current-to-output voltage characteristic of a current detector.
[Explanation of symbols]
30 ... Microcomputer, M31 ... DC servo motor,
EC31 ... encoder, 40 ... current detector,
IC33, IC35 ... differential amplifier, IC34, IC36 ... comparator,
38 ... Absolute value circuit, 33 ... Triangular wave generation circuit,
Q31-34 ... MOS-FETs constituting an H-type bridge circuit

Claims (6)

A rotation speed detection means for detecting a rotation speed including a rotation direction of the 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, and the motor In a servo control device comprising a current detector for detecting a flowing current value, and performing a servo operation by following the target instruction current value with the motor drive current detected by the current detector,
A current-to-output voltage characteristic of a DA converter that converts the target indicated current value into a voltage corresponding to the target indicated current value is obtained through an AD converter that operates with a common reference voltage with the DA converter. A servo control device characterized in that a DC servo motor is prevented from malfunctioning due to the influence of the reference voltage by adjusting to voltage characteristics . 2. The servo control device according to claim 1, wherein the 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 is a microcomputer, the AD converter and The D / A converter is a function of a built-in function of the microcomputer . 3. The servo control device according to claim 1 , wherein the current detector outputs a fixed voltage when no current flows through the motor, and a current in the forward or reverse direction flows through the motor. A servo current detector that detects a value and a direction of a current flowing in the motor by outputting a linear voltage centered on the fixed voltage according to the direction of the current. Control device. 4. The servo control device according to claim 2 , wherein when the value obtained by AD converting the output value of the current detector when the DC servo motor is stopped exceeds a predetermined value, the microcomputer servo controller according to claim Rukoto the operation is stopped. To rotate forward and backward more DC servo motor to the servo control device according to any one of claims 1 to 4, scanner and performs movement control of the reading scanning means of the document by the rotational force. An image processing apparatus comprising the scanner according to claim 5 .

Images