JP2525249B2 - Motor control voltage calculation method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating a motor control voltage, and more particularly to a method for calculating a PWM signal when controlling the rotation speed of a motor by a PWM signal.

<Prior Art> Some motor rotation speed control devices are controlled by a PWM signal.

Such a rotation speed control device is also used in a DC servo motor control device for driving an optical system in a document reading device such as a copying machine.

In the servo motor control device for driving the optical system, in particular, even if the frictional resistance or the like changes with the movement of the optical system and the motor load fluctuates, the servo motor can be kept at a constant speed with good followability and the optical system It is necessary to move at a constant speed.

Conventionally, in order to keep the servomotor at a constant speed, proportional control has been performed in which a PWM signal is obtained by a voltage proportional to the speed difference between the target speed and the actual detected speed.

<Problems to be Solved by the Invention> However, in the conventional proportional control, the acceleration when increasing the motor speed from the actual detected speed to the target speed is
The target speed varies depending on the size of the target speed. The larger the target speed is, the smaller the acceleration is, the longer it takes to reach the target speed, and the poorer the followability is.

 A more specific description will be given.

The equation of motion when a voltage V is applied to a motor is generally Becomes

Solving this for n, if t = 0 and n = Np, Also, Becomes

From this equation, when the sampled speed is Ns, the acceleration a at the time when the voltage V is applied is calculated by substituting Np = Ns, t = 0. Given in.

By the way, the target speed is N and the sampled speed is N
s, and the difference is ΔN, the acceleration a when V = KΔN = K (N-Ns) is applied by normal proportional control is as follows: V = KΔN, Ns = N-ΔN Substituting, From this equation, it can be seen that even if ΔN has the same value, the acceleration a is small when the target speed N is large, and a is large when N is small.

Therefore, the present invention has been made to solve such a drawback, and an object of the present invention is to provide a control signal calculation method capable of controlling a motor with good followability.

<Means for Solving the Problem> The present invention is a method for calculating a control voltage for controlling the rotation speed of a motor, wherein the voltage applied to the motor is the difference between the target rotation speed and the actual rotation speed. Is multiplied by a first constant, a value obtained by multiplying the actual rotation speed by a second constant, and a predetermined offset voltage are calculated, and the control voltage is obtained based on the sum. This is a method for calculating the control voltage of the motor.

<Operation> When the control voltage is obtained according to the present invention, the acceleration is determined by the speed difference between the target speed and the actual speed, regardless of the target speed, and the acceleration constant is selected to be an arbitrary value. A desired acceleration can be obtained with respect to the speed difference.

<Embodiment> In the following, as an embodiment of the present invention, a case where the present invention is applied to a drive control circuit of a DC servo motor for driving an optical system (illumination unit and reflection mirror) of a copying machine will be described as an example.

FIG. 1 is a block diagram showing a configuration example of a drive control circuit of a DC servo motor for driving an optical system of a copying machine. This control circuit is a circuit that uses a PWM signal as a voltage applied to the DC servo motor.

The DC servomotor 10 is of a permanent magnet field type and is rotationally driven by a driver unit 11 to move an optical system 17.

A rotary encoder 12 is connected to the rotary shaft of the servomotor 10. As is well known, the rotary encoder 12 outputs a rotation pulse each time the servo motor 10 rotates a predetermined minute angle. The rotary encoder 12 of this embodiment outputs, for example, 200 rotation pulses when the servo motor 10 makes one rotation.

The rotation pulse of the rotary encoder 12 includes at least an A-phase rotation pulse and a B-phase rotation pulse, and both rotation pulses are equal in number (200 per rotation of the motor) and are 90 degrees in phase with each other. The pulses are out of phase.

The rotation pulse output from the rotary encoder 12 is
It is given to the encoder signal input unit 13. The encoder signal input section 13 is a rotary encoder, as described in detail later.
Servo motor based on rotation pulse given from 12
This is a circuit for detecting 10 rotations. The detection output of the encoder signal input unit 13 is given to the control unit 14.

The control unit 14 is a center for controlling the entire circuit,
The calculation of the control signal of the servomotor 10 and other arithmetic processing are performed.

The control unit 14 includes a memory and a timer used in the control operation described later.

The control unit 14 is also provided with an operation command signal and a command speed. The command speed is supplied to the control unit 14 by applying a speed command clock from a control unit (not shown) of the copying machine main body to the speed command signal input unit 15 for signal processing.

The control unit 14 performs arithmetic processing based on each of these input signals, calculates PWM (pulse width modulation) data and supplies it to the PWM unit 16, and also supplies a driver unit drive signal to the driver unit 11 described above.

The PWM unit 16 outputs the PW based on the supplied PWM data.
This is a unit for changing the pulse width (output duty) of the M signal. The rotation speed of the servomotor 10 is controlled by the PWM signal output from the PWM unit 16. In addition, the driver drive signal determines the rotation direction of the servomotor 10 and brakes.

By the way, in order to accurately rotate the servomotor 10 at a desired speed, it is necessary to accurately detect the rotation speed of the servomotor 10 as a premise.

Therefore, in this drive control circuit, the encoder signal input section 13 is configured as shown in FIG. 2 and the signal reading by the control section 14 is devised so that accurate speed detection can be performed.

Referring to FIG. 2, the encoder signal input section will be described.
13 includes an edge detection circuit 131 to which the A-phase rotation pulse output from the rotary encoder 12 is applied.
The edge detection circuit 131 detects the rising edge of the applied rotation pulse and derives its detection output.

The encoder signal input unit 13 also includes a free running counter 133 of, for example, a 16-bit configuration that counts up a given reference clock, and a capture register 134. The capture register 134 uses the edge detection output of the edge detection circuit 131 as a capture signal, and the free running counter 13 using the capture signal as a trigger.
The number of counts of 3 is read and held.

The reference clock is a reference clock that serves as a reference for the operation timing of the entire circuit shown in FIG. 1, and a machine clock is used when the circuit is configured by a microcomputer.

If there is no such reference clock, a reference clock generation circuit may be provided.

The input unit 13 further includes an up / down detection unit 135 and an up / down counter 136. The up-down detection unit 135 determines the level of the B-phase rotation pulse when the A-phase edge detection output is given, and determines whether the B-phase rotation pulse is at the high level or the low level.
10 (FIG. 1) is to determine whether it is rotating normally or reversely. The up / down counter 136 uses the edge detection circuit 1 based on the discrimination output of the up / down detection unit 135.
The detection output of 31 is up-counted or down-counted.

 Next, the operation of the circuit shown in FIG. 2 will be described.

The contents of the capture register 134 are the capture signal,
That is, it is updated by the rising edge detection signal of the A phase. The up / down counter 136 counts the edge detection signal (the number of rotation pulses).

Therefore, while the up / down counter 136 counts n rotation pulses, the free running counter 13
Measure the count number of the reference pulse counted in 3,
The rotation speed N can be calculated based on this. The rotation speed N is The relationship between the rotation speed N and the error N ′ is Becomes

A graphical representation of these relationships is shown in Figure 3A.

In this embodiment, paying attention to the relationship between the rotational speed N and the error N ′, the control unit 14 devises a sampling timing for reading the count numbers of the capture register 134 and the up / down counter 136, and the rotational speed N is By increasing the rotation pulse number n for calculating the rotation speed N in accordance with the increase, the error N ′ that tends to increase at a square rate with the increase of the rotation speed N is calculated by the formula (7). As shown in the figure, the error N'is suppressed by limiting the increase to the first power so that the rotational speed N can be accurately detected.

More specifically, one sample time from a certain sample timing to the next sample timing is Δt.
Then, in order to detect the rotation speed N, at least one rotation pulse, that is, the output of the edge detection circuit 131 must be derived within the sample time Δt.

For that purpose, the above-mentioned formula (6) must satisfy the condition of X ≦ Δt, and in the end, the rotation speed N and the error N ′ are N ′ ≧ (1 / Δt) N (8) It is necessary to meet the relationship. The relationship of the equation (8) is on the upper side of the straight line N ′ = (1 / Δt) N as shown in FIG. 3B.

Therefore, by setting the sampling time Δt to an appropriate constant time that satisfies the relationship of the equation (8), the thick line portion in FIG. 3B, that is, the range in which the error N ′ is relatively small with respect to the rotational speed N is successfully used Then, the rotation speed N can be detected.

FIG. 4 is a flowchart showing a control operation for the control unit 14 to read the contents of the capture register 134 and the up / down counter 136 at every sampling time Δt to calculate the rotation speed N.

Next, a description will be given with reference to FIG. 2, FIG. 3B and FIG.

The control unit 14 resets the timer (step S2) and reads the contents of the capture register 134 and the up / down counter 136 (step S3) every time the internal timer reaches a certain sample time Δt (step S1).

Then, after subtracting the count number CPT _n-1 of the capture register 134 previously read stored in the memory A from the count number CPT _n of the read capture register 134 to obtain the reference clock number X within one sampling time Δt, CPTn
Is stored in the memory A (step S4).

Further, the count number UDC _n-1 of the previously read up / down counter 136 stored in the memory B is subtracted from the read count number UDC _n of the up / down counter 136 to obtain 1
After obtaining the number of rotation pulses within the sampling time Δt, UDCn is stored in the memory B (step S5).

Then, based on the equation (6) described above, the servo motor
The rotation speed N of 10 is obtained (step S6).

In steps S5 and S6, if the rotation speed N of the servo motor 10 increases, the rotation pulse speed n also increases stepwise within the sampling time Δt. Therefore, the servo motor 10 using the thick line portion in FIG. 3B is used. As described above, the number of rotations N can be detected.

The control unit 14 is the servo motor detected as described above.
The applied voltage represented by the following equation is calculated from the difference between the actual rotation speed Ns of 10 and the command rotation speed N given from the speed command signal input unit.

The acceleration when the voltage expressed by this formula is applied is
Substituting equation (9) into equation (4) above and substituting Ns = N-ΔN, The value obtained by dividing the speed difference ΔN by Δt is the acceleration,
By optimizing Δt, a constant optimum acceleration can be obtained for ΔN.

Next, the process of deriving the above equation (9) will be described.

FIG. 5 is an equivalent circuit diagram of a permanent magnet field type DC servo motor.

When the signal voltage V is applied to the motor armature and the angular displacement θ of the rotating shaft is obtained as the output, the transfer function of the motor is Is led.

Since the induced voltage E of the motor is proportional to the angular velocity ω of the rotating shaft, However, K _E : Induced voltage constant Since the generated torque is proportional to the amateur current I, T = K _T I (13) where K _T : Torque constant Now, the sum of the sensitivity moments of the rotor and the load is J, and the bearing loss is Assuming that the motor braking load including B is B, the load torque T _L is Since the torque generated by the motor is equal to the load torque, Becomes

In general, the inductance La of the amateur circuit is designed to be smaller than the moment of inertia J, so La = 0, and since the braking load B can also be ignored, assuming B = 0, the equation (11) becomes 16) Equation (14) is Becomes

Since it is more convenient to actually use GD ² [kg m ² ] as the moment of inertia J [kg msec ² ], change to GD ² .

GD ² ＝ Mg (2k) ² … (18) Where, G ＝ Mg: Weight of object [kg] M: Mass g: Gravity acceleration [9.8m / sec ² ] k: Radius of gyration Physical moment of inertia J [ Kgmsec ² ], given that the total mass is gathered at a certain distance, it is expressed as J = Mk ² (19). Therefore, GD ² = 4gJ (20)

From equation (17) and equation (20), From equation (21) and equation (22), Becomes

This equation (23) represents the torque required to obtain the change amount ΔN of the rotation speed during the time Δt.

Since the required load torque is the torque generated by the motor,
From equation (13) and equation (23), Is obtained.

Here, although the generated torque and the current are proportional expressions with K _T as a constant from the expression (13), there is a necessary current (no-load current) even in a no-load state.

Now, assuming that the no-load current is I _O Becomes

Next, since the actual drive control method is the voltage drive type by PWM, from equation (16) and equation (25), Becomes

Here, the induced voltage E is E = K _E N where K _{E is the} induced voltage constant, so equation (26) is Becomes

By the way, the equation (27) does not include the terms of the sliding load and the braking load on the motor. Therefore, letting this sliding load and braking load be T _BL , Motor braking load T calculated from no-load current I _O
BM is And the braking load T _B (Bω) in equation (14) is Becomes

Thus, equation (27) becomes Becomes

From the above, when rearranging, when the speed difference ΔN with respect to the target speed N occurs, The voltage V obtained in step 3 may be applied to the motor.

Here, the constants that are determined by determining the load and motor actually used are Ra: Amateur resistance [Ω] K _T : Torque constant [kgm / A] K _E : Induction voltage constant [V / rpm ] I _O : No load current [A] GD ² : Inertia moment due to load and motor [kg cm ² ] T _BL : Sliding load [kgm].

 Therefore, N, ΔN, and Δt are variables during control.

FIG. 6 is a block diagram showing a specific configuration example of the PWM unit 16, and FIG. 7 is a timing chart for explaining the operation of the PWM unit 16.

The PWM unit 16 includes a set signal generator 161, a PWM data register 162, a down counter 163, and an RS flip-flop 164.

The set signal generator 161 generates a set signal at regular intervals. The set signal generation unit 161 is composed of, for example, a ring counter, and is configured to generate a set signal every time a constant number of reference clocks are added.

The PWM data register 162 is a PW provided by the control unit 14.
It is for holding M data. The PWM data given from the control unit 14 is the voltage data obtained by the above-mentioned equation (9). This PWM data is used to determine the duty of the PWM output signal output from the PWM unit 16.

The down counter 163 counts down each time a PWM reference clock (in this embodiment, the PWM reference clock shares the reference clock used by the encoder signal input unit 13 and the speed command signal input unit 15). And
When the set number is measured, a reset signal is output.

The operation of the PWM unit 16 is as follows. When the set signal generator 161 outputs the set signal, the contents of the PWM data register 162, that is, the PWM data given from the controller 14 is set in the down counter 163, and the flip-flop 164 is set by the set signal. To be done.
Therefore, the output of the flip-flop 164, that is, the PWM output signal becomes high level.

Next, the down counter 163 counts down based on the PWM reference clock, and when the set count value becomes “0”, gives a reset signal to the flip-flop 164. Therefore, the output of the flip-flop 164 is inverted to the low level.

As a result, from the PWM unit 16, the PWM data register
The duty is determined by the value held in 162, that is, the voltage data calculated by the equation (9), and the PWM signal is derived.

The present invention can also be applied to the case where the applied voltage is calculated in a form other than the PWM signal.

<Effects of the Invention> Since the present invention is configured as described above, it is possible to calculate a control signal capable of keeping the acceleration constant regardless of the target speed. Therefore, regardless of the target speed, it is possible to control the speed of the motor with good followability.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of a drive control circuit of an optical system driving DC servo motor to which an embodiment of the present invention is applied. FIG. 2 is a circuit block diagram showing a main configuration of a rotation speed detecting device for a DC servo motor for driving an optical system according to an embodiment of the present invention. 3A and 3B are graphs showing the relationship between the motor rotation speed N and the error N '. FIG. 4 is a flow chart showing the rotational speed detecting operation in the embodiment of the present invention. FIG. 5 is an equivalent circuit diagram of a permanent magnet feed type DC servo motor. FIG. 6 is a block diagram showing a specific configuration of the PWM unit. FIG. 7 is a timing chart showing the operation of the PWM unit. In the figure, 10 ... DC servo motor, 11 ... Driver section, 12
… Rotary encoder, 13… Encoder signal input section, 14
... Control unit, 15 ... Speed command signal input unit, 16 ... PWM unit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuji Kajitani 1-2-2 Tamatsukuri, Chuo-ku, Osaka City, Osaka Prefecture Mita Industry Co., Ltd. (56) Reference JP-A-58-119790 (JP, A) JP Flat 2-76686 (JP, A) Actual Open Flat 1-120217 (JP, U)

Claims (1)

(57) [Claims] 1. A method of calculating a control voltage for controlling a rotation speed of a motor, wherein a voltage applied to the motor is a value obtained by multiplying a difference between a target rotation speed and an actual rotation speed by a first constant. And a value obtained by multiplying an actual rotation speed by a second constant and a predetermined offset voltage, and calculating the control voltage based on the calculated value.