JPS61167519A - Control method of injection and follow-up pressure of injection molding machine - Google Patents

Abstract

PURPOSE:To make the changeover from injection speed control to jection pressure control unnecessary, by a method wherein measured torque to be applied to a molding material at the time of injection is calculated through output torque and a speed of a servomotor in an injection molding machine and control is performed so that a calculated result becomes constant. CONSTITUTION:Output torque and driving current of a servomotor are detected by a power source detector and a revolving speed of the servomotor is detected by a detector, in an indication molding machine using the servomotor as a driving source of injection. Control of the titled machine is performed so that measured torque coincides with preset torque by a method wherein a detected revolving speed of the servomotor is differentiated by a differentiator, an output of the differentiator is deducted from that of a power source detector, measured torque to be applied to a molding material is obtained, the measured torque is deducted from a preset torque and a speed instruction or position instruction to the servomotor is made to vary according to the difference.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an injection molding machine that performs injection using a servo motor, and particularly to an injection control method.

2. Description of the Related Art In injection control in conventional injection molding machines, the injection speed, that is, the axial movement speed of the screw is controlled, and then the injection pressure is controlled, that is, the holding pressure is controlled. This control reduces the occurrence of flash, reduces residual stress in molded products, and prevents excessive molding pressure from being applied to the mold. One of the reasons for switching between injection speed control and injection pressure control during injection is that molding resin has not yet accumulated in the mold immediately after injection starts, making it difficult to detect pressure. For this reason, injection control was performed based on the injection speed, and the injection speed was controlled by switching so that the injection pressure remained constant according to the shape of the mold. However, if the injection pressure is constantly detected, injection control can be performed only based on the injection pressure. However, since conventional injection molding machines perform injection using hydraulic pressure, the injection pressure cannot be detected immediately after injection starts, and as described above, injection speed control and injection pressure control are switched. It was controlled by However, the timing of this switching is extremely difficult due to the shape of the mold, etc., and the quality of the molded product is determined by the appropriateness of the timing of this switching.

Problems to be Solved by the Invention The present invention constantly detects the injection pressure in injection control.
Injection control is performed using this injection pressure to obtain injection control that does not require switching from injection speed control to injection pressure control. In particular, the injection pressure can be controlled to be constant at all times.

Means for Solving the Problems In an injection molding machine that uses a servo motor as a drive source for injection, the output torque of the servo motor and the drive current of the servo motor are detected by a current detector, and the rotation of the servo motor is detected by a current detector. Detect the speed with a detector, differentiate the detected rotational speed of the servo motor with a differentiator, subtract the output of the differentiator from the output of the current detector, and find the actual torque applied to the molding material. , this actual torque is subtracted from the set torque, and the speed command or position command to the servo motor is changed according to the difference, so that the actual torque is controlled to match the set torque.

Function: If the product of the capacitor C and the resistor R of the differentiator is set equal to the inertia factor J, which is the sum of the loads of the servo motor and machinery, the output of the differentiator will be equal to the servo motor and the servo motor. This means the torque required to accelerate or decelerate the inertia of a machine such as a screw connected to the motor, and since the output of the current detector means the output torque of the servo motor, the above differential can be calculated from the output of the current detector. By subtracting the output of the machine, the actual torque applied to the molding material can be calculated.
Based on the difference between the actual torque and the set torque, control is performed so that the actual torque matches the set torque by changing the speed command or position command, and control is performed so that a constant injection pressure is always applied to the molding material during injection.

Embodiment The present invention uses a servo motor as the drive source for injection, which moves the screw of an injection molding machine in the axial direction to perform injection. This is a block diagram of a servo circuit having a position control loop for driving and controlling the servo motor. is generally shown in FIG. In this example, a DC motor is shown as the servo motor M, but the same applies to an AC motor, and a pulse encoder P is used as the detector, but other detectors such as resolvers, speed generators, etc. may be used. . The servo circuit shown in FIG. 1 is a servo circuit 1 having a known position control loop.
When a position command a consisting of a pulse train is input as a drive command for the servo motor M as a movement amount per unit time, this position command a and the movement of the servo motor M detected by a detector P such as a pulse encoder are input. ff1b is converted into an analog voltage as a speed command value C by a digital-to-analog converter (hereinafter referred to as a D/A converter) 2.

That is, position command a and servo motor M from detector P
If there is a large difference in the amount of movement, the speed command value C will be a large value.
When the difference is small and the position command a and the movement amount are close to each other, a small speed command value C is output. Furthermore, this servo circuit performs speed feedback in order to improve responsiveness. This converts the signal from the detector P into a voltage using the F/V converter 6, and calculates the actual speed of the servo motor M. The voltage V corresponding to the speed command value C is subtracted from the speed command value C, and the difference therebetween, that is, the error between the command speed C and the actual speed V, is amplified by the error amplifier 3 and output as the command torque e. That is,
This command torque e is output as a voltage corresponding to the current value flowing through the armature of the servo motor M.In order to further improve responsiveness to this command torque e, the armature current of the servo motor M is detected. The voltage f corresponding to the armature current from the current detector 7 is fed back, and the difference between the command torque e and the feedback signal f of the armature current is amplified by the error amplifier 4 and the power amplifier 5. It drives the servo motor M.

Therefore, if the block diagram based on the speed command C is expressed by a transfer function, it will be as shown in FIG. 2. That is, speed command C and actual speed V
The transfer function of the error amplifier 3 that amplifies the difference between is the proportional term KP,
is expressed by the integral term Kl, and the error amplifier 4 in FIG.
．． If the feedback circuit composed of the power amplifier 5 and the current detector 7 has a sufficient frequency band, that is, if it responds immediately to the command, its transfer function can be expressed only by the proportional term Kt. , and the transfer function for converting torque to speed is expressed as r1/JSJ. Here, J is the total inertia factor of the motor and the load. As a result, the block diagram based on the speed command C becomes as shown in FIG. In this block diagram, Td is the disturbance torque, which means the load torque connected to the servo motor, and in this case, the screw of the injection molding machine presses the molding material and means the actual torque Td related to injection. be.

Therefore, assuming that this disturbance torque T' d is "0", as shown by transfer function 12, the torque is integrated and the speed V
Therefore, the torque required to accelerate or decelerate the inertia of the servo motor M or a machine connected to the servo motor M, such as a screw, can be obtained by differentiating the speed d. As a result, the output torque To of the servo motor is expressed by the following equation.

To = Td + J' (dv /dt) --(1
)Td=To −J・(dv/dt)・・・・
...(2) As a result, as shown in equation (2), the actual torque Td required to inject the molding material is the value obtained by differentiating the actual speed from the output torque To of the servo motor M, the servo motor M, and the load. It can be found by subtracting the product of the total inertia factors J. Then, the actual speed ■ is the signal of the detector P
It is obtained from the value converted to voltage by the /V converter 6,
Since the output torque To of the servo motor M is detected by the current detector 7 as a torque command value, that is, a voltage value e with respect to the current value flowing through the armature of the servo motor M, the actual torque Td required for injection is determined by equation (2). It is determined by The present invention performs injection control by always controlling the actual torque Td required for injection, that is, by keeping the injection pressure constant. A first method thereof is shown in FIG. 3.

In Fig. 3, 13 is a differentiator, 14.15 is a subtracter,
3' is an error amplifier, and the difference from the error amplifier 3 shown in FIG. 1 is that the error amplifier shown in FIG. In contrast to the one that amplifies and outputs the difference between the actual speeds V, the error amplifier 3' in FIG. 3 adds the output of the subtracter 15 to the value obtained by subtracting the actual speed ■ from the speed command C. There are differences in

Note that the circuit after the error amplifier 3' is the same as the circuit shown in FIG. In addition, OPI to ○P4 in Fig. 3 indicate operational amplifiers, C1 and C2 are capacitors, and R1-R
11 indicates resistance.

Next, the operation of this embodiment will be explained.

First, the voltage corresponding to the actual speed V output from the F/V converter 6 is input to the differentiator 13. Capacitor 01 of differentiator 13. Let the value of resistor R1 be C1·R1=J. That is, the product of the capacitor C1 and the resistor R1 is set to be equal to the inertia J of the servo motor M and the machine such as the screw of the injection molding machine. As a result, the output of the differentiator 13 is −C1・R1−(dv/dt)=−J・(dv/
dt) (note that since the output V of the F/V converter 6 is a negative voltage, this output has a positive value).

This output is then input to the subtracter 14, and the difference between it and the output f from the current detector 7 is output. That is, the output of the current detector 7 indicates the armature current value f that actually flows through the armature of the servo motor (this value is also output as a negative value), and this armature current value In other words, since f means the voltage value corresponding to the output torque TO of the servo motor M, the subtracter 14 (this subtracter 14 constitutes an adder with the overdrive amplifier OP2,
Since the inputs have different signs, a subtracter is configured. ) is shown by the following equation.

Output of g calculator 14 = -(f + J ・(dv/dt)
)=-(-To+J・(dV/dt))-To-J
・(dv/dt) ...(3) As shown in the above equation (3), the output of the subtracter 14 is the same as the right side shown in the equation (2), so this subtracter 1
The output No. 4 indicates a voltage value corresponding to the actual torque Td required for the injection molding machine to inject the molding material. Thus, the voltage value corresponding to the actual torque Td, which is the output of the subtracter 14, is input to the subtracter 15, compared with the set value -8 (this set value -8 is also a negative voltage value), and a subtracted value is output. Ru. This set value -8 is the voltage that corresponds to the torque that the screw of the injection molding machine applies to the molding material during injection, that is, the set value TS of the injection pressure, and the injection molding machine always injects at this set injection pressure of 1 mTS. The settings are as follows. As shown in equation (4), the subtracter 15 subtracts the voltage value that is input from the set value -8 and corresponds to the output of the subtracter 14, that is, the current actual torque Td, and outputs the difference. .

Output of subtractor 15 = -(-8+Td) = Ts - Td
(4) The output of the subtracter 15 is further input to the error amplifier 3'. As a result, the error amplifier 3
' subtracts the actual speed V from the speed command C from the D/A converter 2, and then subtracts the subtracter 15 shown by the above equation (4).
In other words, a voltage corresponding to the difference between the set torque Ts to be applied to the molding material during injection and the actual torque Td that is actually applied is added, and this is amplified and output from the error amplifier 3 as the command torque e. (Note that the polarity is converted to the appropriate polarity using a sign detector, etc.). As a result, if the difference between the set torque Ts and the actual torque Td is large, the voltage value of the command torque e output from the error amplifier 3' increases, the armature current of the servo motor M increases, and the servo motor M is driven at high speed. Therefore, injection is always performed so that the actual torque Td becomes the set torque TS.

As described above, the first embodiment of the present invention has a speed command C
A speed command is issued by adding the difference obtained by subtracting the actual torque Td from the set torque TS, and injection control is performed so that the injection pressure applied to the molding material during injection is always maintained at the set value.

Next, a second example will be described.

The difference between the second embodiment shown in FIG. 4 and the above first embodiment is that the first embodiment has a set torque Ts and an actual torque Ts.
The second point is that the injection pressure is always maintained at the set value by adding the difference from the speed command C to the speed command C.
In this embodiment, the injection pressure is controlled to always be maintained at the set value by increasing or decreasing the position command amount in accordance with the difference between the set torque Ts and the actual torque Td.

That is, the servo circuit is the same as the circuit shown in FIG. 1, and the difference is that the acceleration/deceleration control unit 20 that generates the position command a of the pulse train output as the amount of movement per unit time is the same as that obtained in the first embodiment. This is the point where the difference between the set torque Ts and the actual torque Td is added.

In Fig. 4, the amount of movement of the servo motor M, that is, the amount of movement of the screw is input, and in order to immediately drive the servo motor M by the commanded movement amount, infinite speed and infinite torque are required. The acceleration/deceleration control unit 20 outputs the movement amount per unit time as a pulse train per unit time while sequentially adding and subtracting the amount of movement per unit time at the rise and fall of the movement command, and further reduces sudden changes in speed by the time constant circuit 21. Then, a position command a consisting of a pulse train as a movement amount per unit time is outputted, and is inputted to the servo circuit shown in FIG. 1 below. Acceleration/deceleration control section 20 described above
．． The time constant circuit 21 has also been implemented before the filing of the present application, but in this embodiment, the difference between the set injection torque and the actual torque of the injection molding machine is further input to the acceleration/deceleration control section 20, and the unit time Pulse train a that commands the amount of movement per
I am trying to increase or decrease the value of.

That is, in FIG. 4, 13, 14, and 15 are the same as the differentiator and subtracter shown in FIG. The resultant output is subtracted by the subtractor 14 from the output f from the current detector 7, which indicates the value of the output torque To of the servo motor M, to calculate the actual amount of material actually added to the molding material for the screw to perform injection. The torque Td is calculated, and the subtracter 15 calculates the difference between the output of the subtracter 14 indicating the value of the actual torque Td and the set value -8 that sets the set torque TS to be applied to the molding material at the time of injection. The difference is converted into a voltage-frequency converter (hereinafter V
/F converter) 22 converts it into a pulse train, and this V/F converter) 22 converts it into a pulse train.
The output of the F converter 22 is input to the acceleration/deceleration control section 20, and the output pulse of the V/F converter 22 is added to the pulse train as the movement amount per unit time which is the output of the acceleration/deceleration control section 20. , change the number of pulses of movement amount per unit time.

As a result, if the difference between the actual torque Td and the set torque Ts is large, the number of movement pulses per unit time, that is, the value of the position command a, will increase, the speed command C will also increase, and the torque command e will also increase. , the armature current of the servo motor M increases, the speed and torque of the servo motor increase, and the value of the actual torque Td applied to the molding material by the screw driven by the servo motor M matches the set torque TS. will be controlled at all times.

That is, the injection pressure is controlled so as to be always maintained at the set value.

In the second embodiment, the subtracter 15 is replaced with a comparator, which outputs when there is a difference between the actual torque and the set torque, and only when there is this output, changes are made to the output pulse train of the acceleration/deceleration control section. A constant pulse may be added.

Effects of the Invention As described above, the present invention calculates the injection pressure applied to the molding material during injection from the output torque of the servo motor, which is the drive source for injection, and the actual speed of the servo motor, and keeps this constant at all times. Since the speed command value or position command value of the servo motor is automatically changed, the injection pressure is always constant, and the injection speed is controlled so that the injection pressure is constant at the initial stage of injection. Since the injection pressure is controlled to be constant during pressure holding, there is no need to control injection speed control and pressure holding control in separate control systems and to switch between them as in the past, making it extremely simple. This allows for excellent injection control.

[Brief explanation of the drawings] Fig. 1 is a block diagram of a servo circuit having a conventionally known position control loop, and Fig. 2 is a block diagram in which the block diagram from the speed command is expressed by a transfer function in the same block diagram. , FIG. 3 is a main part of a block diagram of the first embodiment of the present invention, and FIG. 4 is a main part of a block diagram of a second embodiment of the invention. 13...Differentiator, 14,1.5...Subtractor, 3'.
...Error amplifier, OP1-OP3...Operation amplifier.

Claims (5)

[Claims] (1) In an injection molding machine that uses a servo motor as a driving source for injection, the actual torque that is substantially applied to the molding material during injection is calculated from the output torque and speed of the servo motor, and the actual torque is always constant. An injection/holding pressure control method for an injection molding machine that performs injection control. (2) The injection molding machine according to claim 1, wherein the speed command value to the servo motor is changed according to the difference between the actual torque and the set torque, and control is performed so that the actual torque matches the set torque. injection/holding pressure control method. (3) The frequency of the position command to the servo motor is changed according to the difference between the actual torque and the set torque, so that the actual torque is controlled to match the set torque. Injection/holding pressure control method for injection molding machine. (4) By differentiating the rotation speed of the servo motor detected by the detector with a differentiator, and subtracting the output of the differentiator from the output of the current detector that detects the drive current of the servo motor with a subtracter. 3. The injection/holding pressure control method for an injection molding machine according to claim 2, wherein the actual torque is calculated, the actual torque is subtracted from the set torque value by a subtractor, and the difference is added to the speed command. (5) By differentiating the rotational speed of the servo motor detected by the detector with a differentiator, and subtracting the output of the differentiator from the output of the current detector that detects the drive current of the servo motor with a subtracter. Calculate the above actual torque, subtract the above actual torque from the set torque value using a subtracter, convert the difference into a frequency using a voltage-frequency converter, and convert the output of this voltage-frequency converter to the position of the servo motor. An injection/holding pressure control method for an injection molding machine according to claim 3, which is added to the command.