JP2640666B2 - Pressure control method for electric injection molding machine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a pressure control method for an electric injection molding machine.

2. Description of the Related Art In an injection molding machine, when a pressure-holding step is started after an injection step is completed, the pressure-holding step is divided into several stages, and the pressure of each stage is controlled.

In an electric injection molding machine, a torque command by a torque limit is output to an injection shaft, that is, a motor that moves a screw in a screw axis direction, and pressure in each stage of a pressure-holding process is controlled. I have. The pressure control in this pressure-holding step includes an open-loop control method that controls the pressure-holding pressure by sequentially changing the torque command according to the elapse of time from the start of the pressure-holding, and detecting the pressure actually applied to the resin There is a closed-loop control system of a feedback control system that changes a torque command so as to reach a specified pressure, and each is known.

On the other hand, in general, when pressure control is performed by a closed loop control method, the actual response pressure will overshoot or undershoot with respect to the command pressure, and as a means for eliminating this overshoot or undershoot. , Feedback gain, proportional, derivative,
The so-called PID control for adjusting the integration parameters is performed.

However, even if the parameters are adjusted by the PID control and the gain of the loop is adjusted, the response may saturate depending on the performance and characteristics of the actuator.

The same applies to the pressure control of an injection molding machine. The method of eliminating overshoot and undershoot regardless of open loop control or closed loop control depending on the performance and characteristics of an actuator that applies pressure to the resin, that is, the servomotor for injection. Was difficult. For example, there has been only a method of replacing a servomotor for injection.

Problems to be Solved by the Invention In the case of pressure control during the pressure-holding step of an injection molding machine, pressure control is performed by changing the torque command, whether open-loop control or closed-loop control. Because the speed is early, when the torque command is changed, the response output may overshoot and undershoot due to the inertia of the motor and the screw. The only way to eliminate this is to replace the injection servomotor, as described above.

Therefore, an object of the present invention is to provide a pressure control method that does not cause overshoot or undershoot without replacing the injection servomotor.

Means for Solving the Problems The present invention provides a switching time width from the preceding torque command to the current torque command, and at least in the vicinity of the end of the switching time width, the change value of the torque command value to the servomotor is changed. The above problem has been solved by increasing or decreasing the torque command of the preceding stage to the torque command of the present stage so as to approach zero.

When switching from the previous torque command to the current torque command, the output torque of the servomotor that drives the screw in the axial direction is changed from the previous torque command to the current torque command within the set switching time width. Sequentially increase or decrease,
In addition, near the end of the switching time width, control is performed so that the change value of the output torque gradually approaches zero.

As a result, the torque command does not change abruptly at the time of holding pressure switching, but gradually changes with a predetermined switching time width, and the change value of the torque command is small near the end of the switching time width. Overshoot and undershoot are reduced.

Example Hereinafter, an example of the present invention will be described.

In FIG. 1 showing a main part of an embodiment of an electric injection molding machine for implementing the pressure control method of the present invention, reference numeral 1 denotes an injection / holding pressure by driving a screw 2 of the injection molding machine in a screw axis direction. Servo motor that is the driving source to perform the operation.
A pulse coder 3 is mounted on a motor shaft of the servo motor 1. This electric injection molding machine has a servomotor for each axis, such as a screw rotating shaft and a clamp shaft, similar to a normal one, but these are not directly related to the present invention, and therefore description thereof is omitted. .

Reference numeral 10 denotes a control device of the electric injection molding machine. The control device 10 controls the operation of the entire injection molding machine and the pulse distribution processing of the servomotors of the respective axes.
Central Processing Unit (hereinafter referred to as CPU) 11 and NC
Position of each axis servo motor based on the command value of
Servo CP for servo control to control speed, torque control, etc.
With U12.

The NC CPU 11 has a ROM 13 storing a control program for managing the entire injection molding machine, a sequence program for controlling the sequence operation of the injection molding machine, a program for controlling the servo CPU 12, and the like, and a table T in FIG.
B, the set pressures P1 to PI of each pressure holding stage, the corresponding pressure holding times T1 to TI, the pressure holding switching time width (time constant) ΔT1 to ΔTI, and the various set values and parameters. A RAM 14 storing values and the like, and a manual data input device 15 with a CRT (hereinafter referred to as CRT / MDI) for setting various set values and parameter data in the RAM 14 are connected via a bus 16.

On the other hand, the servo CPU 12 includes a torque command value such as a pulse distribution amount to each axis and a torque limit value output from the NC CPU 11 for each distribution cycle, various data, and an NC signal when the power is turned on.
RAM 17 used for temporary storage of a program for controlling the servo CPU 12 which the CPU 11 transfers via the bus 21, C for NC
A speed control circuit 1 for controlling the speed of the servo motor 1 based on the pulse distribution amount commanded by the PU 11 and a feedback signal from the pulse coder 3 mounted on the servo motor 1
8. Similarly, a torque control circuit 19 for controlling the torque of the servo motor 1 is connected via a bus 20. That is,
In this embodiment, the servo means is constituted by the servo CPU 12, the speed control circuit 18, the torque control circuit 19, and the like. It constitutes a so-called software servo. The servo CPU 12 and the NC CPU 11 are connected via buses 20, 21, and 16.

Next, an outline of the operation principle of the pressure control in this embodiment will be described.

FIG. 3 shows the relationship between the set pressure Pi set in each stage of the pressure holding in the table TB (see FIG. 2) stored in the RAM 14 and the pressure holding time Ti, and the number of the four pressure holding stages is set. It is a figure explaining taking a case as an example. That is, after the injection is completed at the maximum injection pressure P0, the pressure is maintained at the set pressure P1 for T1 seconds, and after the elapse of T1 second and the first-stage dwell is completed, the pressure is maintained at the set pressure P2 for T2 seconds,
Hereinafter, four-stage pressure holding is performed in the same manner.

However, if the NC CPU 11 outputs these set pressures as torque commands in the form shown in FIG. 3, that is, as torque limit values, the pressure applied to the resin by the servo motor 1 Overshoot and undershoot occur due to inertia and the like, and the pressure applied to the resin fluctuates.

Therefore, as shown in FIG. 2, the switching time width until shifting to the next pressure holding stage for each pressure holding stage, that is, the time constant ΔTi
Is set, and the output torque limit value is sequentially changed from the torque limit value at the preceding stage to the torque limit value at this stage according to the elapsed time of the time constant, and the torque limit value changes near the end of the time constant. The pressure is controlled by calculating the torque limit value using a torque limit calculation formula (hereinafter simply referred to as a calculation formula) whose value gradually approaches zero. For example, the maximum injection pressure P0 at the end of injection
To shift to the set pressure P1 of the first stage of dwelling from
As shown in the figure, a time constant ΔTi is provided immediately after the end of the injection, and the torque limit value of the command pressure is calculated based on the above-described equation according to the elapsed time between the time constants ΔT1, and the set pressure is set. Acceleration / deceleration control for smooth control of pressure by eliminating overshoot and undershoot of pressure when switching is performed.

FIG. 5 and FIG. 6 show an example of pressure switching control when shifting from the previous set pressure Pi-1 to the set pressure Pi of the present stage.

The pressure switching control changes the set pressure from Pi-1 based on the elapsed time within the set time constant ΔTi and the above equation.
In this embodiment, the time constant ΔTi is defined as a defined area, and the pressure difference (Pi−
1 to Pi) as a value range, and at least in the vicinity of the end point of the defined range (near the end of the time constant ΔTi), the pressure change value asymptotically approaches zero. Is used. In the equation (1), t is a variable indicating the elapsed time within the time constant ΔTi, and P is a torque limit value as a calculation result. Equation (1) is the seventh
A cosine curve as shown in the figure, y = cosΘ... (2) is parallel in the y-axis direction by (Pi−1 + Pi) / 2, that is, by the average value of the set pressure of the previous stage and the set pressure of this stage. It moves and the amplitude is amplified by (Pi-1-Pi) / 2, that is, the value obtained by dividing the difference between the set pressure of the previous stage and the set pressure of this stage by two.

The equation (1) is set so that the domain 0 ≦ Θ ≦ π where the cosine curve monotonously decreases as shown in FIG. 7 corresponds to the domain 0 ≦ t ≦ ΔTi of the elapsed time within the time constant ΔTi. In (2), a term (π · t / ΔTi) is provided so that (π · t / ΔTi) changes from 0 to π while t changes from 0 to ΔTi.

Therefore, when switching from the set pressure Pi-1 at the preceding stage to the set pressure Pi at this stage, the equation (1) is used according to the elapsed time t (0 ≦ t ≦ ΔTi) having the time constant ΔTi as a defined area. When the torque limit value P is calculated, the torque limit value P at the elapsed time t = 0 becomes Pi-1 and coincides with the set pressure at the previous stage. When the elapsed time t = ΔTi and the time constant ends, the torque limit value P becomes Pi becomes the same as the set pressure at this stage. Also, as described above, the domain 0 ≦ t of the elapsed time t
≦ Ti is the domain 0 ≦ Θ of the cosine curve as shown in FIG.
≤π, the elapsed time t = 0 (（in FIG. 7).
= 0), the rate of change of the torque limit value P is 0
And the rate of change gradually increases as the elapsed time increases,
After reaching the maximum rate of change at the position of t = ΔTi / 2 (corresponding to Θ = π / 2 in FIG. 7), the rate of change is gradually reduced and the elapsed time t
When the differential coefficient asymptotically approaches zero near the end point of the defined area of FIG. 5 and the elapsed time t reaches the time constant ΔTi (corresponding to Θ = π in FIG. 7), the differential coefficient finally becomes zero.

In this manner, the pressure is smoothly switched by gradually approaching the differential coefficient to 0 near the end point of the time constant which is a defined area and gradually decreasing the rate of change of the torque limit value with the passage of time.

Hereinafter, the pressure control operation of this embodiment will be described in detail with reference to the processing flowchart of the NC CPU 11 in the pressure holding operation shown in FIG.

First, as shown in FIG. 2, the number of pressure holding stages I, the pressure holding of each stage,
That is, the torque limit values P1 to PI, the dwell time T1 to TI of each stage,
A time constant ΔT1 to ΔTI, which is a switching time width at the time of pressure switching from the previous stage to this stage, is inputted from the CRT / MDI 15 and the table TB of the RAM 14 is inputted.
To be stored. After setting other various set values, the injection molding machine is driven.

When the injection process is completed and the pressure holding process starts, the NC CPU 11 starts the pressure holding process shown in FIG.

When the injection process is completed and the pressure holding process is completed, the NC CPU 11
First, the counter i indicating the number of pressure holding stages is set to 1 (step S1), and the timer R (t) for monitoring the elapsed time is set to 0 and started (step S2).

Next, from the table TB shown in FIG. 2 stored in the RAM 14, based on the value of the counter i indicating the number of pressure holding stages, the set pressure P1, pressure holding time T1, time constant data ΔT1, Is read and stored (step S3), and based on these data and the elapsed time t indicated by the timer R (t), the torque limit value P is calculated according to equation (1) (step S3). S4) This value is output to the RAM 17 as a command pressure, that is, a torque limit value, and the torque limit value is rewritten (step S4).
Five). On the other hand, the servo CPU 12 passes through the torque control circuit 19,
Based on the torque limit value P rewritten by the NC CPU 11, the torque of the servo motor 1, that is, the holding pressure of the screw 2 is controlled.

Next, it is determined whether or not the elapsed time t indicated by the timer R (t) has reached the time constant data ΔT1 set for the first pressure holding stage (step S6).
Then, based on the stored set pressure P1, the time constant data ΔT1, the maximum injection pressure P0 at the end of injection, and the elapsed time t indicated by the timer R (t), the first (1) The torque limit value P is calculated according to the equation (step S4), and this value is output to the RAM 17 as the command pressure, that is, the torque limit value, and the torque limit value is rewritten (step S5). Hereinafter, in step S6, until it is determined that the elapsed time t indicated by the timer R (t) has reached the time constant data ΔTi set for the first pressure holding stage, the process is performed in steps S4 to S5 to S6. The loop-shaped processing is repeatedly performed, and the torque limit value P is calculated based on the same data in each processing cycle of this loop. However, since the value of the timer R (t) changes, the calculated torque limit value P is calculated. The value of the value P also changes little by little.

Further, since the rewriting of the torque limit value is performed in each processing cycle of the above-mentioned loop processing, the torque limit value output from the torque control circuit 19 to the servomotor 1 becomes a rectangular wave output. Since the processing cycle of the processing is fast, the output change becomes a substantially cosine curve as shown in FIGS. 5 and 6.

Thus, Step S4-Step S5-Step
While the loop-like process formed by S6 is repeated, the elapsed time t indicated by the timer R (t) is set to the time constant data .DELTA.
Is reached, the pressure switching from the maximum injection pressure P0 to the set pressure P1 of the first holding pressure is completed, and the value of the torque limit becomes the set pressure P1 of the first holding pressure.

Next, the process proceeds to step S7, where it is determined whether or not the elapsed time t indicated by the timer R (t) has reached the set dwell time T1 of one dwell, and has reached the set dwell time T1. If not, the apparatus waits for the set pressure holding time T1 of the first set of pressure to elapse, and then applies the set pressure P1.

When the set pressure holding time T1 of the set pressure is passed and the pressure holding process of the set pressure is completed, the process proceeds to step S8, where 1 is added to the value of the counter i indicating the number of pressure levels, and the value is updated. Then, it is determined whether or not the updated value of the counter i exceeds the set value I of the number of pressure-holding stages (step S9). If the value does not exceed the set value I, the process proceeds to step S2, and the process proceeds to the next stage. The pressure switching process is started.

Timer R for monitoring elapsed time in step S2
After setting (t) to 0 and starting, the table TB
Based on the updated value of the counter i that dies the number of pressure holding stages, the set pressure P2, the pressure holding time T2, and the time constant data ΔT2 of the two pressure holding stages are determined.
Then, the set pressure P1 of the first dwell stage is read and stored (step S3). Thereafter, the pressure switching process from the first dwell stage to the second dwell stage and the dwell process of the second dwell stage are performed in the same manner as the previous time. Run.

In this way, while the value of the counter i indicating the number of pressure-holding stages is updated, while the pressure switching process from the pressure-holding i-1 stage to the pressure-holding i-stage and the pressure-holding process in the pressure-holding i-stage are repeated, in step S9 , The updated value of the counter i is the set value I of the number of pressure-holding stages.
, All the pressure holding processes from the first pressure holding stage to the first pressure holding stage are completed.

As described above, a cosine curve is used as an arithmetic expression in which the differential coefficient asymptotically approaches zero at least in the vicinity of the end point of the defined area (near the end of the time constant time) with the time constant as the switching time width as the defined area and the pressure difference from the previous stage to the present stage as the value range. Although the description has been given of the case of using the expression (1) to which the expression (1) is applied, the pressure can be smoothly switched by using another operation expression.

 As an example, a case of a cubic equation will be described.

In the equation (3), P is a torque limit value, Pi-1
Is a set pressure of the preceding stage, Pi is a set pressure of the present stage, ΔTi is a time constant, and t is a variable indicating an elapsed time between the time constants.

Differentiating equation (3) with respect to elapsed time (t), And the discriminant relating to the solution of t when equation (3 ′) is set to 0 is Therefore, Equation (3 ') has two different real solutions with respect to the elapsed time t, that is, Equation (3) defines two extreme values (maximum value and minimum value) with respect to the torquemit value P. It is self-evident. The solution of the elapsed time t when the equation (3 ′) is set to 0 is t = 0, t = ΔTi. Therefore, when the coefficient of the term t ^{3 in} the equation (3) is larger than 0, , i.e., when the Pi-Pi> 0, the (3) the maximum value of expression Pi-1, the minimum value Pi, and the other hand, when the coefficient of the term of t ³ is less than 0, i.e., Pi-1 When −Pi <0, the (3)
The maximum value of the equation is Pi, and the minimum value is Pi-1.

That is, the arithmetic expression shown in Expression (3) is a monotone (increase or decrease) function from Pi-1 to Pi in a closed section of the domain 0 ≦ t ≦ ΔTi of the elapsed time t. = 0,
At t = ΔTi, the differential coefficient becomes 0, and has the same characteristics as the characteristics of Expression (1) shown in FIGS. 5 and 6.

Therefore, even when the expression (3) is used, the pressure can be smoothly switched as in the case of the present embodiment using the expression (1).

As described above, the case where the differential coefficients are both 0 at the elapsed time t = 0 and t = ΔTi, that is, the case where the switching is smoothly performed at the start and the end of the switching control, has been described. It is not always necessary to perform smooth control, and at the end of the switching control, the differential coefficient does not necessarily have to be 0, and it is sufficient that the differential coefficient is asymptotic to 0. The section in which the coefficient asymptotically approaches zero corresponds to the domain of the time constant, the value range at that time is set so as to correspond to the pressure difference from the previous stage to this stage, and an appropriate arithmetic expression may be created.

In any case, step S4 in FIG.
-Step S5-If the cycle of executing the loop processing formed by step S6 becomes longer, the rewrite cycle of the torque limit value becomes longer and smooth pressure switching control becomes difficult, so the calculation in step S4 is performed at high speed. It is desirable to use an arithmetic expression that can be used.

Effect of the Invention The present invention sets a switching time width required for pressure switching at the time of pressure switching in a pressure holding step, and changes a torque command value according to a lapse of time between the set switching time widths to a set torque command value of a preceding stage. From the set torque command value of this stage to the set torque command value, and the control is performed so that the rate of change of the torque command value to the servomotor approaches 0 near the end of the switching time width. , Falling can be controlled smoothly, and overshoot and undershoot accompanying pressure switching can be reliably prevented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main part of an electric injection molding machine for carrying out an embodiment of the present invention, and FIG. 2 is a table of the embodiment.
FIGS. 3 to 7 are diagrams schematically illustrating the principle of operation of the embodiment, and FIG. 8 is a CP for NC according to the embodiment.
It is a flowchart which shows the pressure holding processing operation of U. 1 ... Servo motor, 2 ... Screw, 3 ... Pulse coder, 10 ... Control device, 11 ... NC CPU, 12 ... Servo CPU, 13 ... ROM, 14,17 ... RAM, 15 ... Manual data input with CRT display, 16,20,21 …… Bus,
18: Speed control circuit, 19: Torque control circuit.

 ────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-1-226319 (JP, A) JP-A-61-61820 (JP, A)

Claims (1)

(57) [Claims] 1. A pressure control of an electric injection molding machine for controlling a pressure holding at each stage by a pressure holding time set for each stage of a pressure holding process and an output torque of a servomotor for driving a screw in an axial direction. In the method, a switching time width is provided from the previous set torque command value to the current set torque command value, and at least in the vicinity of the end of the switch time width, the change value of the torque command value to the servomotor gradually approaches zero. A pressure control method for an electric injection molding machine characterized in that the set torque command value at the preceding stage is increased or decreased from the set torque command value at the previous stage.