JPH04244824A - Dwell control method in electric injection molding machine - Google Patents

Abstract

PURPOSE:To prevent the difficulty of holding of set dwell from occurring due to the large movement of a screw, and generation of delay to set dwell when dwell pressure is changed over. CONSTITUTION:A screw is driven in the axial direction by a servomotor, and dwell is controlled. Pressure applied to a resin is detected, difference with set dwell is obtained, and movement corresponding to the difference is output to the servomotor and the servomotor is driven and the position of the servomotor is held. Consequently, dwell pressure is controlled by the position of the screw. Even when dwell pressure set is changed from a large value to a small value, the screw is not retreated by resin pressure largely, and dwell pressure is held at dwell pressure set by positioning the servomotor.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure holding control method for an electric injection molding machine which performs injection and pressure holding by driving a screw in the axial direction.

0002

[Prior Art] Pressure holding control of an electric injection molding machine, which performs injection and pressure holding by driving a screw in the axial direction, has conventionally been carried out by issuing a command to the injection servo motor to move to the tip position of the cylinder. At the same time, the output torque of this injection servo motor is limited to a set holding pressure and driven according to a set holding pressure speed command.

[0003] In the pressure holding stage, even if the output torque of the injection servo motor is limited and the servo motor is driven to drive the screw in the injection direction, the amount of movement is limited because the resin is filled in the mold. is small, and the positional deviation increases. However, even if the positional deviation increases, the output torque of the servo motor is limited to the set value, so the servo motor does not generate torque that exceeds the set value, and in the end, the resin does not respond to the torque limit value. A set holding pressure will be added. In this way, conventional electric injection molding machines control holding pressure by limiting the output torque of the injection servo motor.

0004

[Problems to be Solved by the Invention] In general, the holding pressure is controlled by dividing the holding pressure into several stages. Therefore, when pressure holding control is performed by limiting the output torque of the servo motor for injection as described above, the pressure holding is performed with a balance between the torque output by the servo motor and the resin pressure. If the pressure changes from a large value to a small value,
This balance is disrupted and the resin pressure is greater than the output torque of the servo motor, causing the screw to blow off and retreat, resulting in a problem that proper pressure holding control cannot be performed.

Furthermore, even when changing the holding pressure from a small value to a large value, the output torque of the servo motor increases, but the screw only moves at a predetermined speed according to the holding pressure speed command, and the new setting There is a time delay in reaching the holding pressure, and only holding pressure control with poor responsiveness is possible.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a holding pressure control method in an electric injection molding machine that has good responsiveness and can perform appropriate holding pressure control even when controlling holding pressure in multiple stages. There is a particular thing.

[0007]

[Means for solving the problem] Detect the pressure applied to the resin, find the difference between the set holding pressure and the detected pressure, and issue a movement command according to the difference to the servo circuit that drives and controls the injection servo motor. The above problem was solved by feedback controlling the pressure applied to the resin by outputting it to a set pressure.

[0008]

[Operation] When entering the pressure holding process, the detected pressure applied to the resin and the set holding force are compared, a movement command is issued according to the difference, and the screw moves to the target position according to the movement command. For example, if the detected pressure does not reach the set holding pressure and a pressure deviation occurs, a movement command to move the screw forward according to the deviation is output, the screw moves forward, and as a result, the pressure applied to the resin increases and the set holding pressure is maintained. As the pressure approaches, the smaller the pressure deviation, the less movement of the screw, and eventually the pressure applied to the resin is maintained at the set holding pressure. In addition, even if the holding pressure changes from a large set holding pressure to a small set holding pressure, the above pressure deviation will be a negative value (above the target value).
Therefore, a movement command is issued to move the screw backward, and the screw moves backward. In this case, if the screw moves back more than the movement command due to the pressure of the resin, the sign of the position deviation will reverse and the screw will move forward and maintain the position specified by the movement command. The resin pressure moves to the commanded position and the resin pressure decreases due to this movement, so that the resin pressure approaches the set holding pressure, and feedback control is performed so that the pressure deviation becomes "0". Furthermore, even if the set holding pressure changes from a small value to a large value, the screw moves forward with a movement command according to the pressure deviation, but in this case, the output torque of the injection servo motor that drives the screw is limited. Therefore, it generates a large amount of torque and quickly reaches the target position. Then, the screw position is controlled so that the pressure deviation becomes "0".

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a block diagram of the main parts of an electric injection molding machine embodying the present invention. Only the injection shaft is shown, and other mold clamping shafts, screw rotation shafts, etc. are not shown. 1 is a screw, 2 is an injection servo motor that moves the screw 1 in the axial direction and injects resin, 3 is a pulse coder attached to the injection servo motor 2, 4 is a heating cylinder, 5 is a mold, and 6 is an injection The pressure applied to the resin is detected by a pressure sensor such as a load cell attached to a part of an injection mechanism (not shown) driven by a servo motor 2. Further, 7 is an analog/digital converter (hereinafter referred to as an A/D converter) that converts the analog output of the pressure sensor 6 into a digital signal.

Further, 20 is a numerical control device (hereinafter referred to as an NC device) for controlling the injection molding machine, and the NC device 20 includes an NC processor (hereinafter referred to as a CPU) 21,
Programmable machine controller (hereinafter referred to as PMC)
The NC CPU 21 is connected to a ROM 24 storing a control program, a RAM 25 used for temporary storage of data, and a servo circuit 27 via a servo interface 26 via a bus 36. There is. The PMC CPU 22 also includes a ROM 28 that stores a sequence program, and a RAM that receives and stores the detected pressure from the pressure sensor 6 via the A/D converter 7.
29, and RAM 30 used for temporary storage of data, etc.
are bus-coupled. 23 is a bus arbiter controller (hereinafter referred to as BAC), which includes a nonvolatile shared RAM 31 that stores NC programs and various setting values, and an input circuit 3.
2. Output circuit 33 and the above NC CPU 21, PMC
The CPU 22 is connected via a bus 36 and controls the bus used by the BAC 23. Also, the BA
A CRT/MDI (manual data input device with display device) 3 is connected to the C23 via an operator panel controller 34.
5 is connected, and the servo circuit 27 is connected to the injection servo motor 2.
, connected to the pulse coder 3.

With the above configuration, the NC device 20
The C CPU 22 performs sequence control based on the sequence program stored in the ROM 28, and the NC CPU 21 distributes pulses to the servo circuit of each servo motor based on the control program set and stored in the shared RAM 31 for injection. This is what drives the molding machine.

The resin pressure detected by the pressure sensor 6 is determined by the A/D
The data is converted into a digital signal by a converter and written into the RAM 29 at predetermined intervals, and is successively rewritten as the current resin pressure. In addition, the PMC CPU 22 reads the RAM every predetermined period.
The resin pressure written in 29 is read out and sequentially written to the shared RAM 31 via the BAC 23. Note that the detected resin pressure to be stored can be stored in a register, since it is sufficient to store the current value.

Next, the pressure holding control operation will be explained. First, when setting molding conditions, conditions for the holding pressure step are set. FIG. 3 is an explanatory diagram of the holding pressure condition table TB provided in the shared RAM 31, in which the CRT/MDI 35 is operated to change the CRT screen to the holding pressure condition setting screen, and the number of holding pressure stages I
, the holding pressure P of each stage, and the holding pressure time T of each stage are set and stored in the table TB provided in the shared RAM 31 as shown in FIG. Note that the pointer i is only shown in FIG. 3 for convenience of explanation.

FIG. 1 is a flowchart showing the processing of the pressure holding process of this embodiment. When the injection speed control ends and pressure holding control starts (usually after a set time has elapsed from the start of injection, or depending on the screw position) control start is determined),
The PMC CPU sends pressure holding control commands to BAC23 and shared RA.
Output to NC CPU21 via M31,
Upon receiving the pressure holding control command, U21 executes the process shown in the flowchart in FIG. 1 at predetermined intervals.

[0015] First, the NC CPU 21 sets the flag C to "0".
” (step S1). Note that the flag C
is initially set to "0". If “0”, the above table TB in the shared RAM 31
Pointer i (This pointer i is also initially set as "
At the same time, the actual pressure which is the current resin pressure stored in the shared RAM 31 is read out (step S2), and the actual pressure is subtracted from the set holding pressure Pi. Then, the pressure deviation εp is determined (step S3). Proportional based on this pressure deviation εp,
Performs integral and differential (PID) control. That is, the value obtained by multiplying the above pressure deviation εp by a proportionality constant, the value obtained by integrating the pressure deviation εp detected for each cycle by an integral constant,
Then, a value obtained by multiplying the difference between the pressure deviation εp between the current period and the previous period by a differential constant is added, PID calculation processing is performed, and the sign of the obtained value is converted (steps S4, S5). This sign conversion is performed because the position of the tip of the cylinder is the origin "0" and the direction in which the screw moves toward this origin is the negative direction, so the pressure deviation εp is positive and the actual pressure is set. When the holding force is not reached, it is necessary to move the screw forward (in the direction of the origin), so a sign conversion is performed. If the pressure deviation εp is obtained as (actual pressure-Pi), the process of step S5 is not necessary.

Next, the amount of pulses to be output as a movement command is calculated based on the value obtained by converting the sign of the value obtained by the PID calculation process. Since the pressure deviation εp and the screw position are in a proportional relationship, the output pulse amount is determined by multiplying the sign-converted value obtained by the PID calculation process by a predetermined proportionality constant (step S6). The already output pulse amount is added to the obtained output pulse amount, and it is determined whether or not the screw can be moved (step S7).
). In other words, if the screw moves beyond the movable range (from the tip of the cylinder to the maximum retraction position of the screw), there is a risk of damaging the screw or cylinder, so if the screw moves beyond this range,
If movement is possible without outputting a movement command, the calculated pulse amount is outputted to the servo circuit 27 via the SSU 26 (step S8).

[0017] In addition, when the pressure deviation εp is large and as a result the output pulse amount is large and exceeds the movement range of the screw, it is normal for the screw to follow the movement command with a delay, and the position deviation is Due to this positional deviation, the torque command is sequentially increased by the integrator of the speed loop control of the servo circuit 27, and the output torque of the servo motor is increased, so even if the pulse amount calculated in step S6 is not output, the resin The pressure approaches the set holding pressure.

Next, it is determined whether flag C is "0" or not (step S9), and if it is "0" (initially "0"), table T
The pressure holding time Ti indicated by the pointer i is read from B and set in the timer TE and started, the flag C is set to "1" (steps S10 to S12), and the processing of the corresponding cycle is ended. Incidentally, when the flag is "1" in step S9, the processing in the corresponding cycle is ended without performing the processing in steps S10 to S12.

This is because flag C is set to "1" in the next cycle. The process moves from step S1 to step S13, and it is determined whether or not the timer TE has timed out. If the timer TE has not timed up, since the flag C is "1", the processes of steps S2 to S9 described above are performed and Terminates the process. Thereafter, the above processing will be performed every cycle. As a result, the position of the screw is controlled so that the holding pressure Pi is set for the holding pressure time Ti set in the holding pressure stage corresponding to the pointer i.

Thus, when the timer TE times out (step S13), the pointer i is incremented by "1" (step S14), and the flag is set to "0" (
Step S15), if the pointer i has not reached the set number of holding pressure stages I (step S16), step S2 described above
Execute the following processing. In this case, pointer i is "1"
Since it has been incremented, the set holding pressure Pi of the next stage is read out in step S2, and the flag C is "
Since the timer TE is set to 0, the pressure holding time Ti of the next stage is set in the timer TE in steps S10 and S11. Thereafter, the screw position will be controlled to maintain the set holding pressure Pi until the timer TE times out every cycle. Then, when the timer TE times out, the pointer i is incremented as described above, and the screw position is sequentially controlled for the holding pressure time set for each stage so as to reach the set holding pressure for each stage. When the value of the pointer i reaches the set pressure holding stage number I (step S16), the pointer i is set to "0" (step S17), and the pressure holding process is ended.

As described above, in this embodiment, the screw position is controlled so that the set holding pressure of each stage is achieved.
Even when the setting holding pressure changes from a large holding pressure to a smaller holding pressure when switching the holding pressure stages, the screw will not be blown away by the resin pressure and will not move back significantly. That is, when the set holding pressure changes from a large value to a small value, the pressure deviation εp calculated in step S3 becomes a negative value, and the result of the PID calculation process also becomes a negative value. This is converted into a positive value and outputs a pulse amount that moves the screw in the backward direction.Moreover, since the servo motor has no torque limit, if the screw moves further backward than this output pulse amount, , the servo motor outputs output torque to position the screw so that a negative position deviation occurs and the screw position reaches a position corresponding to the output pulse amount, and the screw does not move back significantly.

Furthermore, even when the set holding pressure changes from a small value to a large value, a positive pressure deviation εp occurs, a pulse amount corresponding to this pressure deviation εp is output, and the servo motor has a torque limit. Since it is possible to output a large output torque, the screw advances so that the resin pressure immediately reaches the set holding pressure, and the screw position is controlled.

In the above embodiment, the PMC CPU 22 once reads the resin pressure detected by the pressure sensor 6,
Although the resin pressure is passed to the NC CPU 21, the resin pressure may be directly read by the NC CPU 21. In this case, even if the output of the A/D converter 7 is read through the input circuit 32, the RA
M may be connected to a bus and the output of the A/D converter may be read through this RAM. Further, the resin pressure within the mold may be detected.

Furthermore, when the servo circuit 27 is configured with a software servo, the CPU of the software servo
Alternatively, the process shown in FIG. 1 may be performed. In this case, the process in step S8 adds the output pulse calculated in step S6 to the positional deviation amount.

[0025] Furthermore, the switching of the set holding pressure is
Using a method equivalent to the method described in Publication No. 226319, overshoot and undershoot of the holding pressure are eliminated by switching from the holding pressure setting of the previous stage to the setting holding pressure of the relevant stage at a certain set time constant. You can do it like this.

Furthermore, as described in Japanese Patent Application Laid-Open No. 1-272432, a switching time width is set when switching the holding pressure from the previous stage to the relevant stage, and within the set time range, the holding pressure is changed from the previous stage to the relevant stage. The holding pressure is controlled to asymptotically approach the set holding pressure of the stage, and near the end of the switching time width, the change value of the command holding pressure is controlled to asymptotically approach zero, thereby preventing overshoot of holding pressure,
Undershoot may also be eliminated.

[0027]

[Effects of the Invention] In the present invention, the holding pressure is controlled by the position of the screw, so when switching the holding pressure, the screw may be blown off by the resin pressure and move back greatly, or the set holding pressure may not be reached. This eliminates the need for large delays. In other words, the servo motor operates to hold the commanded position, so if a deviation occurs between the commanded position and the actual position, it outputs an output torque (up to the maximum torque if necessary) to eliminate the deviation. However, since the commanded position is held, a pressure deviation occurs between the pressure actually applied to the resin and the set holding pressure, and if a movement command corresponding to the pressure deviation is output to the injection servo motor, the commanded position The servo motor is driven so that the screw reaches , and the screw position is controlled so that the set holding pressure is maintained, allowing for appropriate holding pressure control.

[Brief explanation of the drawing]

FIG. 1 is a processing flowchart of pressure holding control according to an embodiment of the present invention.

FIG. 2 is a block diagram of main parts of an electric injection molding machine implementing the same embodiment.

FIG. 3 is an explanatory diagram of a table that stores control conditions for a pressure holding process in the same embodiment.

[Explanation of symbols]

1 Screw 2 Injection servo motor 3 Pulse coder 4 Heating cylinder 5 Mold 6 Pressure sensor 7 A/D (analog/digital) converter 20
numerical control device

Claims (1)

[Claims] Claim 1: In a holding pressure control method for an electric injection molding machine in which a screw is driven in the axial direction by a servo motor to perform injection and holding pressure, the pressure applied to the resin is detected, and the set holding pressure and the detected pressure are determined. A method for controlling pressure in an electric injection molding machine, in which the pressure applied to the resin is feedback-controlled to a set pressure by calculating the difference between the two and outputting a movement command according to the difference to the servo circuit that drives and controls the servo motor. .