Hydraulic servo pump control injection device for injection molding machine and its control method

Abstract

The present invention discloses a hydraulic servo pump-controlled injection device for an injection molding machine and its control method, which includes a hydraulic cylinder and a unidirectional hydraulic pump. The two are connected through an electromagnetic directional valve. The servo motor and the servo driver drive the unidirectional hydraulic pump under the control of the control unit and then drive the hydraulic cylinder to act, realizing functions such as injection, plasticizing, and back pressure control of plasticizing. The injection displacement sensor and the pressure sensor participate in position and pressure closed-loop control. The hydraulic accumulator is used to ensure the oil volume balance between the oil inlet and outlet of the hydraulic cylinder between the rodless cavity and the rod cavity. The control unit performs injection position control and pressure holding control during injection, back pressure control during plasticizing, and plasticizing position control during plasticizing. The present invention can achieve injection displacement accuracy, pressure holding pressure accuracy, plasticizing displacement accuracy, and back pressure control accuracy comparable to those of all-electric injection molding machines, with low cost and high reliability. Compared with the hydraulic injection mechanism controlled by a servo valve, it has better accuracy and energy-saving effects.

Description

Technical Field

[0001] The present invention relates to a hydraulic precision transmission control device, and particularly to a hydraulic servo pump-controlled injection device for an injection molding machine and a control method thereof. Background Art

[0002] The injection action of a traditional injection molding machine usually drives an oil pump to rotate through a motor, controls the oil cylinder through a valve group to drive the oil pressure to realize the injection action, and the injection molding machine computer reads an electronic ruler installed on the injection table or other position feedback components, and controls the action of the valve block according to process requirements, so as to control the displacement stroke, pressure, etc. of the injection. Due to the long delay time of the valve group action (usually reaching dozens to hundreds of milliseconds) and poor repeatability (affected by the actual oil pressure and flow rate), it is difficult to ensure the control accuracy of the injection position, speed, and pressure in the traditional scheme, resulting in large fluctuations in the product accuracy and low qualified rate.

[0003] There are two mainstream schemes for the injection action of all-electric injection molding machines or electro-hydraulic hybrid injection molding machines:

[0004] 1. Use an accumulator + servo valve to control the action of the oil cylinder to achieve high-precision and high-response injection action.

[0005] 2. Use a motor to drive a precision ball screw or a precision gear-rack transmission mechanism, and then push the injection screw to achieve high-precision and high-response injection action.

[0006] The "accumulator + servo valve" scheme has the following deficiencies:

[0007] 1. The motor driving the hydraulic pump must run continuously without interruption to input high-pressure oil into the accumulator, resulting in large energy waste.

[0008] 2. When the servo valve acts, the high-pressure overflow method is used to control the outlet flow rate and pressure, resulting in energy waste.

[0009] 3. The servo valve mechanism is complex, expensive, and difficult to repair, resulting in a significant increase in the purchase, use, and maintenance costs of the system equipment.

[0010] 4. Limited by the working principle of the servo valve, when the injection stroke exceeds the limit, the system does not have the ability to instantly reverse and control the return stroke, resulting in unsatisfactory position control accuracy during the process.

[0011] The "precision ball screw or precision gear-rack" scheme has the following deficiencies:

[0012] 1. The precision ball screw or precision gear-rack is expensive, and its related structural components such as thrust bearings and force sensors for measuring injection thrust are also very expensive.

[0013] 2. Since the injection molding operation is a long-term repetitive action, the wear of the lead screw or the gear-rack is severe, resulting in a limited lifespan of the mechanism. Moreover, due to the overly concentrated end point of the injection molding operation, the wear of the mechanical structure will concentrate at a single point, further shortening the lifespan cycle.

[0014] 3. In the transmission mechanism connecting the mechanism to the motor, the synchronous belts and synchronous pulleys used require high strength, thus having a high cost and a short lifespan. Meanwhile, adopting a direct drive motor solution will also lead to a significant increase in the cost of the motor.

[0015] Patent ZL201710138308.6, "A Servo Pump-Controlled Hydraulic Linear Drive System and Control Method with a Single Motor and Dual Pumps", and Patent 201521053147.3, "A Servo Pump-Controlled Hydraulic Linear Drive System", respectively describe solutions that use a motor to drive a hydraulic pump and then drive a hydraulic cylinder to achieve high-precision and high-frequency response linear reciprocating motion control.

[0016] Although these solutions solve the above problems of traditional hydraulic linear motion mechanisms, they do not conduct reasonable structural planning and motion control algorithm planning in combination with the injection molding operation of the injection molding machine, making it difficult to be directly applied to the field of injection molding machines. Summary of the Invention

[0017] Aiming at the deficiencies of the prior art, the present invention proposes a hydraulic servo pump-controlled injection device for an injection molding machine and its control method. During injection or plasticizing, the push rod of the hydraulic cylinder drives the injection mechanism of the injection molding machine to complete the injection operation, plasticizing operation, and during melting, the hydraulic cylinder is used to achieve back pressure control of melting.

[0018] The hydraulic servo pump-controlled injection device for an injection molding machine according to the present invention has a technical solution including a hydraulic cylinder that drives the injection mechanism to complete the injection operation, plasticizing operation, and back pressure control of melting through a push rod. The pipelines of the inlet and outlet ports of the hydraulic cylinder are connected to the outlet and return ports of a single-direction hydraulic pump through an electromagnetic reversing valve.

[0019] The single-direction hydraulic pump is driven by a servo motor. The servo motor is connected to and controlled by a servo driver through a circuit. The servo driver is connected to and controlled by a control unit through a circuit. The control unit receives various command signals sent by the injection molding machine computer in a wired or wireless manner. The push rod of the hydraulic cylinder is provided with an injection displacement sensor that detects the injection position signal and feeds it back to the control unit. A pressure sensor that feeds back a pressure signal to the control unit is provided on the pipeline of the outlet port of the single-direction hydraulic pump.

[0020] In the above structure, the computer of the injection molding machine downloads all the parameters related to motion control to the control unit. When injection is required, the computer of the injection molding machine sends an injection start instruction to the control unit, and the control unit starts the injection action. After the injection action is completed, the control unit returns an injection completion signal to the computer of the injection molding machine. When plastic extraction is required, the computer of the injection molding machine sends a plastic extraction start instruction to the control unit, and the control unit starts the plastic extraction action. After the plastic extraction action is completed, the control unit returns a plastic extraction completion signal to the computer of the injection molding machine. When plastic melting is required, the computer of the injection molding machine sends a plastic melting start instruction to the control unit, and the control unit starts the back pressure control action for plastic melting (the plastic melting action itself is completed by other mechanisms, and the present invention only provides the back pressure control during the plastic melting process). After the back pressure control for plastic melting is completed, the control unit returns a plastic melting completion signal to the computer of the injection molding machine.

[0021] To prevent oil leakage in the pipeline and the change in the amount of oil caused by the flow of hydraulic oil between the rod chamber and the non-rod chamber of the hydraulic cylinder during various operations, a hydraulic accumulator is provided on the pipeline at the oil return port of the unidirectional hydraulic pump.

[0022] To avoid overloading of the hydraulic pressure in the hydraulic cylinder, safety valves Ⅰ and Ⅱ are respectively connected in parallel in the forward and reverse directions on the pipelines at the inlet and outlet ports of the hydraulic cylinder.

[0023] The method for completing the injection action, plastic extraction action, and back pressure control for plastic melting using the hydraulic servo pump control injection device for an injection molding machine according to the present invention is as follows

[0024] 1. Injection action: After the control unit receives the injection action start instruction sent by the computer of the injection molding machine, it controls the electromagnetic directional valve to switch to the forward injection position. The hydraulic oil output by the unidirectional hydraulic pump enters the non-rod chamber of the hydraulic cylinder, and the oil returned from the rod chamber of the hydraulic cylinder flows through the pipeline to the oil return port of the oil pump. The insufficient part of the oil volume is provided by the hydraulic accumulator. The control unit plans the motor target position setting θ in each control period according to the injection multi-stage speed curve pre-sent by the computer of the injection molding machine, combined with the parameters of the hydraulic cylinder diameter and the displacement of the unidirectional hydraulic pump, according to the position control algorithm n , target angular velocity setting ω n , target angular acceleration setting α n , target jerk setting α n ’, and target jerk differential setting α n ” and then sends them to the servo driver. The servo driver controls the servo motor to drive the unidirectional hydraulic pump, and the unidirectional hydraulic pump drives the hydraulic cylinder to push the injection mechanism of the injection molding machine. The injection displacement sensor feeds back the injection position signal to the control unit to achieve closed-loop control of injection.

[0025] When the injection action enters the stage of changing from injection to holding pressure, the control unit performs a closed-loop control operation based on the actual pressure fed back by the pressure sensor and the preset holding pressure, sends a speed setting value or a torque setting value to the servo driver, controls the servo motor, and then controls the rotational speed of the unidirectional hydraulic pump to achieve the purpose of holding pressure.

[0026] After completing the injection and holding pressure actions, the control unit sends an injection completion signal to the injection molding machine computer.

[0027] 2. Withdrawal action: After receiving the withdrawal action start instruction sent by the injection molding machine computer, the control unit controls the electromagnetic directional valve to switch to the reverse withdrawal position. The hydraulic oil output by the unidirectional hydraulic pump enters the rod chamber of the hydraulic cylinder. Part of the oil returning from the rodless chamber of the hydraulic cylinder enters the oil return port of the unidirectional hydraulic pump, and part enters the hydraulic accumulator. The control unit, based on the withdrawal displacement instruction previously sent by the injection molding machine computer, combines the parameters of the hydraulic cylinder diameter and the displacement of the unidirectional hydraulic pump, and plans the motor target position setting θ n 、target angular velocity setting ω n 、target angular acceleration setting α n 、target jerk setting α n ’ and target jerk differential setting α n ” and then sends them to the servo driver. The servo driver controls the servo motor to drive the unidirectional hydraulic pump. The unidirectional hydraulic pump drives the hydraulic cylinder to push the injection mechanism of the injection molding machine to retreat for withdrawal. The injection displacement sensor feeds back the withdrawal position to the control unit to achieve closed-loop control of withdrawal.

[0028] After completing the withdrawal operation, the control unit sends a withdrawal completion signal to the injection molding machine computer.

[0029] 3. Melting back pressure control: After receiving the melting action start instruction sent by the injection molding machine computer, the control unit controls the electromagnetic directional valve to the forward melting position. The hydraulic oil output by the unidirectional hydraulic pump enters the rodless chamber of the hydraulic cylinder. The oil returning from the rod chamber of the hydraulic cylinder enters the hydraulic accumulator. The control unit performs a pressure closed-loop operation based on the melting back pressure setting value and the feedback value of the pressure sensor, and sends a speed setting to the servo driver, thereby determining the rotational speed of the servo motor. When the melting mechanism drives the screw of the injection molding machine to melt the plastic, the screw will push the molten plastic forward, and the reaction force will push the injection mechanism and then push the push rod of the hydraulic cylinder towards the rodless chamber direction, which will cause the pressure in the rodless chamber, that is, the melting back pressure, to increase. When the control unit detects the increase in the pressure in the rodless chamber through the pressure sensor, it sends a rotational speed setting to the servo driver according to the closed-loop control algorithm, driving the servo motor and the unidirectional hydraulic pump to rotate forward or backward, thereby releasing the pressure through the internal leakage of the pump or reverse rotation, and making the back pressure always equal to the set back pressure value during the melting process.

[0030] In the melt pressure control, if the set value of the back pressure is relatively large or the internal leakage of the pump is relatively large, the servo motor drives the unidirectional hydraulic pump to rotate forward at a low speed; if the set value of the back pressure is relatively small or the internal leakage of the pump is relatively small, the servo motor drives the unidirectional hydraulic pump to operate in the state of forward output but reverse rotation.

[0031] To compensate for the displacement error of the oil pump caused by the internal leakage of the hydraulic pipeline under different pressures, the servo driver controls the servo motor to add compensation for the internal leakage of the hydraulic oil. When the pressure sensor feeds back the pressure as P n , the servo driver automatically adds an angular velocity increment δω n = K×P n to the speed loop, where K is the internal leakage compensation coefficient related to the condition of the system pipeline.

[0032] To measure the internal leakage compensation coefficient K, an automatic measurement program can be written in the control unit. Push the injection mechanism to the mechanical limit position, start the pressure holding control, set different pressure holding pressures, measure the pressure holding speed of the servo motor under the corresponding pressure holding pressures, construct a table, and after measuring the actual pressure during normal control, perform simple table lookup and interpolation operations on the above table to obtain the real-time compensation speed of the servo motor.

[0033] The hydraulic accumulator is pre-applied with an internal pressure P0. When the hydraulic oil is transported from the rodless cavity side to the rodless cavity side under the drive of the unidirectional hydraulic pump, the hydraulic oil inside the hydraulic accumulator flows into the oil circuit to make up for the shortage part compared with the oil discharged from the rod cavity and the oil flowing into the rodless cavity; when the hydraulic oil is transported from the rodless cavity side to the rod cavity side under the drive of the unidirectional hydraulic pump, in addition to a part of the oil discharged from the rodless cavity flowing into the rod cavity, the excess oil flows into the hydraulic accumulator.

[0034] When melting the plastic, the screw mechanism of the injection molding machine will push the injection mechanism backward. To ensure the consistency of the plastic melting amount, on the one hand, the control unit should control the rotation of the unidirectional hydraulic pump according to the pressure setting and pressure feedback, and on the other hand, it should participate in the plastic melting speed control to achieve the position control of the end point of the injection mechanism backward movement during plastic melting. When the injection displacement sensor measures that the plastic melting displacement reaches the target position, the control unit controls the external plastic melting speed to zero and sends a plastic melting completion signal to the injection molding machine computer.

[0035] Advantages of the present invention:

[0036] 1. When implementing the present invention, the control precision of the injection position and pressure can reach the same level as that of the all-electric injection molding machine, the control precision of the plastic withdrawal action position can reach the same level as that of the all-electric injection molding machine, and the control precision of the melt pressure can reach the same level or even higher than that of the all-electric injection molding machine.

[0037] 2. When combined with an external plasticizing mechanism, the present invention can precisely control the plasticizing end point, thus achieving the plasticizing control accuracy of all-electric injection molding machines.

[0038] 3. In the structure of the present invention, there is no problem of local wear of ball screws or precision gear transmission mechanisms, greatly enhancing the reliability.

[0039] 4. In the structure of the present invention, expensive components such as synchronous belts, heavy-duty ball screws, precision sensors, and heavy-duty thrust bearings are eliminated, greatly reducing the cost.

[0040] 5. Compared with the existing servo valve-controlled injection and plasticizing technology, the present invention not only improves the accuracy of various actions but also has a higher energy-saving effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic structural diagram of an embodiment of the present invention.

[0042] Reference numerals: 1. Hydraulic cylinder; 1-1. Push rod; 2. Electromagnetic directional valve; 3. Unidirectional hydraulic pump; 4. Pressure sensor; 5. Servo motor; 6. Servo driver; 7. Control unit; 8. Injection displacement sensor; 9. Hydraulic accumulator; 10. Safety valve I; 11. Safety valve II. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The technical solution of the present invention will be further described below in conjunction with the embodiments shown in the drawings.

[0044] The hydraulic servo pump-controlled injection device for an injection molding machine according to the present invention includes a hydraulic cylinder 1, an electromagnetic directional valve 2, a unidirectional hydraulic pump 3, a servo motor 5, and a hydraulic accumulator 9.

[0045] The push rod 1-1 of the hydraulic cylinder 1 is located in the rod chamber on the left side of the cylinder block, and the right side of the cylinder block is the rodless chamber. An injection displacement sensor 8 for detecting the injection displacement signal of the push rod 1-1 is provided corresponding to the push rod 1-1; the oil inlet of the rodless chamber of the hydraulic cylinder 1 is connected to the normally closed A port in the middle position of the electromagnetic directional valve 2 (connected to the relay for controlling the commutation action through a circuit), the oil outlet of the rod chamber of the hydraulic cylinder 1 is connected to the normally closed B port in the middle position of the electromagnetic directional valve 2, the oil outlet of the unidirectional hydraulic pump 3 is connected to the normally closed P port in the middle position of the electromagnetic directional valve 2, the oil inlet and outlet of the hydraulic accumulator 9 are connected to the normally closed T port in the intermittently open position in the middle of the electromagnetic directional valve 2, the oil return port of the unidirectional hydraulic pump 3 is communicated with the pipeline IV through a pipeline V, and a pressure sensor 4 is provided on the pipeline III, as Figure 1 shown.

[0046] The function of the hydraulic accumulator 9 is to prevent oil leakage from the pipeline and to compensate for the change in the amount of hydraulic oil flowing between the rod chamber and the non-rod chamber of the hydraulic cylinder 1 when performing various actions. The hydraulic accumulator 9 is pre-applied with a certain internal pressure P0. When the hydraulic oil is driven by the one-way hydraulic pump 3 and is transported from the rod chamber side to the non-rod chamber side, the hydraulic oil inside the hydraulic accumulator 9 flows into the oil circuit to make up for the shortage of the amount of oil discharged from the rod chamber compared with the amount of oil flowing into the non-rod chamber. When the hydraulic oil is driven by the one-way hydraulic pump 3 and is transported from the non-rod chamber side to the rod chamber side, in addition to a part of the oil discharged from the non-rod chamber flowing into the rod chamber, the excess amount of oil flows into the hydraulic accumulator 9.

[0047] The drive of the one-way hydraulic pump 3 is a servo motor 5. The output shaft of the servo motor 5 is connected to the rotating shaft of the one-way hydraulic pump 3 through a coupling. The operation of the servo motor 5 is controlled by a servo driver 6. The servo driver 6 receives the instructions issued by the control unit 7, and the control unit 7 receives various instruction signals sent by the injection molding machine computer through wired or wireless means (communication means or I / O ports), such as Figure 1 shown.

[0048] The injection displacement sensor 8 feeds back the injection displacement signal to the control unit 7, and the pressure sensor 4 feeds back the pressure signal to the control unit 7, such as Figure 1 shown.

[0049] A safety valve I is connected in parallel in the forward direction and a safety valve II is connected in parallel in the reverse direction between the pipeline I and the pipeline II, such as Figure 1 shown.

[0050] When the injection molding machine is powered on, the injection molding machine computer sends the injection-related instructions stored in the memory, such as the customer-set data of multi-stage injection speed, injection displacement / time / pressure for each stage, injection end point, injection transfer to holding pressure time or displacement, holding pressure, holding time, etc., to the control unit 7. The injection molding machine computer also sends the withdrawal-related instructions (such as forward withdrawal displacement, backward withdrawal displacement) and the plasticizing-related instructions (such as back pressure pressure setting, back pressure speed setting, back pressure time / displacement setting, etc.) stored in the memory to the control unit 7; when the user modifies the parameters of the injection molding machine computer, the injection molding machine computer also downloads the modified content to the control unit 7 for storage while the user confirms the modification.

[0051] When the injection molding machine meets the operating conditions of injection, withdrawal or plasticizing, the injection molding machine computer can notify the control unit 7 to start the injection action, withdrawal action and plasticizing back pressure control by sending port level signals or communication signals; similarly, after the control unit 7 completes the above actions, it can inform the injection molding machine computer that the corresponding actions have been completed by sending port level signals or communication signals.

[0052] The injection control method of the present invention is as follows:

[0053] 1. Injection action

[0054] After the control unit 7 receives the injection start instruction sent by the injection molding machine computer, it first controls the electromagnetic directional control valve 2 to switch from the intermediate cut-off position to the forward injection position on the left side, that is, to deliver the hydraulic oil output by the unidirectional hydraulic pump 3 to the rodless cavity of the hydraulic cylinder 1, and at the same time deliver the return oil of the rod chamber of the hydraulic cylinder 1 to the oil return port of the unidirectional hydraulic pump 3. The insufficient part of the oil volume is provided by the hydraulic accumulator 9; then the control unit 7 plans the motor target position setting θ in each control cycle according to the injection multi-stage speed curve pre-sent by the injection molding machine computer, combined with the diameter of the hydraulic cylinder 1 and the displacement parameter of the unidirectional hydraulic pump 3, according to the position control algorithm n 、target angular velocity setting ω n 、target angular acceleration setting α n 、target jerk setting α n ’ and target jerk differential setting α n ” (depending on the control algorithm, all or part of the above five variables can be obtained to participate in the control) and sent to the servo driver 6. The servo driver 6 controls the servo motor 5 to drive the unidirectional hydraulic pump 3. The unidirectional hydraulic pump 3 drives the hydraulic cylinder 1 to push the injection mechanism of the injection molding machine. The injection displacement sensor 8 feeds back the injection position to the control unit 7 to achieve closed-loop injection control.

[0055] When the injection action enters the injection-to-holding pressure stage, the control unit 7 performs closed-loop control operation according to the actual pressure fed back by the pressure sensor 4 and the preset holding pressure, and sends a speed setting value or a torque setting value to the servo driver 6 to control the rotation speed of the servo motor 5 to control the unidirectional hydraulic pump 3 to achieve the purpose of holding pressure.

[0056] In order to make up for the oil pump displacement error caused by the internal leakage of the hydraulic pipeline under different pressures, the servo driver 6 controls the servo motor 5 to add internal leakage compensation of the hydraulic oil. When the pressure fed back by the pressure sensor 4 is P n ,the servo driver 6 automatically adds an angular velocity increment δω to the speed loop n =K×P n ,where K is the internal leakage compensation coefficient related to the system oil circuit condition.

[0057] In order to measure the internal leakage compensation coefficient K, an automatic measurement program can be written in the control unit 7, that is: push the injection mechanism to the mechanical limit position, start the holding pressure control, set different holding pressures, measure the holding pressure speed of the servo motor under the corresponding holding pressure, and construct a table. When the actual pressure is measured during normal control, the above table can be simply looked up and interpolated to obtain the real-time servo motor compensation speed.

[0058] When the injection and holding pressure actions are completed, the control unit 7 sends an injection completion signal to the injection molding machine computer.

[0059] II. Plastic Withdrawal Action

[0060] After the control unit 7 receives the plastic withdrawal action start instruction sent by the injection molding machine computer, it first controls the electromagnetic directional control valve 2 to switch from the forward injection position to the reverse plastic withdrawal position on the right side, that is, to deliver the hydraulic oil output by the unidirectional hydraulic pump 3 to the rod chamber of the hydraulic cylinder 1, and at the same time deliver a part of the oil returning from the rodless chamber of the hydraulic cylinder 1 to the hydraulic accumulator 9 and a part to the oil return port of the unidirectional hydraulic pump 3; then the control unit 7, according to the plastic withdrawal displacement instruction pre-sent by the injection molding machine computer, combines the diameter of the hydraulic cylinder 1 and the displacement parameter of the unidirectional hydraulic pump 3, and plans the motor target position setting θ n 、target angular velocity setting ω n 、target angular acceleration setting α n 、target jerk setting α n ’ and the differential component setting α n ” (depending on the control algorithm, all or part of the above five variables can be obtained for control) of the target jerk differential component, and then sends them to the servo driver 6. The servo driver 6 controls the servo motor 5 to drive the unidirectional hydraulic pump 3, and the unidirectional hydraulic pump 3 drives the hydraulic cylinder 1 to push the injection mechanism of the injection molding machine to retreat for plastic withdrawal. The injection displacement sensor 8 feeds back the plastic withdrawal position to the control unit 7 to achieve the closed-loop control of plastic withdrawal.

[0061] When the plastic withdrawal action is completed, the control unit 7 sends a plastic withdrawal completion signal to the injection molding machine computer.

[0062] III. Plastic Melting and Back Pressure Control

[0063] After the control unit 7 receives the plastic melting action start instruction sent by the injection molding machine computer, it controls the electromagnetic directional control valve 2 to switch from the reverse plastic withdrawal position to the forward plastic melting position (i.e., the forward injection position), that is, to deliver the hydraulic oil output by the unidirectional hydraulic pump 3 to the rodless chamber of the hydraulic cylinder 1, and at the same time deliver the oil returning from the rod chamber of the hydraulic cylinder 1 to the hydraulic accumulator 9. The control unit 7 performs a pressure closed-loop operation based on the plastic melting back pressure set value and the feedback value of the pressure sensor 4, and sends a speed setting to the servo driver 6, thereby determining the rotation speed of the servo motor 5. When the plastic melting device (external mechanism) drives the screw of the injection molding machine to melt the plastic, the screw will push the molten plastic forward, and thus the reaction force will push the injection mechanism and then push the push rod 1-1 of the hydraulic cylinder 1 to move towards the rodless chamber direction, which will cause the pressure in the rodless chamber (i.e., the back pressure of plastic melting) to increase. When the control unit 7 detects the increase in the pressure in the rodless chamber through the pressure sensor 4, it outputs a speed setting to the servo driver 6 according to the closed-loop control algorithm, driving the servo motor 5 and the unidirectional hydraulic pump 3 to reverse (if the back pressure set value is relatively large, or the internal leakage of the oil pump is relatively large, it may also be in the low-speed forward rotation state), thereby releasing the pressure, and during the plastic melting process, making the back pressure always equal to the set back pressure value.

[0064] When melting the plastic, the screw mechanism of the injection molding machine will push the injection mechanism backward. To ensure the consistency of the plasticizing amount, on the one hand, the control unit 7 needs to control the rotation of the unidirectional hydraulic pump 3 according to the pressure setting and pressure feedback. On the other hand, it participates in the plasticizing speed control to achieve the position control of the end point of the injection mechanism's backward movement during plasticizing (the specific scheme may vary with the external plasticizing control mechanism). When the injection displacement sensor 8 measures that the plasticizing displacement reaches the target position, the control unit 7 controls the external plasticizing speed to zero and sends a plasticizing completion signal to the injection molding machine computer.

Claims

1. A hydraulic servo pump control method for injection molding machines, characterized in that A hydraulic servo pump-controlled injection device for an injection molding machine is adopted. The hydraulic servo pump-controlled injection device for the injection molding machine includes a hydraulic cylinder (1) that drives an injection mechanism to complete injection, plasticizing, and melt-back pressure control actions through a push rod (1-1). The pipelines of the inlet and outlet ports of the hydraulic cylinder (1) are connected to the outlet and return ports of a unidirectional hydraulic pump (3) through an electromagnetic directional valve (2). The unidirectional hydraulic pump (3) is driven by a servo motor (5). The servo motor (5) is connected to and controlled by a servo driver (6) through a circuit. The servo driver (6) is connected to and controlled by a control unit (7) through a circuit. The control unit (7) receives various command signals sent by the injection molding machine computer in a wired or wireless manner. A injection displacement sensor (8) for detecting the injection position signal of the push rod (1-1) of the hydraulic cylinder (1) and feeding it back to the control unit (7) is provided on the push rod (1-1) of the hydraulic cylinder (1). A pressure sensor (4) for feeding back a pressure signal to the control unit (7) is provided on the pipeline of the outlet port of the unidirectional hydraulic pump (3). A hydraulic accumulator (9) is provided on the pipeline of the return port of the unidirectional hydraulic pump (3). Safety valves I (10) and II (11) are respectively connected in parallel in the forward and reverse directions on the pipelines of the inlet and outlet ports of the hydraulic cylinder (1). The schemes for completing injection, plasticizing, and melt-back pressure control actions are as follows: ①. Injection action After the control unit (7) receives the injection start instruction sent by the injection molding machine computer, it controls the electromagnetic directional valve (2) to the forward injection position. The hydraulic oil output by the unidirectional hydraulic pump (3) enters the rodless cavity of the hydraulic cylinder (1). The oil returning from the rod cavity of the hydraulic cylinder (1) flows through the pipeline to the oil return port of the unidirectional hydraulic pump (3). The insufficient part of the oil volume is provided by the hydraulic accumulator (9). The control unit (7) plans the motor target position setting θ within each control cycle according to the injection multi-stage speed curve pre-sent by the injection molding machine computer, combined with the diameter of the hydraulic cylinder (1) and the displacement parameter of the unidirectional hydraulic pump (3), according to the position control algorithm n , the target angular velocity setting ω n , the target angular acceleration setting α n , the target jerk setting α n ’ and the differential component setting α of the target jerk n ” and then sends them to the servo driver (6). The servo driver (6) controls the servo motor (5) to drive the unidirectional hydraulic pump (3). The unidirectional hydraulic pump (3) drives the hydraulic cylinder (1) to push the injection mechanism of the injection molding machine. The injection displacement sensor (8) feeds back the injection position signal to the control unit (7) to realize the injection closed-loop control; When the injection action enters the injection-to-holding pressure stage, the control unit (7) performs a closed-loop control operation based on the actual pressure fed back by the pressure sensor (4) and the preset holding pressure, sends a speed set value or a torque set value to the servo driver (6), controls the servo motor (5), and then controls the rotation speed of the unidirectional hydraulic pump (3) to achieve the purpose of holding pressure; After completing the injection and holding pressure actions, the control unit (7) sends an injection completion signal to the injection molding machine computer; ②. Plasticizing action After the control unit (7) receives the instruction to start the plastic extraction action sent by the injection molding machine computer, it controls the electromagnetic directional valve (2) to the reverse plastic extraction position. The hydraulic oil output by the unidirectional hydraulic pump (3) enters the rod chamber of the hydraulic cylinder (1). Part of the oil returning from the rodless chamber of the hydraulic cylinder (1) enters the oil return port of the unidirectional hydraulic pump (3), and part enters the hydraulic accumulator (9). The control unit (7) plans the motor target position setting θ in each control cycle according to the plastic extraction displacement instruction pre-sent by the injection molding machine computer, combined with the diameter of the hydraulic cylinder (1) and the displacement parameter of the unidirectional hydraulic pump (3), according to the position control algorithm. n , the target angular velocity setting ω n , the target angular acceleration setting α n , the target jerk setting α n ’ and the target jerk differential setting α n ” and then sends them to the servo driver (6). The servo driver (6) controls the servo motor (5) to drive the unidirectional hydraulic pump (3). The unidirectional hydraulic pump (3) drives the hydraulic cylinder (1) to push the injection mechanism of the injection molding machine to retreat for plastic extraction. The injection displacement sensor (8) feeds back the plastic extraction position to the control unit (7) to achieve the closed-loop control of plastic extraction; After completing the plasticizing operation, the control unit (7) sends a plasticizing completion signal to the injection molding machine computer; ③. Melt-back pressure control After the control unit (7) receives the instruction to start the plasticizing operation sent by the injection molding machine computer, it controls the electromagnetic directional valve (2) to the forward plasticizing position. The hydraulic oil output by the unidirectional hydraulic pump (3) enters the rodless cavity of the hydraulic cylinder (1), and the oil returning from the rod cavity of the hydraulic cylinder (1) enters the hydraulic accumulator (9). The control unit (7) performs a pressure closed-loop operation based on the plasticizing back pressure set value and the feedback value of the pressure sensor (4), and sends a speed setting to the servo driver (6), thereby determining the rotation speed of the servo motor (5). When the plasticizing mechanism drives the screw of the injection molding machine to perform plasticizing, the screw will push the molten rubber material forward, and thus the reaction force will push the injection mechanism and then push the push rod (1-1) of the hydraulic cylinder (1) to move towards the rodless cavity direction. This will cause the pressure in the rodless cavity, that is, the back pressure of plasticizing, to increase. When the control unit (7) detects the increase in the pressure of the rodless cavity through the pressure sensor (4), it sends a rotation speed setting to the servo driver (6) according to the closed-loop control algorithm, driving the servo motor (5) and the unidirectional hydraulic pump (3) to rotate forward or backward, thereby releasing pressure through the internal leakage or reverse rotation of the pump, and making the back pressure always equal to the set back pressure value during the plasticizing process; If the back pressure setting value is relatively large, or the internal leakage of the pump is relatively large, the servo motor (5) drives the unidirectional hydraulic pump (3) to be in a low-speed forward rotation state; if the back pressure setting value is relatively small, or the internal leakage of the pump is relatively small, the servo motor (5) drives the unidirectional hydraulic pump (3) to operate in a state of forward output but reverse rotation.

2. The hydraulic servo pump control injection method for an injection molding machine according to claim 1, characterized in that: To compensate for the displacement error of the oil pump caused by the internal leakage of the hydraulic pipeline under different pressures, the servo driver (6) controls the servo motor (5) to add internal leakage compensation for the hydraulic oil. When the pressure sensor (4) feeds back the pressure as P n , the servo driver (6) automatically adds an angular velocity increment δω to the speed loop n = K × P n , where K is the internal leakage compensation coefficient related to the pipeline condition of the system.

3. The hydraulic servo pump control injection method for an injection molding machine according to claim 2, wherein: In order to measure the internal leakage compensation coefficient K, an automatic measurement program is written in the control unit (7). The injection mechanism is pushed to the mechanical limit position, the holding pressure control is started, different holding pressures are set, and the holding pressure rotation speeds of the servo motor (5) corresponding to the measured holding pressures are measured. A table is constructed. When the actual pressure is measured during normal control, the real-time servo motor compensation rotation speed can be obtained through simple table look-up and interpolation operations on the above table.

4. The hydraulic servo pump control injection method for an injection molding machine according to claim 1, wherein: The hydraulic accumulator (9) is pre-applied with an internal pressure P0. Its function is that when the hydraulic oil is transported from the rod cavity side to the rodless cavity side under the drive of the unidirectional hydraulic pump (3), the hydraulic oil inside the hydraulic accumulator (9) flows into the oil circuit to make up for the insufficient part of the oil discharged from the rod cavity compared with the oil flowing into the rodless cavity. When the hydraulic oil is transported from the rodless cavity side to the rod cavity side under the drive of the unidirectional hydraulic pump (3), in addition to a part of the oil discharged from the rodless cavity flowing into the rod cavity, the excess oil flows into the hydraulic accumulator (9).

5. The hydraulic servo pump-controlled injection control method for an injection molding machine according to claim 1, wherein: When plasticizing, the screw mechanism will push the injection mechanism to move backward. To ensure the consistency of the plasticizing amount, on the one hand, the control unit (7) needs to control the rotation of the unidirectional hydraulic pump (3) according to the pressure setting and pressure feedback, and on the other hand, it needs to participate in the plasticizing speed control to achieve the position control of the end point of the injection mechanism retreating during plasticizing. When the injection displacement sensor (8) measures that the plasticizing displacement reaches the target position, the control unit (7) controls the external plasticizing speed to zero and sends a plasticizing completion signal to the injection molding machine computer.

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