A high-precision control system for the hydraulic straightening machine's downward pressure cylinder

Abstract

This invention discloses a high-precision control system for the downward pressing cylinder of a hydraulic straightening machine, including a hydraulic circuit, a control unit, and a control algorithm running within the control unit. The hydraulic circuit consists of two bidirectional gear pumps (low-pressure high-flow and high-pressure low-flow), a solenoid valve, a relief valve, a cylinder, and a check valve, achieving bidirectional high-speed dynamic adjustment and dead-zone-free high-precision downward pressing positioning. The control unit comprises a pressure sensor, a displacement sensor, a servo motor, a valve-controlled switch, and a real-time controller, providing an RS485 interface and a passive I / O interface to transmit and receive data such as the desired position via protocols, enabling rapid integration with control devices such as computers and PLCs. The control algorithm constructs a calculation model for the desired downward pressing speed based on the position difference and load type, and constructs calculation models for the desired motor speed and solenoid valve action based on the desired downward pressing speed, the pressure in the rodless chamber of the cylinder, and the piston movement speed, achieving overshoot-free, fast, and high-precision control of the hydraulic system.

Description

Technical Field

[0001] The invention belongs to the technical field of electro-hydraulic servo control of a hydraulic straightening machine, and in particular relates to a high-precision control system for a downward pressure cylinder of a hydraulic straightening machine. Background Art

[0002] A straightening machine is a device that straightens shaft parts by repeatedly bending and gradually reducing the bending deflection. When the straightening machine is working, the workpiece with different bending degrees is placed on the two fulcrums of the workbench and the pressing distance is set. The straightening machine quickly presses down to the specified position, keeps the pressure for a while and then withdraws the pressure head to reduce the bending amount of the workpiece. After repeated pressing, the workpiece becomes straight.

[0003] Common driving modes for the straightening machine pressure head are hydraulic drive and motor drive. The hydraulic drive has high power, but the positioning accuracy is not high and the response is slow; the motor drive has high positioning accuracy and quick response, but low power.

[0004] The hydraulic drive straightening machine control system is a nonlinear, underdamped, large inertia servo system. The system is usually controlled by PLC and uses PID algorithm to realize position closed loop. For different loads, this control method has overshoot phenomenon. For nonlinear, underdamped, large inertia servo systems, conventional PID algorithm is difficult to take into account control accuracy, control speed and control robustness at the same time. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine, which uses a hydraulic system to realize a high-power, high-precision, high-response speed, and high-adaptability straightening machine downward pressure component, and facilitates integration with the straightening machine system by providing a signal interaction interface.

[0006] A high-precision control system for a hydraulic straightening machine's downward pressure cylinder comprises a hydraulic circuit, a control unit and a control algorithm running in the control unit.

[0007] The hydraulic circuit is composed of an oil cylinder, a first solenoid valve, a second solenoid valve, a first overflow valve, a second overflow valve, a third overflow valve, a first check valve, a second check valve, a first bidirectional gear pump, a second bidirectional gear pump, and an oil tank. The bidirectional gear pump is used to inject or absorb oil into the rod chamber and the rodless chamber of the oil cylinder to control the position of the oil cylinder piston, thereby achieving rapid downward pressure and withdrawal and high-precision positioning of the straightening machine.

[0008] The control unit is composed of a pressure sensor, a pressure signal transmitter, a displacement sensor, a servo motor, a servo motor driver, a reducer, a first valve-controlled switch, a second valve-controlled switch, and a real-time controller. The real-time controller receives host computer instructions and parameters through the ModbusRTU protocol, reads pressure sensor and displacement sensor data, calculates the desired state of the servo motor and valve-controlled switch, realizes oil flow control, and thus realizes hydraulic circuit control.

[0009] The control algorithm is based on the hydraulic system transmission model, and calculates the expected pressing speed in real time according to the load type and positioning distance difference, calculates the expected oil increase speed according to the expected pressing speed, the current pressing speed, and the rodless chamber pressure, and calculates the expected motor speed and valve control switch state according to the rodless chamber pressure and the expected oil increase speed.

[0010] The first solenoid valve adopts a two-position three-way valve structure, with the P port connected to the rodless chamber oil outlet of the first bidirectional gear pump, the A port connected to the rodless chamber of the oil cylinder, and the B port connected to the oil tank. When the first solenoid valve is in operation, the rodless chamber oil outlet of the first bidirectional gear pump can be unloaded.

[0011] The second solenoid valve adopts a two-position three-way valve structure, with the P port connected to the rod chamber oil outlet of the first bidirectional gear pump, the A port connected to the rod chamber of the oil cylinder, and the B port connected to the oil tank. When it is activated, the rod chamber oil outlet of the first bidirectional gear pump can be unloaded.

[0012] The first relief valve is a low-pressure relief valve, the P port is connected to the oil outlet of the rodless chamber of the first bidirectional gear pump, and the T port is connected to the oil tank.

[0013] The second relief valve is a high-pressure relief valve, the P port is connected to the oil outlet of the rodless chamber of the second bidirectional gear pump, and the T port is connected to the oil tank.

[0014] The third relief valve is a low-pressure relief valve, the P port is connected to the rod chamber oil outlet of the second bidirectional gear pump, and the T port is connected to the oil tank.

[0015] The first one-way valve opens when the piston rod is pressed down and closes when the piston rod is withdrawn. The P port is connected to the oil tank, and the T port is connected to the rod chamber oil outlet of the second bidirectional gear pump.

[0016] The second one-way valve is closed when the piston rod is pressed down, and is opened when the piston rod is withdrawn. The P port is connected to the oil tank, and the T port is connected to the oil outlet of the rodless chamber of the second bidirectional gear pump.

[0017] The first bidirectional gear pump is a low-pressure, high-flow bidirectional gear pump, the rodless chamber oil outlet is connected to the first solenoid valve P port and the first relief valve P port, and the rod chamber oil outlet is connected to the second solenoid valve P port. The fast forward and reverse motion control and loading pressure control of the straightening machine's downward pressure cylinder can be achieved only by the forward and reverse rotation of the first bidirectional gear pump.

[0018] The second bidirectional gear pump is a high-pressure, low-flow bidirectional gear pump, and the rodless chamber oil outlet is connected to the rodless chamber oil inlet of the oil cylinder, the second relief valve P port, and the second check valve T port. The rod chamber oil outlet is connected to the rod chamber oil inlet of the oil cylinder, the second solenoid valve A port, the first check valve T port, and the third relief valve P port. The slow forward and reverse motion control and loading pressure control of the straightening machine's downward pressure oil cylinder can be achieved only by the forward and reverse rotation of the second bidirectional gear pump.

[0019] The first bidirectional gear pump and the second bidirectional gear pump are arranged in series with a through shaft and are synchronously driven by a servo motor. The intervention and bypass of the first bidirectional gear pump are achieved by switching the valve positions of the first solenoid valve and the second solenoid valve, thereby achieving high-speed downward pressure and high-precision positioning.

[0020] The first bidirectional gear pump and the second bidirectional gear pump are controlled by a motor to rotate forward and reversely to realize the downward pressure and retraction of the piston.

[0021] The pressure sensor is installed at the oil inlet of the rodless chamber of the oil cylinder, and sends a signal to the real-time controller through a pressure signal transmitter.

[0022] The displacement sensor adopts a magnetostrictive displacement sensor, the signal converter is fixed on the top of the oil cylinder, the induction element is located at the central deep hole of the piston rod, and the magnetic ring is fixed on the piston. The signal converter is connected to the real-time controller.

[0023] The servo motor drives the first bidirectional gear pump and the second bidirectional gear pump to rotate at the same speed and in the same direction via the reducer. The servo motor is connected to a servo driver. The servo driver servo enable signal, rotation direction signal, and speed pulse signal are connected to the real-time controller.

[0024] The first valve-controlled switch output end is connected to the first solenoid valve, and the control end is connected to the real-time controller.

[0025] The second valve-controlled switch output end is connected to the second solenoid valve, and the control end is connected to the real-time controller.

[0026] The real-time controller runs a data communication program with the host computer, waits for the host computer command and returns the corresponding data. The real-time controller runs a control algorithm, receives pressure sensor data and displacement sensor data, and calculates the motor speed and valve control switch state. The real-time controller has multiple IO ports, outputs passive signals such as valve control switch, motor servo enable, pressure down, withdrawal, fault, pressure alarm, positioning completion, and outputs motor frequency.

[0027] The control algorithm constructs an expected speed curve function, calculates the expected downward pressure speed corresponding to the distance difference according to the difference in damping coefficients of different loads, realizes switching between a no-load or low-load rapid descent mode and a low-speed high-precision positioning mode based on the expected speed, constructs a piston motion model, calculates the oil filling speed based on pressure, piston speed and acceleration, and the expected downward pressure speed, constructs a bidirectional pump model, and calculates the oil pump speed and motor speed based on the expected oil filling speed and pressure.

[0028] The expected speed curve function has the input of the diameter of the shaft workpiece to be straightened, the expected piston position and the current piston position, and the output is the expected pressing speed.

[0029] The piston motion model has inputs of cylinder pressure, piston speed, piston acceleration, and expected downward pressure speed, and an output of expected oil filling speed.

[0030] The piston velocity and acceleration are calculated through a sliding window of the piston position.

[0031] The sliding window includes 5 time units, 10 time units, 20 time units and 50 time units.

[0032] The bidirectional pump model has the input of oil cylinder pressure, bidirectional pump leakage curve and expected oil filling speed, and the output of the model is motor speed.

[0033] The leakage curve of the bidirectional pump is obtained by point-by-point method according to the manual provided by the manufacturer, and then fitted by insertion method.

[0034] Beneficial effects of the present invention: Compared with the unidirectional pump valve control method, the bidirectional pump direct control can achieve fine-tuning without dead zone and high dynamic characteristic adjustment. By connecting large and small pumps in series, both fast feeding and precise position control can be taken into account. By optimizing the hydraulic control unit and control algorithm, the positioning control accuracy, response speed, and control robustness of the hydraulic system are improved while maintaining the high power advantage of the hydraulic straightening machine, the working efficiency and straightening accuracy of the straightening machine are improved, and a communication interface is provided to facilitate the replacement and upgrading of the existing straightening machine's downward pressure components. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Mechanical components of the hydraulic control circuit of the straightening machine's downward pressure cylinder;

[0036] Figure 2 Straightening machine downward pressure cylinder control unit;

[0037] Figure 3 Straightening machine downward pressure cylinder control process diagram;

[0038] Figure 4 Controller expected motor speed and solenoid valve switch calculation flow chart. DETAILED DESCRIPTION

[0039] The following embodiments of the technical solution of the present invention are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore only used as examples, and cannot be used to limit the protection scope of the present invention.

[0040] Figure 1 This is a schematic diagram of the mechanical components of the hydraulic control circuit of the hydraulic cylinder of the straightening machine of the present invention. Figure 1 As shown, the mechanical components of the hydraulic control circuit of the downward pressure cylinder of the straightening machine include a cylinder 1, a hydraulic circuit 2, a magnetostrictive displacement sensor 3, a pressure sensor 4, and a servo motor 5. The cylinder 1 is in contact with the workpiece and is used to straighten the workpiece. The hydraulic circuit 2 is used to transport the oil between the oil tank, the rodless chamber of the cylinder, and the rod chamber of the cylinder to achieve the downward pressure and withdrawal of the cylinder piston. The magnetostrictive displacement sensor 3 is used to measure the position of the cylinder piston. The pressure sensor 4 is used to measure the pressure of the rodless chamber of the piston. The servo motor 5 is used to drive the bidirectional gear pump.

[0041] The oil cylinder 1 is composed of a cylinder body 101, a piston 102, a piston rod 103, and an anti-rotation rod 104. The piston 102 is provided with three through holes, which are located at the center and both sides. The piston 102 is located in the cylinder body 101. The piston rod 103 is connected to the piston 102, and a deep hole is provided at the center. According to the positions of the piston 102 and the piston rod 103 in the cylinder body 101, the oil cylinder 1 is divided into a rodless chamber 105 and a rod chamber 106. The rodless chamber 105 has a rodless chamber oil hole 107 at the upper end, and the rod chamber 106 has a rod chamber oil hole 108 at the lower end. The rodless chamber 105 is filled with oil, the rod chamber 106 is drained of oil, and the piston 102 moves downward. The rod chamber 106 is filled with oil, the rodless chamber 105 is drained of oil, and the piston 102 moves upward. Two anti-rotation rods 104 pass through the through holes on both sides of the piston 102 to prevent the piston 102 from twisting.

[0042] The hydraulic circuit 2 is composed of a first bidirectional gear pump 201 , a second bidirectional gear pump 202 , a first solenoid valve 203 , a second solenoid valve 204 , a first overflow valve 205 , a second overflow valve 206 , a third overflow valve 209 , a first check valve 208 , a second check valve 207 , and an oil tank 210 .

[0043] The first bidirectional gear pump 201 is a low-pressure and high-flow bidirectional gear pump, the forward oil outlet is a rodless chamber oil outlet 2011 , and the reverse oil outlet is a rod chamber oil outlet 2012 .

[0044] The second bidirectional gear pump 202 is a high-pressure and low-flow bidirectional gear pump, the forward oil outlet is a rodless chamber oil outlet 2021 , and the reverse oil outlet is a rod chamber oil outlet 2022 .

[0045] The rodless chamber oil outlet 2011 of the first bidirectional gear pump 201 , the P port of the first overflow valve 205 , and the P port of the first solenoid valve 203 are connected.

[0046] The first bidirectional gear pump 201 has a rod chamber oil outlet 2012 , which is connected to the P port of the second solenoid valve 204 .

[0047] The rodless chamber oil outlet 2021 of the second bidirectional gear pump 202 , the rodless chamber oil hole 107 of the oil cylinder, the A port of the first solenoid valve 203 , the T port of the second one-way valve 207 , and the P port of the second overflow valve 206 are connected.

[0048] The second bidirectional gear pump 202 has a rod chamber oil outlet 2022 , the oil cylinder has a rod chamber oil hole 108 , the A port of the second solenoid valve 204 , the P port of the third relief valve 209 , and the T port of the first check valve 208 are connected.

[0049] The B port of the first solenoid valve 203 , the B port of the second solenoid valve 204 , the T port of the first overflow valve 205 , the T port of the second overflow valve 206 , the T port of the third overflow valve 209 , the P port of the first check valve 208 , and the P port of the second check valve 207 are connected to the oil tank 210 .

[0050] The servo motor 5 is connected in series with the first bidirectional gear pump 201 and the second bidirectional gear pump 202 via a reducer.

[0051] Preferably, the servo motor has a rated power of 5 kilowatts and a rated speed of 3000 rpm, and the reducer has a reduction ratio of 1:2.

[0052] Preferably, the first bidirectional gear pump 201 adopts the CBHYG-F3.5 model.

[0053] Preferably, the second bidirectional gear pump 202 adopts the CBHYG-G3.5 model.

[0054] Preferably, the first solenoid valve 203 and the second solenoid valve 204 are two-position three-way solenoid valves.

[0055] Preferably, the first overflow valve 205 adopts the DBDS6K10 / 31.5 model, and the overflow pressure is 21 MPa.

[0056] Preferably, the second relief valve 206 adopts the DBDS6K10 / 2.5 model, and the relief pressure is 1 MPa.

[0057] Preferably, the third relief valve 209 adopts the DBDS6K10 / 2.5 model, and the relief pressure is 1 MPa, which is greater than the rod chamber pressure when the piston withdraws.

[0058] Preferably, the opening pressure of the first one-way valve 208 is 0 MPa.

[0059] Preferably, the opening pressure of the second one-way valve 207 is 0 MPa.

[0060] The motor 5 rotates forward, the P port of the first solenoid valve 203 is connected to the A port, and the P port of the second solenoid valve 204 is connected to the A port. The hydraulic oil is first sucked out from the rod chamber oil hole 108 of the rod chamber 106 of the oil cylinder, and then flows through the A port of the second solenoid valve 204, the P port of the second solenoid valve 204, the rod chamber oil outlet 2012 of the first two-way gear pump 201, the rodless chamber oil outlet 2011 of the first two-way gear pump 201, the P port of the first solenoid valve 203, and the A port of the first solenoid valve 203, and at the same time flows through the rod chamber oil outlet 2022 of the second two-way gear pump 202, the rodless chamber oil outlet 2021 of the second two-way gear pump 202, and is injected into the rodless chamber 105 of the oil cylinder from the rodless chamber oil hole 107, so as to realize the rapid downward pressure of the piston. When the oil in the rod chamber 106 of the oil cylinder is insufficient, the first one-way valve 208 opens, and the oil flows in from the P port of the first one-way valve 208.

[0061] When the pressure at the P port of the first solenoid valve 203 is too high, the first relief valve 205 opens, and the oil flows from the P port of the first relief valve 205 through the T port of the first relief valve 205 and returns to the oil tank 210 .

[0062] When the P port of the first solenoid valve 203 is connected to the B port, and the P port of the second solenoid valve 204 is connected to the B port, the first bidirectional gear pump 201 is bypassed, and the piston enters a slow and high-precision downward pressure mode.

[0063] When the pressure in the oil hole 107 of the rodless chamber of the cylinder exceeds the output power of the servo motor 5 , the second relief valve 206 opens, and the oil flows from the P port of the second relief valve 206 through the T port of the second relief valve 206 and returns to the oil tank 210 .

[0064] The motor 5 reverses, the P port of the first solenoid valve 203 is connected to the A port, and the P port of the second solenoid valve 204 is connected to the A port. The hydraulic oil is first sucked out from the rodless chamber oil hole 107 of the rodless chamber 105 of the cylinder, and then flows through the A port of the first solenoid valve 203, the P port of the first solenoid valve 203, the rodless chamber oil outlet 2011 of the first bidirectional gear pump 201, the rod chamber oil outlet 2012 of the first bidirectional gear pump 201, the P port of the second solenoid valve 204, and the A port of the second solenoid valve 204, and at the same time flows through the rodless chamber oil outlet 2021 of the second bidirectional gear pump 202, the rod chamber oil outlet 2022 of the second bidirectional gear pump 202, and is injected into the cylinder rod chamber 106 from the cylinder rod chamber oil hole 108, thereby realizing rapid withdrawal of the piston. Since the volume of the rodless chamber is smaller than that of the rod chamber, the first relief valve 205 opens, and the excess oil flows through the P port of the first relief valve 205 and flows back to the oil tank from the T port of the first relief valve 205 .

[0065] When the P port of the first solenoid valve 203 is connected to the B port, and the P port of the second solenoid valve 204 is connected to the B port, the first bidirectional gear pump 201 is bypassed, and the piston enters the slow high-precision withdrawal mode. The second relief valve 206 is opened, and the excess oil flows from the P port of the second relief valve 206 through the T port of the second relief valve 206 and returns to the oil tank 210. When there is no hydraulic oil in the rodless chamber 105 of the oil cylinder, the second check valve 207 is opened, and the oil flows from the P port of the second check valve 207 through the T port of the second check valve 207 and enters the rodless chamber oil outlet 2021 of the second bidirectional gear pump 202. When the piston reaches the top, the third relief valve 209 is opened, and the oil flows from the P port of the third relief valve 209 through the A port of the third relief valve 209 and returns to the oil tank.

[0066] Figure 2 Schematic diagram of the control unit of the hydraulic cylinder of the straightening machine of the present invention. Figure 2 As shown, the straightening machine downward pressure cylinder control unit includes an embedded real-time control system 601, a pressure sensor signal conversion module 602, a rodless cavity pressure sensor 4, a magnetostrictive displacement sensor 3, a first solenoid valve relay 603, and a second solenoid valve relay 604. The embedded real-time control system 601 is used to receive sensor data and control relay action, control motor rotation, and communicate with the host computer. The rodless cavity pressure sensor 4 and the pressure sensor signal conversion module 602 are used to read the rodless cavity oil pressure and convert it into a signal that can be read by the embedded real-time control system 601. The magnetostrictive displacement sensor 3 is used to detect the piston position. The first solenoid valve relay 603 is used to control the valve position switching of the first solenoid valve 203, and the second solenoid valve relay 604 is used to control the valve position switching of the second solenoid valve 204.

[0067] The magnetostrictive displacement sensor 3 is mounted on the oil cylinder 1, and is composed of a signal converter 301, a magnetic ring 302, and a sensing element 303. The signal converter 301 is mounted on the oil cylinder body 101, the magnetic ring 302 is fixed on the piston 102, and the sensing element 303 is connected to the signal converter 301 and passes through the deep hole of the piston 102 and the piston rod 103.

[0068] The magnetostrictive displacement sensor 3 communicates with the embedded real-time control system 601 via the CAN bus.

[0069] The first solenoid valve relay 603 and the second solenoid valve relay 604 have an input signal of 3.3V and are connected to the embedded real-time control system 601 , and an output signal of 24V and are connected to the first solenoid valve 203 and the second solenoid valve 204 .

[0070] The embedded real-time control system 601 uses the STM32F103HZ chip, controls the relay to provide a passive output signal through the 3.3V pin, converts the 3.3V high-speed pulse signal into a 24V high-speed pulse signal through a triode circuit, inputs the motor, and controls the motor speed. It communicates with the magnetostrictive displacement sensor 3 through the CAN bus and communicates with the host computer through the RS485 bus.

[0071] The embedded real-time control system 601 runs control algorithms and communication programs.

[0072] The communication program communicates with the host computer and sends and receives data of the ModBusRTU protocol. The data content includes setting the control mode, setting the target position and load diameter in the positioning mode, setting the expected speed, pressing start command, retracting start command, stop command in the speed mode, reading the motor speed, positioning completion command, relay status, cylinder pressure, piston position, and abnormal information.

[0073] Figure 3 This is the control process diagram of the straightening machine's downward pressure cylinder. After the user sets the target position and load attributes, the controller receives the pressure sensor data and the position sensor data, and calculates the expected motor speed according to the control algorithm. The motor drives the bidirectional gear pump to work, and the bidirectional gear pump injects hydraulic oil into the cylinder. The pressure difference between the rod chamber and the rodless chamber drives the piston to move, and the piston drives the piston rod to straighten the load.

[0074] Figure 4The controller expects the motor speed and the solenoid valve switch calculation flow chart, constructs the expected downward pressure speed curve function according to the control algorithm, calculates the expected downward pressure speed corresponding to the distance difference in real time according to the difference in damping coefficients of different loads, realizes the switching between the no-load or low-load rapid descent mode and the low-speed high-precision positioning mode based on the expected speed, constructs a piston motion model, calculates the oil filling speed based on pressure, piston speed and acceleration, and the expected downward pressure speed, constructs a bidirectional pump model, and calculates the oil pump speed and motor speed based on the expected oil filling speed and pressure.

[0075] The expected speed curve function has the input of the diameter of the shaft workpiece to be straightened, the expected piston position and the current piston position, and the output is the expected pressing speed.

[0076]

[0077] Where D is the load diameter, k is the load diameter coefficient, M is the maximum deceleration distance, Dd is the difference between the target position and the current position, Vmax is the maximum pressing speed, and F(D, Dd) is the expected pressing speed calculation function.

[0078] The piston motion model has inputs of cylinder pressure, piston speed, piston acceleration and expected downward pressure speed, and an output of expected oil filling speed.

[0079] H=G(p,a t ,v t ,V)

[0080] The piston velocity and acceleration are calculated through a sliding window of the piston position.

[0081] The sliding window includes 5 time units, 10 time units, 20 time units and 50 time units.

[0082]

[0083] a t =v t -v t-1

[0084] The input of the bidirectional pump model is the oil cylinder pressure, the bidirectional pump leakage curve, and the expected oil filling speed, and the output is the motor speed.

[0085]

[0086] Where P(p) represents the gear pump leakage under pressure p, and w represents the gear pump displacement.

[0087] The bidirectional pump leakage curve P(p) is obtained by point-by-point method according to the manual provided by the manufacturer, and then fitted by insertion method.

Claims

1. A high-precision control system for a hydraulic straightening machine's downward pressure cylinder, comprising a hydraulic circuit, a control unit, and a control algorithm running in the control unit, characterized in that: The hydraulic circuit is composed of an oil cylinder, a first solenoid valve, a second solenoid valve, a first relief valve, a second relief valve, a third relief valve, a first non-return valve, a second non-return valve, a first bidirectional gear pump, a second bidirectional gear pump, and an oil tank. The bidirectional gear pump is used to inject or absorb oil into the rod chamber and the rodless chamber of the oil cylinder to control the position of the oil cylinder piston, thereby realizing rapid downward pressure and withdrawal and high-precision positioning of the straightening machine. The first solenoid valve adopts a two-position three-way valve structure, the P port of the first solenoid valve is connected to the rodless chamber oil outlet of the first bidirectional gear pump, the A port is connected to the rodless chamber of the oil cylinder, and the B port is connected to the oil tank; when it is in action, the rodless chamber oil outlet of the first bidirectional gear pump can be unloaded; The second solenoid valve adopts a two-position three-way valve structure, the P port of the second solenoid valve is connected to the rod chamber oil outlet of the first bidirectional gear pump, the A port is connected to the rod chamber of the oil cylinder, and the B port is connected to the oil tank; when it is in action, the rod chamber oil outlet of the first bidirectional gear pump can be unloaded; The first relief valve is a low-pressure relief valve, the P port of the first relief valve is connected to the oil outlet of the rodless chamber of the first bidirectional gear pump, and the T port is connected to the oil tank; The second relief valve is a high-pressure relief valve, the P port of the second relief valve is connected to the oil outlet of the rodless chamber of the second bidirectional gear pump, and the T port is connected to the oil tank; The third relief valve is a low-pressure relief valve, the P port of the third relief valve is connected to the rod chamber oil outlet of the second bidirectional gear pump, and the T port is connected to the oil tank; The first one-way valve opens when the piston rod is pressed down, and closes when the piston rod is withdrawn. The P port of the first one-way valve is connected to the oil tank, and the T port is connected to the rod chamber oil outlet of the second bidirectional gear pump. The second one-way valve is closed when the piston rod is pressed down, and is opened when the piston rod is withdrawn. The P port of the second one-way valve is connected to the oil tank, and the T port is connected to the oil outlet of the rodless chamber of the second bidirectional gear pump. The first bidirectional gear pump is a low-pressure and high-flow bidirectional gear pump, the rodless chamber oil outlet is connected to the first solenoid valve P port and the first relief valve P port, and the rod chamber oil outlet is connected to the second solenoid valve P port; the fast forward and reverse motion control and loading pressure control of the straightening machine's downward pressure cylinder can be achieved only by the forward and reverse rotation of the first bidirectional gear pump; The second bidirectional gear pump is a high-pressure and low-flow bidirectional gear pump, and the rodless chamber oil outlet is connected to the rodless chamber oil inlet of the oil cylinder, the second relief valve P port, and the second one-way valve T port; the rod chamber oil outlet is connected to the rod chamber oil inlet of the oil cylinder, the second solenoid valve A port, the first one-way valve T port, and the third relief valve P port; the slow forward and reverse motion control and loading pressure control of the straightening machine's downward pressure oil cylinder can be achieved only by the forward and reverse rotation of the second bidirectional gear pump; The first bidirectional gear pump and the second bidirectional gear pump are arranged in series through the shaft and are synchronously driven by a servo motor. The first bidirectional gear pump is intervened and bypassed by switching the valve positions of the first solenoid valve and the second solenoid valve, thereby achieving high-speed downward pressure and high-precision positioning; The first bidirectional gear pump and the second bidirectional gear pump are controlled by a motor to rotate forward and reversely to realize the downward pressing and retraction of the piston.

2. According to claim 1, a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine is characterized by: The control unit is composed of a pressure sensor, a pressure signal transmitter, a displacement sensor, a servo motor, a servo motor driver, a reducer, a first valve-controlled switch, a second valve-controlled switch, and a real-time controller. The real-time controller receives host computer instructions and parameters through the ModbusRTU protocol, reads the pressure sensor and displacement sensor data, calculates the desired state of the servo motor and the valve-controlled switch, realizes oil flow control, and thus realizes hydraulic circuit control; The pressure sensor is installed at the oil inlet of the rodless chamber of the oil cylinder, and sends the signal to the real-time controller through the pressure signal transmitter; The displacement sensor adopts a magnetostrictive displacement sensor, in which the signal converter is fixed on the top of the oil cylinder, the sensing element is located at the central deep hole of the piston rod, and the magnetic ring is fixed on the piston; the signal converter is connected to the real-time controller; The servo motor drives the first bidirectional gear pump and the second bidirectional gear pump to rotate at the same speed and in the same direction via the reducer; the servo motor is connected to a servo driver; the servo enable signal, rotation direction signal, and speed pulse signal of the servo driver are connected to the real-time controller; The first valve-controlled switch output end is connected to the first solenoid valve, and the control end is connected to the real-time controller; The second valve-controlled switch output terminal is connected to the second solenoid valve, and the control terminal is connected to the real-time controller; The real-time controller runs a data communication program with the host computer, waits for host computer instructions and returns corresponding data; The real-time controller runs the control algorithm, receives the pressure sensor data and the displacement sensor data, and calculates the motor speed and the valve-controlled switch state; the real-time controller has multiple IO ports, outputs passive signals of valve-controlled switch, motor servo enable, downward pressure, withdrawal, fault, pressure alarm, and positioning completion, and outputs the motor frequency.

3. According to claim 2, a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine is characterized in that: The control algorithm is based on the hydraulic system transmission model, and calculates the expected pressing speed in real time according to the load type and positioning distance difference, calculates the expected oil increase speed according to the expected pressing speed, the current pressing speed, and the rodless chamber pressure, and calculates the expected motor speed and valve control switch state according to the rodless chamber pressure and the expected oil increase speed.

4. According to claim 3, a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine is characterized in that: The control algorithm constructs an expected speed curve function, calculates the expected downward pressure speed corresponding to the distance difference according to the difference in damping coefficients of different loads, and realizes switching between a no-load or low-load rapid descent mode and a low-speed high-precision positioning mode based on the expected downward pressure speed; constructs a piston motion model, calculates the oil filling speed based on pressure, piston speed and acceleration, and the expected downward pressure speed, and constructs a bidirectional pump model; calculates the oil pump speed and motor speed based on the expected oil filling speed and pressure.

5. According to claim 4, a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine is characterized in that: The expected speed curve function has the input of the diameter of the shaft workpiece to be straightened, the expected piston position and the current piston position, and the output is the expected pressing speed.

6. A high-precision control system for the downward pressure cylinder of a hydraulic straightening machine according to claim 4, characterized in that: The piston motion model has inputs of cylinder pressure, piston speed, piston acceleration, and expected downward pressure speed, and an output of expected oil filling speed.

7. A high-precision control system for the downward pressure cylinder of a hydraulic straightening machine according to claim 6, characterized in that: The piston velocity and acceleration are calculated through a sliding window of the piston position, where the sliding window includes 5 time units, 10 time units, 20 time units and 50 time units.

8. According to claim 4, a high-precision control system for the downward pressure cylinder of a hydraulic straightening machine is characterized in that: The bidirectional pump model has the input of oil cylinder pressure, bidirectional pump leakage curve and expected oil filling speed, and the output of the model is motor speed.

9. A high-precision control system for the downward pressure cylinder of a hydraulic straightening machine according to claim 8, characterized in that: The leakage curve of the bidirectional pump is obtained by point-by-point method according to the manual provided by the manufacturer, and then fitted by insertion method.

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