A robot leg joint actuation EHA control system and control method

Abstract

The present application relates to an EHA control system and control method for a robot leg joint actuation, wherein the control system includes a host computer, a controller, a signal acquisition system, a driver, a DC motor, a bidirectional fixed-displacement plunger pump, a motor encoder, a symmetrical hydraulic cylinder and a joint angular displacement sensor. The controller is connected to the host computer via a communication interface to receive instructions from the host computer and send instructions to the driver connected thereto. The driver is connected to the DC motor, and the DC motor and the bidirectional fixed-displacement plunger pump are serially installed on the same axis. The motor encoder is installed on the DC motor to collect the motor's speed and position signals. The joint angular displacement sensor is installed at the robot's leg rotation joint to collect joint angle signals and send the collected joint angle signals to the signal acquisition system. The signal acquisition system sends the processed signals to the controller.

Description

Technical Field

[0001] The present application relates to the field of robot control technology, and in particular to a robot leg joint actuation EHA control system and control method. Background Art

[0002] At present, the hydraulic actuation systems of robot joints are mostly based on valve-controlled hydraulic cylinders, which adopt the traditional hydraulic open system drive method and use servo valves to control the flow of hydraulic cylinders or hydraulic motors. Although they have high control accuracy and fast response, they suffer from serious throttling losses and high energy consumption. At the same time, they require supporting pump stations and long oil pipelines, which is not only not conducive to the layout of the robot, but also prone to oil leakage and other problems.

[0003] EHA (Electro-Hydrostatic Actuator) is a linear or rotary motion output device that uses electro-hydrostatic composite transmission technology. It drives the hydraulic pump to rotate through the speed control of the servo motor, thereby controlling the output flow of the pump and controlling the hydraulic cylinder to accurately reach the specified position. It adopts an integrated and modular design, which greatly reduces the size of the system.

[0004] Existing EHA control systems are mostly used in industrial die-casting machines, aircraft control surfaces and other fields. Usually, the hydraulic rod position of the EHA is directly controlled, and the control method is mainly based on the EHA hydraulic rod displacement closed-loop control strategy and its improvement method. In the prior art, EHA has not been applied to the joint angle control and body control of the robot. Summary of the invention

[0005] The present invention aims to provide a robot leg joint actuation EHA control system and control method, so as to improve the joint angle control accuracy and the robustness of the joint angle against load interference, and at the same time enable the robot joint angle to be maintained after power off. The technical problem to be solved by the present invention is achieved through the following technical solutions.

[0006] A robot leg joint actuation EHA control system includes a host computer, a controller, a signal acquisition system, a driver, a DC motor, a bidirectional fixed displacement plunger pump, a motor encoder, a symmetrical hydraulic cylinder and a joint angular displacement sensor. The controller is connected to the host computer through a communication interface to receive instructions issued by the host computer and send instructions to the driver connected thereto. The driver is connected to the DC motor. The DC motor and the bidirectional fixed displacement plunger pump are serially installed on the same shaft. The motor encoder is installed on the DC motor to collect the speed and position signals of the motor. The driver is connected to the motor encoder to receive the speed and position signals fed back by the motor encoder to control the speed of the DC motor, thereby driving the bidirectional fixed displacement plunger pump to rotate. The two output ends of the bidirectional fixed displacement plunger pump are respectively connected to the upper and lower chambers of the symmetrical hydraulic cylinder. The joint angular displacement sensor is installed at the rotating joint of the robot leg to collect joint angle signals and send the collected joint angle signals to the signal acquisition system. The signal acquisition system sends the processed signals to the controller.

[0007] Preferably, the signal acquisition system converts the collected sensor analog signal into a digital signal and sends it to the controller.

[0008] Preferably, the two output ends of the bidirectional fixed-displacement plunger pump are respectively provided with a first pressure sensor and a second pressure sensor, which are connected to the upper and lower chambers of the symmetrical hydraulic cylinder for collecting oil pressure signals of the two chambers of the symmetrical hydraulic cylinder.

[0009] Preferably, the two output ends of the bidirectional fixed-displacement plunger pump are also respectively provided with a first electromagnetic switch valve and a second electromagnetic switch valve, which are connected to the upper and lower chambers of the symmetrical hydraulic cylinder, and the controller sends instructions to its power supply circuit to control the opening and closing of the electromagnetic switch valve.

[0010] A control method for a robot joint EHA control system comprises the following steps:

[0011] The controller receives a command signal from a host computer, wherein the command signal includes information on the target angle trajectory of the joint. The signal acquisition system collects feedback signals from the joint angle displacement sensor and two pressure sensors, and converts them into digital signals and sends them to the controller. The controller processes the feedback signals from the two pressure sensors to form a feedback pressure difference signal. The controller sends instructions to the driver and the first solenoid switch valve and the second solenoid switch valve. The driver drives the DC motor to run and receives feedback signals from the motor encoder. The DC motor drives the bidirectional fixed-displacement plunger pump to rotate.

[0012] Preferably, the control method adopts a three-loop cascade control method, the outermost loop is the joint angle displacement control, and the controller sends the target angle trajectory α ref and the actual angle α of the joint angular displacement sensoract To make a difference, use the feedback PD control method to generate the joint target torque , where K1 is the proportional gain of the angle control loop, and K2 is the differential gain of the angle control loop. At the same time, the EHA output force acting on the robot joint is solved through the parameters of the actual linkage mechanism of the robot and the real-time actual joint angle value to obtain the real-time EHA output force arm l , and then through the symmetrical hydraulic cylinder action area A, the EHA target pressure difference trajectory can be obtained , the second control loop tracks and controls the EHA target pressure difference trajectory, and adopts the feedforward + compensation + feedback method for control, where the feedforward term, leakage compensation term, and feedback term are obtained by the following equations:

[0013]

[0014] in The motor speed of the feedforward project, Motor speed for leakage compensation project, To feedback the motor speed of the project, is the pump displacement, E is the elastic modulus of the hydraulic oil, V is the volume of the symmetrical hydraulic cylinder, is the leakage coefficient, is the actual pressure difference of the EHA symmetrical hydraulic cylinder, is the proportional gain of the pressure control loop, is the integral gain of the pressure control loop, is the differential gain of the pressure control loop, and finally the target speed of the EHA motor is obtained , the motor target speed is sent to the driver for closed-loop feedback control, so that the joint angle moves according to the command trajectory.

[0015] Preferably, it also includes a posture maintaining standing control step. When the leg joint angle of the robot reaches a predetermined position, the controller sends a command to cut off the power to the first solenoid switch valve and the second solenoid switch valve, and then the controller sends a command to the driver to enable the DC motor. Thereafter, the leg joint of the robot can remain motionless.

[0016] Preferably, it also includes a power-off protection control step. When the robot needs to be powered on to run, the driver is powered on first, and the controller sends a command to enable the DC motor. After the DC motor is enabled, the controller sends a command to power on and open the first solenoid switch valve and the second solenoid switch valve. After the solenoid valve is opened, the controller sends the joint angle target trajectory to perform angle trajectory tracking closed-loop control.

[0017] Preferably, if a fault occurs during the power-off protection control process, the first switch solenoid valve and the second switch solenoid valve are first powered off, and then the driver is powered off to check the fault. After the fault is cleared, the power-off protection control step is continued.

[0018] The robot leg joint actuation EHA control system and control method of the present invention have the following beneficial effects:

[0019] 1. The linear displacement sensor and tension pressure sensor in the traditional hydraulic cylinder control are removed, making the robot joint structure simpler and more reliable, while reducing the cost of the robot joint control system;

[0020] 2. The innermost loop of the joint angle closed-loop control adopts the motor speed loop control, so that the flow rate to reach the target joint angle through the speed closed loop is relatively stable during the EHA actuation process under different load conditions, and the dynamic operation process of the system is more stable;

[0021] 3. An electromagnetic switch valve is set up. When the robot fails or loses power, the leg joint angle can still maintain its original position, which greatly reduces the possibility of damage to the robot body and increases the safety of the robot's leg joint movement;

[0022] 4. The robot leg joint angle closed-loop control adopts three-loop cascade control. The outermost loop is the joint angle closed-loop control, the second loop is the EHA pressure difference closed-loop control, and the innermost loop is the motor speed closed-loop control. It can achieve motion control stability and position accuracy under different output torque conditions;

[0023] 5. The arm analysis module from joint target torque to EHA output force is adopted to better solve the nonlinear problem of joint target torque control and increase the stability of control;

[0024] 6. The closed-loop control of joint angle adopts a three-loop cascade control method, which can achieve motion control stability and position accuracy under different joint torque conditions;

[0025] 7. EHA adopts force control method, which has the advantages of fast response and strong load adaptability. At the same time, leakage compensation items are added to EHA force control to reduce the pressure fluctuation of hydraulic cylinder caused by leakage and thus the occurrence of system jitter. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the EHA control system for the robot joint actuation of the present invention;

[0027] Figure 2 Schematic diagram of the EHA control method for closed-loop control of robot joint angles of the present invention;

[0028] Figure 3Schematic diagram of the control steps for the robot's leg posture to maintain standing after reaching a predetermined joint angle;

[0029] Figure 4 This is a step for controlling the power-on operation or power-off protection of the robot of the present invention.

[0030] Explanation of the accompanying drawings: 1. Host computer; 2. Controller; 3. Signal acquisition system; 4. Driver; 5. DC motor; 6. Bidirectional fixed-displacement piston pump; 7. Motor encoder; 8. First pressure sensor; 9. Second pressure sensor; 10. First solenoid switch valve; 11. Second solenoid switch valve; 12. Symmetrical hydraulic cylinder; 13. Joint angular displacement sensor. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

[0032] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0033] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0035] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, or are directions or positional relationships in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0036] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "setting", "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0037] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0038] like Figure 1 As shown, a robot leg joint actuation EHA control system includes a host computer 1, a controller 2, a signal acquisition system 3, a driver 4, a DC motor 5, a bidirectional fixed displacement plunger pump 6, a motor encoder 7, a symmetrical hydraulic cylinder 12 and a joint angular displacement sensor 13. The controller 2 is connected to the host computer 1 through a communication interface to receive instructions from the host computer 1 and send instructions to the driver 4 connected thereto. The driver 4 is connected to the DC motor 5. The DC motor 5 and the bidirectional fixed displacement plunger pump 6 are installed in series on the same shaft. The motor encoder 7 is installed on the DC motor 5 to collect the speed and position signals of the motor. The driver 4 is connected to the motor encoder 7 to receive the speed and position signal fed back by the motor encoder 7, so as to control the speed of the DC motor 5, thereby driving the rotation of the bidirectional fixed displacement plunger pump 6, thereby driving the linear motion of the symmetrical hydraulic cylinder 12, and driving the rotation of the robot joint through the ball joint connection. The two output ends of the bidirectional fixed displacement plunger pump 6 are respectively connected to the upper and lower chambers of the symmetrical hydraulic cylinder 12. The joint angle displacement sensor 13 is installed at the rotation joint of the robot leg, and is used to collect the joint angle signal and send the collected joint angle signal to the signal acquisition system 3, and the signal acquisition system 3 sends the processed signal to the controller 2. In the normal power supply state, the solenoid valve is in an open state. When the first solenoid switch valve 10 is powered off and closed, the upper chamber of the symmetrical hydraulic cylinder is locked, and the oil pressure of the upper chamber is maintained; similarly, when the second solenoid switch valve 11 is powered off and closed, the lower chamber of the symmetrical hydraulic cylinder is locked, and the oil pressure of the lower chamber is maintained.

[0039] Furthermore, the signal acquisition system 3 converts the collected sensor analog signal into a digital signal and sends it to the controller.

[0040] Furthermore, the two output ends of the bidirectional fixed displacement plunger pump 6 are respectively provided with a first pressure sensor 8 and a second pressure sensor 9 , which are connected to the upper and lower chambers of the symmetrical hydraulic cylinder 12 for collecting oil pressure signals of the two chambers of the symmetrical hydraulic cylinder 12 .

[0041] Furthermore, the two output ends of the bidirectional fixed displacement plunger pump 6 are respectively provided with a first electromagnetic switch valve 10 and a second electromagnetic switch valve 11, which are connected to the upper and lower chambers of the symmetrical hydraulic cylinder 12, and the controller 2 sends instructions to its power supply circuit to control the opening and closing of the electromagnetic switch valve.

[0042] A control method for a robot joint EHA control system comprises the following steps:

[0043] The controller receives a command signal from a host computer, wherein the command signal includes information on the target angle trajectory of the joint. The signal acquisition system collects feedback signals from the joint angle displacement sensor and two pressure sensors, and converts them into digital signals and sends them to the controller. The controller processes the feedback signals from the two pressure sensors to form a feedback pressure difference signal. The controller sends instructions to the driver and the first solenoid switch valve and the second solenoid switch valve. The driver drives the DC motor to run and receives feedback signals from the motor encoder. The DC motor drives the bidirectional fixed-displacement plunger pump to rotate.

[0044] like Figure 2 As shown, the control method adopts a three-loop cascade control method, the outermost loop is the joint angle displacement control, and the controller sends the target angle trajectory α ref and the actual angle α of the joint angular displacement sensor act To make a difference, use the feedback PD control method to generate the joint target torque , where K1 is the proportional gain of the angle control loop, and K2 is the differential gain of the angle control loop. At the same time, the EHA output force acting on the robot joint is solved through the parameters of the actual linkage mechanism of the robot and the real-time actual joint angle value to obtain the real-time EHA output force arm l , and then through the symmetrical hydraulic cylinder action area A, the EHA target pressure difference trajectory can be obtained , the second control loop tracks and controls the EHA target pressure difference trajectory, and adopts the feedforward + compensation + feedback method for control, where the feedforward term, leakage compensation term, and feedback term are obtained by the following equations:

[0045]

[0046] in The motor speed of the feedforward project, Motor speed for leakage compensation project, To feedback the motor speed of the project, is the pump displacement, E is the elastic modulus of the hydraulic oil, V is the volume of the symmetrical hydraulic cylinder, is the leakage coefficient, is the actual pressure difference of the EHA symmetrical hydraulic cylinder, is the proportional gain of the pressure control loop, is the integral gain of the pressure control loop, is the differential gain of the pressure control loop, and finally the target speed of the EHA motor is obtained , the motor target speed is sent to the driver for closed-loop feedback control, so that the joint angle moves according to the command trajectory.

[0047] like Figure 3 As shown, it also includes a posture maintaining standing control step. When the leg joint angle of the robot reaches a predetermined position, the controller sends a command to cut off the power to the first solenoid switch valve and the second solenoid switch valve, and then the controller sends a command to the driver to enable the DC motor. After that, the leg joint of the robot can remain motionless.

[0048] like Figure 4 As shown, it also includes a power-off protection control step. When the robot needs to be powered on, the driver is powered on first, and the controller sends a command to enable the DC motor. After the DC motor is enabled, the controller sends a command to power on the first electromagnetic switch valve and the second electromagnetic switch valve. After the electromagnetic valve is opened, the controller sends a joint angle target trajectory to perform angle trajectory tracking closed-loop control. If a fault occurs during the power-off protection control process, the first switch electromagnetic valve and the second switch electromagnetic valve are first powered off, and then the driver is powered off to check the fault. After the fault is cleared, the power-off protection control step is continued.

[0049] It should be noted that the above detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present application belongs.

[0050] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments described in the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0051] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein.

[0052] In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or inherent to these processes, methods, products, or apparatuses.

[0053] For ease of description, spatially relative terms, such as "above", "above", "on the upper surface of", "above", etc., may be used herein to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" may include both "above" and "below". The device may also be positioned in other different ways, such as rotated 90 degrees or in other orientations, and the spatially relative descriptions used herein are interpreted accordingly.

[0054] In the above detailed description, reference is made to the accompanying drawings, which form a part of this document. In the accompanying drawings, similar symbols typically identify similar components unless the context indicates otherwise. The illustrated embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.

[0055] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A control method for a robot joint EHA control system, comprising the following steps: The controller receives a command signal from a host computer, wherein the command signal includes information about the target angle trajectory of the joint. The signal acquisition system acquires feedback signals from the joint angle displacement sensor and feedback signals from two pressure sensors, and converts them into digital signals and sends them to the controller. The controller processes the feedback signals from the two pressure sensors to form a feedback pressure difference signal. The controller issues instructions to the driver and the first electromagnetic switch valve and the second electromagnetic switch valve. The driver drives the DC motor to run and receives feedback signals from the motor encoder. The DC motor drives the bidirectional fixed displacement plunger pump to rotate. The control method adopts a three-loop cascade control method, the outermost loop is the joint angle displacement control, and the controller sends the target angle trajectory α ref and the actual angle α of the joint angular displacement sensor act To make a difference, use the feedback PD control method to generate the joint target torque , where K1 is the proportional gain of the angle control loop, and K2 is the differential gain of the angle control loop. The force arm of the EHA output force acting on the robot joint is calculated through the parameters of the actual linkage mechanism of the robot and the real-time actual joint angle value, and the real-time EHA output force arm is obtained. l , and then through the symmetrical hydraulic cylinder action area A, the EHA target pressure difference trajectory can be obtained , the second control loop tracks and controls the EHA target pressure difference trajectory, and adopts the feedforward + compensation + feedback method for control, where the feedforward term, leakage compensation term, and feedback term are obtained by the following equations: in is the motor speed of the feedforward project, Motor speed for leakage compensation project, To feedback the motor speed of the project, is the pump displacement, E is the elastic modulus of the hydraulic oil, V is the volume of the symmetrical hydraulic cylinder, is the leakage coefficient, is the actual pressure difference of the EHA symmetrical hydraulic cylinder, is the proportional gain of the pressure control loop, is the integral gain of the pressure control loop, is the differential gain of the pressure control loop, and finally the target speed of the EHA motor is obtained , the motor target speed is sent to the driver for closed-loop feedback control, so that the joint angle moves according to the command trajectory.

2. The control method of a robot joint EHA control system according to claim 1, characterized in that: It also includes a posture maintaining standing control step. When the leg joint angle of the robot reaches a predetermined position, the controller sends a command to cut off the power to the first solenoid switch valve and the second solenoid switch valve, and then the controller sends a command to the driver to enable the DC motor. After that, the leg joint of the robot can remain motionless.

3. The control method of a robot joint EHA control system according to claim 1, characterized in that: It also includes a power-off protection control step. When the robot needs to be powered on to run, the driver is powered on first, and the controller sends a command to enable the DC motor. After the DC motor is enabled, the controller sends a command to power on and open the first solenoid switch valve and the second solenoid switch valve. After the solenoid valve is opened, the controller sends the joint angle target trajectory to perform angle trajectory tracking closed-loop control.

4. The control method of a robot joint EHA control system according to claim 3, characterized in that: If a fault occurs during the power-off protection control process, first power off the first switch solenoid valve and the second switch solenoid valve, then power off the driver, check the fault, and continue the power-off protection control step after the fault is cleared.

5. A robot leg joint actuation EHA control system, characterized in that: The invention comprises a host computer (1), a controller (2), a signal acquisition system (3), a driver (4), a DC motor (5), a bidirectional fixed-displacement plunger pump (6), a motor encoder (7), a symmetrical hydraulic cylinder (12) and a joint angular displacement sensor (13), wherein the controller (2) is connected to the host computer (1) via a communication interface to receive instructions issued by the host computer (1) and to send instructions to a driver (4) connected thereto, wherein the driver (4) is connected to the DC motor (5), wherein the DC motor (5) and the bidirectional fixed-displacement plunger pump (6) are serially mounted on the same shaft, wherein the motor encoder (7) is mounted on the DC motor (5) to collect the rotation speed and position signals of the motor, and wherein the driver (4) is connected to the DC motor (5) to collect the rotation speed and position signals of the motor. ) is connected to the motor encoder (7) to receive the speed and position signals fed back by the motor encoder (7), so as to control the speed of the DC motor (5), thereby driving the bidirectional fixed-displacement plunger pump (6) to rotate, the two output ends of the bidirectional fixed-displacement plunger pump (6) are respectively connected to the upper and lower chambers of the symmetrical hydraulic cylinder (12), the joint angular displacement sensor (13) is installed at the rotation joint of the robot leg, and is used to collect joint angle signals and send the collected joint angle signals to the signal acquisition system (3), the signal acquisition system (3) sends the processed signals to the controller, and the control system is controlled by the control method of the robot joint EHA control system described in any one of claims 1 to 4.

6. A robot leg joint actuation EHA control system according to claim 5, characterized in that: The signal acquisition system (3) converts the acquired sensor analog signal into a digital signal and sends it to the controller.

7. The robot leg joint actuation EHA control system according to claim 5, characterized in that: The two output ends of the bidirectional fixed-displacement plunger pump (6) are respectively provided with a first pressure sensor (8) and a second pressure sensor (9), and are connected to the upper and lower chambers of the symmetrical hydraulic cylinder (12) for collecting oil pressure signals of the two chambers of the symmetrical hydraulic cylinder (12).

8. The robot leg joint actuation EHA control system according to claim 5, characterized in that: The two output ends of the bidirectional fixed-displacement plunger pump (6) are respectively provided with a first electromagnetic switch valve (10) and a second electromagnetic switch valve (11), and are connected to the upper and lower chambers of the symmetrical hydraulic cylinder (12). The controller (2) sends instructions to its power supply circuit to control the opening and closing of the electromagnetic switch valves.

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