Hydraulic position closed-loop system nonlinear control method based on servo valve zero offset identification compensation and implementation system of hydraulic position closed-loop system nonlinear control method

Abstract

The invention relates to a hydraulic position closed-loop system nonlinear control method based on servo valve zero offset identification compensation and an implementation system of the hydraulic position closed-loop system nonlinear control method. The control method comprises the following steps: S1, carrying out zero servo control on a hydraulic position closed-loop system by adopting a PID (Proportion Integration Differentiation) control method, and recording a servo valve control input signal at the moment; s2, the accuracy of a servo valve zero offset signal is judged according to the zero position control precision, and if the tracking error is not larger than a preset threshold value, the current servo valve control input is recorded as the zero offset signal; if the tracking error is smaller than the threshold value, the step S1 is repeated after the PID control parameter is adjusted until the precision requirement is met; and S3, designing a nonlinear control law based on a system model, introducing an identified servo valve zero offset signal as a feedforward compensation item in the derivation process of the control law, and performing zero offset compensation on control input to obtain a total control input signal. According to the method, the zero offset signal of the servo valve is accurately identified and introduced into the control law for compensation, so that the comprehensive control precision and stability of the system are improved.

Description

Technical Field

[0001] The present application relates to the field of hydraulic servo control technology, and in particular to a method for identifying and compensating for the zero bias problem of a servo valve in a valve-controlled hydraulic servo system, aiming to improve the control accuracy and stability of the hydraulic position closed-loop system. Background Art

[0002] Hydraulic servo systems, especially valve-controlled hydraulic servo systems, have the advantages of high power density and fast dynamic response, and are widely used in high-end equipment such as aerospace, robotics, and material forming machinery. However, in valve-controlled hydraulic servo systems, the inherent pressure-flow nonlinearity of the servo valve, as well as the parameter uncertainty and uncertain nonlinearity existing in the entire system, will lead to a serious degradation of control accuracy.

[0003] In view of the uncertainty of the flow coefficient of the zero-opening servo valve, the flow coefficient here refers to the flow corresponding to the unit displacement of the servo valve core under no-load conditions. Traditionally, the flow coefficient can be understood as an ideal value. However, due to errors in the processing or installation process, the zero-opening servo valve always has a certain zero offset when it leaves the factory. In addition, with the increase of working time, wear and erosion will also cause the servo valve port to change from the initial sharp angle to a rounded angle, causing the flow coefficient to become an unknown value. At present, it is a common treatment method to directly ignore the zero offset of the servo valve or regard it as a flow parameter uncertainty for adaptive estimation. For the former, the existing valve-controlled hydraulic servo system ignores the servo valve zero offset, believing that it is small enough and not enough to have a fundamental impact on the normal operation of the entire system. In fact, for some hydraulic servo equipment that works for a long time, the servo valve zero offset will gradually increase with the increase of working time. At present, there is no research to indicate to what extent the increase of zero offset will affect the normal operation of the entire system. Therefore, for the existing nonlinear control method of the hydraulic position closed-loop system, it is obviously unreasonable to ignore the servo valve zero offset when it increases. For the latter, due to the inherent contradiction between the convergence speed of parameter estimation and system stability, it is difficult to meet the two indicators of overshoot and adjustment time at the same time. The main defects are as follows:

[0004] 1. For some high-power valve-controlled hydraulic servo systems, the servo valve has a large rated flow rate, and a small zero offset will produce a large flow deviation. If the existing method is used, the controller needs to be adjusted for a relatively long time, which will reduce the overall control accuracy. In addition, when the zero offset is used as an uncertain flow parameter, there is no setting criterion for the convergence speed, which will bring great difficulties to the debugging process;

[0005] 2. Treating the servo valve zero bias as an uncertain flow coefficient and increasing the adaptive rate of this parameter can help speed up the convergence of the flow coefficient to a certain extent, and can achieve the purpose of improving control accuracy to a certain extent, but this also greatly increases the risk of system instability and is not recommended. In addition, parameter adaptation usually needs to meet the conditions of continuous excitation, which will also limit its applicability;

[0006] 3. In addition to the shortcomings in control accuracy, system stability and the need for continuous excitation, the existing methods may also face a certain degree of safety hazards. For some valve-controlled hydraulic servo systems, due to the existence of zero bias, flow will flow into the actuator through the servo valve before the system enters closed-loop control. If it is not discovered and handled in time, it will cause the pressure in the actuator to increase, which can reach the system pressure at most, posing a potential safety hazard.

[0007] It should be noted that the information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or suggestion in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0008] The purpose of the present invention is to provide a nonlinear control method for a hydraulic position closed-loop system based on servo valve zero bias identification and compensation and its implementation system. The method improves the comprehensive control accuracy and stability of the system by accurately identifying the zero bias signal of the servo valve and introducing it into the control law for compensation, thereby overcoming the defects of the prior art.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present application provides a nonlinear control method for a hydraulic position closed-loop system based on servo valve zero bias identification and compensation, comprising the following steps:

[0011] Identification of S1 servo valve zero bias signal: Use PID control method to perform zero-position servo control on the hydraulic position closed-loop system and record the servo valve control input signal at this time; judge the accuracy of the servo valve zero bias signal according to the zero-position control accuracy. If the tracking error is not greater than the preset threshold, record the current servo valve control input as the zero bias signal; if the tracking error is less than the threshold, adjust the PID control parameters and repeat S1 until the accuracy requirements are met;

[0012] S2 Model-based nonlinear control design: Design a nonlinear control law based on the system model. In the derivation of the control law, the servo valve zero bias signal identified in S1 is introduced as a feedforward compensation item to perform zero bias compensation on the control input and obtain the total control input signal.

[0013] Furthermore, the servo valve zero bias signal identification of S1 specifically includes the following steps:

[0014] S11 initialization phase: setting the initial parameters of the PID controller, including the proportional coefficient K p , integral coefficient K i , differential coefficient K d and the preset threshold ε for zero bias identification;

[0015] S12 Zero position servo control: The controller sends a control signal to the servo valve to make the hydraulic motor try to track the zero position target position. During this process, the controller records the control input signal of the servo valve and calculates the tracking error e between the actual position of the hydraulic motor and the zero position target position;

[0016] S13 Zero bias signal identification: If the tracking error e is not greater than the preset threshold ε, the control input of the current servo valve is considered to be the zero bias signal, and the value is saved; if the tracking error e is greater than the threshold ε, the parameters of the PID controller are adjusted according to the preset parameter adjustment strategy, and zero-position servo control and zero bias signal identification are repeated until the accuracy requirements are met.

[0017] Furthermore, in step S13, the parameter adjustment strategy of the PID controller is a trial and error method, that is, the three control parameters of the PID are continuously corrected according to the size of the error: the proportional coefficient K p , integral coefficient K i , differential coefficient K d When the tracking error e is not greater than the threshold ε, stop adjusting the control parameters and record the control parameters at this time.

[0018] Furthermore, in step S13, the adjustment of the PID control parameters includes the proportional coefficient K p , integral coefficient K i , differential coefficient K d adjustments.

[0019] Furthermore, in step S11, the preset threshold ε is adjusted dynamically according to the system accuracy requirements and actual working conditions. If the system control accuracy requirements are high, the threshold ε is set to be smaller; if the system control accuracy requirements are low, the threshold ε can be set to be larger to save adjustment time.

[0020] Furthermore, in step S2, the nonlinear control law adopts a combination of one or more strategies such as sliding mode control, adaptive control or robust control.

[0021] Furthermore, it also includes giving a zero bias compensation signal according to the identified servo valve zero bias signal before the system is started or after the servo control is exited, so as to suppress the risk of the system flow entering the hydraulic actuator through the servo valve.

[0022] In a second aspect, the present application provides a hydraulic position closed-loop system, comprising a hydraulic power source, a servo valve, a hydraulic motor, an encoder and a controller, wherein the controller adopts the control method of the first aspect to perform position closed-loop control on the hydraulic motor.

[0023] Furthermore, the controller adjusts the oil flow from the hydraulic power source to the hydraulic motor by controlling the valve core opening size of the servo valve, and then uses the encoder to measure the angle of the hydraulic motor in real time to achieve closed-loop position control of the hydraulic motor.

[0024] Furthermore, it also includes a monitoring device for real-time monitoring of changes in the servo valve zero bias signal so as to adjust the compensation strategy in time.

[0025] The beneficial effect of the present invention is that: for a valve-controlled hydraulic servo system, a control method based on servo valve zero bias identification and compensation is proposed, which can be applied to the servo control of a hydraulic position closed-loop system. The method can improve the comprehensive control accuracy of the hydraulic position closed-loop system, especially the control accuracy at the start-up moment, and has a significant improvement effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 According to some embodiments of the present application, a schematic diagram of a hydraulic position closed-loop control system is shown;

[0027] Figure 2 According to some embodiments of the present application, a schematic diagram of a nonlinear control system of a valve-controlled hydraulic motor is shown;

[0028] Figure 3 According to some embodiments of the present application, a method diagram of servo valve zero bias identification is shown (where a is a closed-loop tracking diagram and b is a control input diagram);

[0029] Figure 4 According to some embodiments of the present application, a zero bias compensation control block diagram of a servo valve is shown;

[0030] Figure 5 According to some embodiments of the present application, a control effect diagram of servo valve zero offset compensation is shown. DETAILED DESCRIPTION

[0031] The technical features and advantages of the present invention are described in more detail below in conjunction with the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0032] In order to more specifically illustrate the implementation process and effect of the nonlinear control method of the hydraulic position closed-loop system based on servo valve zero bias identification and compensation of the present invention, a detailed embodiment is provided below.

[0033] 1. System composition

[0034] The hydraulic position closed-loop control system in this embodiment is mainly composed of a hydraulic power source, a high-precision servo valve, a hydraulic motor, a high-resolution encoder, and an embedded controller. The hydraulic power source provides the power required by the system, the servo valve is responsible for controlling the flow direction and flow of the hydraulic fluid, the hydraulic motor acts as an actuator to convert hydraulic energy into mechanical energy, and the encoder is used to monitor the position information of the hydraulic motor in real time and feed it back to the controller for closed-loop control. The controller controls the valve core opening size of the servo valve to adjust the oil flow from the hydraulic power source to the hydraulic motor, and then uses the encoder to measure the angle of the hydraulic motor in real time to achieve closed-loop position control of the hydraulic motor. Please refer to Figure 1 , showing the schematic diagram of the hydraulic position closed-loop control system of this embodiment; Figure 2 The schematic diagram of the nonlinear control system of the valve-controlled hydraulic motor system is shown.

[0035] 2. Identification of servo valve zero bias signal

[0036] 1. Initialization phase: First, the system is in standby mode, the controller starts the initialization program, sets the initial parameters of the PID controller (including the proportional coefficient K p , integral coefficient K i , differential coefficient K d ) and the preset threshold ε for zero bias identification.

[0037] 2. Zero position servo control: The controller sends a control signal to the servo valve to make the hydraulic motor try to track the zero position target position. During this process, the controller records the control input signal of the servo valve and calculates the tracking error e between the actual position of the hydraulic motor and the zero position target position. Figure 3 As shown, Figure 3 (a) is the closed-loop tracking curve when the servo valve is in zero position. Figure 3 (b) is the control input curve when the servo valve is in zero position.

[0038] 3. Zero bias signal identification: According to Figure 3Observe the accuracy of the current zero-position control and record the servo valve control input at this time. If the tracking error e is not greater than the preset threshold ε, it is considered that the zero-position control accuracy of the current servo valve is good, then record the control input of the current servo valve as the zero bias signal u0, and save the value. If the tracking error e is greater than the threshold ε, adjust the parameters of the PID controller according to the preset parameter adjustment strategy, and repeat the zero-position servo control of step 2 and the zero bias signal identification of step 3 until the accuracy requirements are met, until the expected zero-position control accuracy (that is, e≤ε) is obtained, and then record the corresponding servo valve control input as the zero bias signal. In this embodiment, the preset PID control parameter adjustment strategy can be a trial and error method, that is, continuously correcting the three control parameters of the PID (proportional coefficient K p , integral coefficient K i , differential coefficient K d ), specifically, we can first give a set of relatively small control parameters, let p = 1, i = 1, and then observe the size of the error e, and adjust the control parameters according to the size of the error; when the tracking error e is not greater than the threshold ε, stop adjusting the control parameters, and record the control parameters at this time as the zero bias signal u0.

[0039] In this embodiment, according to the system characteristics and control accuracy requirements, the preset threshold ε of zero bias identification is set to 0.01°. If the zero position control accuracy is good, that is, the tracking error e is not greater than 0.01°, the control input of the current servo valve is recorded as the zero bias signal u0. Figure 3 As shown in (b), in the system of this embodiment, the zero bias signal u0 identified by the above method is approximately 0.17V.

[0040] 3. Model-based nonlinear control design

[0041] 1. System modeling: According to the physical characteristics and dynamic equations of the hydraulic position closed-loop system, a mathematical model of the system is established. The model should include factors such as the flow characteristics of the servo valve, the dynamic response of the hydraulic motor, and the damping and stiffness of the hydraulic system. The specific mathematical model is as follows:

[0042] Define the angle of the hydraulic motor as γ and the load pressure P L =P1-P2, so the state variable is defined as Then the state equation of the system can be expressed as:

[0043]

[0044] In the above formula, f r represents the friction torque, which is assumed to be a smooth function and only a function of x2;

[0045] V1=V 10 +D γis the control volume of the hydraulic motor V1 chamber, V 10 is the initial control volume of the V1 cavity;

[0046] V2=V 20 -D γ is the control volume of the hydraulic motor V2 chamber, V 20 is the initial control volume of the V2 cavity;

[0047] D is the displacement of the motor, B is the viscous friction coefficient, β e is the effective bulk modulus of the hydraulic oil, K v is the flow coefficient of the servo valve, C t is the internal leakage coefficient of the hydraulic motor, u is the control input of the servo valve, J is the inertia of the hydraulic motor shaft, s v (·) is a user-defined function:

[0048]

[0049] 2. Nonlinear control law design: Based on the established system model, a nonlinear control law including zero bias compensation is designed. The obtained servo valve zero bias signal u0 is introduced into the feedforward channel for compensation, that is, the control input u is compensated for zero bias to obtain the total control input u t =u+u0. In this embodiment, the derivation process of the control law is as follows:

[0050] In order to facilitate the design of control law, the mathematical model of the system is first re-expressed as

[0051]

[0052] In the above formula, θ1=D / J, θ2=1 / J, θ3=β e K v ,θ4=β e D, θ5 = β e C t , f1=(R1 / V1+R2 / V2), f2=(1 / V1+1 / V2)x2f3=(1 / V1+1 / V2)x3.

[0053] definition θ=[θ1,θ2,θ3,θ4,θ5] T The estimated value of , and θ satisfies:

[0054]

[0055] In the formula, θ min =[θ 1min ,θ 2min ,θ 3min ,θ 4min,θ 5min ] T and θ max =[θ 1max ,θ 2max ,θ 3max ,θ 4max ,θ 5max ] T are the minimum and maximum values ​​of the estimated parameters, respectively, and both are assumed to be known.

[0056] Define the following intermediate variables:

[0057]

[0058] In the formula, x 1d is a sufficiently smooth control instruction, and k1 is a positive feedback coefficient.

[0059] The designed virtual control law is:

[0060]

[0061] Where k2 is a positive feedback coefficient.

[0062] The designed control law is:

[0063]

[0064] Where k3 is a positive feedback coefficient.

[0065] The parameter estimates used in the virtual control law and the control law are obtained through the parameter adaptation law of the following formula.

[0066]

[0067] Where Γ is a positive definite diagonal matrix,

[0068] In this embodiment, an adaptive robust control strategy is adopted, and the parameter adaptive law and control law are constructed in combination with the identified servo valve zero bias signal u0. In the control law, the zero bias compensation u0 term directly acts on the control input to offset the influence of the servo valve zero bias on the system performance. Figure 4 The servo valve zero bias compensation control block diagram shown in the figure performs a closed-loop test similar to the sinusoidal tracking instruction. After setting the model parameters and control parameters, the system is run. Through the assistance of state feedback and encoder, the servo valve zero bias compensation control effect diagram is obtained, as shown in Figure 5 shown.

[0069] according to Figure 5(a), the black dotted line in the figure is the command signal, the red solid line is the sampling signal with zero bias compensation in this embodiment, and the blue solid line is the sampling signal without zero bias compensation using the prior art. By making a difference with the command signal respectively, the tracking error curve shown in 5(b) is obtained, the red solid line is the tracking error with zero bias compensation proposed in this application, and the blue solid line is the tracking error without zero bias compensation using the prior art. Obviously, in the startup stage, the zero bias compensation method proposed in this embodiment has a more obvious effect on suppressing the tracking error, and after entering the steady-state stage, although the tracking error is only slightly better than the prior art, the method proposed in this embodiment does not need to meet the continuous excitation condition, nor does it need to consider how to adaptively set the flow coefficient. According to the servo valve control input shown in 5(c), introducing the zero bias signal into the control law does reflect a more reasonable result.

[0070] This embodiment verifies the effectiveness and superiority of the nonlinear control method of the hydraulic position closed-loop system based on servo valve zero bias identification and compensation in the embodiment of the present invention through detailed steps and experimental data. This method not only improves the control accuracy and stability of the system, but also reduces the requirements for the servo valve performance, and has broad application prospects.

[0071] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A nonlinear control method for a hydraulic position closed-loop system based on servo valve zero bias identification and compensation, characterized in that: The following steps are involved: S1 Identification of servo valve zero bias signal: Use PID control method to perform zero-position servo control on the hydraulic position closed-loop system and record the servo valve control input signal at this time; judge the accuracy of the servo valve zero bias signal according to the zero-position control accuracy. If the tracking error is not greater than the preset threshold, record the current servo valve control input as the zero bias signal; If the tracking error is less than the threshold, adjust the PID control parameters and repeat S1 until the accuracy requirement is met; S2 Model-based nonlinear control design: Design a nonlinear control law based on the system model. In the derivation of the control law, the servo valve zero bias signal identified in S1 is introduced as a feedforward compensation item to perform zero bias compensation on the control input and obtain the total control input signal.

2. The control method according to claim 1, characterized in that: The servo valve zero bias signal identification of S1 specifically includes the following steps: S11 initialization phase: setting the initial parameters of the PID controller, including the proportional coefficient K p , integral coefficient K i , differential coefficient K d and the preset threshold ε for zero bias identification; S12 Zero position servo control: The controller sends a control signal to the servo valve to make the hydraulic motor try to track the zero position target position; during this process, the controller records the control input signal of the servo valve and calculates the tracking error e between the actual position of the hydraulic motor and the zero position target position; S13 Zero bias signal identification: If the tracking error e is not greater than the preset threshold ε, the control input of the current servo valve is considered to be the zero bias signal, and the value is saved; if the tracking error e is greater than the threshold ε, the parameters of the PID controller are adjusted according to the preset parameter adjustment strategy, and the zero-position servo control S12 and zero bias signal identification S13 are repeated until the accuracy requirements are met.

3. The control method according to claim 1, characterized in that: In step S1, the adjustment of the PID control parameters includes the proportional coefficient K p , integral coefficient K i and the differential coefficient K d adjustments.

4. The control method according to claim 2, characterized in that: In step S11, the value of the preset threshold ε is set according to the system accuracy requirements and actual working conditions, and can be adjusted dynamically.

5. The control method according to claim 1, characterized in that: In step S2, the nonlinear control law adopts one or a combination of strategies such as sliding mode control, adaptive control or robust control.

6. The control method according to claim 1, characterized in that: The method also includes providing a zero bias compensation signal according to the identified servo valve zero bias signal before the system is started or after the servo control is exited, so as to suppress the risk of the system flow entering the hydraulic actuator through the servo valve.

7. A hydraulic position closed loop system, characterized in that: It comprises a hydraulic power source, a servo valve, a hydraulic motor, an encoder and a controller, wherein the controller adopts the control method described in any one of claims 1 to 6 to perform closed-loop position control on the hydraulic motor.

8. The system according to claim 7, characterized in that The controller adjusts the oil flow from the hydraulic power source to the hydraulic motor by controlling the valve core opening size of the servo valve, and then uses the encoder to measure the angle of the hydraulic motor in real time to achieve closed-loop position control of the hydraulic motor.

9. The system according to claim 7, characterized in that It also includes a monitoring device for real-time monitoring of changes in the servo valve zero bias signal so as to adjust the compensation strategy in time.

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