Abstract : An electro-hydraulic servo system is an automatic control system based on the principle of hydraulic transmission and employing an electro-hydraulic servo mechanism. By establishing a mathematical model of the electro-hydraulic servo position system, a single-neuron adaptive PID intelligent control algorithm is proposed, taking into account the characteristics of a double-acting hydraulic servo position control system. The control system parameters are optimized through Simulink simulation. Finally, real-time measurement and control software is developed using LabVIEW, and real-time position servo control experiments of the hydraulic cylinder are conducted. Experimental results show that the electro-hydraulic servo system has significant advantages such as fast response speed, high output power, and high control accuracy, effectively improving the position control performance of the system.
Keywords: electro-hydraulic servo; LabVIEW; robot; Matlab
1. Introduction
Hydraulic transmission technology is widely used in various types of engineering machinery. With the development of computer technology, hydraulic transmission technology has evolved into a complete automation technology encompassing transmission, detection, and control. Due to the significant uncertainties and interference inherent in electro-hydraulic servo systems, high demands are placed on electro-hydraulic servo testing systems.
Electro-hydraulic position servo systems are the most basic and commonly used type of hydraulic servo system, used in applications such as machine tool table positioning, strip thickness control in strip mills, strip misalignment control, aircraft and ship steering gear control, radar and artillery control systems, and vibration test benches. In control systems for other physical quantities, such as speed and force control systems, position control loops are often used as components within a larger loop.
Electro-hydraulic position servo systems are primarily used to solve position tracking control problems. Their fundamental task is to achieve timely and accurate tracking of the controlled variable to the given variable through an actuator, while maintaining sufficient control precision. The dynamic characteristics of an electro-hydraulic servo system are a crucial indicator for evaluating its design and debugging level. It consists of an electrical signal processing unit and several hydraulic components. The dynamic performance of these components interacts and restricts each other, and the system itself contains nonlinearities, resulting in complex dynamic performance. Therefore, the design and simulation of electro-hydraulic servo control systems are receiving increasing attention.
2. Brief Description of Hydraulic System Characteristics
With the continuous development and advancement of hydraulic technology and the expanding application areas and scope, the requirements for system flexibility and various performance aspects are becoming increasingly stringent. Traditional system designs that focus on completing predetermined actuator cycles and are limited by static system performance are far from meeting these demands. Therefore, it is essential for modern hydraulic system design researchers to study the dynamic characteristics of systems, understand and master the dynamic working characteristics and parameter changes of hydraulic systems, in order to improve the system's response characteristics, control accuracy, and operational reliability.
The dynamic characteristics of a hydraulic system refer to its properties as it moves from a previous equilibrium state to a new one. These characteristics are primarily caused by changes in the transmission and control systems, as well as external disturbances. During this process, the performance of each system parameter over time determines the quality of the system's dynamic characteristics. The dynamic characteristics mainly manifest as stability (the instantaneous peak and fluctuation of pressure in the system) and transient response quality (the response quality and speed of the actuators and control mechanisms).
The main research methods for the dynamic characteristics of hydraulic systems include transfer function analysis, simulation, experimental research, and digital simulation. Digital simulation utilizes computer technology to study the dynamic characteristics of hydraulic systems. First, a digital model of the hydraulic system's dynamic process—the state equations—is established. Then, the time-domain solutions of the main variables in the system during the dynamic process are obtained on a computer. This method is applicable to both linear and nonlinear systems, and can simulate the changes in various system parameters under the action of input functions. This provides a direct and comprehensive understanding of the system's dynamic process, allowing researchers to predict the dynamic performance of the hydraulic system during the design phase. This enables timely verification and improvement of the design results, ensuring the system's performance and reliability. It offers advantages such as accuracy, adaptability, short cycle time, and low cost.
3 System Principles and Modeling
3.1 System Composition and Principle
The electro-hydraulic position servo control system uses liquid as the power transmission and control medium, and utilizes electrical signals for control input and feedback. As long as a specific input signal is input, the actuator can start and quickly and accurately reproduce the change pattern of the input quantity. The control system structure diagram is shown in Figure 1.
Figure 1. Structure diagram of the electro-hydraulic position servo control system
3.2 Modeling of Electro-hydraulic Position Servo System
A typical electro-hydraulic servo control system model for a workpiece fatigue testing machine was established. Based on the MATLAB/Simulink environment, the general method for modeling an electro-hydraulic servo control system is introduced. As shown in Table 1, the design requirements and given parameters of the electro-hydraulic servo control system for the workpiece fatigue testing machine are as follows: a double-rod hydraulic cylinder is used to load the specimen, and a force sensor is used for detection and feedback, thus forming a closed-loop electro-hydraulic control system for servo valve control of the hydraulic cylinder. Its principle is shown in Figure 2.
Table 1 Design requirements and given parameters for the electro-hydraulic servo control system of the workpiece fatigue testing machine.
Figure 2. Schematic diagram and block diagram of the electro-hydraulic servo control system for workpiece fatigue testing.
The parameters of the hydraulic power components should meet the dynamic characteristics required by the entire system, and should also consider optimal matching with the load parameters to ensure minimal power consumption and high efficiency. Based on the design requirements shown in Table 1, the selected and calculated power component parameters are shown in Table 2.
Table 2 Main parameters of the components
System modeling typically utilizes differential and difference equations, but for typical electro-hydraulic servo control systems, the transfer functions can be directly established using the mathematical models of the system's components or parts. The transfer functions of the components or parts of the electro-hydraulic servo control system for a workpiece fatigue testing machine are as follows:
Servo amplifier transfer function: ΔI(s)/Uc(s) = Ka
Electro-hydraulic servo valve transfer function: KsvGsv(s)=Q0/ΔI=Ksv (considered as a proportional element)
The transfer function between the hydraulic cylinder and the load is: Fg=KpAp(s2/ωm2+1)/(s/ωr+1)(s2/ω02+2*ξ0/ω0s+1)
In the formula, ωm is the natural frequency of the load.
ωr — the corner frequency of a first-order inertial element;
ω0 — the natural frequency of the second-order oscillatory element;
ξ0 — Damping ratio of the second-order oscillating element.
Based on the system's given parameters and by consulting the servo valve sample, the main parameters in the transfer function are calculated when the load spring Ks = 180000 N/cm: amplifier gain Ka = 40000 mA/V, ωr = 0.588 rad/s, ωm = 200 rad/s, ω0 = 674 rad/s, ξ0 = 0.005. Then, based on the transfer functions of each component of the system, a Simulink dynamic model of the system is established, as shown in Figure 3.
Figure 3 Simulink dynamic simulation model of the system
4 Software Design
The LabVIEW online control process begins with data acquisition. The acquired data is the displacement from the displacement sensor, which is converted into voltage and sent to the analog input terminal AI0 of the data acquisition card. The program configures the analog input channel, including the sampling channel number, maximum and minimum values, and sampling mode (differential or single-ended), and outputs the sampled waveform. Next, the PID algorithm is used, setting the parameters of P, I, and D, as well as the upper and lower limits of the output. Then, the analog output is configured. The program configures the analog output channel, setting the output port to AO0 and configuring the maximum and minimum values. The calculated value is sent to the input of the servo amplifier, driving the servo valve to move the hydraulic cylinder forward or backward, thus completing the position control of the electro-hydraulic servo system.
During data acquisition, the front panel is configured with physical channels, maximum and minimum values for analog inputs, and single-ended configuration. Real-time waveform curves of the acquired data are plotted. In this experiment, the object of real-time data acquisition is the feedback value from the displacement sensor, which is sent to the analog input terminal of the N1-6008 data acquisition card. The acquisition system subroutine is shown in Figure 4.
Figure 4. Flowchart of the data acquisition subroutine
5. Conclusion
(1) The modeling process and simulation results show that by establishing a correct mathematical model of the system and conducting analysis and simulation, the dynamic characteristics of the system can be analyzed, the output of the system can be effectively predicted, the working state of the system can be understood, the efficiency of system design and analysis can be improved, and a certain foundation can be laid for further control system, improving response speed and control accuracy.
(2) As can be seen from the above, the reliability verification of the system provided by MATLAB/Simulink simulation accurately simulates the working state of the actual system, and will surely be widely used in the field of electro-hydraulic servo control. LABVIEW visual programming makes the system simpler and easier to operate.
