Foreword
Electro-hydraulic servo systems possess advantages such as fast response speed, wide speed range, high power density, and high durability, and have been widely used in vehicles, ships, engineering machinery, and weapon systems. To analyze, simulate, and control electro-hydraulic servo systems, it is necessary to construct a model of the system. However, inherent nonlinear factors such as flow-pressure curves, liquid compression, electromagnetic conversion, and saturation friction make it difficult to obtain an accurate model of the electro-hydraulic servo system.
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.
This paper implements position control of a servo cylinder using a proportional directional valve, and incorporates a displacement sensor to form a closed-loop position control system. A NI USB-6008 data acquisition card is used to complete multiple functions such as data acquisition and data output control. Good real-time control functionality is achieved through mixed programming of LabVIEW and MATLAB.
1. 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.
2. System Principles and Modeling
2.1 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 electro-hydraulic position servo control system
2.2 Modeling of Electro-hydraulic Position Servo System
Establishing the transfer function of an electro-hydraulic servo system requires establishing functional expressions for components such as the servo valve and hydraulic cylinder based on physical laws such as the continuity equation and Newton's laws, and then solving these expressions simultaneously to derive the system's transfer function. Specifically, this involves establishing the servo valve equation, the servo valve flow equation, the continuity equation, and the force balance equation.
2.2.1 Servo Valve Equation
Servo valves are devices with complex, high-order nonlinear characteristics. In practice, servo valves can often be simplified and equivalent to a first-order system (low frequency) or a second-order system (high frequency):
Figure 2. System Simulink dynamic simulation model
3. Software Design
LABVIEW Online Control Process: First, data acquisition is performed. The acquired data is the displacement of the displacement sensor, which is converted into voltage and sent to the analog input terminal AI0 of the data acquisition card. The analog input channel is configured in the program, mainly including configuring the sampling channel number, maximum and minimum values and sampling mode (differential, single-ended), and outputting the sampling waveform. Next is the PID algorithm, which requires setting the parameters of P, I, and D and the upper and lower limits of the output. Then comes the analog output. The analog output channel is configured in the program, the output port is configured as AO0 port, and the maximum and minimum values of the output are configured. The value obtained after the program calculation is sent to the input terminal of the servo amplifier to drive the servo valve, so that the hydraulic cylinder moves forward or backward, completing the position control of the electro-hydraulic servo system [3]. The flowchart of the data acquisition system is shown in Figure 3.
Figure 3. Flowchart of the data acquisition system
4. MATLAB simulation
An intelligent model of an automated constant-depth electro-hydraulic servo system was constructed based on a multi-layer feedforward neural network. The hidden layer had 8 nodes, and the model was trained 1000 times. Because the uncertainty of the initial values of the neural network parameters can lead to variations in network performance, the neural network was trained 10 times, and the average value of performance indicators such as root mean square error was used. The neural network can fit the dynamic characteristics of the electro-hydraulic servo system well, and the model has high generalization ability. Simulation results are shown in Figure 4.
Figure 4. Output and actual values of the neural network of the electro-hydraulic servo system.
5. Conclusion
Electro-hydraulic servo systems are a typical type of nonlinear system. This paper employs mechanistic modeling and intelligent modeling methods to lay the foundation for further control of electro-hydraulic servo systems. The modeling process and simulation results demonstrate that establishing a correct mathematical model and conducting analysis and simulation of the system's dynamic characteristics can effectively predict the system's output, achieving an understanding of the system's operating state. This improves the efficiency of system design and analysis, and lays a foundation for further control systems, enhancing response speed and control accuracy.
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