Abstract: This paper introduces the current status, development trend, and system design method of electro-hydraulic motion control system, and illustrates the characteristics of the method by combining the application of the electro-hydraulic motion control system of a wave generator. Keywords: Electro-hydraulic control; Servo system; Design method Introduction Motion control originated from early servo control. Simply put, motion control is the real-time control and management of the position, speed, or force of mechanical moving parts, so that they move according to the expected motion trajectory and specified motion parameters. Motion control technology has mainly developed along with the development of CNC technology, robotics technology, and factory automation technology. Electro-hydraulic servo control technology, as a bridge connecting modern microelectronics technology, computer technology, and hydraulic technology, has become an important component of modern control technology. Due to its significant advantages such as good linearity, small dead zone, high sensitivity, good dynamic performance, fast response, and high accuracy, it has been widely used. Electro-hydraulic motion control mainly uses hydraulic cylinders and hydraulic motors as actuators, proportional valves or servo valves with amplifiers as power amplification units, position, speed, or pressure sensors as signal feedback units, and necessary control units to form a closed-loop motion control system. It is widely used in packaging, printing, textile, steel, and assembly industries. 1. Current Status and Development Trends Currently, widely used motion control systems mainly consist of two types: electro-servo/stepper motors (hereinafter referred to as electro-motors) and electro-hydraulic systems, with electro-motors being the most widely used. Electro-motor control primarily comprises two main actuators: servo motors and stepper motors (for linear motion, devices such as ball screws can be added to the mechanism for conversion), motion control cards, and controllers. This type of system structure is relatively mature and widely used in various aspects of industrial control. In comparison, electro-hydraulic motion control systems have distinct characteristics, excelling at handling position and force control, primarily linear motion. However, due to limitations in application scenarios and scale, they lag behind electro-motor motion control systems in terms of maturity and system application promotion. Furthermore, their system design is complex and requires highly skilled technicians. Therefore, many large-scale electro-hydraulic automation equipment in China currently relies heavily on imports. Currently, most components of electro-hydraulic motion control systems are commercialized and standardized. For example, proportional valves and servo valves have corresponding product series and related standards; displacement and pressure sensors have universal alternative products from various manufacturers; and actuators such as servo cylinders and hydraulic motors have relevant product series and standards for selection and design reference. In system design, the core of the system that designers need to focus on is the hydraulic motion controller. Currently, the controller is characterized by diversification, variety and non-universality. Electro-hydraulic motion controllers are mainly divided into the following three categories in terms of structure: 1) motion controllers based on computer standard buses; 2) soft open motion controllers; 3) embedded structure motion controllers. At present, domestic motion control system designers mainly design electro-hydraulic motion control systems for specific projects. Because the scale is small and the number is small, the system designers first meet the functional requirements of the system, and then consider the requirements of system universality, reliability, debugging convenience and operation performance. Therefore, the motion control system developed for specific projects has poor portability, no universality, and no relevant standard constraints, which is not conducive to technological progress and will also lead to resource waste. Its main problems are: (1) The design and development cycle is long, the developed system does not have universality, and the performance of the electro-hydraulic motion control system may not be optimized. It does not help the promotion of electro-hydraulic motion control technology. (2) The system reliability is not high and the universality is not strong, resulting in resource waste. Therefore, it is necessary to design a modular electro-hydraulic motion control system to simplify system design and debugging. The future development direction of electro-hydraulic motion control systems will gradually shift from traditional amplification execution units to open modular systems (see Figure 1). Modular dedicated controllers will replace the current major types of controllers. These controllers will require a bus interface for information exchange with a host computer, an open software structure for setting hydraulic system parameters and related motion parameters, and a general-purpose electrical interface for electrical connections to modulators, sensors, etc. 2. Design Considerations Before system design, the control requirements and accuracy must be clearly defined. Then, components are selected, and the hydraulic system and electrical control schematics are designed. Finally, the schematics are presented in physical form, and the preset requirements are achieved through on-site debugging. 2.1 Hydraulic System When designing a hydraulic system, the parameters that need to be controlled must first be determined. Key considerations include system flow rate, pressure, the direction and speed of the drive components, the stopping position of the drive components, and whether acceleration control is required. Then, hydraulic components are selected according to specific requirements. If the accuracy requirement is not high, a proportional control system can be considered. Generally, a proportional control system can achieve a position control accuracy of 3mm, a speed control accuracy (with pressure compensator) of 3%, and an acceleration/deceleration (hill time) of 0.5s. For higher control requirements, a high-frequency response proportional valve control system can be selected to maintain the setpoint unaffected by external interference, ensure stable speed under different working pressures, guarantee the same position under different output forces, and perform synchronous movement under off-center loads. Its control accuracy can achieve: position error less than 1mm, pressure error less than 0.1MPa. If even higher control requirements are needed, the system's control accuracy can be improved by selecting a servo valve with faster response and better performance. 2.2 Electrical System The electrical system works in conjunction with the hydraulic system to fulfill the motion control requirements. It mainly consists of sensor signal detection circuits, control signal circuits, and a control calculation unit. The selection of sensors must meet the requirements of having a resolution five to ten times higher than the required control accuracy, excellent linear performance, and low hysteresis (high dynamic response); otherwise, control failure or failure to meet requirements may occur. The controller should be an open-structure modular controller with a higher resolution (AD/DA bit depth). If a digital controller is used, the scan time should be as short as possible. The control algorithm should be optimized specifically for hydraulic transmission and requires a modular integrator that can be switched on and off for position control, a nonlinear gain for valve spool smoothing and masking compensation, tracking error compensation, PID control, and valve output controller, etc. 2.3 By comprehensively selecting general-purpose hydraulic and electrical components and modular hydraulic controllers, the motion control system shown in Figure 2 is constructed. In addition to the hydraulic and electrical requirements mentioned above, it also needs to meet the requirements for mechanical installation and field signal anti-interference. 3 Application Example The design concept of an electro-hydraulic motion control system example is introduced below. Before introducing the system, the process of the example is briefly explained for ease of explanation. This example is a hydraulic wave generator system. Figure 3 is the wave generator flow chart. As can be seen from the figure, the system generates a control signal for the desired wave according to the given requirements. This signal is then converted into a discrete voltage timing signal and sent to the hydraulic servo drive module. The hydraulic control unit compares the detected displacement signal with the given control signal to derive a 'deviation' signal, which drives the wave generator to produce water waves within the control cycle. During the wave generator's wave generation process, there is inevitably a certain error, and the waves will deform during propagation. Therefore, the waves generated by the wave generator directly driven by the calculated control signal will differ from the input value, requiring multiple adjustments to achieve the desired waveform. The structure diagram of the electro-hydraulic motion control system is shown in Figure 4. Its control logic sequence is as follows: the host computer in the motion control room sends a switch signal to control the start/stop signal of the hydraulic power system, which is then sent to the PLC via the fieldbus to control the start/stop of the motor and pressure protection devices. After the system pressure is established, the host computer converts the waveform into a discrete control timing signal based on the user-required wave waveform and related conversion relationships, and sends the control signal to the hydraulic controller in a time-division manner. Within the control cycle, the controller receives the motion control parameters from the host computer and completes its control to drive the actuator. This process repeats continuously; the controller sequentially completes the motion control parameters sent by the host computer, driving the actuator to move the wave generator. Superimposing these wave generator movements yields the desired waveform. Simultaneously, the controller transmits the detected displacement and pressure signals back to the host computer for system monitoring. The hydraulic controller's role in the system is as follows: it receives the control signal from the host computer, compares it with the detected displacement signal to obtain a deviation signal, uses this signal as the controller's input, and calculates the control voltage (PID control algorithm) to send to the proportional servo valve amplifier. The amplifier uses this voltage signal to control the movement of the proportional servo valve's "valve core," thus controlling the hydraulic cylinder. The hydraulic cylinder, through a four-bar linkage, pushes a wave-making plate to generate the required water waves. During the control process, the controller collects the hydraulic cylinder's pressure signal for system monitoring and pressure protection. During control, the three hydraulic cylinders must move synchronously. The overall control structure of the control system adopts a separate loop control. By detecting the hydraulic cylinder pressure, the system's thrust can be effectively controlled, achieving synchronous control requirements while also providing system protection and correction functions. In field applications, this system effectively meets the parameter requirements and demonstrates good control performance. 4 Conclusion There are certain differences between the design of electro-hydraulic motion control system and ordinary hydraulic system. The open electro-hydraulic control module can simplify the system design, enrich the system debugging means, and effectively improve the system control accuracy. After the system designers master this modular design method, it will be of great benefit to the promotion of technology and the application of the system. References [1] Help Document of WIN-PED. Application Version 5.10, Rexroth Bosch Group, 2004 [2] Rexroth Company Sample Manual [3] Wu Genmao, Qiu Minxiu, Wang Qingfeng "Newly Compiled Practical Electro-hydraulic Proportional Technology" [M]. Zhejiang University Press, 2006