I. Overview of Electro-hydraulic Servo Systems
Electro-hydraulic servo systems are important control devices in the field of automation, widely used in industrial control applications requiring high precision and high output power. While liquids, as the medium for power transmission and control, have some disadvantages and are more expensive than electricity, they offer advantages such as fast response speed, high power-to-weight ratio, and high load stiffness. Therefore, electro-hydraulic servo systems possess unique advantages in control applications demanding high precision and high output power. An electro-hydraulic servo control system is an automatic control system for mechanical quantities that uses hydraulic power and electrical means to achieve signal transmission and control. Based on the different controlled mechanical quantities, it can be further divided into three types: electro-hydraulic position servo systems, electro-hydraulic speed servo control systems, and electro-hydraulic force control systems.
my country's electro-hydraulic servo technology is still in its early stages of development. While we have made some progress in conventional electro-hydraulic servo control technology, we lag significantly behind developed countries in high-end electro-hydraulic servo products and applications. Electro-hydraulic servo technology is a comprehensive technology integrating mechanics, hydraulics, and automatic control. Developing domestic electro-hydraulic servo technology requires simultaneous advancement across various technical fields, including mechanics, hydraulics, automatic control, and computer science.
II. Components of an Electro-hydraulic Servo
An electro-hydraulic control system is short for an electro-hydraulic control system, which consists of two parts: electrical control and hydraulics. In electro-hydraulic hybrid drive technology, energy flow is controlled electronically and transmitted through a hydraulic circuit, fully combining the advantages of both electronic control and hydraulic transmission technologies while avoiding their respective shortcomings.
(1) Characteristics of electronic drive technology
① High precision, high efficiency, low energy consumption, and low noise
② High-performance dynamic energy control
③ Stable temperature performance
④ Energy regeneration and feedback grid
⑤ No energy loss occurs during the cycle's idle time.
(2) Characteristics of hydraulic drive technology
① High (force/work) density
② Compact structure
③ Hydraulic motors (cylinders) are high-power and economical actuators.
④ There is significant energy loss when performing pressure control in the hydraulic system.
Hydraulic system: Using liquid as the transmission medium, motion and energy transfer are achieved through the pressure energy of the pressurized liquid. Based on the principle of hydraulic transmission, the system can control controlled variables such as position, speed, acceleration, and force with a certain precision according to the requirements of mechanical equipment. Furthermore, it can operate stably and accurately even under external interference, achieving the predetermined technological objectives. (Industrial Control Network)
A hydraulic servo valve is a hydraulic control valve whose output and input quantities have a specific functional relationship and can respond quickly. It is a crucial component of a hydraulic servo system. Hydraulic servo valves typically consist of an electromechanical converter (torque motor).
It consists of three parts: a hydraulic motor, a hydraulic amplifier, and a feedback or balancing mechanism. Currently, hydraulic servo valves mainly refer to electro-hydraulic servo valves. After receiving electrical analog signals, they output modulated flow and pressure. It is both an electro-hydraulic conversion element and a power amplification element. It can convert weak electrical input signals with low power into high-power hydraulic energy (flow and pressure) output, realizing the conversion of electro-hydraulic signals and hydraulic amplification.
The core of the system is the electro-hydraulic servo valve. This valve is a key component of the electro-hydraulic servo control system, serving as both the interface between the electrical control and hydraulic actuation parts and an amplifying element that enables high-power control with small signals. Among various types of electro-hydraulic servo valves, the dual-nozzle baffle two-stage electro-hydraulic servo valve is the most widely used, with flow rates ranging from a few tenths of a liter per minute to hundreds of liters per minute. The electro-hydraulic servo system also exhibits typical characteristics of a servo system (also known as a follow-up system), meaning it is an automatic control system with power amplification. Even with high uncertainty in the given value, the system's output always accurately tracks changes in the input. Furthermore, the fundamental function of a follow-up system is to amplify the signal, ensuring sufficient energy to drive the load (controlled object) to move according to the input signal and keeping the deviation between input and output within acceptable error ranges.
Electro-hydraulic servo systems are widely used in various fields of industrial production due to their advantages such as high output power and high control precision. As the core component of an electro-hydraulic servo control system, the performance of the electro-hydraulic servo valve directly affects the performance of the entire system. An electro-hydraulic servo valve is a hydraulic control valve whose output and input quantities have a certain functional relationship and can respond quickly. It is a crucial component of a hydraulic servo system. After receiving an electrical analog signal, it outputs modulated flow and pressure. It acts as both an electro-hydraulic conversion element and a power amplification element, converting low-power, weak electrical input signals into high-power hydraulic energy (flow and pressure) output, thus realizing electro-hydraulic signal conversion and hydraulic amplification.
The control of an electro-hydraulic servo valve is achieved by controlling the current flowing through its coil, which in turn controls the force generated by the torque motor, thereby controlling the flow rate and converting it into a corresponding displacement.
Electrical Components: In an electro-hydraulic servo system, the detection, correction, and initial amplification of deviation signals are all achieved using electrical and electronic components. An electro-hydraulic servo system is an automatic control system that utilizes an electro-hydraulic servo mechanism and is based on the principle of hydraulic transmission. In this system, the movement of the actuator changes in response to changes in the control signal.
Servo amplifiers are an important component of electro-hydraulic servo control systems, used to improve the steady-state and dynamic performance of electro-hydraulic control elements or systems. A servo amplifier is a DC power amplifier that drives an electro-hydraulic servo valve; its preamplifier stage is a preamplifier circuit, and its power stage is a current negative feedback amplifier circuit.
The function of the servo amplifier is to amplify and process the deviation signal obtained by comparing the input voltage signal with the feedback signal, and output a control current that is a function of the deviation signal voltage. This current is then input into the servo torque motor coil to drive the servo valve. This further reduces the signal, allowing the system to achieve the required control accuracy.
III. Electro-hydraulic Servo Control Principle
The working process of an electro-hydraulic servo system mainly involves the control computer calculating the current control signal based on the target position provided by the system. After D/A conversion, the signal is transmitted to the servo amplifier. The output current of the servo amplifier drives the valve core of the electro-hydraulic servo valve to move, and the hydraulic power source provides power to drive the hydraulic cylinder to achieve the loading function. The actual position of the load is fed back to the servo amplifier via a displacement sensor, forming a complete closed-loop control system to achieve target position tracking. This is the most common electro-hydraulic servo control principle.
This is the most common type of electro-hydraulic servo control, where the valve amplifier is used as a closed-loop controller. Its advantages are that it is simple to operate and easy to learn. Its disadvantages are that adjusting the PID parameters is inconvenient and inflexible, and the actual feedback is not visible, which is not conducive to real-time monitoring.
Therefore, it is recommended to integrate closed-loop control into a PLC or motion controller, using the amplifier only as a power amplifier. Command values and parameters are set via a computer or touchscreen, sensor signals are collected, processed, and then transmitted to the PLC or motion controller. By comparing the error between the given command signal and the feedback signal, and performing appropriate PID control calculations, a displacement closed-loop control system is formed. The feedback signal is compared with the input signal to obtain the deviation signal, causing the system to change in the direction of reducing the deviation. This continuously corrects the signal transmitted to the servo valve until the deviation is equal to zero or sufficiently small, thus ensuring that the actual output of the system matches the expected value until the performance requirements are met. This achieves the closed-loop control function.
In recent years, the structure and performance of PLCs have been continuously improved. With the continuous improvement of the performance-price ratio of PLCs, many small PLCs have the computing speed and advanced functions that were once only available in mainframe computers. Therefore, they have been widely used. The method proposed in this paper is universal and is derived from specific engineering practices.
Electro-hydraulic position servo systems are primarily used to solve position tracking control problems. They utilize electro-hydraulic servo valves to control the position of servo cylinders, and incorporate displacement sensors to form a closed-loop position control system. Position sensors (linear displacement sensors) measure the actual position signal and convert it into a corresponding current or voltage signal, which is then sent to the PLC or controller as a feedback signal. The fundamental task is to achieve timely and accurate tracking of the controlled variable to the given variable through the actuator, while maintaining sufficient control precision. The dynamic characteristics of an electro-hydraulic servo system are a crucial indicator of its design and debugging quality. It consists of an electrical signal processing device and several hydraulic components. The dynamic performance of these components interacts and constrains each other, and the system itself contains nonlinearities, resulting in complex dynamic performance.
IV. Electro-hydraulic Servo Applications
Electro-hydraulic servo control technology, serving as a bridge connecting modern microelectronics, 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 precision, it has been widely applied.
Electro-hydraulic servo systems possess outstanding advantages such as fast response speed, high output power, and high control precision, leading to their widespread application in aerospace, military, metallurgy, transportation, and engineering machinery fields. The advent of electro-hydraulic servo valves ushered in the electro-hydraulic servo era for hydraulic servo technology, significantly expanding its application areas. Electro-hydraulic position servo systems are the most basic and commonly used type of hydraulic servo system, used for applications such as machine tool table positioning, strip mill thickness control, and aircraft and ship steering. Electro-hydraulic position servo control systems are suitable for controlling high-speed, high-power objects with large load inertia. They have been applied in aircraft attitude control, aircraft engine speed control, radar antenna azimuth control, robot joint control, strip misalignment and tension control, material testing machines, loading devices, radar and artillery control systems, and vibration testing benches.
