The way the valve controls the fluid can be divided into two types: one is on/off control, either fully open or fully closed. Examples for this type of valve include ordinary direct-acting solenoid valves, solenoid-operated directional valves, electro-hydraulic directional valves, etc.
The other is continuous control. The valve can be opened to any opening degree as needed, thereby controlling the flow rate. This type of valve can be manually controlled, such as a throttle valve, or electronically controlled, such as a proportional valve and a servo valve.
The following will introduce to you the differences among the proportional valves, solenoid valves and servo valves.
1. Basics about Proportional Valves, Solenoid Valves, and Servo Valves
1.1 What is a Solenoid Valve?
1-Electromagnet 2-Control piston 3-Spool valve core 4-Valve body 5-Return spring 6-Air outlet 7-Manual button 8-Electromagnet connector
A solenoid valve is a device that uses electromagnetic principles to control fluid flow. It generally consists of an electromagnet, a valve body, a valve core, a spring and other components. When the electromagnet is energized, it generates magnetic force to move the valve core and control the opening and closing of the valve, thus controlling the flow, pressure, direction and other parameters of the medium.
1.2 What is a Servo Valve?
1-Torque motor; 2-Nozzle; 3-Spool valve; 4-Coil; 5-Armature; 6-Torque tube; 7-Flapper; 8-Variable orifice; 9-Feedback spring
A servo valve, mainly composed of a torque motor and hydraulic amplifier, controls the flow and pressure of the fluid by adjusting the relative position between the valve core and the valve seat through external electrical signals. Servo valves work similarly to proportional valves, but with greater precision and sensitivity.
Servo valves are mainly used in high-precision fields such as aerospace and military industry including aircraft, rockets and missile weapons. At the same time, they are also used in electro-hydraulic servo valve systems with higher requirements in the industrial field. The future servo valve will develop in the direction of miniaturization, high reliability, intelligence and greenness.
1.3 What is a Proportional Valve?
With the development of hydraulic transmission and hydraulic servo systems, the hydraulic systems have appeared in production practice that require continuous control of pressure, flow and direction, but do not require high control accuracy.
Since ordinary hydraulic components cannot meet certain servo requirements, and the use of electro-hydraulic servo valves is too wasteful because of low system control accuracy requirements, in recent years there have been products between ordinary hydraulic components (on/off control) and servo valves (continuous control), that is, proportional control valves.
Proportional valve, also known as electro-hydraulic proportional valve, is a new type of electro-hydraulic control component between on-off control and servo control. Based on ordinary pressure valves, flow valves and directional valves, proportional electromagnets are used to replace the original control parts, and the pressure, flow or direction of the fluid can be continuously, proportionally, and remotely controlled according to the input electrical signal.
Proportional valves generally have pressure compensation performance, and the output pressure and flow rate are not affected by load changes. They can also control output parameters such as temperature and flow rate.
2. Proportional Valve VS Solenoid Valve
 vs. proportional valve (right)-min.png)
Working Principle
The solenoid valve generates a magnetic field through an electromagnet to control the opening/closing state of the valve to control the flow and direction of the medium.
The proportional valve uses the proportional control technology to achieve precise positioning of the valve opening by adjusting the magnitude of the output signal, so that the medium flow and pressure can be continuously adjusted.
Control Accuracy
Since the proportional valve adopts the proportional control technology, it can achieve more precise position control, making the control accuracy higher.
The solenoid valve can only control the opening and closing status of the valve. The control is relatively simple and the accuracy is limited.
Application Scope
Since proportional valves have the advantages of more precise control and wider application fields, they have been used more in fields that require high-precision control, such as industrial automation, aerospace, and medical equipment.
3. Proportional Valve VS Servo Valve
In hydraulic control systems, proportional valves and servo valves are often used in some precision control systems.
As a valve type between general valves and servo valves, proportional valves have the advantages of low manufacturing cost and strong anti-pollution ability, and are suitable for some small and medium-power, low-frequency hydraulic applications.
The servo valve has the characteristics of fast response and high accuracy, and is suitable for high-precision and high-demand hydraulic control systems.
Below we will look at the differences between the two in detail from different perspectives.
Response Speed
The frequency response speed of the servo valve is higher, up to about 200HZ. It is suitable for scenarios that require high system response speed. The frequency response speed of proportional valves is generally only a few tens of Hertz at most, with poorer dynamic characteristics and slower response speed.
Manufacturing Accuracy and Cost
The manufacturing accuracy of servo valves is very high, while the manufacturing accuracy of proportional valves can be relatively lower, the same as general valves. Because servo valves are more difficult to manufacture, they are relatively more expensive, while proportional valves are relatively more economical.
Hydraulic Oil Requirements and Anti-Pollution Capabilities
Servo valves have high requirements for hydraulic oil and require precision filtration to operate normally, otherwise they are prone to be clogged. The proportional valve has lower requirements for hydraulic oil, has strong anti-pollution ability, and shows better adaptability in some harsh working environments.
Control Accuracy
The servo valve has no deadband and can achieve higher precision control; while the proportional valve has a deadband and the control accuracy is relatively lower.
Application Scope
Proportional control is mainly suitable for open-loop systems and can control the hydraulic system simply and effectively; while servo control is mainly suitable for closed-loop systems and plays a better role when the hydraulic system requires high control accuracy.
4. Conclusion
We have learned that proportional valves, solenoid valves and servo valves have their own advantages and disadvantages. During use, we need to choose the appropriate valve type according to different needs and application scenarios to ensure the performance of the system. Although servo valves have many advantages, they are more expensive. Therefore, it is necessary to consider the balance between cost and performance when selecting valves.
