Proportional control technology is a control technology developed in the late 1960s that is reliable, inexpensive, and has control accuracy and response characteristics that can meet the actual needs of industrial control systems. Electro-pneumatic proportional control technology is a control method between the on-off control technology of ordinary pneumatic valves and electro-pneumatic proportional control technology. It can continuously and proportionally follow the control signal to change the air pressure. Therefore, the control performance of the electro-pneumatic proportional control technology is better than the on-off control of ordinary pneumatic valves. Comparing with the servo valve, due to the proportional valve has a dead zone in neutral position, there is still a slight gap in the control accuracy and response speed. However, its obvious advantage is that it has strong anti-pollution ability, which greatly reduces the working failure of pneumatic system caused by contaminant. On the other hand, the cost of proportional valve is lower than servo valve, and the structure is also simple, which has been widely used in many occasions.
After decades of continuous development, proportional control technology has reached a relatively complete level. It is mainly manifested in three aspects. First, feedback and electrical correction methods such as pressure, flow rate, displacement and dynamic pressure are adopted, which improves the steady-state accuracy and dynamic response quality of the valve. It indicates that the design principle of proportional control has been perfected. Second, Proportional technology and cartridge valve have been combined to give birth to proportional cartridge technology. Last, the birth of proportional volume components represented by proportional control pumps further expands the application of proportional control technology.

1. Types of Electro-Pneumatic Proportional Valves and how they work
There are many structural forms of electro-pneumatic proportional valve, similar to electro-pneumatic pressure regulator, usually composed of electro-mechanical converter, pneumatic amplifier (pilot valve and power main valve) and detection feedback mechanism. If it is a single valve, there is no pilot valve or it is a multi valve.
Electromechanical converters such as proportional electromagnetic valve, force motors or torque motors are used to convert the input electrical signal into force or torque through a proportional amplifier to generate displacement or rotation angle that drives the movement of the pilot valve. The pilot valve, also known as the pre-stage (can be a spool valve, a poppet valve, a nozzle flapper valve or a cartridge valve), is used to receive the displacement or rotation angle signal input by a low-power electrical-mechanical converter to convert the mechanical quantity. The main valve (spool valve, poppet valve or cartridge valve) converts the pneumatic pressure of the pilot stage valve into flow or pressure output, the detection feedback mechanism (which can be hydraulic, mechanical, electrical feedback, etc.) The pressure, flow or spool displacement of the main valve control port is fed back to
input end of pilot valve or input end of proportional amplifier to reach the balance of input and output.
The core of proportional control is proportional valve. The input unit of proportional valve is an electro-mechanical converter, which converts the input signal into a mechanical quantity. Converters are in the form of servo motors and stepper motors, force motors and torque motors, and proportional electromagnetic valve. However, most common proportional valves use proportional electromagnetic valve. The proportional electromagnetic valve is designed according to the principle of
electromagnetic valve, so that the mechanical quantity (force or torque and displacement) generated input electrical signal (current), and then continuously controlled the position of hydraulic valve spool, thereby reaching continuous control of pressure, direction and flow of pneumatic system. The structure of proportional electromagnetic valve is shown in Figure 2. It consists of a coil, an armature, a putter,etc. When there is a signal input to the coil, the magnetic field in the coil produces a force on the armature, and the armature is in the magnetic field according to the magnitude and direction of signal current. It moves proportionally and continuously, and then drives the push rod to move through fixed pins, so as to control the movement of spool valve. The most widely used proportional electromagnetic valve is the high voltage DC proportional electromagnetic valve.

The types of proportional electromagnetic valves are mainly divided into the following categories according to their working principles:
(1) Force control type
The stroke of this type of electromagnetic valve is short, only 1.5mm, and the output force is proportional to the input current. It is often used in the pilot control stage of the proportional valve.
(2) Stroke control type
It is composed of a force control type plus a load spring. The force output by the electromagnetic valve is converted into an output displacement through the spring. The output displacement is proportional to the input current. The working stroke reaches 3mm and has good linearity. It can be used in direct Controlled proportional valve.
(3) Position adjustment type
After the position of the armature is detected by the sensor in the valve, an in-valve feedback signal is sent out, and the position of the armature is re-adjusted after the comparison in the valve, forming a closed-loop control in the valve, with high precision. Regardless of the force, it can almost be compared with the servo valve in terms of accuracy. The proportional valve produced by many famous companies in the world adopts this structure.
Proportional valves are classified into four categories according to their main functions: proportional pressure regulators, flow control valves, directional control valves, and proportional compound control valves:
(1) Electro-Pneumatic Pressure Regulator
There are relief valve, pressure reducing valve, sequence valve, which can continuously adjust the system pressure.
(2) Proportional directional valve
The polarity of the input current determines the flow direction of the air flow, the stroke of the valve core is proportional to the size of the input current, and the directional valve is divided into two categories: with position sensor and without position sensor.
(3) Proportional flow control valve
There are proportional speed regulating valve and proportional overflow flow control valve, which can continuously adjust the system flow or speed.
(4) Proportional composite control valve
Generally, it is composed of two kinds of valves with different functions, suitable for use in open-loop control systems. Each type can be divided into two structural forms: direct control and pilot control. Direct control is used in low-flow and low-power systems, and pilot control is used in large-flow and high-power systems to form electro-pneumatic proportional valves.

2. Selection of proportional valve
(1) Select the type of proportional valve according to the purpose and the controlled object.
(2) Understand the dynamic and static indicators of proportional valve correctly, mainly including rated output, initial current, hysteresis, repeatability, rated pressure loss, temperature drift, response characteristics, frequency characteristics, etc.
(3) Select the accuracy of proportional valve according to the working accuracy requirements of actuator, and the valve with feedback closed loop has good steady-state and dynamic quality. If the inherent characteristics of proportional valve, such as hysteresis, nonlinearity, etc., cannot make the controlled system achieve the desired effect, software programs can be used to improve the performance ofsystem.
(4) If you choose a proportional valve with a pilot valve, notice the requirements of pilot valve on oil pollution degree. Generally, it should meet the ISO18/15 standard, and install a 10 oil inlet oil filter on the oil road.
(5) The orifice of proportional valve should be the flow of actuator at the maximum speed. If the orifice is too large, the resolution of system will be reduced. The proportional valve must use the matching amplifier. The distance between the valve and the amplifier should be as short as possible. The amplifier adopts current negative feedback, and sets the ramp signal generator to control the boosting, decreasing time or motion acceleration and deceleration. When power is off, the spool can be placed in a safe position.

3. Proportional control system
The proportional control system has direct proportional control and electro-pneumatic proportional control, which is similar to the servo system control in essence, and can be analyzed with reference to the servo system. According to whether there is feedback, it is divided into open-loop control and closed-loop control. The proportional valve-controlled hydraulic cylinder or motor system can control parameters such as speed, displacement, rotational speed and torque.

4. Comparison of Electro-hydraulic Servo System and Proportional Servo System

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