Pneumatic control components and basic circuits
In pneumatic systems, control elements are crucial components for controlling and regulating the pressure, flow rate, flow direction of compressed air and sending signals. By using them, various pneumatic circuits can be formed to ensure that pneumatic actuating elements operate normally as required. Pneumatic control components can be classified into three major categories based on their functions and applications: pressure control valves, flow control valves, and directional control valves. In addition, there are pneumatic logic components that achieve various logical functions by changing the direction and on-off of the airflow.
①Pressure control valve and pressure control circuit
Pressure control valves are mainly used to control the pressure of gases in the system and meet various pressure requirements. Pressure control valves can be classified into three types: The first type is the pressure reducing valve that serves to reduce and stabilise pressure; The second type is the safety valve that serves to limit pressure and provide safety protection, namely the relief valve. The third type is a sequence valve that performs certain controls based on different gas line pressures.
1. Safety valve
The safety valve plays a role in safety protection in the system. When the system pressure exceeds the specified value, the safety valve opens to release a portion of the gas into the atmosphere, ensuring that the system pressure does not exceed the allowable value and thus preventing accidents caused by excessive pressure in the system. The structure and graphic symbol of the safety valve are shown in the figure.
Figure: Structure and graphic symbol of the safety valve
2. Pressure-reducing valve
The function of the pressure-reducing valve is to reduce the pressure of the gas supply source to the pressure required by the device and ensure that the pressure value remains stable after pressure reduction. The basic performance of a pressure reducing valve includes the pressure regulating range, pressure characteristics and flow characteristics. Pressure characteristics and flow characteristics are two important features of a pressure-reducing valve and serve as crucial bases for its selection and use. When selecting a pressure-reducing valve, its type and pressure regulation accuracy should be determined based on the usage requirements, and then its diameter should be selected according to the maximum output flow required. The structure of the pressure-reducing valve is shown in the Figure. The air source pressure of the valve should be greater than the maximum output pressure by 0.1MPa. The pressure-reducing valve is generally installed after the water separator and air filter and before the oil mist lubricator, as shown in the Figure. Please note not to reverse its inlet and outlet. When the valve is not in use, the knob should be loosened to prevent the diaphragm from being frequently deformed under pressure, which may affect its performance.
Figure： The structure of the pressure-reducing valve
Figure： Installation position of the pressure-reducing valve
3. Pressure control circuit
The pressure control circuit is a fundamental circuit that keeps the pressure within the circuit within a certain range or enables the circuit to obtain pressures of different levels. The commonly used ones include primary pressure control circuits and secondary pressure control circuits.
Primary pressure control circuit
The primary pressure control circuit is used to control the pressure of the gas storage tank so that it does not exceed the specified pressure value. External control relief valves and electrical contact pressure gauges are often used to control the start and stop of air compressors, keeping the pressure in the air storage tank within the specified range. Electrical contact pressure gauges are adopted, which have high requirements for the motor and control. They are often used for the control of small air compressors, as shown in Figure.
Figure： Primary pressure control circuit diagram
2) Secondary pressure control circuit
The secondary pressure control loop mainly controls the air source pressure of the pneumatic system. In pneumatic transmission, the water separator and air filter, pressure reducing valve and oil mist lubricator are often collectively referred to as pneumatic three-piece sets. As shown in the Figure, it is a secondary pressure control circuit composed of pneumatic three-piece sets.
Figure： Secondary pressure control circuit
② Flow control valve and speed control circuit
To ensure the smooth and reliable operation of the cylinder, the movement speed of the cylinder should be controlled. A common method is to use a flow control valve to achieve this. The flow control valve controls the movement speed of the pneumatic actuator by regulating the gas flow rate, and the control of gas flow is achieved by changing the flow area of the flow control valve. Commonly used flow control valves include throttle valves, one-way throttle valves, exhaust throttle valves, etc.
One-way throttle valve
The one-way throttle valve is A combined control valve composed of a one-way valve and a throttle valve in parallel. Its structure and graphic symbol are shown in the Figure. When the airflow flows from port P to port A, it is throttled through the throttle valve. When flowing from A to P, the check valve opens without throttling. One-way throttle valves are often used in the speed regulation and delay circuits of cylinders.
Figure： Structure and graphic symbol of the one-way throttle valve
2. Speed control loop
Double-acting cylinders have two adjustment methods: intake throttling and exhaust throttling. The figure shows the intake throttling adjustment circuit. During intake throttling, when the load direction is opposite to the piston direction, the piston movement is prone to an unbalanced phenomenon, that is, a crawling phenomenon. When the load direction is consistent with the piston direction, the load is prone to running dry, causing the cylinder to lose control. Therefore, the intake throttling adjustment circuit is mostly used for vertically installed cylinders. For horizontally installed cylinders, the adjustment circuit generally adopts the exhaust throttling adjustment circuit, as shown in the Figure. As shown in the Figure, it is the speed control circuit diagram composed of throttle valves. When compressed air is intake from end A and exhausted from end B, the check valve of the one-way throttle valve A opens to rapidly inflate the rodless cavity of the cylinder. Since the one-way valve of the one-way throttle valve B is closed, the gas in the rod cavity can only be discharged through the throttle valve. By adjusting the opening degree of the throttle valve B, the movement speed when the cylinder extends can be changed. Conversely, adjusting the opening degree of throttle valve A can change the movement speed of the cylinder when it retracts. This control method ensures the stable operation of the piston and is the most commonly used one.
Figure： Unidirectional adjustment circuit for double-acting cylinder
Figure： Speed control circuit composed of throttle valves Figure
③ Electromagnetic directional control valve and pneumatic control circuit
1. Directional control valve
The directional control valve is used to control the flow direction of compressed air and the air flow interruption. Pneumatic directional control valves can be classified into different types based on the structure of the valve core, such as slide valve type, globe type, flat surface type, plug type and diaphragm type, among which the globe type and slide valve type are more widely used. According to different control methods, they can be classified into electromagnetic control type, pneumatic control type, mechanical control type, manual control type and time control type, etc. According to their functional characteristics, they can be classified into the unidirectional type and the reversing type. According to the number of ports and the number of valve core working positions, it can be classified into various types such as two-position two-way, two-position three-way, and three-position five-way, as shown in the Table.
Table: Ports and Working Positions of Directional Control Valves
2. Electromagnetic directional control valve
The electromagnetic directional control valve uses the suction force of an electromagnet to push the valve core to change the working position of the valve, thereby controlling the flow direction of the airflow. As it can be controlled by signals sent by push-button switches, limit switches, proximity switches, etc., it is easy to achieve electro-pneumatic combined control and can be operated remotely, with a wide range of applications. The most common classification of solenoid valves is based on the number of ports and the working position of the valve core, including two-position two-way, two-position three-way, three-position five-way, and many others. According to the number of coils driven by the electromagnet, solenoid valves are classified into single-controlled and double-controlled types. Valve electromagnets are classified into three types according to the different power sources used: AC type, DC type and local type. This type is the AC local rectifier type. This electromagnet itself is equipped with a half-wave rectifier, which can directly use AC while having the structure and characteristics of a DC electromagnet. When in use, the appropriate electromagnetic directional control valve should be selected according to the control requirements.
The figure shows a schematic diagram of the working principle of a direct-acting single electrically controlled two-position three-way electromagnetic directional control valve.
Figure： Working principle diagram of the direct-acting single electrically controlled electromagnetic directional control valve
Working principle: When the electromagnet is de-energised, the valve core is pushed to the upper end by the spring, connecting 7 and A. When the electromagnet is energised, the iron core pushes the valve core to the lower end through the push rod, connecting P and A.
The figure shows the working principle diagram of a direct-acting, double electrically controlled two-position five-way electromagnetic directional control valve. The figure shows the working principle diagram of the pilot-operated double electrically controlled directional control valve.
Figure： Working principle diagram of a direct-acting double electrically controlled two-position five-way solenoid valve
Figure： Working principle diagram of pilot-operated double electrically controlled Directional control valve
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