How the pilot control stage works with manually and mechanically actuated directional control valves
Piloted directional control valves use a pilot valve to indirectly actuate the main valve piston. This means that a relatively small force is required to operate the valve. A small duct connects the supply port to the pilot valve. When the pilot valve is opened, compressed air flows to the diaphragm and moves the valve disc of the main valve downwards. When the pilot valve is closed, the compressed air is exhausted through the guide bush of the stem. Piloted directional control valves with spring return have a spring that returns the valve disc of the main valve to its normal position when the pilot valve is closed.
In other words, piloted directional control valves are easy to operate and can be used to control large flows of compressed air. They are often used in industrial applications where high performance and reliability are required.
Piloted 3/2-way roller lever valve
When the roller lever is actuated, the pilot valve opens. The valve is switched in two phases: first port 2 to 3 is closed and then port 1 to 2 is opened. The valve is reset by releasing the roller lever.This type of valve can either be used in closed or open position. You simply need to swap ports 1 and 3 and rotate the actuation setup by 180°.
How the pilot control stage works with solenoid actuated directional control valves
When the coil is de-energised, the armature is pressed into its lower sealing seat by the spring. The chamber on the top of the piston is exhausted. When a current flows through the coil, the magnetic force exerted by the solenoid pulls the armature upwards. The chamber on the top of the piston is pressurised.
Piloted 3/2-way solenoid valve
Figure below shows two sectional views of a piloted 3/2-way solenoid valve.
- In the normal position, only the atmospheric pressure acts on the upper piston area which means that the spring force pushes the piston upwards. Ports 2 and 3 are connected.
- When a current flows through the solenoid coil, the chamber above the valve piston is connected to supply port 1. The force above the valve piston increases and the valve piston is pushed downwards. The connection between ports 2 and 3 is closed and the connection between ports 1 and 2 is opened. This switching position is maintained for as long as current flows through the solenoid coil.
- When the solenoid coil is de-energised, the valve switches back to its normal position.
A minimum supply pressure (pilot pressure) is necessary to actuate the piston of a piloted valve against the spring force. This pressure is specified in the technical documentation for the valve and, depending on the valve type, is approx. 200 to 300 kPa (2 to 3 bar).
Comparison of piloted and directly actuated valves
To achieve a sufficient opening cross section and therefore a sufficient flow rate, a
comparatively large armature is needed. Accordingly a strong return spring is required and the solenoid
must apply a large amount of force. It is therefore big in terms of size and has a high electric power
consumption.
With a piloted valve, the flow rate to the consuming device is opened by the main stage. The valve piston is moved via the air duct. A low flow rate is sufficient for this, which means that a comparatively small armature with a low actuating force can be used. The solenoid can be smaller compared with a directly actuated valve. The electric power consumption and the thermal output are lower.
The advantages with respect to electric power consumption, size of the solenoid and thermal output mean that almost exclusively piloted directional control valves are used in electropneumatic control systems.