How to choose a pneumatic valve?

A directional control valve is identified by the number of its main connections and possible switching positions. For example a "3/2" valve corresponds to three ports and two switching positions. In addition to the different functions of the ports and switching positions, the mechanical and functional design of the directional control valve also varies. DCV design types include: plugs (ball plugs, flat plugs), valve shafts, flat slides and rotary slides. Even within the same type, it is still possible to apply different design methods and materials. For example, a spool valve may have a seal ring along its valve shaft or in the valve body in place of the seal. Design characteristics affect valve service life, flow rate, actuation device, actuation force, size, and price.

While a directional valve is used to perform all switching operations, a proportional valve (PV) regulates pressure and flow rate. The most basic characteristic of a proportional valve is to convert an input signal into a specific output signal. Proportional valves usually consist of two components - a pilot control and a valve unit. The pilot control consists of two 2/2-way pilot valves or a proportional solenoid (solenoid) and also includes electronics for evaluation. The valve unit contains only integrated pneumatic functions.

A DC voltage or current (eg 0~10V voltage or 0~20mA current) is applied to the control block connector. The voltage or current then corresponds to a specific controlled pressure. The actual pressure at the working port is measured by an integrated pressure sensor and sent to the evaluation electronics, which compares the actual pressure with the desired pressure. The electronic circuit unit then switches the operating positions of the pilot valve and the main valve in sequence according to the pressure deviation until the desired pressure is obtained.

Proportional directional control valve has more functions

For "standard" directional control valves there is only a single switching position, but proportional directional control valves also allow intermediate positions. This means, for example, in addition to end positions such as "open" or "closed", intermediate positions such as "slightly open" are possible.

The actuator used to switch the working position is a proportional electromagnetic coil. When the electromagnetic coil is excited by the electronic circuit, the coil voltage will change accordingly, and the position of the control piston will change according to the intensity of the electromagnetic field. As a result, each air passage is simultaneously opened or closed to varying degrees by means of the action of the control piston. This not only initiates open-loop or closed-loop control of the cylinder's movement direction, but also controls the cylinder's movement speed. Therefore, it can be considered that this type of valve functions as a throttling, and at a switching frequency of about 100 Hz, this type of valve can achieve a fairly high dynamic performance.

Proportional valves have a wide range of applications, for example, when the material is uncoiled, the application of proportional valves can pneumatically maintain a constant tension during the uncoiling process. Proportional directional control valves have a further application in cylinder positioning, where the cylinder is equipped with a displacement encoder that provides a feedback signal of the actual position of the piston. To achieve a specific position, a proportional directional control valve controls the volume flow into or out of the cylinder.