Pneumatic valve choices

Valves are at the heart of pneumatic control systems, with directional and proportional control valve varieties serving as major choices. Directional control valves (DCVs) are arguably the most frequently used valves. They control the direction of compressed air (or other suitable fluid), for instance, to determine whether a cylinder stroke advances or retracts.

08/01/2004


Valves are at the heart of pneumatic control systems, with directional and proportional control valve varieties serving as major choices. Directional control valves (DCVs) are arguably the most frequently used valves. They control the direction of compressed air (or other suitable fluid), for instance, to determine whether a cylinder stroke advances or retracts.

DCVs are identified according to the number of their main connections and possible switching positions. For example, a "3/2 valve" refers to three ports and two switching positions. Besides their function according to ports and switching positions, directional control valves differ in mechanical and functional design. DCV design types include: poppet (ball poppet, flat-seal poppet), spool, flat slide, and rotary slide. Even among these types, other design methods and materials may be used. For example, a spool valve can have sealing rings along its spool or the seals could be housed in the valve body instead. Design features influence the valve's service life, flow rate, actuating elements, actuating forces, size, and price.

While directional valves are used to perform complete switching operations, proportional valves (PVs) regulate pressures and flow rates. Conversion of an input signal into a specific output signal is the most important basic characteristic of proportional valves. PVs normally consist of two elements—a pilot control and a valve unit. Pilot control consists of two 2/2-way pilot valves or a proportional solenoid, and includes the evaluating electronics. The valve unit consists solely of integrated pneumatic functions.

An electrical dc voltage (such as 0-10 V or 0-20 mA current) is applied at the control block connection. Voltage or current then corresponds to a specific pressure to be controlled. Actual pressure at the working port is measured by the integrated pressure sensor and transmitted to the evaluating electronics, which compares actual pressure with desired pressure. The electronics unit then switches the pilot valves according to the pressure deviation and, in turn, the operating positions of the main valve until the desired pressure is obtained.

Proportional DCVs do more

With "standard" directional control valves, only the individual switching position can be assumed. Proportional directional control valves also permit intermediate positions. This means that apart from the end positions, such as "OPEN" or "CLOSED," intermediate "SLIGHTLY OPEN" positions are also possible.

The actuating element for the switching of working positions is a proportional solenoid . When electronically actuated, the solenoid changes the voltage in its coil segments and the position of the control piston changes according to the altered field strengths. As a result, individual air passages simultaneously open or close to varying degrees via the control piston. This not only enables open- or closed-loop control of a cylinder's direction of movement, but also its speed. Here, the valve assumes a so-called throttling function. With a switching frequency of approximately 100 Hz, these valves can achieve extremely high dynamics.

Proportional valves have wide application, such as pneumatically maintaining a constant tension force during the unwinding of lengths of material. Proportional directional control valves have further application in the positioning of cylinders. In such a case, the cylinder is equipped with a displacement encoder, which supplies a feedback signal of the actual piston position. To obtain a specific position, the valve controls volumetric flow into or out of the cylinder.


Author Information
Frank Langro is manager, Product Management, and Frank Latino is product manager, Valve Terminals & Electronics, at Festo Corp.




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