Pneumatic Actuator

(in Russian, pnevmoprivod), a pneumatic device for remote operation of a control element, such as a valve, damper, or stopcock, in automatic control systems.

Pneumatic actuators may act on the control element through translatory or rotary motion. Actuators with translatory motion are more common in industrial pneumatic automatic control. They may be of the two-position or multiposition type. A two-position pneumatic actuator is used to shift a control element from one extreme position to the other, usually according to a signal from a relay element. A multiposition actuator places the control element in various positions according to the level of the control signal. Actuators may be of the piston or diaphragm type.

A piston pneumatic actuator consists of a cylinder in which a piston and a rod are moved by the action of compressed air or a spring. The action may be single or double. In a single-action pneumatic actuator the working stroke is provided by the action of compressed air, and the return stroke by a spring; in a double-action actuator the piston is moved in both directions by compressed air. If substantial forces must be produced using cylinders of small diameter, multiple actuators (double, triple, and so on) are used.

The main parameter of a pneumatic actuator is the force developed on the rod, which is determined by the pressure of the compressed air and the dimensions of the cylinder. Piston pneumatic actuators often have not only the main slide valve but also one or more additional valves, which are used to produce the required control logic. A slave piston pneumatic actuator, which is a variety of servomechanism in which the pneumatic piston mechanism is the working member, is used in cases where a substantial displacement of the control element or a large force is required—for example, in autopilots or for the opening and closing of slide valves in blast furnaces.

A diaphragm pneumatic actuator consists of a sealed chamber divided into two operating cavities by a diaphragm; the rigid center of the diaphragm is connected to the rod. Like the piston pneumatic actuators, diaphragm actuators may have single or double action. They may be single, double, or multiple, depending on the force required from the rod. The travel and the force produced on the rod of a diaphragm pneumatic actuator are considerably less than for piston types; they depend mainly on the effective area of the diaphragm and the pressure acting on it. Diaphragm pneumatic actuators are sometimes equipped with an amplifying device called a positioner.

In cases where a faster response is required, vibrating relay actuators are used. The relay characteristic of such actuators is linearized by applying the high-frequency vibrations to the control device, which may be a slide valve, jet tube, or nozzle and baffle. The shorter the linear portion of the characteristic for such a pneumatic actuator, the faster its response.

The Russian term pnevmoprivod also refers to the pneumatic drive of working machines.