pneumatic actuator

A Pneumatic actuator mainly consists of a piston or a diaphragm which develops the motive power. It keeps the air in the upper portion of the cylinder, allowing air pressure to force the diaphragm or piston to move the valve stem or rotate the valve control element. The valves input is the “control signal.”

A simplified diagram of a pneumatic actuator is shown in Figure a. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem.

A rubber diaphragm separates the actuator housing into two air chambers. The upper chamber receives supply air through an opening in the top of the housing.

The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the upper chamber. Finally, a local indicator is connected to the stem to indicate the position of the valve.

The position of the valve is controlled by varying supply air pressure in the upper chamber. This results in a varying force on the top of the diaphragm. Initially, with no supply air, the spring forces the diaphragm upward against the mechanical stops and holds the valve fully open.

As supply air pressure is increased from zero, its force on top of the diaphragm begins to overcome the opposing force of the spring. This causes the diaphragm to move downward and the control valve to close. With increasing supply air pressure, the diaphragm will continue to move downward and compress the spring until the control valve is fully closed.

Conversely, if supply air pressure is decreased, the spring will begin to force the diaphragm upward and open the control valve. Additionally, if supply pressure is held constant at some value between zero and maximum, the valve will position at an intermediate position. Therefore, the valve can be positioned anywhere between fully open and fully closed in response to changes in supply air pressure.

A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It does this by comparing the actuator’s demanded position with the control valve’s actual position.

The demanded position is transmitted by a pneumatic or electrical control signal from a controller to the positioner. The pneumatic actuator in Figure a is shown in Figure b with a controller and positioner added.

The controller generates an output signal that represents the demanded position. This signal is sent to the positioner. Externally, the positioner consists of an input connection for the control signal (4-20mA), a instrument supply air input connection, a supply air output connection, a supply air vent connection, and a feedback linkage.

Internally, it contains an intricate network of electrical transducers, air lines, valves, linkages, and necessary adjustments. Other positioners may also provide controls for local valve positioning and gauges to indicate supply air pressure and control air pressure (for pneumatic controllers – old controlling methods).

In Figure b, the controller responds to a deviation of a controlled variable from setpoint and varies the control output signal accordingly to correct the deviation. The control output signal is sent to the positioner, which responds by increasing or decreasing the supply air to the actuator.

Positioning of the actuator and control valve is fed back to the positioner through the feedback linkage. When the valve has reached the position demanded by the controller, the positioner stops the change in supply air pressure and holds the valve at the new position. This, in turn, corrects the controlled variable’s deviation from setpoint.

For example, as the control signal increases, a valve inside the positioner admits more supply air to the actuator. As a result, the control valve moves downward. The linkage transmits the valve position information back to the positioner.

This forms a small internal feedback loop for the actuator. When the valve reaches the position that correlates to the control signal, the linkage stops supply air flow to the actuator.

This causes the actuator to stop. On the other hand, if the control signal decreases, another valve inside the positioner opens and allows the supply air pressure to decrease by venting the supply air. This causes the valve to move upward and open. When the valve has opened to the proper position, the positioner stops venting air from the actuator and stops movement of the control valve.

An important safety feature is provided by the spring in an actuator. It can be designed to position a control valve in a safe position if a loss of supply air occurs. On a loss of supply air, the actuator in Figure b will fail open.

This type of arrangement is referred to as “air-to-close, spring-to-open” or simply “fail-open.” Some valves fail in the closed position. This type of actuator is referred to as “air-to-open, spring-to-close” or “fail-closed.” This “fail-safe” concept is an important consideration in hazardous areas.

What is a pneumatic actuator?

A pneumatic actuator is a device that converts energy typically in the form of compressed air into mechanical motion. Within the industry, pneumatic actuators are recognised by several different names including pneumatic cylinders, air cylinders, and air actuators; all of which are one and the same.

Consisting of a piston, cylinder, and valves or ports, a pneumatic actuator can convert energy into linear or rotary mechanical motions. This is dependent on whether the application is using a pneumatic rotary actuator or a linear actuator.

Linear actuators are well suited for fitting to angle seat control valves built for high temperature and steam applications, whereas the pneumatic rotary actuators are better suited for fitting to quarter-turn valves depending on the specification of the application.

How a pneumatic actuator works?

Pneumatic actuators are reliant on the presence of some form of pressurised gas or compressed air entering a chamber where pressure is built up. Once this exceeds the required pressure levels in contrast to the atmospheric pressure outside of the chamber, it creates a controlled kinetic movement of a piston or gear which can be directed in either a straight or circular mechanical motion.

Pneumatic actuators are well suited to a wide variety of application types, serving across many different industry areas. Some of the most common applications include

Combustible automobile engines
Air compressors
Packaging & production machinery
Railway application
Aviation.

Pneumatic Actuators

A set of devices into with one or more pneumo engines, which are determined to start mechanisms or some other objects by means of pressed working gas is called pneumatic actuator,or pneumo actuator.The devices intended for transformation of potential and kinetic energy of the stream of compressed gas in mechanical energy of the output link that can be, for example, a rod of the piston, a shaft of the turbine or the case of the jet device is called pneumatic engines of the automated actuator.All pneumatic actuators can be subdivided into the following types:

diaphragm pneumatic actuators;
pneumatic power cylinders;
gas-engine pneumatic actuators;

The principle of transformation of potential or kinetic energy of the gas stream into mechanical energy of the engine output link of the engine provide the base for division into types

Advantages of Pneumatic Actuators

simplicity of realization relatively to small back and forth motions;
sophisticated transfer mechanisms are not required;
low cost;
high speed of moving;
ease at reversion movements;
tolerance to overloads, up to a full stop;
high reliability of work;
explosion and ﬁre safety;
ecological purity;
ability to accumulation and transportation.

Disadvantages of Pneumatic Actuators

compressibility of the air ;
impossibility to receive uniform and constant speed of the working bodies movement ;
diﬃculties in performance at slow speed;
limited conditions – use of compressed air is beneﬁcial up to the deﬁnite values of pressure;
compressed air requires good preparation

pneumatic actuator types, working advantages and disadvantages