1. Introduction
Actuators are the final elements in a control system. They receive a low power command signal and energy input to amplify the command signal as appropriate to produce the required output. Applications range from simple low power switches to high power hydraulic devices operating flaps and control surfaces on aircraft; valves, car steering, etc.

2. Types of Actuator
Actuators may be grouped in a number of ways:

Key features of pneumatic and hydraulic systems are summarised below:

Feature Hydraulic actuators Pneumatic actuators
Medium Fluid, can be oil, oil/water mix (non flammable) or water + corrosion inhibitors
- virtually imcompressible
- viscosity heavily temperature dependent
Usually air
- compressible
- separate lubrication probably required.
- viscosity fluctuations not important
Pressure range Up to about 30 MPa
- 200 MPa for diesel injectors
Up to about 1 MPa
Applications Positioning with high load rigidity and precision in closed loop control systems Devices with lower power and force / torque requirements, positioning by mechanical stops in open loop systems.

In most applications fluid mechanical actuator drives are in the form of hydrostatic energy converters. These operate on the displacement principle converting the pressure energy of the fluid into mechanical work and vice - versa. Automotive brakes are an important example.

The hydrodynamic transformer works by converting flow energy (kinetic energy of the moving fluid) into mechanical work. An important automotive application is the hydrodynamic torque converter used in most automatic transmission systems.

Other types of actuator may be based on the piezoelectric effect and and the behavior of shape memory alloy wire or strip.

3. Current Trends
A significant trend is the move away from hydraulic to electrical devices. This is driven partly by the desire to have cleaner systems (no hydraulic fluid) and making integration with other (normally electrical) control systems easier to achieve. New cars are often now fitted with electric power assisted steering rather than the hydraulic system that was the only system available till recently. Developments are progressing with electrical assistance of car braking systems. This trend to electrical systems is also present in the aviation industry, but the very high power densities and forces required from some actuators mean that this will be more difficult.

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David J Grieve, 28th July 2003.