Electromagnetic Actuators
The solenoid is the most common electromagnetic actuator. A DC solenoid actuator consists of a soft
iron core enclosed within a current carrying coil. When the coil is energized, a magnetic field is established
that provides the force to push or pull the iron core. AC solenoid devices are also encountered, such as
AC excitation relay.
A solenoid operated directional control valve is shown in Figure 9.10. Normally, due to the spring
force, the soft iron core is pushed to the extreme left position as shown. When the solenoid is excited,
the soft iron core will move to the right extreme position thus providing the electromagnetic actuation.
Another important type is the electromagnet. The electromagnets are used extensively in applications
that require large forces.
Hydraulic and Pneumatic Actuators
Hydraulic and pneumatic actuators are normally either rotary motors or linear piston/cylinder or control
valves. They are ideally suited for generating very large forces coupled with large motion. Pneumatic
actuators use air under pressure that is most suitable for low to medium force, short stroke, and highspeed
applications. Hydraulic actuators use pressurized oil that is incompressible. They can produce very
large forces coupled with large motion in a cost-effective manner. The disadvantage with the hydraulic
actuators is that they are more complex and need more maintenance.
The rotary motors are usually used in applications where low speed and high torque are required. The
cylinder/piston actuators are suited for application of linear motion such as aircraft flap control. Control
valves in the form of directional control valves are used in conjunction with rotary motors and cylinders
to control the fluid flow direction as shown in Figure 9.10. In this solenoid operated directional control
valve, the valve position dictates the direction motion of the cylinder/piston arrangement.

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