An actuator is the final control element that operates on the low-level control signal to produce a signal containing enough power to drive the plant for the intended purpose. The armature-controlled dc motor, the hydraulic servomotor, and the pneumatic diaphragm and piston are common examples of actuators.
Electromechanical Actuators
An electromechanical system consisting of an armature-controlled dc motor driving a load inertia. The rotating armature consists of a wire conductor wrapped around an iron core. This winding has an inductance L. The resistance R represents the lumped value of the armature resistance and any external resistance deliberately introduced to change the motor’s behavior. The armature is surrounded by a magnetic field. The reaction of this field with the armature current produces a torque that causes the armature to rotate. If the armature voltage v is used to control the motor, the motor is said to be armature controlled. In this case, the field is produced by an electromagnet supplied with a constant voltage or by a permanent magnet.
An actuator especially suitable for digital systems is the stepper motor, a special dc motor that takes a train of electrical input pulses and converts each pulse into an angular displacement of a fixed amount. Motors are available with resolutions ranging from about 4 steps per revolution to more than 800 steps per revolution. For 36 steps per revolution, the motor will rotate by 10∘ for each pulse received. When not being pulsed, the motors lock in place. Thus, they are excellent for precise positioning applications, such as required with printers and computer tape drives. A disadvantage is that they are low-torque devices. If the input pulse frequency is not near the resonant frequency of the motor, we can take the output rotation to be directly related to the number of input pulses and use that description as the motor model.
Hydraulic Actuators
Hydraulic servomotor with a load.
Machine tools are one application of the hydraulic system shown in figure. The applied force f is supplied by the servomotor. The mass m represents that of a cutting tool and the power piston, while k represents the combined effects of the elasticity naturally present in the structure and that introduced by the designer to achieve proper performance. A similar statement applies to the damping c. The valve displacement z is generated by another control system in order to move the tool through its prescribed motion. The spool valve shown in figure had two lands. If the width of the land is greater than the port width, the valve is said to be overlapped. In this case, a dead zone exists in which a slight change in the displacement z produces no power piston motion. Such dead zones create control difficulties and are avoided by designing the valve to be underlapped (the land width is less the port width). For such valves there will be a small flow opening even when the valve is in the neutral position at z = 0. This gives it a higher sensitivity than an overlapped valve.
Many hydraulic servomotors use multistage valves to obtain finer control and higher forces. A two-stage valve has a slave valve, similar to the pilot valve but situated between the pilot valve and the power piston.
Rotational motion can be obtained with a hydraulic motor, which is, in principle, a pump acting in reverse (fluid input and mechanical rotation output). Such motors can achieve higher torque levels than electric motors. A hydraulic pump driving a hydraulic motor constitutes a hydraulic transmission.
A popular actuator choice is the electrohydraulic system, which uses an electric actuator to control a hydraulic servomotor or transmission by moving the pilot valve or the swashplate angle of the pump.
Pneumatic Actuators
Pneumatic actuators are commonly used because they are simple to maintain and use a readily available working medium. Compressed air supplies with the pressures required are commonly available in factories and laboratories. No flammable fluids or electrical sparks are present, so these devices are considered the safest to use with chemical processes. Their power output is less than that of hydraulic systems but greater than that of electric motors.
Formerly, many control systems utilized pneumatic devices to implement the control law in analog form. Although the overall, or higher level, control algorithm is now usually implemented in digital form, pneumatic devices are still frequently used for final control corrections at the actuator level, where the control action must eventually be supplied by a mechanical device.
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