Monday, February 19, 2018

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INDUSTRIAL CONTROL HANDBOOK - 2.4 HYDRAULIC ACTUATORS AND VALVES

Hydraulic actuator and valve choices are very similar to pneumatic choices, except that the actuators are more powerful, faster, and must be more robustly built. There is a much wider choice of hydraulic rotary actuators than is the case with pneumatically powered actuators, because the high pressures used in hydraulic systems allow them to develop reasonable torque.

Since hydraulic oil (and the water-based substitutes that are sometimes used) are relatively incompressible, position control is possible. A well-designed hydraulic system is not subject to the same jerky actuation that is sometimes seen in pneumatic systems.

For position control, special types of hydraulic valves are often used. The servo-valve and proportional valve (see Figure 2-10) are both spool valves that open proportionally with analog DC.

The several types of true servovalves all include internal servomechanisms to ensure precise proportionality. An electric input signal causes a torque motor to move a spool-control mechanism that provides pressure to move the spool. The feedback link readjusts the spool-control mechanism as the spool reaches its desired position. Feedback links do not have to be mechanical as in this simple diagram; many are hydraulic.

Figure 2-10 Filter-regulafor-lubricators in a compressed air supply system

Figure 2-10 Filter-regulator-lubricators in a compressed air supply system

Figure 2-10 Servovalves: (a) one type of true servovalve; (b) proportional valve

Figure 2-10 Servovalves: (a) one type of true servovalve; (b) proportional valve

Figure 2-11 Hydraulic power supply

Figure 2-11 Hydraulic power supply

Proportional servovalve spools are moved directly by torque motors, roughly proportionally with the input signal. Proportional servovalves have improved over recent years so that many now come with spool position sensors and external controllers that ensure their output is as proportional to input as a true servovalve's would be.

Hydraulic power supply units, such as that shown in Figure 2-11 must include a pump, a pressure control valve, an oil reservoir tank, and a filter to clean the hydraulic fluid as it returns. If precision components such as servovalves are used, the fluid in the tank should be agitated and cleaned continuously.

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