One of the advantages of electric actuators is that it is not necessary to install and maintain a compressed air system in the field. This means that a totally electric connection from the process control system to the valve is possible. Pneumatic actuators, however, provide a means for fail-safe action which for electric actuators may mean dc motors with battery backup which are both cumbersome and expensive.

Figure 9.7 - Electrohydraulic actuator on quarter-turn valve. (Courtesy of Elo-o-matic, Inc.)

With new developments in hydraulic components including electronic-hydraulic devices these actuators are now able to interface with computer-controlled systems for both on-off and modulating applications. Most high-pressure hydraulic actuators are being built on a custom basis, but some hydraulic component advancements have helped make low-pressure systems more practical.

For these applications some manufacturers offer a self-contained electrohydraulic spring-return actuator. As the name implies, this actuator uses an electrical supply to pressurize through a pump the hydraulic fluid required to stroke a spring-return actuator (see Fig. 9.7).

With self-contained electrohydraulic actuators a compressed air system is not necessary and power wiring is like that for a spring-return pneumatic actuator providing, in effect, a fail-safe electric actuator.

Electrohydraulic rotary actuator.
A typical electrohydraulic rotary actuator package includes rotary actuator, hydraulic pump, solenoid valve, limit switch, and reservoir. The system operates as follows (see Fig. 9.8).

The electrohydraulic power unit is connected to both sides of the piston chambers of a rotary actuator. When the hydraulic system is energized, pressurized hydraulic fluid enters the spring opposed side of the actuator, causing the two pistons to start transmitting a rotary motion to the central drive shaft by means of a rack and pinion. As the actuator moves through 9O^o the hydraulic oil on the low-pressure side of the pistons (spring side) returns to the power unit reservoir maintaining a constant level of oil and providing a totally sealed hydraulic system. When the actuator is in the desired position, the hydraulic pump within the power unit is deenergized through limit switches, and the actuator is held in position by locking pressure within the unit.

The return direction is achieved by deenergizing the normally open solenoid vent valve which allows oil in the pressure chamber to flow back into the power unit reservoir, thus reducing the pressure and allowing the opposing return springs to force the pistons together and provide a reverse rotary motion.

Figure 9.8 - Operating schematic of electrohydraulic actuator.