A variable displacement pump is more expensive, but it adds tremendous options for the design of a fluid power circuit. A servo valve can be used to change pump displacement and thus change actuator speed. The open-loop system shown in Fig. 11.1 can be made a closed-loop circuit by mounting a transducer on the output shaft of the gearbox. The resulting voltage is compared to a voltage corresponding to the set speed. The difference between the two voltages is used to increase (or decrease) pump displacement. The error signal causes the torque motor to rotate and move the servo valve spool. Fluid is directed to the control pistons, which extend (or retract) and change the angle of the swash plate, thus changing the displacement of the pump. If the various components (servo valve and associated circuitry) work together correctly, the pump output is continuously adjusted to maintain constant load speed as the load changes within some range.
When a servo valve is mounted on a variable displacement pump, the combination is known as a servo pump. The requirement to create a pressure drop across the servo valve (and dump fluid across the relief valve) to control actuator speed is eliminated. A servo pump is much more efficient for controlling actuator speed; however, it does not provide as accurate positioning as the servo-actuator configuration discussed in the previous section.