Design and control of direct drive servo-valve operated by the piezostack actuator

Electrohydraulic servo valves have been widely used in various automatic systems which need high precision of flow rate or pressure control to provide excellent static and dynamic control performance. The servo valves are generally classified into single-stage valve and two-stage valve. Direct drive servo valve (DDV) is a kind of single-stage valve in which the actuator is directly connected to the spool of the valve. In the conventional DDVs, the spool is generally actuated by electromagnetic actuator. Therefore, performance characteristics such as the accuracy and bandwidth of the DDVs are limited due to the inherent characteristics of the actuator. In this paper, a new type of the DDV operated by piezostack actuator is proposed and the goal of the proposed DDV is to achieve an accurate control of the flow rate at high frequency. The proposed DDV consists of a piezostack actuator, a lever mechanism to amplify displacement from the piezostack actuator and a spool part. A dynamic model of piezostack driven DDV system is derived by considering the flow force. After formulating the governing equation of the piezostack driven DDV system, a sliding mode control algorithm is designed to enforce the spool position to the desired position trajectories by activating the piezostack actuator. For the computer simulation, the specific geometric dimensions of the spool are chosen in order to achieve operation requirements: spool motion amplitude: over than 0.5mm; flow rate: over than 12liter/min; operating frequency: over than 200Hz. After confirming the maximum displacement of the spool and the flow rate of the valve at 200Hz, control performances are evaluated in time domain.