This paper presents a two-stage electrohydraulic servovalve with a nozzle-flapper pilot stage, which is controlled by stack-type piezoelectric elements. The flapper moving mechanism proposed in this research compensates for the thermal expansion of the piezoelectric elements and applies preload to the piezoelectric elements. The experimental result shows that this flapper moving mechanism has a frequency response of over 500 Hz without oil pressure, and a simplified servovalve system using this flapper moving mechanism has a frequency response of about 150 Hz at the supply pressure of 210 bar.
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