The Use of Derivative Pressure Feedback in High Performance Hydraulic Servomechanisms

Hydraulic servomechanisms are sometimes used to drive a load member which is predominantly inertia. The usual overriding requirements for output disturbance discrimination and high power efficiency dictate a simple closed center, flow type, servo valve, and a positive displacement actuator. The resulting transfer function relating output velocity to servo valve input current invariably includes an underdamped quadratic lag due to fluid compliance. In simple hydraulic servo systems, the corner frequency of this quadratic lag represents the absolute limit to system bandwidth. Pressure feedback systems have been devised to damp the fluid resonance so effectively that bandwidth extension beyond the quadratic corner frequency is entirely feasible. Unfortunately, such a scheme destroys the natural output disturbance discrimination inherent in the closed center hydraulic systems. A hybrid method of compensation is proposed whereby pressure feedback occurs only in the region of the resonant frequency, effectively preserving the natural output disturbance discrimination characteristics at the lower frequencies. The pressure drop across positive displacement type hydraulic actuators is a good measure of acceleration. Therefore, the technique involves feeding back this load differential pressure, sensed by electromechanical transducers, through a simple RC high pass (derivative) filter. The effectiveness of the damping is determined by the filter time constant and loop gain. Experimental results verify linear predictions of the possibility of extending the closed loop bandwidth beyond the uncompensated resonant frequency.