Spool Hydraulic Stiffness and Flow Force Effects in Electrohydraulic Servo-Valves

Abstract

This paper deals with an analytical and experimental assessment of the flow force effects on electrohydraulic servo-valve steady state characteristics. The system mathematical model is derived, and special consideration is given to non-linearity of the feedback wire stiffness and magnetic saturation of the armature. The ‘spool hydraulic stiffness’ is defined and expressed in terms of the servo-valve parameters and supply pressure to allow clear interpretation of the nature of flow force effects. Experimental results of spool displacement decrement due to flow force versus net displacement are in good agreement with the predicted performance in a wide range of input current up to almost full magnetic saturation. The results provide evidence, on an alternative basis, of the non-linear feedback behaviour. Correlation is also made between flow force and limits of stable operation, and an expression is derived for the maximum allowable supply pressure.