This paper proposed a nonlinear mathematical model for a 4-port pneumatic nozzle-flapper type servo valve with dual fixed orifices and dual nozzles. In this model, the influence of the flow force was also included. The determination of the flow force on the flapper was proposed using an experimental approach. The effectiveness of the proposed model was verified by comparing experimental and simulated results. This comparison confirmed the influence of flow force on the static and dynamic behavior of the servo valve. A linear model, which was derived from the nonlinear model, showed an applicable range of about ±30 % of rated input current. A practical, order-reduced linear model, which neglects the dynamics of the torque motor and the armature-flapper, was also proposed. The order-reduced model was suitable for systems with large load volumes.

Keywords: nozzle-flapper type pneumatic servo valve, nonlinear model, flow force, linear model