Modelling the Dynamics of a Direct-Drive Servovalve for Fail-Operative Primary Flight Control Actuators

This paper deals with the modelling and the experimental characterisation of the dynamics of a quadruple Direct-Drive servovalve used in the horizontal tail tandem actuator of a modern Fly-By-Wire advanced jet trainer. By means of a specifically designed test equipment, the servovalve dynamic response is characterised for the normal operative condition as well as for the fail-operative conditions, which have been simulated by artificially injecting one or more electrical failures (open circuit and short circuit) into the four coils of the servovalve Linear Force Motor. In parallel with the experimental activities, an accurate model of the dynamics of the servovalve has been developed in the Matlab/Simulink environment. The simulation results are compared with the experimental data and a good agreement is achieved over a wide range of tested conditions. The effects of the electrical failures on the dynamic performance of the servovalve are examined and evaluated. In order to overcome the performance degradation in the fail-operative conditions, the applicability of a simple compensation technique is discussed. The model is designed as a multiple-state machine, where the transition from a state to another is managed using the Simulink/Stateflow charts. This choice, together with the use of Simulink/S-functions for the execution of part of the simulation code, allowed to obtain a real-time executable model, which can be used for real-time monitoring purposes as well as for real-time hardware-in-the-loop simulations