Cascaded second order sliding mode observer for state and friction dynamics of a control valve

In this paper, higher order sliding mode (HOSM) observers have been applied for state observation of an engine air path actuator. The cascaded observation scheme is based on a detailed physical model of the actuator, ([28], [1])which includes LuGre friction model. Two cascaded observers provide actuator dynamics and friction dynamics separately. Equivalent Output Injection, a property of HOSM observers, has been exploited to calculate friction force as an unknown external input. Taking advantage of this additional information, a method to identify the parameters of the LuGre model has also been presented. The identification has been verified using simulation and experimental results.