Sliding Mode Control of a Hydraulically Actuated Load Application Unit With Application to Wind Turbine Drivetrain Testing

This paper presents a sliding mode control strategy for a load application unit (LAU) found at Clemson University´s Wind Turbine Drivetrain Testing Facility in North Charleston, SC, USA. The LAU is a multiinput, multioutput, overactuated, nonlinear, dynamic system and is part of a 7.5-MW wind turbine nacelle test bench. First, a plant model of the system is developed. Second, a sliding mode control strategy is developed. This control strategy uses a pseudoinverse of the input influence matrix to account for the system being overactuated. The resulting reference signals are inputs to a sliding controller responsible for controlling the system´s 24 hydraulic actuators. The results show that the proposed control strategy performs well and that the LAU may be suitable for replicating full-scale time-varying wind loads on the main shaft of a test nacelle. Also discussed are key areas for improvement and future research directions.