Experimental validation of robust nonlinear tracking control for a hydrostatic transmission with unknown disturbances

This paper presents a robust flatness-based control strategy in combination with a nonlinear reduced-order state and disturbance observer for tracking control of a hydrostatic transmission, which is typically used in the drive trains of working machines. The corresponding control-oriented system model is characterized by unknown lumped disturbances: the leakage volume flow and the torque acting on the motor. These disturbances, together with unmeasurable state variables, the normalised swashplate angle and the normalised bent axis angle, are estimated by a nonlinear observer. Furthermore, parametric uncertainty as well as model errors are taken into account by introducing an additional robust control action. Experimental results indicate that a high tracking accuracy can be achieved for the normalised bent axis angle and the angular velocity of the motor as controlled variables.