This paper pertains to the nonlinear control of a high-precision hydrostatic actuation system known as the ElectroHydraulic Actuator (EHA). It describes the action of the controller in reducing the steady state error resulting from flow and force disturbances. The EHA uses inner-loop pump velocity feedback to achieve an unprecedented level of accuracy for a hydrostatic system. A published mathematical model of the EHA is reviewed and expanded to produce an equation that predicts the response of the EHA to both desired inputs as well as flow and force disturbances. This equation suggests that the use of a proportional outer-loop controller should result in steady-state error as a result of these disturbances, but that a PI outer-loop controller should eliminate the steady-state error. Experimental results from a prototype of the EHA demonstrate that due to the nonlinear friction present in the actuator, the use of a conventional proportional or PI controller is not sufficient to effectively deal with these disturbances. However, a nonlinear proportional outer-loop controller does result in a substantial performance improvement in regards to disturbance rejection for positional accuracy. Experiments conducted on the prototype using the nonlinear controller reveal that it is capable of a positional accuracy of 1 µm for a load of 20 kg.

Keywords: actuator, electrohydraulic, hydrostatic, micro-precision, disturbance rejection