Asymptotically Stable Disturbance Observer-Based Compliance Control of Electrohydrostatic Actuators

This article proposes a disturbance observer (DOB)-based compliance control strategy for electrohydrostatic actuators (EHAs), particularly those with high gear reductions and long hydraulic pipelines, which could lead to a high performance interactive robot system. EHAs suffer significantly from internal leakage and friction, which hampers the application of existing compliance control methods to EHAs. Therefore, a two-degree-of-freedom (DoF) DOB is introduced to compensate for both undesirable effects separately; one degree of freedom handles friction observations and the other monitors internal leakage. Then, a compliance controller is designed without affecting the closed-loop stability. To this end, the state of the nominal plant, not the real plant, is used in the designs of the internal leakage observer and the compliance controller as it guarantees closed-loop stability. The asymptotic convergence of the closed-loop system is evaluated via simulations and experiments.