Feedback parameter selection for impedance control of series elastic actuators

The interest of series elastic actuators (SEAs) for legged robots has recently increased to achieve compliant interactions and efficient gaits. However, control of legged robots with SEAs is difficult due to the need to design controllers that take into account both torque and impedance feedback loops. The work presented here addresses this issue by proposing a critically-damped fourth order system gain selection criterion for a cascaded SEA control structure with inner torque and outer impedance feedback loops. Velocity filtering and feedback delays are taken into consideration for stability and impedance performance analysis. We observe and analyze the interdependence between torque and impedance feedback gains to achieve the desired closed loop performance. Based on this analysis we derive a simple gain design criterion to maximize the tracking performance of SEAs. Our final goal is to maximize the output impedance capabilities of SEAs in order to fulfill a wide range of application needs. In contrast to low impedance design studies, we focus here specifically on achieving the highest possible impedance gains of SEAs. Finally, experiments using our UT-SEA are conducted to verify our proposed approach. This study serves as a stepping stone towards utilizing and designing humanoid robots with SEA actuators for mobile behaviors and interaction with cluttered and unknown environments.