Stiffness-fault-tolerant control strategy for elastic actuators with interaction impedance adaptation

Elastic actuators have the potential to enable safe interaction and energy efficient mobility, making them suitable for physical human-robot interaction. However, their increased complexity makes technical faults and their prevention a relevant research topic, particularly considering faults in elastic and kinematic elements. In this article we investigate a stiffness-fault-tolerant control strategy for elastic actuators, based on impedance control, which compensates for internal faults and adapts to a desired interaction impedance behavior. We analyze the control strategy regarding its stability, and adapt it to the dynamic characteristics of two systems: a mechanically adjustable compliance actuator (MACCEPA) and a series-parallel elastic actuator (+SPEA), highlighting the strategy's general applicability to multiple actuator designs, considering nonlinear and redundant characteristics. Experimental validation with these systems shows that the control strategy is capable of accurately tracking reference output trajectories and adapting interaction characteristics, under fault and disturbance conditions, showcasing the versatile applicability of the strategy while achieving fault-tolerance.