Design and analysis of ℋ∞ force control of a series elastic actuator for impedance control of an ankle rehabilitation robotic platform

Impedance control based on robust force control can improves rehabilitation therapies. In this paper, we formulate and validate a model of a series-elastic-actuator-based ankle rehabilitation device that includes a second-order description of the human joint dynamics. In order to control the output impedance of the platform we used a cascade configuration with a PD position controller as outer loop and a force controller in the inner loop. We present the design and analysis of an ℋ
_∞
force controller that guarantees robust stability in the inner loop of the impedance control strategy. ℋ
_∞
mixed-sensitivity synthesis method was used for robust control loop-shaping design of the force controller. Experimental results of human-robot interaction tests are presented. Performance and stability analysis using frequency and temporal responses are conducted. We found that our controller, actually any fixed-gain controller, can not guarantee the same performance among the different human-robot conditions. Results obtained in this work are a departure point to improve performance of the force controllers and therefore impedance-controlled rehabilitation therapies.