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

Abstract

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> force controller that guarantees robust stability in the inner loop of the impedance control strategy. ℋ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> 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.