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నైరూప్య

Variable Impedance Control Based on Impedance Estimation Model with EMG Signals during Extension and Flexion Tasks for a Lower Limb Rehabilitation Robotic System

Baoping Yuan, Masashi Sekine, Jose Gonzalez, Jose Gomez Tames and Wenwei Yu

Rehabilitation robotic devices could be used as an effective tool to restore impaired motion functionality. Due to the human-robot cooperative nature of rehabilitation, these systems are expected to be user-oriented i.e., they should be controlled considering users dynamic characteristics. In this article, we proposed a variable impedance control, in which desired impedance of a system was setup to match human joint stiffness estimated from Electromyogram (EMG) signals recorded. Two experiments were performed in this work. The objective of the first experiment (Experiment-1) was to study the relationship between EMG and changing impedance in knee joint extension and flexion tasks. Based on the recorded data, a nonlinear model was proposed to express the relationship between EMG and changing impedance. The results show that Root Mean Square of the EMG signals (RMS-EMG) of target muscles increases, as specified elastic modulus increases for both tasks, but there is a significant difference (p<0.01, t-test) between the extension and flexion task. The second experiment (Experiment-2) was to confirm the effectiveness of the variable impedance control with the motion-dependent models acquired in Experiment-1. Four different control policies were tested, i.e., NA: No Assist; FO: using EMG-impedance model from the Flexion Task; EO: using EMG-impedance graph from the Extension Task; FE: using two EMG-impedance models from Flexion and Extension task correspondently. Results indicate that the proposed control model (FE) achieved a smaller discrepancy (p<0.01, t-test) between desired angle and the reached angle than the control with EO or NA cases. Moreover, a small sum of RMS_EMG from the variable impedance control with motion- dependent models denoted less effort required than the NA (p<0.01, t-test) case or the control with FO (p<0.01, t-test). Results also indicate that the proposed nonlinear and motion-dependent variable impedance control method achieved a smaller angular discrepancy (P<0.05, t-test) than linear variable impedance control.