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Hybrid Impedance-Sliding Mode Switching Control of the Indego Explorer Lower-Limb Exoskeleton in Able-Bodied Walking

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Abstract

This paper proposes a novel hybrid controller for promoting safe human-robot interaction. The hybrid controller modifies a model-based impedance controller such that it uses impedance control but switches to sliding mode control under non-nominal conditions. Each control law is formulated with an inner-loop controller for feedback linearization and an outer-loop feedback controller for trajectory tracking. The outer-loop feedback torque is theoretically proven to have a smaller magnitude in hybrid control than in impedance control under an assumed condition, suggesting it may be the safer approach. To validate the mathematical assumption and purpose of the controller, a walking experiment is conducted where a healthy able-bodied subject using a lower-limb exoskeleton is randomly subjected to either hybrid or impedance control. Perturbations are induced through sudden changes in treadmill speed, resulting in operation outside nominal conditions for 15.9% of the experiment. The assumption made in the theory holds true for the majority of the experiment, failing only 14.3% of the time. The main results show a statistically significant reduction in average feedback torque magnitudes by 7.9%. This is accomplished without drastically affecting gait, with joint angle root-mean-square differences being 0.36° for the hip and 0.64° for the knee. This demonstrates how the hybrid controller can achieve similar gait patterns with lower feedback torque magnitudes, suggesting it is a promising alternative to impedance control.

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Acknowledgments

The authors would like to thank the individuals at the Human Motion and Control Division at Parker Hannifin Corporation for their technical expertise and support with the Indego Explorer exoskeleton.

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Authors and Affiliations

  1. Center for Rotating Machinery Dynamics and Control, Department of Mechanical Engineering, Cleveland State University, Cleveland, OH, 44115-2214, USA

    Curt A. Laubscher, Anthony Goo & Jerzy T. Sawicki

  2. Department of Engineering, Messiah University, Mechanicsburg, PA, 17055-6706, USA

    Ryan J. Farris

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  1. Curt A. Laubscher
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  2. Anthony Goo
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  4. Jerzy T. Sawicki
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Contributions

C.A.L. developed the controller and composed the manuscript. C.A.L. and A.G. conducted the experiments. R.J.F. provided technical expertise on the exoskeleton. J.T.S. supervised and oversaw the work. All authors reviewed the manuscript.

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Correspondence to Jerzy T. Sawicki.

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Laubscher, C.A., Goo, A., Farris, R.J. et al. Hybrid Impedance-Sliding Mode Switching Control of the Indego Explorer Lower-Limb Exoskeleton in Able-Bodied Walking. J Intell Robot Syst 104, 76 (2022). https://doi.org/10.1007/s10846-022-01583-7

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