The aim of this work is to demonstrate the feasibility of a novel two degree-of-freedom suspension system as a rehabilitation device for people with lower-limb injuries through nonlinear controls-mechanism design integration. The proposed mechanism, which combines a basic four-bar parallelogram linkage with an extra link and two tension springs, is inspired from the gravity compensation for space research as it provides a reduced weight or even a weightless experience for the suspended person. &nbsp;Compared to the currently more complicated, commercially available alternatives such as treadmill machines, programmable footplates, robotic manipulators, and wearable exoskeletons, the simplicity of this novel design reduces the load acting on the actuators, which means that power consumption and maintenance costs will be much lower. The initial concept of the linkage mechanism design has been presented and combined with a model-based, nonlinear, feedback control algorithm design under a simple impulsive force scenario [1]. In this work, the linkage mechanism is reconsidered in a different configuration, and the nonlinear controller is re-designed and tested under human walking pattern scenarios to allow for an application for physical rehabilitation. To facilitate the nonlinear control re-design, the dynamic model of the mechanism is first modified based on the new configuration using the Lagrange’s equations in non-conservative form. An inverse kinematic analysis is performed separately to extract the target joint variables that correspond to the vertical displacement of the center of mass of the patient under ideal suspension condition with a constant lifting force provided by the device. An open-loop numerical simulation is then performed to demonstrate the sensitivity of the lifting force against the link inertia under dynamic conditions. Additional parametric uncertainties are also added on the link and patient body weights to test the robustness of the controller and to imitate the conditions of a real, experimental implementation. Finally, the system dynamic model is incorporated into nonlinear robust control design based on the sliding-mode control (SMC) strategy to keep the lifting force constant and robust for different patients under different phases of recovery. The performance of the proposed nonlinear controller is tested in closed-loop numerical simulations using the generated walking pattern trajectory as the target trajectory to track. The performance improvement under feedback control is demonstrated through the comparison of the lifting force in the open-loop and closed-loop dynamic simulations with parametric uncertainties and initial condition errors added. The promising performance results motivate the implementation of the proposed controller in a real system to allow for a medical implementation.[1] Ilhan, Z.O., &amp; Chew, M. "Nonlinear Control Design for a Gravity Compensation Mechanism for Human Lower Limb Rehabilitation."&nbsp;Proceedings of the ASME 2020 International Mechanical Engineering Congress and Exposition.&nbsp;Volume 7A: Dynamics, Vibration, and Control. Virtual, Online. November 16–19, 2020.

Nonlinear Robust Control Design for a Gravity Compensation Mechanism Under Human Walking Pattern Scenarios

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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