The aim of this work is to demonstrate the feasibility of a two degree-of-freedom suspension system as a rehabilitation device for people with lower limb injuries. The proposed mechanism, which combines a basic four bar linkage with an extra link and two springs, is inspired from the gravity compensation approach for space research as it provides a reduced weight experience for the suspended person. Compared to the more complicated, commercially available alternatives such as treadmill machines, programmable foot plates, robotic manipulators, and wearable exoskeletons, the novelty of this simple design reduces the load acting on the actuators, which means that the power consumption and the maintenance costs will be much smaller. This is because, the linkage suspension system itself is able to provide static compensation of any portion of the body weight of the patient, and of the links, while the joint torques need to be adjusted only to counter small performance deviations under dynamic conditions due to the presence of the link inertia.  Therefore, the controller design should be made to handle the nonlinearity of the system dynamics, while adding robustness against small performance deviations due to the link inertia, modeling uncertainty and/or external disturbances such as joint friction. For this reason, a nonlinear feedback control approach is proposed in this work based on the sliding-mode control (SMC) strategy to drive the links of the mechanism so that the lifting force provided by the device is kept constant and robust against the vertical displacements of the center of mass of the different body weights of the patients under different stages of rehabilitation. To facilitate control design, the dynamic model of the mechanism is first obtained based on the Lagrange’s equations under a constant force scenario. Using the system dynamic model, the sensitivity of the system against changing model parameters and body weights is demonstrated in open-loop numerical simulations. The system dynamic model is then incorporated into feedback control design based on the nonlinear, sliding-mode control strategy. The control tuning to guarantee tracking convergence is provided based on the Lyapunov stability theory. A walking pattern analysis is carried out separately to determine the vertical displacement of the center of mass, which is then converted into the joint space through inverse kinematic analysis to generate a target trajectory for control testing purposes. Finally, 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. Initial condition errors as well as some parametric uncertainties are also added to test the robustness of the controller. The promising performance results motivate the implementation of the proposed controller in a real system to allow for a medical implementation.


Nonlinear Control Design for a Gravity Compensation Mechanism for Human Lower Limb Rehabilitation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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