UC Berkeley

Safety in the execution of the sit-to-stand movement is a key feature for wide adoption of powered lower limb orthoses that assist the mobility of patients with complete paraplegia. This work provides techniques for planning the motion of these medical devices to yield biomechanically sound configurations, designing tracking controllers for the reference trajectories of the movements, evaluating the robustness of the controllers against parameter uncertainty, and assessing the ability of a proxy for the user to coordinate with the control input during rehabilitation and physical therapy sessions. Although our ideas can be applied to analyze any powered orthosis in the market, the featured numerical simulations consider a minimally actuated orthosis at the hips.

The orthosis and its user are modeled as a three-link planar robot. The reference trajectories for the angular position of the links are defined from the desired behavior for the Center of Mass of the system, and the corresponding input trajectory is obtained using a computed torque method with control allocation. With the Jacobian linearization of the dynamics about the reference trajectories, a pool of finite time horizon LQR gains are designed assuming that there is control authority over the actuators of the orthosis, and the torque and forces that are applied by the user. Conducting reachability analysis, we define a performance metric for the robustness of the closed-loop system against parameter uncertainty, and choose the gain from the pool that optimizes it. Replacing the presumed controlled actions of the user with an Iterative Learning Control algorithm as a substitute for human experiments, we find that the algorithm obtains torque and forces that result in successful sit-to-stand movement, regardless of parameter uncertainty, and factors deliberately introduced to hinder learning. Thus we conclude that it is reasonable to expect that the superior cognitive skills of real users will enable them to synchronize with the controller of the hips through training. Further tests are performed to verify the robustness of the system in feedback with the LQR gain in the presence of measurement noise, and model uncertainty.

We believe that our tests can set a good benchmark to systematically choose actuators for fitting a large variety of users, and develop a protocol for assessing the robustness of the sit-to-stand movement in clinical trials. This would then help to close the gap between these medical devices and standing wheelchairs, which still remain the most reliable mobility solution for patients with complete paraplegia.