Fast replanning of a lower-limb exoskeleton trajectories for rehabilitation

Abstract

This paper exposes a control architecture enabling rehabilitation of walking impaired patients with the lower-limb exoskeleton Atalante. Atalante's control system is modified to allow the patient to contribute to the walking motion through their efforts. Only the swing leg degree of freedom along the nominal path is relaxed. An online trajectory optimization checks that the muscle forces do not jeopardize stability. The optimization generates reference trajectories that satisfy several key constraints from the current point to the end of the step. One of the constraints requires that the center or pressure remains inside the support polygon, which ensures that the support leg subsystem successfully tracks the reference trajectory. As a result of the presented works, the robot provides a non-zero force in the direction of motion only when required, helping the patient go fast enough to maintain balance (or preventing him from going too fast). Experimental results are reported. They illustrate that variations of ±50% of the duration of the step can be achieved in response to the patient's efforts and that many steps are achieved without falling. A video of the experiments can be viewed at https://youtu.be/_1A-2nLy5ZE.

Enabling safe walking rehabilitation on the exoskeleton Atalante: experimental results

“…They stress that, using the OP strategy, the proposed controller is completely preserving the balance of the system for velocities about as low as 70% of the nominal velocity, and velocities as low as 50% of the nominal velocity provided the change duration is lower or equal to 300 ms. These results are in complete alignment with the previous stability results obtained in simulation and reported in [23].…”

“…Problem 1 is in fact a minimal time problem for an input-constrained linear dynamics of dimension 4. Its phase plane analysis, decoupling the X and Y directions, has been performed in [23], covering all possible cases of initial and final conditions. The main finding of [23] is that T is the union of at most two intervals, such that the solutions T opt and u opt can be easily determined using a bisection method, granting high-numerical efficiency.…”

“…It is worth noting that of all these schemes, only [21], [22] solves a non-linear MPC problem at 1kHz as we do (with a CPU twice as fast as Atalante onboard computer). The simplicity of the LIP model allows us to use a fast resolution method for this nonlinear OCP, relying on the theoretical study of [23].…”