Improving Low-Level Control of the Exoskeleton Atalante in Single Support by Compensating Joint Flexibility.

This paper describes a novel low-level controller for the lower-limb exoskeleton Atalante. The controller implemented on the commercialized product Atalante works under the assumption of full rigidity, performing position control through decentralized joint PIDs. However, this controller is unable to tackle the presence of flexibilities in the system, which cause static errors and undesired oscillations. We modify this controller by leveraging estimations of the position and velocity of the flexibilities, readily available on Atalante through the use of strapdown IMUs. Instead of considering feedback on the motor position only, we perform feedback on both the joint position and the flexibility angle, keeping a decentralized approach. This enables compensation of both the static error present at rest, and rapid damping of the oscillations. To tune the gains of the proposed controller, we use a linearized model of an elastic joint to which we apply a steady-state LQR, which creates desirable robustness to the flexible model. The proposed controller is experimentally validated through various single support experiments on Atalante, either empty or with a user. In all cases, the proposed controller outperforms the state-of- the-art controller, providing improved trajectory tracking and disturbance rejection.