Improved assistive profile tracking of soft exosuits for walking and jogging with off-board actuation

In this paper, we present a design and control approach of a modular off-board actuation system with mono-articular hip exosuit, enabling the instantaneous assistive profile modification of magnitude, shape, and timing. The off-board system consists an actuation unit with two degrees of freedom that can transmit forces to our soft exosuit via Bowden cables. To perform accurate force tracking and cable slack management, we implemented a switching admittance-position control approach that utilizes the advantages of both admittance and position control. In particular, feedforward models (FFMs) are added to the admittance control to compensate variability of individual kinematics, nonlinear hip exosuit stiffness and actuator transmission loss to enable a more accurate force tracking. This system allows us to track force profiles accurately and robustly under normal walking or jogging speed. The performance of force tracking was evaluated on three subjects walking on a treadmill at 1.25 m/s with various magnitude, shape, and timings of assistive profiles and jogging on a treadmill at 2.3 m/s with varied magnitude of a representative assistance profile. Results showed that assistance could be delivered reliably across different profiles. In case of walking experiments, the errors of the magnitude, RMS-E, and timing were within 2.0 N, 8.4 N, and 1.8% when varying the magnitude, shape, and timing respectively. In case of jogging experiments, the RMS-E was within 3.0 N. This system will be used for future research examining how to individualize assistive profiles for different wearers during walking and jogging.