A Compliant Transition Control Strategy for Plantarflexion-Dorsiflexion Switch in a Bidirectional Cable-Driven Ankle Exoskeleton

Ankle exoskeletons play a vital role in gait rehabilitation for stroke patients. While cable-driven systems provide high compliance, single-motor bidirectional actuation presents challenges during directional transitions, including abrupt tension changes and torque overshoot that impair interaction comfort. This study proposes a hybrid control framework based on a finite-state machine combining position, pretension, and torque control. The core innovation is a compliant transition strategy employing a jerk-continuous S-curve velocity trajectory for the pretensioning phase during plantarflexion-dorsiflexion switching. By regulating motor velocity, the method achieves smooth tension buildup and a stable initial state for subsequent torque control. Benchtop experiments under four test conditions show that the proposed S-curve-based planning transition (SPT) control strategy reduces torque overshoot by an average of 12.25% compared with direct reversal control, improving compliance and stability. Seated trials with the exoskeleton further confirm smooth adaptation of the human-machine interface to dynamic direction changes, enhancing comfort. This study offers a practical and effective framework for improving compliant control and cable pretension regulation in tendon-driven exoskeleton systems.