Time-Varying Acceleration Constraint-Based Admittance Control for Robot-Aided Bilateral Human-Compliant Ankle Rehabilitation

Robot-assisted bilateral rehabilitation has demonstrated efficacy in facilitating ankle function recovery. This article develops a control framework to ensure safe bilateral human-compliant ankle rehabilitation training. First, a bilateral ankle-coupled rehabilitation training mode (BACRTM) is established, which dynamically couples the human–robot interaction (HRI) torques of both ankles. When one ankle performs active training, resistive or assistive torques are synchronously applied to the contralateral side, forming a coordinated bilateral movement mechanism. Second, a time-varying acceleration constraint (TVAC) strategy integrating error feedback is designed based on Lyapunov stability theory. By limiting the upper and lower bounds of reference acceleration, system stability is ensured during high-dynamic HRI. Theoretical analysis proves that the TVAC strategy can produce stable and bounded tracking errors. Experimental results demonstrate that under the BACRTM, the TVAC strategy can reduce the bilaterally rendered inertia by an average of 99.05%, while maintaining robustness to varying subject inputs and dynamics. Furthermore, the strategy features adjustable acceleration boundary effects, enabling adaptation to patients with ankle injuries at different rehabilitation stages.