A PID Controller Approach to Explain Human Ankle Biomechanics across Walking Speeds

Lower-limb robotic prostheses and exoskeletons depend on controllers to function in synchrony with their users. Recent advancements in control technology permit embodiment and more intuitive control for the user. In this study, we utilize a control engineering perspective to propose a phase-dependent muscle-driven proportional, integral, and derivative (PID) controller to regulate human ankle joint trajectories across walking speeds. We calculated the correlation coefficients that relate the tibialis and gastrocnemius muscle activation to the ankle joint angle error, integral of the error, and rate of change of the error between an average ankle joint trajectory and the ankle angle at two walking speeds: 1.5 m/s and 2.0 m/s. We noted that preswing (PSW) was the only gait period that had high absolute values for the correlation coefficients (> 0.7) across all three relationships. Other gait periods had varying high and low correlation coefficients across the different relationships. These results present a promising justification to utilize the classic control technique in a non-conventional manner. A phase-dependent and muscle-driven PID controller influenced by the PSW phase may be used to modulate the ankle joint trajectory with muscle activation across walking speeds in lower-limb robotic prostheses and exoskeletons.