Development a multi-loop modulation method on the servo drives for lower limb rehabilitation exoskeleton

Abstract

This paper proposed a multi-loop modulation method (MMM; 3M) on the servo drives applied for the lower limb rehabilitation exoskeleton (LLRE). This proposed 3M included the current, speed and position 3-loop with the auto-tuning, inertia identifier and external load torque observer. The controller gains by searching the optimal bandwidth and by identifying the inertia of the LLRE system were derived from the 3-loop auto-tuning. The controller gains could be instantly adjusted according to the system's oscillation during the LLRE gait motion. In particular, a concept of the phase margin (PM) was introduced in the calculation of the controller gain to ensure system stability. The proposed method could avoid system turbulence due to the excessive bandwidth. On the other hand, a torque observer and inertia identifier were adopted to compensate the external load. Compared the traditional parameter design method and traditional auto-tuning method with this proposed 3M in terms of tracking error and root mean square error (RMSE), the result showed that this proposed 3M provided better response and stability for gait tracking. Also, this proposed 3M could be adapted to variations in different loads.