Field-Based Assist-as-Needed Control Schemes for Rehabilitation Robots

This article presents assistive control schemes for rehabilitation robots. The objective was to develop control strategies to follow a predefined path by maximizing the active participation of impaired subjects during training sessions. For this purpose, the path following task was defined in the velocity domain by constructing a desired velocity field around the path, and two different controllers were designed, which adjust the intervention of the robot according to the performance of the users, delivering an assist-as-needed (AAN) feature with adequate timing freedom. First, we developed an impedance controller for velocity tracking, which gains the AAN property by means of modulating the target impedance. Second, we designed a velocity tracking controller, which adjusts its contribution, in an AAN way, by monitoring the tracking error. Utilizing force sensors, the first method does not require a compliant robotic system, while the second approach does not need force sensors but provides its best AAN performance whenever the system is compliant. The controllers were featured by a force field component to avoid large position errors. The performance of the controllers was evaluated through experimental tests performed on a planar upper-limb training robot.