High-fidelity rendering of virtual objects with the ReHapticKnob - novel avenues in robot-assisted rehabilitation of hand function

Rehabilitation robots can provide intensive and motivating therapy after stroke in order to further promote recovery of sensorimotor function. To provide the necessary patient-robot interaction for the assessment and training of the hand throughout the different phases of recovery, high-fidelity haptic interfaces with a wide impedance width (Z-width) are required. In this paper the Z-width and haptic interaction quality of a 2 degree-of-freedom (DOF) end-effector based hand rehabilitation robot called the ReHapticKnob are evaluated and strategies to improve these parameters are investigated. An impedance-based controller with force feedback was implemented to modulate the apparent impedance of the robot's end-effector. Additionally, a discrete-time adaptive velocity estimator was used to increase the Z-width of the device. The resulting impedance is evaluated and compared to a commercial haptic device (Phantom Premium 1.5) and the achieved Z-width is analyzed in frequency space and on a K-B-plot. With the proposed control strategy the ReHapticKnob shows similar transparent behavior as a Phantom Premium 1.5 but can render much higher impedances, resulting in a unique high-fidelity patient-robot interaction capable of adapting to different impairments and presenting various haptic stimuli.