Performance comparison of interaction control strategies on a hand rehabilitation robot

Numerous upper-limb rehabilitation robots have been developed to complement physical therapy following stroke. Most systems have focused on providing passive or assisted movement therapy, largely neglecting the haptic aspects of interaction with the environment. We argue that, especially for the training of object manipulation, rehabilitation robots should be able to stably render a broad dynamic range of impedances. Furthermore, the apparent dynamics of a rehabilitation robot might alter the motion of a patient or impact assessments performed with the device, and should therefore be characterized in detail. We implemented and compared different interaction controllers on a hand rehabilitation robot, the ReHapticKnob, and characterized the dynamic performance using conventional performance metrics as well as with a human in the loop. We further propose transparency planes, an extension to the regression of interaction data proposed in the literature, as a simple performance metric allowing a visual appreciation and comparison of the apparent dynamics of a device in transparency mode. Impedance control with force feedback outperformed the other control schemes and achieved a dynamic impedance range of 82 dB at 0.5 Hz and 35 dB at 10 Hz along the linear degree of freedom of the ReHapticKnob. The implications of these results are discussed in the context of robot-assisted assessment and therapy.