Design and evaluation of a multi-finger skin-stretch tactile interface for hand rehabilitation robots

TL;DR

This work tackles the scarcity of tactile feedback in robotic neurorehabilitation by introducing a four-finger, two-dimensional skin-stretch interface integrated with the PRIDE hand rehabilitation robot, paired with a compact magnetic-field-based force sensor for closed-loop rendering. The system combines a two-DoF planar platform, a small magnetic sensor, and a 500 Hz control loop to render precise lateral skin-stretch cues across four fingertips. Experimental results show sub-centimeter trajectory precision, moderate hysteresis and creep in the sensor, and steady-state force errors in the tens of millinewtons with gain-crossovers around 4–8 Hz, demonstrating viable real-time tactile rendering suitable for ADL-like tasks. This approach advances rehabilitation by providing richer, more ecological haptic feedback, with potential to improve therapy transfer and outcomes.

Abstract

Object properties perceived through the tactile sense, such as weight, friction, and slip, greatly influence motor control during manipulation tasks. However, the provision of tactile information during robotic training in neurorehabilitation has not been well explored. Therefore, we designed and evaluated a tactile interface based on a two-degrees-of-freedom moving platform mounted on a hand rehabilitation robot that provides skin stretch at four fingertips, from the index through the little finger. To accurately control the rendered forces, we included a custom magnetic-based force sensor to control the tactile interface in a closed loop. The technical evaluation showed that our custom force sensor achieved measurable shear forces of +-8N with accuracies of 95.2-98.4% influenced by hysteresis, viscoelastic creep, and torsional deformation. The tactile interface accurately rendered forces with a step response steady-state accuracy of 97.5-99.4% and a frequency response in the range of most activities of daily living. Our sensor showed the highest measurement-range-to-size ratio and comparable accuracy to sensors of its kind. These characteristics enabled the closed-loop force control of the tactile interface for precise rendering of multi-finger two-dimensional skin stretch. The proposed system is a first step towards more realistic and rich haptic feedback during robotic sensorimotor rehabilitation, potentially improving therapy outcomes.

Figures (3)

• Figure 1: Overview of the tactile interface and custom sensor. (a) The tactile interface is integrated into the PRIDE rehabilitation robot, with the fingerpads touching the moving platform and pressure applied on the fingers' dorsal side with the device's quick release. (b) Custom force sensor with the magnet embedded in the silicone. (c) Schematic showing the sensor elastomer deforming under a force F. (d) Front view of the tactile interface with the moving platform. (e) Back view of the tactile interface with linear rods, DC motors, and force sensor.
• Figure 2: Experimental setup used to calibrate and evaluate the custom sensors and tactile interface.
• Figure 3: Bode plots of the tactile interface's frequency response for the X and Y axes.