A Sense of Touch: Three-Axis Sensing for Robotic Prosthetic Fingertips

Abstract

Skin-level force detection in humanoid robotics is a demanding design problem benefitting from soft sensors that are compact, conform to curved surfaces, and sense multiaxis forces. Interactions with people and fragile objects make the ability to detect multiaxis force important for dexterous manipulation and safety. This work presents a three-axis capacitive force sensing soft skin integrated into the fingertips of the PowerHand, a 1-degree-of-freedom prosthetic/robotic hand, enabling real-time measurement of directional interaction forces during object manipulation. Advances include the integration of a curved and compact sensor into the five fingers of the hand, along with wireless communication. Composed of stretchable electrodes and a soft dielectric on a flexible printed circuit board, the capacitive sensor is unobtrusively mounted on the finger, with a 2 mm "skin" thickness and a 6 mm radius of curvature. It has low power consumption (<5 mW) and can employ wireless communication (15 mW Bluetooth Low Energy). The outer surface of the sensor is soft due to the elastomer substrate and elastomer-carbon blend electrodes. The sensitivity at 6 mm radius of curvature is 2 %/N normal force and 7.2 %/N in shear up to 1.75 and 0.6 N in each––ranges typical for fine dexterous manipulation. The wireless readout system refreshes at 34 Hz. The hand is used to demonstrate three-axis forces involved in gently grasping a soft rubber ball and a paper cup. Multiaxis grasping forces may inform development of sophisticated closed-loop control in the future, including slip detection and adaptive grasp stabilization.