Rendering Fine Tactile Feedback With a Novel Hydraulic Actuation Method for Wearable Haptic Devices

Tiny contact transients prove very informative for dexterous, fine manipulation, both in virtual environments and teleoperation. Yet, it is challenging to obtain such quality of the rendering in compact and highly wearable haptic devices. To this aim we propose a novel miniature hydraulic actuator, aiming at low-noise rendering of tiny tactile signals, fully enclosed in a fingertip haptic device. Its novelty is in the use of ferrofluidic sealings, embedded within a miniature linear electromagnetic motor. The solution has two main advantages: it shows no static friction, due to the magnetohydrodynamic levitation effect of the ferrofluid, and a noiseless hydraulic reduction. Besides, the hydraulic actuator enables the use of a soft and compliant interface for transmission of signals to the fingerpad tissues. We evaluate here a prototype of the actuator implemented in a compact and soft haptic thimble: the low-noise rendering capabilities are evaluated in experiments at the bench and through perception study. Then, in a simplified teleoperation setup, we match one-to-one the pressure rendered by the device with a sensitive pneumatic pressure sensor mounted on a robotic fingertip. Results show that haptic feedback alone (no vision) is able to improve results over the visual condition in terms of maximum exerted pressure.