Tactile Display Device Using Distributed Lateral Skin Stretch

In the past, tactile displays were of one of two kinds: they were either shape displays, or relied on distributed vibrotactile stimulation. A tactile display device is described in this paper which is distinguished by the fact that it relies exclusively on lateral skin stretch stimulation. It is constructed from an array of 64 closely packed piezoelectric actuators connected to a membrane. The deformations of this membrane cause an array of 112 skin contactors to create programmable lateral stress fields in the skin of the finger pad. Some preliminary observations are reported with respect to the sensations that this kind of display can produce. INTRODUCTION Tactile displays are devices used to provide subjects with the sensation of touching objects directly with the skin. Previously reported tactile displays portray distributed tactile stimulation as a one of two possibilities [1]. One class of displays, termed “shape displays”, typically consists of devices having a dense array of skin contactors which can move orthogonally to the surface of the skin in an attempt to display the shape of objects via its spatially sampled approximation. There exist numerous examples of such displays, for recent designs see [2; 3; 4; 5]. In the interest of brevity, the distinction between “pressure displays” and shape displays is not made here. However, an important distinction with regard to the focus of this paper must be made between displays intended to cause no slip between the contactors and the skin and those intended for the opposite case.1 Displays which are intended to be used without slip can be mounted on a carrier device [6; 2]. 1Braille displays can be found in this later category. Another class of displays takes advantage of vibrotactile stimulation. With this technique, an array of tactilly active sites stimulates the skin using an array of contactors vibrating at a fixed frequency. This frequency is selected to maximize the loudness of the sensation (200–300 Hz). Tactile images are associated, not to the quasi-static depth of indentation, but the amplitude of the vibration [7].2 Figure 1. Typical Tactile Display. Shape displays control the rising movement of the contactors (resp. the force applied to). In a vibrotactile display, the contactors oscillate at a fixed frequency. Devices intended to be used as general purpose tactile displays cause stimulation by independently and simultaneously activated skin contactors according to patterns that depend both on space and on time. Such patterns may be thought of as “tactile images”, but because of the rapid adaptation of the skin mechanoreceptors, the images should more accurately be described as “tactile movies”. It is also accepted that the separation between these contactors needs to be of the order of one millimeter so that the resulting percept fuse into one single continuous image. In addition, when contactors apply vibratory signals to the skin at a frequency, which may range from a few Hertz to a few kiloHertz, a perception is derived which may be described 2The Optacon device is a well known example [8]. as “buzzing” and which depends strongly on the waveform of the stimulating signal [9]. The purpose of this paper is to describe initial exploration in a third direction. It is known that the skin’s mechanoreceptors responds to a wide variety of stimuli, again for brevity see [10]. Of particular interest is the ability of the skin to respond to lateral stretch (resp. lateral compression), both physiologically and mechanically [11]. In the next section, we will observe that stretch corresponds to sensations that differ from what might be expected. In order words, mechanical stretch does not correspond to a sensation of stretch. An important consideration which motivates our approach is purely of technological nature. The practical realization of tactile displays based either on shape or vibrotactile sensations is a considerable technical challenge. Difficulties arise (1) from fabricating micro-scale actuators (millimeter scale) which can be packed in dense arrays, and provide for displacements commensurate to their form factors. They must also provide high levels of energy densities, which implies low levels of dissipation; (2) from the necessity to fabricate large quantities of such actuators preferably in an integrated manner; and (3) from operation in environmental conditions resulting from the contact with humans.