A New Mult-Finger Tactual Display

A multi-finger positional display (the T ACTUATOR ) was developed to study communication through the kinesthetic and vibrotactile aspects of the tactual sensory system of the hand. The display consists of three independent single contact-point actuators interfaced (individually) with the fingerpads of the thumb, the index finger, and the middle finger. Each actuator utilizes a disk-drive headpositioning motor augmented with angular position feedback from a precision rotary variable differential transformer (RVDT). A floatingpoint DSP system provides real-time positional control using a digital PID controller. Stimuli from threshold to about 50 dB SL can be delivered throughout the frequency range from near DC to above 300 Hz , thereby encompassing the perceptual range from gross motion to vibration. Actuator frequency and step responses are well modeled as a second-order linear system. Distortion is low allowing delivery of arbitrary stimulus waveforms, e.g., 25 mm low-frequency motion with superimposed high-frequency vibration. System noise and inter-channel crosstalk are also small. As one example of behavioral performance verification, absolute thresholds measured with the stimulator are in general agreement with values in the literature. Overall, the T ACTUATOR accurately follows its drive waveforms and is well suited for a variety of multi-finger tactual perceptual studies. INTRODUCTION This work was motivated by our interest in using the sense of touch as an alternative communication channel. One application area for this work is sensory substitution for individuals who are hearingimpaired and/or visually-impaired. It also provides a new haptic interface for exploring novel human-computer interactions through the tactual channel. The potential to receive information tactually is well illustrated by some natural (i.e., non-device related) methods of tactual speech communication. Particularly noteworthy is the so-called Tadoma method that is employed by some individuals who are both deaf and blind. In Tadoma, one places a hand on the face and neck of a talker and monitors a variety of actions associated with speech production. Our previous research has documented the remarkable abilities of experienced Tadoma users (Reed, Rabinowitz, Durlach, Braida, Conway-Fithian, & Schultz, 1985); these individuals can understand everyday speech at very high levels, allowing rich two-way conversation with both familiar and novel talkers. Conversely, attempts to develop artificial tactual speech communication devices have had only limited success, with none achieving performance anywhere near that demonstrated by Tadoma (e.g., Reed, Durlach, Delhorne, Rabinowitz, & Grant, 1989). One problem with most previous tactual devices concerns the nature of the output display. These displays have generally been composed of multiple stimulators that deliver high-frequency vibration to the cutaneous sensory system. Such “homogeneous” displays have few distinctive perceptual qualities. Furthermore, for practical and/or technical reasons, the displays have rarely engaged the hand, the most sensitive and richly innervated receiving site. In contrast, Tadoma is received by the hand and a talking face is perceptually rich, simultaneously displaying various stimulation qualities that engage both the kinesthetic and cutaneous sensory systems. Recognition of the need for richer tactual displays is now prevalent. Our group has developed an artificial mechanical face display, built around a model plastic skull (Reed et al. , 1985), that has shown promise in conveying information important in Tadoma (Leotta, Rabinowitz, Reed, & Durlach, 1988; Rabinowitz, Henderson, Reed, Delhorne, & Durlach, 1990). As a more general display for studying haptic perception by the hand, the “OMAR” system was recently described by Eberhardt, Bernstein, Barac-Cikoja, Coulter, & Jordan (1994). It was designed to deliver kinesthetic as well as cutaneous stimulation to one or more fingers. The present research is directed at a display that shares some features with OMAR. Our display, the “T ACTUATOR ”, aims at a