Tactile Displays

1. Tactile Displays Kaczmarek, K.A. and Bach-Y-Rita, P. (1995), Tactile displays, in Virtual Environments and Advanced Interface Design, Barfield and Furness, pp. 349-414. Summarized by Geb Thomas

2. Your 2m2 of skin • 90% hairy, 10% glabrous (hairless) • Accessible • Richly innervated • Precise discrimination • Adaptable to spatial and temporal displays

3. Covered Here • Present and potential applications • Mechanisms of normal touch perception • Technology for producing tactile displays • Practical considerations

4. Traditional displays for the blind • Braille (6-dot matrix, 2.3mm separation, 125 words*min-1) • Sign language: finger spelling: 6 letters* sec-1, American Sign Language: 4-5 syllables*sec-1) • Tadoma 3 syllables*sec-1

5. Tactile Feedback from tactile sensors • For people with poor haptic perception in their hands (Hansen’s disease, suited astronauts) • strain guages on a glove to forhead electrodes: can detect shape and texture! • Movable pins, enhanced fingerpads, tactile pads, glove-hand adhesion, removable glove fingertip • Sample task: no feedback: 92s, force feedback 63s, barehanded: 14s

6. Tactile auditory substitution • Auditory prosthesis which adjusts the perceived intensity of 16 electrodes, each corresponding to the sound intensity of a given passband in the audio spectrum. • Improve speech clarity for deaf children • Improve auditory discrimination and comprehension in older patients

7. Tactile vision substitution (TVS) • Television Camera to users skin with a vibrotactile or electrotactile stimulators array. • Stimulation intensity is controlled by grayscale • Distal attribution -- with practice, user perceives the stimulation to be in front of them

8. Tactile Reading • Optacon 6x24-row vibrating fingerpad • 90 words*min-1 exceptional 28 words*min-1 normal • Now discontinued • Significant underground calling for its resurgance

9. Static tactile displays • 64-solenoid, four level display presenting graphical information • Another model has one prime mover and many piezoelectric latches • Muscle wire display

10. Virtual tactile tablet • Fingerpad vibrotactile stimulation array on a mouse • 5x20 array of pin vibrotactors mounted directly above t-shaped mouse • Minsky’s sandpaper display

11. Human Tactile Perception • Six types of cutaneous receptors, four functions • Fast adapting, broad receptive field (FAII) -- high-frequency vibration • Fast adapting, small receptive field (FAI) --localized movement fine form and texture • Slow adapting, large-field (SAII) -- maybe not involved in haptics • Slow adapting, small-field (SAI) -- form and roughness

13. Measures • Smallest amount of pressure • Two-point limen (two point discrimination threshold TPDT) • Affected by location, practice, fatigue, distraction • Modeling attempts include convolving integral, low-pass filter or Gaussian blur

14. Design Criteria • Static tactile displays • Vibrotactile displays • Electrotactile displays

15. Static • High power consumption • Rapid adaptation to static stimuli • 12-20mm height to match Optacon accuracy

16. Vibrotactile • Threshold: 5 micro-m at 25-650 Hz for small areas (<.05 cm2) • Adaptation to strong stimuli • Full recovery requires 2 min. • 160 Hz is best • 10dB over threshold • 1 mm diameter stimulator with .5mm movement

17. Electrotactile Displays • Current through skin • Current-limited • Balanced, biphasic pulses with zero net DC current • Electrodes to produce appropriate ions (gold, platinum, silver) • Electrode size is important • Some are implantable

18. Important Issues • Pain threshold • Skin condition • Sensory adaptation • Subjective magnitude of electrotactile stimulation