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Introduction

Introduction to Haptic Display: Tactile display

by Robert Howe, Harvard University

Skin sensation is essential for many manipulation and exploration tasks. To handle flexible materials like fabric and paper, we sense the pressure variation across the finger tip. In precision manipulation, perception of skin indentation reveals the relationship between the hand and the grasped tool. We perceive surface texture through the vibrations generated by stroking a finger over the surface. Tactile sensing is also the basis of complex perceptual tasks like medical palpation, where physicians locate hidden anatomical structures and evaluate tissue properties using their hands.

Tactile display devices stimulate the skin to generate these sensations of contact. The term "tactile display" is sometimes used to describe any apparatus that provides haptic feedback, but it's useful to distinguish between systems for vector force feedback and devices that convey distributed sensations. The skin responds to several distributed physical quantities; the most important are perhaps high-frequency vibrations, small-scale shape or pressure distribution, and thermal properties.

Vibrations can relay information about phenomena like surface texture, slip, impact, and puncture. In many situations, vibrations are experienced as diffuse and unlocalized, so a single vibrator for each finger or region of skin may be adequate. The frequency range of interest is a roughly a few Hertz to a few hundred Hertz, and effective single-channel devices are relatively easy to build.
Small-scale shape or pressure distribution information is much more difficult to convey. The most common design approach is an array of closely-spaced pins that can be individually raised and lowered against the finger tip to approximate the desired shape. To match human finger movement speeds, bandwidths from DC to several dozen Hertz may be required, and to match human perceptual resolution, pin spacings of less than a few millimeters are appropriate. In addition, the display often must be small and light enough to mount on a force- reflecting interface. To convey a range of spatial scales across a finger tip may thus require dozens of fast actuators in a few cubic centimeters, a serious design challenge.
Thermal display is a relatively new area of research. Because human fingers are often warmer than the "room temperature" objects in the environment, thermal perceptions are based on a combination of thermal conductivity, thermal capacity, and temperature. This allows us to infer material composition as well as temperature difference. A few thermal display devices have been reported, usually based on Peltier thermoelectric coolers.
Many other tactile display modalities have been demonstrated, including electrorheological devices for conveying compliance, electrocutaneous stimulators, ultrasonic friction displays, and rotating disks for creating slip sensations.

Current research on tactile displays has much in common with previous work on sensory substitution for the disabled. This includes tactile pin arrays to convey visual information to the blind, and vibrotactile displays of auditory information for the hearing impaired. Few of these sensory substitution techniques have gained wide acceptance in the intended user community. Tactile displays for teleoperation and virtual environments may fare better, because the goal is replication of stimuli in the original sensory modality, rather than mapping phenomena from one modality to another.

Selected References:

VIBROTACTILE DISPLAYS

Kontarinis DA, Howe RD. Tactile display of vibratory information in teleoperation and virtual environments. Presence, 4(4):387-402, 1995.

Minsky M, Lederman, SJ. Simulated Haptic Textures: Roughness. Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, ASME International Mechanical Engineering Congress and Exposition, Atlanta, GA, Nov. 17-22, 1996, K. Danai, ed., Proceedings of the ASME Dynamic Systems and Control Division, DSC-Vol. 58, p. 451-458.

SHAPE/PRESSURE DISPLAYS

Cohn MB, Lam M, Fearing RS. Tactile feedback for teleoperation. Proc. Telemanipulator Technology, H. Das, Editor, Boston, Proc. SPIE 1833, p. 240-254, 1992.

Hasser C, Weisenberger JM. Preliminary evaluation of a shape memory alloy tactile feedback display. Proc. Symposium on Haptic Interfaces for Virtual Environments and Teleoperator Systems, ASME Winter Annual Meeting, Kazerooni H, Adelstein BD, Colgate JE, Editors, New Orleans, LA, p. 73-80, 1993.

Howe RD, Peine WJ, Kontarinis DA, Son JS. Remote palpation technology. IEEE Engineering in Medicine and Biology, 14(3):318-323, May/June 1995.

THERMAL DISPLAYS

Caldwell G, Gosney C. Enhanced tactile feedback (tele-taction) using a multi-functional sensory system. Proc. IEEE International Conference on Robotics and Automation, Atlanta, GA, 2-6 May 1993, p. 955-60.

Ino S, Shimizu S, Odagawa T, Sato M, Takahashi M, Izumi T, Ifukube T. A tactile display for presenting quality of materials by changing the temperature of skin surface. Proc. Second IEEE International Workshop on Robot and Human Communication Tokyo, 3-5 Nov. 1993, p. 220-4.

HUMAN TACTILE SENSATION

Boff, KR, Lincoln JE (Eds.). Engineering Data Compendium: Human Perception and Performance. Ohio: H. G. Anderson Aerospace Medical Research Laboratory, 1988.

Johansson RS, Vallbo AB. Tactile sensory coding in the glabrous skin of the human hand. Trends in Neuroscience, 6(1): 27-32, 1983.