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.
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.