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.
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.