Biomedical Engineering Reference
In-Depth Information
FIGURE 7-36
Image acuity as a
function of the array
size [Adapted from
(Wicab 2009), with
permission.]
FIGURE 7-37
Schematic diagram
showing the
construction of a
single Peltier
element.
7.7.2.5 Thermotactile Displays
Peltier elements consist of p-doped and n-doped semiconductor material separated by a
metallic interconnect, as shown in Figure 7-37. Current flows through the n-type material
before crossing a metallic interconnect and passing into the p-type material. Electrons in
the n-type material move against the direction of current flow whereas holes in the p-type
material move in the direction of current, both remove heat from one side of the device
and deposit it on the other. If the polarity is reversed, then the positions of the hot and cold
junctions are reversed.
In commercial thermotactile displays, an array of closely spaced Peltier elements can
be controlled to generate either hot or cold regions on the upper surface of the array, as
shown in Figure 7-38. Control electronics monitors the temperature of each to maintain a
reasonably constant element temperature. The main advantages of this form of display are
that there are no moving parts and hence nothing to wear out compared with vibrotactile
displays. They are also less intrusive than electrotactile displays.
Because the distribution of temperature-sensitive nerve fibers is lower than that of
some of the tactile corpuscles, the practical resolution of thermotactile displays will never
be as high.
These devices are still in the experimental phase but can be used to produce Braille or
standard letters as well as the simple graphics shown in the thermal images of the display
visible in Figure 7-39.
