Biomedical Engineering Reference
In-Depth Information
FIGURE 7-22
Representations of
the sensory cortex
as a function of its
association with
parts of the body
(a) Sensory
homunculus model.
(b) Graphical model.
FIGURE 7-23
Conventional
vibrotactile display.
(a) Photograph.
(b) Diagram of a
single solenoid
based tactor.
(Courtesy of
Forschungszentrum
Karlsruhe.)
Unfortunately, most of the tactile displays built to date fail to convey meaningful
tactile information and to be practical at the same time. The devices are too big; they do
not yield enough force or are constrained too low bandwidth; they are limited to a small
number of actuators with low-spatial density; they require constant maintenance or are
simply too expensive and too complex. These failures are reflected by the relative scarcity
of tactile displays that have made it to the commercial world.
Frequency of vibration is a parameter that influences the quality and intensity of
perception, with the sensitivity affected by factors like body position, skin temperature,
and underlying tissue type (bone, fat, muscle, or a combination). Values between 50
and 300 Hz are generally used as being in the range of frequencies where the tactile
corpuscles are most sensitive. Oscillating pressure also adds new degrees of freedom to
the design of vibrotactile stimuli. These include waveform shape, for example sinusoidal
or square, and amplitude modulations (at different modulation frequencies) of the carrier
frequency.
