Biomedical Engineering Reference
In-Depth Information
camera module. The camera module consists of 144 phototransistors arranged in a 24
6
configuration along with two small rollers for easy movement and two small lights for
illumination. The electronics unit includes a vibrotactile display onto which the user places
his finger and then tracks along a line on a printed page with the camera module, which
feeds the letter-sized images back to the display. The tactile array contains a 24
×
6 array
of 0.08 mm diameter pins, each of which can be independently vibrated at about 230 Hz by
a PZT bimorph reed. As the user moves the camera, tactile images of print letters are felt
moving across the array of rods under his finger (Efron, 1977). Reading speed is limited
to about 100 wpm compared with 250 wpm or more using Braille. But that was not the
point, the Optacon offers access to innumerable sources of the written word not previously
available to the blind—labels on jars, CD covers, computer screens (with a special lens
attachment), ordinary newspapers, and a host more.
A number of derivative products using similar principles have been developed to
extend the capability of these vibrotactile systems because they have proven to be so
useful to blind people. Devices like the videoTIM have a much larger tactile display
and are therefore more amenable to representing simple graphics, handwritten text, and
formulas. They are also able to read keys and messages on mobile phone and computer
displays if optical character recognition (OCR) software is not available.
Vibrotactile research at Johns Hopkins University has included the development of
a 400-element array for fingertip stimulation. This consists of four planes of actuators
slightly staggered so that the pins can all pass through a uniform array of holes in a
fingertip sized plate. This large number of actuators in a small area is required for research
as it is of the same order as the number of tactile corpuscles on the fingertip.
The McGill Haptic lab has been working on the development of an up-to-now unex-
plored mode of interaction for tactile displays. Their researchers believe that lateral skin
stretch is sufficient to give the impression of skin indentation. The objective is therefore to
create a device capable of conveying meaningful tactile sensations by lateral skin stretch
at the fingertip. The project is based on the simple realization that it might be possible
to compensate for both our limited knowledge of touch and the current shortcomings of
haptic technology by making use of tactile illusions that are less complex to implement
with state-of-the-art actuators.
This research has led to the development of a tactile display called the Stimulator
of Tactile Receptors by Skin Stretch (STReSS). Various prototypes of the device have
been built based on the principles shown in Figure 7-27 (Pasquero, 2003), and these show
potential.
STReSS is a computer-peripheral device that stimulates the fingertip by lateral skin
stretch. It is composed of a miniature array of 100 (prototype #1) or 50 (prototype #2)
bimorph PZT actuators that induce time-varying programmable strain fields at the fingertip
to convey tactile information such as texture or small-scale shape. Tactile signals are
generated on a personal computer and then fed to the display via a universal serial bus
(USB) port.
Vibrotactile devices are also used for haptic feedback on hand grips, as can be seen in
Figure 7-28. Haptic devices are often used in teleremote control applications where visual
and audio feedback is insufficient to generate the required immersion. These are discussed
in more detail in a later chapter.
From a visual prosthetic perspective, such devices can replace the handle of a cane
where they become sensory substitutes and convey visual information to the palm of the
hand.
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