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Source: http://web.mit.edu/press/2013/wearable-tactile-displays.html
Can you feel me now?
New array measures vibrations across the skin, may help engineers design optimal, wearable tactile displays.
CAMBRIDGE, Mass. — In the near future, a buzz in your belt or a
pulse from your jacket may give you instructions on how to navigate your
surroundings.
Think of it as tactile Morse code: vibrations
from a wearable, GPS-linked device that tell you to turn right or left,
or stop, depending on the pattern of pulses you feel. Such a device
could free drivers from having to look at maps, and could also serve as a
tactile guide for the visually and hearing impaired.
Lynette
Jones, a senior research scientist in MIT’s Department of Mechanical
Engineering, designs wearable tactile displays. Through her work, she’s
observed that the skin is a sensitive — though largely untapped — medium
for communication.
“If you compare the skin to the retina, you
have about the same number of sensory receptors, you just have them over
almost two square meters of space, unlike the eye where it’s all
concentrated in an extremely small area,” Jones says. “The skin is
generally as useful as a very acute area. It’s just that you need to
disperse the information that you’re presenting.”
Knowing just
how to disperse tactile information across the skin is tricky. For
instance, people may be much more sensitive to stimuli on areas like the
hand, as opposed to the forearm, and may respond best to certain
patterns of vibrations. Such information on skin responsiveness could
help designers determine the best configuration of motors in a display,
given where on the skin a device would be worn.
Now Jones has
built an array that precisely tracks a motor’s vibrations through skin
in three dimensions. The array consists of eight miniature
accelerometers and a single pancake motor — a type of vibrating motor
used in cellphones. She used the array to measure motor vibrations in
three locations: the palm of the hand, the forearm and the thigh. From
her studies with eight healthy participants, Jones found that a motor’s
mechanical vibrations through skin drop off quickly in all three
locations, within 8 millimeters from where the vibrations originated.
Jones
also gauged participants’ perception of vibrations, fitting them with a
3-by-3 array of pancake motors in these three locations on the body.
While skin generally stopped vibrating 8 millimeters from the source,
most people continued to perceive the vibrations as far away as 24
millimeters.
When participants were asked to identify specific
locations of motors within the array, they were much more sensitive on
the palm than on the forearm or thigh. But in all three locations,
people were better at picking out vibrations in the four corners of the
array, versus the inner motors, leading Jones to posit that perhaps
people use the edges of their limbs to localize vibrations and other
stimuli.
“For a lot of sensory modalities, you have to work out
what it is people can process, as one of the dictates for how you
design,” says Jones, whose results will appear in the journal
IEEE Transactions on Haptics.
“There’s no point in making things much more compact, which may be a
desirable feature from an engineering point of view, but from a
human-use point of view, doesn’t make a difference.”
Mapping good vibrationsIn
addition to measuring skin’s sensitivity to vibrations, Jones and
co-author Katherine Sofia ’12 found that skin has a strong effect on
motor vibrations. The researchers compared a pancake motor’s frequency
of vibrations when mounted on a rigid structure or on more compliant
skin. They found that in general, skin reduced a motor’s vibrations by
28 percent, with the forearm and thigh having a slightly stronger
dampening effect than the palm of the hand.
The skin’s damping
of motor vibrations is significant, Jones says, if engineers plan to
build tactile displays that incorporate different frequencies of
vibrations. For instance, the difference between two motors — one
slightly faster than the other — may be indistinguishable in certain
parts of the skin. Likewise, two motors spaced a certain distance apart
may be differentiable in one area but not another.
“Should I
have eight motors, or is four enough that 90 percent of the time, I’ll
know that when this one’s on, it’s this one and not that one?” Jones
says. “We’re answering those sorts of questions in the context of what
information you want to present using a device.”
Creating a buzzJones
sees promising applications for wearable tactile displays. In addition
to helping drivers navigate, she says tactile stimuli may direct
firefighters through burning buildings, or emergency workers through
disaster sites. In more mundane scenarios, she says tactile displays may
help joggers traverse an unfamiliar city, taking directions from a
buzzing wristband, instead of having to look at a smartphone.
Using
data from their mechanical and perceptual experiments, Jones’ group is
designing arrays that can be worn across the back and around the wrist,
and is investigating various ways to present vibrations. For example, a
row of vibrations activated sequentially from left to right may tell a
driver to turn right; a single motor that buzzes with increasing
frequency may be a warning to slow down.
“There’s a lot of
things you can do with these displays that are fairly intuitive in terms
of how people respond,” Jones says, “which is important because no
one’s going to spend hours and hours in any application, learning what a
signal means.”