Lynette Jones, a senior research scientist in Massachusetts Institute of Technology's Department of Mechanical Engineering, designs wearable tactile displays.
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
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.
used the array to measure motor vibrations in three locations: the palm of the hand, the forearm and the thigh.
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.
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."
In 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 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?
work sets up a methodology and potential guidelines for effective pattern displays."
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."