YOUR eyes tell you that your hand is locked in a vice-like mechanical device, but your fingertips tell you you’re stroking fur. Welcome to the world of haptics, where nothing is quite how it feels.
As neuroscientists decode how we process signals from nerves that sense touch, engineers are beginning to use their discoveries to dupe us into feeling something that isn’t there. Given the right kind of manipulation, a smooth surface can be made to mimic the feel of a range of materials, and a solid slab can be made to feel like shifting sand.
As well as producing weird tactile illusions, haptics have practical uses. For example, tactile feedback can make touchscreen devices more intuitive to use, says , head of haptics at the Institute of Intelligent Systems and Robotics at Pierre and Marie Curie University in Paris, France. Such systems are already in use on some smartphones in which actuators within the touchscreen produce a basic “clicking” sensation when the screen is pressed.
Advertisement
, a company in San Jose, California, is attempting to push haptics further. In a system planned for later this year, users will be given the tactile illusion that touchscreen buttons protrude from the surface. This will be achieved by using a piezoelectric motor to vibrate the screen laterally, though beyond that Immersion is not revealing how its system will work.
Hayward’s team has been working on similar systems, using surface vibrations to generate sensations of texture. By altering the frequencies of the vibration, they are able to make the surface feel rougher or smoother at will.
Others, like , a neuroscientist and computer engineer based in Mexico City, have used vibrating surfaces to simulate sensations of sharpness, again by using motors to impart lateral movement to a smooth, flat surface. This produces a sharp change in the resistance a user’s finger feels as they move it across a particular portion of the screen, and this change is perceived as a sharp edge.
Meanwhile Ed Colgate, a mechanical engineer at Northwestern University in Evanston, Illinois, has used vibrations to achieve a very different effect – making objects feel more slippery. His system vibrates their surface at high frequency with an amplitude of a mere 2 micrometres. “It’s not much but it’s enough to act like a pump, pumping a little bit of air underneath a finger when touched.”
Under normal circumstances, our sense of touch combines input from different kinds of sensory nerves to build up a model of what we are touching. Some of the nerves in our skin sense pressure, while others detect stretching of the skin.
“Our brain combines input from different kinds of nerves to build up a model of what we are touching”
Systems like Robles De La Torre’s show that it is not necessary for both kinds of nerves to be stimulated. Though his device only mimics the way an edge stretches the skin, the brain is fooled into feeling pressure.
A force-feedback system devised by at the University of Exeter, UK, expoits pressure-sensitive nerves, rather than stretch-sensitive ones. It is able to simulate the feel of a range of flexible materials, including silk, hessian and fur. Subjects place their hand in a constraining device inside which are 24 computer-controlled actuators that make contact with the skin. “The computer imagines you are moving each bit of your finger over the material, and works out what mechanical input would be applied to your finger,” says Summers. “With something like fur it doesn’t have to press very hard.”
Our sense of touch can also use temperature changes to helps us identify materials. Haptics researcher at the Massachusetts Institute of Technology has devised a mouse-like contraption that exploits this, in which the temperature changes are produced by the Peltier effect – the heating or cooling that occurs when current flows between two dissimilar metals. Running current through strips of metal laid one on top of another on the surface of the mouse allows rapid changes to be made to the temperature sensed by the subject’s fingers when they graspit. “You can get them to respond very quickly, on the order of milliseconds,” says Jones. This can be used to induce rapid changes of skin temperature, simulating the different rates at which heat is transferred to and from the skin by different materials. Jones says her team has identified how different materials conduct heat, and how to adjust current in their device to convince subjects that they are touching metal or plastic, for example.
Jones is exploring how this could help people with impairment to their sensory systems, such as the nerve damage caused by diabetes. Haptic devices could be used to retrain their senses, by tricking them to grasp objects more tightly than their damaged nerves suggest is needed.
It is not just the sense of touch in our fingers that is attracting the attention of haptics researchers. Instead, at the Centre for Intelligent Machines at McGill University in Montreal, Canada, is focusing on the feet, and has developed a novel surface designed to simulate walking on different types of ground. It uses a series of 30-centimetre tiles, each with sensors at its corners and an actuator similar to a loudspeaker coil mounted beneath it. By modelling the properties of various surfaces and calculating what vibrating forces the coil should apply as different parts of the foot make contact with it, Visell has been able to mimic the sensation of walking on solid ground, gravel or sand.
Visell’s tiles could be used to help rehabilitate people who have difficulty walking. By making them feel as if they are walking on a soft, compliant surface like sand, for example, their muscles could be retrained to lift the foot higher to ensure that it clears the ground between steps.
So far, haptics researchers have concentrated on individual facets of our sense of touch, but Hayward looks forward to future applications which will combine them. For example, by vibrating a Peltier device it should be possible to convey temperature and texture information in a single surface.
