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The Cutting Edge of Haptics

Touch-based interfaces get to the point.

Scientists have found a way to trick the body’s senses into thinking a flat surface is actually sharp or pointed. Their findings will be presented next month at the IEEE International Symposium on Robot and Human Interactive Communication in Hatfield, England.

This new touch interface appears to show that it’s possible to fool our sense of touch into feeling very fine and detailed sensations of pressure without pushing down against the skin. A virtual knife edge could bring an added sense of realism to touch-based haptic systems, such as those used in surgical simulators.

The goal of this field of touch technology, which is often called haptics, is to be able to simulate any shape, texture, or sensation, says Gabriel Robles-De-La-Torre, a neuroscientist and computer engineer based in Mexico City, who founded the International Society for Haptics and led the research. It’s an important step toward developing haptic “displays,” he says.

Sile O’Modhrain, an expert in haptics formerly at Queen’s University of Belfast, Northern Ireland, agrees. She says the work should make haptics feel more realistic. “It’s a way of improving the perceptual quality of the rendering of surfaces.”

Although theoretically it may be possible to design a machine that could change its actual texture and shape to simulate a wide range of shapes and textures, such a mechanism would be unfeasibly complex and large. Instead, researchers like Robles-De-La-Torre have been exploring ways to exploit our sensory system’s ability to be deceived.

“It’s just a way of taking advantage of human perception,” says Vincent Hayward, an electrical and computer engineer who works on haptics at McGill University in Montreal. It is somewhat similar to the illusion we experience when our eyes perceive a wide range of colors on a video display, even though the image consists of just three different colored pixels, he says.

With haptics, the illusion is created with applied forces. Typically, when someone moves his or her finger over a sharp or pointed edge, both vertical and lateral forces are applied to the skin, says Robles-De-La-Torre. But he found that the brain could be fooled into thinking it’s being poked simply by applying lateral forces.

To create this illusion of sharpness, Robles-De-La-Torre, working with Carlo Alberto Avizzano and colleagues at the Scuola Superiore Sant’Anna in Pisa, Italy, used a haptic interface called GRAB, which consists of a thimble connected to the end of a motorized, extendable arm. A user is able to move the thimble freely when placing their forefinger in it. Then carefully controlled motors provide force feedback, so the thimble’s movement is impeded in ways that create “virtual” surfaces.

By setting up the system so subjects can move only their finger along one axis, from left to right, the researchers were able make people feel like they were running a finger over a range of different sharp and pointed edges, just by applying lateral resistance to their movement. The sensation was so convincing that the subjects were even able to match the shapes of the edges to images of them, such as a saw tooth or a hump with a pointed peak.

Robles-De-La-Torre believes this illusion occurs because of a tradeoff that exists between different types of sensory information. Specifically, when we explore objects that contain small details, the force information is more important than proprioception, the sensory information that comes from our muscles and skin to tell our bodies how far a finger or limb has moved. As a result, we may feel like our finger is actually moving as it rides up one of these sharp edges, if they’re not too big, he says.

Queen’s O’Modhrain says that a lot of work has been done in rendering either large geometric shapes using force feedback, or fine-grained surfaces using vibrating skin actuators. But she says this recent work shows that it should be possible to render surfaces that lie between these two extremes without having to apply pressure to the skin.

It may even be possible to simulate the feelings of pain that can accompany a sharp object, says Robles-De-La-Torre, because the short pulse of lateral force that stimulates the sharpness sensation may also stimulate skin receptors normally associated with pain. But to prove this hypothesis, more tests are needed. “Pain is a complex phenomenon,” he says.

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