Inexpensive sensors could capture your every move

Video games such as Dance Dance Revolution could soon require more than just fancy footwork. Small, cheap sensors for tracking the movement of a person’s entire body could lead to what researchers call whole-body interfaces for controlling computers or playing games. Conventionally, motion capture makes use of reflective dots or small LEDs attached at key points on a person’s torso, limbs, and head. Capturing the movements of these points using an array of cameras allows animators to create a computerized skeleton, which can then guide the movements of an animated character, for example. These systems, however, “need controlled situations to work” and typically cost tens of thousands of dollars, Rolf Adelsberger of the Swiss Federal Institute of Technology (ETH) in Zurich told the New Scientist’s Mason Inman. Adelsberger and colleagues at ETH, along with researchers at Massachusetts Institute of Technology (MIT), and Mitsubishi Electric Research Laboratories, both in Cambridge, Massachusetts, decided to create a cheaper, more versatile motion capture system that works outside the lab or studio. Their new motion capture sensors work even while a person is driving or skiing. It could make computer animation or movie effects more lifelike, the researchers say, and perhaps even help doctors analyze movements of patients going through physical therapy.

Several sensors measuring about 2.5 centimeters on each side are attached to a person’s legs and arms. The sensors detect movement in two different ways: accelerometers and gyroscopes measure motion, but ultrasonic beeps are also emitted. Tiny microphones mounted on the torso pick up these beeps, allowing a laptop computer, carried in a backpack, to calculate the distance to the sensor. The system is similar to, but much simpler than, bats’ ultrasonic echolocation, and together with the motion sensors provides a more accurate overall picture of body movement. The small backpack also holds the batteries that power the system. “The sensors are all off-the-shelf parts,” Adelsberger says, making the system much cheaper than other motion-capture technology. It costs about $3,000 currently, but this could come down to a few hundred dollars, he says, if the sensors are mass-produced. In tests, the system compared well with a commercial motion capture system called Vicon. It got the angles of the body’s joints almost exactly correct, but experienced some “drift,” in which the system erroneously thought that the body as a whole had shifted or rotated from its actual orientation. For Vicon to work, it requires a controlled, indoor environment and a number of fixed cameras. The new system works in a variety of different situations, for example, when someone is skiing or biking.

The new system does not work when people make very sudden movements, however, because the relatively cheap sensors used are not yet accurate enough to compensate. They are quickly improving, Adelsberger says.

“I think the biggest impact of this system is in easier data collection in everyday situations,” says Christoph Bregler, an expert on motion capture at New York University. “This system could record many new activities for sports medicine, behavioral studies [and other fields] that were impossible before,” he says. Details of the project were presented at SIGGRAPH, a computer graphics and interactive technologies conference held in San Diego in August.