Video: The precise twists and turns of a skier鈥檚 body during a downhill run have been captured for the first time by fusing the data from a multitude of body-worn sensors
The precise twists and turns of a skier鈥檚 body during a downhill run have been captured for the first time by fusing the data from a multitude of body-worn sensors. The information could help skiers hone their technique before important competitions.
Traditionally, motion capture technology relies on cameras to record body movement. But for a skier racing down a kilometre-long ski run, up to 40 synchronised and calibrated cameras would be needed to keep the skier constantly in sight.
Matthew Brodie at in Wellington, New Zealand, decided to dispense with the cameras altogether and instead use motion-capturing sensors strapped to the body to record the skier鈥檚 moves.
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The skier wears up to 15 inertial motion units (IMUs) on the limbs, torso, pelvis and head. Within each matchbox-sized IMU there are three gyroscopes to measure up down or sideways movements, from which it鈥檚 possible to reconstruct limb and body orientation in three dimensions.
However, these gyroscopes tend to drift with time. In extreme cases, that can give rotation errors of up to 180掳.
Fusing data
To improve their accuracy, the IMUs also contain that measure the forces the skier experiences as he or she moves, including gravity.
Another device can also measure magnetic north, which, combined with the direction of gravity, provides a frame of reference that can be used to reduce the rotation error to less than 1掳 when the skier is stationary.
The task becomes harder during a run because the skier鈥檚 body twists and turns, and these accelerations make it harder to use an accelerometer to work out which way is down.
To counter this, Brodie uses data from a GPS receiver attached to the skier鈥檚 helmet to measure their velocity and any changes in direction during the run. This can then be used to distinguish the forces caused by twisting and turning from the force of gravity.
A central hub on the skier鈥檚 torso takes data from all of these sources and uses Brodie鈥檚 鈥淔usion Motion Capture鈥� algorithms to accurately calculate the skier鈥檚 movement and position during the ski run.
Improved performance
鈥淣one of the data streams are accurate enough by themselves,鈥� Brodie says. 鈥淏ut by fusing the different data streams, accurate measurements [of the skier鈥檚 motion] are possible.鈥�
The information is then transmitted to a computer, which takes less than a minute to construct and display the skier鈥檚 body movements, position, velocity and acceleration during the run.
That information can be analysed to help the skier improve their technique and shave vital hundredths of a second off their personal best times.
Although up to 15 IMUs are strapped to the skier, the devices don鈥檛 interfere with the skier鈥檚 performance, says Brodie. 鈥淭he sensors are small and light 鈥� about 35 grams,鈥� he says. 鈥淭he skiers who have used the prototype system forget that they are wearing it.鈥�
at Mitsubishi Electric Research Laboratories in Cambridge, Massachusetts, is impressed with the system.
鈥淐ollecting the data is usually difficult when it鈥檚 done outside the lab,鈥� he says. 鈥淎ll parts of the system need to be very robust to perform in the alpine environment 鈥� snow, high accelerations and stress. Under these conditions, the reduction of the error at the joints in their algorithm looks very good.鈥�
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