Motion Tracking with Ascension Technologies Flock of Birds Motion Sensors

The Boker Lab has two sets of Ascension Technologies Flock of Birds (FOB) motion trackers: a sixteen sensor MotionStar system with a long-range transmitter magnet and a three sensor portable Flock of Birds with two short-range transmitters.

The sixteen sensor FOB is generally used to record two individuals at once. Each person is outfitted with eight sensors: one attached to a cap, one strapped to the sternum, one strapped to each forearm just below the elbow, one attached to the back of each hand and one strapped to each shin just below the knee.


Since each sensor records both position and orientation we can track the motion of the whole body in this way as long as the person is seated. We have been using a sampling rate of 80Hz, which in our lab minimizes the RMS error of a motionless sensor.

Seated participants wear a lightweight nylon jacket that helps reduce the intrusiveness of the sensors during conversational interaction. The long-range transmitter can be seen in the background of this photo. Generally, the transmitter magnet is placed on the other side of a foam partition during interaction so that participants need not be informed that motion tracking is being performed. A cover story of "magnetic fields produced by the body" has been used to explain the sensors, and after more than 200 sessions, no participant has yet guessed during debriefing that we have been tracking motion.


The portable, three sensor Flock of Birds is used in experiments in natural environments outside the lab. The photo below shows the setup for a study of postural stability that is part of our research into posture and aging. The checkered tablecloth is a high contrast visual target for studying the effects of vision on postural control. Head, sternum and hip measurements are acquired at 60 Hz from young and old subjects as they attempt to stand motionless. These experiments have yielded substantial data that are now being used to inform and constrain models of postural control.