The Nest of Birds motion tracker from Ascension Technology Corp., Burlington, Vt., features four pulsed-dc magnetic sensors that simultaneously track the location of arms, heads, bodies, or objects in real time. The four sensors each measure six degrees of freedom (position and orientation). The plug-and-play motion tracker also comes with a USB interface.

Magnetic tracking is preferred for measuring motion in computer graphics applications. Magnetic tracking has a number of advantages over mechanical, optical, and acoustic methods. For starters, it gives six-degrees-of-freedom tracking without limits in angular coverage. The sensing surfaces are small, ranging from 25 down to 5-mm square, and range of motion is unlimited with no mechanical or line-ofsight limits.

Two basic magnetic tracking methods are ac electromagnetic and pulsed dc. Ac and dc trackers respond differently to conductive metals such as stainless steel and aluminum. The dc approach is effective in environments with metal. Transmitting a magnetic field induces eddy currents in the metal, which interfere with the transmission of the tracker magnetic field, distorting position and orientation measurements.

The Nest of Birds is suitable for interactive broadband use such as virtual reality (VR), interactive gaming, and in CAVE environments.