Biomedical Engineering Reference
In-Depth Information
Figure 2.2.
Active Optotrack NDI sensors (http://www.ndigitaJ.com)
Magnetic capture
is based on the creation of an
electromagnetic field by an "antenna" generally composed of
three electric wire coils at perpendicular axes (whose size
can be adapted to different volumes), and on sensors which
are also made up of coils disturbing the magnetic field. The
calculation, performed in real time, of the differences in
potential between each sensor and the antenna provides both
its position and orientation. The advantage, unlike previous
systems, is having access to six degrees of freedom in the
segment with a single sensor, whereas optical systems
(active or passive markers) require the positioning of at least
three non-aligned markers on each body segment of interest.
Nevertheless, as for systems with photosensitive cells, the
automatic recognition of sensors is based on the multiplexing
of the signal, which requires that a compromise is found
between the number of sensors monitored and sampling
frequency. The accuracy obtained with regard to sensor
position is lower (to approximately 1 mm) but above all, any
metallic object in the field of measurement or any type of
electromagnetic distortion affects the measurement, which
makes this type of system unusable in certain environments.
