Biomedical Engineering Reference
In-Depth Information
adapted to the motion velocity of certain markers, or when
certain markers are partially hidden or fuse if they are too
close together.
To limit systematic errors, for a given optoelectronic
system, the experimental setup must be defined carefully,
i.e. the volume of work, the number and optimal position of
cameras, marker size, etc. It is also important to take time to
scan all the required space well during dynamic calibration
(see Chapter 2).
Correcting random errors is essentially based on
smoothing or filtering the marker trajectories [DE 08].
Instrumental errors are similar to white noise spread over a
wide frequency range; a “low-pass”-type filter is quite often
used to eliminate part of the noise in system measurements,
without losing information on the motion analyzed, at lower
frequencies. However, in sporting applications, it is not
realistic to consider signals corresponding to marker
trajectories as stationary, and more advanced techniques,
based, for example, on the Wigner distribution function
[GIA 00] are recommended instead.
Under appropriate operating conditions, instrumental
errors in the spatial position of a marker for current systems
are approximately 1/3,000th of the diagonal of the calibrated
volume. These values are acceptable in most motion analysis
applications.
4.2. Experimental errors
Experimental errors are not directly linked to the system
used but rather to its implementation. They are essentially
due to interfering movements of soft tissues, positioned
between the bone and the skin, and due to the poor
