Page 153 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

130 BIOMECHANICS OF THE HUMAN BODY

three-degree-of-freedom electrogoniometers of this type are also available. Strain sensitive wires are
mounted within a connecting element and electrically connected to form a Wheatstone bridge. Each
strain-sensitive wire is, in effect, a resistor and is sensitive to strains in particular directions. Hence,
when the electrogoniometer is forced to rotate about the axis of rotation as drawn in Fig. 6.3, the
bridge circuitry becomes unbalanced. This “unbalancing” is noted as a change in the output voltage
of the bridge and is proportional to the amount of rotation. The design is clever because pure rotation
about axes perpendicular to the axis of rotation depicted in Fig. 6.3 will not unbalance the bridge.
Another interesting and practical characteristic of this device is that it does not have to be positioned
over the joint axis of rotation as is the case for rotatory potentiometers. Note, however, that the base
of the mounting blocks must lie in the plane of rotation without necessarily being centered over
the axis of rotation.
Electrogoniometers of this type can be configured to display joint angles in real time and/or inter-
faced with a computer for data storage. Additionally, data can be saved to a storage unit (i.e., data
logger) strapped to the subject. The data logger is ideal for recording dynamic movements in the
field. The stored data can be uploaded to a computer at a later time and converted to joint angles for
analysis.

Limitations of Electrogoniometers. There are advantages and disadvantages associated with the
use of electrogoniometers. In their favor are ease of use and cost. On the other hand, they are less
accurate than other systems used to record movement. In addition, both designs (i.e., potentiometer
and strain gauge) require placement over the joint, which may interfere with the natural kinematics
due to cumbersome cabling and/or method of attachment. Another drawback of these devices is that
while they provide a relative measure of joint angular position, the data do not lend themselves to an
inverse dynamics analysis in which joint reaction forces and moments are of interest, the computation
of which requires knowledge of the absolute positions of the body segments.

6.3.2 Electromagnetic Tracking Systems
Electromagnetic tracking technology originated in the defense industry and has since become widely
used in the entertainment industry (e.g., motion pictures, animation, and gaming). The use of
electromagnetic tracking has become increasingly popular in the academic environment as evinced
by the growing number of research publications using this technology.
Electromagnetic tracking is based on Faraday’s law of magnetic induction. That is, electrons in a
conductor experience a spontaneous magnetic force when moved through a magnetic field. The
magnitude of the induced force (i.e., electromotive force or EMF) is proportional to the strength of
the magnetic field through which the conductor is moved. The magnitude of the EMF (i.e., voltage)
is also related to the speed the conductor is moving. If the conductor is in the shape of a loop, the
same principles apply with an induced EMF proportional to the strength of the magnetic field
perpendicular to the cross-sectional area of the loop. The induced EMF is related to the magnetic
flux (Φ ) as noted in Eq. (6.1).
B
dΦ
EMF =− B (6.1)
dt
Conceptually, the strength and direction of a magnetic field can be thought of as the density and
direction of magnetic field lines. The magnetic flux will vary as the conductor moves closer/further
to the source of the magnetic field and also as it rotates relative to the magnetic field lines. The more
field lines passing through the loop of the conductor, the greater the induced EMF. This principle
forms the basis for electromagnetic tracking. That is, the general idea is to move a conducting
sensor through a magnetic field and record the induced voltage.
The basic components of an electromagnetic tracking system consist of an active transmitter and
passive sensors. The transmitter is stationary and contains three orthogonal coils (i.e., antennae) that
are activated in sequence, with only one antenna generating a magnetic field at a time. Interestingly,