Game Development Reference
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Electronic control
Electromagnetic field
Figure 6.2 Devices of an electromagnetic location sensor
Calculation of location parameters as per the measurements taken;
Filtering the measurements, if required, to remove random noise, but this increases
the response time;
Transmission of the calculated values to the computer connected to the device.
The tracker is all the more efficient as it can take a large number of accurate measure-
ments per second (120) and as it transfers the obtained values at the quickest possible
speed. The order of magnitude of the latency time: about ten milliseconds with a sin-
gle receiver and without filtering, but more with filtering and multiple receivers (refer
to the following paragraph for the comparison with other type of electromagnetic
The main constraint of this type of tracker is that it cannot function if there are
metal parts in the vicinity. In the presence of conductive parts, the magnetic field, which
varies with time, creates induced currents in these parts (Eddy currents). The strengths
of these currents are proportional to the frequency of the magnetic field and create
additional magnetic fields which create disturbances in the measurements. LIBERTY
tracker can be used on a large range of measurements and with sixteen operating
6.3.2 Electromagnetic trackers using impulsive field
To eliminate or at least to reduce the Eddy currents, we can take measurements with
stationary magnetic fields that do not generate such currents. The formulae given
earlier for magnetic field can be used by keeping I
I o constant.
µ o NI o R 2 cos( θ ) / 2 r 3
µ o NI o R 2 sin( θ ) / 4 r 3
B r =
B θ =
with geometric positions: r
R and r
L , coil of N windings, of radius R and length
L ; vacuum permeability µ o .
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