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Figure 6.3 Ascension tracker, Bird. (Illustration: Immersion S.A., with permission)
The magnetic field of a coil of the transmitter is impulsive for about 2.5ms. Eddy
currents are created at the beginning of this time interval, which are subsequently
reduced when the magnetic field becomes stationary. At this point, the measurements
are taken on the three coils of the receiver. This sequential cycle is repeated twice for
the other two coils of the receiver. The fourth interval of 2.5ms is reserved for the
measurement of the electromagnetic field of the place, which is determined in this type
of tracker (it can thus be varying). A complete cycle of measurements of a receiver posi-
tion takes less than 10ms. Apart from the shape of winding, the hardware structure
of this tracker is similar to the previous tracker. The transmitter and the receiver both
have three coils wound around a cube. The receiver also has an additional cylindrical
winding at the centre of the cube to increase the magnetic energy received. The man-
ufacturer of this type of tracker suggests using a transmitter with multiple receivers
(up to 30) without decreasing the number of measurements per second (Figure 6.3).
We can also use more than one transmitter to increase the scope of measurements.
Northern Digital Inc (NDI) has launched a new electromagnetic sensor, Aurora,
with receivers as small as 0.55mm in diameter. This sensor can monitor eight (5DOF)
or four (6DOF) receivers simultaneously.
The accuracy of the 5DOF version is 0.9mm in translation motion and 0.3 in
rotation. It has a measurement frequency of 40Hz, irrespective of the number of
receivers. Electromagnetic disturbances are detected and reported using a software
6.3.3 Characteristics of electromagnetic trackers
The electromagnetic trackers give good results in terms of measurement accuracy.
They are the most commonly used trackers in virtual reality applications as they are
transparent for the user. Following are their main drawbacks:
High cost - The prices did not go down, though the market for less-expensive
6DOF trackers was supposedly expanding and the components used were mainly
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