Game Development Reference
based on different principles to create location trackers measuring three or six degrees
of freedom. Refer to the paragraph on compasses.
Accelerometers, as their name suggests, are based on the measurement of a force
coming from the acceleration of a mass. They can be in an open loop or in a closed
loop: in an open-loopmeasurement, the force is measured by slightly stretching a spring
supporting the mass. This measurement is detected by the piezoelectric, piezoresistive,
strain gauge or capacity variation principle. In this type of open-loop accelerometer,
the classic problem of compromising between accuracy and speed of response arises
often. In case of closed-loop accelerometers, we create a force which is opposite to the
force of inertia cancelling the movement of the mass. This opposite force is created
by a motor system whose control depends on this force, and hence on acceleration.
A highly accurate accelerometer can be created on the basis of this principle.
The measurement of location of a moving object is obtained by double integration,
which causes a quadratic increase in position errors. The temporal drifts are thus the
main inconvenience of this type of trackers. Acceleration due to gravity must be taken
into account, for example in an inclinometer.
Movea designs andmarkets products that measure movement using accelerometers
and magnetometers. Its expertise and its technology developed at the French research
institute CEA-Leti place it among the world leaders in this industry. Movea products
use on-body micro-sensors which are embedded in the subject's body. Today, these sys-
tems combine micro-accelerometers and micro-magnetometers to capture movements.
In the second generation systems, gyroscopes will also be incorporated to improve the
accuracy of measurements. Measurement frequency is about 200Hz; static accuracy
is 2 degrees on an average and with a latency of 5ms.
A special and well-known device is the Wiimote, Nintendo's Wii, which is a loca-
tion sensor used in a gaming console. It is a hand held lever having a number of control
buttons. It has multiple sensors which help it to position itself in space. The Wiimote
measures accelerations along three axes. Its expansion device, Wii MotionPlus, uses a
gyroscope for better accuracy. Wiimote is connected via Bluetooth.
6.3.1 Electromagnetic trackers using alternating
This type of tracker has a transmitter, a receiver and an electronic unit. The transmitter
is a set of three coils which propagate electromagnetic fields in a surrounding sphere
of a few meters. The receiver also has three small coils which receive magnetic flux
depending on their position with respect to the transmitter (Raab & Blood, 1979).
For a point that is away from a coil, where an alternating current I is passing, with
maximum current I o , time t and angular frequency ω
I o exp(