Game Development Reference
In-Depth Information
6.2.2.1 Inclinometers
The earth is the source of gravitational field for this type of tracker. When the incli-
nometer is not moving, it indicates its angular position with the vertical direction of
the place. But when it is moving it indicates the direction of the total external force
on it, caused by the acceleration of the movement and the earth's gravity. This type
of sensor is used with other sensors based on different principles to create location
trackers measuring three or six degrees of freedom.
6.2.2.2 Gyroscopes and rate gyros
The most common gyroscope is the one based on the mechanical principle of a rotor,
spinning at a high speed, whose axis maintains a constant direction. However, other
physical phenomena are used to measure angular rotational velocity. In the following
part, we will describe only those devices which can be miniaturised and are affordable
to be used in virtual reality.
Gyroscope with Cardan suspension is a device that indicates a constant direction
thanks to a mass (spinner) rotating around its axis of revolution. The spinner is con-
nected at its centre of inertia to a Cardan suspension which helps the spinner to turn in
any direction. The friction in the joints is reduced to a minimum. In most cases, a motor
driving the spinner is provided to correct the drift. The operating principle is based
on the following property (gyroscopic approximation): Since a mass of revolution of
axis has a relatively higher angular velocity around , it is accepted that the angular
momentum of this mass remains parallel to . Applying the angular momentum theo-
rem, we find that the direction of is fixed. Theoretically, a gyroscope always points
to the same direction. This, however, is difficult in practice due to the friction at the
suspension. A large number of devices do not measure the rotation angle of the support
with respect to axis , but its angular rotational velocity, such as the rate gyro. The
measurement of location of a moving object is obtained on the basis of speed by inte-
gration, which results in the increase in errors of position in time. Manufacturers have
succeeded in miniaturising these devices, which can be used in various fields (model
making, virtual reality, etc.). The flaw in these sensors is that they have a temporal
drift of several degrees per minute. But in virtual reality applications such as head
rotation measurement, knowing the relative rotation is more important than knowing
the absolute rotation. The weight can be less than 50 grams while maintaining a good
resolution for the tracker.
The piezoelectric gyroscope created by Murata is based on the principle of Coriolis
acceleration. It consists of a small triangular piezoelectric prism with excitation on one
surface and reception on the other two surfaces. The actuator provides periodic pulses
which are received on the other two surfaces, in the same way as when the device is
idle. Any rotational movement of the prism produces Coriolis forces which increase
the amplitude of the reception signal on a detector with respect to the other.
The difference between the two detectors is proportionate to the rotational speed.
It should thus be integratedwith the inherent problems of this method to obtain angular
rotation: the drift of measurements makes it necessary to reset the device if we need
an absolute measurement of position. Two such devices are used in the gyroscopic
tracker manufactured by VR Systems. This type of sensor is used with other sensors