Information Technology Reference
In-Depth Information
2.1 Sensors
Laser range sensors are widely used nowadays. Simple devices measure distances
for a few 3D directions, but more sophisticated sensors exist. For example, there
are devices that are used in conjunction with rotating mirrors to scan a 360°
field of view. For our platform, we use the rotating laser sensors manufactured
by B.E.A. (see Figure 1).
Fig. 1.
The laser sensor used in our experiments (the LZR P-200 manufactured by
B.E.A. S.A.,
http://www.bea.be
)
These laser range sensors are completely independent of the lighting condi-
tions, as they rely on their own light sources. They are able to measure the
distance between the scanner and surrounding objects by sending and receiving
laser pulses in a plane. The measurement process is discrete; it samples the an-
gles with an angular precision of 0
.
35° and covers an angular aperture of 96°. The
plane is scanned 60 times per second. In practical terms, these sensors deliver a
signal
d
t
(
θ
) where (1)
d
is the distance between the sensor and the object hit by
the laser ray, (2)
θ
denotes the angle in the scanning plane (0
≤
θ
≤
96°), and
(3)
t
(=
60
s
for
k
=0
,
1
,
2
,...
) is the time index.
The information that these sensors provide has a physical meaning (the dis-
tance is given in millimeters) and relates only to the geometrical configuration;
the color and texture of objects have no impact on the measurements. Note
also that these sensors have been designed to be integrated in dicult industrial
environments, like revolving doors, where a camera might not fit as well.
2.2 Towards the Concept of Virtual Curtain
Theoretically, a single scanner suces to build a 2D shape. However, we have
decided to use two scanners to reduce the shadowing effects resulting from a
single scanner. The sensing system is made of two laser scanners located in
the two upper corners of the frame of a door (see Figure 2). Consequently,
distances are measured in a plane that comprises the vertical of the gravity and
the straight line joining the two sensors. This is our concept of
virtual curtain
.
To some extend, it can be seen as a wide traversable waterfall, except that you
don't get wet if you cross it!
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