Where am I? Sensors and Methods for Mobile Robot Positioning

160 Part II Systems and Methods for Mobile Robot Positioning
d
d
d
R
d d
R
R
Figure 6.7: Retroreflective beacon array
configuration used on the mobile robot Hilare.
(Adapted from [Banzil et al, 1981].)
Figure 6.8: A confirmed reflection pattern as depicted
above was required to eliminate potential interference
from other highly specular surfaces [Banzil et al., 1981].
6.3.3 NAMCO LASERNET
The NAMCO LASERNET beacon tracking system (Figure 6.9) employs retroreflective targets
distributed throughout the operating area of an automated guided vehicle (AGV) in order to measure
range and angular position (Figure 6.10). A servo-controlled rotating mirror pans a near-infrared
laser beam through a horizontal arc of 90 degrees at a 20 Hz update rate. When the beam sweeps
across a target of known dimensions, a return signal of finite duration is sensed by the detector. Since
the targets are all the same size, the signal generated by a close target will be of longer duration than
that from a distant one.
Angle measurement is initiated when the
scanner begins its sweep from right to left;
the laser strikes an internal synchronization
photodetector that starts a timing sequence.
The beam is then panned across the scene
until returned by a retroreflective target in
the field of view. The reflected signal is
detected by the sensor, terminating the
timing sequence (Fig. 6.11). The elapsed
time is used to calculate the angular position
of the target in the equation [NAMCO,
1989]
= Vt - 45( (6.2)
b
where
= target angle
V = scan velocity (7,200(/s)
T b = time between scan initiation and target
detection.
Figure 6.9: The LASERNET beacon tracking system.
(Courtesy of Namco Controls Corp.)