Laser ranging

Laser ranging

Ultrasonics introduces acoustic energy into the environment and measures
the time of flight of the signal to return. The same principle can be used with
lasers; a laser beam can emitted and then the reflectance measured. Unlike a
sonar beamwhich has a very wide field of view(almost 30 ), a laser produces
an almost infinitesimal field of view. If the laser is directed in a scan, just like
a raster scan on a CRT, the device can cover a reasonable area and produce
an image, where the image function produces depth values. Devices which
use lasers to produce a depth map LASER RADAR or image are often called laser radar, ladar,
LADAR or lidar. They can generate up to one million range pixels per second,52 with
LIDAR a range of 30 meters and an accuracy of a few millimeters. The mechanical

scanning component makes lidars very expensive, on the order of $30,000 to
$100,000 USD. A less expensive solution for navigation is to create a planar
laser range finder.
A lidar produces two images: intensity and range. Fig. 6.27 shows the
images produced by a Odetics laser range (LADAR) camera. The intensity
map is essentially a black and white photograph and measures the intensity
of the light reflected or absorbed by objects in the scene. This corresponds to
how humans perceive the scene The image function for the range image is
depth from the camera. Pixels that are black, or have a value of 0, are closer
than white pixels. A flat floor usually appears as a radiating set of semicircles
going from near to far; trigonometry is then used to compute that the
circles represent a flat RANGE SEGMENTATION surface. This process is called range segmentation and
can be quite difficult.
Lidars have some problems in practice. For example, Fig. 6.27 shows an
area on the range image that is pure black or very near. But as can be seen
from the intensity image the area is actually far away. Likewise, the black
moulding between thewall and floor appear to be very far away on the range
image. The errorswere due to out of range conditions, absorption of the light
(not enough light returned), or to the optical equivalent of specular reflection
(light hitting corners gets reflected away from the receiver).
A planar laser range finder, such as the Sick shown in Fig. 6.28, provides a
narrow horizontal range map. The map is essentially a high resolution polar

plot. Robots such as Nomads and Pioneers originally came with Sick lasers
mounted in parallel to the floor. This was useful for obstacle avoidance (as
long as the obstacle was tall enough to break the laser plane), but not particularly
helpful for extracting 3D information. Also, as with sonars, robots ran
the risk of being decapitated by obstacles such as tableswhich did not appear
in the field of view of the range sensor but could hit a sensor pod or antenna.
To combat this problem, researchers have recently begun mounting planar
laser range finders at a slight angle upward. As the robot moves forward,
it gets a different view of upcoming obstacles. In some cases, researchers
have mounted two laser rangers, one tilted slightly up and the other slightly
down, to provide coverage of overhanging obstacles and negative obstacles.