Page 130 - Introduction to Autonomous Mobile Robots

115
Perception
f
where is the distance of the lens to the imaging plane. In the limit, the ratio of image res-
olution to range resolution is defined as the triangulation gain G p and from equation (4.12)
is given by

⋅
u ∂ bf
----- = G = --------- (4.13)
z ∂ p z 2
This shows that the ranging accuracy, for a given image resolution, is proportional to
source/detector separation and focal length , and decreases with the square of the range
f
b
z . In a scanning ranging system, there is an additional effect on the ranging accuracy,
caused by the measurement of the projection angle . From equation 4.12 we see thatα
∂ α bsin α 2
------ = G = ----------------- (4.14)
z ∂ α z 2

We can summarize the effects of the parameters on the sensor accuracy as follows:

b
b
• Baseline length ( ): the smaller is, the more compact the sensor can be. The larger b
is, the better the range resolution will be. Note also that although these sensors do not
suffer from the correspondence problem, the disparity problem still occurs. As the base-
line length is increased, one introduces the chance that, for close objects, the illumi-
b
nated point(s) may not be in the receiver’s field of view.
f
• Detector length and focal length ( ): A larger detector length can provide either a larger
field of view or an improved range resolution or partial benefits for both. Increasing the
detector length, however, means a larger sensor head and worse electrical characteristics
(increase in random error and reduction of bandwidth). Also, a short focal length gives
a large field of view at the expense of accuracy, and vice versa.
At one time, laser stripe-based structured light sensors were common on several mobile
robot bases as an inexpensive alternative to laser rangefinding devices. However, with the
increasing quality of laser rangefinding sensors in the 1990s, the structured light system has
become relegated largely to vision research rather than applied mobile robotics.

4.1.7 Motion/speed sensors
Some sensors measure directly the relative motion between the robot and its environment.
Since such motion sensors detect relative motion, so long as an object is moving relative to
the robot’s reference frame, it will be detected and its speed can be estimated. There are a
number of sensors that inherently measure some aspect of motion or change. For example,
a pyroelectric sensor detects change in heat. When a human walks across the sensor’s field