Page 132 - Introduction to Autonomous Mobile Robots

Perception

2f vcos θ 117
t
∆f = f – f = --------------------- (4.17)
t
r
c
∆f c⋅
v = ------------------ (4.18)
2f cos θ
t
where
∆f = Doppler frequency shift;
θ = relative angle between direction of motion and beam axis.

The Doppler effect applies to sound and electromagnetic waves. It has a wide spectrum
of applications:

• Sound waves: for example, industrial process control, security, fish finding, measure of
ground speed.
• Electromagnetic waves: for example, vibration measurement, radar systems, object
tracking.
A current application area is both autonomous and manned highway vehicles. Both
microwave and laser radar systems have been designed for this environment. Both systems
have equivalent range, but laser can suffer when visual signals are deteriorated by environ-
mental conditions such as rain, fog, and so on. Commercial microwave radar systems are
already available for installation on highway trucks. These systems are called VORAD
(vehicle on-board radar) and have a total range of approximately 150 m. With an accuracy
of approximately 97%, these systems report range rates from 0 to 160 km/hr with a resolu-
tion of 1 km/hr. The beam is approximately 4 degrees wide and 5 degrees in elevation. One
of the key limitations of radar technology is its bandwidth. Existing systems can provide
information on multiple targets at approximately 2 Hz.

4.1.8 Vision-based sensors
Vision is our most powerful sense. It provides us with an enormous amount of information
about the environment and enables rich, intelligent interaction in dynamic environments. It
is therefore not surprising that a great deal of effort has been devoted to providing machines
with sensors that mimic the capabilities of the human vision system. The first step in this
process is the creation of sensing devices that capture the same raw information light that
the human vision system uses. The next section describes the two current technologies for
creating vision sensors: CCD and CMOS. These sensors have specific limitations in per-
formance when compared to the human eye, and it is important that the reader understand
these limitations. Afterward, the second and third sections describe vision-based sensors