Page 63 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 4

A microwave motion detector emits a UHF radio signal which reflects off objects in
the environment and is then received back at the detector where it is mixed with
the outgoing signal. If nothing moves in the environment, the two frequencies are
the same and no sum or difference “beat” signal is produced. However, if some of the
returning signal was reflected from a moving object, then a frequency (beat note) is
produced that is proportional in frequency to the velocity of the target. This signal is
then amplified, rectified, and compared to a threshold. If it is greater in amplitude
than an adjustable threshold, then a relay closes to signal an alarm.

A PIR alarm, on the other hand, is purely passive. Black body radiation (including
body heat) in the 8 to 12 micron wave lengths is passed through a Freznel lens that
has multiple facets. The images from these lenses are focused onto two elements of a
detector that are connected in opposite polarity. As the heat source moves, the dan-
cing images on the detector cause a pulsating signal. This signal is then amplified,
rectified, and compared to a threshold just as is done in microwave systems.

Microwave systems can be falsely triggered by stray signals from other systems, and
such systems could actually detect large moving vehicles outside of the building that
did not represent true intruders. Passive infrared sensors can be falsely triggered by
heat from forced air heaters, as well as by blowing curtains and certain other signal
sources. Initially, manufacturers attempted to reduce false alarms with signal-process-
ing techniques such as time delays. Despite these marginal improvements, the market
wanted a system whose false alarms were much less frequent than either system
could achieve.

As the market grew, eventually the cost of these systems became low enough that it
was possible to incorporate both technologies into a single alarm without the price
becoming prohibitive. The result is shown in Figure 4.1. Note that the symbol
shown as an “Amp” is both an amplifier and detector, which puts out a fluctuating
DC signal proportional to the disturbance magnitude.

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