Page 64 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Thinking More Clearly Through Fuzzy Logic

+V

M W Amp. Coil
+V
Comp.

Microwave Horn
MW Threshold

Differential PIR +V
Detector
Amp. Coil
+ +V
- Comp.
-
+
PIR Threshold

Figure 4.1. A dual technology motion detector

Since the disturbances that cause PIR technology to false alarm are not related to
those that cause microwave technology to false alarm, the false alarm rate of a com-
bined system is much lower than for either technology alone. The Boolean equation
for an alarm is stated as:

Alarm = (MW > MW Threshold) AND (PIR > PIR Threshold)
Or:

Alarm = (MW > MW Threshold) · (PIR > PIR Threshold)
Notice that the comparators convert the analog signals to binary (on/off) signals. In-
terestingly, most early systems actually contained two relays. The Boolean AND
function was implemented by wiring the contacts of these relays in series. Why this
was not done with simple logic is puzzling, as the extra relay is much more expensive
than a couple of transistors or even diodes. Even so, the dual technology alarm was
an immediate success and has remained a mainstay of the industry.

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