Page 65 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 4

The dual technology detector is a classical example of Boolean logic in a signal pro-
cessing application, but it had a logical weakness. To understand this weakness, let’s
arbitrarily call a signal 100% when it is precisely at the threshold level. Now con-
sider a case when the microwave signal is 99% and the PIR signal is 200%. This
would not generate an alarm, yet it would be more likely to represent a true detec-
tion than the case when an alarm would be generated by both signals being 101%.
There is a better way to combine these signals.

MW Gain
M W Amp.

+V
Microwave Horn +
Amp. Coil
+ +V
Comp.
Differential PIR
Detector
Amp.
+ Alarm Threshold
- PIR Gain
-
+

Figure 4.2. Dual technology motion detector using fuzzy logic equivalence

The circuit of Figure 4.2 demonstrates how we might sum the two sensor signals of
the motion detector before comparing the result to a threshold. Instead of convert-
ing the signals to Boolean values before ANDing them, we sum their analog levels
and then compare the result to a threshold. Two gain potentiometers have been pro-
vided so that we can balance the two signals to appropriately represent their relative
sensitivities.

If the final threshold is set higher than either sensor signal can achieve by itself,
then both signals must be present to at least some degree to cause an alarm. The
advantage to this approach is that a much wider range of valid alarm input combina-
tions is possible. What we have done is to create an electrical equivalent to a fuzzy logic
solution.

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