b. Detection of Vibration. This type of detection can operate on one
of two principles.
The first, known as Piezo Electric, converts energy
exerted by the intruder against the structural material into an electrical
signal, proportional to the energy exerted, and triggers an alarm.
The
second is seismic mass, or small weight, and rests on two contacts. When
force of impact is applied, the seismic mass is vibrated off of the
contacts, breaking the circuit and triggering an alarm. Sensitivity of this
alarm cart be adjusted.
(1) Characteristics.
(a) Advantages.
Easily
installed,
economical,
flexible
in
application and salvageable.
(b) Disadvantages. Can only be installed where minimal vibration
exists. Heavy construction, railroads, or heavy vehicular traffic creates
false alarms. Cannot be used effectively outdoors."
11. Detection of Motion. Motion is detected by ultrasonic sound, microwave
(radiowaves), or infrared energy. These types of sensors are volumetric in
nature, meaning these sensors are capable of covering a large volume of
area.
a. The ultrasonic motion sensor (UMS) operates on a "Doppler Frequency
Shift" principle much like police radar. A pattern of inaudible sound waves
is transmitted and monitored by the system receiver.
Intruders motion
within the covered area disturbs the sound wave pattern, altering its
frequency or speed. The receiver detects the frequency or "Doppler Shift"
and signals an alarm. Caution must be taken not to aim the sensors toward
each other unless separated by at least 60 feet. The transmitters/receivers
will "talk" or signal each other and they will become ineffective."
(1) Characteristics.
(a) Advantages.
This system is easily installed by unskilled
personnel and requires minimum installation time. Routine maintenance cost
is low.
If security interest ends, complete recovery of equipment is
possible.
(b) Disadvantages.
Sensitivity controls must be carefully
adjusted and frequently checked; nuisance alarms may lead security personnel
to reduce the system's sensitivity.
At low sensitivity, it is sometimes
possible to enter a protected area without activating the alarm. This can
be done by staying beneath the level of tables or desks; it can also be done
by moving so slowly that the ultrasonic waves are not shifted. This system
may not be adaptable for use in areas where quantities of absorbent
materials are stored, since these absorb sound waves.
MP1002
4-8
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