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 receiver "fingers" from having any heating or air-conditioning devices
within its field of view. Positioning is critical in order to achieve
optimum PIR performance.
4.2.1.6 Advantages of PIR Sensors. PIRs are often selected for areas where
EMI/RFI is a consideration, yet high security is a requirement. This is
because PIRs are passive devices which, if properly positioned and
installed, provide optimum protection since they are relatively immune to
the environmental effects which often degrade the performance of other
volumetric sensors.
4.2.2 Ultrasonic Sensors. This discussion is limited to "active"
ultrasonic volumetric sensors. "Passive" ultrasonic sensors which are used
to detect glass breakage penetration were discussed in paragraph titled
"Glass Breakage Detectors." This sensor is termed "active" because it has a
transmitter which emits ultrasonic energy (inaudible to the human ear) and
sets up a "standing" energy field which is sensed by a receiver. The
transmitter and receiver are normally located separately within the
protected area, but may also be collocated within one unit called a
transceiver. As depicted in Figure 22, the transmitter and receiver
locations determine the coverage pattern. This type of sensor works on the
Doppler principle; movement within a stable energy pattern or field will
produce a detectable change of state. Any change in the frequency of the
"standing" energy field will be detected by the receiver and cause an alarm.
The energy field is normally sound waves at a frequency of about 19.2 kHz
per second (19 to 35 kHz are manufactured). Electronics within the sensor
permit adjustment of the detection alarm threshold. Thus, the sensors can
distinguish electronically between movements of a small object, such as a
bird, and larger motions of a human. While the motion produced by a human
will cause an alarm, these sensors can compare electronically what is
transmitted versus what is received and can also make limited allowances for
such sources of nuisance alarms as ambient environmental changes. Normally,
the electronics are adjusted to declare an alarm with a human intruder
moving at one step per second for not more than four steps. The ultrasonic
energy will not penetrate, but is reflected by walls, glass, and other
objects so that an intruder will be shielded. Hard surfaces will reflect
the energy and extend the area of coverage; soft surfaces will absorb the
energy and reduce the coverage area. Because of masking or absorption,
multiple units may be required to achieve the coverage required. While the
energy pattern is easily contained, the big drawback of ultrasonic sensors
is that air turbulence and other environmental effects produce nuisance
alarms. Areas with large volumes of moving air, such as from Heating,
Ventilating, and Air-Conditioning (HVAC) systems, may be unsuitable for
application of ultrasonic sensors. Slow-moving intruders or a highly
sophisticated intruder can defeat an ultrasonic sensor, especially a sensor
not optimally positioned and adjusted to compensate for the ambient
environment. The following other application and installation
considerations apply to ultrasonic sensors:
a) The sensor should be applied to a stable surface and the
pattern should avoid objects whose surfaces can vibrate. To obtain maximum
efficiency, aim the sensor as closely and directly as possible to the
objects requiring protection.
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