handbook of modern sensors

228 6 Occupancy and Motion Detectors
12. Infrared motion detectors: devices sensitive to heat waves emanated from warm
or cold moving objects
13. Microwave detectors: active sensors responsive to microwave electromagnetic
signals reflected from objects
14. Ultrasonic detectors: similar to microwaves except that instead of electromagnetic
radiation, ultrasonic waves are used
15. Video motion detectors: video equipment which compares a stationary image
stored in memory with the current image from the protected area
16. Video face recognition system: image analyzers that compare facial features with
a database
17. Laser system detectors: similar to photoelectric detectors, except that they use
narrow light beams and combinations of reflectors
18. Triboelectric detectors: sensors capable of detecting static electric charges carried
by moving objects
One of the major aggravations in detecting the occupancy or intrusion is a falsepositive
detection. The term “false positive” means that the system indicates an intrusion
when there is none. In some noncritical applications where false-positive
detections occur occasionally, (e.g., in a toy or a motion switch controlling electric
lights in a room), this may be not a serious problem: The lights will be erroneously
turned on for a short time, which unlikely do any harm. 1 In other systems, especially
used for security and military purposes, the false-positive detections, although generally
not as dangerous as false-negative ones (missing an intrusion), may become
a serious problem. While selecting a sensor for critical applications, considerations
should be given to its reliability, selectivity, and noise immunity. It is often a good
practice to form a multiple-sensor arrangement with symmetrical interface circuits.
It may dramatically improve the reliability of a system, especially in the presence of
external transmitted noise. Another efficient way to reduce erroneous detections is to
use sensors operating on different physical principles [2] (e.g., combining capacitive
and infrared detectors is an efficient combination, as they are receptive to different
kinds of transmitted noise.
6.1 Ultrasonic Sensors
These detectors are based on the transmission to the object and receiving reflected
acoustic waves. A description of the ultrasonic detectors can be found in Section 7.6
of Chapter 7. For the motion detectors, they may require a somewhat longer operating
range and a wider angle of coverage.
6.2 Microwave Motion Detectors
The microwave detectors offer an attractive alternative to other detectors when it
is required to cover large areas and to operate over an extended temperature range
under the influence of strong interferences, such as wind, acoustic noise, fog, dust,
1 Perhaps just stirring up some agitation about the presence of a ghost.