█ LARRY GILMAN
In security applications, a motion sensor is a device that detects human presence, usually inside a building or in the immediate vicinity of a building. Not all devices classified as "motion" sensors actually sense motion; for instance, passive infrared systems (PIRs) detect the infrared light (heat radiation) emitted by human beings. "Presence detectors" might be a more accurate term for this class of devices.
The simplest type of motion sensor sets up a circuit or closed electrical path partly composed of a beam of light. This beam is directed across an open space to be monitored to a photoelectric detector, which converts the incident light beam to a voltage signal; any interruption in the beam is detected as an interruption in the voltage signal. This straightforward design, still in use, has the disadvantage of monitoring only the space occupied by the beam itself. This means that several beams must be used to secure a doorway or passageway, and many beams must be used to monitor a large, complex space (e.g., armory). Further, a beam-circuit system cannot distinguish between intruders of different sizes; a moth can interrupt a beam as effectively as a leg.
PIRs are also commonplace. Low-grade PIRs are often used for automatic lights, while more complex models are used for building security. PIRs do not detect motion, as mentioned above, but relatively rapid changes in the overall amount of infrared light in a scene. Slow, overall changes in the amount of infrared light, such as would occur when the sun goes behind a cloud or a room heats up, should not trigger the sensor; the sudden change caused by a human being entering the sensor's field of view should trigger the sensor. PIR sensors can fail to detect intruder movement that is (a) primarily towards or away from the sensor, rather than across its field of view, or (b) slow.
PIRs are passive because they detect energy that the scene emits of its own accord. Active motion detectors, in contrast, illuminate the scene with laser light, ultrasound (sound waves pitched too high for the human ear to detect), or microwaves (radar). Regardless of the type of energy used to illuminate the scene, there are two basic ways of using reflected energy to detect presence or intrusion: (1) monitor for relatively rapid changes in wave energy reflected from the scene, and (2) monitor for Doppler shifts in wave energy reflected from the scene. A Doppler shift is a change in frequency of a wave that is reflected from or emitted by a moving object or is measured by a moving observer. As in the motion-detection case, the detector is stationary, any frequency shift in waves reflected from the scene must result from the motion of objects in the scene.
Both PIRs and active detectors overcome the inability of the beam-circuit detector to distinguish between a tiny intruder and a large one, as these detector types can be set to trigger a light, door, alarm, or other device only if a certain threshold in signal intensity is crossed. However, there is no one, correct threshold. When designing a system to detect intrusion or presence, one wishes to avoid both a high false-alarm rate (which will over activate lights or doors or, in the case of a security system, eventually cause human operators to ignore the system) and a high likelihood of real presence detection. Yet, these two goals are in opposition; an insensitive detector will fail to detect persons that enter its field of view, but a too-sensitive detector will detect not only intruders, but also insects, vibrations in its mounting bracket, and other causes of minor signal variation. A compromise sensitivity must be chosen for each device type and application.
A more complex but potentially more informative class of motion-detection systems applies computer analysis to video images. By looking for changes from one image frame to the next, a computer can easily detect motion in a scene; the difficulty is to design algorithms that can distinguish between important motion (a man climbing a fence) and unimportant motion (leaves rustling in trees, cloud-shadows moving over the ground, etc.). A reliable video-based motion-detection scheme, therefore, requires the application of artificial intelligence techniques. Such systems are under development, but not yet widely deployed.
█ FURTHER READING:
Lester, Andrew J., and Clifton L. Smith. "Analyses of Performance of Volumetric Intrusion Detection Technologies." Proceedings, 33rd Annual International Carnahan Conference on Security Technology. Oct. 12–14, 1999: 111–58.