FUJI & CO.(Piezo Science)
INTRODUCTION AND GENERAL PRINCIPLES OF OPERATION
A passive infrared (PIR) sensor as used by the security industry is a special purpose radiometer used to detect the body heat of an intruder. It is an almost ideal sensor because it is passive. Its presence cannot be detected as is the case for active sensors such as ultrasonic or microwave. The design goal is quite simple, To create a radiometer with a high probability of detecting an intruder within a defined area while not responding to anything else. The area of detection is defined by the optics and pyroelectric detector element geometry. The probability of detection is enhanced by the efficient discrimination of the intruders body heat by the optics and electronic signal processing while not responding to anything else is the art and/or science of rendering the detection area background invisible and hardening the sensor against all other stimuli.
Detection, general principles
For detection, an intrusion sensor is sensitive to changes in infrared energy rather than absolute levels. It accommodates itself to the background conditions in the room and perceives the intruder as a change in this state of equilibrium. This change principle is fundamental to the detection process and PIR sensors are designed to maximize this by a process known as chopping. Conventional radiometers, those designed to measure temperature, sometimes have a built-in mechanical chopper wheel which alternately cuts the external field-of-view on and off. This renders everything external to the radiometer visible by reference to the chopper wheel which in effect becomes its reference temperature or background. Another way is use an electronic chopper which alternatively switches the signal processor from the external source to an internal reference at a suitable frequency modulating the reference.
Intrusion sensors cleverly use the real background as a reference completely avoiding the use of a chopper. By optically dividing the area to be protected into a number of separate and separated fields-of -view, when an intruder moves through the area he appears and disappears from view and by doing so modulates the reference condition. Now you see him, now you don't; now you see him, now you don't! The signal produced is proportional to the difference in temperature between the intruder and the background.
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