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General 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.

The diagram shows the target moving across the fields-of -view and the electrical signal generated. The overshoot as the target leaves the field is caused by partial accommodation of the target which occurred when the target occupied the field.

If the sensor were not designed in this way it would be ineffective. An internal chopper or scanning method would render the entire protected area visible and it is not the intent of an intrusion sensor to detect rooms.

MFOVani.gif (10623 bytes)
Without the defined separate fields the intruder and the room would be rendered together and he could move around with little or no perceivable change in radiation. In other words he becomes part of the background. He might be seen as he first enters the area but this transitory event might be missed especially if he approached very slowly and from a distance. This change of radiation principle implies accommodation and once accommodated the target becomes invisible. The rate of accommodation is only a few seconds, a fact taken advantage of in trying to render the background invisible to the sensor. With good optics the accommodation effect for the intruder can be minimized but it is inherent in the technology. If the intruder walks very slowly the sensor will accommodate him. If he runs very fast there may be to little time on target for detection. SFOVani.gif (6479 bytes)
The range of speeds over which he can be detected is called the target velocity range.  It is a function of distance from the sensor because the optics which determine the horizontal field-of-view cause it to diverge from almost zero close to the sensor to a larger value some distance away. The frequency of the electrical signal produced by the detector for the same walking speed will change with both distance and direction. The diagram shows this for a typical optical system. TVrange.gif (12896 bytes)



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Last modified: October 25, 2009