Forever IR : Low-Cost Intrusion Detection

Two decades ago, who would have guessed that the humble pyroelectric infrared (IR) detector would still be a mainstay of 21st century security systems! Surveillance cameras now dominate the market, yet sales growth continues for intrusion motion sensors based on dual-element pyroelectric IR detectors. Continuous improvement of IR detectors supports this trend, with clever sensor designs harvesting better and better information from these simple, capable detectors.

Simplicity brings popularity. Tens of millions of pyroelectric IR detectors are sold every year. People emit 6-14 micronwavelength IR light, and these detectors can ¨see〃 it. In passive infrared (PIR) motion sensors, they can detect people, day or night, without illumination. Incidentally, television remote controls and beam-type security sensors are active IR devices. These emit about 1 micron IR light, which does not interfere with PIR motion sensors.

Simplicity also means low cost. Pyroelectric IR detector prices range from US$0.30 to $1.00, depending on type. Once assembled with optics and electronics, these detectors are the ¨eyes〃 of PIR motion sensors ranging in price from $3.50 to $80.00.

Pyroelectric IR detectors also offer high sensitivity. Via a 7mm round lens, a motion sensorˇs IR detector can detect a person 20 meters away, even at night. This presents a better option than illuminating a large area to support a camera vision system.

From Burglar's Glow to Siren's Wail

The standard pyroelectric IR detector has two elements. For varying IR light intensity (as provided by warm persons moving in a cooler background), one element generates a (-) polarity signal, while the other generates a (+) signal. In a PIR sensor, one lens in front of the detector creates two fields-of-views, shaped like the two elements, and expanding with distance from the sensor. As a person walks through each view, the detector generates an electrical signal wave. The PIR sensor ˇs processing circuitry  analog or digital  determines if the waveˇs size indicates human motion.

PIR sensors employ multi-lens arrays, which yield many views over a wide angle. Moving persons thus produce several signal waves for detection processing, and then  the alarm. This simple function remains at the heart of the classic alarm market, yet improvements remain possible.

For example, in the views, changing-temperature stationary objects also can cause detector signal waves, leading to erroneous detection  and false alarms. Changing-temperature stationary objects include hot water pipes, metal furnace registers, surfaces flashing visible/near-IR light, and so on.

Application Challenges

Because of false alarms, why not design an IR camera with many elements or pixels, to confirm motions more clearly? Actually, multi-element detector arrays are available, yet their prices remain far above the sub-$2.00 range needed for PIR sensors.

Instead, to confirm motion and reduce false alarm rates, IR detectors are often combined with microwave Doppler motion detectors to form dual-technology sensors. Alternately, for applications where a motion sensor ˇs price is a significant percentage of a cameraˇs price, a camera might be used instead, along with illumination of the monitored area.

However, it is not always camera or PIR sensor  together they bring major benefits! For example, a security system company may have hundreds or thousands of cameras at customersˇ facilities, and may employ several operators to monitor and evaluate images arriving from these numerous sites. If each camera has a PIR sensor, then the system can alert operators only to images pre-qualified by sensorbased image-processing reports of zone-entrance, curtain-crossing, directional-walking, etc. Thus, fewer operators can monitor many sites  staff costs are reduced.

Improving Sensors for a Growing Market

Thus, two growing applications foster improvements in PIR sensors: sensor-based image processing for PIR camera applications, and reliable detection in classic alarm applications. Also, a fast-growing alarm sub-sector is outdoor monitoring, where false alarm prevention is more challenging than indoors. For all applications, the best PIR sensors will provide more and more information, at ever-lower false-alarm rates.

Most basic are single detector/ optics systems , offered with dual-element detectors by virtually all PIR sensor manufacturers. Clever optics and signal processing give the better models a reasonable degree of false-alarm resistance.

Beyond the basics, there are double detector/optics systems (each detector with its own lens array). In the simplest type, two identical systems monitor the same area, and two active signals are required for an alarm. Thus, single-detector noise causes no false alarms. However, false alarm sources remain: small animals, such as pets, and changingtemperature stationary objects.

Another double system combines a wide-angle view array and a narrower curtain array. For an alarm, motion must be detected first by the wider-array system, and then by the curtain system. This system can ignore changing temperature stationary objects, though application is tricky: all possible intruder paths must enter the monitored area first through the wider-array system.

A vertically interleaved double detector/optics system offers an excellent advantage. An erect human is detected by both systems simultaneously, while a small animal is detected by only one system at a time. Thus, this system distinguishes humans from small animals.

Double detector/optics systems are not always needed, though. Four-element (or simply quad) detectors are available, which produce two signals  one from each pair of elements. Over the years, there have emerged several two-signal, quad PIR sensor types. In the most basic sensor, the two sets of detector (+) and () elements are one above the other, behind a single lens array. The resulting view pattern resembles that of the vertically interleaved double detector/optics system, and this can also be used in a sensor to tell pets from humans.

Another quad system has four detectors side-by-side to produce two signals with a 90-degree phase difference, which shows motion direction. This system also provides some resistance to changing-temperature stationary objects. A new type of quad detector system (Figure1) also uses four detectors side-by-side, and produces two processed signals of very different frequencies (often in a 2:1 ratio). Only moving objects crossing the views produce dual-frequency signals, while changing-temperature stationary objects produce same-frequency signals. Thereby, the sensor can indicate an alarm only upon actual motion. This design, called Dual-Frequency InfraRed, is exceptionally resistant to false alarms.

All told, it is no wonder that PIR sensors remain popular. Manufacturersˇ diligent work has yielded many fine products, with improvements ongoing. Also, more companies continue to develop all-new PIR sensors for many applications.
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