The pros and cons of volumetric burglar alarm sensors

Nov 1, 1999 12:00 PM, Lou Fiore


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Volumetric burglar alarm sensors fall into several types: ultrasonic, microwave, passive infrared (PIR) and combinations of the above. You don't hear a lot about ultrasonic sensors these days. But microwave and especially PIRs are common.

The PIR has gained enormous popularity because of its cost; relatively few components at low cost are its trademark. PIRs are very inexpensive at the low end of the scale and are affordable at the high end of the scale.

What differentiates the various "grades" of PIR performance? The passive infrared unit attempts to differentiate the motion of radiant heat as a source moves through its field-of-view. Usually, one sensor is at the heart of the unit. The designer of the unit creates variations in the field-of-view by fashioning a lens. The quality and type of the lens are key to the unit's stability and performance.

There are two types of lenses: fresnel and mirrored optics. My favorite for performance is the mirrored optics type. The mirror, usually a molded and coated piece of plastic, yields more infrared energy to the sensor, is more stable and yields a crisper field-of-view. Usually, a mirrored unit is more expensive, but worth the price. It performs better because it needs less amplification in the electronic circuit following the infrared sensor. The overall unit is, therefore, less prone to interference from static electricity, radio energy or lightning.

I do not wish to discount fresnel lenses. In a more benign environment, such as the home, they work fine because the typical interference sources found in a commercial environment are not present in the smaller rooms of a home. The resulting combination, along with lower cost, makes the fresnel unit a good choice for a residential system.

At the top of the list of PIRs is a dual technology device - a marriage with the microwave sensor. This device is often touted as the answer to the false alarm problem caused by volumetric sensors. "Just put it up and don't worry about it" is what some would like you to think. The main premise here is that the additional cost of the unit is far offset by the additional savings of fewer false alarms and, even more important, fewer service calls.

At the risk of creating controversy, my stand is that this device is more difficult to install than a single technology unit. Why? First let's look at the two technologies individually. The instructions for mounting a PIR would have the installer mount it not facing a heat source, while aiming its fingers across the direction of potential motion. The instructions for mounting a microwave would have the installer not point it at sources of motion such as fans, or looking out toward a public area - a window, door or hallway, for example. In other words, the entire range of the unit should be limited to the area of protection.

In my opinion, carelessly using a dual technology device would be analogous to fastening a seat belt, then driving carelessly, thinking you are safe. If the unit was just thrown up on a wall at random, one of the two technologies would most likely be over-taxed and in alarm much of the of time, leaving the unit at the mercy of the other technology. My point is that for a dual technology unit to be properly installed, both technologies need to be optimized, so that the full false alarm immunity is realized. A dual technology unit is, therefore, more difficult to install.

But once installed properly, a dual technology volumetric device will offer a great deal of false alarm immunity. It is achieved because the false alarm mechanisms for a microwave and a PIR are different. Therefore, what causes a false alarm for one probably will not for the other.

Various manufacturers have optimized the design of a dual device. In its basic form, the two act as an "AND" gate: one technology must alarm AND then the other, within a small time window, to create an alarm.

At the low end of this product's spectrum is a device that is a combination of an inexpensive PIR and an inexpensive microwave: Effective, but not optimum. Some manufacturers will raise the sensitivity of the microwave when a PIR disturbance is sensed.

For me the best dual technology device is a combination of the best PIR combined with the best microwave sensor that a manufacturer can design. Whatever nuance is used to effect the combination, the resulting volumetric unit, if installed properly, is very effective.

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