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False Alarm Immunity



False alarm stimuli fall into two categories, those that are thermally related and those that are not. It is an important design goal that alarms from non-thermally related stimuli be completely eliminated, thus if a false alarm does occur it would be assured that its cause was local to the sensor and associated with its primary detection characteristic, heat. While good progress has been made by some companies, there are still improvements to be made in meeting this objective.

Avoiding false alarms from thermally related stimuli require that their thermal effects are perceived to be somehow different from that of an intruder. This differentiation can be made both optically and electronically and is the purpose of signal processing.

Examples of false alarm hazards.

Thermally related

bulletUnstable background
bulletSun shining on wall
bulletCloud shadows passing across a surface heated by the sun through a window.
bulletAir drafts from various sources acting on a background surface.
bulletSmall animals, pets.
bulletEasily seen when crawling across sensor optics. Acts as a chopper making the background visible.
bulletHeat sources in FOV
bulletElectronic appliances
bulletAir drafts, hot and cold directed at sensor.
bulletOpen window
bulletForced hot air heating systems
bulletRapid shift in ambient temperature

Non-thermally related

bulletRadio Frequency Interference (RFI)
bulletPolice or Fire trucks can get very close to an installed sensor.
bulletElectrostatic Discharge
bulletLightning strikes
bulletElectromagnetic Discharge
bulletMechanical shock
bulletPounding on other side of wall on which sensor is mounted.
bulletObject falling on sensor
bulletDetector Spike Noise.
bulletVisible and Near Infrared radiation
bulletFlash lights.

Some means of controlling these and some specifics of detection are discussed in other parts of the technical notes.  Just a description of false alarm causes suggest that much in avoiding false alarms in a given installation depends on the analytical skill of the installer. Nonetheless a good sensor must have high inherent immunity.


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Last modified: October 25, 2009