DESIGNING FOR 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
|
 | Unstable background
 | Sun shining on wall |
| Cloud shadows passing across a surface heated by the sun through a window. |
| Air drafts from various sources acting on a background surface. |
|
| Small animals, pets. |
| Insects
| Easily seen when crawling across sensor optics. Acts as a chopper making the background
visible. |
|
| Heat sources in FOV
| Radiators |
| Electronic appliances |
|
| Air drafts, hot and cold directed at sensor.
| Open window |
| Forced hot air heating systems |
|
| Rapid shift in ambient temperature |
|
Non-thermally related
|
| Radio Frequency Interference (RFI)
| Police or Fire trucks can get very close to an installed sensor. |
|
| Electrostatic Discharge
| Lightning strikes |
|
| Electromagnetic Discharge |
| Mechanical shock
| Pounding on other side of wall on which sensor is mounted. |
| Object falling on sensor |
|
| Vibration. |
| Detector Spike Noise. |
| Visible and Near Infrared radiation
| Flash 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.
FUJI &
CO.(Piezo Science)
