Intrusion alarm system with improved air turbulence compensation

Details Intrusion alarm system with improved air turbulence compensation Intrusion alarm system with improved air turbulence compensation

1974-07-08

1976-06-29

Trafton, David L.

Communications: electrical

Systems

Call station

340 1R, G08B 1316

Patent

active

039672603

ABSTRACT:
A comprehensive analysis of the effect of air turbulence on the acoustic performance of an ultrasonic intruder alarm system indicates that air turbulence does not cause Doppler frequency shifts in the received signal in the frequency region 2 - 100 Hz, as has been universally assumed in prior art systems but that air turbulence can only produce amplitude modulations in the received signal over this frequency region. It is also disclosed that changes in air velocity, or accelerations of the air flow, cause only very low Doppler frequency shifts in the signal, below 2 Hz for a 20 kHz carrier, and only moving targets can produce higher Doppler frequency shifts. A processing system is disclosed which is immune to false alarms in the presence of air turbulence without sacrificing threshold sensitivity and without introducing long time delays in the signal processing circuit as has been necessary in prior art systems. The disclosed signal processing system removes the amplitude modulation from the received signal and then discards all very low Doppler frequency shifts below approximately 5 Hz thereby completely eliminating any effect of air turbulence on the operation of the system. Doppler shift frequencies, which can only be produced by moving targets, are utilized to activate the alarm.

REFERENCES:
patent: 2767393 (1956-10-01), Bagno
patent: 2791756 (1957-05-01), Wood et al.
patent: 3202960 (1965-08-01), Galbraith
patent: 3629812 (1971-12-01), Amato

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