Patent GB-803368-A (GB)
Title: Compensation for turbulence and other effects in intruder and flame detection systems
IPC: G08B19/00
Filed: 1956-01-10
Abstract: 803,368. Intruder and fire alarm systems. Jan. 10, 1956 [Jan. 24, 1955], No. 801/56. Class 118 (1). An intruder moving within a space containing a radio field distribution will produce a spectrum of Doppler frequencies in a receiver since not all parts of his body will be moving at the same speed, but the amplitudes of these frequencies will be substantially uniform (see curve D, Fig. 3) when compared with the amplitude/Doppler frequency curves A and B caused by turbulence, the amplitudes of turbulence frequencies varying considerably from high to low frequency. Flame Doppler frequencies produce a curve C of an amplitude/frequency relationship between the curves due to an intruder and turbulence, but the amplitude of the flame Doppler frequency is substantially constant up to a much higher frequency than is the case with turbulence, so that a flame simulates an intruder. In the apparatus of the invention therefore an alarm is actuated if a signal is detected in which the amplitude at the higher frequency is substantially the same as or does not differ greatly from the amplitude at the lower frequency. The apparatus will also eliminate the effects of arcing, shock sounds, line voltages changes and so on. The spectrum analysis is carried out as follows. The detected Doppler frequency is divided into high- and lowfrequency components, the low-frequency component is then attenuated relative to the highfrequency component (by amplifying the high frequency) and the amplitudes of the two signals are then opposed in a relay, an appreciable resultant setting off the alarm. Thus turbulence will not set off the alarm as the low-frequency component has a higher amplitude than the high-frequency component, and when attenuated and matched give no appreciable resultant. An oscillator 2 in the space to be protected generates frequencies in the ultrasonic range. These frequencies are received by a pick-up 8 and are amplified by stages 10, 12, passed through a high-pass filter 16 and are beaten in a detector 18 against the original signal. Any frequency difference (Doppler frequency) is passed by a low-pass filter 20, through the ampliffer 10 and sensitivity control 22 to the second amplifier 12. It is then rejected by the high-pass filter 16 and passes through a lowpass filter 14 to a third amplifier 24. This circuit so far is known. From the amplifier 24 there extends a low-frequency line 26, with a filter 30 which will pass only low Doppler frequencies to a detector 38, and a high-frequency line 28 having a high-pass filter 34. The output from the filter 30 is detected at 32 and has a positive sign, whilst the output from the filter 34 is amplified at 36 and passes its signal to a detector 38, the output of which is negative in sign. These two outputs are fed to an integrator 40 and a relay tube 42 controlling an alarm relay 44. Since the high Doppler frequencies are amplified at 36 whereas the low Doppler frequencies are not, then the tube 42 is biased normally to pass current and energize the relay 44 to keep the alarm off. If a turbulence signal is received in which the high Doppler frequencies have a much lower amplitude than the low Doppler frequencies, then those high frequencies even after amplification are cancelled by the low frequencies and the tube 42 continues to conduct. On the other hand, if the high and low Doppler frequencies have substantially the same amplitudes, then after amplification a negative bias will be given to the tube 42 and the alarm will be energized. Since the high-frequency signals pass through the circuitry more rapidly than the low-frequency signals then an artificial time delay is incorporated into the high-frequency side. In addition, it is noted that the high-frequency signals last for a longer time so that the high and low sides must be provided with substantially the same time constant.