GB1327016A - Respirator control apparatus - Google Patents

Abstract

1327016 Mask supported microphones DEFENCE SECRETARY OF STATE FOR 22 June 1972 [28 July 1971] 35475/71 Heading H4J [Also in Divisions A5 and G1] Respirator control apparatus comprises a mask 1 in which a microphone 7 is placed protruding into the interior of the mask. The microphone consists of a diaphragm 8, a magnetic core 9 and a winding 10 mounted on the core 9 and is responsive to both the low frequency pressur e changes caused by the user breathing and also the higher frequency pressure changes caused by speech or sound vibrations. The output from the microphone is fed into a control circuit shown in Fig. 2. The winding 10 forms one arm of a bridge circuit, an adjustable inductance 22 and resistances 23, 24 forming the other three arms. A 30 KHz carrier frequency oscillator 20 has two outputs of opposite phase which are connected across the bridge and also supply the reference signal input of a phase sensitive demodulator 21. The output of the bridge which is a modulated 30 KHz signal is connected to a 30 KHz amplifier 25 which also feeds the demodulator 21. The output of the demodulator passes through one of two filters 27 and 29. Filter 27 has a band width of 300 Hz to 3500 Hz and the signals passing through this filter represent the sound or speech vibrations of the user which are fed into a conventional intercom amplifier. The second filter 29 allows signals with frequencies between 0 and 20 Hz to pass through representing the pressure variations caused by the breathing of the user. These signals are fed into an amplifier the output of which controls an electromechanical oxygen flow regulator 30 which controls the flow of oxygen from a supply 31 to the pipe 4. A transducer 32 provides a signal indicative of the rate of oxygen flow, which signal provides a negative feedback signal to the amplifier 28 via a shaping network 33.