GB1402156A - Pulse generating apparatus - Google Patents

Abstract

1402156 Fluid-flow measuring WESTINGHOUSE BRAKE & SIGNAL CO Ltd 20 Nov 1973 [2 Dec 1972] 55784/72 Heading G1A [Also in Division G4] In apparatus which measures fluid-flow in a forward direction and emits electric pulses corresponding thereto, the effect of any periods of reverse flow is eliminated. Two counters 10, 11, Fig. 1, are provided which are both driven forwards when fluid-flow drives transducer 1 in the forward direction; as long as this happens, the two counter outputs are equal and a comparator 14 drives a bi-stable 16 to maintain AND gate 12 open so that output pulses can be taken on line 20 representing flow. If flow reverses, counter 10 ceases to count as described below, so that the bi-stable changes state and AND gate 12 closes to prevent output pulses. When flow in the forward direction resumes counter 11 stops, and no output pulses are obtained until counter 10 has caught up with counter 11; at that point the bistable switches state, AND gate 12 opens, and the two counters continue to count. The period of reverse flow is eliminated, and the net forward flow is measured. The transducer 1 has a GaAs source and two photo-transistors separated by a chopper; it gives two phase separated outputs to drive Schmitt triggers 5, 6 whose outputs are shown at Figs. 2(a), 2(b) respectively. The trigger outputs are connected to gates 7, 8, the latter via inverter 9. The gate outputs pass to counter 10, AND gate 12, OR gates 13, 18, counter 11, and bi-stable 16 via inverter 19. In normal forward operation the phasing of the Schmitt triggers is such that gate 8 remains closed, and gate 7 drives counter 10 and, via gates 12 and 13, counter 11. In reverse flow the phasing reverses so that gate 7 ceases to drive counter 10 but gate 8 drives counter 11 forwards via gate 13. When forward flow restarts, the phasing reverts to its initial state and gate 8 cannot drive counter 11; also AND gate 12 now prevents gate 7 driving counter 11 until counter 10 reaches the same count. When that happens the bi-stable is reset and normal forward counting occurs.