Patent GB-689108-A (GB)
Title: Improvements relating to electrical signalling systems
IPC: G08B21/00
Filed: 1950-06-13
Abstract: 689,108. Electric indicating systems. ELECTROFLO METERS CO., Ltd., and HARDY, R. June 7, 1951 [June 13, 1950], No. 14798/50. Class 40 (i). In an electrical telemetering system the rotatable disc of an induction motor is slotted round its periphery and rotated at a speed dependent on the magnitude of a variable quantity (but independently of supply voltage and. frequency) in co-operation with photoelectric, electromagnetic, or electrostatic apparatus responsive to the slotted rotary disc to produce an alternating current whose frequency is proportional to the speed of rotation and therefore to the variable quantity and which is remotely transmitted to a receiver responsive to frequency changes and insensitive to amplitude and waveform changes, whereby indicating, recording, or controlling effects are produced. Fig. 2 shows a variable frequency telemetering transmitter responsive e.g. to the position of a water level float 17 which varies a resistance 16 in the excitation circuit of winding 15 of an A.C. disc induction motor of the kind described in Specification 211,237, [Class 37], wherein a conducting disc 7 is rotated by torque due to the interacting fields of electromagnet 14 excited by winding 15 and of electromagnets 8, 9 excited by windings 10, 11 from an A.C. source 12. The magnetic flux due to windings 10, 11 is large compared with that due to winding 15 and is in quadrature therewith, and the rotational speed of the disc is proportional to the ratio of the two fluxes, thus to the ratio between the current in coils 10, 11 and the current in coil 15, independently of supply voltage and frequency variations. The disc 7, rotating at a speed inversely proportional to the value of the float operated resistance 16 has radial slots 22 in its periphery (Fig. 3) which co-operate with similar slots in fixed masks 23, 24 to interrupt the passage of light from a source 25 to a photocell 26, whose output frequency (of the order of 100 c.p.s.) is dependent on the liquid level, and may be transmitted direct by land line, or by modulation of a carrier frequency for radio or landline transmission. At the remote station the directly transmitted or demodulated signal is amplified in transformer coupled valves VTI, VT2, (Fig. 4) and applied in antiphase to excite the grids of series connected valves VT3, VT4, which are normally biased to cut off for zero input signal. The valves conduct on opposite half cycles of the input so that on one half cycle the condenser C across valve VT4 charges, and discharges on the next half cycle through the anode milliammeter MA. For large excitations of VT3, VT4 and constant stabilized anode voltage, the average condenser discharge current in MA is proportional to the frequency of the input and to the quantity to be remotely indicated. The amplifiers may be R.C. coupled and may comprise an amplitude limiter, while the anode current of VT4 may operate recording instruments, alarm, or control circuits. Plural signals may be transmitted over a single landline or radio link by carrier working. The transmission signal may be produced by rotating the radial projections of the disc in co-operation with fixed elements to vary an inductance or capacity at the required frequency, and the frequency sensitive receiving circuit may comprise a pulse counter or frequency responsive bridge circuit.