GB621384A - Improvements in or relating to electric signalling systems - Google Patents

Abstract

621,384. Electric indicating systems. STANDARD TELEPHONES & CABLES, Ltd., McWHIRTER, E. M. S., DUNN, R. H., and LENNOX, P. W. Feb. 5, 1946, No. 3616. [Class 40 (i)] An electrical value is converted into a representative D.C. potential which is converted into a start-stop telegraph code. General arrangement.-The electric quantity is expressed as a signal D.C. potential which is compared with a reference potential obtained from a potentiometer progressively stepped on in association with a first wiper moving over a contact bank. When the reference potential just exceeds the signal potential, the wiper is stopped, and the reference potential is reduced to be below the signal potential and again increased by a vernier potentiometer stepped on in association with a second wiper moving over a contact bank. Once again, this wiper is stopped when the reference potential just exceeds the signal potential, and the positions of the two wipers determine the transmitted signal code as in Specification 555,999. Expressing the electric quantity as a signal D.C. potential.-An alternating voltage is applied to transformer T, Fig. 1, rectified at R, and smoothed by condensers CA, CB and resistances YA, YB to produce the desired potential at tapping P; a limiter valve CV may be used to subtract a constant voltage from the signal voltage to correct a zero error. A similar circuit is used to develop a D.C. potential from an A.C. current, Fig. 2 (not shown). Where the electric quantity is the operation of a kilo-watt-hour meter, the meter actuates contacts M, Fig. 3, to charge condenser CA to a constant level from source e and discharge it into circuit CB, XA feeding a cathode follower valve V whose cathode potential is fed back to the source e so that CA is always charged by contact M to the voltage of e plus that of condenser CB to obtain linearity. Where the signal D.C. potential is rapidly fluctuating it may be applied for short periods to a condenser whose potential is then used as the signal D.C. potential, Fig. 4 (not shown). Comparison of signal and reference voltages.- The reference potential is taken from a wiper sam<SP>1</SP>, Fig. 9, stepped over a potentiometer R ... R by stepping magnet SAM operated by depression of key KT. This potential is applied to the grid of valve VB, Fig. 7, which has a common cathode load YD with valve VA fed with the signal potential. Initially, the signal potential is higher than the reference potential, and valve VA conducts while valve VB is non-conducting; when the reference potential just exceeds the signal potential, valve VA ceases to conduct and valve VB conducts to actuate relay R, which changes contacts BAC, Fig. 9, to stop stepping magnet SAM and actuate relay CO. Relay CO holds on over contact co<SP>4</SP>, changes contacts co<SP>3</SP> to prepare stepping magnet SBM for actuation, closes contact co<SP>2</SP> to connect wiper sbm<SP>1</SP> to wiper sam<SP>2</SP> which is two steps behind wiper sam<SP>1</SP>, and opens contact co<SP>1</SP> inserting resistance R into the potentiometer R ... R so that the reference potential has dropped to a value below that at which it operated relay R, Fig. 7, by an amount equal to the potential across a section R of potentiometer- RR. As a consequence, valve VB ceases to conduct, and relay R de-energizes to change back contacts BAC, Fig. 9, to actuate stepping magnet SBM to step on wiper sbm<SP>1</SP> over vernier potentiometer r . . . r until the reference potential just exceeds the signal potential when valve VB conducts to energize relay R, change over contacts BAC and stop the stepping magnet SBM. Wipers sam<SP>3</SP>, sbm<SP>2</SP> then register in tens and units the magnitude of the reference potential which is equal to the magnitude of the signal D.C. potential and by the arrangement of Specification 555,999, this position can be translated into a teleprinter code. Keys HK<SP>1</SP>, HK<SP>2</SP> close homing circuits for wipers SAM, SBM. Preferably, the potentiometer R ... R includes more than ten sections and wiper sam<SP>4</SP> registers hundreds; this is of value when intrument readings are being transmitted as percentages, a signal potential greater than 100 per cent indicating overload. In a modification, the signal and reference potentials are applied in opposition to a single valve, Fig. 6 (not shown), to actuate a relay when the reference potential exceeds the signal potential. In a further modification, the output of PA, Fig. 7, is applied to trip an oscillator whose output is amplified to actuate the relay R, Fig. 8 (not shown).