GB2114307A - Measurement of direct current - Google Patents

Abstract

Apparatus for measuring the algebraic sum of a number of direct electric currents, for example in an inter-exchange telephone signalling system, each current is supplied to respective secondary windings 16, 17 of each of a pair of matched transformers 10, 11. The current causes a change in flux in the primary windings (12, 13) in a detection circuit which changes their inductance. This inductance is measured by a negative feedback method, supplying current to bias windings (20, 21) to cancel the change in inductance. The current to the bias windings providing a measurement of the algebraic sum of the currents. The apparatus may be used as a replacement for mechanical Carpenter relays in telephone signalling systems. <IMAGE>

Description

SPECIFICATION Measurement of direct electric current electric This invention relates to apparatus for measuring a direct electric current and for measuring the algebraic sum of a plurality of direct electric currents, and may find particular application in inter-exchange signalling in a telephone system.

In the British Telecommunications telephone system, a direct current signalling system known as DC2 is used on inter-exchange circuits and this allows signaliing over 2-wire or 4-wire physical circuits of up to 1 50 kilometres in length. In this system current reversals are used to signal across the junction between the calling and the called exchanges.

These current reversals are used to operate relays to effect the usual operations involved in setting up and terminating a call. Since, in this environment, there are high common mode currents and voltages which float with respect to the reference level, it is necessary to detect the direction of direct current by detecting the differential current in the two wires of the junction circuit.

In this system, polarised relays, known as Carpenter relays, are used. A Carpenter relay is an electromagnetic device in which a light armature is pivoted between two matched permanent magnets. The pole pieces of the magnets project into the gap in which the armature is located at both ends of the armature, so that pole pieces of the same polarity are disposed opposite one another across the gap at the pivoted end and at the free end of the armature.

A lower limb extending between the pole pieces of the two magnets, and forming the core, has the signal coils wound upon it.

Current flowing in the signal coil or coils produces flux which strengthens one of the permanent magnets and weakens the other.

The armature pivots to one side and a lever spring attached to the armature makes a connection with one of a pair of contacts.

The suspension springs of the armature are made sufficiently thin so that the armature is unstable in a central position and assumes stability in either side position whereupon one of the contact positions is engaged. The armature remains on one side even when no current flows and remains there until current in the coil produces a flux to move it to the other side.

There are usually three windings on the core, two of these are the line wires and the third is a bias winding. The direction of current flow in these windings determines which pole piece is strengthened and hence which side position the armature will assume. If the currents in the signal wires are of opposite polarity and roughly equal then the direction of the flux will be determined by the current in the bias winding, however, this current is less than the line current and so will not overrule the effect of a line current when one line current only is flowing.

The present invention seeks to provide apparatus for measuring a diret electric current. This apparatus may find application as a detector of differential current and in particular may be used as part of an alternative to the Carpenter relay described above.

In a first aspect the present invention provides apparatus for measuring a direct electric current comprising a pair of matched transformers each having a first winding connected in a detection circuit and a second winding connected in a circuit in which the current to be measured flows, wherein the pair of windings in each circuit are substantially matching and are connected such that the current flowing in both windings of a pair are equal, and the windings comprising one of said pairs of windings being disposed or connected in opposite senses with respect to the windings of the other pair;; the apparatus further including, a pair of matched bias windings, one on each transformer, disposed in opposite senses with respect to the windings of the detection, and a feedback loop from the detection circuit to a source of current applied to the bias windings and operative-to feed current, dependent upon the change in inductance in the detection windings, to the bias windings in a direction to oppose the change in flux produced by the current to be measured.

It will be understood that the opposition of the dispositional sense of one pair of windings may be accomplished by disposing the windings of either the detector circuit or signal circuit in the same way while disposing the windings of the other opposite one another.

The effect of this is that induced emfs due to varying currents in either circuit will cancel.

In a second aspect the invention provides apparatus for measuring the algebraic sum of a plurality of direct electric currents comprising a pair of matched transformers, each having a first winding connected in a detection circuit and a plurality of second windings, said direct electric currents each being fed to a respective one of the plurality of second windings on each transformer; wherein the pair of windings in the detection circuit are substantially matching and the corresponding pairs of second windings, are substantially matching; and the windings comprising each pair of the plurality of second windings being disposed or connected in opposite senses with respect to the windings in the detection circuit, the detection circuit being operative to provide a measurement of said algebraic sum by providing an indication of the inductance of said windings in the detection circuit.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

In the drawings: Figure 1 is a circuit diagram of a basic transductor network; Figure 2 is a graph of magnetic flux density (B) against magnetic field strength (H) for a transformer core; Figure 3 is a graph of the inductance (L) of a coil of the transformer against the magnetic field strength.

Figure 4 is a circuit diagram of a circuit in accordance with the present invention for detecting the algebraic sum of a plurality of direct electric currents, and Figure 5 is a schematic diagram of part of an inter-exchange telephone signalling system.

In Fig. 1 a basic transductor network is shown and it comprises two matched transformers 1 and 2. One matched pair of windings 1A, 2A is part of a first series circuit, and the other matched pair of windings 1 B, 2B is part of a second series circuit. The second pair of windings need not contain the same number of turns as the first pair. The series windings 1A and 2A in the first circuit are disposed in opposing configuration. As a result there is no coupling between the circuits because the induced voltages in each transformer are equal and opposite.

Each circuit is equivalent to an inductor whose inductance is proportional to the magnetic flux density of the transformer cores.

Fig. 2 shows a typical B-H curve and Fig. 3 shows the change in inductancie produced in response to a change in the magnetic field in the transformer cores.

This arrangement provides the basis of a method of measuring a direct electric current, since the current through the windings of either circuit will result in a change in inductance in the windings in the other circuit, and this can be measured.

To measure the differential current in two circuits, as is required in the direct-current telephone signalling system described above, two series windings from each circuit are connected one on each transformer. These two series windings are configured oppositely as in Fig. 1 to prevent any net induced emf in the other circuit, but in addition to this the winding from the two circuits on the same transformer are oppositely configured, so that the resultant fluxes tend to cancel. Fig. 4 shows the full circuit, comprising two transformers 10 and 11 each having wound thereon one of a pair of matched coils 1 2 and 1 3 of a detector circuit; these coils being connected in series and wound in the same sense on their respective cores.Pairs of terminals 14 and 1 5 are connected to the two circuits whose differential current is to be measured. Terminals 14 are connected across two series windings 1 6 and 1 7 connected respectively on transformers 10 and 11. The winding 17 is connected in the opposite sense to winding 1 6.

Terminals 1 5 are similarly connected across series windings 18 and 19, and winding 1 9 is connected in the opposite sense to winding 18.

The polarities of the terminals in each pair 14 and 1 5 are such that currents which, in the context of the circuit in use, are considered to be in the same direction at both pairs of terminals, flow in opposing directions in each transformer. Thus at each transformer the fluxes produced by the windings will cancel each other if the currents flowing in each circuit are equal in direction and magnitude.

Any difference will produce a net flux and consequently a change in the inductance of the coils 12 and 13.

A matched pair of bias winding 20 and 21 are included one on each transformer, these windings being connected in series and disposed in opposing senses. As in the case of the other signal windings no net induced voltage is produced by the direct current flowing through these windings, but they serve to provide a bias flux level so that the change in inductance produced in the detector coils 1 2 and 1 3 by the differential current varies above or below the level set by the bias current.

The detector coils 12 and 1 3 are connected in parallel with a capacitor 22 to form a resonant circuit. A source of clock pulses 23 is connected to the resonant circuit via a resistor 24. The value of the capacitor 22 is chosen so that, with the inductance produced when only the bias current flows in the transformer coils, the resonant frequency of the circuit equals the clock frequency. By comparing the phase difference between the clock pulses on either side of the resistor 24 the inductance of the coils 1 2 and 1 3 relative to the bias level, and hence the differential current in the signal windings may be measured.

A negative feed-back method of detecting this phase difference is employed.

The phase shifted signal produced by the resonant circuit is converted into digital format by a zero-crossing detector 25. The output from this detector is fed to a phase comparator 26 whose other input is connected to the source of clock pulses 23. The comparator 26 comprises a phase locked loop, the output terminal of which is connected to a low pass filter 27. The output current from the filter 27 comprises the detector output signal.

The output current from the low pass filter 27 is also fed back to the bias windings 20 and 21 via a summing buffer 28 which also has connected as a further input a source of a bias current I. The detector output current is given by: N2 (B-A)+K N1 where N1 and N2 are the number of turns on the bias windings and signal windings respectively, A and B are the signal currents at input terminals 14 and 15, and K is a constant which can be reduced to zero by adjusting the bias current 1B The effect of this feed-back current is to maintain the flux in the transformer cores at a constant value.The phase comparator 26 detects any difference between the actual flux and this constant value and causes a current to be produced which is fed back and acts to bring the flux back to the constant value. The differential current is measured by measuring this feed-back current at the output of the low pass filter 27.

It will be appreciated that by maintaining the net flux in the coils at a constant level, good linearity can be achieved. It is preferable to choose the frequency of the clock 23 to be much higher than the highest repetition rate that occurs in the signal to be measured so that any coupling between the ac and dc circuits which may arise from mismatch in the transformers is suppressed.

Referring to Fig. 5, part of an interexchange direct-current signalling system comprises part of an outgoing relay set 30 and part of an incoming relay set 31. The operation of the differential current detection aspect of the system will be illustrated by a consideration of what happens when dialling is commenced.

The outgoing relay set includes a relay D controlled by three coils 32, 33 and 34. The connections to these coils comprise the terminals 14, 15 in the circuit of Fig. 4 and the bias windings. The arrows on the coils 32-34 show the direction of current flow which causes the switch of relay D to make with its 'high' contact. The coils 32 and 33 of relay D are connected to the two line wires 40, 41 respectively and it will be appreciated that if current flows in a loop through these two wires then current will cause opposing fluxes in the cores of the relay coils. The bias winding 34 is disposed and connected to a source of direct current such that, in the absence of current in the other two coils it will cause the switch of relay D to make with the high contact.

The exchange sides of the coils 32 and 33 are each connected via resistors to negative battery terminals. Each also has a connection on this side to a respective contact of a relay AA whose switch connects either one of these contacts to earth.

At the input of the part of the outgoing relay set shown is a relay A which controls the relay AA. The operation of the relay A is analogous to a similarly placed relay A in the incoming relay set which will now be described. The relay A also comprises a transductor circuit as described with reference to Fig. 4, but in this case the two sets of signal windings 35, 36 are disposed or electrically connected such that when a loop current flows in the line wires in a direction through windings 36 to windings 35 the fluxes produced are additive. The coils 35 and 36 are connected on their exchange side, and their junction is connected via switches 39 to earth. The contact of relay A controls a relay AA, the contact of which is at the output of the incoming relay set.

At the commencement of dialling the first break pulse causes relay AA in the outgoing set to release. The AA contact switches to the position shown and loop current flows in the line wires in the direction shown by the arrow.

The coils 32 and 33 receive current which tends to produce cancelling fluxes, and the differential current measurement circuit previously described with reference to Fig. 4 detects this. The current in the bias winding predominates and causes the D contact to remain high.

In the incoming set relay A receives the loop current in a direction such that both coils 35 and 36 act to release the relay and its contact removes the holding earth from the relay AA in the incoming set. The break pulse is thus transmitted via the relay contact AA.

When a make pulse is received at the outgoing relay, the relay AA operates and the contact AA reverses the direction of loop current. The currents through the windings 32 and 33 still produce cancelling fluxes and the bias current causes contact D to remain 'high'. Relay A in the incoming set becomes operated and a loop is extended forward. This series of make and break pulses is continued until dialling has ceased.

When the called subscribers unit answers, a reverse signal is sent to the D relay of the incoming set, and, although not shown in detail, this causes the contacts 39 connected to the coils 36 and 36 to close, thus connecting an earth to both wires. Relay D in the outgoing relay set now receives a current to coil 32 only, and a differential current exists between the windings 32 and 33. This is detected by the detection circuit as previously described and its direction is such as to cause the contact D togo to the 'low' position.

Successive switching operations then extend the reversal signal to local equipment.

Similarly the incoming relay can backwardbusy the junction circuit when not in use by closing the contacts 39. At the outgoing relay set, the contact AA is as shown. Relay D in the outgoing relay set now receives a current to coil 33 only and a differential current exists between the windings 32 and 33. This is detected and causes contact D to go to the 'low' position which results in busying the outgoing relay set.

The bias winding in the Carpenter relay does not have a counterpart in the present differential current measurement system; biasing windings in the latter functioning as feedback windings. Hence, unlike the side-stable Carpenter relay the differential current measurement system is capable of distinguishing between both positive and negative differential currents and the absence of a differential current. It can, therefore, effectively replace two traditional side-stable Carpenter relays.

Claims (8)

1. Apparatus for measuring a direct electric current comprising a pair of matched transformers each having a first winding connected in a detection circuit and a second winding connected in a circuit in which the current to be measured flows; wherein each pair of windings in each circuit are substantially matching and are connected such that the current flowing in both windings of a pair are equal, and the windings comprising one of said pairs of windings being disposed or connected in opposite senses with respect to the windings of the other pair, the apparatus further including, a pair of matched bias windings, one on each transformer, disposed in opposite senses with respect to the windings of the detection circuit, and a feed-back loop from the detection circuit to a source of current applied to the bias windings and operative to feed current, dependent upon the change in inductance in the detection windings, to the bias windings in a direction to oppose the change in flux produced by the current to be measured.

2. Apparatus for measuring the algebraic sum of a plurality of direct electric currents comprising a pair of matched transformers, each having a first winding connected in a detection circuit and a plurality of second windings, said direct electric currents each being fed to a respective one of the plurality of second windings on each transformer; wherein the pair of windings in the detection circuit are substantially matching and the corresponding pairs of second windings are substantially matching and, the windings comprising each pair of the plurality of second windings being disposed or connected in opposite senses with respect to the windings in the detection circuit, the detection circuit being operative to provide a measurement of said algebraic sum by providing an indication of the inductance of said windings in the detection circuit.

3. Apparatus as claimed in claim 2 including a pair of matched bias windings, one in each transformer, and disposed in opposite senses with respect to the windings of the detection circuit.

4. Apparatus as claimed in claim 3 including a feed-back loop from the detection circuit to a source of current applied to the bias windings and operative to feed current, dependent upon the change of inductance in the detection windings, to the bias windings in a direction to oppose the change in flux produced by the algebraic sum of said currents.

5. Apparatus as claimed in claim 1 or claim 4, wherein the detection circuit comprises a resonant circuit to which a periodically varying electrical potential is applied, and said current fed to the bias windings is controlled by the phase shift produced on said varying potential by the resonant circuit.

6. A telecommunications system including apparatus as claimed in any one of the preceding claims forming part of a current detector for operating a relay.

7. A telecommunications system as claimed in claim 6 in which the system is a direct-current, hard-wire inter-exchange telephone signalling system and said relay forms part of a telephone exchange.

8. Apparatus for measuring a direct electric current substantially as hereinbefore described with reference to and as illustrated in Figs. 4 and 5 of the accompanying drawings.