Circuit using a multi-path magnetic core with common output limb

An AND function gating circuit for A.C. signals comprising several magnetic circuits around a common limb, each circuit having a separate input winding and a single output winding on the common limb. The magnetic flux level in the common limb is greatly increased when all input windings are simultaneously energized by A.C. signals of the same frequency and in phase relative to the output achieved under other input signal conditions.

The invention relates to an A.C. "AND" Gate, and particularly to such a 
gate useful for combining the outputs of parallel redundant channels or 
circuits in a fail-safe manner. 
According to the invention there is provided a circuit for performing a 
logical AND function of A.C. signals comprising a plurality of magnetic 
circuits disposed around a common limb, a first signal winding inductively 
coupled with the common limb and inductively coupled with each magnetic 
circuit one of a plurality of further signal windings such that an output 
induced in the first winding by flux through said common limb is 
significantly greater, relative to other conditions, when all said further 
windings are energised simultaneously by A.C. signals substantially in 
phase and of the same sense. 
Preferably the common limb has an air-gap or the like providing said limb 
with reluctance value greater than the value of the remainder of the 
magnetic circuits to increase the ratio of output in the first winding. 
In one form of the invention the magnetic circuit comprises a central 
common limb and a plurality of further parallel limbs connected with the 
common limb by end sections disposed at angular intervals around the 
central limb.

Referring now to FIG. 1, there is shown in the drawing the basic "AND" gate 
according to the invention. A magnetic circuit indicated at 1 consists of 
core of magnetically susceptible material constructed in the general 
configuration of a figure of eight, having two end sections 2, 3 and three 
cross limbs 4, 5 and 6 forming a magnetic circuit with two apertures. The 
cross limbs 4, 5 and 6 carry multi-turn electrical windings 7, 8, 9 
respectively. 
For an understanding of the operation of the device consider the effect of 
an alternating signal connected to winding 7 carried by the outer short 
limb 4. The flux produced thereby is conducted around the flux path of the 
magnetic circuit by the laminated core 1, at the junction the flux divides 
between a shorter path through limb 5, carrying the winding 8, and a 
longer path through limb 6 which carries winding 9. The proportion of flux 
in each path is governed by several factors, firstly, by the reluctance of 
the centre limb and the influence of the load impedance; also the centre 
limb may have a reduced cross-section to increase its inherent reluctance, 
and alternatively, or even additionally an air gap may be provided in the 
centre limb, e.g. by means of a layer 10 of non-magnetic material. As a 
result the paths, one through the centre limb and the other through the 
peripheral limbs, will share the total flux according to the ratio of the 
path reluctances, in a typical example of a single circuit as described 
this ratio of the change in flux density of the centre limb is in the 
division of aproximately 1:4 at least and preferably is greater. 
In the case where winding 9, carried by the opposite outer limb 6, is 
connected to an alternating signal source, a corresponding flux path is 
established in which roughly one quarter of the total flux is conducted 
through each of the centre limb 5 and three quarters through the outer 
limb 4. 
Consider now the situation when both the windings 7 and 9 carry alternating 
signals of substantially similar amplitude, of the same frequency, 
substantially in phase and in the same sense. Two flux paths are 
established in the magnetic circuit and, assuming the windings to be 
appropriately sensed, of opposite circularity. Instead of the division of 
flux in each path at the junction with the centre limb 5, the combined 
effect of two opposing flux paths is to force both flux paths through the 
centre limb of the magnetic circuit, the result being to increase the flux 
density in the centre limb by a substantial factor when input signals are 
present on both windings relative to the density level present when only 
one winding is energised. 
Generally, full output is delivered to a load connected to winding 8 when 
two A.C. inputs are present on the windings 7 and 9 and a greatly reduced 
output when either of the A.C. inputs is absent. 
Thus, subject to a load connected to the output winding 8 having a suitably 
arranged threshold or level discriminating function, the magnetic circuit 
may be used to produce an "AND" gating function for A.C. signals connected 
to the windings 7 and 9. 
In a preferred construction of the magnetic circuit of the device of FIG. 
1, the circuit comprises two symmetrical halves each of which consists of 
three sets of "C" core laminations, two smaller sets in which the 
dimension of the open part of the "C" is equal to the required limb 
spacing and a larger set in which the corresponding dimension embraces 
both the smaller sets. The two halves of the circuit abut one against the 
other in the assembled device and are held firmly in postion by means of 
circumferential banding, as is well known in the field. 
An embodiment of the invention will now be described with reference to 
FIGS. 2 and 3 of the accompanying drawings. The illustrated arrangement 
forms part of a computer based railway signal interlocking system, and it 
is particularly concerned with fault detection in the control signal 
output stage. 
Referring firstly to FIG. 2 the interlocking system comprises three 
parallel interlocking processors 20, 21 and 22 which operate to produce 
serial control signal outputs in parallel. These outputs comprise serially 
multiplexed signals consisting, in respect of each control, an address 
portion and a data or information portion. In the particular system being 
described, the output of processor 20 is nominated as the preferred system 
output, the processor 22 is designated as a "hot stand-by" and is used to 
provide the system output in the event that processor 20 has developed a 
fault and its output is disqualified, and processor 21 is used purely in a 
checking procedure. The processors are interconnected by a data highway 
and exchange their outputs for the purpose of determining, by majority 
voting, if the output of processor 20 is a correct signal. The resulting 
interlocking system serial outputs are distributed to the controlled 
devices in the railway system by means of demultiplexing circuits in the 
housings 23a and 23b. These housings are identical in construction and in 
the circuits each contains, although identity is not essential. In the 
following description like parts are given like references individually 
identified by the suffixes "a" and "b". Corresponding demultiplexed 
outputs are combined or correlated by means of an AND gate of the present 
invention, the output winding of which is connected to control, by the 
signal it produces, one device. There is, therefore, for each device one 
AND gate connected to receive two corresponding demultiplexed control 
signals which operate switches 42a, 42b to pass A.C. signals which must 
correspond in frequency substantially in phase and of sufficient amplitude 
if the device is to be operated to a predetermined state. 
In the housings 23a and 23b the output of processor 20 is received by 
circuit by 24a and 24b respectively and the output of processor 22 is 
connected to similar circuits 25a and 25b which are arranged to connect 
the selected processor output for distribution to the control devices in 
the railway system. The selected serial output is circulated on a data 
highway in the housing to a multiplicity of data output cards which are 
divided into two types: 26a and 26b providing a data output "information" 
signal and 27a and 27b for providing a data output "control" output. The 
housing 23a and 23b also include a multiplicity of data input cards for 
"information" signals 28a and 28b, and for "control" signals 29a and 29b, 
these latter cards receive feed-back signals which will be further 
described below. Corresponding pairs of cards in the housings 23a and 23b 
which concern "information" are connected to modules 30 including a 
transformer AND gate of the present invention, and which provides an 
output signal to a controlled device represented by a load 31 in the 
drawing. Corresponding pairs of "control" cards in the housings 23a and 
23b are similarly connected with further modules 32, also including a 
transformer and AND gate of the present invention and which provide an 
output energising a control relay 33. This control relay 33 has a pair of 
closed-when-energised contacts connected in series with the output circuit 
of modules 30. The purpose of this will be further described below. 
The circuits of modules 30 and 32 in FIG. 2 are identical and shown in 
greater detail in FIG. 3, to which reference will now be made. In the AND 
gate modules there is a transformer AND gate, of the type shown in FIG. 1, 
indicated by reference 1, this has an output winding 8 connected via 
output terminals 40 with a remote load 31. The input windings 7 and 9 are 
connected in respective identical A.C. circuits which will now be 
described. These circuits are identical the references of individual 
components will be appropriately identified by suffixes "a" and "b". Each 
of the A.C. circuits comprises the secondary winding of a transformer 41a 
and 41b a current switching circuit 42a, 42b and current detection circuit 
43a and 43b all adapted for full wave operation and connected in a closed 
loop with windings 7 and 9. 
The switching circuits 42a, 42b include bridge type rectifier arrangements 
44a, 44b in which a first pair of diagonal bridge nodes are connected with 
adjacent circuit components and a current switching transistor 45a, 45b 
connected between the opposite nodes to provide a selectively controlled 
current path which may be switched by means of a signal on lines 46a, 46b 
applied to transistor base drive circuits 47a, 47b respectively. The 
signal lines 46a, 46b are connected to data output circuits in the housing 
23a , 23b, in the module 30 these are connected to the "information" 
circuits 26a, 26b, and in the control module 32 these are connected to the 
control circuits 27a, 27b. 
The transformer secondary windings 41a, 41b may form part of separate 
supply transformers or be supplied by a common primary transformer 
winding, either being acceptable since correct operation of the 
transformer AND gate 1 requires the supply to windings 7 and 9 of the same 
frequency and in phase. 
In series with the switching circuits 42a, 42b there is also provided, as 
mentioned, similar current detection arrangements 43a , 43b comprising two 
anti-parallel half-wave diode circuits 48a, 48b, 49a, 49b; each consisting 
of a plurality of rectifier diodes connected in series, the combination 
being connected in parallel with light emitting diodes 50a, 50b, 51a, 51b. 
The light emitting diodes 50a and 51a are paired with light sensitive 
transistors 52a and 53a respectively which are connected in parallel to 
provide a feed-back signal at terminals 54a which are connected back to 
data input circuits "information" 28a or "control" 29a from modules 30 and 
32 respectively. Similarly feed-back signals are provided at terminals 54b 
to corresponding circuits in the housing 23b. 
Referring again to FIG. 3 there are also provided test circuits generally 
indicated at 56a, 56b which are constructed and function in identical 
manner to switching circuits 42a, 42b but receive an attenuated supply 
voltage from transformer secondary windings 41a, 41b respectively. The 
transistor based circuits of these test circuits 56a, 56b receive 
operating pulses from the housings 23a, 23b for the purpose of 
periodically injecting a test signal into the transformer magnetic 
circuit, which because of the attenuation of the A.C. supply is 
insufficient to fully energise windings of 7 and 9 and thereby produce 
sufficient output at terminals 40 to clear the threshold discriminating 
level of load 31. Nevertheless this subliminal testing of the transformer 
magnetic circuit is sensed by the optical detection circuits and feed-back 
signals pass via terminals 54a, 54b to the housings 23a, 23b for 
correlation with the original test pulses to ensure that the detection and 
control gate circuits remain fully operational. 
In operation of the system, and assuming there is no fault present, 
identical control signals will appear at terminals 46a, 46b thus switching 
transistors 45a, 45b into conduction permitting A.C. current to flow 
through windings 7 and 9. Being drived from the same A.C. supply these 
signals are of the same frequency and in phase, and in the absence of 
fault, also of similar amplitude, therefore output winding 8 will be 
energised and an output signal will appear at terminals 40 being supplied 
to the load 31, providing contacts 55 of control relay are closed. Under 
identical signal conditions, described above, the module 32 behaves in 
exactly the same manner as module 30 and the output winding 8 of its 
transformer AND gate provides a current at its output to energise the coil 
of control relay 33 enabling the contacts 55 to close. The relay 33 is 
connected in series with a delay timing circuit having a relatively short 
delay but which is sufficient to accommodate discrepancy between the 
output signals provided by housing 23a and 23b under initial power-on 
conditions. Thus, there is a short delay following initial power-on before 
output signal will appear. The period of the timer is sufficent to allow 
the system to establish the absence of faults before permitting connection 
of the system output to the controlled devices. 
The circuits 28a and 29a in housing 23a, and also circuits 28b and 29b in 
housing 23b act as interfaces between 24a ad 25a and 24b and 25b which, 
correlate the feed-back signals received from the modules 30, 32 
respectively with the output signals provided by circuits 26a, 27a and 
26b, 27b thereby ensuring that the control and detection circuits are 
functioning correctly, during both normal control operation and subliminal 
test phases. 
In the event that housings 23a and 23b do not produce signals to 
transformer AND gate module 30 of substantially identical amplitude the 
output level of the control signal from winding 8 will be reduced, and at 
some level of imbalance, will fall below the threshold level of the load 
31. The AND gate magnetic circuit may be designed, e.g. by inclusion of an 
air-gap in the central limb, to have a high to low level ratio of say 
12:1. Thus a fault in the demultiplexing or output circuits is quickly 
detected by circuits 24 and 25 via light emitting diodes detectors and 
circuits 28a or 28b at the control relay is de-energised disconnecting the 
output design for slight delay before this disconnection any output for 
the AND gate would be significantly before the load threshold. 
A modification to the transformer AND gate is shown in FIG. 4 in which 
additional sense windings 60a, 60b are provided for the purpose of 
continuously monitoring the performance of the magnetic circuit, and 
therefore of the whole gated output circuit including the multiplexer 
output circuits. These sensing windings 60a, 60b may also be used in 
conjunction with the subliminal test pulses for the purpose of detecting 
faults at the earliest opportunity so that appropriate action can be taken 
for example to shut-down the fail system, and to alert to the need for 
remedial action. 
In a further variation of the circuit the winding 60a, 60b may be used for 
the purpose of test monitoring by periodic injection of a test signal into 
the magnetic circuit and observing its momentary effect of inducing 
voltages in the windings 7 and 9 of the device, this test signal may be 
pulsed or alternatively a continuous of frequency. In either correlation 
of the original test and sensed pulses is carried out in the housings 23a 
and 23b as shown in FIG. 2. In similar manner a sense or test winding may 
also be provided on the common limb of the transformer gate magnetic 
circuit. 
In an alternative embodiment of the invention the transformer AND gate may 
be employed in a railway track circit arrangement, as an alternative to a 
double element vane relay. In such an arrangement an original A.C. signal 
is connected to one of the windings 7 or 9, and a signal taken from the 
track running rails is connected to the other winding of the pair, either 
input being provided via phase adjusting circuits as necessary to 
establish the connect phase relationship. The output produced across the 
terminals of winding 8 is therefore greatest when both signals are of 
substantially the same amplitude, frequency and in phase. This amplitude 
condition only exists when the track section is unoccupied. A significant 
reduction in the track signal input will result in a drop in output level 
from the AND gate which when less than the load threshold is taken to 
indicate that the track section being occupied. The frequency and phase 
properties of the AND gate ensre that potentially interfering signals, 
e.g. traction currents, do not cause false track circuit operation, 
providing of course that such signals are not of the same frequency and 
phase as the gate of A.C. supply frequency.