GB2034990A - Phase sensitive product detector - Google Patents

Abstract

A phase sensitive detector receives two a.c. signals VL1, VT1 and actuates a relay TR if both signals are present at the same time and are in phase. The detector may include an optocoupler D2, PRC as shown to effect multiplication of the two signals. Alternatively a Hall Effect Device or varistor may be used. The detector is used for detecting the occupancy of a railway insulated track section, the first signal being received directly from an a.c. source, and the second being received from the same source via the track section rails. <IMAGE>

Description

SPECIFICATION A.C. signal product detector arrangements for railroad track circuits This invention relates to product detector circuit arrangements for detecting the presence or absence of at least one of a pair of signals and particularly to railroad track circuits employing such product detector circuit arrangements.

in an alternating current (AC) electrified railroad, severe signal to noise ratios at the receiving end of ACtrackcircuits are prevalent as a result of the AC propulsion current flowing in the rails. This propulsion current can be at very high levels when a train is close or the catenary becomes shorted to the rails. It has been design policy to consider filtering by itself to be insufficient to guarantee an adequate ratio between the track circuit signals and the accompanying propulsion noise. Therefore, a synchronous detection arrangement was developed based on a two-phase motor as the track circuit receiving device.One phase of such apparatus is fed from the track circuit receiving end by the rail current while the second phase is supplied over wayside line wires with a sample signal from the track circuit voltage source at the transmitter end of the track section.

The detection of train occupancy or non-occupancy of the track section then depends upon the shaft speed of this motor apparatus. When the motor speed is high enough, with both signals applied at the proper levels, the centrifugal force closes contacts to register an unoccupied condition of the associated track section. For this reason, the apparatus has been known in the art as a centrifugal relay. However, the proper operation of this type relay requires that one of the applied voltage signals be phase-shifted relative to the other signal, which is an inconvenience and requires special circuitry and/or apparatus. Centrifugal relays also require frequent preventative maintenance to assure proper operation. A conventional product detector also produces an output when supplied with two similar and synchronized input signals, that is, from the basic source.It is to be noted that, if a product detector is used with a railway signalling system, particularly with the corresponding track circuits, the product detector arrangement must be fail-safe. The relay operates to indicate an unoccupied track section only when both windings are energized by track circuit frequency currents having a phase angle relationship within predetermined limits.

Aceordingly, it is an object of the present invention to provide a passive circuit network for controlling a vital relay which serves as the track relay means for alternating current track circuits in electrified railroads.

According to the present invention, there is provided a product detector circuit arrangement for detecting the presence or absence of at least one of a pair of signals comprising, a first input means coupled to receive a first signal from a first source of alternating current having a preselected frequency, a second input means coupled to receive a second signal from a second source of alternating current having said preselected frequency and having a variable phase relationship with respect to said first source, AC signal product detecting means having first and second inputs connected respectively to said first and second input means and responsive only to simultaneous input of synchronized and substantially in phase signals of said preselected frequency from said first and second sources to produce at an output a DC output signal, and registry means connected to said output and responsive to the presence or absence of the output signal to register the presence of both signals or the absence of at least one input signal.

In one embodiment of the invention the AC signal product detecting means comprises a product detector circuit arrangement wherein the AC signal product detecting means comprises a light-emitting diode coupled to said first source and responsive to the first signal for emitting light radiation on each of selected half cycles of said first signal and a photoresistive device coupled to said second input means and positioned to be actuated by radiation from said light-emitting diode for producing a direct current output signal of at least a predetermined magnitude only when both said first and second signals are present, are of said preselected frequency, and are within a predetermined phase relationship.

In a further embodiment, the AC signal product detecting means comprises a product detector circuit arrangement wherein the AC signal product detecting means comprises a Hall effect device having two input and one output connections coupled respectively to the first and second input means and the registry means.

In yet another embodiment, the AC signal product detecting means comprises a product detector circuit arrangement wherein the AC signal product detecting means comprises a frequency selective varistor device in a circuit network tuned to said preselected frequency and coupling the first and second input means, the varistor device and the registry means in series.

Fig. 1 is a circuit diagram of a vital circuit arrangement including an optical coupler for registering the presence or absence of two input signals by the position of a relay element.

Fig. 2 is a chart illustrating the wave form of the output signal from the circuit arrangement illustrated in Fig. 1, which is used to energize the registry relay.

Fig. 3 illustrates schematically the use of the optical coupler circuit arrangement of Fig. 1 as a dual input track relay means in an alternating current track circuit.

Fig. 4 is a chart illustrating the relative DC output level of the Fig. 1 circuit arrangement as a function of the phase relationship between the input signals.

Fig. 5 is another chart showing the DC output of the Fig. 1 circuit arrangement as a function of the frequency of the track input signals.

Fig. 6 is a partially schematic circuit diagram of a railroad track circuit using a product detector incorporating a Hall effect device.

Fig. 7 is a schematic diagram of a simple Hall effect device usable as a product detector in the track cir cuit of Fig. 6.

Fig. 8 is a diagrammatic illustration of a modification of the product detector output portion of the track circuit arrangement of Fig. 6, required under certain track conditions.

Fig. 9 is a circuit diagram of a vital circuit arrangement including a varistor, for registering the presence or absence of two input signals by the operating position of a vital relay, which arrangement embodies the invention.

Fig. 10 illustrates schematically the use of the varistor circuit arrangement of Fig. 9 as a dual input track relay means in an alternating current track curcuit.

Fig. 11 is a chart showing the level of current flow in the Fig. 9 circuit as a function of the product of the input signal levels.

Fig. 12 is another chart illustrating the output level of the Fig. 9 circuit as a function of the phase shaft between the input signals.

Fig. 13 is another chart depicting the relative output of the Fig. 9 circuit as a function of the frequency of the track voltage signal.

In each of the drawings, like parts are given like references.

Referring to the drawings, in Figs. 3,6 and 10 the two rails of a railroad track are shown by conventional single line symbols 1 and 2. These rails are set off by the insulated joints 3 into a track section T which is electrically isolated from the adjacent track sections. This railroad is assumed to be electrified, that is, trains are powered by alternating current energy, for example, of the commercial frequency supplied from an overhead catenary wire. The return circuit for the propulsion current is, as is conventional, through the rails. However, the impedance bond connections to pass propulsion current around the various insulated joints are only shown schematically since they do not enter specifically into the arrangement of our invention and the network is well known in the railroad art.To detect a train occupying section T for safety signalling purposes, a track circuit is provided. Asource of energy S forthe track circuit is coupled to the rails at the left end of section T which thus becomes the transmitter end.

Since this is an alternating current track circuit, source S provides AC energy of a selected frequency which is different than the frequency of the propulsion power. For example, the track circuit energy may have a frequency of 100 hertz or be in the audio frequency range. Source is coupled to the rails through a track transformer TT with a limiting resistor Z placed in series in the leads from the trans former secondaryto the rails, all conventional practice. To avoid interference with track circuit operation bythe propulsion current, it is necessary to provide synchronous detection of the track current at the receiver end of section T, that is, at the right end as shown in Figs. 3,6 and 10.One existing method uses a centrifugal type track relay jointly energized by the rail current and by a sample voltage signal received direct from the track circuit source over line wires. A 90" phase shift of one signal provides the necessary torque for relay operation when both signals are present. The present invention substitutes a product detector for the centrifugal type relay or its equivalent. This product detector is illustrated by a conventional box 24, so labelled, at the right of the section. This product detector then controls a vital relay TR which registers the track occupancy condition in the same manner as a track relay directly connected to the rails.

Referring to Fig. 1, the first illustrated embodiment comprises a vital optical coupler circuit arrangement having two principal or controlling elements, a light emitting diode D2 and a photo resistive cell or device PRC. The LED element is shown by conventional symbol and has the normal expected characteristic of being actuated to emit light when current flows in the conventional positive direction through the unit.

The photo resistive cell PRC is also shown by a conventional symbol and has the characteristic of reducing its series resistance when illuminated by a light source, the reduction being to a relatively low level to allow a greater magnitude of current to flow through the corresponding circuit. As to physical mounting, the LED D2 is either a single element or a series connected cluster of high intensity diode emitters which are focused on device PRC so that the emitted light actuates the photo resistive cell to lower its resistance. Although not specifically shown in the drawings, the LED element is preferably a cluster of such units so that the photo resistive cell is actually over-driven to assure complete response.

This over-driving compensates for aging of the LED units, for output variations due to temperature changes, and provides some voltage regulation of the input signals. Subsequent references to LED D2 include the cluster arrangement.

The entire circuit arrangement is designed to detect or indicate the presence of both of two input signals VL, and VT, or the absence of at least a selected one of the two, each supplied through an input means from a selected source. These input means are transformers Ti and T2. Transformer T1 is a step-down transformer while transformer T2, of the saturable type as shown, is a step-up transformer so that the two secondary outputs VL2 and VT2 will be of the same general range under normal conditions. The corresponding instant polarity of the various windings of these two input transformers is designated in the conventional manner by the dot symbols.

The primary winding of transformer T1 is connected to a local source of AC signal energy to receive input signal VL1 which has a preselected frequency, for example, 100 Hz. The step-down characteristic of transformer T1 supplies secondary voltage signal VL2 which is of a comparable level with that supplied by the other transformer, to be discussed shortly. The secondary of transformer T1 is connected, in series with a noise rejecting band pass filter comprised of capacitor C and inductor L and series tuned to the frequency of signal VL1 to supply signal VL2 to LED D2. A resistor R in this circuit, together with the impedance of inductor L, limits the current flowing through diode D2. A conventional diode D1 is connected in parallel with the diode D2 but with opposite polarity to protect the LED against excess reverse polarity voltage in the circuit. When signal VL2 is present, which is normally continuously, diode D2 is turned on to emit light every positive half cycle, that is, when current flows through diode D2 in its low resistance direction. This LED, either a single element or a cluster, is positioned so that its output is focused on the photo resistive cell PRC. Thus the resistance of unit PRC decreases periodically to a relatively low level during each positive half cycle of signal VL2.

The primary winding of transformer T2 is con nectedto a second source of alternating current, having the same frequency as and synchronized with the local source previously mentioned to receive the second input signal VT,. In the specific example herein, this primary winding is connected to a transmission channel, e.g. the track rails, which is supplied with energy from the same central source as the local energy supply connected to transformer T1. As mentioned, transformerT2 is a step-up transformer and has saturable characteristics to provide an amplitude limiting feature for the circuit arrangement.In the principal use of this arrangement in track circuits, the saturable characteristic of transformer T2 limits excessive voltage levels of signal VT1 when an insulated joint failure allows the transmitted signal from the adjoining track section to feed direct into this receiver apparatus.

The secondary of transformer T2 supplies signal VT2, which is of the same order of magnitude normally as signal VL2, to circuit network consisting of device PRC and a biased direct current vital relay TR.

This relay, as shown, is connected between the output terminals R + and R-, for the circuit arrangement, with the polarity such that conventional current flows in the proper direction through the relay winding as designated by the small arrow therein. A current arrow I is shown associated with this circuit network in order to provide a reference for correlation with the charts in the other figures. When signals VL1 and VT1 are in phase, and of course are of the same frequency, the current I flowing through the network from the secondary of transformer T2 is shown by the solid line in Fig. 2. This is a modified alternating current of the frequency of signal VT2 and its wave form is determined by the periodic change in the resistance through photo resistive cell PRC.

This current has a direct current component shown by the dash line designated as the AVERAGE DC LEVEL, which energizes relay TR. Thus this optical coupler network acts as a synchronous rectifierto provide the DC component in current I which is of the proper polarity to energize relay TR when the input conditions or signals have the proper characteristics.

A principal use for the optical coupler circuit arrangement of Fig. 1 is in a railroad track circuit as illustrated in Fig. 3. In this drawing, a track section T of a stretch of electrified railroad is shown with its rails 1 and 2 illustrated by conventional single line symbols. The rails of section Tare electrically insu lated from the rails of the adjoining sections by the insulated joints 3, also illustrated by conventional symbols. In order to provide a return circuit for the propulsion current, impedance bond windings 4 are connected across rails 1 and 2 at each end of section T and the associated ends of the adjoining sections.

Centre taps of each associated pair of bond windings 4 are connected by a lead 5 to provide a conventional circuit path through section Tfor propulsion current.

It is here assumed that the frequency of the AC propulsion power is the commonly used 25 or 60 Hz.

A signalling system for this stretch of railroad is based on continuous train detection using an AC track circuit for each track section such as section T, as described above. At the receiving end of section T, the optical coupler circuit arrangement of Fig. 1 is connected across the rails and to the line circuit. This circuit is illustrated by a dashed block with input and output terminals designated by the same references as in Fig. 1. For example, the terminals VL, are connected across line wires 6 and 7 to receive energy direct from source S at this local location. Terminals VT1 are connected across rails 1 and 2 at the same point as bond winding 4 at this end of the track section.The track relay TR, which is of the same biased vital type as in Fig. 1, is connected across terminals R + and R- with proper polarity for energizing the relay when output is present.

Considering now the operation of the track circuit, it is to be remembered that the vital optical coupler circuit network connected within the track circuit of Fig. 3 acts as a two element AC track relay means to register the absence or presence of a train within section T. This device will also detect the presence of a broken rail within the section which interrupts the normal flow of track current. The track circuit is adjusted with minimum ballast conditions (wet weather, low resistance) so that the track and local signals (VT1 and VL1) at the receiver end are in phase.

Under these conditions, track current is approximately at the minimum level which will still pick up relay TR. In other words, the averaged DC output is at the relative 1.0 level of the charts shown in Figs. 4 and 5. Under dry weather conditions, with high ballast resistance, the track current is at a maximum level. The phase of the track signal VT1, under these maximum current conditions, leads the phase of the local signal VL1. In Fig. 4, the chart illustrates that the output DC of the optical coupler network is attenuated from its maximum value 1.0 under phase shift conditions. However, with maximum track current, the largerVT1 input signal compensatesforthe reduced multiplier function, from the phase shift curve, so that sufficient output remains to energize track relay TR. In other words, the out-of-phase attenuation is counter-balanced by the higher level of the track current Of course, the biased relay TR will not respond to a reverse polarity output, i.e., phase shifts beyond -t or -90 . Thus any extreme phase shift, or a moderate shift without increased rail current, due to a fault condition, results in the release of relay TR.

When a train occupies section T and shunts the rails, the reception of rail current at the receiver end is inhibited. With no input signal VT1 and thus signal VT2 absent, no energy is available to supply the current through the device PRC - relay TR network, even though LED D2 is periodically activated by the continuously supplied local signal VL1. Track relay TR is thus de-energized and releases to register the track occupied indication. In other words, the circuit detects and registers the absence of the one selected signal, i.e., VT1, which may also be caused by a broken rail condition.

This optical coupler circuit arrangement has extremely sharp rejection of signals at other than the track circuit frequency. This is illustrated in the chart of Fig. 5 forthe specific assumed example of a track circuit frequency of 100 Hz. It is to be noted that the circuit arrangement rejects large undesired propulsion current signals of either 25 or 60 Hz. frequency, and the common second and third harmonics thereof, without any additional filtering. Even on the amplified vertical scale used in Fig. 5, the averaged DC output at these unwanted frequencies is not measurable. This results from the natural or inherent synchronous filter characteristics of the disclosed circuit arrangement.In other words, the synchron- ous rectifier action of LED D2 and photo resistive cell PRC results in a registerable output from the arrangement only when both input signals are atthe track circuit frequency.

This circuit arrangement is also vital, that is, failsafe, since any failure in the LED drive circuit connected to the secondary of transformer T1, either an open or a short circuit, results in a decreased light pulse level. If the photo resistive cell opens, the relay current ceases. Further, should this photo cell short out, the relay will receive an alternating current and will not respond since it is a biased DC relay. A short or open circuit failure in transformer TR2 will also result in a failsafe condition, that is, the track relay releasing to indicate an occupied section. Thus any failure or circuit element fault within the arrangement results in the release of track relay TR to indicate an occupied track section, which is a safe condition.The optical coupler circuit arrangement of the invention thus provides for an improved track circuit for electrified railroads. The two element circuit network uses passive circuit elements except for the final registry track relay which is a conventional and readily available type of high reliability and low maintenance requirements. Preventative maintenance for the entire track circuit is thus reduced andthe reliability increased. The excellent phase angle and frequency response of prior art apparatus is retained so that broken rails can be detected and high level propulsion current signals rejected. This results in an efficient and economical track circuit apparatus.

In the second illustrated embodiment of Figs. 6,7 and 8, the product detector 24 comprises a Hall effect device. This Hall effect device as illustrated schematically in Fig. 7 includes a magnetizable core 15 of open rectangular shape with an air gap in the top portion of the rectangle. A winding or coil 16 is wound around one leg of this core 15 as illustrated diagrammatically in the bottom portion of the rectangle. When energized by the alternating current coil voltage Vc, an alternating flux is created which flows through core 15 and across the air gap, this flux having the same frequency as voltage Vc. The active Hall effect element is the probe plate or bar 17 shown positioned approximately midway in the air gap and which is preferably made of indium anti monide or indium arsenide.Another material which exhibits the Hall effect characteristic is germanium.

The probe voltage signal V0 is applied to the end surfaces or terminals of bar 17 as shown schematically. Voltage V0 is also an alternating current voltage having the same frequency as voltage signal Vc.

When signals V0 and V0 are synchronized and in phase, a direct current voltage is produced across the other side surfaces of bar 17 perpendicular both to the frow of current resulting from voltage V0 and the flux flow resulting from signal Vc. This produced voltage appears across the output leadsor connec- tions as an output voltage signal V,, as indicafed in the drawing. It is to be noted that the application of signals V0 and Vp may be reversed and equivalent operation obtained.

Returning to Fig 6, input V0 of the product detector is connected across the rails at the receiving end of section T through at least one filter unit 18 which is tuned to pass only current at the track circuitfre- quency. The filter output is amplified in unit 19 to assure a sufficient signal level for the product detector operation. A similar coupling through filter 20 and amplifier 21 provides a signal for the V0 input of the product detector from the track circuit voltage source 14 at the transmitter end of section T.The input network for signal V0 is shown as connected across the secondary of the track transformer at the transmitter end in the same fashion as the rails of the section but this connection can also be direct to source S with the proper limiting resistance in the circuit connections. Under fault conditions, a large amount of propulsion current will flow in the rails so that a larger than normal voltage of this same frequency may be induced in the wayside line wires, that is, in the circuit network applying signal Vc to the product detector.If a design computation or actual operation shows filtering by units 18 and 20 to be insufficient, it is possible to add additional filters 22 and 23, shown by conventional dotted blocks, immediatelyadjacentthe inputstothe product detector and to make amplifiers 19 and 21 of the type which will limit the gain if the input reaches a level only slightly larger than normal. By this arrangement, the overpowering of the product detector by large propulsion currents flowing in the rails and induced into the wayside circuits may be overcome.

The track circuit network shown in Fig. 6 operates as follows. With no train occupying any portion of section T, signals V0 and V0 are applied at the product detector inputs. If the applied signals are in syn chronization.and at the proper levels, the product detector 24 produces an output signal V0 which is a DC voltage to energize track relay TR. This relay picks up to register an unoccupied section T, that is, provides a non-occupancy indication. The operation is the same with only a single filter and amplifier network at each input or with thetwo filter and limiting gain amplifier network specifically illustrated.

When a train occupies section Tand shunts the rails, there is no signal V0 input to the product detector.

Since the unit requires both inputs to produce an output, signal V0 disappears and relay TR is deenergized and releases to register an occupied track section. If an insulated joint 13 fails, that is, breaks down and no longer isolates the adjacent rail sections, the track current will have an added compo next from the adjacent section so that signals V0 and V0 are not synchronized. Again, this results either in no output signal V0 or a signal with a large AC component so that relay TR releases, in a failsafe manner, to indicate an occupied track section whether or not a train actually is within the area. It is to be noted that there is no failure mode of the Hall effect device which will produce a direct current output from a single alternating current input.In other words, any fault or failure in bar 17 (Fig. 7), including physical damage, will produce only an alternating current output or no output so that relay TR cannot be improperly energized to register a non-occupied indication in an unsafe manner.

There are other uses in railway signalling for the product detector arrangement illustrated, that is, the filter, amplifier, Hall effect device, and relay TR network of Fig. 6. For example, it may be used to detect defective insulated joints. If the arrangement or apparatus isto be used for the joints 13 at the right of section T, a circuit path to input connections V0 of the product detector is connected across the joint 13 in rail 11, that is, between rail 11 in section T and the adjacent section to the right. The circuit path to the input connections V0 of the product detector is then coupled across the joint 13 in rail 12 in a similar manner.Normally, a potential exists across each insulated joint since the polarity of the alternating current supply voltage to the rails is reversed from section to section for safety reasons. Therefore, with two good insulated joints, a pair of synchronized inputs is provided to the product detector which results in the production of output voltage V0 to hold a relay such as TR energized. If either joint fails, there is no potential across that particular joint so that there is no corresponding input to the product detector. The absence of one input cancels the generation of output signal V0 and causes the release of the detector relay to indicate that there has been a failure of one or the other of the insulated joints at that location.

It has also been discovered that, in many track circuit installations, it is not practical to drive the track relay TR direct from the product detector as in Fig. 6.

Normally the two inputs to the product detector, that is, the Hall effect device, are adjusted for minimum pickup of relay TR under wet track conditions, that is, when the rail current at the receiver end is at its minimum level. Dry track conditions may then cause an increase in signal Vp to a level which causes excess heat to develop within the Hall device which will destroy or at least damage the element. Under this situation, the Hall device must be operated at low level currents in the coil and probe circuits and an additional electronic detector network inserted between the output and the track relay. Such a network is shown schematically in Fig. 8. The heart of the arrangement is the low energy transistorized oscillator element using a super 'B' transistor such as type 2N5962 or 2N5210, available in the commercial market.This oscillator element is supplied with a low level DC input signal V0 from the product detector as the driving voltage. A conventional transistor oscillator circuit, for example, a tuned collector type with a bypass filter to eliminate the propulsion current and/or other ripple components in signal VO, may be used but must be a vital circuit for failsafe purposes. The output from the oscillator is then amplified and rectified and applied to a level detectorto drive track relay TR, as shown by the conventional symbols in Fig. 8. The amplifier and level detector circuitry must also be vital, failsafe arrangements for use in the track circuit.

Operation of the resulting arrangement is similar to that of Fig. 6. When both track and line inputs to the product detector exist, it produces a DC signal V0 which is applied to the oscillator element. The oscillator produces an output which is amplified and rectified and actuates the level detector to energize relay TR to register a non-occupied indication. If there is no output from the product detector, for example, signal V0 is absent because of a train shunt of the rails, there is no oscillator output signal and relay TR releases to register an occupied section T.

In the third illustrated embodiment of Figs. 9, 10, 11, 12 and 13, the product detector 24 comprises a vital power varistor circuit arrangement. The two input transformers T1 and T2 are used to couple signals from two energy sources into the operating circuit network. The primary of transformer T1 is connected across the local source of alternating current energy to supplythe received input signal designated as signal VL1 and, in a commonly used signal system, its frequency is 100 Hz. Again, transformer T1 is a step-down transformer so that its secondary voltage signal is of a comparative level or range with that provided by the other input means.

Transformer T2 is of the saturable type and is a step-up transformer with its primary winding connected to the second signal source, supplying an input signal VT,,through a noise rejection filter path comprising the inductor L2 and the capacitor C2. The resulting series LC filter element, including this inductor and capacitor and the primary winding of transformerT2, provides a band pass arrangement tuned to the signal source frequency. Signal VT,, as previously described, is supplied from a distant point, by the same central source that supplies signal VL1 over a transmission means extending between the two locations.The filter element, inductor L2, capacitor C2 and the primary of transformer T2, is tuned to the 100 Hz. frequency to limit excessive noise amplitude which results, at least partly, from the electrical propulsion current flowing in the rails of the track section. To swamp outthe nonlinear loading influence of the varistor element, which will be discussed shortly, a heavy duty, single turn winding with a short circuit connection is added to transformer T2 to present a low impedance load to the transformer under all conditions. As previously noted, transformer T2 is a step-up transformer whose output signal VT2 from the secondary winding is of the same order of magnitude as signal VL2 under normal conditions in the input signals, for example, from the track circuit and the local source.

The saturable characteristic of transformer T2 limits the input signal when excessive voltage levels occur in the second source. By way of specific example, this situation may occur, in track circuit use, when an insulated joint failure allows the transmitted signal from the adjoining track section to feed direct into the receiving apparatus.

The secondary windings of transformers T1 and T2 are connected in series to supply the sum of the input signals VL2 and VT2 to energize the remainder of the passive circuit network. This series network includes a series tuned band pass filter element comprising inductor L1 and capacitor C1 and tuned to 100 Hz. In this specific example, a varistor device RV of the metal oxide type, the input terminals of a full-wave rectifier Q, and an instrument type fuse F.

A vital type, biased, direct current relay TR is connected to the output terminals of rectifier Q, designated by the symbols R+ and R-, to complete the circuit network. Relay TR is the registry means for the input signals when they have subsequently discussed characteristics. A current arrow I, shown between varistor RV and rectifier Q, designates a flow of current in this circuit network to provide a reference to relate circuit operation to the charts of Figs. 11,12 and 13.

The voltage (V) - current (I) characteristics of the varistor are illustrated by the dashed curve shown in Fig. 11. The associated solid curve illustrates the V-l characteristics of the varistor plus other circuit network resistances. In the presence of a local input1 only, the output of the transformer T1 secondary, the signal VL2, is applied to the circuit network and produces only the small output currentvector8 shown immediately to the left of the vertical axis of the chart of Fig. 11. When the second or track input is also active, an equal amplitude voltage is produced atthe transformerT2 secondary and this signal VT2 is applied to the network relatively in phase with signal VL2.The two voltage vectors add in phase, as illustrated by the designated symbols in the lower right quadrant of the chart, thus shifting to the dynamic resistance part of the curve to produce the large current vector 9 shown to the left of the vertical axis line. In other words, a result of the non-linear circuit transfer characteristics of the varistor, adding the two input voltage vectors produces a total current roughly proportional to the voltage product As another means of understanding this action, this calculation is performed similarto the manipulation of a conventional slide rule wherein logarithmic distances are added to provide a product. The actual current through the biased relay TR then is a pulsed direct current produced by the full-wave rectifier 0.

Said in another way, this varistor circuit network acts as a synchronous detector having a direct current output relationship, with respect to the relative phase angle between the input signals, illustrated by the chart of Fig. 4.

In considering the operation of the track circuit of Fig. 10, it is to be remembered that the vital varistor network shown in Fig. 9, when connected within the track circuit of Fig. 10, acts as a two-element alternating current track relay device. The track circuit is adjusted under minimum ballast conditions (wet weather and low ballast resistance) so that the track and local signal currents are in phase at the receiver end. Under these conditions, track current is near the minimum level requried to pick up relay TR. That is, direct current output from the dashed block is at the relative 1.0 level shown in the charts of Figs. 12 and 13, the peak of each curve.With no train shunt or broken rail in section T, the VL2 and VT2 signal vectors (Fig. 11) are both present and add in phase so thatthecurrentfunction is art a high level on the curve in the upper right quadrant and sufficient energy is output to pick up relay TR. When a train occupies section T and creates a shunt between the rails, signal VT2 is absent and signal VL2 alone produces insufficient output energy, since the current I is on the low, flat part of the varistor curve, so that relay TR releases to register an occupied track condition. When ballast resistance ofthe section is high (dry conditions) the track current and thus signal VT is at a maximum level.In addition, the phase of the track current leads that of the local current supplied from circuit 6,7 (signal VL2) so that the direct current output is attenuated, that is, the function shifts to the right, on the chart of Fig. 12, along the curve to a relative position less than 1.0. However, the larger VT1 input signal under dry ballast conditions compensates for the reduced multiplier function, from the phase shift curve, so that sufficient output still exists to energize relay TR to pick up to register the unoccupied condition of section T.

This varistor circuit arrangement also has the ability to reject high level track signals at the propulsion frequency, as shown in Fig. 13. This chart is based on the assumed specific condition of the use of the common 100 Hz. track circuit. As shown in the chart, the varistor circuit network response peaks at this frequency. However, both the solid normal and dashed high track input signal level curves illustrate a shart rejection, that is, a reduction of the direct current output, at both 25 Hz. and 60 Hz. propulsion power and the common second and third harmonics of these frequencies. Even with five times the normal signal level input from the track circuit at 120 Hz, the second harmonic of the proposed 60 Hz. propulsion current, the relative direct current output is reduced below the 0.5 level of its peak at the tuned frequency of100Hz.

The varistor circuit network is considered vital for several reasons. Any input transformer failure, either an open or short circuited winding, results in a decreased output signal level into the varistor network. The single shorted turn of transformer T2 is considered vital since, because of the large size wire used, it cannot open circuit. An open or short circuit failure in inductors L1 or L2 or capacitors C1 or C2 results in decreased relay current. The varistor also consists of a bulk material which, like a resistor, can open but cannot short circuit. To insure that the varistor cannot be electrically abused and its characteristics altered, the instrument fuse F is used in the circuit network for current limiting purposes. If a bridge diode in rectifier 0 open circuits, the relay experiences a decrease in the energy supplied. If a diode short circuits, the resulting alternating current output decreases the average direct current level and causes the biased relay to release.

Claims (17)

1. A product detector circuit arrangement for detecting the presence or absence of at least one of a pair of signals comprising, a first input means coupled to receive a first signal from a first source of alternating current having a preselected frequency, a second input means coupled to receive a second signal from a second source of alternating current having said preselected frequency and having a variable phase relationship with respect to said first source, AC signal product detecting means having first and second inputs connected respectively to said first and second input means and responsive only to simultaneous input of synchronized and substantially in phase signals of said preselected frequency from said first and second sources to produce at an output a DC output signal, and registry means connected to said output and responsive to the presence or absence of the output signal to register the presence of both signals or the absence of at least one input signal.

2. A product detector circuit arrangement as claimed in Claim 1, wherein the AC signal product detecting means comprises a light emitting diode coupled to said first source and responsive to the first signal for emitting light radiation on each of selected half cycles of said first signal and a photoresistive device coupled to said second input means and positioned to be actuated by radiation from said light emitting diode for producing a direct current output signal of at least a predetermined magnitude only when both said first and second signals are present, are of said preselected frequency, and are within a predetermined phase relationship.

3. A product detector circuit arrangement as claimed in Claim 1, wherein the AC signal product detecting means comprises a Hall effect device having two input and one output connections coupled respectively to the first and second input means and the registry means.

4. A product detector circuit arrangement as claimed in Claim 3, wherein the Hall effect device has coil and probe input connections coupled alternatively to the first and second input means.

5. A product detector circuit arrangement as claimed in either Claim 3 or Claim 4, and further including a low energy oscillator circuit means coupled to said Hall effect device output connection and activated for generating a signal only in response to a direct current signal received from said device, and a circuit network coupled for amplifying and rectifying a signal generated by said oscillator means and for driving said registry means.

6. A product detector circuit arrangement as claimed in Claim 1, wherein the AC signal product detecting means comprises a frequency selective varistor device in a circuit network tuned to said preselected frequency and coupling the first and second input means, the varistor device and the registry means in series.

7. A product detector circuit arrangement as claimed in any preceding claim, wherein said second source comprises transmission means over which said second signal is supplied from said first source.

8. A product detector circuit arrangement as claimed in Claim 7, wherein said transmission means comprises the rails of an insulated track section of a railroad.

9. A product detector circuit arrangement as claimed in Claim 8, wherein the railroad is an electrified railroad and the rails also carry propulsion return current of a frequency different from said preselected signal frequency.

10. An arrangement as claimed in any of Claims 8 and 9, connected to function in operation as a railway track circuit to detect whether the track section is occupied or unoccupied.

11. An arrangement as claimed in any preceding claim, wherein the registry means comprises a vital, biased, direct current relay operable between a first released, or normal, position when de-energized corresponding to the absence of at least one of the input signals, and a second energized position corresponding to the presence of both input signals.

12. A railway track circuit arrangement as claimed in Claim 11, when dependent on Claim 10, wherein said registry relay registers an unoccupied track section only when energized.

13. An arrangement as claimed in any preceding claim, wherein said first input means is a transformer connected to couple said first source to said detector circuit, and said second input means is a saturable transformer for coupling said second source to said detector circuit.

14. An arrangement as claimed in Claim 13, wherein the saturable transformer includes a shortcircuited single turn winding effective to provide a constant low impedance load for amplitude limitation of the signal induced in the saturable transformer secondary winding.

15. A railway track circuit arrangement, substantially as described herein with reference to Figs. 1, 2, 3,4and5.

16. A railway track circuit arrangement, substantially as described herein with reference to Figs. 6,7 and 8.

17. A railway track circuit arrangement, substantially as described herein with reference to Figs. 9, 10,11, 12 and 13.