US3752426A - Train detector - Google Patents

Abstract

A track monitoring circuit for detecting the presence of a train on a section of track by means of a pair of separated current transformers whose primary circuits are coupled to a portion of a common rail in either side of an audio frequency transmitter coupled across the track. The separation of the transformers is greater than the distance between the first and last wheels of one truck of a railway vehicle but less than the distance between the last wheel of the leading truck and the first wheel of a rear truck. The output of the transformers are converted respectively into first and second binary logic signals which are fed to first logic circuit means for providing an output signal indicative of the NOT AND function. The output of the first NOT AND logic circuit means is fed into two flip-flop circuits through a differentiator. Each flip-flop is respectively triggered by opposite edges of the output pulse of the first NOT AND logic gate. The outputs of these flip-flops are fed to a second NOT AND logic circuit means for providing an output which is adapted to activate suitable utilization or indicating means.

Description

United States Patent I191 Pal [ Aug. 14, 1973 TRAIN DETECTOR [75] Inventor: Ajoy Kumar Pal, Downers Grove, Ill.

73 Ass ignee: Portec, Inc., Oak Brook, 111.

[22] Filed: May 7, 1971 [21] App1.No.: 141,270

[52] U.S. Cl. 246/249, 246/40 [51] Int. Cl B6ll 1/02 [58] Field of Search 246/34 CT, 40, 125-130, 246/34 R, 249

I 56] References Cited UNITED STATES PATENTS 3,333,096 7/1967 Ohman et al. 246/34 CT 3,471,689 10/1969 Wetmore 246/125 2,993,116 7/1961 Utt 246/34 CT FOREIGN PATENTS OR APPLICATIONS 915,701

7/1954 Germany 246/34 R Primary Examiner-Gerald M. Forlenza Assistant Examiner-George I-l. Libman Attorney-Emory L. Groff and Emory L. Groff, Jr.

[5 7] ABSTRACT A track monitoring circuit for detecting the presence of a train on a section of track by means of a pair of separated current transformers whose primary circuits are coupled to a portion of a common rail in either side of an audio frequency transmitter coupled across the track. The separation of the tran sformers is greater than the distance between the first and last wheels of one truck of a railway vehicle but less than the distance between the last wheel of the leading truck and the first wheel of a rear truck. The output of the transformers are converted respectively into first and second binary logic signals which are fed to first logic circuit means for providing an output signal indicative of the NOT AND function. The output of the first NOT AND logic circuit means is fed into two flip-flop circuits through a differentiator. Each flip-flop is respectively triggered by opposite edges of the output pulse of the first NOT AND logic gate. The outputs of these flip-flops are fed to a second NOT AND logic circuit means for providing an output which is adapted to activate suitable utilization or indicating means.

9 Claims, 3 Drawing Figures l8 I numo FREQ. TRAIN 3| 2e TRANSMITTER IO 24 29 RAIL gn L1 |e-l w 30 SIG/8 f? bzsl l4 -34 |2 32 L22 L ML BRIDGE A B BRIDGE R RECTIFIER RECTIFIER 0) L FLIP u. FLOP 66 TRAIN DETECTOR BACKGROUND. OF THE INVENTION 1. Field of the Invention This invention relates to railway track apparatus and more particularly to means for detecting occupancy of a predetermined stretch of railway track.

2. Description of the Prior Art The variousknown techniques for detecting the presence of'a train along a stretch of railroad track generally utilize either an audio frequency current applied directly to the rails or an audio frequency current supplied to an AC bridge circuit embedded. in the track. The former'checks whether or not asufflcient current is flowing through receiver units placed at each end of the detectingzone and additionally includes electromechanical relays which remain energized so long as the current flowing therethrough exceeds a predetermined threshold. These electromechanical relays are adapted to become. deenergized when car wheels and axles shunt the received terminals. The latter measures the inductance of a detector loop which is embedded in the track. The detector loop is approximately-of the same length-as the detector zone. Whenever a car or a'locomotiveapproaches the detector loop, the inductance of the loop increases gradually. The presence of a. car or a locomotive will be indicated when the value of this inductance exceeds. a' predetermined threshold value.

The-limitation of the first technique Iies mainly in the also prone to provide false indication inasmuch as.

other factors can influence the change in-the loop inductance apart from the presence of a railway vehicle.

SUMMARY I Recognizing the limitations of the prior art, the present invention discloses and claims an improved system for the detection of a railway vehicle in a predetermined tracksection. The invention comprises, inter alia, a pair of AC shunts coupled across the rails at a first and-second point along the track. An audio frequency transmitter is coupled across the track rails intermediate the shunts and preferably halfway therebe tween, and applies a constant current, sinusoidal signal thereto. A first anda second current transformer has its primary winding coupled in parallel to a portion of a rail at a track joint respectively on either side of the audio frequency transmitter. The primary winding is of such a nature that it will offer less resistance and inductive reactance to the audio frequency current transmitted along the rail. The output winding of both transformers then provides an output-signal which is dir'ectly proportional to the current flowing through the rail.

least equal to or greater than the distance between the first and last wheels of one truck of a railway vehicle such as a locomotive or freight car and less than the distance between the last wheel of the leading truck and the first wheel of the next or following truck. AC rectifier means are coupled to each secondary winding to respectively provide a DC output of a first magnitude when the wheels of a railway vehicle do not shunt the primarywinding and-a second magnitude when the primary winding is shunted thereby. Thus a binary logic v The separation between the two primary windings is at signal output is provided from each rectifier which is then coupled into a first binary logic circuit means providing the NOT AND or NAND logic function. Two bistable or flip-flop circuits are coupled to theoutput of the first logic circuit through a differentiator. The respective outputs of the two flip-flops are connected to a second NOT AND or NAND function circuit which provides an output pulse which control suitable circuit means coupled thereto for providing either an indication of track occupancy or for operating suitable railway protection devices.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 isa block diagram illustrative of a preferred embodiment of the subject invention;

FIG. 2is=a diagram-illustrating the relative separation distance between transformers in relation to the wheels of a railway vehicle; and

FIG. 3 is a timing diagram of voltage waveforms produced by the embodiment shown in FIG. I which are illustrative of the operation of the subject invention.-

DESCRIPTION OF THE PREFERRED EMBODIMENT Referringnow to the drawingsand more particularly to FIG. 1, there is disclosed a portion of railway trackcomprised-of the rails 10 and 12 across which is connected apair of AC shunts l4 and 16. The shunts 14 and 16. are both comprised of a series circuit including an inductance and a capacitance which have a minimum value impedance at their series resonant frequency as-determined by their respective component values. Intermediate the AC shunts l4 and 16 and preferably midway therebetween is an audio frequency transmitter 18 which is coupled across the rails 10 and 12 at the connections 20 and 22. The resonant frequency-of the AC shunts l4 and 16 is chosen to be substantially equal to the frequency of the audio frequency output from the transmitter 18 which comprises a sinusoidal substantially constant current AC signal. The signal current for example coupled to the rail 10 flows in the opposite-directions along the rail 10 through the shunts 14 and 16, returning to the audio frequency transmitter 18 along rail 12. The maximum distance of the audio frequency current flow through the rails 10 and 12, isthus limited by the position of the shunts l4 and 16 on the track.

A first and a second current transformer" is coupled to the rail 10 on either side of .the transmitter connection 20 within the track boundary determined by the shunts 14 and-16. The transformers 24 and 26 are located preferably substantially equal distances away from the connection 20 and are preferably identical in configuration. The term current transformer" is meant to designate that type of electrical transformer wherein the voltage appearing across its secondary winding is proportional to the current flowing through its primary winding. Accordingly, the primary windings 28 and 30 of the transformers 24 and 26, respectively, are connected'as parallel branch elements to the rail 10 at the location of the joint sections 29 and 31 which may be insulated or regular bar members. The primary windings are comprised of sheet metal which exhibits relatively less resistance and inductive reactance to the audio frequency. currenttransmitted along the rail 10 from the transmitter 18. Therefore substantially all of the current in the two loops will flow through windings 28 and 30. To this end the primary windings 28 and 30 are further illustrated as being single turn primary windings having relatively less resistance to current flow than the rails themselves. The secondary windings 32 and 34 of the transformers 24 and 26, respectively, are comprised of multi-turn windings which are coupled into AC rectifiermeans 36 and 38 which are each comprised of, for example, full-wave bridge rectifiers. Such circuits are well known to those skilled in the art and are comprised of interconnected solid state devices such as semiconductor diodes.

The separation of the current transformers 24 and 26 is designated by a distance d in FIG. 1 with the transmitter connections 20 and 22 being located at the midpoint therebetween. Referring now briefly to FIG. 2, the distance d is selected to be at least equal to but preferably greater than the distance d which is the distance between the first or leading wheels 40 and the rear wheels 42 of the leading truckof a railway vehicle 44 which may be for example a locomotive but less than the distance d which is the distance between the rear wheels 42 of the leading truck and the leading wheels 46 of the following truck. The distance d denotes the distance between the aforementioned trucks and is shown measured between the rear wheels 42 and 48.

Referring now back to FIG. 1, as long as no train occupies the section of track between the current transformer 24 and 26, the bridge rectifiers 36 and 38 provide a DC output of a constant or fixed magnitude since the transmitter 18 is operating in a constant current mode. However, whenever the wheels of a train pass the location of either of the transformers 24 or 26, the respective primary windings 28 and 30 will be shunted by means of a short circuit provided by a pair of wheels respectively in contact with the rails and 12 and the axle upon which the wheels are mounted. When this condition occurs the output of the respective bridge rectifier 36 and 38 falls to zero. Such an output corresponds to one of two possible binary logic states while the non-shunted condition constitutes the other binary state. Accordingly, a binary output signal A appears on circuit lead 50 from the bridge rectifier means 38 which is coupled to one input of an AND circuit 52 whose other input constitutes the binary output signal B from the bridge rectifier means 36 which appear on circuit lead 54. The AND gate 52 is a well known logic circuit and is often referred to as a coincidence gate. It operates to provide an output, for example, a binary 1 signal only when the inputs A and B are also a binary l simultaneously, however, if either of the inputs is a binary 0, the output A'B will be a binary 0. The output of the AND gate 52 corresponding to the logic signal A-B appears on circuit lead 56 and is applied to a logic inverter or NOT circuit 58 which provides an output which is the complement of the logic functionA'B or NOT AB and which is expressed simply as H. The combination of the AND circuit 52 and inverter or NOT circuit 58 when desirable can be configured as a single NAND logic gate 60 which is also well known to those skilled in the art.

The complementary output A 'B appears on circuit lead 62 and is applied to a differentiator circuit 64 which provides an output comprised of two spikes or triggers signals of opposite polarities (positive and negative). This output is applied to the diodes 66 and 68 which are connected to bistable multivibrators or flipflop circuits 70 and 74 respectively. Due to polarity connection of the diodes, flip-flop 70 receives a positive trigger while flip-flop 74 receives a negative trigger which cause the respective circuits to change state each time a trigger is coupled thereto. The outputs of these flip-flops designated as Z and Z appear on circuit leads 72 and 76 and are fed to a second digital logic NOT AND circuit comprised of a second AND gate 78 which provides an output on the lead 80 corresponding to the Boolean function Z 'Z and a NOT gate 82 which provides the complement. The signal NOT Z Z appearing on circuit lead 84 is adapted to control a relay 86 which in turn actuates a mechanical switch assembly, not shown, in the railroad classification yard or a railroad crossing gate and/or warning indicator apparatus.

The operation of the preferred embodiment of the subject invention shown in FIG. 1 can best be understood when considered in conjunction with the waveforms shown in FIG. 3 and the relative distances shown in FIG. 2. Also a binary l is defined as the state of signals A or B whenever there .is a DC output of predetermined amplitude from the respective bridge circuits 36 and 38 indicative of an unshorted primary winding. A binary 0 designates a zero or no DC output which is indicative of a shorted primary winding.

Normally when there is no railway vehicle within the detecting zone corresponding to the distance d shown in FIGS. 1 and 2, current transformers 24 and 26 will be energized by an audio frequency AC signal current transmitted from the transmitter 18 and consequently the output signals A and B from the bridge rectifiers 38 and 36, respectively, will exhibit a binary I state. The logic signal appearing at the output of the NOT circuit 58 is shown by waveform 88 of FIG. 3 and indicates that it is in a binary 0" state. This corresponds to the time t Additionally, the output states of the flipflop circuits 70 and 74 as shown by waveform 92 and 94 of FIG. 3 indicates that at the time t both flip-flops are in a binary 1" state as opposed to a binary 0" state. The flip-flop circuit 70 is responsive to the leading edge (positive trigger) of the input signal 90 whereas the flip-flop 74 is responsive to trailing edge (negative trigger) of the signal 90 and both change states in accordance therewith. The signal 90 represents the output of the differentiator 64. The output of the logic inverter circuit 82 is illustrated by waveform 96.

The following truth table is helpful in understanding the operation of the logic circuitry as it pertains to the detection of a train within the zone defined by the transformers 24 and 26.

TABLE I TRUTH TABLE Condition 1 Condition 2 Condition 3 Condition 4 Condition 5 Condition 6 Condition 7 Condition 8 Condition 9 When the zone between transformers 24 and 26 is unoccupied (condition 1 the signals A and B are both I whereupon the logic function FD as referenced by waveform 88 shown in FIG. 3 is a 0. Waveforms 92 and 94 signifying Z andZ remain in a l state.

When a train enters the detection zone (condition 2) from left to right as shown in FIGS. 1 and 2, the wheels 40 and axle not shown associated therewith shorts out the primary winding 30 whereupon signal A switches to binary while signal B-remains in a binary l state. The signal A E however, now is a binary l as indicated by waveform 88 at the time,t This state will continue untilvthe wheels 40 reach the connections 20 and 22 (condition 3) whereupon the signals A and B both become a-binary 0. As the front wheels of the leading truck advance beyond the connections 20-and 22 (condition 4), signal B will remain a logic 0; however, signal A'will become a binary 1 unless the rear wheels 42 of the leading-truck have in the meantime shorted primary winding 30, whereupon condition 2 and the 3 will again obtain. When both wheels 40 and 42 of the leadingrtruck pass beyond the primary winding 28, signals A and B will again revert to a. logic fl" as shown by condition-5 of the truth table and which corresponds to the time t shown in FIG. 3. Condition 5 of the truth table will exist until the front wheels 46 of the following truck short out the primary winding 30 whereupon conditions 6 occurs, which is identical to'condition 2, and signal A B'a'gainassumes a binary 1 value until the rear truck'cle ars the zone (condition 9) whereupon the rear wheels 48 no longer short out the primary winding 28. This occurs at the time 4 shown in FIG. 3.

Since the flip- flop circuits 70 and 74 are-respectively made responsive to the leading and trailing edge of the ing zone to the right; whereas the flip-flop 74 remains at this time in a l state. Consequently, Z wouldbe a binary *1 .f When the leading truck 'clears out the 'dete'cting zone, the pulse A? drops down to 0" resulting in the'change of state of the flip-flop'74 to a binary 0 state from a l-at time t,. Since the second pulse of waveform 88' isgenerated at time as the rear .truck including the wheels 46'and 48 travel over the detecting zone. The flip-flop 70 switches-to a the flip-flop 74 remains in a 0" state and thereby the pulse Z will remain in a binary l The trailing edge of the second pulse corresponding tothe time t does not occur until the rear wheel 48 of the second truck clears the primary winding 28. At this-time the-flip-flop circuit 74 is triggered back to its l state, whereas the flip-flop 70 is in a 1 state and-the pulse 2 will come back to fO as shown in the FIG. 3.

Thus an energizing signal is coupled-from the NOT gate 82 to, forexample, the relay 86 from a timewhen the first or leading truck enters the detectingzone after passing over the transformer 26 until the rear wheels'of the second or rear truck have cleared the detecting zone on the right of transformer 24. The set-reset" operations of the flip- flops 70 and 74 controlling the on-off switching-of the relay 86 with the help of aNQT AND logic circuitry shown in FIG. 1 can be utilized not only for a railroad crossinggate activation system, but is equally adapted to be used in a railroad classification yard where it is" desirable to countthe number of train cars. This can easily be done knowing the number of trucks in each car. I

,What has been shown and described, therefore, is a simple train detection system utilizing digital logic circuitswhich are particularly adapted to be fabricated state' whereas i from solid state devices which will providefar greater reliability than all other presently known circuits for such detection.

Having disclosed what is at present considered to ,be

the preferredtembodiment of the subject invention,

I claim as my invention: I w

1. Apparatus for detecting the presence of a railway vehicle within apredetermined. track zone comprising in combination:

a section of track including a pair conducting electrical current;

first and second AC shunt means coupled across said rails and being separated a predetermined distance onsaid track section;

a source of alternating current of a selected frequency coupled across said rails intermediate said first and second shunt means;

a'first and second electrical transformer each having a primary winding and a secondary winding and includingmeans for coupling the primary winding of said transformers in parallel with separate rail'portions intermediate said first and second shunt means on opposite sides from the point of coupling of said source of AC current across said rails and being separated by a distance at least equal to the distance between the first and last wheels in one truck of a railway vehicle and less than the distance between the last wheel of the leading truck and the firstwheel of a rear truck of said vehicle, said transformers being adapted to provide an AC output signal across respective secondary windings of a signal transmitted from said source but becoming deenergized by the shorting effect of the train wheels across said rails anywhere between the respective primary winding and the point of coupling of said AC source to said rail;

first andsecond rectifier means respectively coupled of rails capable of acrosssaid secondary winding of said first and second' transformer providing a DC output of a first magnitude when the respective primary winding is energized from said source and a DC output of a secondmagnitude when said primary winding is deenergized;

first NAND'digital logic circuit means having a pair of inputs respectively coupled to the outputs A and B of said first and second rectifier means and providing alogic output signal corresponding to F3;

a differentiator circuit coupled to the logic output signal of said first NAND logic circuit means and providing a positive and a negative going trigger signal;

first and second bistable circuit means coupled to said differentiator circuit and including means for being respectively triggered by said positive .and negative going trigger signals to produce a respective first and second square wave output signal Z and. Z

second NAND digital logic circuit means having a pair of inputs respectively coupled to said first and second square wave output signals of said bistable circuit means and providing a logic output signal m Z for operating external utilization and indicating means.

, 2. The invention as defined by claim 1 wherein said primary winding of said first and second transformer exhibits relatively less electrical resistance than the rail portion to which it is coupled in parallel so as to carry substantially most of the current flowing in the rail from said source.

3. The invention as defined by claim 2 and wherein said secondary winding of said first and second transformer provides an AC voltage thereacross which is proportional to the current flowing through said respective primary winding.

4. The invention defined by claim 3 wherein said source of AC current comprises an audio frequency transmitter operating in a constant current mode.

5. The invention as defined by claim 1 and wherein said first and second bistable circuit are comprised of flip-flop circuits.

6. The invention as defined by claim 1 wherein said track includes a first and second rail joint respectively located on opposite sides of the coupling point of said source of AC current and wherein said first and second primary winding are respectively coupled across said first and second rail joint.

7. The apparatus as defined by claim 1 wherein said source of alternating'current is coupled across said rails midway between said first and second electrical trans former.

8. The apparatus as defined by claim 1 and additionally including first and second diode means respectively coupled between aid differentiator circuit and said first and second bistable circuit meansand being conductive in mutually opposite current directions to respectively couple said positive and negative trigger signals to said first and second bistable circuit means.

9. Apparatus as defined in claim 1 wherein said first and second NAND digital logic circuit means each comprises a dual input AND logic gate circuit and a logic inverter circuit coupled to the output of said AND gate.

Claims (9)

1. Apparatus for detecting the presence of a railway vehicle within a predetermined track zone comprising in combination: a section of track including a pair of rails capable of conducting electrical current; first and second AC shunt means coupled across said rails and being separated a predetermined distance on said track section; a source of alternating current of a selected frequency coupled across said rails intermediate said first and second shunt means; a first and second electrical transformer each having a primary winding and a secondary winding and including means for coupling the primary winding of said transformers in parallel with separate rail portions intermediate said first and second shunt means on opposite sides from the point of coupling of said source of AC current across said rails and being separated by a distance at least equal to the distance between the first and last wheels in one truck of a railway vehicle and less than the distance between the last wheel of the leading truck and the first wheel of a rear truck of said vehicle, said transformers being adapted to provide an AC output signal across respective secondary windings of a signal transmitted from said source but becoming deenergized by the shorting effect of the train wheels across said rails anywhere between the respective primary winding and the point of coupling of said AC source to said rail; first and second rectifier means respectively coupled across said secondary winding of said first and second transformer providing a DC output of a first magnitude when the respective primary winding is energized from said source and a DC output of a second magnitude when said primary winding is deenergized; first NAND digital logic circuit means having a pair of inputs respectively coupled to the outputs A and B of said first and second rectifier means and providing a logic output signal corresponding to A.B; a differentiator circuit coupled to the logic output signal of said first NAND logic circuit means and providing a positive and a negative going trigger signal; first and second bistable circuit means coupled to said differentiator circuit and including means for being respectively triggered by said positive and negative going trigger signals to produce a respective first and second square wave output signal ZF1 and ZF2; second NAND digital logic circuit means having a pair of inputs respectively coupled to said first and second square wave output signals of said bistable circuit means and providing a logic output signal ZF1 . ZF2 Zout for operating external utilization and indicating means.

2. The invention as defined by claim 1 wherein said primary winding of said first and second transformer exhibits relatively less electrical resistance than the rail portion to which it is coupled in parallel so as to carry substantially most of the current flowing in the rail from said source.

3. The invention as defined by claim 2 and wherein said secondary winding of said first and second transformer provides an AC voltage thereacross which is proportional to the current flowing through said respective primary winding.

4. The invention defined by claim 3 wherein said source of AC current comprises an audio frequency transmitter operating in a constant current mode.

5. The invention as defined by claim 1 and wherein said first and second bistable circuiT are comprised of flip-flop circuits.

6. The invention as defined by claim 1 wherein said track includes a first and second rail joint respectively located on opposite sides of the coupling point of said source of AC current and wherein said first and second primary winding are respectively coupled across said first and second rail joint.

7. The apparatus as defined by claim 1 wherein said source of alternating current is coupled across said rails midway between said first and second electrical transformer.

8. The apparatus as defined by claim 1 and additionally including first and second diode means respectively coupled between aid differentiator circuit and said first and second bistable circuit means and being conductive in mutually opposite current directions to respectively couple said positive and negative trigger signals to said first and second bistable circuit means.

9. Apparatus as defined in claim 1 wherein said first and second NAND digital logic circuit means each comprises a dual input AND logic gate circuit and a logic inverter circuit coupled to the output of said AND gate.

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