Railroad Grade Crossing Protection System

Abstract

A railroad crossing warning indicator which predicts the time of arrival of trains to a grade crossing is described. Two voltages are derived from the track reactance magnitude and the impedance magnitude and are both indicative of the distance of the train. By summing the difference between the impedance voltage and the reactance voltage with the impedance voltage, a new distance voltage is obtained whereby errors are reduced due to the nonlinearity of the signals due to ballast resistances in the tracks.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to warning systems for railroad grade crossings and more particularly to an improvement in warning predictor systems used to predict the time of arrival of an approaching train.

2. Discussion of the Prior Art

A typical grade crossing predictor is set forth in U. S. Pat. No. 3,246,143 and U.S. Pat. application Ser. No. 807,626, filed Mar. 17, 1969, by the same inventor and assigned to the same assignee as this invention. Much of the same system's electronics of that patent are used in the invention discussed herein.

The system of the above patent provides a railroad crossing warning system whereby delay to cross traffic is minimized. This is achieved in an arrangement wherein the railroad track is considered as a shorted transmission line in which the short is provided by the train. An alternating current signal which is a substantially constant current level is applied to the tracks at the location of the grade crossing. The voltage existing across the tracks as the train, and therefore the short, approaches the grade crossing, will diminish. Thus, the amplitude of this voltage provides a measure of the distance of the train from the crossing while the rate at which this voltage diminishes provides a measure of the velocity of the train. With these parameters it becomes possible to estimate the time of the train's arrival at the crossing. Knowing the time of arrival, the system can start warning signals at such a time as will provide the least possible delay to cross traffic. The signal representative of distance and the signals derived therefrom representative of velocity are combined to provide a third voltage representative of the time required for the train to arrive at the railroad grade crossing.

It has been found that the input impedance of the shorted railroad track section, having infinitely high ballast resistance, varies linearly with track length. The Grade Crossing Predictor, as set forth in the above patent, for example, uses this principle to develop a voltage which is the measure of the distance of the train to the predictor probe location. The voltage is derived from the reactance component of the input impedance. The rate at which this voltage diminishes as a train approaches, provides a measure of the speed of the train. These two voltages are then combined to estimate the time of the train's arrival at the crossing. Knowing the time of arrival of the train, the aforesaid system can initiate warning signals before the arrival thereof.

Since, in actual practice, ballast resistance is low enough to cause the input impedance, and in particular the reactance component, to vary nonlinearly with track length, an error is introduced into the distance voltage and thus the speed voltage. These two errors cause the predictor to err in the estimate of the arrival time of the train, thus as the ballast decreases, the error increases. Thus, a need has arisen to reduce the error in the estimate of arrival time of a train when ballast resistance is low.

The system as described in the copending application makes use of a second distance voltage which differs from the first in that it is developed from the impedance magnitude of the input impedance. It is this second voltage which is used to measure the speed of the train. The first distance voltage is used to measure the distance to a train.

SUMMARY OF THE INVENTION

Briefly described, the distance-to-train voltage (E.sub.D) is developed in the computing circuit by two other distance voltages. One of the two voltages which is the reactance voltage (E.sub.DX) is developed from the reactive component of the track input impedance. The other voltage is derived from the impedance component track input impedance magnitude. Because of the low ballast resistance, neither of these voltages provide an acceptable voltage due to the nonlinearity thereof.

In accordance with this invention, the computing circuit combines the E.sub.DZ and the E.sub.DX voltage in a unique manner to overcome the nonlinearity problem and to provide a voltage from the track which changes substantially linearly as the distance between the train and the receiver changes. The computing circuit subtracts the E.sub.DX voltage from the E.sub.DZ voltage and adds the difference to the E.sub.DZ voltage to provide a near linear E.sub.D. Thus

E.sub.D = 2E.sub.DZ - E.sub.DX

DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages will become more apparent to those skilled in the art when taken into consideration with the following detailed description, wherein like reference numerals indicate like and corresponding parts throughout the several views, and wherein:

FIG. 1 is a block diagram of the preferred embodiment of the invention; and

FIG. 2 is a graph of the voltage versus distance and the error reduction linearity realized by this invention.

DESCRIPTION OF ONE PREFERRED EMBODIMENT

Turning now to FIG. 1, there is shown a block diagram of the preferred embodiment of this invention. The train 10 has a motion in a direction represented on a pair of track rails 12. The train is at a distance L from the origin point P, P', which represents the location of a grade crossing, for example. The train motion occurs from left to right. The velocity V and the acceleration A factors are, therefore, represented as going from left to right on the drawing. The method of computing the time of arrival is set forth fully in the aforesaid U. S. Pat. No. 3,246,143 and in the copending application aforesaid.

A computer, in accordance with the aforesaid, includes an oscillator 16 which oscillates at a suitable frequency. The output of the oscillator 16 is applied to excite a power amplifier 22. A resistor 18 connects one side of the power amplifier to one of the rails at a point P'. The other side of the power amplifier 22 connects to the other rail P. The power amplifier 22, together with the resistor 18, comprises a constant current generator. This delivers an input to the track at substantially a constant current.

It should be appreciated that as the train 10 approaches the points P, P' on the track to which current from the constant current generator is applied, the impedance of the tracks looking toward the train from these points is continuously being diminished. Thus, the train comprises a short across the tracks 12, which is moved toward the points P, P'. With current being maintained constant, the voltage at the points P, P' will continuously decrease to a minimum when the train reaches the points P, P'. Therefore, by measuring the voltage across the tracks 12, an indication is obtained of the distance of the train 10 from the points at which the voltage is impressed. The change, with respect to time of this voltage, can provide velocity information and a second derivative of this voltage information provides information as to the acceleration of the train 10.

Accordingly, a narrow band-pass amplifier 26 centered at the frequency of the oscillator 16, which is connected to the same points of the tracks 12 as the constant current generator, receives a voltage representative of length of track L or distance between the train 10 and the points P, P'. This voltage is an alternating current which is modulated by the motion of the train 10 toward the points P, P'.

The output of the band-pass amplifier 26 is applied to a quadrature detector 28 which also has a reference input applied from the oscillator 16 through a phase shift network 30. The output of the quadrature detector 28 is applied to a summing amplifier 38. The output of the band-pass amplifier 26 is also coupled to an amplitude detector 32. The amplitude detector 32 provides a DC voltage proportional to the impedance of the track 12. The output of the amplitude detector 32 provides a voltage - E.sub.DZ developed by the track input impedance. The output of the quadrature detector 28 provides a voltage E.sub.DX which is developed from the reactive component of the track input impedance. The -E.sub.DZ voltage and the E.sub.DX voltage are summed in the summing amplifier 33 to produce E.sub.D = (2E.sub.DZ - E.sub.DX). Circuit 34 provides the rate of change of that voltage to produce E.sub.D = dE.sub.D /dt. The output of the circuit 34 is coupled through an amplifier 36 to a summing amplifier 38, where it is summed with E.sub.DX. The summing amplifier 38 receives the time rate of change E.sub.D of the linearized distance voltage E.sub.D from the output of the differentiating circuit 34 which is equal to the speed of the train 10. The output of the summing amplifier 38 is connected to a high gain amplifier 40. The output of the high gain amplifier is applied to an amplitude comparator 42 wherein it is compared with a signal from the reference voltage source 44. The output of the amplitude comparator 42 is connected to a relay amplifier 46 which operates the warning relay when the signal applied into it has a sufficient magnitude.

To complete an operative embodiment of the system, an override circuit is also provided, and this includes an amplitude discriminator 52 which receives the output from the quadrature detector 28 and compares it to the output of a reference voltage source 54. The output of the amplitude discriminator 52 is connected to a relay amplifier 56 which drives a minimum distance override relay 58. The input to the differentiator circuit 34 and summing amplifier 38 are voltages proportional to the distance L between the train and the excitation points P and P'. When differentiated, this voltage gives a voltage proportional to train speed. The output of the quadrature detector 28 is a voltage proportional to the reactance component across the track which is a measure of the distance to a train from points P and P'.

FIG. 2 illustrates the difference between the distance voltage derived from the reactance magnitude provided by the quadrature detector 28 and the distance voltage derived from the impedance magnitude provided by amplitude detector 32. The sum of distance voltage E.sub.DX derived from the reactance magnitude and the time rate of change of the linearized distance voltage E.sub.D derived from the reactance and impedance magnitudes is provided by the summing amplifier 38.

When the ballast resistance is very high (R.sub.B = .infin.) the two distance voltages have the same slope. When the ballast resistance is decreased (R.sub.B = 1.5 ohms, for example, lumped at the predictor) it can be readily seen that the slope of the impedance magnitude is much improved over the slope of the reactance magnitude, as shown in the two graphs in FIG. 2. Thus, the error in estimate of the arrival of a train by the predictor is also much improved. The reason that the impedance magnitude provided by amplitude detector 32 is less affected by low ballast resistance than the reactance magnitude is apparent in the following example:

Z.sub.in = R + J X

If we assume that R = 0.5X, which provides a high ballast resistance condition, then

Z.sub.in = 1.12 X< 63.4.degree.

If we assume a low ballast resistance condition (R.sub.B - 2 X) where the ballast resistance is in parallel with Z.sub.in then

Z.sub.in = 0.85 X <41.6.degree.

therefore, the impedance magnitude

Z.sub.in = 0.83/1.12 or a 26 percent reduction in its magnitude

while the reactance magnitude

or a 45 percent reduction in its magnitude for the same given low ballast condition.

When the system in accordance with this invention generates the voltage

E.sub.D = 2E.sub.DZ - E.sub.DX

by subtracting, in effect, the E.sub.DX distance voltage from the E.sub. DZ voltage and adds the difference back to the E.sub.DZ a more ideal and linear slope is generated as shown in FIG. 2. This slope is nearly as linear at times as the R.sub.B = .infin. slope. This improved slope gives a more correct calculation of train speed and thus warning time at grade crossing is more nearly correct.

Thus, there has been provided by the improvements set forth herein a time of arrival predictor computer which has a lower error as compared to the prior art systems. The output, as provided by this predictor from summing amplifier 38, is sent through a high gain amplifier 40 and compared in a comparator 42 to a reference voltage provided by 44. If the sum of these voltages is above the reference voltage, then the relay amplifier 46 enables a relay 48, which, in turn, either sounds an alarm or lowers a crossing gate, or the like.

Claims

Having thus described but one preferred embodiment of this invention, what is claimed is:

1. In a system for deriving from railroad tracks information for predicting the time required for arrival at a given location of a distant train which is moving on said track towards said location, comprising:

means for applying an AC signal of a constant current level on said track from said location;

means for deriving a first voltage proportional to the reactance component across said tracks indicative of the distance of said train from said location;

means for providing a second voltage proportional to the impedance component across said tracks;

means for providing a voltage developed by the difference between said first voltage and said second voltage;

means for combining said difference voltage with said first voltage to provide a linearized distance voltage;

means for differentiating said linearized distance voltage to obtain a voltage representative of the instantaneous speed of said train; and

means for combining said first voltage and said differentiated voltage to provide a third voltage which is a function of the time required for said train to arrive at said given location.

2. The system as defined in claim 1 and further comprising means for utilizing said third voltage for operating a warning device at said location.

3. The system as defined in claim 1 wherein said means for deriving said first voltage is a quadrature detector.

4. The system as defined in claim 1 wherein said means for deriving said second voltage is an amplitude detector.

5. The system as defined in claim 1 wherein said means for differentiating is an operational amplifier differentiator.

6. The system as defined in claim 1 and further comprising:

means for providing a reference voltage; and

comparator means responsive to said reference voltage and said third voltage provided by said combining means, said comparator means being adapted to provide an output when said third voltage exceeds said reference voltage.

7. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said comparator means for operating a warning device at said location.

8. In a system for predicting the time of arrival of a train on a track comprising:

a source of AC signals at a constant current level, said source being coupled across said tracks at a selected location;

a quadrature detector adapted to receive signals from said track at said selected location, said quadrature detector being adapted to provide an output voltage proportional to the reactance component across said track;

an amplitude detector adapted to receive signals from said track at said selected location, said amplitude detector being adapted to provide an output voltage proportional to the impedance component across said tracks;

a first summing amplifier responsive to said amplitude detector and said quadrature detector for providing a voltage difference between the impedance output voltage and the reactance output voltage and adding this difference to the impedance output voltage;

a differentiator circuit responsive to the output voltage of said first summing amplifier adapted to provide an output indicative of the instantaneous speed of said train; and

a second summing amplifier responsive to the output voltage of said differentiator circuit and the output voltage of said quadrature detector, said summing amplifier being adapted to provide an output voltage which is a function of the time required for said train to arrive at said location.

9. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said summing amplifier for operating a warning device at said location.

10. The system as defined in claim 6 and further comprising:

a reference voltage source adapted to provide an output voltage of a predetermined level; and

an amplitude comparator being responsive to the output voltage of the said summing amplifier and to the output voltage of said reference voltage source and being adapted to provide an output signal when the voltage provided by said summing amplifier exceeds the voltage provided by said reference voltage source.

11. The system as defined in claim 8 wherein said source of AC signals includes:

an oscillator adapted to provide an output signal at a predetermined frequency; and

a power amplifier coupled between said oscillator and said tracks.

12. The system as defined in claim 11 and further including a band-pass amplifier coupled between said track and said quadrature detector and said amplitude detector.

13. The system as defined in claim 12 and further comprising means for utilizing the output voltage of said amplitude comparator for operating a warning device on said location.

14. A system for determining the distance to a vehicle on a railroad track which has an electrical current thereon, including;

means coupled to said track for generating a first voltage indicative of the track input impedance;

means coupled to said track for generating a second voltage indicative of the reactance component of said track input impedance;

means for generating a third voltage proportional to the difference between said first voltage and said second voltage; and

means for combining said third voltage with said first voltage to provide a fourth voltage indicative of said distance.

15. The system as defined in claim 14 wherein said means for generating said first voltage comprises an amplitude detector.

16. The system as defined in claim 14 wherein said means for generating said second voltage comprises a quadrature detector.

17. The system as defined in claim 14 wherein:

said means for generating said first voltage comprises an amplitude detector; and

said means for generating said second voltage comprises a quadrature detector.