Railway switches and signals – Roadway-defect protection
Reexamination Certificate
2002-02-15
2003-12-02
Morano, S. Joseph (Department: 2859)
Railway switches and signals
Roadway-defect protection
Reexamination Certificate
active
06655639
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the fail-safe detection of dangerous rail conditions such as broken railroad tracks.
BACKGROUND OF THE INVENTION
The danger of broken rails has been obvious to the railroads almost since their inception. The resulting potential for trains to derail has been the subject of considerable efforts aimed at the detection of broken rails and the subsequent automatic warning or control of trains in response to the danger. The classical approach to this problem has been through the use of track circuits, even though the detection of broken rails is not their primary function. Track circuits were developed in the 1870's for the purpose of determining whether a given section of track was clear of trains.
Referring to
FIG. 1
, a track circuit consists of the two rails [
1
] of a section of track electrically isolated by insulated joints [
2
] that determine the boundaries of the track section in which trains are to be detected. A battery [
3
] and a “track” relay [
4
] are each connected to the respective rails. When no train is present, current produced by the battery [
3
] flows though rails [
1
] and the track relay [
4
], thereby energizing the track relay [
4
]. If a train is present, its wheels and axles [
5
] provide a low impedance path in parallel with the track relay [
4
], effectively “shunting” it and thereby de-energizing it. Contacts [
6
] of the track relay [
4
] that are closed when the track relay [
4
] is energized, i.e. “front” contracts, are used as an input to signal or train control systems to provide positive and fail-safe indication that the track section is clear of trains when such contacts [
6
] are closed.
The choices of arranging the track relay [
4
] such that it is energized when no train is present, as well as the use of front contacts [
6
] for the indication of the track section being clear, are both made to ensure fail-safety. If the battery [
3
] were to fail, or if a wire were to break, the track relay [
4
] would assume the de-energized position, which corresponds to the presence of a train. Signal and train control systems are almost universally designed to stop trains or to restrict their speed when the associated track relays are de-energized, and therefore respond to the presence of trains and to track circuit failures in an identical and fail-safe manner.
Shortly after the initial development of the track circuit, an important shortcoming was discovered. Referring to
FIG. 1
, if a rail were to break at point [
7
], the shunting effect of the train [
5
] would be isolated from the track relay [
4
], which would now fail to detect the train. This situation would give rise to a non-fail-safe state in which the track section would falsely be indicated as being clear.
In response to this possibility, a modification to the track circuit design was made. Referring to
FIG. 2
, as compared to
FIG. 1
, the track relay [
4
] has been relocated to the end of the track circuit opposite to that of the battery [
3
]. As a result, the unsafe situation referred to above could no longer exist because the rail break [
7
] would no longer prevent the shunt [
5
] of the train from de-energizing the track relay [
4
]. A secondary benefit of this revised configuration is that broken rail detection is provided because any break [
7
] in an unoccupied track circuit would occur between the battery [
3
] and the track relay [
4
], thereby de-energizing the track relay [
4
].
It can be seen, therefore, that broken rail detection is largely a by-product of the basic design of a track circuit. This is underscored by the fact that track circuits in some applications are arranged with insulated joints on one rail only. These “single rail” track circuits are simpler than the more common “double rail” track circuits described above, but they only detect breaks in one rail.
In practice, there are considerable challenges to be met in the proper design and operation of track circuits. Each track circuit must be adjusted such that sufficient energy reaches the track relay [
4
] so as to energize it, while simultaneously being such that the shunt [
5
] of a train, which may be a single wheel set, will de-energize the track relay [
4
]. In addition, continuously and widely varying track ballast impedance that also tends to shunt the track relay energy must be contended with. These phenomena make the adjustment of track circuits very critical, resulting in reduced reliability. In fact, track circuit failures represent a significant proportion of signal and train control system failures.
More recently, electronic track circuits were developed that do not require insulated joints for track section delineation. These operate on the principal of applying audio range electronic signals with different frequencies and/or modulating schemes on the track and detecting these signals with matching receivers. These track circuits are associated with relatively complex circuitry to generate and decode the electronic signals because many such signals may be present due to the absence of insulated rail joints.
An additional complication in the design of track circuits is the requirement for compatibility in electrified territory. In such applications, the rails are used not only as part of the track circuits, but for the return of train propulsion current to substations. This requirement is usually met through the use of “impedance bonds” at the ends of each track circuit. These provide a very low impedance to the traction return current while maintaining a nominal impedance in the approximate range of one to ten ohms across the rails so that track circuit operation may be maintained. The presence, however, of these otherwise undesired impedances across the rails results in even further criticality in track circuit adjustment. In electrified territory, track circuits generally utilize a special 100 Hz power source eliminate any possibility of interference between track circuits and traction power or adjacent commercial power lines.
Beyond train detection and broken rail detection, there is a third function of conventional track circuits that is employed in some systems. This is to apply coded cab signals to the tracks so as to be received by equipment onboard trains where it is decoded into discrete speed commands. Cab signals generally consist of a current that is modulated on and off at one of several distinct rates in the range of 50 to 420 cycles per minute, each rate corresponding to a defined allowable speed or signal “aspect” to be displayed in the train cab. This requires elaborate equipment to not only to generate and apply the codes, but to distinguish between them and the normal track circuit energy.
It can be readily seen that the prior art currently provides technically complex and costly solutions to the broken rail detection problem because of the need for track circuits to perform other functions.
The advent of new technologies in train control applications, such as Communications-Based Train Control (CBTC) or Positive Train Control (PTC), can provide for the detection of the precise location of trains without track circuits as well as the control of the speed of trains without cab signals. This removes the requirement for two of the three classical functions of track circuits. An opportunity therefore exists for the development of a broken rail detector that satisfies the remaining requirement. With this, the elimination of costly and maintenance intensive insulated rail joints and impedance bonds is made possible.
It is therefore desirable to provide a practical track-based broken rail detector that is simpler and more reliable than conventional track circuits, which is the subject of the present inven
Abelman ,Frayne & Schwab
Grappone Technologies Inc.
McCarry, Jr. Robert J.
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