The present invention relates to a train detection apparatus for detecting the location of a train which is running on rails and a train-location detection system having a plurality of the apparatuses disposed along the rails.
Detection of the locations of a train is of importance for the railway operation for the purpose of performing safety operation. For example, information that a train is approaching is required to control railroad crossings to open/close, control points disposed in a forward station, guidance for passengers and assure safety of operators. Information that a train has passed and moved away is required for the operation of following trains.
The railway system in which trains having steel wheels run on steel rails is usually adapted a method called a "track circuit" for detecting the location of a train. The track circuit is structured so that two ends of two rails disposed in parallel and used in a pair are electrically insulated from one another. Moreover, a predetermined voltage is always applied to a position between the two rails. When wheels joined to the two ends of a steel wheel shaft are placed on the circuit, the two parallel rails are electrically short-circuited. Thus, the voltage between the two rails is made to be zero. The foregoing fact is used to detect whether or not a train exists.
When the above track circuit is employed, the rails must be cut at required intervals so as to electrically insulate the rails from each other. However, the long rails each having several kilometers and taken for granted today encounters a limit of the maximum lengths. Moreover, special joints called "expansion joints" are required, thus causing the cost to be enlarged. What is worse, there is apprehension that an accident happens such that the travel of the train is obstructed because of an electrical insulation failure occurring in the insulated portion.
To overcome the problem experienced with the conventional track circuit, a train detection method using a sound wave which propagates through the rail has been disclosed in, for example, Japanese Patent Laid-Open No. 10-02951.
FIG. 1 is a block diagram showing a conventional train-approach-detection apparatus disclosed in Japanese Patent Laid-Open No. 10-002951 and using a sound wave. FIG. 1 shows a state in which a train 2 runs on rails 1 in a direction indicated by an arrow. Reference numerals h1 and h2 represent the train-approach-detection apparatuses which have corresponding acceleration sensors S1 and S2 and magnetostrictive oscillators M1 and M2 which are connected to the rails 1. The acceleration sensors S1 and S2 detect oscillations of the rails 1, while the magnetostrictive oscillators M1 and M2 transmit sound waves to the rails 1.
The operation of the foregoing conventional train-approach-detection apparatus will now be described. The train-approach-detection apparatus h1 operates the magnetostrictive oscillator M1 to transmit a sound wave having a specific frequency to the rail 1. A sound wave reflected by the train 2 is received by the acceleration sensor S1. The train-approach-detection apparatus hi measures required time to multiply the measured time by a known propagation speed of a sound wave through the rail 1 so that the distance to the train 2 is calculated.
Since the train 2 is always moved, the train-approach-detection apparatus h1 repeats the foregoing process at predetermined time intervals so as to always detect the location of the train 2. Also the train-approach-detection apparatus h2 performs a process similar to the process which is performed by the train-approach-detection apparatus h1 so as to always detect the location of the train 2.
If the train-approach-detection apparatuses h1 and h2 are disposed apart from each other for a relatively short distance, for example, several hundred meters, incorrect recognition and overlap of the sound waves take place between the two train-approach-detection apparatuses h1 and h2 in a case where the frequency of the sound waves which are employed by the two train-approach-detection apparatuses h1 and h2 are the same. In this case, the distance cannot accurately be measured. Therefore, the frequencies of the sound waves which are employed by the two train-approach-detection apparatuses h1 and h2 must be different from each other. Since a fact has been found that frequencies included in a relatively narrow range is easy to be propagated through the rail 1, the two train-approach-detection apparatuses h1 and h2 must use substantially the same frequencies. Therefore, the conventional train-approach-detection apparatuses h1 and h2 mist be disposed apart from each other at a considerably long distance.
Even if the train is detected by the above-mentioned method, the conventional apparatus must have signal cable arranged along the rail so as to commutate a result of the detection to another apparatuses. Thus, a railway company must bear a great cost. The foregoing cable can easily be gnawed and damaged by mice. To prevent the damage, another large cost is required What is worse, the travel of the train is obstructed.
The sound waves which are generated by the magnetostrictive oscillators M1 and M2 are mainly composed of elastic waves. If the rail 1 is hit with a hammer, elastic waves having frequencies in a relatively wide range are generated. Therefore, the frequencies of sound waves which is easy to be propagated through the rail 1 were examined.
Waveforms of sound waves (elastic waves) measured at points apart from a position at which the rail 1 was hit by the hammer at distances of 50 m and 150 m by an acceleration sensor and results of Fourier transformation were shown in graphs in FIGS. 2A, 2B, 3A and 3B. As can be understood from FIG. 2, sound waves (elastic waves) having frequencies in a relatively wide range exist at the position 50 m away from the hit point. As can be understood from FIG. 3, a fact can be understood that sound waves (elastic waves) having frequencies near 3 kHz intensely remains at the position 150 m apart from the hit point. Although a fact has theoretically been found that the frequency of the sound wave (the elastic wave) which is easy to be propagated through the rail 1 considerably depends on the intervals of sleepers, a fact was found that only sound waves having the frequencies near the base frequency (which as 3 kHz) were easily propagated.
As described above, the train detection method using sound waves in place of the conventional track circuit encounters the problem in that the distance to the train cannot be detected or incorrect detection is performed if the two apparatuses are disposed apart from each other at a relatively short distance.