The present invention refers to a method and apparatus for determining the occupation state of the circuit of a direct current track circuit in a railway line. In the railway industry language, a direct current track circuit is a section of a railway track of variable length (ranging from a few metres to several kilometres) electrically insulated from the adjacent track sections and having a low voltage applied on the set of two tracks. Application of voltage onto the electrified tracks occurs at an end of the track section, while a voltage detector is applied parallel at the end opposite to such power supply point. When there are no vehicles on that track section, the detector extracts a non-zero voltage given that the electrified tracks are not short-circuited by the presence of the train axes, while should any railway vehicle reach that track section it short-circuits the tracks with each other through its axes, and triggers current circulation on the axes causing the detection of an ideally zero-voltage on the detector. Thus, such technology allows continuously generating information regarding the presence in a track section for vehicles (standstill or mobile) , and used as it is for automating the control of a railway line. The information received from the direct current track circuit is reliable if electrically insulated from the earth, or from an adjacent direct current track circuit. Furthermore, the measurement environment is extremely noisy - just imagine the traction noise of electric locomotives - and the measurement method must be practically immune to noise signals even of high levels. In addition in case there is no train in the circuit, the current supplied by the generator is not zero due to the dispersion resistance between the insulating joints, due to the dispersion resistance between the tracks (depending on the ballast for example) .

Document US20060155433 proposes a method and a system for detecting a train on a direct current track, circuit wherein an unambiguous code is added to the signal transmitted to the circuit. In particular, added to the voltage signal at the carrier frequency sent on the direct current track circuit is a preset number of information bits which - if identified in the reception step - signal the absence of the train in that direct current track circuit. It is obvious that in a noisy environment like the one described above even this system could fail to signal the correct information in that the added word could erroneously not be received. The object of the present invention is that of overcoming the abovementioned drawbacks and finding a reliable method for accurately detecting the signal for identifying the occupation state of a direct current track circuit. The present invention proposes a method and an apparatus wherein in a direct current track circuit part of a railway line the low alternating voltage signal is modulated in amplitude and/or in phase with an unambiguous digital code word. Each circuit shall have its own unambiguous digital word.

Such encoding is currently used for encoding signals in mobile phones and it is substantially a technique for access, by several sources of information, to the same transmission channel, by using the same frequency band, which consists in associating - to the information transmitted by each of these sources - a code capable of identifying it in an unambiguous manner with respect to the rest and capable of allowing a receiver, to selectively extract information associated to each of them.

In such manner, verifying - in reception - the correlation between the received signal and the unambiguous digital word associated to that direct current track circuit, among all possible words, there will be no doubts as to whether the received signal is due to the absence of a train on the direct current track circuit in that the power of the detected signal shall be the maximum one. An aspect of the present invention regards an apparatus for verifying the occupation state of a direct current track circuit in a railway line, having the characteristics of claim 1. A further aspect of the present invention regards a method for verifying the occupation state of a direct current track circuit in a railway line, having the characteristics of claim 4.

Characteristics and advantages of the method and apparatus according to the present invention shall be more clear and apparent from the following exemplifying and non-limiting description of an embodiment provided with reference to the attached figure which represents a block diagram of the apparatus according to the present invention.

With reference to the abovementioned figure the apparatus according to the invention is applied on a direct current track circuit which essentially comprises a pair of tracks R made of parallel metal sections B usually T-shaped, mounted transversely with respect to a bearing structure made up of crosspieces made of wood, steel or cement. Each direct current track circuit is separated from the adjacent one through electrically insulating joints G. In a railway line there is a plurality of adjacent direct current track circuits and insulated with respect to each other.

The apparatus according to the present invention for each direct current track circuit comprises a transmission block 2 capable of transmitting an alternating voltage signal, to which an unambiguous code word is associated, to the circuit. Such code word consists in providing a preset number of positive and negative half-waves of such alternating current according to a predetermined sequence. In particular, a so-called spread word S, for example made up of 16 chips, wherein each chip is equivalent to a sinusoidal cycle at the considered frequency, wherein the chip with value "1" provides for the sinusoidal cycle at the considered frequency with zero-phase, while the chip with value "0" provides for the sinusoidal cycle with phase 180° . Advantageously, an unambiguous digital word of a specific direct current track circuit is orthogonal to the unambiguous code words of the adjacent direct current track circuits. According to the present invention, the term "orthogonal" is used to indicate that the scalar product between two orthogonal words is zero. Such voltage signal on which the code word is overlapped or associated is the traditional 50 Hz or 83.3 Hz voltage signal or other frequencies, which is used for detecting the passage of the train on that direct current track circuit .

Exploiting such property, the apparatus of the present invention further comprises a block 3 for extracting or detecting an ideally zero-voltage signal when the presence of the train closes the track electric circuit and a block 4 for processing such signal . Under normal conditions (no train) such signal must contain the unambiguous word regarding its direct current track circuit alongside the noise associated thereto caused by the abovementioned phenomena. The detected signal is sent to the processing block which comprises a microprocessor and at least one analogue/digital converter for such extracted signal . By carrying out the scalar product between such signal and the unambiguous word assigned to that specific direct current track circuit the presence of the correct unambiguous word may be verified and an energy value is obtained. If lower than a predetermined threshold value, such energy value causes the emission of an occupation signal which reveals the occupation state of the track.

Barzanό & Zanardo Milano S. p.A.