Patent Publication Number: US-2006010979-A1

Title: Failure detecting system

Description:
This application is a continuation of PCT/JP00/08549 filed on Dec. 1, 2000. 
    
    
     TECHNICAL FIELD  
      The present invention relates to a failure detecting system for detecting failures such as breaks and the like in rails or pipes laid over long distances by using sound wave. In particular, the present invention relates to a technique for achieving simplification of such a system.  
     BACKGROUND ART  
      For example, in the field of railway signalling, a detection of train is performed by using track circuits. However, as a secondary function, these track circuits are provided with rail failure detecting functions for detecting whether there is a break in the rails.  
      Incidentally, in recent years, train detecting systems using radio transmission for the purpose of facilitation of safety have been considered. In the case where a train detecting system using radio transmission is adopted, since this system does not have a function for detecting rail breaks, it is necessary to provide separately a facility for detecting rail failures such as breaks and the like. An advantage of a train detecting system using radio transmission is that signal line cabling is not required. Accordingly, it is desirable for a rail failure detecting apparatus used together with a train detecting system using radio transmission to be one that does not require the signal line cabling.  
      For such a rail failure detecting system, systems using ultrasonic waves are well known (refer to International Publication WO98/7610 and U.S. Pat. 5,743,495).  
       FIG. 1  shows an example of a conventional break detecting system for detecting rail breaks over a long distance using ultrasonic waves.  
      In  FIG. 1 , a large number of terminal devices  2   1 ,  2   2 ,  2   3 , . . . are installed at intervals along a rail  1 . The terminal devices  2   1 ,  2   2 ,  2   3 , . . . are provided with ultrasonic wave transmitter-receivers  3   1 ,  3   2 ,  3   3 , . . . and communication units  4   1 ,  4   2 ,  4   3 , respectively, and are connected to a central processing unit  5  via a communication line  6  so as to be able to communicate with the central processing unit  5 .  
      In this conventional system, ultrasonic waves are transmitted from each terminal device to the next terminal device such that the terminal device  2   1  transmits ultrasonic waves to the next terminal device  2   2 , the terminal device  2   2  transmits ultrasonic waves to the next terminal device  2   3 , and the terminal device  2   3  transmits ultrasonic waves to the next terminal device. Each of the terminal devices  2   1 ,  2   2 ,  2   3 , . . . periodically informs the central processing unit  5 , through each of the communication units  4   1 ,  4   2 ,  4   3 , . . . , whether ultrasonic waves have been transmitted or received.  
      For example, if a rail break occurs between the terminal device  2   1  and the terminal device  2   2 , an ultrasonic wave signal transmitted from the terminal device  2   1  is not received by the terminal device  2   2 . If there is no information from the terminal device  2   2  to the central processing unit  5  that the signal has been received, then the central processing unit  5  judges that there is a break in the rail between the terminal device  2 , and the terminal device  2   2 .  
      Furthermore, in the case where the location of a rail break is detected by the system in  FIG. 1 , when each of the terminal devices  2   1 ,  2   2 ,  2   3 , . . . receives reflected waves, it immediately informs the central processing unit  5  that reflected waves have been received. The central processing unit  5  can calculate the location where reflection occurred, that is, the location of a break in the rail, based on the time the ultrasonic waves were transmitted and the time the reflected waves were received.  
      Incidentally, the transmission distance of ultrasonic waves depends on the form and installation condition of an ultrasonic transmission medium. For example, in the case of a pipe, equipments for fitting pipe increase the attenuation of ultrasonic waves. Furthermore, in the case of a rail, the attenuation of ultrasonic waves becomes large by sleeper and rail fastenings. Generally, the detection range in the case of pipe failure detection using reflection of ultrasonic waves is approximately few score meters, and the detection range in the case of rail break detection is from 1 to 2 km.  
      Therefore, in the case of detecting breaks in a rail or pipe installed over a long distance, in the conventional system in  FIG. 1 , if the number of terminal devices connected to the central processing unit via the communication line is increased, there is a problem in that control of the network comprising terminal devices, communication lines and a central processing unit becomes complicated.  
      A method has also been reported for performing long distance transmission by generating powerful ultrasonic waves. However, since there are possibilities of the ultrasonic wave generating apparatus becoming too big, and the ultrasonic transmission medium itself getting damaged, it is difficult to utilize such a method for detecting breaks in pipes and rails.  
      The present invention takes such conventional problems into consideration, and has an object of providing a failure detecting system capable of reducing the number of terminal devices connected to a communication line of a network, and also simplifying the network.  
     DISCLOSURE OF THE INVENTION  
      In order to achieve the above object, the construction of a failure detecting system of the present invention is such that a detecting unit, relay units and a terminal unit are arranged along a detection object at intervals, sound wave transmitted from the detecting unit is relayed by the relay units to be transmitted to the terminal unit using the detection object as a transmission medium, and when the terminal unit receives the transmitted sound wave, the sound wave is returned from the terminal unit, and relayed by the relay units to be transmitted to the detecting unit, so that it is judged whether or not there is a failure in the detection object, based on the sound wave reception state in the detecting unit.  
      According to such a construction, the sound wave reception state may be monitored in the detecting unit, and even in the case where failures such as breaks and the like in a detection object are detected over a long distance, only the detecting unit need be connected to a central processing unit or the like via a communication line. Therefore, it is possible to simplify the communication network.  
      Furthermore, each of the relay unit is provided with a first transmission and reception section that transmits and receives sound wave on the detecting unit side, and a second transmission and reception section that transmits and receives sound wave on the terminal unit side, wherein when the first transmission and reception section receives sound wave from the detecting unit side, the second transmission and reception section transmits the sound wave to the terminal unit side, and when the second transmission and reception section receives the sound wave from the terminal unit side, the first transmission and reception section transmits the sound wave to the detecting unit side. If the construction is such that when the relay unit receives sound wave from the detecting unit side, the first transmission and reception section returns the sound wave to the detecting unit side, then in a case where there is no failure in the detection object, since reception signals corresponding to the number of installed relay units and terminal unit are received, it is possible to monitor the operating states of the relay units and the terminal unit. Moreover, when reflected sound wave is received from a failure, since a location of failure can be specified based on the time from the reception signal immediately beforehand to the reception signal by the reflected sound wave, it is possible to suppress errors in detecting the location of failure caused by a difference in timing from receiving to transmitting in the relay unit, and hence the location of failure can be specified accurately. Furthermore, in the detecting unit, it is possible to correct a change in sound wave propagation speed caused by the installation environment and the like, based on the time from when the detecting unit transmits sound wave to when it receives the sound wave transmitted to the direction of the detecting unit by the adjacent relay unit that received the sound wave, and hence the location of failure can be specified accurately.  
      If the sound wave transmission levels of the relay unit and the terminal unit are set to be almost the same as the reflected sound wave level in the vicinity of where these units are arranged, then it is possible to check whether the detecting unit and the relay unit maintain the ability to receive reflected sound wave, respectively.  
      Furthermore, the construction may be such that the relay unit comprises at least a transmission and reception section that transmits and receives sound wave in the directions of both the detecting unit side and the terminal unit side.  
      In such a construction, it is becomes possible to easily install a transmission and reception section for a detection object.  
      In this case, the relay unit is constructed to transmit sound wave with a preset time delay after received the sound wave. If the construction is such that the time delay is different between when sound wave is received for the first time and when sound wave is received for a second or later time, it is possible to differ the transmission timing of each relay unit from each other, so that interference due to received sound wave can be avoided.  
      Furthermore, the construction may be such that when the relay unit receives a sound wave signal from the terminal unit, it transmits the same signal as the sound wave signal returned from the terminal unit, and after transmission, the relay of the signal is stopped.  
      In such a construction, it is possible to avoid unnecessary transmission of sound wave.  
      Moreover, the construction may be such that when the relay unit receives reflected sound wave from a failure in a detection object, it transmits a signal indicating reception of the reflected sound wave, which is different from any of a sound wave signal transmitted from the detecting unit and a sound wave signal returned from the terminal unit.  
      Furthermore, the construction may be such that the transmission and reception sections of each the detecting unit, the relay unit and the terminal unit are installed on at least a pair of detection objects that are separated acoustically from each other, a sound wave signal transmitted from the detecting unit is transmitted to each of the pair of detection objects alternately to be relayed to the terminal unit via the relay unit, returned sound wave from the terminal unit are transmitted to each of the pair of detection objects alternately to be sent to the detecting unit via the relay unit, and also a sound wave signal is transmitted from the detecting unit to each of the pair of detection objects alternately.  
      According to such a construction, by propagating sound wave to each of the pair of detection objects alternately, it is possible to propagate sound wave bypassing a break, and it is possible to receive reflected sound wave from the break with certainty, regardless of a location of break, so that it is possible to specify the location of break even in a case where respective units are arranged at maximum intervals between which signals can be transmitted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing an example of a conventional failure detecting system.  
       FIG. 2  is a block diagram showing a first embodiment of the present invention.  
       FIG. 3  shows a configuration example of a relay unit.  
       FIG. 4  is an operation time chart at a time of normal operation, in the first embodiment.  
       FIG. 5  is an operation time chart for when there is a break, in the first embodiment.  
       FIG. 6  shows the installation state of each unit when there is a joint in a rail.  
       FIG. 7  shows the installation state of each unit when breaks in a pair of rails are checked at the same time.  
       FIG. 8  is an operation time chart at a time of normal operation, in a second embodiment of the present invention.  
       FIG. 9  is an operation time chart for when there is a break, in the second embodiment.  
       FIG. 10  is an explanatory diagram of the principle of a reflected sound wave reception function checking of a unit.  
       FIG. 11  shows a third embodiment of the present invention, and is a block diagram of a case where sound wave is propagated in bi-directions.  
       FIG. 12  shows a configuration example of a relay unit in the third embodiment.  
       FIG. 13  is an operation time chart at a time of normal operation, in the third embodiment.  
       FIG. 14  is a block diagram of a fourth embodiment of the present invention.  
       FIG. 15  is an operation time chart at a time of normal operation, in the fourth embodiment.  
       FIG. 16  is an operation time chart for when there is a break, in the fourth embodiment.  
       FIG. 17  is an operation time chart for when there is a break, in a fifth embodiment of the present invention.  
       FIG. 18  is a diagram for explaining the relationship between sound wave transmission distance and reflected wave reception range in a case where respective units are arranged at maximum intervals between which signals can be transmitted.  
       FIG. 19  is a block diagram of a sixth embodiment of the present invention.  
       FIG. 20A  and  FIG. 20B  are explanatory diagrams of signal propagation operation, in the sixth embodiment.  
       FIG. 21  is a time chart at a time of signal propagation operation in  FIG. 20A .  
       FIG. 22  is a time chart at a time of signal propagation operation in  FIG. 20B .  
       FIG. 23A  and  FIG. 23B  are explanatory diagrams showing an example of signal propagation operation when there is a break, in the sixth embodiment.  
       FIG. 24  is a time chart at a time of signal propagation operation in  FIG. 23A .  
       FIG. 25  is a time chart at a time of signal propagation operation in  FIG. 23B . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      As follows is a description of preferred embodiments of a failure detecting system according to the present invention, with reference to drawings.  
       FIG. 2  shows a first embodiment of the present invention, and shows an example applied to a rail break detection.  
      In  FIG. 2 , along a break detection region of a rail  11 , being a detection object, a detecting unit  20  is arranged at the start of the detection region and a terminal unit  40  is arranged at the end of the detection region. Furthermore, an appropriate number of relay units  30 A and  30 B (the present embodiment shows an example with two units arranged) is arranged at intervals in accordance with the length of the detection region. Here, needless to say, the number of relay units is not limited to two.  
      The detecting unit  20  is provided with an ultrasonic transducer  21  attached in contact with the rail  11 , being a transmission medium, so as to be enable to transmit ultrasonic waves as sound wave to the direction of relay unit  30 A, and to receive ultrasonic waves from the direction of relay unit  30 A. Furthermore, based on transmission and reception information from transmitting and receiving circuits connected to the ultrasonic transducer  21  as described later, the detecting unit  20  judges whether or not there is a break, specifies the location of the break or the like, and transmits information of the judgment or specifying result to a central processing unit or the like (not shown in the figure) via a communication line.  
      The terminal unit  40  is provided with an ultrasonic transducer  41  attached in contact with the rail  11 , so as to enable to transmit ultrasonic waves to the direction of relay unit  30 B, and to receive ultrasonic waves from the direction of relay unit  30 B. The ultrasonic transducer  41  is connected to internal transmitting and receiving circuits, which are not shown in the figure. The construction of the terminal unit  40  is such that when receiving ultrasonic waves transmitted by the relay unit  30 B, it returns the ultrasonic waves to the relay unit  30 B.  
      The relay units  30 A and  30 B each is provided with an ultrasonic transducer  31   a , being a first transmission and reception section, attached in contact with the rail  11 , so as to enable to transmit ultrasonic waves to a direction of detecting unit  20  and to receive ultrasonic waves from the direction of detecting unit  20 , and an ultrasonic transducer  31   b , being a second transmission and reception section, attached in contact with the rail  11 , so as to enable to transmit ultrasonic waves to the direction of relay unit  30 B (terminal unit direction) and to receive ultrasonic waves from the direction of the relay unit  30 B (terminal unit direction). The construction of the relay unit  30 A is such that when receiving ultrasonic waves transmitted by the detecting unit  20  at the ultrasonic transducer  31   a , it transmits the ultrasonic waves to the relay unit  30 B via the ultrasonic transducer  31   b , and when receiving ultrasonic waves transmitted by the relay unit  30 B at the ultrasonic transducer  31   b , it transmits the ultrasonic waves to the detecting unit  20  via the ultrasonic transducer  31   a . The relay unit  30 B is constructed similarly. When receiving ultrasonic waves transmitted by the relay unit  30 A at the ultrasonic transducer  31   a , the relay unit  30 B transmits the ultrasonic waves to the terminal unit  40  via the ultrasonic transducer  31   b , and when receiving ultrasonic waves transmitted by the terminal unit  40  at the ultrasonic transducer  31   b , it transmits the ultrasonic waves to the relay unit  30 A via the ultrasonic transducer  31   a.    
       FIG. 3  shows a configuration example of the relay unit  30 A. The relay unit  30 B has the same construction, and its description is thus omitted.  
      In  FIG. 3 , the relay unit  30 A comprises a receiving circuit  33 A and a transmitting circuit  33 B that can be connected selectively to the ultrasonic transducer  31   a  via a change-over switch  32 , a receiving circuit  35 A and a transmitting circuit  35 B that can be connected selectively to the ultrasonic transducer  31   b  via a change-over switch  34 , and control circuits  36  and  37  that switching control the change-over switches  32  and  34 , respectively, based on information indicative of the signal reception from the receiving circuits  35 A and  33 A.  
      Next is a description of operation of the first embodiment with reference to time charts in  FIG. 4  and  FIG. 5 .  FIG. 4  shows a time chart for a case where the rail is normal, and  FIG. 5  is a time chart for a case where there is a break between the relay unit  30 A and the relay unit  30 B.  
      Firstly, there will be described the operation during normal status, where there is no break in the rail  11 .  
      The detecting unit  20  transmits ultrasonic waves to the direction of the relay unit  30 A at a predetermined period. A signal S 1  transmitted from the detecting unit  20  is received by the ultrasonic transducer  31   a  of the relay unit  30 A, and a reception signal R 1  is generated. When the receiving circuit  33 A receives the signal R 1 , the relay unit  30 A outputs information indicative of the signal reception to the control circuit  37 . Based on the information indicative of the signal reception, the control circuit  37  switches the change-over switch  34  to the transmitting circuit  35 B side. In this manner, ultrasonic waves are transmitted to the relay unit  30 B via the ultrasonic transducer  31   b . Accordingly, the transmission signal S 1  from the detecting unit  20  is relayed as a signal S 2  by the relay unit  30 A, to be transmitted to the direction of the relay unit  30 B. Similarly, the transmission signal S 2  from the relay unit  30 A is received by the ultrasonic transducer  31   a  of the relay unit  30 B, and a reception signal R 2  is generated. The reception signal R 2  is relayed as a transmission signal S 3  by the relay unit  30 B, to be transmitted to the terminal unit  40 . The terminal unit  40  receives the signal S 3 , generates a transmission signal S 4  due to the generation of a reception signal R 3 , to send the transmission signal S 4  to the direction of the relay unit  30 B. The transmission signal S 4  is received by the relay unit  30 B, and relayed as a transmission signal S 5  due to the generation of a reception signal R 4 . The signal S 5  is received by the relay unit  30 A, and relayed as a transmission signal S 6  due to the generation of a reception signal R 5 . The signal S 6  is received by the detecting unit  20 , and a reception signal R 6  is generated.  
      In this manner, if there is no break in the rail  11 , the signal S 1  transmitted from the detecting unit  20  is relayed by the relay units  30 A and  30 B, to be transmitted to the terminal unit  40 . An ultrasonic wave signal returned as information indicative of the signal reception from the terminal unit  40  is transmitted to the detecting unit  20  via the relay units  30 A and  30 B, so that the reception signal R 6  is generated in the detecting unit  20 . Since it is possible to calculate in advance the time for the ultrasonic wave signal to go forth and back between the detecting unit  20  and the terminal unit  40  via the rail  11 , it is possible to set in advance the generation time of the reception signal R 6  in the detecting unit  20  when the rail  11  is normal. Accordingly, by setting a time range in advance based on a predicted generation time in the detecting unit  20 , and by monitoring whether or not the reception signal R 6  is generated within the preset time range, it is possible to monitor whether or not there is a break, and hence if the signal R 6  is generated within the set time range, the rail  11  can be judged to be normal.  
      Next is a description of the case where the rail  11  has a break with reference to the time chart in  FIG. 5 .  
      In the case where there is a break in the rail  11  between the relay units  30 A and  30 B, the transmission signal S 2  from the relay unit  30 A, based on the transmission signal S 1  from the detecting unit  20 , is reflected at the break. As a result, the relay unit  30 A receives reflected waves from the break at the ultrasonic transducer  31   b , and a reflected reception signal “a” is generated as shown in  FIG. 5 . When the signal “a” is generated, the relay unit  30 A transmits a signal “b” to the detecting unit  20  via the ultrasonic transducer  31   a  by operations of the receiving circuit  35 A, the control circuit  36  and the change-over switch  32 . The detecting unit  20  generates a reception signal “c” on receiving the signal “b”. The reception signal “c” which is based on the reflection from the break, becomes outside the set time range earlier than the generation time of the reception signal R 6  (shown by dashed lines in  FIG. 5 ). Hence, it is possible to judge that the reception signal “c” is generated by reflected waves from the break.  
      In the case where it is judged that the reception signal “c” is the one based on the break, in the detecting unit  20 , it is possible to calculate a rail break location X from the equation X=C·T/2 where T is the time from the transmission of the signal S 1  to the reception of signal “c” and C is the propagation speed of ultrasonic waves. Hence, it is possible to specify the break location. Information such as the abovementioned judgment of normal operation and the break location is transmitted to the central processing unit connected to the detecting unit  20  through the communication line, although not shown in the figure.  
      As above, in the first embodiment, it is monitored whether or not the reception signal R 6  is generated due to a return signal from the terminal unit  40 . If the signal R 6  is generated, it is judged that the rail  11  is normal, and if the reception signal “c” is generated, it is judged that there is a break. Furthermore, if the detecting unit  20  does not receive a signal, it means that some abnormality has occurred in the ultrasonic wave transmission system.  
      In a case where there is a joint in the middle of the detection region of the rail  11 , as shown in  FIG. 6 , the two ultrasonic transducers of the relay unit  30  may be installed on the rail  11  so as to cross over a joint  11   a . If there is the joint  11   a  in the detection region, there is a reflected wave from the joint  11   a . However, if the reception signal R 6  is generated based on the transmission from the terminal unit  40 , it is judged that the rail is normal.  
      Furthermore, in a case where breaks in a pair of rails  11  are monitored at the same time, then as shown in  FIG. 7 , the detecting unit  20  may be arranged on one of the rails and the terminal unit  40  may be arranged on the other rail, at one end of the detection region of the rails  11 , and also the relay unit  30  may be arranged at the other end of the detection region so that the two ultrasonic transducers are installed across both rails.  
      Here, in the case where the constructions are as in  FIG. 6  and  FIG. 7 , the transmission operation of ultrasonic wave signals is similar to the case in  FIG. 2 . Therefore, the description is omitted.  
      In the case of the first embodiment described above, when the number of relay units  30  is increased, the cumulative time of a time delay from the reception of signal to the transmission of signal in each relay unit  30  is increased, and hence the error in the reception time of the reflected signal “c” from the break is increased. Therefore, there is a possibility that the accuracy of detecting the location of a break is reduced.  
      Next is a description of a second embodiment of the present invention that can detect the location of a break accurately regardless of the number of relay units.  
      When a signal is received from the direction of detecting unit, each relay unit of the present embodiment relays and transmits the signal to the direction of terminal unit, and also transmits it back to the direction of detecting unit via an ultrasonic transducer  31   a . In this case, the construction may be such that each relay unit also outputs information indicative of the signal reception from the receiving circuit  33 A to the control circuit  36 , as shown by the broken line in  FIG. 3 . The constructions of the detecting unit and the terminal unit are similar to the first embodiment.  
      Next is a description of operations of the second embodiment with reference to operation time charts in  FIG. 8  and  FIG. 9 . The description hereunder assumes that the detecting unit, relay unit and terminal unit are arranged similarly to those in the first embodiment as shown in  FIG. 2 . In the figures, the same symbols are used for the same items as in  FIG. 4  and  FIG. 5 .  
       FIG. 8  is an operation time chart in a normal case where there is no break in the rail  11 . In  FIG. 8 , on receiving a signal S 1 , the relay unit  30 A generates a reception signal R 1  and transmits ultrasonic waves to the relay unit  30 B as a signal S 2 , similarly to the first embodiment, and at the same time also, as shown in the figure, transmits it as a signal S 1 ′ to the detecting unit  20 . On receiving the signal S 2 , the relay unit  30 B generates a reception signal R 2  and transmits ultrasonic waves to the terminal unit  40  as a signal S 3 , and at the same time, as shown in the figure, transmits it as a signal S 1 ″ to the relay unit  30 A. On receiving the signal S 1 ″, the relay unit  30 A generates a signal R 1 ″, and transmits it to the detecting unit  20  as a signal S 2 ″. In the case where the rail  11  is normal, as shown in the figure, the detecting unit  20  generates reception signals R 1 ′ and R 2 ′ based on the transmission signals S 1 ′ and S 1 ″ from the relay units  30 A and  30 B, and the reception signal R 6  based on the transmission signal S 4  from the terminal unit  40 , similarly to the first embodiment. Accordingly, if the reception signal R 6  is generated within a set time range, the detecting unit  20  can judge that the rail is normal.  
      Next is a description of a case where the rail has a break using the time chart in  FIG. 9 .  
      In the case where there is a break between the relay units  30 A and  30 B in the rail  11 , the relay unit  30 A generates normally the transmission signals S 2  and S 1 ′ similarly to  FIG. 8  due to the generation of the reception signal R 1 , and the reception signal R 1 ′ is generated in the detecting unit  20 . The transmission signal S 2  of the relay unit  30 A is reflected by the break, a signal “b” is transmitted from the relay unit  30 A to the detecting unit  20  based on a reflected reception signal “a”, and the detecting unit  20  generates a reception signal “c” based on the reflected waves earlier than the time when the reception signal R 2 ″ is generated at normal times. In the present embodiment, in this case, time T when the signal “c” is received is measured with the time when the reception signal R 1 ′ is generated as a reference, and the location of the break in the rail is calculated from this time T and the propagation speed C. In this manner, it is possible to eliminate an influence of delay time between the reception of signal and the transmission of signal in each relay unit  30 , and also it is possible to specify the location of a break more accurately than in the first embodiment.  
      Furthermore, in the second embodiment, it is possible to compensate for a propagation speed C of ultrasonic waves using the transmission signal S 1 ′ of the relay unit  30 A.  
      That is to say, if a distance between the detecting unit  20  and the relay unit  30 A is L, then the time “to” from when the signal S 1  is generated by the detecting unit  20  to when the reception signal R 1 ′ is generated is represented by “to”=2L/C+tx. Here, “tx” represents a time delay from when the relay unit  30 A receives ultrasonic waves to when it transmits them, which is a fixed value, being fixed by the design. If the time “to” is measured in accordance with the above-described equation, the propagation speed C at that time can be calculated. In this manner, since it is possible to correct the propagation speed C of the ultrasonic waves, if the corrected propagation speed C is used for detecting the location of a break, it is possible to specify the location of a break more accurately.  
      Furthermore, in the case of the second embodiment, as shown in the time chart of  FIG. 8 , since the reception signals corresponding to the number of relay units between the detecting unit and terminal unit are generated in the detecting unit  20 , then by counting the number of generated reception signals, the number of relay units installed can be monitored sequentially on the side of the detecting unit  20 . Therefore, by checking the number of signals to be received and the reception times on the side of the detecting unit  20 , it is possible to monitor the operating status of all of the relay units  30  and the terminal unit  40 .  
      Moreover, in the construction of the second embodiment, if there is a break in the rail, the detecting unit does not receive transmission signals from the relay units and the terminal unit positioned behind the break. Therefore, by monitoring the number of reception signals in the detecting unit  20 , it is possible to detect whether or not there is a break and the approximate location of the break.  
      In the case where there is a break between the last relay unit and the terminal unit, it is not possible to detect whether there is a break from the number of reception signals. In this case, it may be judged whether the reception signal is caused by the reflection wave at the break based on the time that the final reception signal was generated or is caused by the return signal from the terminal unit.  
      Furthermore, in the construction of the present invention, by setting the levels of the ultrasonic wave transmissions in the direction of detecting unit of each relay unit  30  and the terminal unit  40  appropriately, it is possible to monitor whether or not the functions for receiving reflected waves from the break are normal in the detecting unit  20  and the relay units  30 .  
      Such a monitoring function will be described in  FIG. 10  using the transmitting and receiving operation between the detecting unit  20  and the relay unit  30 A as an example.  
      It is assumed that a distance between the detecting unit  20  and the relay unit  30 A is Lx. A transmission signal from the detecting unit  20  is attenuated before it reaches the location of the relay unit  30 A, which is the distance Lx away, and after reached, it is further propagated the distance Lx while being attenuated, to be received by the detecting unit  20 . Therefore, the reception ability of the detecting unit  20  is set so as to enable to detect reflected waves generated at the maximum distance Lx, that is, the location of the relay unit  30 A. To check whether or not the detecting unit  20  has an ability to receive reflected waves generated at the location of the relay unit  30 A, the arrangement may be such that, as shown in  FIG. 10 , the transmission level from the relay unit  30 A to the detecting unit  20  is sufficiently lower than the transmission level of the detecting unit  20 , in the direction of relay unit  30 A, that is, almost the same level as the reflected waves at the location of the relay unit  30 A. Here, to check the reflected wave reception function of the other relay units, the level of transmissions to the direction of detecting unit in the adjacent relay unit and the terminal unit may be set similarly.  
      In this manner, it is possible to monitor whether or not the reflected wave reception functions in the detecting unit  20  and the relay unit  30  are normal, at the same time as the break detection operation, and hence the reliability of the failure detecting system is improved.  
      Each of the above-described embodiments shows a configuration example in which sound wave is propagated to be relayed through the transmission medium in one direction from the detecting unit side to the terminal unit side, and alternatively from the terminal unit side to the detecting unit side. As follows is a description of a configuration example for a case where sound wave is propagated in bi-directions through the transmission medium.  
       FIG. 11  shows a third embodiment of the present invention in which sound wave is propagated in bi-directions through the transmission medium.  
      In  FIG. 11 , a detecting unit  50 , relay units  60 A and  60 B, and a terminal unit  70 , which are arranged along a break detection region of the rail  11 , being the transmission medium, have ultrasonic transducers  51 ,  61  and  71 , respectively, as their transmission and reception sections. The ultrasonic transducers  51 ,  61  and  71  of the units  50 ,  60  and  70  are mounted, respectively, such that the ultrasonic wave transmission and reception faces thereof are almost at right angles to the propagation direction of the rail  11 , being the transmission medium. In this manner, ultrasonic waves as sound wave, transmitted from each of the ultrasonic transducers  51 ,  61  and  71 , are propagated in bi-directions through the rail  11 , the right and left directions in the figure.  
      The detecting unit  50 , which has the same construction as the detecting unit  20 , judges whether or not there is a break, specifies the location of the break or the like, based on transmission and reception information of the ultrasonic waves, and transmits information of the result to a central processing unit or the like (not shown in the figure) via a communication line.  
      The terminal unit  70  is constructed to, when receiving a transmission signal from the relay unit  60 B, transmit an ultrasonic wave signal that is different from the transmission signal from the detecting unit  50  in order to distinguish it from the transmission signal from the detecting unit  50 . A different signal means a signal whose generation time is differentiated by varying the number of pulses for example. Furthermore, The terminal unit  70  stops relay operation after the transmission, that is to say, does not perform an operation in which a signal is transmitted whenever received.  
      The relay units  60 A and  60 B have the same construction, and have, as shown in  FIG. 12 , a receiving circuit  63  and a transmitting circuit  64 , which can be selectively connected to the ultrasonic transducer  61  via a change-over switch  62 , and a control circuit  65 , which switching controls the change-over switch  62  based on information that a signal has been received from the receiving circuit  63  and information of the reception signal form, and also controls the transmission mode of the transmitting circuit  64 .  
      That is to say, in each of the relay units  60 A and  60 B, when a signal is received, the change-over switch  62  is switched to the transmitting circuit  64  side by the control circuit  65  to transmit a signal, and thereafter is returned to the receiving circuit  63  side. Then, if the received signal is a signal from the detecting unit  50 , a signal identical to the signal from the detecting unit  50  is transmitted, while if the received signal is a signal from the terminal unit  70 , a signal identical to the signal from the terminal unit  70  is transmitted. After transmitting the signal identical to the signal from the terminal unit  70 , the relay operation is stopped, and the transmission operation is not performed even if a signal is received.  
      A signal relay operation in the third embodiment will be described with reference to a time chart in  FIG. 13 . Here, the white squares in the figure indicate signals identical to those from the detecting unit  50 , and the black squares indicate signals identical to those from the terminal unit  70 .  
      The detecting unit  50  transmits an ultrasonic wave signal S 1  in a predetermined cycle. A reception signal R 1  is generated in the relay unit  60 A due to the signal S 1 , and the relay unit  60 A transmits a signal S 2  identical to the signal S 1 . Since the signal S 2  is propagated in bi-directions through the rail  11 , reception signals R′ and R 2  are generated in the detecting unit  50  and the relay unit  60 B, respectively, as shown in the figure. The relay unit  60 B transmits a transmission signal S 3  in response to the generation of the reception signal R 2 , and reception signals R 2 ′ and R 3  are generated in the relay unit  60 A and the terminal unit  70 , respectively. Since the relay unit  60 A transmits a transmission signal S 2 ′ in response to the generation of the reception signal R 2 ′, a reception signal R 2 ″ is generated in the detecting unit  50 , and a reception signal R 3 ′ is generated in the relay unit  60 B. Furthermore, the terminal unit  70  transmits a signal S 4 , which is different, for example in pulse numbers, from the signal S 1  from the detecting unit  50 , in response to the generation of the signal R 3 , and a reception signal R 4  is generated in the relay unit  60 B in response to this signal S 4 . The relay unit  60 B transmits a signal S 5  in response to the generation of the reception signal R 4 . The relay unit  60 A transmits a signal S 6  in response to the generation of the reception signal R 5  due to the signal S 5 , and a reception signal R 6  is generated in the detecting unit  50 .  
      Here, since the terminal unit  70  stops the relay operation after transmitting the signal S 4 , then even if a reception signal R 4 ′ is generated due to the signal S 5  from the relay unit  60 B, it does not perform the transmission operation. Similarly, since the relay unit  60 B also stops the relay operation after transmitting the signal S 5  due to the signal S 4  from the terminal unit  70 , even if a reception signal R 5 ′ is generated by receiving the signal S 6 , it does not perform the transmission operation.  
      In this third embodiment, if the reception signal R 6  is generated within a predetermined time range, the detecting unit  50  judges that the rail  11  is normal. In the case where there is a break in the rail  11 , it is possible to detect the break similarly to the second embodiment as shown in  FIG. 9 . Furthermore, similarly to the second embodiment, since the same number of reception signals is generated in the detecting unit  50  as the number of relay units between the detecting unit and terminal unit, by counting the number of reception signals generated, the number of relay units installed can be sequentially monitored in the detecting unit  50 . Therefore, by checking the number of reception signals and the reception times, it is possible to monitor the operating status of the relay units  60 A and  60 B and the terminal unit  70 . It is easier to mount ultrasonic transducers on the transmission medium compared with the construction in which ultrasonic waves are propagated and relayed in a single direction through the transmission medium, and hence there is an advantage that there is no restriction in the mounting.  
      Incidentally, in the case of the third embodiment, as shown in  FIG. 13 , the transmission timing of the signal S 4  from the terminal unit  70  and the transmission timing of the signal S 2 ′ from the relay unit  60 A overlap with each other, so there is a possibility that the reception signals R 4  and R 3 ′ interfere with each other in the relay unit  60 B.  
      Next is a description of a fourth embodiment of the present invention, wherein the problem of reception signal interference as described above is avoided.  
      In the fourth embodiment, the transmission operation after receiving a signal is delayed in the relay operation of each relay unit, and a delay time is changed. To be specific, the construction is such that the delay time from when the relay unit receives a signal to when it transmits a signal is different between the first time it receives a signal and the second or later time it receives a signal. Furthermore, each relay unit has a construction to transmit a signal identical to a signal from the terminal unit when receiving reflected waves from a break.  
      A signal transmission operation in the fourth embodiment will be described with respect to a case where three relay units  60 A,  60 B and  60 C are provided as shown in  FIG. 14  with reference to a time chart in  FIG. 15 . In  FIG. 15 , T 2  designates a delay time of each of the relay units  60 A through  60 C when receiving a signal for the first time, and T 3  designates a delay time when receiving a signal for a second or later time. T 1  designates a signal propagation time between units.  
      As shown in  FIG. 15 , each of the relay units  60 A through  60 C transmits a signal after the lapse of time T 2  from when receiving a first reception signal, and transmits a signal after the lapse of time T 3  from when receiving a second or later time reception signal. Furthermore, the terminal unit  70  transmits a signal after the lapse of time T 2  from when receiving a signal from the relay unit  60 C. Relay operations other than changing the transmission timing are similar to the case of the third embodiment.  
      In such a construction, when the relay unit  60 B receives a signal from the relay unit  60 A and a signal from the relay unit  60 C as shown in A and C of  FIG. 15 , and when the relay unit  60 C receives a signal from the relay unit  60 B and a signal from the terminal unit  70  as shown in B of  FIG. 15 , the reception timing of reception signals is delayed, and hence it is possible to prevent reception interference even in the case where ultrasonic waves are propagated in bi-directions.  
      Next is a description of signal relay operation in the case where there is a break between the relay units  60 B and  60 C in the fourth embodiment, based on the time chart in  FIG. 16 .  
      In a case where there is a break between the relay units  60 B and  60 C, as shown by the broken line in  FIG. 14 , a transmission signal “a” from the relay unit  60 B is reflected by the break, and a reception signal “b” is generated due to the reflected waves. The relay unit  60 B transmits a signal “c” identical to a signal from the terminal unit  70  after the lapse of time T 3  from when the reception signal “b” is generated. In response to the generation of this signal “c”, a reception signal “d” based on the reflected waves is generated in the detecting unit  50  by the relay of the relay unit  60 A. In the detecting unit  50 , after a reception signal a′ is generated based on the transmission signal “a” from the relay unit  60 B, the reception signal “d” is generated with a delay of a time (Tr+T 3 ). Here, the time Tr designates a time after the transmission signal “a” is generated from the relay unit  60 B to when the reception signal “b” is generated due to the reflected waves.  
      Accordingly, in the detecting unit  50 , in this case, by calculating the reception time (Tr+T 3 ) of the signal “d” due to the reflected waves based on the time when the reception signal a′ is generated, it is possible to detect that there is a break between the relay units  60 B and  60 C, at a location corresponding to a propagation time Tr/2, ahead of the relay unit  60 B.  
      In the fourth embodiment, the construction is such that when receiving reflected waves, each of the relay units  60 A through  60 C transmits a signal identical to a signal from the terminal unit  70 . However, the construction may be such that a signal different from the transmission signal from the detecting unit  50  and the transmission signal from the terminal unit  70  is transmitted.  
       FIG. 17  shows a time chart of signal relay operation in a fifth embodiment of the present invention with such a construction. The location of a break is the same as that in  FIG. 14 . Here, the fifth embodiment has a construction in which a signal is transmitted immediately after receiving reflected waves.  
      In  FIG. 17 , when a reflected wave reception signal “b” is generated based on a transmission signal “a”, the relay unit  60 B transmits a signal “e” of a kind different from the signal from the terminal unit  70  after the lapse of the time T 3 . On receiving this transmission signal “e”, the relay unit  60 A transmits a signal “f” without a delay. Accordingly, in the detecting unit  50 , a signal “g” based on the signal “f” from the relay unit  60 A is generated with the time delay Tr, after the reception signal a′ is generated based on the transmission signal “a” from the relay unit  60 B.  
      In such a construction, the detecting unit  50  can specify the location of a break by calculating the reception time Tr of the signal “g” due to the reflected waves based on the time when the reception signal a′ was generated. Therefore, it is not necessary to use time T 3 , compared with the case of the fourth embodiment in  FIG. 16 , and hence the specifying operation of the break location becomes simplified.  
      Incidentally, if each unit is arranged at maximum interval over which a signal can be transmitted, it is possible to reduce the number of relay units as minimum as possible. That is to say, as shown in  FIG. 18 , each of units SA, SB and SC is arranged at the maximum interval L over which a signal can be transmitted. However, in the case where ultrasonic waves are transmitted bi-directions of a rail, even if the reflectance of a signal (ultrasonic wave) is assumed to be 100%, the range where each of the units SA, SB and SC can receive reflected waves is about half the distance L between units, that is, a distance range of L/2 from the location of each unit. Accordingly, if there is a break in the location shown by a broken line in the figure, for example, the unit SB cannot receive reflected waves from the break, and hence although the detecting unit can detect that there is a break because it does not receive a reception signal from the terminal unit, it cannot specify the location of the break.  
       FIG. 19  shows a sixth embodiment of the present invention, which can specify the location of a break regardless of the location of the break using the unit arrangement configuration as shown in  FIG. 18 .  
      The sixth embodiment has a construction in which at least a pair of transmission media, for example, a pair of rails  11 A and  11 B, are used as shown in  FIG. 19 , and by switching the transmission media through which sound wave is transmitted alternately, a signal is transmitted to a unit that can receive reflected waves from a break, so that the location of the break can be specified.  
      In  FIG. 19 , the detecting unit  50 , the relay units  60 A through  60 C and the terminal unit  70  have two ultrasonic transducers each,  51 A and  51 B,  61 A and  61 B, and  71 A and  71 B, respectively. One set of ultrasonic transducers  51 A,  61 A and  71 A is installed on the rail  11 A side, and the other set of ultrasonic transducers  51 B,  61 B and  71 B is installed on the rail  11 B side. Furthermore, the constructions of the relay units  60 A through  60 C and the terminal unit  70  are the same as those shown by the solid lines in  FIG. 3 . Each ultrasonic transducer transmits ultrasonic waves when it receives a signal from the other ultrasonic transducer. Here, similar to the fourth embodiment, the signal generating state and the relay operation of each unit are such that signals generated by the detecting unit and terminal unit are different from each other. At a first time of receiving a signal, a signal is transmitted after the lapse of time T 2 , and at a second or later time of receiving a signal, a signal is transmitted after the lapse of time T 3 . Moreover, when receiving reflected waves from a break, the relay unit transmits a signal indicating that reflected waves have been received.  
      Signal relay operations of the present embodiment in the case where there is no break will be described based on  FIG. 20 .  
      In the sixth embodiment, the transmission operations of  FIG. 20A  and  FIG. 20B  are executed alternately. That is to say, the signal transmission operation from the ultrasonic transducer  51 A of the detecting unit  50  is executed as in  FIG. 20A , and then the signal transmission operation from the ultrasonic transducer  51 B is executed as in  FIG. 20B .  
      In  FIG. 20A , after a transmission signal from the ultrasonic transducer  51 A side is propagated to the terminal unit  70  in the order of solid line arrows A through D in the figure, a signal from the terminal unit  70  is propagated to the detecting unit  50  in the order of broken line arrows E through H in response.  
      That is to say, the transmission signal from the ultrasonic transducer  51 A is propagated through the rail  11 A as shown by the arrow A, to be received by the ultrasonic transducer  61 A of the relay unit  60 A, and a signal is transmitted to the other rail  11 B from the ultrasonic transducer  61 B of the relay unit  60 A as shown by the arrow B, to be received by the ultrasonic transducer  61 B of the relay unit  60 B. Next, a signal is transmitted from the ultrasonic transducer  61 A of the relay unit  60 B to the rail  11 A as shown by the arrow C, to be received by the ultrasonic transducer  61 A of the relay unit  60 C, and a signal is transmitted from the ultrasonic transducer  61 B of the relay unit  60 C to the rail  11 B as shown by the arrow D, to be received by the ultrasonic transducer  71 B of the terminal unit  70 . On receiving the signal, the terminal unit  70  transmits a signal different from the signal from the detecting unit  50 , from the ultrasonic transducer  71 B to the rail  11 B as shown by the arrow E. This signal is received by the ultrasonic transducer  61 B of the relay unit  60 C, and a signal is transmitted from the ultrasonic transducer  61 A to the rail  11 A as shown by the arrow F, to be received by the ultrasonic transducer  61 A of the relay unit  60 B. Next, a signal is transmitted from the ultrasonic transducer  61 B of the relay unit  60 B to the rail  11 B as shown by the arrow G, to be received by the ultrasonic transducer  61 B of the relay unit  60 A. Next, a signal is transmitted from the ultrasonic transducer  61 A of the relay unit  60 A to the rail  11 A as shown by the arrow H, to be received by the ultrasonic transducer  51 A of the detecting unit  50 . Thus, the signal from the terminal unit  70  is transmitted to the detecting unit  50  in response.  FIG. 21  shows a detailed time chart for the transmitting and receiving operations of each unit in the case of  FIG. 20A .  
      When the operations in  FIG. 20A  are completed, the signal transmission operations in  FIG. 20B  are executed. In  FIG. 20B , after a transmission signal from the ultrasonic transducer  51 B side of the detecting unit  50  is propagated to the terminal unit  70  in the order of solid line arrows A′ through to D′ in the figure, a signal is propagated from the terminal unit  70  to the detecting unit  50  in the order of broken line arrows E′ through H′ in response. The transmission operations in  FIG. 20B  and  FIG. 20A  are the same except that the transmitting and receiving order of each ultrasonic transducer in each unit is reversed. Therefore, the description is omitted.  FIG. 22  shows a detailed time chart for the transmission operations of each unit in the case of  FIG. 20B .  
      Next is a description of the transmission operations in the case where there is a break in the vicinity of the relay unit  60 C on the rail  11 A between the relay unit  60 B and the relay unit  60 C, for example, with reference to  FIG. 23  through  FIG. 25 .  
       FIG. 23A  shows the transmission operations when a signal is transmitted from the ultrasonic transducer  51 A in  FIG. 20A , and  FIG. 24  shows a detailed time chart for the transmitting and receiving operations of each unit in this case.  FIG. 23B  shows the transmission operations when a signal is transmitted from the ultrasonic transducer  51 B in  FIG. 20B , and  FIG. 25  shows a detailed time chart of the transmitting and receiving operations of each unit in this case.  
      The transmission operations when a signal is transmitted from the ultrasonic transducer  51 A as shown in  FIG. 23A  will be described.  
      Similarly to the case of  FIG. 20A , a transmission signal from the ultrasonic transducer  51 A of the detecting unit  50  is relayed in the order of solid line arrows A and B in  FIG. 23A , to be received by the ultrasonic transducer  61 B of the relay unit  60 B, and a signal is transmitted from the ultrasonic transducer  61 A of the relay unit  60 B in bi-directions of the rail  11 A as shown by a solid line arrow C. However, if there is a break, the signal of the arrow C transmitted from the ultrasonic transducer  61 A of the relay unit  60 B is not propagated to the relay unit  60 C since it is reflected by the break. As a result, the relay unit  60 C and the terminal unit  70  cannot receive ultrasonic waves, as shown in  FIG. 24 .  
      On the other hand, the signal of the arrow C, which has been propagated from the relay unit  60 B in the left hand direction in  FIG. 23 , is received by the ultrasonic transducer  61 A of the relay unit  60 A similarly to the case where there is no break, transmitted from the ultrasonic transducer  61 B of the relay unit  60 A to the rail  11 B again as shown by a solid line arrow D, to be received by the ultrasonic transducer  51 B of the detecting unit  50  and the ultrasonic transducer  61 B of the relay unit  60 B again. In the case where there is no break, since the relay unit  60 B receives a signal from the relay unit  60 C after receiving the signal of the arrow D as shown in  FIG. 21 , it transmits a signal after the lapse of time T 3  from when it receives the signal of the arrow D. However, in the case where there is a break, since there is no signal transmitted from the relay unit  60 C, when time T 3  has elapsed after receiving the signal of the arrow D as shown in  FIG. 24 , the relay unit  60 B transmits a signal to the rail  11 A as shown by a solid line arrow in  FIG. 23A . As a result, in the case where there is a break, the second transmission timing of the relay unit  60 B becomes earlier than the case where there is no break. The transmission signal of the arrow D is received by the relay unit  60 A, and transmitted from the relay unit  60 A to the rail  11 B as shown by a solid line arrow F, to be received by the detecting unit  50 . Accordingly, the reception timing of the reception signal based on the second transmission operation of the relay unit  60 B in the detecting unit  50  is earlier than the case where there is no break, and thus the detecting unit  50  detects this earlier reception timing, to judge that there is a break.  
      When judging that there is a break, the detecting unit  50  transmits a control signal (shown by a black square in  FIG. 24 ) to advise the relay units to stop relay operations. The control signal is transmitted to the relay units  60 A and  60 B in the order of broken line arrows G, H and I in  FIG. 23A . On receiving the control signal, the relay units  60 A and  60 B stop relay operations after transmitting similar signals. Afterwards, the detecting unit  50  executes a transmission operation from the ultrasonic transducer  51 B as shown in  FIG. 23B  in order to confirm whether or not the relay operation is normal so that ultrasonic waves can be propagated to the terminal unit  70 .  
      Similarly to  FIG. 20B , the transmission signal from the ultrasonic transducer  51 B of the detecting unit  50  is relayed to the relay unit  60 C via the relay unit  60 A and the relay unit  60 B in the order of solid line arrows A′ through D′ in  FIG. 23B , and the signal is transmitted from the ultrasonic transducer  61 A of the relay unit  60 C in bi-directions of the rail  11 A as shown by a solid line arrow D′ in the drawing.  
      The signal of the solid line arrow D′, which has been transmitted from the relay unit  60 C and propagated in the right hand direction in the figure, is reflected by the break, and the reflected waves are received by the ultrasonic transducer  61 A of the relay unit  60 C. The relay unit  60 C that has received the reflected waves, immediately transmits a reflected wave reception signal (shown by a symbol A in  FIG. 25 ) indicating that reflected waves have been received, from the ultrasonic transducer  61 B to the rail  11 B as shown by a broken line arrow F′ in the figure, and the relay unit  60 B that has received this reflected wave reception signal, transmits a similar reflected wave reception signal A immediately from the ultrasonic transducer  61 A to the other rail  11 A as shown by a broken line arrow G′, and the relay unit  60 A that has received this, similarly transmits a reflected wave reception signal A immediately from the ultrasonic transducer  61 B to the rail  11 B as shown by a broken line arrow H′, to be received by the detecting unit  50 . The detecting unit  50  calculates the time (=T 2 −T 3 +2T 1 +Tr) from the reception of the previous signal to the reception of the reflected wave reception signal, and specifies the location of the break.  
      Here, the signal propagated from the relay unit  60 C in the right hand direction in the figure is received by the terminal unit  70 , a transmission signal from the terminal unit  70  is propagated to the detecting unit  50  in the order of the broken line arrows E′ through H′ similarly to the case where there is no break as shown in  FIG. 22 , and relay operations of the detecting unit  50  and each of the relay units  60 A through  60 C are stopped. Furthermore, by receiving the signal from the terminal unit  70 , the detecting unit  50  judges that the operation of each unit is normal.  
      As described above, in a construction in which relay units are disposed between a detecting unit and a terminal unit, ultrasonic waves transmitted from the detecting unit are relayed to the terminal unit by the relay units using a failure detection target section as a transmission medium, and a signal from the terminal unit is transmitted to the detecting unit via the relay units in response, it is possible for only the detecting unit to be connected to a central processing unit by a communication line, and hence it is possible to simplify a failure detecting system network. Furthermore, in a construction in which ultrasonic transducers are installed such that ultrasonic waves are propagated in bi-directions, the ultrasonic wave transmitting and receiving faces of the ultrasonic transducers may be simply installed at right angles to the transmission medium. Therefore, installation environment restrictions are reduced, and hence the work of installing ultrasonic transducers is facilitated.  
      Moreover, for example, in a wireless communication network in which a large number of base stations are installed, since electrical power supplies and communication equipment for network connections are installed in the base stations, if detecting units are installed near the base stations, it is possible to utilize the existing network for another purpose, and also the interconnection length of the communication lines can be shortened. Accordingly, it is also possible to use the wireless train detecting system network as described above, it being ideal to use the failure detecting system in common with the wireless train detecting system.  
      Furthermore, similarly to the conventional system in  FIG. 1 , in the case where the relay units and the terminal unit are also connected to the network in the construction in  FIG. 2 , failures can be monitored by both a failure detecting system using a network such as the conventional system, and a failure detecting system according to a construction of the present invention, in which the detection object is used as an information propagation medium. Such a system structure is a multiple redundancy system using physically different devices, and so safety is improved for failures occurring in either of the two systems.  
      The applications of the failure detecting system of the present invention are not limited to rails, and the system is applicable to pipes for fluid transfer, such as pipelines and the like, provided they are structures constructed over a long distance, through which ultrasonic waves can be propagated.  
     INDUSTRIAL APPLICABILITY  
      The present invention can simplify the structure of a failure detecting system for rails, pipelines and the like, which are constructed over a long distance. Therefore, its industrial applicability is high.