Abstract:
An ultrasonic rail inspection system includes an ultrasonic transducer mounted on a yoke for attachment to a frame of a rail inspection vehicle. The ultrasonic transducer transmits ultrasonic pulses and receives reflected ultrasonic pulses. A control device controls the ultrasonic transducer. A clock device provides clock signals to the control device. The control device controls the ultrasonic transducer to transmit the ultrasonic pulses at a fixed pulse repetition period.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to the art of detecting flaws, particularly cracks, in railroad rails. 
       BACKGROUND OF THE INVENTION 
       [0002]    For safety reasons, it is important that railroad rails are inspected regularly for the presence of flaws or defects. A flaw or defect may be an existing small crack, or a location where a crack could arise. Cracks have the tendency to grow, and a broken rail may have catastropic consequences, so it is important to detect potential crack sites as early as possible. Many cracks are rarely visible from the outside; if they are, the rail is probably completely broken, and this is a situation that is to be avoided. Hence, a technology is required that is capable of detecting flaws in the rail. 
         [0003]    Such technology is ultrasonic measurement. Briefly said, an ultrasonic pulse is coupled into the rail, and the reflection of this pulse is captured. The pulse reflects from material surfaces, such as the outside rail surface but also the internal surface of a flaw. Thus, the reflection pattern in the case of a flaw differs from the reflection pattern in the case of an undisturbed rail. However, although the measurements could be performed stationary from a technical point of view, this is not desirable from a practical point of view, because the railroad should remain open for railroad traffic. Therefore, mobile systems have been developed that include a railroad vehicle carrying a mobile ultrasonic transducer. 
         [0004]    Such mobile systems, however, introduce the complication that the ultrasonic transmitter and the ultrasonic receiver are moving with respect to the rail under inspection. One aspect of this complication is that it is more complicated to achieve a good signal coupling between rail and transducer. Another aspect of this complication is that the pulse measurement itself requires some measuring time, basically caused by the travelling time of the pulse from the ultrasonic transmitter to the reflection surface and back to the ultrasonic receiver. This sets restrictions on the minimum repetition frequency of the measurements. On the other hand, the repetition frequency of the measurements, or better: the time period between successive measurements, determines the spatial distance between the investigated rail locations, indicated as “inspection pitch”. With a certain maximum requirement for the inspection pitch (i.e. the pitch should be a certain value or shorter), a certain maximum operational speed for the inspection vehicle results (i.e. the operational speed can not be higher than a certain value). Given the fact that the railroad tends to be used more and more intensely, with regular trains driving faster and/or at closer mutual distance, it is desirable that the maximum operational speed for the inspection vehicle is as high as possible. If the inspection vehicles are too slow, they either disrupt regular service, or inspection can be done only during the night when traffic is reduced. 
         [0005]    Mobile ultrasonic rail inspection systems can be distinguished in two basically different types. Both types require a liquid coupling substance between the ultrasonic transducer and the rail. A first type of inspection system is indicated as a contact-type system: in these systems, the transducer is held at a small distance from the rail, with a thin film of coupling liquid in between. A drawback of this type of system is the relatively large consumption of coupling liquid. This drawback is avoided in the second type of inspection system, which is indicated as a wheel-type system: in these systems, the transducer is positioned inside a wheel-like container filled with coupling liquid and riding on the rail, which wheel-like container has a flexible wall that is able to conform to the shape of the rail head. 
         [0006]    In practical circumstances, with an inspection pitch of about 3 mm, the maximum operational speed for the inspection vehicle is about 72 km/h for the contact-type system and about 37 km/h for the wheel-type system. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to improve on the existing technology so that the maximum operational speed for the inspection vehicle can be substantially increased or the inspection pitch can be reduced, or both. The principle proposed by the present invention is applicable in each type of wheel-type inspection system, and will result in an improvement in each system as compared to the capabilities of such system without the invention, ceteris paribus, but the extent of the improvement may depend on the actual system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
           [0009]      FIG. 1A  is a schematic cross-section of an ultrasonic rail inspection system; 
           [0010]      FIG. 1B  schematically illustrates the disposition of several transducers in an ultrasonic rail inspection system; 
           [0011]      FIG. 2  is a graph showing ultrasonic signals as a function of time; 
           [0012]      FIG. 3  is a graph showing ultrasonic signals as a function of time; 
           [0013]      FIG. 4  is a schematic block diagram of an ultrasonic rail inspection system; 
           [0014]      FIG. 5  is a schematic cross-section of an ultrasonic rail inspection system; 
           [0015]      FIG. 6  is a graph comparable to  FIG. 2 ; 
           [0016]      FIG. 7  is a graph comparable to  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1A  schematically shows a cross-section of a rail  1 , having a central rail body or web  2 , a rail foot  3  and a rail head  4 . A rail inspection vehicle  100  comprises a yoke  21  mounted to a frame  20 . The vehicle  100  comprises an ultrasonic rail examination system  30 , which includes at least one ultrasonic transducer  33  mounted on the yoke  21  within a rotating container  31  filled with coupling liquid  32 , typically water or glycol or a mixture thereof. The container  31  is pressed onto the rail head  4 . The wall of the container  31  is flexible, at least to a certain extent, so that the container wall conforms to the top surface of the rail head  4 . A contact area of the container wall is indicated at  34 . With travel of the vehicle  100  over the rail  1 , the container  31  rolls over the rail and is therefore also indicated as “wheel”. For a more detailed description of an example of such wheel-type system, reference is made to U.S. Pat. No. 5,419,196 by way of example. 
         [0018]    It is noted that the figure only shows one transducer  33 , but in practice the system  30  may comprise multiple transducers, directed to the rail  1  under different angles.  FIG. 1B  is a schematic side view of the wheel  31  on the rail  1 , showing multiple transducers  33  positioned within the wheel  31 . It can be seen that the transducers aim their ultrasonic beams  41  to the same contact area  34  under different angles. These angles are standardised; typical angles are 0°, 40°, 70°, as will be known to persons skilled in the art. And transducers may look forward or backward. 
         [0019]    The basic operation is as follows. At a certain moment in time, the transducer  33  sends an ultrasonic pulse  41  into the rail head  4  via the contact area  34 . The ultrasonic pulse  41  reflects from a reflective surface within the rail  1 . The reflected pulse  42  is received by the transducer  33 . The reflective surface may be the bottom of the rail foot  3 , as shown in the figure, but may also be a flaw such as for instance a crack. 
         [0020]      FIG. 2  is a graph showing the ultrasonic signals as a function of time. The horizontal axis represents time, the vertical axis represents signal strength, in arbitrary units. At time t 0 , the transducer  33  sends the ultrasonic pulse  41  to the rail head  4 . At time t 0 R, the reflected pulse  42  is received by the transducer  33 . 
         [0021]    The distance between transducer  33  and contact area  34  may be in the order of about 135 mm. The sound velocity in the medium  32  is in the order of about 1700 m/s. Consequently, the travel time of the pulses  41  and  42  in the medium  32  is in the order of about 160 μs. This travel time will be indicated as internal delay Δi. 
         [0022]    In the rail  1 , the sound velocity is in the order of about 5900 m/s for the longitudinal mode and in the order of about 3200 m/s for the transversal mode. The travel distance (back and forth) in the rail depends on the angle of the ultrasonic beam and on the presence of defects. Theoretically, the travel distance can be zero. Without defects, the travel distance for instance for a 40° transducer may be in the order of 450 mm, in which case the travel time of the pulses  41  and  42  in the rail is in the order of about 140 μs for the transversal mode. This travel time will be indicated as external travelling time Δe. Thus, the total time lapse Δt=Δi+Δe from t 0  to t 0 R may vary within a reflection range  43  from 160 to 300 μs. The width of this reflection range  43  corresponds to the maximum expected external travelling time Δe MAX . 
         [0023]    It should be clear to a person skilled in the art that the above calculation is given by way of example, and that the precise values for a given concrete system may deviate depending on the precise system design. 
         [0024]    When performing a rail inspection, there may be requirements or regulations defining the defect resolution. A typical required maximum pitch is 3 mm, which basically means that the ultrasonic pulses should be transmitted for each 3 mm of travel progress of the vehicle. In the currently practiced art, the vehicle is provided with a sensor that very accurately measures the travel distance of the vehicle, and that sends a trigger pulse to the ultrasonic transducer after a predetermined distance, for instance a trigger pulse for every 1, 2 or 3 mm of progress. However, a subsequent ultrasonic transmission pulse  44  at time t 1  should not interfere with the reflection pulses, which can be expected up to 300 μs from the previous ultrasonic transmission pulse. This means that the trigger pulses may not be less than 300 μs apart. In the example of a 3 mm inspection pitch, this translates to a maximum vehicle speed of 37 km/h. 
         [0025]      FIG. 3  is a graph comparable to  FIG. 2 , showing the ultrasonic signals as a function of time for a system proposed by the present invention. Instead of transmitting the subsequent ultrasonic transmission pulse  44  after termination of the reflection range  43 , the present invention proposes to transmit the subsequent ultrasonic transmission pulse  44  at time t 1  before start of the reflection range  43 . Further, instead of triggering the ultrasonic transducer by trigger pulses derived from a travel distance measurement, the ultrasonic transducer is fired at a constant repetition frequency, or in other words at a constant repetition period t 1 −t 0 =P. 
         [0026]      FIG. 3  also shows a next subsequent ultrasonic transmission pulse  45  at time t 2 =t 0 +2P. The figure illustrates that the key of the present invention is that the reflection range associated with an N-th transmission pulse is always located between the (N+1)th and (N+2)th transmission pulses. It will be seen that the following formulas apply: 
         [0000]        P≦Δi   (1)
 
         [0000]      2 P≧Δi+Δe   MAX   (2)
 
         [0027]    From these formulas, it follows that the maximum expected external travelling time Δe MAX  should be smaller than the internal delay Δi, and that the ultrasonic pulse repetition period P can be set in the range from (Δi+Δe MAX )/2 to Δi. In practice, it may be that the compression of the container  31  varies during the travel of the vehicle. This will have as consequence that the internal delay Δi varies. In order to accommodate for this variation, it may be preferred that P be set in the centre of said range. 
         [0028]    For the example as described above, with Δi=160 μs and Δe MAX =140 μs, a preferred value for the ultrasonic pulse repetition period P would be 155 μs. This means that a 3 mm inspection pitch will be achieved at a vehicle speed of about 70 km/h. 
         [0029]    It is noted that JP-58151554 discloses a method for detecting defects in the bottom part of a railroad rail. An ultrasonic pulse a1 is emitted into the rail under a certain angle, and will reflect from the rail bottom as bottom pulse b1. In case there is no defect, this bottom pulse b1 will not reach the ultrasonic transducer. In case there is a defect, the pulse will reflect from the defect as reflected pulse c1 towards the transducer. The time duration between emission of a1 and detection of c1 is indicated as TE. The document discloses that the next ultrasonic pulse a2 is emitted before the reflected pulse c1 reaches the transducer. The time between a1 and a2 is indicated as T, and the formula T&lt;TE applies. Further, the formula 2T&gt;TE applies. However, the document does not specify a wheel-type inspection system, consequently does not mention the travel of the sound wave in the liquid medium of the inspection wheel, and is silent about any reflection originating from the upper surface of the rail. More particularly, the document does not specify that the next ultrasonic pulse a2 should be transmitted before detection of the reflection originating from the upper surface of the rail. 
         [0030]      FIG. 4  is a schematic block diagram of the ultrasonic rail examination system  30  according to the present invention. Reference numeral  51  indicates a control device, for instance a suitably programmed microprocessor. 
         [0031]    Reference numeral  52  indicates a clock, for providing clock signals to the control device  51 . The clock  52  may be an external component to the control device  51 , but may also be an integrated component of the control device  51 . On the basis of the clock signals, the control device  51  is designed to provide trigger signals for the transducer  33  at a predetermined pulse repetition period P. 
         [0032]    Reference numeral  53  indicates a sensor for sensing travel distance of the vehicle  100 . Such sensor may for instance include a tachometer for sensing rotation angle of a wheel axle of the vehicle  100 . The sensor output signal is provided to the control device  51 . 
         [0033]    Reference numeral  54  indicates an output device. The output device may include for instance a printer, a plotter, a display monitor, a memory. The control device  51  is programmed to generate an output signal that contains information from the ultrasonic reflection signal received from the transducer  33  in conjuction with the location information (distance along the rail) as obtained from the travel distance sensor  53 . The information from the ultrasonic reflection signal received from the transducer  33  may include the original receipt signal as received by the transducer  33 , or a processed signal that shows artefacts. In conjunction with the distance information received from sensor  53 , the control device  51  may translate the ultrasonic reflection signal to a displayed signal as function of location. Use of the sensor  53  allows the vehicle to be driven at varying speed: at lower speed, the displaced signals will be closer together and at higher speed they will be further apart, but they will at all times be correlated to distance, which is very intuitious for the personnel using the system. 
         [0034]    Although the above explanation contains a description for a single transducer  33  only, it should be clear that the same explanation applies to a system that comprises two or more transducers. 
         [0035]      FIG. 5  is a schematic cross-section of an ultrasonic rail inspection system  230  of a rail inspection vehicle  200 . For the ultrasonic rail inspection system  230 , the same explanation as above may apply. The figure shows a transducer  33  mounted on a carrier  221  that has an inverted U-shape with arms  222 ,  223  and a bridge  224 . The arms  222 ,  223  are connected to two aligned halves of a horizontal shaft  225 . The wheel  31  rotates around the shaft  225 . The ends of the shaft  225  are mounted in a frame mount  226  having an inverted U-shape and being attached to vehicle frame  20 . It is noted that the system comprises a plurality of transducers, but the figure only shows one for sake of simplicity. 
         [0036]    It is easily possible to exchange the set of transducers: by detaching the frame mount  226  from the vehicle frame  20 , the entire unit of frame mount  226 , shaft  225 , carrier  221 , transducers  33 , container  31  with liquid  32  can be removed and a different unit can be mounted in place. When mounting such unit, it is important that the unit is mounted in the correct orientation: the transducers should have correct orientations with respect to the rail. Each transducer is designed to direct an ultrasonic beam to the contact area  34  between container  31  and rail head  4  under a specific angle to detect specific artefacts at specific locations within the rail  1 , and a mis-alignment of the carrier  221  will result in mis-alignment of the transducers  33  which in turn may result in corrupted measuring results. It is customary that the transducers are adjusted with respect to a rail: for instance, the alignment is adjusted until the 0° transducer shows a maximum in reception strength, which indicates that the beam is perpendicular to the rail. However, practical rails are hardly suitable for performing such adjustment, and therefore this alignment needs to be done in a laboratory setting with a special test rail portion. This is very cumbersome. 
         [0037]    In order to provide a solution to this problem, the present invention proposes to attach a first 3D-gyrosensor  241  to the carrier  221  and a second 3D-gyrosensor  242  to the frame mount  226 . In an adjustment situation, which may be in a laboratory setting and which does not require the presence of a train, the transducers are aligned accurately with respect to the carrier  221 , and the carrier  221  is aligned with respect to the frame mount  226  using the first gyrosensor  241 . The second gyrosensor  242  is now set to zero. Consequently, whenever the second gyrosensor  242  shows a zero-reading, it is certain that the transducers  33  are aligned correctly. 
         [0038]    When this unit is mounted on an inspection vehicle, it may be that the mounting accommodation of the vehicle is not aligned accurately. This may show as a deviation of the second gyrosensor  242 , and can be compensated by adjusting the mounting of the frame mount  226  to the vehicle frame  20  until the second gyrosensor  242  shows a level reading. 
         [0039]    In the following, a variation of the present invention will be discussed. 
         [0040]    As mentioned before, the system typically comprises multiple transducers, and one of these transducers typically is the so-called 0°-transducer, which is a transducer that sends its ultrasonic beam  41  vertically downward. In  FIG. 1B , this specific transducer is indicated at reference numeral  133 . It is typically a transducer for generating longitudinal waves. 
         [0041]    Apart from the reflections generated by reflective surface within the rail, there will also be a reflection from the interface between liquid  32  and contact wall area  34  and a reflection from the interface between contact wall area  34  and the top surface of rail head  4 . In view of the relatively small thickness of the wall  31 , the two reflection pulses resulting from these reflections substantially coincide or at least overlap, and can be considered as one reflection pulse, that is indicated as interface pulse. 
         [0042]    Under normal, i.e. flawless conditions, there will subsequently be a reflection from the bottom surface of the rail foot  3 , as indicated in  FIG. 1A . This reflection will be indicated as bottom pulse. With for instance a rail height of 159 mm, the bottom pulse (longitudinal mode) is typically expected about 54 μs later than the interface pulse. It is noted that the bottom pulse itself will partly reflect from the top surface of rail head  4  and then again reflects from the bottom surface of the rail foot  3 , so that a so-called “repeated bottom pulse” is expected 54 μs later than the first bottom pulse. In fact, this repeated reflection up and down will result in a train of pulses 54 μs apart, but each subsequent pulse has an amplitude much smaller than the corresponding previous pulse, so that in practice only the first and second bottom pulse are relevant. 
         [0043]      FIG. 6  is a graph comparable to  FIG. 2 , showing the interface pulse at reference numeral  61 , showing the bottom pulse at reference numeral  71 , and showing the repeated bottom pulse at reference numeral  72 . Δv TB  indicates the travel time from top to bottom of the rail and back along a vertical path. This figure also shows that, in prior art, the subsequent pulse  44  would be transmitted after the train of bottom reflection pulses  71 ,  72  have been received. 
         [0044]      FIG. 7  is a graph comparable to  FIG. 3 , showing a subsequent transmission pulse  44  that largely coincides with the interface pulse  61 . This means that the pulse repetition period P is equal to Δi. It may be that control is set to a pulse repetition period equal to Δi, but it is also possible that the control device  51  detects the arrival of the interface pulse  61  and in response triggers the transmission of the next pulse. It is noted that all transducers are triggered at the same time. 
         [0045]    It is noted that all transducers  33  are preferably arranged along a circular arc around the contact area  34 , so that the internal delay Δi is equal for all transducers, in which case the control device  51  can simply trigger all transducers at the same time. If needed, timing corrections for the individual transducers  33  can be effected, either in hardware or in software, for instance by displacing a transducer. In any case, the pulse repetition frequency is now determined by the length of the liquid path for the 0°-transducer  133 . 
         [0046]    It is important to correlate the transmitted/reflected pulse with a specific position on the rail. After all, if a flaw is detected, repair or replacement should be done at the position of the flaw. For this correlation purpose, a tacho sensor is present, giving pulse signals after a certain travel distance, for instance each 0.1 mm. Effectively, each tacho pulse corresponds to a well-defined rail position. The tacho pulses can be input to the control device  51 , so that the control device  51 , whenever it generates a trigger signal and processes the corresponding reflection signals, can correlate the reflection signals and any possible flaw information contained therein to the tacho signals and the position information contained therein, as should be clear to a person skilled in the art. 
         [0047]    In  FIG. 7 , it will be seen that the bottom reflection pulses  71 ,  72  from an N-th transmission pulse  41  of the 0°-transducer  133  will be received between the (N+1)th and (N+2)th transmission pulses  44  and  45 , i.e. in the time interval needed for the (N+1)th transmission pulse  44  to travel towards the interface contact area  34  and back. The time interval Δv TB , needed for the longitudinal sound wave to travel from the top surface of the rail to the bottom surface of the rail and back, will obviously depend on the rail geometry. Any reflections from defects or objects in the rail head, web or foot will be received within this time interval. 
         [0048]    It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, two or more functions may be performed by one single entity, unit or processor. Even if certain features are recited in different dependent claims, the present invention also relates to an embodiment comprising these features in common. Any reference signs in a claim should not be construed as limiting the scope of that claim. 
         [0049]    In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.