Patent Publication Number: US-2010127133-A1

Title: Method and system for detecting impacts on areas to be monitored on a running vehicle

Description:
The invention relates to the field of monitoring impacts on a running vehicle, and in particular ballast impacts on a railway vehicle circulating at high velocity. 
     A running vehicle, of the railway vehicle type, is by nature exposed to the external environment when moving on traffic ways, such as railway tracks lying on ballast. 
     Ballast is the stone or gravel layer onto which the rails lie. Its function is to transmit the stresses generated by railway vehicles circulating on the ground, without the latter becoming distorted through packing. The ballast has also a function to embed rails so as to provide some resistance to longitudinal distortions. 
     When a railway vehicle is circulating, a drawing force, depending on the velocity of the railway vehicle, is created between the vehicle and the track it is circulating on. Because of such a drawing force, the ballast lifts off the ground, onto which it lies, and is propelled at high velocity on the vehicle or in the vicinity of the tracks. The ballast collides with the floor and the axles of the railway vehicle. Such impacts are detrimental and decrease the life of railway equipment. Moreover, such projections are likely to wound maintenance technical personnel located in the vicinity of the traffic tracks. Such a phenomenon is traditionally referred to as a “ballast flight”. 
     Such a phenomenon, although well known, has a higher and higher importance for the railway transport professionals because of the higher and higher velocity of railway vehicles on the rails. In order to better understand such a phenomenon, it has been suggested to measure and model the ballast flight. 
     Methods for measuring ballast projections upon the circulation of a railway vehicle have already been implemented but have not made it possible to reach satisfactory results. 
     Thus, it is known for example to hang microphones inside a railway vehicle in the vicinity of the floor of said vehicle so as to be able to listen to the various impacts on the external surface of the floor. Unfortunately, it is difficult to distinguish the noise of the ballast impacts from the background noise, in particular vibrations of the railway vehicle upon its circulation on the tracks. The results of such a method are inaccurate and do not allow to draw satisfactory conclusions. 
     Another drawback of this type of method lies in the fact that when an impact is detected by the microphone, it is not able to be geographically located. Otherwise stated, it is not known where on the floor of the railway vehicle the ballast has hit the floor. In fact, the noise of the impact is received nearly simultaneously by all the microphones, preventing from accurately determining the part of the floor of the vehicle being damaged. 
     In order to overcome such a problem, it has been suggested arranging video recording cameras filming the external surface of the vehicle floor so as to film the impacts. Due to the high velocity of the ballast when it is projected against the vehicle floor, it is appropriate to use so-called “fast” cameras, because of the high number of pictures being recorded per second. Thus, in comparison with a traditional camera, a fast camera records a much higher number of data on a similar period of time and thereby allows capturing the path of the ballast upon its flying. 
     For true condition testing on a railway vehicle path, the memory storing capacity of the fast camera is limited and it is not possible to continuously film the ballast impacts for the duration of the test. 
     In order to overcome such a drawback, a solution comprises continuously taking pictures, but only capturing video recordings when an impact occurs, i.e. when an impact noise is being detected by the microphone. In order to visualize the impact, the camera is associated with a pre-triggering system allowing to, upon reception of a control message, obtain a video recording prior to the impact. 
     The pre-triggering system comprises, to this end, a buffer or volatile memory comprising data as filmed by the camera during the last recording seconds. Conventionally, the buffer memory of the camera comprises data being recorded during the last 5 to 10 seconds. When an impact is detected, the contents of the buffer memory is stored in a storing memory (non volatile memory also referred to as read-only memory), the storing memory comprising the impact pictures. 
     Such impact detecting and visualizing method requires an operator able to trigger the video recording at each impact detection. Because of the background noise and the subjectivity of the operator, detecting impacts is very random, numerous impacts being not detected while false alerts are emitted. 
     As a result of such a drawback, it is necessary to visualize all the video recordings, in order to discard non relevant recordings resulting from detection false alerts. Moreover, it is necessary to perform such a step again for all the recording cameras. Such a method is thus difficult to be implemented and does not allow for reliably used results to be obtained. 
     In order to solve at least some of such problems, the Applicant provides a method for detecting impacts on areas to be monitored of a running vehicle, created by objects being projected as a result of the velocity thereof, wherein vibration sensors are provided in direct contact with the areas to be monitored on the running vehicle, and signals from the vibration sensors are analyzed for detecting impacts. 
     Such a detection system advantageously allows for detecting with accuracy the impacts while directly measuring the vibration resulting from the impact on an area to be monitored on the running vehicle. Moreover, an impact detected by a vibration sensor is able to be located on the running vehicle, enabling to model the ballast flight phenomenon. 
     This invention is the result of the discovery of damage caused by ballast impacts on a railway vehicle but it is understood that this invention applies to any circulating vehicle (car, plane, either taking off/landing, etc.), being able to project objects (ballast, small stones, etc.) because of the velocity thereof. 
     Preferably, an impact coefficient is measured for each vibration signal provided by a vibration sensor, each impact coefficient is compared to a predetermined threshold value and an impact detection message is emitted when said impact coefficient exceeds said predetermined threshold value. 
     Measuring an impact coefficient on a vibration signal allows overcoming vibrations on the running vehicle detected by vibration sensors when the vehicle is running. Indeed, a neural network could have been used for determining whether a vibration signal, measured by a sensor, represents an impact signal, but such a neural network is sensitive to noises and vibrations of the vehicle and does not provide satisfactory results. 
     Still preferably, an impact coefficient of said vibration signal is measured on a time window with a predetermined time duration. 
     Measuring the impact on a time window allows, on the one hand, to limit the number of data from the vibration signal to be considered for calculating the impact coefficient and thereby, allows detecting in real time the impact. On the other hand, this allows to very finely detect the impacts on a time window with a predetermined size and thus, to accurately determine the instant of the impact. 
     Preferably, the window size is determined as a function of the duration of the vibration of an area to be monitored after the impact. 
     More preferably, after an impact has been detected, the impact detection is locked, as long as the impact coefficient of said vibration signal is not lower than said threshold value. 
     Still preferably, the impact coefficient corresponds to the peak to peak measurement of the vibration signal provided by the vibration sensor. 
     A conventional solution would have been to compare the effective value of the signal to a threshold value. However, such a measurement is noise sensitive. As an impact results in intense vibrations in a very short period of time (close to the pulse), a peak to peak measurement allows characterizing an impact in a vibration signal faster and more reliably. 
     Indeed, an impact signal corresponds to a pulse noise, the average level of which is low and the peak to peak level is high. Moreover, such a measurement is easy to calculate and could be carried out rapidly. By means of such a measurement, it is possible to detect an impact in real time. 
     According to a particular embodiment of this invention, the threshold value is function of the area to be monitored on the running vehicle. 
     Using an adaptive threshold value for implementing the impact detecting method according to the invention allows adapting the detection as a function of, for example, the nature of the area to be monitored (material, density, surface), as well as of its response in vibration. Thus, areas to be monitored in cast iron and aluminium do not exhibit the same threshold value, an impact being detected under conditions being specific to the area wherein the vibration sensors are arranged. 
     Preferably, the impact detection message comprises the reference of the area to be monitored having received the impact. 
     This advantageously allows locating for each detected impact, the area to be monitored on the running vehicle having detected the impact. Thus, a map could be achieved of impact locations on the running vehicle and the ballast flight could be modelled relevantly. 
     According to a particular embodiment of this invention, each area to be monitored is associated with at least one video recording camera able to film said area. From the impact detection message, the reference to the area to be monitored having received the impact is extracted and only the video recording camera associated with the reference of said area to be monitored is activated. 
     This advantageously allows to only activate the video recording cameras likely to visualize the projection of the object on the vehicle, thereby avoiding to make use of a systematic study foe all the recordings. Processing the recorded data thus become easier. 
     This invention also relates to a system for detecting impacts on areas to be monitored of a running vehicle, created by objects projected because of the velocity thereof. The system comprises vibration sensors in direct contact with areas to be monitored on said running vehicle and a data processing unit, connected to the plurality of vibration sensor, arranged as to receive vibration signals provided by the plurality of vibration sensors and detect impact signals from said vibration signals for each of the areas to be monitored on the running vehicle. 
     Preferably, the processing unit comprises a discrimination module arranged for:
         measuring an impact coefficient for each vibration signal provided by a vibration sensor;   comparing said impact coefficient to a predetermined threshold value; and   transmitting an impact detection message when said impact coefficient exceeds said predetermined threshold value.       

     Still preferably, the processing unit comprises a matching table, connected to the discrimination module, associating for each vibration sensor the area to be monitored in which it is arranged. 
     The matching table thus allows putting in relation the reference of the sensors with the location thereof. 
     According to a particular embodiment of this invention, the processing unit comprises a video management module, the input of which is connected to the discrimination module and the output of which is connected to a plurality of video recording cameras, the video management module being arranged to control the plurality of video recording cameras according to the impact detection messages transmitted by the discrimination module. 
     Preferably, the video management module is connected to a location table wherein each area to be monitored on the running vehicle is associated with at least one video recording camera able to film said area, the video management module being arranged for receiving an impact detection message from the discrimination module; extracting from the impact detection message the reference of the area to be monitored which received the impact; transmitting said reference of the area to be monitored to the location table, said location table returning in response the reference of the recording camera associated with said area to be monitored and controlling only the actuation of the recording camera corresponding to the reference of the camera being provided. 
    
    
     
       This invention will be better understood through the appended drawing wherein: 
         FIG. 1  shows a sectional view of a railway vehicle provided with a monitoring system according to this invention wherein the vibration sensors of said monitoring system are fixed on the floor of said railway vehicle; 
         FIG. 2  shows a schematic side view of  FIG. 2 ; 
         FIG. 3  shows a schematic diagram of the monitoring system of  FIGS. 1 and 2 ; 
         FIG. 4  shows a signal measured by a vibration sensor of the monitoring system according to this invention, the signal being displayed on a graph with its abscissa indicating the time in seconds and the coordinate, the value of vibrations as measured in the vertical direction (m/s 2 ); and 
         FIG. 5  shows another signal measured by a vibration sensor of the monitoring system according to this invention, the threshold value being represented. 
     
    
    
     The system for detecting and monitoring the impacts in a running vehicle according to this invention will now be described for a railway vehicle. It is understood that this invention applies to other running vehicles, in particular automotive vehicles. 
     In this example, the system is arranged so as to detect and monitor ballast projections on the railway vehicle and in particular on the external surface of the floor thereof as well as on the axles thereof. This invention applies to any objects projected onto the vehicle as a result of its velocity (ballast, small stones, etc.). 
     Referring to  FIG. 1 , a system  1  for detecting and monitoring the ballast projection is arranged on a railway vehicle  2 , running on longitudinal rails embedded in ballast  3 . The ballast  3  is in the form of pebbles, having a diameter ranging from 5 to 10 cm. 
     As previously indicated, ballast  3  is the stone or gravel layer onto which the rails lie. Its function comprises transmitting the stresses generated by railway vehicles circulating on the ground, without the latter becoming distorted through packing. The ballast has also a function of embedding rails so as to provide some resistance to longitudinal distortions. 
     In this example, a railway vehicle  2  is considered, with its floor  21  comprising axles onto which there are mounted wheels lying on the rails. The floor  21  comprises an internal surface, facing the interior of the railway vehicle  2 , and an external surface facing the ground and the ballast  3 . 
     Referring to  FIGS. 1 and 2 , the monitoring system  1  comprises a plurality of vibration sensors  11   a - 11   d  arranged directly on the internal surface of the floor  21  of the railway vehicle  2  and a data processing unit  12  mounted on the railway vehicle  2 . The data processing unit  12 , connected to a plurality of vibration sensors  11   a - 11   d , is arranged for receiving vibration signals provided by the vibration sensors  11 A- 11   d . The vibration sensors  11   a - 11   d  are here wire-connected to the processing unit  12 , but it is to be understood that a radio link could be appropriate as well. 
     The vibration sensors  11   a - 11   d  are mounted in the railway vehicle  2  for sensing vibrations from the floor  21  of the railway vehicle  2  in the vertical direction, i.e. in the direction orthogonal to the plane wherein the floor  21  of the railway vehicle  2  extends. The vibration measurement direction by the vibration sensors  11   a - 11   d  is referred to as Z on  FIG. 2 . 
     In this example, the floor  21  of the railway vehicle  2  comprises a plurality of supporting structural plates  21   a - 21   c , or supporting metal sheets, forming the floor  21 . Each plate  21   a - 21   c  forms a vibration unit because a ballast impact  3  on part of a plate  21   a - 21   c  makes said plate vibrate as a whole. Otherwise stated, vibrations resulting from a ballast impact  3  on a given plate  21   a - 21   c  are not transmitted to the adjacent plates  21   a - 21   c  of said plate  21   a - 21   c  having received the impact. 
     The vibration sensors  11   a - 11   d  have here the form of accelerometers. The vibration sensors  11   a - 11   d  are here chosen so as to be able to measure the vibrations resulting from an impact on a plate  21   a - 21   c  of the floor  21  in order to avoid the vibration sensor  11  from becoming saturated. The frequency range of the sensors here varies from 5 Hz to 1 kHz. 
     The vibration sensors  11   a - 11   d  are here distributed on areas to be monitored of the floor  21  of the railway vehicle  2 , the areas to be monitored corresponding to the supporting structural plates  21   a - 21   c  of the floor  21 . 
     In contrast to a sound measurement as previously carried out with a microphone, measuring vibrations using one or more vibration sensors  11   a - 11   d  directly arranged on a supporting structural plate  21   a - 21   c  allows to associate for each impact detected by the processing unit  12  the plate  21   a - 21   c  being hit by a ballast projection  3 . As each plate  21   a - 21   c  forms a vibration unit, detecting an impact by a vibration sensor  11  makes possible to infer which plate  21   a - 21   c  has received the impact. 
     The detection and monitoring system  1  of this invention not only allows counting ballast impacts but it also allows defining in real time the impact location. This system is then referred to as an impact located detection system. 
     The processing unit  12  of the monitoring system  1  is mounted preferably in the railway vehicle  2  and is connected to the vibration sensors  11   a - 11   d  distributed on the different supporting plates  21   a - 21   c  of the floor  21  on the railway vehicle  2 . Referring to  FIG. 1 , the processing unit  12  is mounted on a vertical wall of the railway vehicle  2  but also lie on the internal surface of the floor  21  in the railway vehicle  2 . 
     The processing unit  12  here is in the form of a set of calculators, such as computers and servers, but it is to be understood that one single calculator could be appropriate as well. 
     Referring to  FIG. 3 , the processing unit  12  comprises a discrimination module  121  having as a main function to gather the different vibration signals provided by the vibration sensors  11   a - 11   d  and to detect, among such signals, a ballast impact  3 . In other words, the role of the discrimination module  121  is to extract from an overall vibration signal from a vibration sensor a vibration signal characteristic of a ballast impact  3 . 
     To this end, referring to  FIG. 4 , the discrimination module  121  is arranged so as to receive a vibration signal  110  and to analyse the latter in real time on a time window with a predetermined size T. Otherwise stated still, each vibration signal  110  is cut sequentially into signal portions with a predetermined duration T. Each portion of vibration signal  110  is then analyzed by the discrimination module  121  so as to distinguish a signal characteristic of a ballast impact. 
     Referring to  FIG. 3 , in a first step of the analysis of the portion of vibration signal  110 , an impact index Ci is measured corresponding, in the present case, to the peak to peak value Vcc of the vibration signal  110  upon the predetermined duration. Then the impact index Ci is compared to a predetermined threshold value Vs, also referred to as threshold Vs. The threshold Vs, previously calculated, depends on several parameters of the area to be monitored  21   a - 21   c , such as its material, its density, its response in vibration or its vibration surface. 
     The position of the different vibration sensors  11   a - 11   d  on the different areas to be monitored  21   a - 21   c  of the floor  21  on the railway vehicle  2  are referenced in a matching table  123  stored in the processing unit  12 , as shown on  FIG. 3 . The different thresholds Vs are also associated with the different areas to be monitored  21   a - 21   c  of the floor  21  in said matching table  123  (see table hereinbelow). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Matching table 
               
            
           
           
               
               
               
            
               
                   
                 Area to be 
                   
               
               
                 Vibration sensor 
                 monitored 
                 Threshold value 
               
               
                   
               
               
                 11a 
                 21a 
                 Vs1 
               
               
                 11b 
                 21b 
                 Vs1 
               
               
                 11c 
                 21c 
                 Vs2 
               
               
                 11d 
                 21c 
                 Vs2 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 3 , the railway vehicle ( 2 ) comprises three areas to be monitored ( 21   a - 21   c ), a threshold value Vs having been previously determined for each one. Conventionally, a vibration threshold Vs is empirically defined for each family of areas to be monitored (steel structural plates, cast iron structural plates). Thus, as above indicated in table 1, the area to be monitored  21   c  has a threshold value Vs 2  being different from that of areas to be monitored  21   a - 21   b . In this example, the supporting structural plate, corresponding to the area to be monitored  21   c , is formed in a different way from that of the supporting structural plates corresponding to the areas to be monitored  21   a - 21   b.    
     Referring to the example of  FIG. 4  showing a vibration signal coming from the vibration sensor  11   b  mounted on the area to be monitored  21   b  of the floor  21 , the threshold value Vs 1  for the vibration signal  110  of said area to be monitored  21   b  is equal to 300 m/s 2 . 
     For each vibration signal  110  received by the discrimination module  121 , this latter determines the vibration sensor  11   a - 11   d  from which is coming the vibration signal  110 . By interrogating the matching table  123  to which the discrimination module  121  is connected, the discrimination module  121  determines the area to be monitored  21   a - 21   c  on which is placed the vibration sensor  11   a - 11   d , the vibration signal  110  of which is analyzed. Thus, in case of detection of a ballast impact  3 , the discrimination module  121  immediately provides the area to be monitored  21   a - 21   c  hit by the ballast  3 . 
     Upon the comparing step, referring to  FIG. 4 , in the time window T 1 , the impact index Ci is higher than the threshold value Vs 1  of the zone to be monitored  21   b . An impact is detected by the discrimination module  121  that send back to the processing unit  12  a detection message Mi wherein it is indicated that an impact has occurred, mentioning the area to be monitored having received the impact and the sensing time for the ballast impact  3 . Here, the detection message Mi indicates that an impact has occurred at the time t 1  on the area to be monitored  21   b.    
     On the contrary, still referring to  FIG. 4 , in the time window T 2 , the impact index Ci is lower that the threshold value Vs 1 , no impact is detected by the discrimination module  121  and no detection message Mi is sent. 
     The discrimination module  121 , parameterized by the threshold values Vs adapted to each one of the areas to be monitored  21   a - 21   d , allows detecting precisely a ballast impact on the floor  21  of the rail vehicle  2 . 
     The adaptive threshold values Vs make possible to overcome the background noise made by the running vehicle  2  vibrations while running on rails, such a monitoring and detection system  1  being advantageously able to limit the number of false alarms. 
     In a particular implementation of the invention, the discrimination module  121  is arranged in order to distinguish two successive impacts. For this purpose, the discrimination module  121  is parameterized to count an impact when the impact index Ci is higher than the threshold value Vs for a first time window. No other impact can be counted until the impact index Ci is not lower than the threshold value Vs for a second time window, posterior to the first one. This locking mechanism allows avoiding a same impact to be counted several times by the monitoring and detection system  1 . 
     Referring to  FIG. 5 , the impact index Ci, corresponding to the peak to peak measurement Vcc, is higher than the threshold value Vs for the time windows T′ 1 , T′ 2  and T′ 3 . Further to the impact detection in the time window T′ 1 , a locking signal is activated, so preventing to count a new impact for the time windows T′ 2  and T′ 3 . 
     The locking signal is deactivated for the time window T′4 wherein the peak-to-peak measurement Vcc is lower than the threshold value Vs. It advantageously allows detecting a new impact in the time window T′ 12 . 
     The detection and monitoring impact system  1  can be associated with a geolocation system and a mapping. During the rail vehicle path, the detection system transmits the number of impacts detected and the locus of the impact on the vehicle. This information is transmitted to the geolocation system which reproduces on a map of the traveled area the number of impacts being received. Thus, one can detect on which travel part the number of impacts is the most important. 
     Advantageously, one can represent on the map the travel sectors that are likely to favour the ballast flight. For instance, the travel sectors favouring the ballast flight are shown in red. The coloured map is then transmitted to the track maintenance agent who can thus arrange the traffic ways to reduce the ballast flight phenomenon. 
     In a preferred embodiment of the invention, the detecting and monitoring system  1  comprises a plurality of video recording cameras  17   a - 17   d  which are mounted on the rail vehicle  2  in order to visualize one or several areas to be monitored  21   a - 21   c  in the rail vehicle  2 . 
     Each video recording camera  17   a - 17   d  is here mounted on the outer surface of the floor  21  of the rail vehicle  2  so that at least one area to be monitored  21   a - 21   c  on the rail vehicle  2  is comprised in the view angle of one of the video recording cameras  17   a - 17   d.    
     As shown in the preamble of the present application, the video recording cameras  17   a - 17   d  used are so-called “fast” cameras due to the high number of pictures they can register per second (from 100 to 250 pictures per second). 
     Each video recording camera  17   a - 17   d  is associated with a pre-triggering system allowing, upon reception of a detection message Mi, to obtain a video recording being anterior and posterior to the impact. 
     The pre-triggering system comprises to this end a buffer or volatile memory which comprises data captured by the camera during the last second recordings. Conventionally, the buffer memory of the camera comprises data registered during the last 5 to 10 seconds. When an impact is detected, the content of the buffer memory is stored in a storage memory (non-volatile memory), the storage memory comprising the pictures of the impact. 
     In practice, the pre-triggering system is arranged for storing in memory the data filmed 5 seconds before and 5 seconds after the effective impact of ballast  3  on the floor  21 . 
     Referring to  FIGS. 1 and 2 , the video recording cameras  17   a ,  17   b  are mounted in different positions on the floor  21  of the rail vehicle  2  and are connected to the processing unit  12 , the video recording cameras  17   c ,  17   d  being not visible on these figures. The cameras  17   a - 17   d  are here wire-connected but it is evident that they could also be connected by a radio link. 
     A location table  124 , shown on the following table 2, associates for each area to be monitored  21   a - 21   c  on the running vehicle  2  at least one video recording camera  17   a - 17   d  being able to film said area  21   a - 21   c , that is the areas to be monitored  21   a - 21   c  that are visible by said video recording camera  17   a - 17   d . 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Location table 124 
               
            
           
           
               
               
            
               
                 Area to be monitored 
                 Recording camera 
               
               
                   
               
               
                 21a 
                 17a 
               
               
                 21b 
                 17a, 17b 
               
               
                 21c 
                 17a, 17b, 17c, 17d 
               
               
                   
               
            
           
         
       
     
     Thus, the video recording camera  17   a  illustrated on  FIG. 1  films the supporting structural plates  21   a  and  21   b  while the video recording camera  17   b  only films the supporting structural plate  21   b.    
     Referring to  FIG. 3 , the processing unit  12  of the monitoring system  1  further comprises a management module  122  arranged for controlling the different video recording cameras  17   a - 17   d  according to the detection messages Mi transmitted by the discrimination module  121 . To this purpose, the management module  121  is connected at the input to the output of the discrimination module  121  and at the output to the video recording cameras  122  also communicating with the location table  124 . 
     As previously indicated, when an impact is detected by a vibration sensor  110 , the discrimination module  121  transmits a detection message Mi in which it is indicated that an impact has occurred on the area to be monitored  21   a - 21   c  at a determined time. Said detection message Mi is transmitted to the management module  122  of the processing unit  12  which extracts from the detection message Mi the impact time as well as the area to be monitored  21   a - 21   c  having received the impact. The reference of the area  21   a - 21   c  is then introduced into the location table  124  which returns back in response the references of the video recording cameras  17   a - 17   d  which are oriented toward said referenced zone  21   a - 21   c.    
     Thus, also with the preceding example, when a ballast impact  3  is detected by the vibration sensor  11   b , the discrimination module  121  of the processing unit  12  transmits a detection message Mi, stating that the supporting structural plate  122  receives the detecting message Mi and looks up the location table  124  to determine which video recording cameras have to be activated. Here, as the supporting structural plate  21   b  has received an impact, the video recording cameras  17   a  and  17   b  are activated (see table 2). 
     Thus, only the video recording cameras  17   a ,  17   b  that are likely to film the ballast flight  3  are activated. The visualization of the video records is then facilitated, the number of records corresponding to false alarms (detection errors) being very limited. 
     In this example, the management module  122  of the processing unit also comprises a video recording database, not shown, wherein the different video recordings registered by the video recording cameras  17   a - 17   d  are stored. 
     It has been described here vibration sensors under the form of accelerometers, but it is to understood that strain gauges could also be appropriate. 
     Here, it has been described vibration sensors mounted on the lower surface of the floor but it is evident that they could also be mounted on the outer surface. In this case, it is necessary to protect the sensors from outdoor conditions (rain, frost, etc.) but also from impacts due to the ballast.