Patent Publication Number: US-7708232-B2

Title: Railroad car lateral instability and tracking error detector

Description:
CROSS REFERENCE APPLICATIONS 
   This application is a non-provisional application claiming the benefits of provisional application No. 60/682,537 filed on May 19, 2005. 

   FIELD OF INVENTION 
   The present invention relates to using a series of inductive proximity sensors to determine the trajectory of railroad car wheel sets over a section of straight railroad track. The trajectory is analyzed to determine if the wheel sets exhibit an unstable lateral motion or exhibit eccentric lateral tracking positions relative to the track. 
   BACKGROUND OF THE INVENTION 
   Freight and passenger railroad car wheel sets can develop sustained lateral oscillations, commonly referred to as high-speed lateral instability or “hunting”, while operating on railroad track at elevated speeds. The consequences of wheel set lateral instability include: 
   1. Excessive suspension wear. 
   2. Damage to lading carried by railroad vehicles, particularly finished automobiles, electronic products or any items that are sensitive to sustained vibrations. 
   3. Increased derailment risk. 
   4. Increased fuel consumption of trains with hunting cars. 
   5. Reduced train operating speeds. 
   Lateral instability is a natural consequence of the typical railroad car wheel set design ( FIG. 1   a ) that consists of a pair of conical shaped wheels  4 , 5  mounted rigidly to a solid axle  6 . This design is inherently unstable as the wheel set rolls on the rails as shown in  FIGS. 1   a  and  1   b . A slight lateral displacement of the wheel set  4 ,  5 ,  6  toward the left rail  2  causes the effective rolling radius of the two wheels of the wheel set to change, with the effective rolling radius of the left wheel  4 , r left , increasing and that of the right wheel  5 , r right , decreasing. Because the wheels  4 , 5  are connected via a rigid axle  6 , they cannot rotate independently of one another. The difference in their rolling radii (r left &gt;r right ) caused by the lateral shift creates longitudinal and lateral creep forces F creep  at the wheel/rail contact area  100 ,  101  that act to restore the wheel set back to its equilibrium position on the rails. 
   However, due to insufficient damping forces in this simple mechanical system the wheel set will tend to oscillate laterally around its equilibrium position, as shown in  FIG. 1   a . The magnitude and frequency of this lateral oscillation depends on several factors, including the amount of taper (T 1 , T 2 ) of the wheel tread cross section, the friction between the wheels  4 ,  5  and rails  2 ,  3  the lateral alignment of the railroad track, the design and condition of the railroad car&#39;s suspension and, most importantly, the weight and speed of the railroad car, which is shown traveling into the page for  FIGS. 1   a ,  1   b . Lateral instability tends to increase as railroad car weight decreases and speed increases. 
   Railroad cars have suspensions commonly referred to as “trucks” or “bogies”. Several different types of trucks are currently used in railroad cars, but most consist of two or more rigid axle wheel sets contained within a framework that rotates horizontally under the railroad car body to negotiate curves.  FIG. 2  shows top views of a typical railroad car truck  7  with laterally unstable wheel sets at five locations L 1 →L 5  along the track. The lateral oscillations of the wheel sets  4 , 5 , 6  are shown, and their trajectories  20 ,  21  are represented as the dashed lines passing through each wheel. L 1  shows truck  7  veering left. L 2  shows truck  7  veering right. L 3  shows truck  7  veering about straight. L 4  shows truck  7  starting to veer left again. L 5  shows truck  7  returning past straight again before veering right again. 
   Attempts have been made to minimize wheel set lateral instability in railroad cars by several methods: 
   1. The use of cylindrical wheel shapes or wheels with very little tread taper. 
   2. Increasing the yaw resistance of railroad car suspensions to prevent lateral wheel set oscillations. 
   3. Adding yaw dampers to railroad car suspensions to damp out the lateral wheel set oscillations. 
   Unfortunately these methods also tend to degrade the ability of railroad car suspensions to negotiate curves, and they increase the cost and maintenance of railroad car suspensions. Thus, the vast majority of freight railroad cars in service in North America are not equipped with any special equipment to control wheel set lateral instability. As a consequence high-speed instability is remedied by simply replacing wheel sets and truck components when lateral instability is detected. 
   Truck tracking errors occur when one or more wheel sets in a truck run with a lateral offset toward one rail or the other. The causes of this behavior include: 
   1. The two wheels of a wheel set have worn to different diameters. 
   2. Different side/side wheel set center distances (d 1 , d 2 ) due to defects in the truck frame ( FIG. 2 , L 1 ) 
   3. Truck frames  7 , locked in misalignment with the railroad car and track due to rotational binding or friction at their pivot point  22  ( FIG. 2 , L 1 ). 
   Three truck-tracking situations are illustrated in  FIGS. 3   a ,  3   b ,  3   c .  FIG. 3   a  shows the top view of a truck  7   a  in proper alignment with the track. Both wheel sets  30 ,  31  are in rolling alignment with the track and are centered between the rails.  FIG. 3   b  shows a truck  7   b  that is not tracking properly. The truck center member  22  is locked in a rotated position such that neither wheel set  32 ,  33  in the truck  7   b  is aligned with the track. The misalignment causes both wheel sets  32 ,  33  to track toward the left rail. In  FIG. 3   c  the truck tracking error is characterized by the leading wheel set  35  tracking toward the right rail, and the trailing wheel set  34  tracking toward the left rail. 
   The current invention utilizes the same array of inductive proximity sensors as the lateral instability detector to detect wheel sets that are tracking toward one rail or the other. The invention also employs an algorithm that evaluates the wheel set trajectory to determine if a wheel set is tracking consistently toward one rail or the other. 
   Several methods have been previously developed to detect and quantify the lateral instability of railroad cars. Prior art involved placing acceleration or force sensors on individual railroad cars and monitoring these sensors in a series of track tests under controlled conditions. These “on-board” methods of detecting and quantifying lateral instability are not practical for the large number of railroad cars in operation on the freight railroads. 
   Another lateral instability detection device has been developed for commercial applications by Salient Systems, Inc. This device employs strain gauge force sensors applied to lengths of rail that sense the lateral forces applied by railroad car wheel sets. Proprietary computer algorithms are applied to the wheel set lateral force data to detect lateral force patterns associated with lateral instability. 
   The lateral force measurement method of detecting lateral instability suffers from the following problems: 
   1. Lateral force measuring sensors must be applied to the rails and calibrated periodically. 
   2. The lateral force sensors cannot be removed and reapplied to the rails for track maintenance. 
   3. Certain track maintenance activities destroy the lateral force sensors. 
   4. The lateral force sensors are susceptible to voltage surges that propagate along the rails. 
   5. Lighter railroad cars may generate lateral wheel forces that are below the sensitivity threshold of the sensors and will not be detected even though the railroad car wheel sets are laterally unstable. 
   The advantages of the lateral displacement measurement method of detecting lateral instability of the present invention compared to the lateral force method include: 
   1. The lateral displacement sensors of this invention are easily removed from the rails and do not require periodic calibration. 
   2. The inductive proximity sensors are well isolated from the rails and are less susceptible to damage from voltage surges in the rails. 
   3. The lateral displacement sensor detection capability is not affected by the magnitude of the lateral wheel force, and very light railroad cars (those more inclined to hunt) are detected as reliably as heavier railroad cars. 
   The shape of the sinusoidal trajectory of a laterally unstable wheel set is more uniform and easier to characterize compared to the wheel set lateral force time series. 
   Prior art for detecting truck tracking errors consists of a commercial product offered by Wayside Inspection Devices Inc. (http://www.wid.ca) called the T/BOGI™ system (U.S. Pat. No. 5,368,260). This device consists of a laser/camera range finder system that scans the side of passing railroad car wheel sets to measure their angular orientation and tracking disposition relative to the track. 
   The disadvantage of this prior art is the complexity and cost of the laser/camera range finder system and the need for periodic cleaning and maintenance. In addition, the T/BOGI™ system obtains one instantaneous measurement of the wheel set tracking position at a single point on the track. 
   The current invention evaluates the tracking position of the wheel set at several points along the track. Furthermore, the present invention detects light railroad cars, which are most prone to hunt. The present invention is easier to maintain and more resistive to damage caused by voltage surges in the rails. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention is to provide a railroad car lateral instability detection system using an array of inductive proximity sensors located at several points along a length of railroad track and oriented to measure the lateral position of wheel sets relative to the track. 
   Another aspect of the present invention is to provide a reliable computer algorithm that evaluates the set of wheel set lateral position sensor readings to detect an oscillating pattern indicating lateral instability. 
   Another aspect of the present invention is to provide a computer algorithm that fits a sinusoidal curve equation to the oscillating pattern of lateral wheel set positions. 
   Another aspect of the present invention is to provide a computer algorithm that evaluates the sinusoidal curve equation to develop a severity index that is related to the lateral acceleration of the unstable wheel set. 
   Another aspect of the current invention is to provide a remote alarm communication sub-system connected to the lateral instability detector. 
   Another aspect of the current invention is to provide a truck tracking error detector within the same system. 
   Another aspect of the current invention is to provide an algorithm that evaluates the wheel set trajectory to determine if a wheel set is tracking consistently toward one rail or the other, thereby indicating a truck tracking error. 
   Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views. 
   An array of inductive proximity sensors are attached to both rails along a length of railroad track and oriented to sense the lateral position of railroad car wheel sets relative to the track. The proximity sensor voltage signals are monitored by a computer running an automatic data collection and control (ADCC) system. 
   As a train passes over the section of track the lateral positions of the wheel sets in the railroad cars are recorded at each proximity sensor pair by the ADCC system. 
   After the train passes, the ADCC system applies an algorithm to the data that evaluates the lateral position data set of each wheel set to determine if an oscillating pattern exists. If so, then a second algorithm fits a sinusoidal curve equation to the oscillating pattern of lateral wheel set positions. A third algorithm evaluates the sinusoidal curve equation to develop a severity index that is related to the lateral acceleration of the unstable wheel set. 
   If an oscillating pattern is not found in the lateral position data of a wheel set, then the ADCC system applies an algorithm that evaluates the data for consistent tracking of the wheel set toward one rail or the other, thereby indicating a truck tracking error. 
   Concurrent with the data collection activity, the ADCC system scans the car identification radio tags of passing railroad cars as a reference for reporting any cars that exhibit lateral instability or truck tracking errors. 
   The ADCC program generates electronic reports of any railroad cars exhibiting lateral instability or truck tracking errors and transmits these reports over the railroad communication network to the appropriate destinations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a , prior art, is a front plan view of a centered railroad car wheel set. 
       FIG. 1   b , prior art, is the same view as  FIG. 1   a  illustrating the variation in rolling radii as the wheel set shifts to the left. 
       FIG. 2 , prior art, is a top view of a laterally unstable railroad car truck at five positions along the track. 
       FIGS. 3   a ,  3   b ,  3   c , prior art, are top views of three truck tracking dispositions. 
       FIG. 4  is a schematic view of the lateral instability/tracking error detection system. 
       FIG. 5  is a detailed top view of an inductive proximity sensor. 
       FIG. 6   a  is an end view of the inductive proximity sensor preferred mounting arrangement on standard North American 136-lb rail. 
       FIG. 6   b  is a top view of the proximity sensor preferred mounting arrangement shown in  FIG. 6   a.    
       FIGS. 7   a ,  7   b  show a typical wheel flange profile on the rail at two lateral positions relative to the inductive proximity sensor detection envelope. 
       FIGS. 8   a ,  8   b ,  8   c  are views of the wheel set on the rails at three lateral positions relative to the inductive proximity sensors. 
       FIGS. 9   a ,  9   b ,  9   c  are three top views of a wheel set on the track with the inductive proximity sensor arrays. 
       FIG. 10   a  shows plots of the trajectories of the wheels in an unstable wheel set, the inductive proximity sensor detection envelopes, and the resulting sensor voltage signals. 
       FIG. 10   b  shows equations used to calculate lateral acceleration. 
       FIGS. 11   a ,  11   b  are two top views of wheel sets exhibiting different tracking positions on the track, the inductive proximity sensors and the resulting sensor voltage signals. 
       FIG. 12  is a logic flowchart of the program that collects and analyzes the inductive proximity sensors data and railroad car identification codes. 
   

   Before explaining the disclosed embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
   DETAILED DESCRIPTION OF THE DRAWINGS 
   Referring first to  FIGS. 1   a ,  1   b  an example of wheel set lateral instability is shown. A disturbance or perturbation in the track  2 ,  3  causes the wheel set  4 ,  5 ,  6  to shift laterally from its centered position ( FIG. 1   a ) toward the left rail  2  ( FIG. 1   b ). Due to the tapered or conical shape of the wheel rim (T 1 , T 2 ), the lateral shift causes the left wheel  4  to roll on a larger rolling radius than the right wheel  5 . Being connected by a rigid axle  6  the wheels  4 ,  5  are forced to rotate at the same speed. This causes the left wheel  4  to generate longitudinal creep forces as it tries to “pull ahead” of the right wheel  5  because of its larger rolling radius. Consequently the left wheel “steers” the wheel set toward the right rail (traveling into the page) and restores the wheel set to an equilibrium position as shown by force arrow F creep . However, insufficient damping in the suspension of the truck containing the wheel set  4 ,  5 ,  6  may allow the lateral oscillations to continue, and the wheel set  4 ,  5 ,  6  becomes laterally unstable. 
   Referring next to  FIG. 2  top views of a laterally unstable truck  7  and wheel sets  4 ,  5 ,  6  are shown at five positions (L 1 -L 5 ) along the track  2 ,  3 . The individual wheel  4 , 5  trajectories are represented by the dashed lines  20 ,  21 . 
   Referring next to  FIGS. 3   a ,  3   b ,  3   c  three truck tracking dispositions are shown.  FIG. 3   a  shows a properly tracking truck  7   a  with the wheel sets  30 ,  31  properly aligned and centered between the rails  2 .  FIG. 3   b  shows a truck  7   b  with tracking errors in which both wheel sets  32 ,  33  track toward the left rail.  FIG. 3   c  shows a truck  7   c  with tracking errors in which the leading wheel set  35  tracks toward the right rail, and the trailing wheel set  34  tracks toward the left rail. 
   Referring next to  FIG. 4  a lateral instability/tracking error detection system  1000  consists of an array of inductive proximity sensors  800  mounted on the left rail  2 , and an array of sensors  900  mounted on the right rail  3 . Voltage signals from the sensors&#39; arrays are continuously monitored by the automatic data collection and control (ADCC) system  10 . 
   The ADCC system  10  concurrently monitors the railroad car identification system  1001  comprised of the radio identification tag reader  13  and wheel detector  12 . The wheel detector  12  generates a voltage pulse as a railroad car wheel passes over the detector  12 . These pulses are recorded by the ADCC system  10 . 
   Electronic alerts or reports of railroad cars exhibiting lateral instability or truck tracking errors can be sent by the ADCC system via the phone, internet, radio or microwave link  11  to the appropriate destinations on the railroad communication network. 
   Referring next to  FIG. 5  the details are shown of an inductive proximity sensor  8 . The sensors used in the preferred embodiment of the current invention are unmodified commercial inductive proximity sensors manufactured by TURCK Inc. Part #Bi50U-Q80-RP6X2-H1143 with a nominal detection range of 0-50 mm, an internal switching relay, a switching frequency of 250 Hz. and an operating voltage range of 10-30 VDC. The sensor  8  employs a high-frequency electrical field generated by a coil embedded within the sensor body. When a ferrous object such as a wheel flange enters the sensor&#39;s electrical field, the amplitude of the field voltage decreases and triggers a relay circuit incorporated in the sensor  8 . The relay circuit switches an applied voltage to generate a signal that is recorded by the ADCC system. 
   Nominal dimensions are d 10 =3.150 inch, d 11 =2.550 inch, d 12 =3.150 inch, d 13 =2.550 inch. Mounting holes H are used to mount the sensor  8  to a bracket. Active face  500  is placed near the flange tip point of a passing wheel. The cable connector  501  receives a cable (not shown). 
   Referring next to  FIGS. 6   a ,  6   b  an inductive proximity sensor  8  and mounting bracket  14 , clamp block  15  and clamp bolt  16  are shown for an installation of the invention on standard 136-lb railroad rail  2 . The dimensions of the sensor mounting bracket  14  must be adjusted according to the rail&#39;s size and wear to maintain the sensor  8  at the detection distances from the rail head as shown in  FIGS. 6   a ,  6   b . Shims  17  are used to adjust the height of the sensor  8  in the field to account for railhead wear. Nominal dimensions are d 20 =2.34 inch, d 21 =1.6 inch. 
   Referring next to  FIG. 7   a  the wheel flange profile  4  is shown on rail  2 . Inductive proximity sensor  8  is shown in its preferred mounting position relative to the rail  2 . The detection envelope of the proximity sensors have been precisely mapped relative to the wheel “flange tip point” (FTP) using steel targets having the dimensions of a railroad car wheel flange. In  FIG. 7   a  and subsequent figures representing the sensors and wheel flange profiles, the flange tip point FTP must fall within the sensor detection envelope E to trigger the sensor relay. 
   In  FIG. 7   a  the wheel flange profile  4  is shown in a position when the wheel set  4 ,  5 ,  6  is centered between the rails  2 ,  3 . The flange tip point resides inside the sensor  8  detection envelope E triggering the internal relay and generating the 10 volt signal shown. 
     FIG. 7   b  shows the wheel flange profile  4  shifted 0.25-inch toward the rail  2 . The flange tip point FTP resides outside of the sensor  8  detection envelope E such that the internal relay is not triggered, and the sensor output voltage remains at 0 volts. The sensors are mounted such that a 00.25-inch lateral shift of the wheel set from the nominal center position toward the rail will result in the wheel flange moving outside of the sensor detection envelope, thereby changing the sensor output from 10 to 0 volts. 
   Referring next to  FIG. 8   a  wheel flange profiles  4 , 5  are shown on left and right rails  2 , 3  passing over inductive proximity sensors  8 ,  9 . The axle  6  joining the wheels  4 , 5  in a wheel set is not shown for clarity. The sensor mounting brackets have been omitted for clarity. 
   In  FIG. 8   a  the wheels in the wheel set have shifted toward the right rail  3 . The flange tip point of left wheel profile  4  is within the left sensor  8  detection envelope E triggering the relay circuit in the sensor and generating the 10-volt output signal  18  shown for the left rail sensor  8 . The flange tip point (FTP) of right wheel profile  5  has moved toward the right rail  3  and out of the right rail sensor  9  detection envelope such that the right sensor  9  relay is not triggered, and the output signal  18  remains 0 volts. The sensor voltage signal pattern  18  of 10 volts from the left rail sensor  8  and 0 volts from the right rail sensor  9  indicates that the wheel set is shifted toward the right rail. 
   In  FIG. 8   b  the wheels in the wheel set are centered between the rails  2 ,  3  such that the gaps G 1  and G 2  are equal. The flange tip point (FTP) of left wheel flange profile  4  is within the left sensor  8  detection envelope E triggering the relay circuit in the sensor  8  and generating the 10-volt output signal  19  shown for the left rail sensor  8 . The flange tip point (FTP) of right wheel flange profile  5  is also within the right rail sensor  9  detection envelope E triggering the relay circuit and generating the 10-volt output signal  19 . The sensor voltage signal pattern  19  of 10 volts from the left rail sensor  8  and 10 volts from the right rail sensor  9  indicates that the wheel set  4 ,  5  is centered between the rails  2 ,  3 . 
   In  FIG. 8   c  the wheels in the wheel set have shifted toward the left rail  2 . The flange tip point FTP of the left wheel flange profile  4  has moved toward the left rail  2  and out of the left rail sensor  8  detection envelope E such that the sensor relay is not triggered, and the output signal  20  remains at 0 volts. The flange tip point of the right wheel flange profile  5  is within the right sensor  9  detection envelope E triggering the relay circuit in the sensor and generating the 10-volt signal  20  shown for the right rail sensor  9 . The sensor voltage signal pattern  20  of 0 volts from the left rail sensor  8  and 10 volts from the right rail sensor  9  indicates that the wheel set  4 , 5  is shifted toward the left rail  2 . 
   Referring next to  FIGS. 9   a - 9   c  the proximity sensor array voltage signal patterns that correspond to different wheel set trajectories are shown.  FIG. 9   a  shows a stable wheel set  4 , 5 , 6  tracking properly between the rails  2 , 3  through the test zone. The pattern of left rail sensors voltage signals  21  and right rail sensors voltage signals  22  corresponding to this trajectory are shown. All 16 sensors output 10-volt signals when the wheel passes over. 
     FIG. 9   b  shows a wheel set  40 ,  50 ,  60  exhibiting a slight lateral oscillation through the test zone. The pattern of sensor voltage signals  23 , 24  that correspond to this trajectory are shown. Sensors at positions  8  and  9  on the right rail  3  output 0-volt signals as the wheel passes over because the wheel set has moved toward flange contact with the right rail  3 . Sensors at positions  1  and  16  on the left rail  2  output 0-volt signals as the wheel set passes over because the wheel has moved toward flange contact with the left rail  2 . 
     FIG. 9   c  shows a wheel set  41 ,  51 ,  61  exhibiting more severe lateral oscillations through the test zone. The pattern of sensor voltage signals  25 , 26  corresponding to this trajectory are shown. Sensors at positions  7 - 10  on the right rail  3  output 0-volt signals as the wheel set passes over because the wheel set has moved toward flange contact with the left rail. Sensors at positions  1 , 2  and  15 , 16  on the left rail  2  output 0-volt signals as the wheel set passes over because-the-wheel set has moved toward flange contact with the left rail. 
   Comparing the patterns in  FIGS. 9   b  and  9   c  reveals that the amplitude of the wheel set lateral oscillations is related to the pattern of inductive proximity sensor voltage signal outputs. The greater the number of adjacent proximity sensors with voltage signals of 0 volts the greater the lateral oscillation amplitude of the wheel set. 
   Referring next to  FIG. 10  an example analysis of the inductive proximity sensor voltage signals for an unstable wheel set is shown. The lateral (y) scale of the plot in  FIG. 10  is greatly exaggerated for clarity. 
   The algorithm first scans the left and right rail sensor voltage signals to find 0-volt readings that correspond to the wheels of an oscillating wheel set moving laterally toward the rail and outside of the sensor detection envelopes. In this example the wheel set shifted toward the left rail at sensor locations  2 - 6  and toward the right rail at sensors locations  11 - 14  as indicated by the 0-volt signals from these sensors. 
   Next, the algorithm determines the set of distance indices (L F ,L L ,R F ,R L ) corresponding to the positions of the first and last sensors signaling 0 volts according to EQS.1-4 in  FIG. 10   b . The distance indices of these locations are required to determine the approximate wavelength and lateral amplitude of the wheel set trajectory. Because the locations at which each wheel moved beyond the sensor detection envelopes may not occur precisely over a sensor, the algorithm assumes that these locations are half the distance between the outer 0-volt reading sensors and the adjacent 10-volt reading sensors on each rail as shown. This results in acceptably small errors in calculating the wheel set trajectory wavelength and lateral amplitude. 
   The wavelength λ of the lateral wheel set sinusoidal oscillation is calculated from the average distance between the indices according to EQ. 5 in  FIG. 10   b.    
   Next, the right and left rail chord lengths C R  and C L  are calculated according to EQ. 6 and EQ. 7 in  FIG. 10   b.    
   The wavelength λ, right rail chord length C R  and sensor lateral detection distance A R ′ from the nominal wheel lateral tracking line are used in EQ. 8 of  FIG. 10   b  to calculate the maximum amplitude of the wheel set lateral oscillation toward the right rail A R . Likewise the wavelength λ, left rail chord length C L  and sensor lateral detection distance A L ′ are used in EQ. 9 of  FIG. 10   b  to calculate the maximum amplitude of the wheel set lateral oscillation toward the left rail A L . A R  and A L  are then averaged according to EQ. 10 in  FIG. 10   b  to obtain the lateral amplitude A of the wheel set trajectory. 
   Next, the oscillatory frequency ω of the wheel set is calculated according to EQ. 11 in  FIG. 10   b  using the wheel set forward velocity V and the lateral oscillation wavelength λ. 
   The maximum amplitude of the wheel set lateral acceleration a max  is calculated from the average lateral oscillation amplitude A and the lateral oscillatory frequency ω according to EQ. 12 in  FIG. 10   b . The lateral instability detection system uses the maximum lateral acceleration a max  as an indicator of the relative severity of the unstable lateral oscillations because the forces imposed on the railroad car suspension, on the railroad car lading and on the railroad track are proportional to the lateral acceleration. 
   Referring next to  FIGS. 11   a ,  11   b  the proximity sensor array voltage signal patterns  27 , 28  that correspond to different wheel set tracking trajectories are shown.  FIG. 11   a  shows the sensor array voltage signal patterns for wheel set  4 , 5 , 6  centered between the rails  2 , 3  and tracking properly through the test zone. The sensor voltage signal patterns  27 , 28  on both rails show all sensors signaling 10 volts as the wheel set passes over. 
     FIG. 11   b  shows the sensor array voltage signal patterns  29 ,  30  for a wheel set  40 ,  50 ,  60  tracking consistently toward the left rail  2 . The right rail sensor voltage pattern  30  shows all sensors signaling 10 volts while the left rail sensor voltage pattern  29  shows all sensors signaling 0 volts. The patterns  29 ,  30  of  FIG. 11   b  would reverse if the wheel set  4 ,  5 ,  6  tracked consistently toward the right rail. 
   The tracking detector algorithm scans the sensor voltage signals for consistent readings of 10 volts from every sensor in the array on one rail and 0 volts from every sensor in the array on the other rail. If such patterns are found, then the wheel set is flagged as having a tracking error, and a report is issued identifying the wheel set by its position in the railroad car and the railroad car identification code. 
   Referring next to  FIG. 12  a logic flowchart for the program that collects and analyzes the sensor data and the railroad car identification codes is shown. The logic flow sequence is as follows: 
   1. Block  1000  monitors the inductive proximity sensors for high (10-volt) signals that indicate a train has arrived at the test zone. 
   2. When a sensor signal goes high block  1001  records the train time. 
   3. Block  1002  records the wheel detector ( 12  of system  1001 ) signals. 
   4. Block  1003  records the railroad car radio identification tag reader ( 13  of system  1001 ) data. 
   5. Block  1004  records the inductive proximity sensor array voltage signals. 
   6. Block  1005  monitors the elapsed time since the last sensor high signal to determine when the train has left the test zone. The program flows back to block  1002  if the train is still in the test zone. 
   7. If block  1005  determines that the train has left the test zone the program proceeds to block  1006  which records the end of file times and closes the files containing raw proximity sensor voltage signal data, wheel detector data and railroad car identification code data. 
   8. An algorithm operates in block  1007  that associates the wheel detector and railroad car identification code data with the proper proximity sensor array data for each wheel set and railroad car. 
   9. Block  1008  checks the proximity sensor array voltage patterns of the wheel set for lateral instability. 
   10. If block  1008  finds the current wheel set to be unstable then it proceeds to block  1010 , which scans the proximity sensor array voltage patterns for the 0-volt index locations. 
   11. Block  1011  calculates the lateral oscillation wavelength for the left and right rails based on the locations of the 0-volt indices. 
   12. Block  1012  calculates the average lateral oscillation amplitudes for the left and right rail wheels. 
   13. Block  1013  calculates the oscillatory frequency of the wheel set from its linear velocity and oscillation wavelength and the maximum lateral acceleration of the wheel set. 
   14. Block  1014  writes the wheel set lateral instability records to a file on disk. 
   15. If the wheel set is found to be stable in block  1008  then the program proceeds to block  1009  which checks the proximity sensor array voltage signal patterns of the wheel set for tracking errors. 
   16. Block  1014  writes the wheel set tracking error records to a file on disk. 
   17. After the records for all of the wheel sets in the train are analyzed block  1015  generates an electronic report of the wheel sets and associated railroad cars that exhibit instability or tracking errors. 
   18. Block  1016  transmits the electronic report via the railroad communication network. 
   19. The program proceeds back to block  1000  to wait for proximity sensor signals indicating that a train is present at the test zone. 
   PREFERRED EMBODIMENT OF THE INVENTION 
   The preferred embodiment of the invention to detect lateral instability and tracking errors in North American freight railroad train service is shown in  FIG. 4  and consists of: 
   1. Arrays of 16 inductive proximity sensor pairs  8 , 9  mounted on the left and right rails  2 , 3  of a railroad track  1  with a spacing of approximately 24 inches between sensor pairs. 
   2. Inductive proximity sensors  8 , 9  with a nominal detection range of 50 mm, an internal switching relay, a switching frequency of at least 250 Hz. and an operating voltage range of 10-30 VDC. 
   3. Inductive proximity mounting brackets ( 14 - 17  in  FIGS. 6   a ,  6   b ) that mount the inductive proximity sensors on the rails such that the sensor face resides 1.60 inches below the top of rail and centered laterally 2.34 inches from the inside edge of the rail head. 
   4. A railroad car identification system  1001  in  FIG. 4  comprised of the radio identification tag reader  13  and wheel detector  12 . 
   5. An automatic data collection and control computer  10  in  FIG. 4  that monitors and records the signals from the inductive proximity sensors, applies algorithms to analyze the sensor signals, records the railroad car radio identification tag information, generates reports and transmits them over the communication link  11  to the railroad network. 
   6. A straight section of railroad track with minimal surface and alignment deviations and average train speeds above 50 mph. 
   Alternative embodiments of the invention are appropriate for other railroad applications such as high-speed passenger trains. In this application the inductive sensor design and spacing would be modified to detect the longer wavelength lateral oscillations and higher operating speeds of passenger railroad cars. Another less expensive embodiment would only use a single left or right array to compare a pattern deviating from a chosen normal pattern of wheel segments either in or out of a set of proximity sensor envelopes. 
   Although the present invention has been described with reference to preferred embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred. Each apparatus embodiment described herein has numerous equivalents.