Patent Publication Number: US-6701228-B2

Title: Method and system for compensating for wheel wear on a train

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to railroads generally, and more particularly to a system and method for determining wheel size to compensate for wheel wear. 
     2. Discussion of the Background 
     Controlling the movement of trains in a modern environment is a complex process. Collisions with other trains must be avoided and regulations in areas such as grade crossings must be complied with. The pressure to increase the performance of rail systems, in terms of speed, reliability and safety, has led to many proposals to automate various aspects of train operation. For example, positive train control (PTC) and automatic train control (ATC) systems have been widely discussed in recent years. 
     Some automated systems rely on global positioning system (GPS) receivers for indications of train speed and position (as used herein, “global positioning system” and “GP S” refer to all varieties of global positioning system receivers, including, but not limited to, differential global positioning system receivers. Still other systems use inertial navigation systems (INSs) for determining speed and location. However, GPS receivers and INSs sometimes fail, and for that reason it is desirable to have a back-up system. 
     One method that can be used in case of a positioning system failure is to measure the rotation of motor, axle or wheel rotation to determine the speed at which a train is traveling and/or the distance which a train has traveled. Each time the wheel makes a compete revolution, the distance traveled by the wheel is equal to its circumference in the absence of any slippage. Thus, if the radius R of the wheel is known, the distance traveled for each revolution of the wheel is equal to 2πR. However, the radius of a wheel changes over time due to wheel wear. For example, a standard train wheel can decrease in size from 40 inches to 36 inches over its useful life. Therefore, the distance traveled in each wheel revolution can vary between 125.7″ and 113.1″, a difference of approximately 12.6″ or 10%. This error is significant. 
     What is needed is a method and system that compensates for wheel wear. 
     SUMMARY OF THE INVENTION 
     The present invention meets the aforementioned need to a great extent by providing a method and system for compensating for wheel wear in which wheel rotation information from a revolution counter or a tachometer and position and/or speed information from an independent positioning system such as GPS or INS are measured over a predetermined distance and used to determine the size of the train wheels. This process is performed periodically to compensate for wheel wear. 
     In one aspect of the invention, the system includes a map database and the position information from the independent positioning system is used to as an index to ensure that the rotation data used for the speed/position comparison between the position system and rotation data is collected in an area of straight and flat track so as to exclude errors in the rotation data caused by wheel slippage and turns. 
     In another aspect of the invention, the data used for the comparison between the speeds/distances indicated by the positioning system and by the rotation data is collected over a long distance to minimize known errors in the positioning system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant features and advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a logical block diagram of a train control system according to one embodiment of the invention. 
     FIG. 2 is a flowchart showing a wheel wear compensation technique according to one embodiment of the invention. 
     FIG. 3 is a logical block diagram of a train speed signal distribution system according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be discussed with reference to preferred embodiments of train control systems. Specific details, such as wheel sizes and types of positioning systems, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments and specific details discussed herein should not be understood to limit the invention. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 is a logical block diagram of a train control system  100  according to the present invention. The system  100  includes a control module  110 , which typically, but not necessarily, includes a microprocessor. The control module  110  is connected to a revolution counter  120 . The revolution counter  120  measures rotation of a locomotive wheel (not shown in FIG. 1) on a train. The revolution counter  120  may be of any type, including mechanical, magnetic, and optical. The revolution counter  120  may measure the rotation of a wheel directly, or may measure rotation of an axle to which the wheel is connected, or may measure rotation of a motor driveshaft or gear that powers the wheel. 
     Also connected to the control module  110  is a positioning system such as a GPS receiver  130 . The GPS  130  receiver can be of any type, including a differential GPS receiver. Other types of positioning systems, such as inertial navigation systems (INSs) and Loran systems, can also be used. [As used herein, the term “positioning system” refers to the portion of a positioning system that is commonly located on a mobile vehicle, which may or may not comprise the entire system. Thus, for example, in connection with a global positioning system, the term “positioning system” as used herein refers to a GPS receiver and does not include the satellites that are used to transmit information to the GPS receiver.] The GPS receiver  130  provides position and speed information to the control module  110 . 
     The control module  110  uses the position information from the GPS receiver  130  as an index into a map database  140 . The map database  140  provides information including track grade and curvature to the control module  110 . As will be explained in further detail below, this information is used in some embodiments to ensure that rotation information from the revolution counter will not include rotation information that is corrupted due to wheel slippage and/or errors due to track curvature. 
     Referring now to FIG. 2, a flowchart  200  illustrates operation of a wheel wear correction method according to one embodiment of the present invention. The control module  110  determines whether track conditions are acceptable at step  210 . In some embodiments, this is accomplished by obtaining the current position from the GPS receiver  130  and indexing the map database  140  to determine the track grade and curvature over a predetermined length of upcoming track over which rotation information is to be collected. 
     The predetermined length of track is preferably of a sufficient length such that any errors introduced by the inaccuracy of the global positioning system receiver  130  are minimized. Obviously, it is advantageous to use as great a length as possible since the effect of positioning systems errors are decreased as the length is increased. However, there is a trade-off that must be made because if the length is too great, the time required to complete the wheel correction algorithm is too long and/or the amount of curvature and grade in the track segment over which the data is to be taken preclude running the algorithm over too much track in the system In some embodiments, the predetermined length of track is 100,000 meters. In such an embodiment, with a global positioning system having a position error on the order of 30 meters, the total error is equal to (30+30)/100,000=0.0006=0.06%. 
     In the embodiment described by FIG. 2, the determination as to whether track conditions are acceptable is made at the start of the algorithm. In other embodiments, rotation data is only collected if the train is traveling greater than some minimum. The reason behind this is that most wheel slippage occurs at slow speeds as a locomotive is attempting to accelerate. Most locomotives use electric induction motors, and most electric motors used in locomotives have torque curves with torques decreasing as speed increases such that it is not possible for the locomotive to generate enough torque to cause the wheels to slip above certain speeds. In some embodiments, the minimum speed at which data will be collected is 15 m.p.h.; in other embodiments, the minimum speed is 20 m.p.h. 
     In yet other embodiments, the wheel acceleration is monitored to detect wheel slippage. If an acceleration exceeds a threshold, the collected information is discarded and the entire process is started over. 
     In still other embodiments, the system notes the upcoming sections of the track in which either the grade or curvature is above a corresponding threshold and does not include those distances and any corresponding rotation information collected over those distances in the calculations. Such embodiments are particularly useful for railroads in which long, straight and level sections of track are not present in many areas. 
     If the track conditions are not favorable at step  210 , the system delays for a period of time at step  220  and repeats step  210  until track conditions are favorable. When track conditions are favorable at step  210 , the control module  110  determines a start position from the global positioning receiver  130  at step  230  and counts rotations as measured by the revolution counter  120  at step  240 . When a threshold (which may be a number of rotations and/or a time period) has been reached at step  250 , the control module  110  determines a stop position from the global positioning receiver  130  at step  260 . Next, at step  270 , the control module  130  calculates the distance D traveled based on the start and stop positions measured at steps  230  and  260 , respectively. Then the control module  130  determines the radius R of the wheel at step  280  according to the equation R=D/2πT r , where T r , is the total number of rotations counted over the distance D. The control module  110  then delays, at step  290 , for a period of time such as a day (it is not necessary to run the algorithm often as train wheels wear slowly). 
     In the above-discussed embodiments, a predetermined distance is used. It should be noted that the predetermined distance will vary depending upon the accuracy of the positioning system used and the particular environment in which the invention is used. 
     In the foregoing embodiments, data is not collected when the system determines that track conditions are not favorable. However, in cases where curvature exceeds the threshold, it is also possible to allow data collection to occur and correct the data for the curvature. 
     In the foregoing embodiments, positional inputs from the positioning system are used; however, it will be readily apparent that speed can also be used. For example, if the current speed S of the train is known from the positioning system, then the wheel size can be determined according to the equation S=DF r =2πRF r , where D is the distance traveled in each rotation, F r , is the rotation frequency of the wheel, and R is the radius of the wheel. In practice, the speed from the global positioning system may be read a number of times and the wheel size corresponding to each reading may be averaged. It should be noted that using speed rather than position information allows the wheel size to be determined more rapidly than using position information and is therefore preferable when wheel size is needed quickly (such as when a gross error has been detected). However, using position information, especially over a long distance, results in greater accuracy. Accordingly, in some embodiments, speed is used to rapidly generate an initial estimate and position is used to generate a better estimate at a later time. 
     Furthermore, while track curvature and grade were determined by referencing a map database in the embodiments discussed above, it will be readily recognized by those of skill in the art that curvature and grade can be determined from altitude and direction information provided by the global positioning system. For example, the track curvature may be determined by recording the train&#39;s position as reported by the positioning system at several times during the period in which data is collected. This position information can be used to construct a curvature profile so that the amount of curvature can be determined after the data is collected. If the curvature is greater than a threshold, the data can be ignored, or, in some embodiments, can be corrected for the curvature. The same techniques can be used to construct a grade profile. 
     It should also be noted that the invention may be incorporated into various types of train control systems, including the aforementioned PTC and ATC systems as well as many others. 
     In another embodiment of the invention, the wheel wear compensation method is incorporated into a wheel revolution sensor signal distribution/conversion system such as the QUIP™ system manufactured by the assignee of the present invention, Quantum Engineering. There may be several systems on board a train that input a signal representative of the wheel rotation and use that signal to calculate speed. For example, many locomotives that have been back-fitted with a train control system also are equipped with a separate speed display. Such systems typically require the conductor/engineer or maintenance personnel to measure the diameter of the train wheel to which the wheel sensor is attached and set DIP switches or otherwise configure the devices to indicate the wheel size. Because the wheel size changes over time as discussed above, these other devices must be reconfigured on some periodic basis, thereby increasing labor costs. 
     Because there may be several systems that require the wheel sensor signal which together constitute a larger electrical load than the wheel sensor is capable of handling, and because some of these systems require an input signal of a different form than is supplied by the wheel sensor, signal conversion/distribution systems such as the aforementioned QUIP™ distribution/conversion system have been devised. A substantial savings can be realized by modifying these distribution/conversion systems to output a modified signal that is representative of a wheel sensor signal would be generated by a wheel of a fixed size. Thus, for example, if the conversion/distribution system outputs a modified wheel sensor signal that is representative of a 40 inch wheel, each of the other systems that use the wheel sensor signal could be configured once for a 40 inch wheel and would thereafter not need to be periodically reconfigured. 
     Such a conversion/distribution system  300  is illustrated in FIG.  3 . The system includes a control unit  110  connected to a wheel revolution sensor  320 . In some embodiments, the wheel sensor  320  outputs a square wave, with each rising edge representing a revolution of the wheel. Thus, the time between leading edges represents the time taken for one full revolution of the wheel. It will be readily understood that the signal output by the wheel sensor  320  may be of many forms, analog or digital, and that the particular form of the signal is not important. Also connected to the control unit  110  is a GPS receiver  130  and a map database  140 . The control unit  110  is configured to determine the wheel size using the method described in FIG. 2 or one of the other methods described herein. The control unit  110  determines the speed of the train, which can be taken from the GPS receiver  130  or can be determined with the knowledge of the previously determined wheel size. Using the actual speed of the train, the control unit  110  then determines the parameters necessary for a signal that would be representative of the signal that would be generated by the wheel sensor  320  if the wheel were a predetermined size such as 40″. For example, where the wheel sensor outputs a square wave signal as discussed above, the period of the square wave when the train is traveling 30 m.p.h. would be the distance traveled by one revolution, 2*π*20 inches, divided by the train speed, 30 m.p.h. or 528 inches/sec, which is equal to 125.7/528=0.238 seconds. This 0.238 second period is supplied by the control unit  110  to a signal generator  180 , which generates a square wave of the type discussed above with a period of 0.238 seconds. The signal generated by the signal generator  180  is then supplied to other systems A,B and C  191 - 193 . Because the signal output by signal generator  180  will always be representative of a 40 inch wheel, it is not necessary to reconfigure the other systems  191 - 193  once they have been configured for a 40 inch wheel, thereby substantially reducing labor costs associated with these operations. 
     In the embodiment discussed above, speed is determined as part of the process of determining the parameters of the signal to be generated by the signal generator  180 . It will be readily apparent to those of skill in the art that the parameters can be determined without actually calculating the speed. For example, once the wheel size is determined using the method of FIG. 2, that wheel size can be used to form a ratio of the predetermined wheel size to the actual wheel size. Thus, for example, if the predetermined wheel size is 40 inches, and the actual wheel size is 36, the ratio would be 40/36. The control unit can then measure the period of the square wave and multiply the period by the ratio to determine the period of the signal hat would be generated by the wheel sensor  320  if the wheel were actually 40 inches, and supply this period to the signal generator  180  to generate this signal. 
     As discussed above, it is possible to generate a signal for the other devices without using the signal from the wheel sensor  320 . That is, the speed can be determined from the positional system (e.g., GPS receiver  130 ) and the parameters of the desired signal can be sent to the signal generator so that a signal can be generated and distributed to the other systems, all without an actual wheel rotation sensor  320 . This allows the system to serve as a back up for situations where the wheel sensor fails. This also allows the wheel sensor to be replaced, but such a system has the drawback that it will not provide a correct signal when the GPS system is not operational. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.