Patent Publication Number: US-2005137761-A1

Title: Two-axis accelerometer used for train speed measurement and system using the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to the field of two-axis accelerometers, in particular the present invention is directed to two-axis accelerometers used for train speed measurement.  
      2. Discussion of Related Art  
      Currently, a need exists for a train speed measurement system that accurately measures the acceleration and grade of a locomotive assembly. Existing train speed measurement systems use a one-axis accelerometer including a mechanical sensor arrangement to provide velocity, acceleration, adhesion, and speed sensor tracking. However, due to the dependence, in these systems, on the acceleration measurements upon a rail-wheel adhesion factor, the current systems must compensate for the loss of adhesion during the measurement phase of operations.  
      The loss of rail-to-wheel adhesion during the course of train operations renders the measuring of speed parameters difficult and unreliable when utilizing conventional systems. Because measured acceleration is dynamically biased with the current grade, a need exists to properly compensate when taking speed measurements, requiring additional algorithms for obtaining an accurate measurement of the necessary data. These factors culminate in a train speed measurement system that has an estimated error measurement of more than 10%.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to two-axis accelerometers used for train speed measurement, which address the problems discussed above. Specifically, the present invention is a two-axis accelerometer which separates the train&#39;s grade component from the train&#39;s acceleration component. This configuration and structure allows for a tighter tracking of train&#39;s primary speed sensor, a more reliable detection of the loss of adhesion and more accurate compensation during loss of adhesion.  
      In the present invention, a two-axis accelerometer is mounted in a longitudinal plane at an angle to a reference plane. The two-axis accelerometer measures the train&#39;s acceleration and gravity components along both an x and y axis, with respect to the mounted accelerometer. In using the two-axis accelerometer of the present invention, the train&#39;s acceleration is a function of the mounting angle and actual acceleration measurements.  
      The two-axis accelerometer, of the present invention, provides a train acceleration measurement that is independent of the grade on which the train is traveling, while determining a dynamic measurement of the grade. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiment of the invention which is schematically set forth in the drawing, in which:  
       FIG. 1  is a graphical representation of a coordinate system and vectors for a train acceleration/deceleration and grade measurement with a two-axis accelerometer of the present invention;  
       FIG. 2  is a diagrammatical representation of a train acceleration and grade monitoring system incorporating a two-axis accelerometer of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way.  
      Turning now to  FIG. 1 , this Figure depicts a vector representation of the train acceleration at with respect to a normal reference axis x-y. In the embodiment of the present invention, the two-axis accelerometer is mounted in a longitudinal plane, with respect to the train, and at an angle δ with respect to the x-axis (i.e. horizontal). The y axis denotes a vertical reference while the x axis denotes a horizontal reference. It is noted that the angle δ of mounting can be at any point between 0° to 90°, but is preferably within the range of 35° to 55°, and most preferably at an angle of 45°.  
      When the train is accelerating/decelerating and/or is on a graded portion of the track, the two-axis accelerometer measures the train&#39;s acceleratioti/deceleration and gravity g components along both the a x  and a y  axis of the accelerometer, as shown in  FIG. 1 . Based on these measurements, obtained from the two-axis accelerometer, both the acceleration/deceleration at and the grade γ of the train can be calculated. The equations that can be used to perform these calculations are as follows: 
 
 a   t   =a   x cosδ+ a   y ±{square root}{square root over ( g   2 −( a   x sinδ− a   y cosδ) 2 )}  (1) 
 
             γ   =       arctg   ⁡     (           a   t     ⁢   sin   ⁢           ⁢   δ     -     a   y             a   t     ⁢   cos   ⁢           ⁢   δ     -     a   x         )       -   δ   -     π   2               (   2   )             
 
      As shown in the above equations the acceleration a t  of the train is dependent only on actual measurements taken by the two-axis accelerometer measurements and known constants, such as the mounting angle δ and gravity g. Additionally, by using the above equations the current grade γ can be dynamically measured.  
       FIG. 2  depicts, train acceleration and grade monitoring system incorporating a two-axis accelerometer of the present invention, shown on a train  100 . As indicated above, the system uses a two-axis accelerometer  120  which is mounted in a longitudinal plane of the train  100 . The mounting of the two-axis accelerometer  120  is in accordance with the required mounting standards for the particular accelerometer  120  used, and preferably should be in a vertical orientation, as shown in  FIG. 2 . An example of a two-axis accelerometer which can be used is the ADXL202 from Analog Devices.  
      In a preferred embodiment, the two-axis accelerometer  120  uses iMEMS technology to integrate two accelerometers which are positioned 90 degrees, with respect to teach other, and provides outputs proportional to the tilt of each of the integrated accelerometers. The accelerometer  120  has a plurality of outputs to provide the needed data. Two of the outputs, one each for the X and Y directions, are in PWM format, where the output has a nominal 50% duty cycle for a 0 degree tilt. These outputs are used in a microcontroller based system. Two additional outputs, again one each for the X and Y directions, are in analogue format, providing a DC voltage which is proportional to the tilt.  
      As shown in  FIG. 2 , the system for monitoring the train acceleration uses a speed and distance processor card  130  and two sensors, the two-axis accelerometer  120  and a tachometer  110 . The two-axis accelerometer  120  is mounted in a longitudinal plane of the train  100  with the Ox and Oy axis rotated, most preferably, at 45 degrees from the horizontal and vertical axis, respectively.  
      The two PWM outputs from the two-axis accelerometer  120  are coupled to the processor card  130 , and the processor card  130  determines, from these inputs, the true acceleration of the train  100 , eliminating the grade component. This determination is made in accordance with the previous discussion, regarding  FIG. 1 .  
      Additionally, the processor card  130  receives a signal from the tachometer  110 , which can be a digital tachometer. The tachometer  110  is mounted on an axle or wheel of the train  100 . The tachometer  110  outputs a predefined number of pulses for each complete rotation of the axle, (or wheel depending on the configuration). Thus, the tachometer  110  provides the information on the distance traveled by the train  100  under normal conditions, i.e. when the wheel-to-rail adhesion is sufficient and no slipping or sliding is occurring.  
      With the above information, the processor card  130  determines the speed of the train and then differentiates the distance traveled in time, using the tachometer  110  input, and integrates the acceleration in time, using the acceleration input from the accelerometer  120 . Then, the two speed values (from each of the sensors  110  and  120 ) are continuously cross-compared to ensure that the two are in agreement, within a predetermined or defined tolerance. If the tolerance is exceeded (i.e. the difference in speed values between the sensors  120  and  110  is too great) a slippage or sliding condition is detected and the processor card  130  compensates the train&#39;s  100  dynamics values. Stated differently, when the difference between the values from the two sensors  110  and  120  is over a threshold the card  130  determines that the train  100  is slipping or sliding, and then the card  130  corrects the dynamic values (i.e. speed and distance traveled) of the train  100 . This permits the train&#39;s systems to accurately monitor the train&#39;s progress and data during conditions or times when the train  100  has lost adhesion with the rails.  
      It is noted that although the above embodiment is discussed within the context of monitoring the dynamic values of a train, it is understood and contemplated that the above described system can be used on additional modes of transportation, including passenger vehicles, freight vehicles and the like.  
      It is of course understood that departures can be made from the preferred embodiments of the invention by those of ordinary skill in the art without departing from the spirit and scope of the invention that is limited only by the following claims.