Abstract:
An apparatus and method for predicting attractive magnetic levitation force comprising measuring flux density for a component of an attractive magnetic levitation system and computing a predicted attractive magnetic levitation force from the flux density.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/468,700, entitled “Magnetic Levitation Force Control”, filed on May 6, 2004, and the specification thereof is incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
   Not Applicable. 
   COPYRIGHTED MATERIAL 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention (Technical Field) 
   The present invention relates to force control for attractive magnetic levitation, particularly for trains. 
   2. Background Art 
   The primary parameter that must be controlled in attractive magnetic levitation is lift force. Lift force can be measured with strain gauges, but these devices are extremely sensitive to the thermal environment. Force can be predicted as a function of gap, lateral displacement, and current, but the prediction is very sensitive to measurement errors in gap and current. Better, more robust force prediction is needed, which is provided by the present invention. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is of an apparatus and method for predicting attractive magnetic levitation force, comprising: measuring flux density for a component of an attractive magnetic levitation system; and computing a predicted attractive magnetic levitation force from the flux density. In the preferred embodiment, computing comprises calculating a polynomial equation, most preferably a second order polynomial equation. A predicted attractive magnetic levitation force is also calculated from measured lateral displacement, preferably by calculating a polynomial equation, most preferably a third-order polynomial equation. The two predictions are then combined. A prediction from measured magnetic gap can also be calculated and combined with the prediction from the flux density and/or lateral displacement. A flux sensing coil is employed, preferably with each of the plurality of bogies of the attractive magnetic levitation system. The predictions by the invention have an error of less than or equal to approximately 1 percent, and even less than or equal to approximately 0.1 percent. 
   Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
       FIG. 1  is a graph of actual force and force predicted from flux density; 
       FIG. 2  is a graph of actual force and force predicted from lateral displacement; and 
       FIG. 3  is a schematic diagram of the apparatus of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the present invention, which is of an apparatus and method for predicting force in an attractive magnetic levitation system, force is predicted by measurement of flux density, magnetic gap, and lateral displacement. In this approach, force is a strong function of SQRT (Flux) and a weak function of gap and lateral displacement. 
   In present systems, force is predicted by measuring gap, lateral displacement, and current. In other systems, flux density is controlled directly, with inherent force errors of 15% to 25%. Augmenting flux density with lateral displacement and gap information results in force prediction with less than 1% error. This approach eliminates the requirement for expensive secondary suspension systems on attractive magnetic levitation vehicles. 
   A typical prior art force controller is based upon a polynomial using magnet gap, current, and lateral displacement. It is very sensitive to errors in gap measurement. This results in poor ride quality. The present invention employs flux density, gap, and lateral displacement. It has low sensitivity to gap errors. Ride quality is improved by a factor of approximately twenty-three. 
   Preferably, a flux gauge is used to measure flux density (“B”). Lift force is equal to K√{square root over (∫B)} for zero lateral displacement, where K is magnetic stiffness. A flux sensing coil is preferably added to each lift magnet, which coil measures total integrated flux. Current control is then replaced with flux control. 
   Referring to  FIGS. 1 and 2 , flux density alone or in conjunction with lateral displacement (and/or gap) is a much better way to predict magnet force. The relationship to flux density is strong while the relationship to gap and lateral is weak. In  FIG. 1  (force versus flux density), the curve is the prediction (y=67.968x 2 −12.235x+6.7459) and the points are measured data. The peak error is less than 0.1%. In force vs. lateral displacement ( FIG. 2 ), results are similar (y=−0.386x 3 +0.5812x 2 −0.0074x+0.9997). The peak error in this case is less than 1%. In prior force control methods, it is not unusual to see force errors as large as 40% due to gap measurement errors alone. As understood by one of ordinary skill in the art, the estimates provided by flux density can be combined with the estimates provided by lateral displacement to provide estimates with even less error than either along. The same is the case with the addition of predictions based on measured gap, which are known in the art, to the method. 
   As readily understood by one of ordinary skill in the art, the method of the invention can be implemented in a variety of electronic control systems, whether analog, digital via microprocessor, digital via microcode, digital via programmable logic controller, and the like. 
   One embodiment of an apparatus  10  according to the present invention is shown in  FIG. 3 . A rail  13  carries an attractive magnetic levitation device, particularly a carriage/train  11 . A flux sensing coil  12  is added to the lift magnet (a coil is preferably added to all of the lift magnets, each requiring its own primary and secondary coil drivers and demodulation circuitry, not shown)  14  within bogie  15 . (Note that the extent and location of the windings in  FIG. 3  are not meant to be limiting.) The flux density reported by the coil is then employed by control means  20  according to the method of the invention. 
   Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents.