Abstract:
A system for determining the position of a vehicle includes a plurality of sensors for measuring the magnetic field strength of a magnet, means and is configured for determining the position of the magnet with respect to a plurality of sensors wherein the plurality of sensors are arranged in a two-dimensions array.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for determining the position of a vehicle. 
     The invention further relates to a vehicle comprising such a system. 
     The invention also relates to a method for determining the position of a vehicle. 
     BACKGROUND OF THE INVENTION 
     Systems for determining the position of a vehicle are known for many years. One such known system makes use of a grid of magnets that is provided in the surface whereon the vehicle moves. The system comprises a plurality of sensors that are able to sense the strength of a magnetic field. The plurality of sensors is arranged in a lateral direction of the vehicle, in order to ensure that a magnet arranged in the ground surface is detected when the vehicle passes over the magnet. The signals from the sensors are periodically sampled and provided to computation means to calculate the position of the sensed magnet with respect to the sensors. The described detection of magnets is usually not sufficient to determine the position, as individual magnets can normally not be distinguished from each other. Therefore, in one prior art system, rotational counters are installed on at least two wheels to keep track of the wheel rotation in order to determine a position change and therewith the position. The detection of the magnets prevents the occurrence of an accumulation of errors in the determination of the position and therefore position drift. 
     A drawback of this known system is that when the vehicle negotiates a bend, the sensors at the outer side of the bend are undersampling the magnetic field strength, while the sensors at the inner side of the bend are oversampling the magnetic field strength. Although the latter causes less problems with the processing power available to present day digital signal processors, the former causes a deterioration of the accuracy of the determined position when the vehicle negotiates a bend. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a system for determining the position of a vehicle, wherein the deterioration of the accuracy when the vehicle negotiates a bend, is substantially less compared to the described prior art. 
     This object is realised by the present invention by providing a system for determining the position of a vehicle comprising: a plurality of sensors for measuring the magnetic field strength of a magnet; computation means for determining the position of the magnet with respect to the plurality of sensors; wherein the plurality of sensors are arranged in a two-dimensions array. 
     In the system according to the prior art, the acquisition of samples from the sensors is fixed in time and the position of the samples in the space domain is dependent on and varies with the movement of the vehicle. In the present embodiment the plurality of sensors no longer solely serves to prevent missing a magnet when a magnet passes underneath the vehicle off-centerline, but serves to take a complete snapshot in two-dimensional space of the magnetic field of the magnet. To this end it is preferable that the array of sensors is arranged substantially parallel to the surface comprising the magnets, when the system is in use. This way determining the position of the magnet with respect to the sensors does not depend on any time related sampling rate and therefore the movement of the vehicle, but solely on the sensor arrangement. Because the determining of the position is independent of the vehicle movement, also the accuracy of the determined position is independent of the vehicle movement. This way undersampling and oversampling cannot occur, not when travelling in a substantially straight line, nor when negotiating a bend. The accuracy (for determining a single position) is fixed by the sensors&#39; arrangement. 
     In a further embodiment, the present invention provides a system, wherein the sensors are evenly interspaced in both dimensions. This way the samples are evenly spaced. 
     According to another embodiment, a system is provided, further comprising means for fitting the magnetic field strengths measured by the plurality of sensors to a spatial model of the magnetic field of a magnet. Although computationally expensive, fitting the acquired magnetic field strengths to a spatial model of a magnet, results in high accuracy positions. 
     In a further embodiment, the present invention provides a system, wherein the sensitivity of the sensors for a magnetic field component perpendicular to the plane wherein the plurality of sensors are arranged is substantially higher than the sensitivity for magnetic field components parallel to this plane. Such an embodiment has the advantage that inaccuracies due to spatial variations in the earth&#39;s magnetic field are minimised, as this perpendicular component can be considered constant within a typical action radius of a surface bound vehicle. In a further embodiment, the system estimates a constant background magnetic field and compensates during the measurement of the magnets for this constant background field. This background field may constitute a magnetic field caused by components of the vehicle itself, but also a remaining perpendicular component of the measured magnetic field of the earth. 
     In again a further embodiment, the present invention provides a vehicle comprising a system as described above. 
     In a further embodiment, the system is arranged such that the positions of multiple magnets are determined from a single measurement by the two-dimensional array of sensors. In order for this feature to be advantageous, the magnets in the surface are to be arranged sufficiently close in order for the array to detect at least part of the magnetic fields of the multiple of magnets. 
     In again a further embodiment, the system is arranged to decode information that is coded by means of a mutual arrangement of a group of magnets in the surface and/or the polarity of the individual magnets in a group of magnets. To that end, it is preferred to arrange the magnets in clusters, wherein each cluster of magnets provides position information or for deriving position information analogous to the description of the previous embodiments, and wherein the arrangement of magnets in the cluster and/or the polarity of magnets in the cluster provides additional information, either additional positional information, or non-positional information. In one particular embodiment, position coordinates are directly encoded in the mutual arrangement. However, in a preferred alternative arrangement, the mutual arrangement encodes for a key for a lookup table that is stored in the vehicle. In accordance with another aspect of the present invention, a method is provided for determining a position of a vehicle, comprising the steps of: acquiring from a plurality of sensors, the magnetic field strengths measured by the sensors; determining the position of a sensed magnet with respect to the plurality of sensors; wherein the plurality of sensors are arranged in a two-dimensional array. 
     According to another aspect of the invention a method is provided, wherein the position of a sensed magnet is determined by fitting the acquired magnetic field strengths to a spatial model of the magnetic field of a magnet. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further embodiments and advantages thereof will be described below with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a vehicle according to the present invention. 
         FIG. 2  shows a system according to the present invention that is comprised in the vehicle of  FIG. 1 . 
         FIG. 3  shows a hierarchy for data-processing produced by a system according to the present invention. 
         FIG. 4  shows the magnetic field components of the magnetic field vector at a sensor of the vehicle of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Further embodiments and advantages thereof will be described below with reference to the accompanying drawings, wherein: 
     When a magnetic marker element  14  passes under a certain minimum number of sensors  20 , the position of the vehicle  12  relative to the magnetic marker element  14  can be estimated. The minimum number of sensors required can be obtained by means of, for example, the Fisher Information Matrix. The magnetic field generated by the magnetic marker element  14  is sensed in one or more of the magnetic sensors  20 . The magnetic sensors  20  are connected to a digital signal processor (DSP)  24 . The DSP  24  fits the signals coming from the magnetic sensors  20  to a 3-dimensional model of the magnetic field of a magnetic marker element  14 . From the fitted model, the position of the magnetic marker element  14  is obtained relative to the array  10  of magnetic sensors. Consequently, from a known position of a magnetic marker element  14 , the position of the vehicle  12  is obtained, which is done by the navigation computer  26 . 
     The position of the magnetic marker elements  14  is shown in  FIG. 1  as a grid-like pattern. For the present invention, it is not necessary for the pattern of magnetic marker elements  14  to be grid-like. It is sufficient if the position of the magnetic marker elements  14  is known. A regular pattern like the grid-like pattern in  FIG. 1  is advantageous though, as no position of individual marker elements need to be stored, as the positions are easily derivable from the pattern. In one particular embodiment the vehicle travels along a predetermined path, and the grid is a one dimensional grid that extends along the predetermined path. 
     The pattern of the magnetic sensors  20  is not bound to any specific pattern either. In principle, the pattern could be random, as long as the position estimation algorithm takes the location of the individual magnetic sensors  20  within the array into account. 
     The hierarchical functionality  30  provided by the DSP  24  and the navigation computer  26  is the following ( FIG. 3 ): the lowest level is the 10 level  32 . The 10 level  32  is responsible for obtaining the magnetic field strength readouts from the sensors  20 . 
     In the next level, the scanner level  34 , a position within the array  10  of sensors  20  is associated with each individual magnetic field strength measurement. As these positions are fixed in two-dimensions, this activity is rather straightforward in contrast to the embodiments from the prior art, that only employ magnetic sensors in a single dimension, as time does no longer play a role. 
     Subsequently, the set of magnetic field strength positions is fitted in a fitter-level  36  to a 3-dimensional model of a magnetic marker element  14 . This results in an estimated position of the magnetic marker element  14  relative to the array  10  of sensors  20 . 
     Finally, the estimated relative position of the magnetic marker element  14  is used in the last level, the coding level  38 , to determine the position of the vehicle  12 . In one particular embodiment the position is related to a particular moment in time, for example as a time elapsed since the occurrence of a time synchronisation signal. 
     The magnetic field  40  of magnet  14 , and the magnetic field vector  42 , and its components, at sensor  20  are shown in  FIG. 4 . As seen here, the magnetic field component  44  that is perpendicular to the plane wherein the sensors is arranged is substantially higher than the magnetic field component  46  that is parallel to the plane. 
     It should be noted that the embodiments shown and described only serve as example embodiments of the invention, and that the invention is not limited to these embodiments. Many modifications of and variations on the embodiments shown and described are possible without departing from the scope of the present invention. For example, it is without limitation, possible to combine embodiments shown and described. The scope of protection sought is therefore only determined by the following claims.