System and method for electromagnetic navigation in the vicinity of a metal object

Abstract

A system and method for performing object localization based on the emission of electromagnetic fields. The electromagnetic fields are simultaneously emitted from different transmitters. One electromagnetic field is emitted at a base frequency; the remaining waves are emitted at frequencies that are harmonics of the base frequency. The composite magnetic fields are measured by sensors. The signal generated by each sensor is subject to a Fourier analysis to determine the strengths of the individual electromagnetic fields forming the composite electromagnetic field. These individual measure field strength data are then used to determine the position and orientation of the sensors relative to the transmitters.

1 . A system for determining the position and orientation of an object, said system comprising: 
 at least two transmitters each said transmitter capable of emitting a magnetic field, said transmitter configured so that said transmitters emit navigation magnetic fields at different frequencies, the frequencies being harmonic frequencies of a base frequency;    at lest two sensors, each said sensor configured to simultaneously measure the navigation magnetic fields emitted by said transmitters and to generate a sensor signal reprehensive of a composite magnetic field measured by said sensor; and    a processor connected to said sensors to receive the sensor signals and configured to, based on the sensor signals, compute position and orientation data for said sensors relative to said transmitters.    
   
   
       2 . The system of  claim 1 , wherein said transmitters are configured to emit magnetic fields at frequencies of 1,500 Hz or less.  
   
   
       3 . The system of  claim 1 , wherein said processor is configured to: 
 for each sensor signal, perform a frequency component decomposition process to determine the strengths of the individual magnetic fields that comprise the magnetic field measured by said sensor that generated the sensor signal; and    based on the strengths of the individual magnetic fields, compute position and orientation data for said sensors relative to said transmitters.    
   
   
       4 . The system of  claim 3 , wherein the frequency component decomposition process said processor is configured to perform is a Fourier transformation.  
   
   
       5 . The system of  claim 1 , wherein there are said transmitter assembly has three said transmitters.  
   
   
       6 . The system of  claim 1 , wherein there are three said receivers.  
   
   
       7 . The system of  claim 1 , wherein: 
 said transmitter assembly is further configured so that at least one said transmitter emits a surveillance magnetic field simultaneously with said navigation magnetic field at a frequency less than the frequency at which the navigation magnetic field is emitted    said sensors measure the surveillance magnetic field emitted with the navigation magnetic field;    said processor is further configured to: 
 determine the field strengths of the measured navigation magnetic field and the surveillance magnetic field;  
 based on the field strengths of the measured magnetic field, generate data representative of corrected navigation magnetic field strength measurements; and  
 based on the data representative of the corrected navigation magnetic field strength measurements, compute the position and orientation data for the said sensor relative to said transmitters.  
   
   
   
       8 . The system of  claim 1 , wherein: 
 one said transmitter emits navigation magnetic fields at the base frequency; and    the remaining said transmitters emit navigation magnetic fields at harmonic frequencies of the base frequency.    
   
   
       9 . The system of  claim 1 , wherein: 
 at least one of the harmonic frequencies of the base frequency is a frequency at which electromagnetic interference fields are present; and    said transmitters do not emit magnetic fields at the at least one harmonic frequency of the base frequency at which magnetic interference fields are present.    
   
   
       10 . A system for determining the position and orientation of an object, said system comprising: 
 a transmitter assembly comprising at least two transmitters, each said transmitter capable of emitting magnetic fields, said transmitter assembly configured so that: 
 said transmitters simultaneously emit navigation magnetic fields at different frequencies;  
 at least one said transmitter simultaneously emits at least one surveillance magnetic field, the at least one surveillance magnetic field being emitted at a frequency lower that the frequency of the navigation magnetic field emitted by said at least one transmitter;  
   at least two sensors, each said sensor configured to simultaneously measure the magnetic fields emitted by said transmitters and to generate a sensor signal representative of the composite magnetic field emitted by said sensor; and    a processor connected to said sensors to receive the sensor signals, said processor configured to: 
 based on the sensor signals, generate corrected navigation magnetic field strength measurement data; and  
 based on the corrected navigation magnetic field strength measurement data, compute the position and orientation of said sensors relative to said transmitters.  
   
   
   
       11 . The system of  claim 10 , wherein said transmitter assembly is further configured so that: 
 emit the at least one surveillance magnetic field at a base frequency; and    emit the navigation magnetic fields at harmonic frequencies of the base frequency.    
   
   
       12 . The system of  claim 10 , wherein: 
 said transmitter assembly is further configured so that said at least one transmitter simultaneously emits a plurality of surveillance magnetic fields at different frequencies; and    said processor, based on components of said sensor signal representative of magnetic fields of the plural simultaneously emitted surveillance magnetic fields, generates the data representative of the corrected navigation magnetic field strength measurements.    
   
   
       13 . The system of  claim 10 , wherein: 
 said transmitter assembly is further configured so that said at least one transmitter simultaneously emits a plurality of surveillance magnetic fields at different frequencies; and    said processor, based on components of said sensor signal representative of magnetic field strengths of the plural simultaneously emitted surveillance fields determines if excessive spurious magnetic fields are present.    
   
   
       14 . The system of  claim 10 , wherein said processor is further configured to: 
 based on the sensor signals, generate data representative of the measured navigation magnetic field emitted by said at least one transmitter and the measured surveillance magnetic field simultaneously emitted by said at least one transmitter; and    based on the measured navigation magnetic field data and the surveillance magnetic field data, determine if excessive spurious magnetic fields are present.    
   
   
       15 . A system for determining the position and orientation of an object, said system comprising: 
 a transmitter assembly comprising at least two transmitters said transmitter assembly configured so that: 
 said transmitters simultaneously emit magnetic fields at different frequencies; and  
 at least one transmitter alternatingly emits a low frequency magnetic field and a high frequency magnetic field  
   at least two sensors, each said sensor configured to simultaneously measure the magnetic fields emitted by said transmitters and to generate a sensor signal representative of a composite magnetic field measured by said sensor; and    a processor connected to said sensors to receive said sensor signals and configure to: 
 based on the sensor signals, compute position and orientation data for said sensors relative to said transmitters; and  
 based on the sensor signals representative of the low and high frequency magnetic fields emitted by said at least one transmitter, determine if spurious magnetic fields are present.  
   
   
   
       16 . The system of  claim 15 , wherein said processor, based on the sensor signals representative of the low and high frequency magnetic fields emitted by said at least one transmitter, computes the sensor position and orientation data.  
   
   
       17 . The system of  claim 15 , wherein said transmitter is configured so that: 
 the low frequency magnetic field emitted by said at least one transmitter is emitted at a base frequency; and    the remaining magnetic fields emitted by said transmitters are at frequencies that are harmonics of said base frequency.    
   
   
       18 . The system of  claim 15 , wherein, said processor does not use the sensor signal based on the emitted low frequency magnetic field to determine the sensor position and orientation data.  
   
   
       19 . A method for determining the position or orientation of an object, said method including the steps of: 
 emitting a calibration magnetic field pulse from a transmitter;    measuring the strength of the calibration magnetic field pulse over time with a sensor;    based on said measurement of the strength of the calibration magnetic field pulse, determining a measurement start time;    emitting a navigation magnetic field pulse from the transmitter from a time t NPS  to a time t NPE ;    measuring the strength of the navigation magnetic field pulse with the sensor from a measurement start time t NPMS  based on the determined measurement start time to time t NPE , wherein time t NPS <time t NPMS <time t NPE ;    based on said measuring of the navigation magnetic field pulse, determining the strength of the navigation magnetic field; and    computing the position or orientation of the object based on the determined strength of the navigation magnetic field.    
   
   
       20 . The method of  claim 19 , wherein the navigation magnetic field pulse is emitted for a time less than the time the calibration magnetic field pulse is emitted.  
   
   
       21 . The method of  claim 19 , wherein, in said step of emitting a navigation magnetic pulse, the navigation magnetic pulse is emitted for a variable time as a function of the measurement start time t NPMS .  
   
   
       22 . A method of determining the position and orientation of an object, said method including the steps of: 
 emitting electromagnetic fields from a plurality of transmitters;    measuring the strengths of the emitted electromagnetic fields with a plurality of sensors;    based on the strengths of the measured electromagnetic fields, compute position and orientation data for the sensors; and    based on data generated as a result of said computation of the position and orientation data, determine if significant extraneous electromagnetic fields are present.    
   
   
       23 . The method of  claim 22 , wherein: 
 eigenvalues of a matrix based on a measurements of the magnetic fields by said sensors are generated; and    in said step of determining if significant extraneous electromagnetic fields are present, said determination is made by comparing the eigenvalues to each other.    
   
   
       24 . The method of  claim 22 , wherein: 
 as part of said step of computing position and orientation data for the sensors, a rotational matrix that defines the orientation of said sensors is generated; and    in said step of determining if significant extraneous magnetic fields are present, said determination is based on an evaluation of the rotational matrix.    
   
   
       25 . A method of determining the position and orientation of an object, said method including the steps of: 
 emitting electromagnetic fields from a plurality of transmitters;    simultaneously measuring the strengths of the emitted electromagnetic fields with a first set of a plurality of sensors and with a second set of a plurality of sensors spaced from the first set of sensors;    based on the strengths of the measured electromagnetic fields, compute position data and orientation data for the for the first set of sensors;    based on the strengths of the measured electromagnetic fields, compute position data for the for the second set of sensors and    based on the position data for the first and second sets of sensors, determine if significant extraneous electromagnetic fields are present.    
   
   
       26 . A method of determining the position and orientation of an object, said method including the steps of: 
 emitting electromagnetic fields from a first set of a plurality of transmitters;    simultaneously with said emission of the magnetic fields from the first set of transmitters, emit magnetic fields from a second set of a plurality of transmitters spaced from the first set of transmitters;    simultaneously measuring the strengths of the emitted electromagnetic fields with a plurality of sensors;    based on the strengths of the measured electromagnetic fields, compute position and orientation data for the sensors, the position and orientation data indicating the position and orientation of the sensors relative to the first set of transmitters and relative to the second set of sensors; and    based on the position and orientation data of said sensors relative to the first and second sets of transmitters, determining if significant spurious electromagnetic fields are present.