Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-03-19
2001-01-23
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207170
Reexamination Certificate
active
06177792
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to apparatus for generating and detecting electromagnetic fields, and specifically to non-contact, electromagnetic methods and devices for tracking the position and orientation of an object.
BACKGROUND OF THE INVENTION
Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
For example, U.S. Pat. No. 4,054,881 describes a tracking system using three coils to generate electromagnetic fields in the vicinity of the object. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time, frequency or phase. The signal currents flowing in three orthogonal sensor coils are used to determine the object's position, based on an iterative method of computation.
U.S. Pat. No. 5,391,199, filed Jul. 20, 1993, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describe a system for generating three-dimensional location information regarding a medical probe or catheter. A seconsr coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
Unpublished PCT patent application No. PCT/US95/01103, filed Jan. 24, 1995, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of non-concentric sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates. As in the case of the '539 patent application described above, the radiator coils operate simultaneously at different frequencies, for example at 1000, 2000 and 3000 Hz, respectively.
The above tracking systems rely on separation of position-responsive signals into frequency components, wherein each such component is assumed to correspond uniquely to a single radiator coil, in a known position, radiating in a narrow, well-defined frequency band. In practice, however, the radiator coils also generate magnetic fields at the frequencies outside the desired bands, for example due to mutual inductance effects. These mutually-induced fields lead to errors in determining the position of the object being tracked.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved electromagnetic radiator coils and driver circuitry therefor, for use in conjunction with object tracking systems, so as to increase the accuracy of object tracking.
It is a further object of the present invention to provide magnetic field generator coils and associated driver circuitry that generate fields of having narrow bandwidths in the frequency domain.
In one aspect of the present invention, narrowed field bandwidth is achieved by eliminating mutual inductance effects among a plurality of coils, which generate magnetic fields at different frequencies.
Still a further object of the present invention is to provide a method for improving the accuracy of a position determined by an object tracking system, by correcting the position determination so as to account for mutual inductance effects.
In preferred embodiments of the present invention, a plurality of radiator coils generate magnetic fields at a plurality of different respective driving frequencies.
In some preferred embodiments of the present invention, one or more additional shimming coils are adjacent to each of the radiator coils. Preferably the shimming coils are interwound with windings of the radiator coils. The shimming coils of each radiator coil are driven by driver circuitry so as to generate magnetic fields that are substantially equal in amplitude and frequency, and opposite in direction, to magnetic field components induced along the axis of that radiator coil by other radiator coils.
In other preferred embodiments of the present invention, driver circuitry is associated with each of the coils and generates electrical driver currents therein, wherein for each coil the current comprises a major component at the coil's respective driving frequency, and minor components at other frequencies. The minor components are substantially equal in amplitude and frequency and 180° out of phase with currents induced in the coil due to magnetic fields generated by other radiator coils, so as to substantially cancel the effect of the induced currents.
In one such preferred embodiment of the present invention, driver circuitry includes sensing apparatus, which measures the amplitude, frequency and phase of induced currents in a coil. The driver circuitry further includes an adaptive current supply, which receives the amplitude, frequency and phase data measured by the sensing apparatus, and generates the out-of-phase minor current components to substantially cancel the effect of the induced currents.
In still other preferred embodiments of the present invention, driver circuitry comprises ideal current sources, which function to maintain a constant current in each of the coils at its fixed, respective driving frequency.
In other preferred embodiments of the present invention, an object tracking system comprises one or more sensor coils adjacent to a locatable point on an object being tracked, and a plurality of radiator coils, which generate magnetic fields when driven by driver circuitry at a plurality of different respective driving frequencies in a vicinity of the object. The sensor coils generate signals responsive to the magnetic fields, which signals are received by signal processing circuitry and analyzed by a computer or other processor. The signals include position signal components responsive to the magnetic fields generated by the radiator coils at their respective driving frequencies, and parasitic signal components responsive to magnetic fields generated due to mutual inductance among the radiator coils. The computer processes the signals to separate the position signal components from the parasitic components, preferably using an iterative method, as described below, and uses the position signal components to determine the position of the object.
In one such preferred embodiment of the present invention, the signals, including position components and parasitic components, are used to compute approximate location and orientation coordinates of the object. The strengths of magnetic fields generated at this location by the radiator coils due to mutual inductance effects among the coils are then calculated theoretically, and are used to estimate theoretical strengths of the parasitic components. Corrected signals are calculated by subtracting these theoretical parasitic component strengths from the signals, and these corrected signals are used to calculate corrected location and orientation of the object. These steps are repeated iteratively until the calculation of the corrected location and orientation converges.
In some preferred embodiments of the present invention, mutual inductances among radiator coils in an object tracking system are determined by empirical measurement, using methods known in the art. Thus, for example, the mutual inductance of a pair of radiat
Fenster Maier
Govari Assaf
Osadchy Daniel
Bisense, Inc.
Capezzuto Louis J.
Patidar Jay
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