Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2000-03-21
2003-03-04
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207170, C702S150000
Reexamination Certificate
active
06528989
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to embodiments of an AC magnetic tracker with frequency-based compensation for distortion caused by metals.
As is well known, eddy currents are induced in conductive metals through the presence of changing magnetic fields. This is a particularly serious problem where AC current is used since where the AC current is operating at any frequency, eddy currents are induced in nearby conductive metals throughout each measurement period.
In systems using magnetic fields to measure position and orientation of objects in a prescribed space in six degrees of freedom, some have attempted to solve some of the problems inherent in AC-based devices by devising systems that use direct current (DC). In such systems, eddy currents are still induced by the rising and falling edges of the DC magnetic pulse. However, delaying of field measurements until sometime after the rising edge allows eddy currents to decay significantly and for measurements to be taken that are substantially free of eddy current distortions. However, use of DC-based systems increases the length of time expended in conducting each cycle of measurements.
Systems have been devised in which the environment in and adjacent the measuring space is “mapped” so that the locations of metallic objects are known as is their effect on magnetic measurements. A look-up table is provided to store these measurements so that compensation may be made for prospective eddy current distortion. Such a method is quite tedious and is only effective if the environment in and around the measuring space stays the same at all times, that is, no metallic object or objects are moved from their original locations of placement and no such objects are moved adjacent the measuring space at any other time. In addition, such measurements depend upon a particular defined location for the transmitter coils. Movement of any objects and/or the transmitter coils requires re-measuring and mapping of the environment.
Ferrous objects (with a relative permeability &mgr;
r
>1), when placed in a magnetic field, acquire magnetization that is referred to as induced magnetization. The magnitude and direction of the induced magnetization is a function of the primary field strength, ferromagnetic susceptibility of the body, its shape, and its orientation with respect to the primary field. Ferrous objects may also have permanent or intrinsic magnetization, usually called remnant magnetization. Remnant magnetization is a function of metallurgical properties of the object as well as its thermal, mechanical, and geomagnetic history. Remnant magnetization can be very difficult to model since its strength and direction are often unknown for an individual object. No prior art system teaches compensation for distortion caused by ferrous metals.
As such, a need has developed for a position measuring system that can accurately measure the position and orientation of objects in a measuring space in the six degrees of freedom x, y, z, azimuth, elevation and roll, and which can do so regardless of the particular locations of metallic objects in and adjacent the measuring space. A need has also developed for such a system which can operate quickly without the need to wait for eddy current distortion to decay and wherein the system is less sensitive to ferrous metals, magnetic materials, dynamic filtering and environmental noise.
The following prior art is known to Applicant:
U.S. Pat. Nos. 4,346,384 and 4,328,548 and 4,314,251 and 4,298,874 and 4,054,881 and 4,017,858 and 3,983,474 and 3,868,565 all teach the use of AC magnetic fields in order to measure the position and orientation of a sensor. These patents teach use of different algorithms including 1) Nutating vectors, 2) Far-field, 3) Near-field, 4) Iterative solutions, and 4) Direct solutions.
Some of the patents teach the use of mapping the environment, that is, before the position and orientation measurements are made, storing the mapping data in memory and using look-up tables in order to compensate for eddy current distortion caused by metals in the environment during position and orientation measurements. No direct compensation for the metallic distortions, during position and orientation measurements, are made.
U.S. Pat. Nos. 4,287,809 and 4,394,831 teach position and orientation systems utilizing AC magnetic fields. They utilize non-coplanar transmitter coils and non-coplanar receiver coils. They teach mapping of the environment before the position and orientation measurements are made, storing the mapping data in memory and the use of look-up tables in order to compensate for eddy current distortion for metals in the environment during position and orientation measurements. No direct compensation for the metallic distortions is made during position and orientation measurements.
U.S. Pat. No. 4,829,250 teaches the use of multifrequency transmitters in a position and orientation system. The system utilizes AC magnetic fields and curvefitting is used in order to compensate for the eddy current distortion in conductive metals. The eddy current distortion of conductive metals varies as a function of the frequency and the conductivity. The eddy current distortion decreases with lower frequencies and is zero for DC fields. This method works well if only one conductive metal is present in the work area. If multiple conductive metals are present in the work area, problems may arise. If ferrous metals are present in the work area, a serious problem arises, since the magnetic distortion from ferrous metals is a function of the permeability and the frequency. This distortion decreases with higher frequencies directly opposite the eddy current distortion for conductive metals. The metallic distortion for ferrous metals is not equal to zero at DC. This means that the system will not compensate for metallic distortion if ferrous metals are present. No compensation is made for ferrous metallic distortion.
U.S. Pat. No. 5,347,289 teaches the use of a rotating multifrequency AC magnetic field in a position and orientation system. The system utilizes a special timing sequence of the rotating field in order to compensate for the eddy current distortion from conductive metals. The eddy current distortion decreases with lower frequencies and is zero for DC fields. This method works well if only one conductive metal is present in the work area. If multiple conductive metals are present in the work area, problems may arise. If ferrous metals are present in the work area, a serious problem arises, since the magnetic distortion from ferrous metals is a function of the permeability and the frequency. This distortion decreases with higher frequencies directly opposite with respect to the eddy current distortion for conductive metals. The metallic distortion for ferrous metals is not equal to zero at DC. This means that the system will not compensate for metallic distortion if ferrous metals are present. No compensation is made for ferrous metallic distortion.
U.S. Pat. Nos. 5,694,041 and 5,457,641 teach the use of AC magnetic fields in a position and orientation system. These patents teach the need for an initial mapping of the environment and storing of this data into memory. The data is then used to correct the position and orientation measured during operation. No direct compensation is made for the conductive and ferrous metal distortion.
U.S. Pat. Nos. 5,646,524 and 5,646,525 teach the use of AC magnetic fields in a position and orientation system. They utilize a rotating magnetic field and teach the use of mapping the environment before the position and orientation measurements are made, storing the mapping data in memory, and the use of look-up tables in order to compensate for eddy current distortion for metals in the environment during position and orientation measurements. No direct compensation of the metallic distortions is made during position and orientation measurements.
In U.S. Pat. Nos. 4,849,692 and 4,945,305, a remote object's position and orientation are determined.
Aurora Reena
Lefkowitz Edward
Skysense, Ltd.
Spiegel H. Jay
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