JP2002365010A - Magnetic position measurement system provided with magnetic field closure means - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a distortion effect by a permeable material in a magnetic position measurement system. SOLUTION: A metal body is to practically include a magnetic field from one or more transmission elements so as to reduce the magnetic field in outside region of an operation space in the region generally existing, to enclose it and to change the direction. A thin barrier produced from ferrite or Mumetal for example, which are of high permeability and practically non-conductive materials, is put on a conductive plate 15. On the top of the permeable barrier 13, a three-axis rhombic transmitter 11 is put. The permeable barrier may be a flat and plane structure. On the contrary, it may be produced similarly to a pancake having a flat center region which has a lifted peripheral edge in cross section. In other way, it may be a flat structure having the peripheral edge with a outside taper from the top surface to the bottom surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic position measuring system having magnetic field confinement means.

[0002]

BACKGROUND OF THE INVENTION In biomechanical engineering and medical diagnostic technology, the concept of using transmitting and receiving components with electromagnetic coupling to mount a sensor assembly at a point of interest and determine the position of this point relative to a fixed transmitter is well known. It is. This information is used by the computing system to accurately indicate the relative movement of the points, which allows the equipment to be accurately positioned with respect to the human body in a medical sense and within one another. In addition, this makes it possible to carry out advanced new methods of surgery and diagnosis.

[0003]

When conductive materials are present (often located above, below, or near the operating table), these materials generate an eddy current magnetic field. This eddy current magnetic field distorts the waveform of the received magnetic field. If the system does not use distortion reduction or compensation techniques, this magnetic field waveform will distort the output of the system. When magnetically permeable materials are present, these magnetically permeable materials bend or distort the magnetic field with the same effect as conductive materials. In the operating room, there are significant amounts of conductive and magnetically permeable materials. These materials are a key component of many operating tables surrounding equipment, such as carts, and are present in movable spotlights used to illuminate the surgical area. Many operating tables have many positional and angular degrees of freedom to allow the surgeon to optimally position the surgical area, and are designed to be extremely stable and robust while supporting heavy human bodies. ing. As a result of these conditions, the operating table includes a number of mechanisms that allow for anterior, posterior, upper, lower, lateral, rolling and tilting movements. Because these mechanisms are physically rugged and are generally made of steel, they have significant magnetic field distortion characteristics. The components present may include screws, rack and pinion gears or scissor type actuators. The operating table surface may be integral or divided into several parts, each part being able to move relative to other parts, and having various stresses and relative stresses for a particular surgical or diagnostic procedure. The body is flexed so that the anatomical position is optimal. The operating table installation base is very diverse in structure, and operating tables are often used for decades, so there are many distributors, and each distributor offers many different types of operating tables. I have. This creates significant problems for magnetic position tracking systems used in critical surgical environments. That is, the working space for the tracker is generally in the body resting on top of the operating table. This means that the tracking system operates in close proximity to the metal structures above, below and around the operating table. These metal structures distort the magnetic field, resulting in large errors in the reported magnetic sensor positions. The extreme variety of operating table structures makes it difficult to predict the severity of the distortion that will occur in a given operating table. This is an unacceptable condition in a surgical environment. Attempts have been made to compensate for these adverse effects with different degrees of effectiveness.

One method used so far is to map the entire working space each time the system is used. This method is extremely time consuming and costly, as the distortion is severe and if the working space is large, potentially thousands of points must be accurately measured. Also, the shape of the operating table often changes during a surgical or diagnostic procedure, which changes the relationship of the operating table metal structure to the tracking system and requires a new map if errors cannot be tolerated. Not reliable. The map becomes invalid as medical and diagnostic equipment is introduced or removed near the tracking system. Also, if the distortion is severe, the map may be totally invalid because the system may determine that the sensors are at the same location at the spatial point of two different physical sensors. In this case, the use of output data is minimal.

Another known method, commonly described in the prior art, is to use an AC magnetic field against a conductive ground plane. This ground plane attenuates the magnetic field below the ground plane to approximately zero. This method has the advantage that the system can be unaffected by metal objects below the ground plane. For a dipole transmitter, to calculate the theoretical magnetic field vector for a plane,
An "image method" is used and this magnetic field vector is used to generate the sensor position. This method has several disadvantages. One disadvantage is that the magnetic field strength near the ground plane is almost zero, the vector intersection angle is small,
This severely reduces the accuracy and noise performance of the system. The net result is that the sensor must be kept several centimeters (several inches) above the ground plane. Also, the dipole must be positioned at a predetermined distance from the ground plane to reduce signal loss and reduced vector intersection angle in the working space. For volume of about 0.02832m ³ (1 cubic foot), the bottom of the transmitter, to ensure that performance is acceptable if located approximately 5.08 cm (about 2 inches) above the ground plane No. The transmitter thickness must be added to this 5.08 cm (2 inch) number to calculate the height to raise the patient when the patient is on the transmitter. The size of the transmitter is determined by the required signal level in the working space. The size of the sensor coil for minimally invasive surgery is about 1m in cross section
m × 5 mm, which is extremely small. The conditions for precise, low-noise surgery at the edges of the working space require that relatively large magnetic field amplitudes be generated in order to induce a sufficiently strong signal in a small coil. The size of the transmitter depends greatly on how much magnetic field the transmitter must output. Since the transmitter is generally cubic, a practical transmitter size of 5.08 cm per side (for each side) to obtain sufficient signal within a volume of about 0.02832 m ³ (1 cubic foot) with a small receive coil 2 inches). An effective transmitter assembly in the prior art, including a ground plane, has been found to be 4 inches thick. In a surgical environment, the patient must be raised to such a height that it is uncomfortable for the surgeon to see. Further, if the patient has to lie flat on the operating table, extra pads may be required. The operating table must have both the transmitter and the pad secured. In short, this configuration is bulky and may not position the patient in an optimal manner.

This is undesirable, as placing the transmitter above the working space potentially interferes with the surgical area. Further, since the transmitter is installed at a distance from the ground plane, the size of the ground plane should be approximately 4
When fixed to a 5.7 cm (about 18 inch) cube,
The ground plane is no longer effective in reducing the effect of metal objects near the working space. Since the metal housing of the surgical lighting device is close to both the transmitter and the receiver, the distortion effect is greater in the upper part of the working space. This is an unacceptable condition because instruments used during surgery, including the operating table, create potentially life-threatening distortions.

[0007] Position measurement depends on the magnitude of the relative vectors from the x, y and z coils. Even in this method, the distortion effect can be eliminated by using a method such as mapping. As the magnitudes of the magnetic vectors transmitted from the x, y and z coils become more similar, the magnitude of the predetermined error in the measurement results in a greater error in the position output. When considering a limited case, if the sizes are equal, position measurement becomes impossible. Such a combined effect of the small crossing angle of the transmission vector and the small difference in the magnitude of the transmission vector is known to those skilled in the art as a vector dilution effect. The use of a conductive ground plane under the transmitter creates a vector dilution effect. As the transmitter approaches the ground plane, the magnitude of vector dilution increases, even as the receiver moves away from the transmitter. This vector dilution effect generally sets a practical limit on how close a transmitter of a magnetic tracking system can be to a conductive ground plane. 0.028
For a cubic foot motion box of 32 m ³ (2 cubic feet), the transmitter is 5.08 cm (2.
If closer, the vector dilution approaches an unacceptable level. Even in non-dipole transmitter structures, this vector dilution effect exists, and so does its effect.

The applicant has recognized the following prior art. U.S. Pat. No. 4,849,692 to Brad discloses a method for eliminating eddy current distortion effects caused by conductive objects and other objects having a large surface area, such as the surface of a stainless steel operating table. The problem of the distortion effect of permeable metals is not solved by this system. This means that metal structures in, around, and below the surgical area of the system will distort the received magnetic field and reduce the performance of the system. Further, the eddy current decay time of a wide, thick sheet of conductive metal such as aluminum can exceed 200 milliseconds. If the system uses three time division multiplexed axes and all off-axis periods to compensate for terrestrial magnetism as described in the preferred embodiment, the latest rate is 1/4 * (200 m
S) = 1.25 Hz. Such values are unacceptably slow for many applications.

US Patent No. 5,767, issued to Hansen et al.
No. 669 discloses a method of compensating for eddy current magnetic fields without having to compensate for geomagnetic effects. This system has no means to reduce the effect of proximity permeable metal, and the disadvantage of having to slow down the latest rates while operating near wide and thick sheets of highly conductive metal. Have not solved.

US Patent No. 5,600, issued to Brad et al.
No. 330 discloses a magnetic tracking system based on a non-dipole loop transmitter. This system is
The magnetic field from the smaller object drops as 1 / r ³ (where r is the receiving distance from the object), while the magnetic field from the larger transmit loop drops as 1 / r ² ,
5 shows low sensitivity to small metal objects in the working space when the effect from small metal objects is reduced. However, a wide sheet of metal has a larger effective loop area than the loop of the magnetic transmitter, thereby reducing the benefit of the rate of magnetic field reduction, which makes the system less susceptible to large metal objects. Has the general effect of being totally insensitive, reducing the advantage of the magnetic field reduction rate. Furthermore, metal objects parallel to and close to the metal loop generate very large eddy currents, which reduce the signal level in the working space. To reduce the effect of metal objects near the transmitter in this system, the transmission coil should be grounded to reduce the signal loss that occurs when the wire loop approaches a conductive ground plane parallel to the plane of the loop. Must be separated by a certain distance. In this system, when the structure of the transmitter is planar, a planar ground plane can be installed a predetermined distance below the transmitting coil. At zero distance, the reduction of the magnetic field in the working space is almost total, so the effective transmitter thickness and signal loss, defined as the sum of the thickness of the transmitting coil, the ground plane, and the spacing between them, We have to find a compromise between. Further, due to the technique of having a large eddy current loop area on the ground plane with respect to a single transmitting coil area, there is still another degrading effect when the sensor moves away from the transmitter. Regardless of which transmitting coil is active, the current distribution in the ground plane is similar. This means that the magnetic field base of the eddy current in the ground plane is similar. The magnetic field at any point in the working space is the vector sum of the magnetic field of the transmitting coil minus the ground plane eddy current field and the effective radius of the ground plane magnetic field larger than the radius of the transmitting coil, so the plane of the transmitter The further away from, the greater the magnetic field is measured by the ground plane current. This true effect is 3
The vectors from the two transmit coils are not very different, which makes the system more susceptible to noise and metal distortion if the system uses differences in vector magnitude and direction to determine position. is there. As these differences become smaller, the apparent change in the receiving position can be larger as a result of the smaller change in one of the vectors.

US Patent No. 5,752, issued to Ecker et al.
No. 513 shows a system that is a subset of the system described in US Pat. No. 5,600,330 to Brad, whose operation is in many respects the same in non-dipole transmitter characteristics and metal sensitivity.

US Pat. No. 5,550,09 issued to Fukuda et al.
No. 1 shows a system using a so-called "Helmholtz" device for generating a controlled magnetic field in the working space. The disadvantage of this system is that it is bulky and requires that the working space be enclosed by a Helmholtz coil assembly. A second disadvantage of this system is that when the system is mounted on a metal object, for example a steel operating table, the magnetic field from the transmitting coil is distorted in the working space.

US Pat. No. 5,64 to Anderson
No. 0,170 discloses a method for positioning a dipole in a special structure, spiral, on the ground plane. The dipole transmitter in this system must be centered on the spiral ground plane assembly, which makes it more difficult to position the patient in a clinical set. This positioning may interfere with the operative area during a given operation. The advantage of this method is that it is possible to bring the transmitter closer to the ground plane, without having to use an "image method" to resolve the position. However, the disadvantage of placing the transmitter in a spiral / ground plane assembly is very similar to the case with a ground plane only.

US Pat. No. 5,198,768 to Keren
Numbers indicate surface coil arrays for use for NMR. This system does not measure position and does not use any methods to reduce the effects of nearby metal objects.

The present invention is directed to the prior art associated with such transmitting / receiving position and orienting devices, as long as they meet the conditions of being insensitive to metal objects below and adjacent to the transmitter assembly without exhibiting the disadvantages of signal degradation. It is fundamentally different.

[0016]

SUMMARY OF THE INVENTION The present invention substantially reduces the magnetic field from one or more transmitting elements so as to attenuate the magnetic field in a region outside the working space within a region where metallic objects are generally present. And an embodiment of a magnetic field position and orientation measurement system having means for confining and redirecting the same.

The present invention relates to a device for measuring the position of a receiving antenna with respect to a transmitting antenna using a magnetic field. In particular, such devices have six degrees of freedom positions,
In other words, the purpose is to measure movement (position) in three coordinate directions, that is, translational movement and rotational movement (orientation) on three coordinate axes, but the present invention is not limited to this. Position is generally defined by the X, Y, and Z linear coordinates, referred to as three mutually perpendicular directions, and the orientation is pitch, roll, and roll in three mutually perpendicular axes that usually coincide with the three mutually perpendicular directions. Generally described by azimuth rotation angle coordinates.

The present invention includes the following interrelated objects, aspects and features. (1) In a preferred embodiment, the magnetic flux vector is increased inside the sensor working space, and the magnetic flux vector is reduced below and adjacent to the transmitter plane, so that a metal system below and near the transmitter is used. Flux confinement means is used to redirect the direction of the magnetic flux vector so as to reduce the effect on the magnetic flux vector. Since the magnetic flux vector from the transmitter is distorted by the magnetic flux confinement means in a stable and repeatable manner, the characteristics of the distorted magnetic field can be determined accurately and repeatably. Once the exact vector distribution from the transmitting assembly is known, solving for the position and orientation from the receiving means is a straightforward task for those familiar with magnetic position tracking techniques. One reliable way to characterize such a vector is to use a finite element analysis method to calculate the magnetic field vector from the transmitter. Another reliable method is to use one of several so-called mapping techniques, which are well known to those skilled in the art.

(2) The preferred embodiment of the present invention teaches a method for forming a typical magnetic transmission assembly with reduced effects from metal objects below and adjacent to the working space of the system. Things. This preferred embodiment further reduces the vector dilution effect of the conductive ground plane to a level that is not problematic. This reduction in vector dilution effect substantially reduces distortion caused by metal objects in the working space while maintaining insensitivity to metal objects below the transmitter and low sensitivity to objects adjacent to the working space. A system which is not affected by the above is obtained. The transmitting means may include a wire loop, solenoid or permanent magnet arranged in a suitable shape and position to determine the position of the receiver within the volume.

(3) The present invention satisfies the requirements for a system that can be installed on a surface of any width and configuration without deteriorating the accuracy of reading a position from a sensor located in a desired working space. The present invention achieves this goal for exciting AC and DC transmitters, which was not possible at all using conventional ground plane based compensation methods. Further, the present invention achieves this goal while significantly increasing the magnetic field strength in the working space, which was not possible using conventional ground plane based compensation methods. The present invention also eliminates the problem of the vector dilution effect that occurs when placing a conductive ground plane near the transmitter.

(4) In a preferred embodiment of the present invention, a magnetically permeable barrier made of a highly permeable but substantially non-conductive material, such as ferrite or mu metal, is placed on top of the conductive plate. . In a preferred embodiment,
The thickness of the magnetically permeable layer when manufactured from ferrite is 1.2.
It is in the range of 7 mm to 6.35 mm (0.05 inch to 0.25 inch), but the thickness can be made smaller than 0.254 mm (0.01 inch) if mu metal is used. The conductive plate, preferably made of an aluminum alloy, is about 4.76 mm to 6.35 mm (3/16
Inches to 1/4 inch). When the mu metal is used in the magnetically permeable layer, it is not important to determine the thickness for mechanical support, so that the thickness of the conductive plate can be reduced. A planar 3-axis rhombic transmitter is mounted on top of a ferrite or mu metal barrier, which is described in detail in US Pat. No. 5,600,33.
No. 0. In a preferred embodiment, the transmitter comprises a printed circuit board on which the transmitter is etched. About 0.79 in thickness
Printed circuit boards from 4 mm to 3.175 mm (0.0125 to 0.125 inches) can be used.

(5) If desired, the magnetically permeable barrier may have a flat, planar structure. Alternatively, the cross-section may be made to resemble a pancake having a flat central region with elevated peripheral edges. Further alternatively, the magnetically permeable barrier may be a substantially flat structure having a peripheral edge that tapers outwardly from the top surface to the bottom surface, wherein the taper forms an angle with the bottom surface, preferably in the range of 30-85 degrees. You may do it.

(6) If a conductive object adjacent to or in the area below the transmitter receives an AC magnetic field,
Eddy currents are induced in this object. The eddy currents thus induced generate a magnetic field component which, by adding a vector, combines with and distorts the magnetic field near the object, usually without metal. The magnitude of such a parasitic eddy current magnetic field is proportional to the magnitude of the AC magnetic field near the conductive object.

(7) Thus, while the magnitude of the magnetic field in the area adjacent to and below the transmitter assembly is reduced, the magnitude and direction of the magnetic field vector in the working space above the transmitter assembly is constant. If it does, it can be seen that the effects of metal objects in this area on the magnetic field in the working space above the transmitter assembly are proportionally reduced. While the magnitude of the magnetic field in the area adjacent to and below the transmitter assembly remains constant, the effect of reducing distortion when the magnitude of the magnetic field in the working space above the transmitter assembly increases is similar. It is. Thus, the ratio of the magnitude of the magnetic field in the working area above the transmitter assembly to the magnitude of the magnetic field below and below the area adjacent to the transmitter assembly can be used to predict the effect of the metallic object. The same holds true for the ferromagnetic distortion effect when there is a distorted object in the area adjacent to and below the transmitter assembly.

(8) If the value of the relative magnetostriction sensitivity of a single transmission coil having a structure as shown in FIG. 13 can be set as a normal value, the relative distortion sensitivity of an object adjacent to the working space is indicated. The sex number Ma can be determined. here,
Ma is in the area above the transmitter assembly,
Dividing (the magnetic field of the system shown in FIG. 2) (in the area above the transmitter assembly, the magnetic field of the system shown in FIG. 5) and (in the area adjacent to the transmitter assembly, FIG. 5) and (magnetic field of the system of FIG. 5 in the area adjacent to the transmitter assembly). The system shown in FIG. 1 has a sensitivity index of 1 so as to select FIG. 11 as the reference system.

(9) Similarly, the term Mb can be defined to compare objects below the transmitter assembly. This M
b is divided by (the magnetic field of the system of FIG. 2 in the area above the transmitter assembly, the magnetic field of the system shown in FIG. 5 in the area above the transmitter assembly), Divided by (the magnetic field of the system of FIG. 2 below the transmitter assembly) and divided by (the magnetic field of the system of FIG. 5 in the area below the transmitter assembly). Using these exponential numbers Ma and Mb, several different structures can be evaluated to determine their relative sensitivity to metal objects in the area adjacent to and below the transmitter assembly.

Accordingly, it is a first object of the present invention to provide a magnetic position measurement system having a magnetic field confinement means.

It is another object of the present invention to provide such a system wherein a thin magnetically permeable barrier is mounted above a thin conductive plate. Yet another object of the present invention is to provide
It is to provide such a system having a substantially high magnetic permeability and a substantially non-conductive barrier having an upwardly curved peripheral edge.

It is yet another object of the present invention that the substantially magnetically permeable, substantially non-conductive barrier has a peripheral edge, which edge points downward from the top surface to the bottom surface. It is to provide such a system that is tapered. It is yet another object of the present invention to provide such a system in which a thin rhombic transmitter is mounted above a magnetically permeable barrier.

Yet another object of the present invention is to provide a system for quantitatively measuring the position of a receiving antenna with respect to a transmitting antenna without having the disadvantage of being affected by a metal object directly below the transmitter. Is to do. It is yet another object of the present invention to provide a system that is insensitive to metal objects that lie in the plane of or below the transmitter and extend as horizontally as possible.

It is yet another object of the present invention to provide such a system that eliminates transmission field strength losses in the intended working space. It is yet another object of the present invention to provide such a system in which the performance is not significantly degraded by the vector dilution effect. Still another object of the present invention is to provide D
It is an object of the present invention to provide such a system that can use C or AC transmitter excitation techniques and is not affected by magnetic objects located below the transmitter structure.

The foregoing and other objects, aspects and features of the present invention will become better understood from the following detailed description of the preferred embodiment, read in conjunction with the accompanying drawings.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, a system, generally designated by the numeral 1, is shown to include a conventional three-axis dipole transmitter, which is generally designated by the numeral 2. Shown and suspended above the conductive plate 3. The eddy currents induced in the conductive plate 3 by the X and Y coils of the transmitter 2 are substantially identical to each other with respect to magnitude, direction and distribution on the conductive plate 3. Due to such similarities, the eddy current magnetic field vectors at points inside the working space 4 are exactly the same in both magnitude and direction. As the transmitter 2 approaches the conductive plate 3, the magnitude of the eddy current field relative to the transmitted magnetic field at any point inside the working space 4 increases. Since the eddy current field from the conductive plate 3 is the same, this results in a more similar total field vector from the X and Y coils, which
The angle formed by the intersection of the two vectors decreases. Magnetic dipole systems utilize the angle of intersection of three different vectors from three orthogonal transmitter coils to an inductive configuration. If these angles are distorted due to the presence of eddy current fields, the system will output distorted orientation values.

In order to eliminate such distortions, the prior art used a magnetic field mapping process well known to those skilled in the art to eliminate such errors. However, such a magnetic field mapping process has serious disadvantages when applied to a system as shown in FIG. For a given amount of error in determining these three intersection angles, the system outputs an error in the orientation output. If this error is due to a noise source, the orientation output contains noise.
As the crossing angles of the vectors transmitted from the X, Y and Z coils decrease, the orientation measurement of the receiver becomes more susceptible to noise or other errors. In the extreme case, where the coils are nearly identical and the crossing angle is nearly zero, orientation measurements are not possible as the susceptibility to errors and noise approaches infinity.

The method of comparing expected magnetic field distortion levels in a given metal environment is useful in evaluating different systems. One such method utilizes a magnetic field strength ratio, which is defined as the magnetic field strength in the region to be measured divided by the magnetic field strength in a region outside the region. The latter volume is generally chosen as the volume immediately adjacent to the conventional volume. To further facilitate such analysis, a single point is selected to represent the total magnetic field in each volume. The theoretical basis of this method is as follows.

When a conductive object adjacent or in the area below the transmitter receives an AC magnetic field, eddy currents are induced in the object. The induced eddy currents generate a magnetic field component that combines and distorts the normal metal-free magnetic field near the object by adding the vectors. The magnitude of such a parasitic eddy current magnetic field is proportional to the magnitude of the AC magnetic field near the conductive object.

Thus, the magnitude and direction of the magnetic field vector in the working space above the transmitter assembly remains constant, while the magnitude of the magnetic field in the area adjacent to and below the transmitter assembly is reduced. It can be seen that if reduced, the metallic objects in that area will have a proportionally reduced distortion effect on the magnetic field in the working space above the transmitter assembly. Distortion decreases when the magnitude of the magnetic field in the working space above the transmitter assembly increases, while the magnitude of the magnetic field in the area adjacent to and below the transmitter assembly remains constant. The effect is the same. Thus, the sensitivity to metal objects (by metal objects) is determined by using the ratio of the magnitude of the magnetic field in the working space above the transmitter assembly to the amplitude of the magnetic field in the area adjacent to and below the transmitter assembly. Impact) can be predicted. The same is true for the ferromagnetic distortion effect if the strain-causing object is located in an area adjacent to and below the transmitter assembly.

If the value of the relative magnetostriction sensitivity of the single transmission coil having the structure shown in FIG. 13 can be set as a normal value, the relative distortion sensitivity indicated by Ma becomes:
(The magnetic field of the system shown in FIG. 2 in the area above the transmitter assembly) is divided by (the magnetic field of the system shown in FIG. 5 in the area of the magnetic field of the transmitter assembly), and A point is reached that is equal to the value divided by (the magnetic field of the system shown in FIG. 2 in the area adjacent to the assembly) and divided by (the magnetic field of the system in the structure of FIG. 5 in the area adjacent to the transmitter assembly). it can. The system shown in FIG. 11 has a sensitivity of a sensitivity of 1 which selects, for example, FIG. 1 as the reference system.

Similarly, to compare the sensitivity of the strain to metal objects below the transmitter assembly, the (magnetic field of the system of FIG. 2 in the area above the transmitter assembly) is compared to (the magnetic field above the transmitter assembly). 5), divided by (the magnetic field of the system of FIG. 2 below the transmitter assembly), and divided by (the magnetic field of the system of FIG. 2 below the transmitter assembly). The term Mb can be defined equal to the value divided by The exponential numbers Ma and M
By using b, several different structures can be evaluated to determine the relative sensitivity to metal objects in the area adjacent to and below the transmitter assembly.

Referring to FIG. 13, the diameter is 1
Yet another prior art system is shown in which a 7.5 inch single turn transmit coil 6 operates at a frequency of 20 KHz and a current of 1 amp rms.
The magnetic field vector can be calculated using a number of methods, one of which is known as the so-called finite element method. The basic tool for this calculation is a software program that uses Maxwell's equations as boundary conditions and magnetic field properties. Create a model to be analyzed using a computerized drafting program. This model consists of both the geometry of the system and the properties of the material, as well as any excitation characteristics. This model is computed by a numerical finite element solver that simulates the behavior of electromagnetic fields in and around the model. This results in accurate quantitative values for the magnetic field at all points in the model.
The skillful creation of models, which is easily achieved by those skilled in the art, allows for the analysis of various combinations of materials and geometries. Furthermore, it is possible to select an exact spatial position on the model, and obtain an accurate numerical value for the magnitude and direction of the magnetic field vector at this position. By selecting these same locations and changing the model parameters, one can see the effect of material properties and geometry on the magnetic field at a particular spatial location. In order to show the behavior of the magnetic field in each volume, the working space 7,
One point is selected in each of the adjacent space 8 and the area 9 below the transmitting coil. Select the geometric origin of 0,0,0 at the center of the transmission loop 5. (0,0,
The working space 7 is displayed by the point of (6), and (0,
The adjacent space 8 is displayed by the point (12, 0), and the area 9 is displayed by (0, 0, 6). By using this method, the magnitude of the magnetic field vector for the working space 7 is (9.12e-17 Tesla), the adjacent space 8 is (7.2e-17 Tesla), and the area below the transmitting coil 6 9 becomes (6.4e-15 Tesla). Such a configuration is selected as the reference configuration, so that Ma and Mb are equal to one.

Referring now to FIG. 5, there is shown another conventional system in which a flat transmitter 5 floats over a conductive plate 3 over a length of 0.3 inches. Have been. In such a conventional technique, the conductive plate 3 has a thickness of about 6.35 mm (about 0.25 inch) and is made of aluminum. Therefore, the total height from the top surface of the conductive plate 3 to the top of the transmitter 5 is about 1
3.97 mm (0.55 inch).

When comparing the structure of FIG. 5 with the structure of FIG. 13, when the structure of FIG. 5 is compared with the magnetic field level of the structure of FIG. 13, the magnetic field 7 in the working space is reduced to 27%. The adjacent space 8 has been reduced to 40% and the area 9 below the conductive plate 3 has been reduced to 0.14%. From such data, Ma is equal to 0.68,
It can be concluded that Mb is equal to 193. The value of Ma is 0.6
Equal to 8 indicates that the system is susceptible to greater distortion due to the presence of metal objects in region 8. The value of Mb means that the system is completely unaffected by metal objects located below the conductive plate 3 in the area 9. A serious disadvantage of the structure of FIG. 5 is that the magnetic field working space 7 has been reduced to 27% of its original value. This means that for a given noise level in the tracking system and its environment, the position output always drops. Increasing the transmission current by a factor of 3.6 can compensate for such losses, but
The drive system will be more demanding and costly. Further, when the transmission loop 5 is not a superconductor,
This loop dissipates power equal to I ^{2 R,} or 12.9 times, for a given transmit loop structure. Thus, larger wire dimensions and / or equipment may be required to remove heat from the wires of the transmission loop 5. Both of these create distinct disadvantages. The system described in FIG. 5 also has the problem of a substantial vector dilution effect when energizing any of the three transmission loops, due to a similar eddy current field from the conductive plate 3. Thus, the conductive plate 3 is sensitive to metallic objects in the working space magnetic field 7.

Referring now to FIGS. 2, 3 and 4, a preferred embodiment of the present invention is generally designated by the numeral 10 and is mounted on top of a conductive plate 15 and has a magnetically permeable barrier 13 thereon. It is shown to include a planar rhombic transmitter 11 provided above.

In the preferred embodiment, transmitter 11 includes a printed circuit board having a three-axis transmitter etched on its surface. In the preferred embodiment, the thickness of the printed circuit board is about 1.625 mm (0.0625 inch), but the thickness of the printed circuit board is between 0.794 and 3.175 mm (0.003125 inch and 0.125 inch). Can be used appropriately.

For proper operation, the magnetically permeable barrier must not be the primary eddy current magnetic field source. For a given material having a bulk resistivity p, as the frequency decreases, a point is reached where the eddy currents in the material decrease to the point where the distortion of the incident magnetic field decreases. It can be seen that in the extreme case of a DC transmitter, the conductivity of the magnetically permeable barrier is not important. It is clear that there is a predetermined relationship between the frequency used in determining the operating frequency of the transmitter and the material used in selecting the required bulk resistivity of the magnetically permeable barrier. This relationship can be described by the value obtained by dividing the bulk resistivity p of the material in units of ohmmeter by the operating frequency f of the transmitter. That is, Rfc = p
/ F. Rfc is about 2e-1 with respect to steel
If it is greater than zero, the parasitic eddy current field from the magnetically permeable barrier is sufficiently low that it is advantageous to use a steel barrier against an aluminum or copper ground plane. For cold rolled steel this is about 50
It can occur at a transmission frequency of 0 Hz. If Rfc is greater than 2e-9, it is generally advantageous to use steel or stainless steel for the ferrite, unless it is necessary to completely optimize the transmission field characteristics.
For cold rolled steel, this occurs at a transmission frequency of about 50 Hz. If Rfc is greater than 1e-8, the material will behave essentially as a pure magnetically permeable barrier when steel or stainless steel. The transmission frequency is 10 Hz for cold-rolled steel. In this case, there is no improvement in performance even if cheap and strong steel is replaced by expensive and fragile ferrite.

The magnetically permeable material 13 has a thickness of 1.27 to 6.35.
mm (0.05 to 0.25 inch), but 3.81 to
A thickness range of up to 6.35 mm (0.15 to 0.25 inch) is preferred. The magnetically permeable barrier 13 can be made from a highly magnetically permeable material, but can also be made from a substantially non-conductive material.
One such material is ferrite. This material has a relative permeability in the range of 50-25000 compared to the permeability of air. This material has a typical resistivity in the range of 0.1 ohm / m to 10 < ⁸ > ohm / m, depending on the commercial composition used. In a specific implementation of the preferred embodiment, suitable materials include 10 ⁴ ohm / m at 25 ° C.
And a ferrite-type MN67 having a relative magnetic permeability of 2500 at 25 ° C. This material is 5.08 mm (0.2 inch) thick, 45.72 cm (18 inch) in diameter, and is coaxial about the transmitter 11 formed by the three rhombic transmission loops 14, 16 and 18. It is. As can be seen from FIG. 2, a transmission driver 21 is connected to the transmitter 11 via a conductor 23. This driver 21 is 20
Each of the loops of the transmitter 11 is sequentially energized at a frequency of 1 KHz at 1 Amps rms. Another material to be used for the permeable barrier is mu metal. This material is a nickel iron alloy and contains another small amount of metal. This material is specially formulated and annealed to produce a relative magnetic permeability Ur of 75,000 to 300,000, but is conductive. Such characteristics can be compensated for by a magnetic permeability higher than that of ferrite, and mumetal has been found to be an extremely effective magnetic permeability barrier. A typical commercial product is A
It is called D-MU-80 Mu Metal and is manufactured by Advance Magnetics. In an experiment, a thick sheet of this material was used as the ferrite barrier 13, and the effect on the magnetic field of the transmitter was analyzed. At a transmission frequency of 3 KHz from DC, AD-MU-80
Mu Metal is 5.08mm (0.2 inch) thick M
It has been found that this produces a performance approximately equal to that of the N-67 ferrite material. At frequencies from 3 KHz to 19 KHz, the AD-MU-80 mu metal reduces adjacent and lower magnetic field strength by the same percentage as MN67 ferrite, but has a small increase in magnetic field strength in the operating region. At frequencies higher than 19 kHz, AD
-MU-80 mumetal showed the same magnetic field reduction as MN-67 ferrite in the lower and adjacent regions,
In the operating region, the magnetic field strength decreased. DC and 5MHz
AD-MU at all tested frequencies, including
The -80 mu metal exhibited a much lower vector dilution effect in the operating region than the conductive ground plane, and exhibited a higher transmit field strength.

Mumetal has extremely effective mechanical properties as compared with ferrite. Since this mu metal has a magnetic permeability generally 30 times that of ferrite at a frequency lower than several KHz, it can be manufactured thinner than ferrite while acting similarly as a magnetically permeable barrier. Unlike ferrite, mumetal is not a fragile ceramic but a flexible metal. As a result, the mu metal is not destroyed even when stress similar to that of the ferrite is applied, so that the hard support packing required for the ferrite can be made relatively thin. Because the permeability barrier 13 can be made thinner, it saves weight over ferrite with obvious advantages. Further, mu metal is cheaper than ferrite, and has a permeability barrier 13.
Is easy to mold, form, machine and weld into a suitable shape to form As a result of these other advantages, the permeability barrier 13 can be effectively used in place of ferrite, even in lower performance gains where economic and mechanical reasons offset the performance difference.

The conductive plate 15 is provided directly below and substantially in contact with the magnetic permeability barrier 13. In a preferred embodiment, the conductive plate 15 is manufactured from an aluminum alloy 6061 T-6 and has a thickness of about 4.76-
It has a thickness of about 0.1875 to 0.25 inches.

Therefore, the transmitter 11 and the permeable barrier 13
And the conductive plate 15, the combined thickness is about 7.62 to 15.9 mm (about 0.3 to 0.6 mm).
25) inches, resulting in an extremely compact assembly. Such a transmitter 11 and a magnetically permeable barrier 13
And the conductive plate 16 can be generally referred to as a transmitter assembly 25.

FIG. 4 shows the transmitter assembly 25 mounted on the operating table 27 with the patient 30 resting on the transmitter assembly 25. The receiver 3 is provided inside the patient 30.
1 has been inserted and the receiver has a transmitter assembly 2
5 and transmit the signal to a computer (not shown) via lead 33 so that the position and orientation of the receiver 31 can be accurately determined.

FIG. 16 shows a graph of a cross section 37 of the equipotential contour of the magnetic field resulting from points 0, 0 when the magnetic field extends above and below the x-axis line 36. Referring to FIG. 17 for comparison, placing the magnetically permeable barrier 25 above the line 39 will result in a magnetic field equipotential profile such that substantially none of the magnetic field 37 will extend below the line 39. The shape of the cross section 37 changes. FIG.
8 shows the further attenuation of the cross section 37 of the equipotential contour of the magnetic field below the line 39 with the addition of the conductive plate 41 below the magnetically permeable barrier 25. This effect results from the operation of the preferred embodiment shown in FIGS.

Referring to FIG. 6, there is shown an embodiment of the transmitter assembly 25, which in this embodiment includes 5.x.
A transmitter 11 is provided directly on top of a ferrite layer 13 made of MN67 ferrite material having a thickness of 0.8 mm (0.2 inch), and the ferrite layer 13 has a thickness of 6.35 mm.
It rests directly on top of a conductive plate 15 of 0.25 mm (0.25 inch) thick aluminum. As described above,
13, the working space 7 is 159% of the working space of FIG. 13 and the adjacent space 8 is 60% of the working space of FIG. The area 9 below is 0.11% of the working space in FIG. From these results, it is clear that Ma is equal to 2.65 and Mb is equal to 1445. Thus, it should be understood that the structure of FIG. 6 performs better than the systems of FIGS. 13 and 5 with respect to the expected sensitivity to metallic objects in regions 8 and 9. In the working space 7, the signal level is also 151 in comparison with FIG.
Increase by%.

FIG. 7 shows that the ferrite plate is 5.08 mm.
(0.2 inch) thick ferrite plate 1 made of MN67 type ferrite
3 shows a transmitter 11 provided directly on top of 3. The total thickness of the transmitter assembly of FIG. 7 is 5.08 mm (0.2 inch)
It is. Compared to the magnetic field levels in FIG. 13, the magnetic field 17 in the working space is 191%, the magnetic field in the adjacent space 8 is 81%, and the magnetic field inside and below the operating table 9 is 4.3%.
It is. From these data Ma is equal to 2.35 and M
b will be equal to 44.4. From these data,
It is expected that this system will be much less susceptible to metal objects in region 8 and less susceptible to metal objects in region 9 as compared to FIG. Further, the magnetic field in the working space 7 is 191 more than the original magnetic field.
%, Which results in improved signal to noise performance. The vector dilution effect is negligible. The signal level in the working space is only 83% of the signal level in FIG.
However, the system of FIG. 6 is better suited for applications where the area 9 is constructed from the area inside and below the operating table. The reason for this is that if the structure of the region 9 changes, the system will not experience significant magnetic field distortion in the region 7. Since the vector dilution effect is negligible, the sensitivity to metal objects in the working space 7 does not decrease with respect to FIG.

[0054]

Table 1: Comparison of Ma, Mb and working space magnetic field strengths for four representative planar non-dipole magnetic transmitters System Ma Mb Working volume magnetic field based on the magnetic field of FIG. 1 1 FIG. 5 0.68 193 0.27 FIG. 6 2.65 1445 1.59 FIG. 7 2.35 44.4 1.91

Another advantage of the conductive plate 15 is that it provides a physical mechanical support for the generally brittle ferrite layer 13. Of course, the undesirable signal loss effect of the eddy current effect from the conductive plate 15 can be substantially eliminated. Ideally, the conductive plate 15 is the structure 25 of the transmitter.
Is to select a thickness of a few skin depths at the operating frequency so as to produce the maximum magnetic field attenuation at the bottom. For very low frequency excitations, including DC excitations where the skin depth is very large, the purpose of the conductive plate 15 is purely to mechanically support the transmitter.

Applicants have discovered that the operation of the present invention can improve the performance of non-dipole systems. The enhanced non-dipole exhibited high field strength in the working space with concomitantly reduced output noise. Realistically, it is totally insensitive to metal objects located below the transmitter, for example, in the area indicated by number 9. Such a system exhibits low sensitivity to metal objects adjacent to the working space and reduces the vector dilution effect compared to the shielding method based on the ground plane, and is therefore inherently affected by metal objects in the working space. Hardly affected by noise.

FIG. 8 shows another non-planar magnetically permeable barrier 50 having a shallow cross-section, consisting of two parts 51 and 53 meeting at an intersection line 55. These portions 51 and 53 are at an angle of 15 degrees to the horizontal and are angled downward from the central upper terminal. As shown in FIG. 8, the transmitter 57 is floating above them.

FIG. 9 shows another embodiment of the present invention, designated by the numeral 60, in which the magnetically permeable barrier 61 has peripheral edges 65, 67 of the transmitter 63 and the peripheral edges 62 and 64 of the magnetically permeable barrier 61. The transmitter 63 is floating on the barrier in a state where the transmitter 63 is overlapped.

FIG. 10 shows the transmitter 70 floating above the magnetic barrier 71, and further shows the magnetic flux pattern for this structure. FIG. 11 shows the peripheral edge 8 bent upward so that the cross section of the barrier 81 resembles a pancake.
3 shows a system 80 having a magnetically permeable barrier 86 with a 3. This transmitter is suspended in the volume formed by the peripheral edge 83. With the use of a magnetically permeable barrier as indicated at 81, it has been found that the magnetic field is concentrated around its upper edge and that certain advantages are obtained when this structure is mounted on a permeable sheet, for example plate steel. Was. The advantage of a thin transmitter is somewhat compromised in this case, as is the magnetic field shape around the raised peripheral edge 83, as well as the intensity distribution.

In another variation, reference is made to FIG. 15, which shows a ferrite permeable barrier 90. The barrier 90 has a main body 91 and a peripheral edge 93, which are
Is tapered outwardly from a top surface 94 to a bottom surface 95. The peripheral edge 93 is preferably between 30 and 85
Make an angle in the range of degrees. As this angle gets smaller,
Performance results are also improved, but a decreasing return point is reached as this angle decreases for two reasons.
The first reason is that it becomes increasingly difficult to manufacture a barrier 91 having a shallower peripheral edge 93. Further, the angle of the peripheral edge is generally 9
Decreasing from 0 degrees to 45 degrees can increase about 99%, but this is achievable. The use of such angled peripheral edges reduces the magnitude of the fringe effect at the edges. The bevel of the angled peripheral edge 93 moves the strain closer to the edge of the permeable barrier 91 and further down into the operating table, for example, near the system of the present invention.

FIG. 12 shows another variant of the assembly shown in FIGS. 2 to 4 in which the transmitting coil 11 is floating above the magnetically permeable barrier 13. FIG. 14 shows that the use of the magnetically permeable barrier 13 reduces the vector dilution effect of the ground plane by providing a low reluctance flux path for the magnetic field generated by the transmitter 2. . As a result, the conductive plate 1
The magnetic field adjacent to 5 is effectively attenuated to a modest level, so that the vector dilution effect can be greatly reduced while maintaining insensitivity to metal objects below the transmitter. X, Y and Z
The magnetic field transmitted from the coil is distorted, but this distortion is not severe and can be easily removed by magnetic field mapping techniques.

It has been discovered that by using an extremely low reluctance ferrite barrier, the magnetic field can be primarily passed through the low reluctance path below the ferrite material, provided by an effective shielded body of ferrite material. The advantages of using mu metal for magnetic permeability barriers have been described in detail above.

It is advantageous to use aluminum for the conductive plate. The reason for this is that aluminum acts as a support for the fragile ferrite permeable barrier and attenuates the magnetic field. Ferrite or mu-metal permeable barriers provide 95% of the benefits of the present invention with the variations and modifications disclosed herein,
An additional 5% of the perimeter of the magnetically permeable barrier and the benefits of allowing the use of aluminum conductive plates can be provided.

A typical intended use of the present invention is on an operating table. There is a lot of steel inside the operating table, and the operating table is heavy and cantilevered. The invention amplifies the magnetic field in the working area above the operating table,
It reduces the magnetic field adjacent to the transmitter and below the top of the operating table.

If desired, the transmitter 11 and the magnetically permeable barrier 1
3 and the aluminum conductive plate 15 may be laminated with a material such as silicone or epoxy adhesive. A laminated assembly finished as described above may have a thickness of less than about 5/8 inch, thus making the operating room more convenient.

The present invention has been disclosed above with respect to the preferred embodiments satisfying each of the objects of the invention as described above.
The present invention provides a new and effective magnetic position measurement system with more novel and useful magnetic confinement means.

Of course, those skilled in the art will be able to make various modifications, alterations, and modifications within the scope of the present invention without departing from the scope of the present invention. Thus, the present invention is limited only by the terms of the appended claims.

[Brief description of the drawings]

FIG. 1 shows a schematic diagram of a conventional system.

FIG. 2 shows a perspective view of a preferred embodiment of the present invention, in which the Rhombic transmitter is shown schematically.

FIG. 3 shows a side view of the preferred embodiment shown in FIG.

FIG. 4 shows a perspective side view of the preferred embodiment of FIGS. 2 and 3, also showing the patient supported above the preferred embodiment on an operating table.

FIG. 5 shows an area selected to predict the sensitivity of a conventional non-dipole transmitter on a conductive ground plane to metal.

FIG. 6 shows regions selected to predict the sensitivity to the metals of the invention shown in FIGS.

FIG. 7 shows a region selected to predict the sensitivity of the transmitter to metal as predicted in FIG. 3 without a plate of conductive material below the magnetically permeable barrier.

FIG. 8 shows a variant of the invention using a non-planar, magnetically permeable barrier.

FIG. 9 shows another variation in which the transmitter extends beyond the periphery of the magnetically permeable barrier.

FIG. 10 shows a magnetic flux pattern in which the transmitter is a dipole transmitter.

FIG. 11 shows a region having raised edges on the periphery and selected to predict metal sensitivity in a variant of a magnetically permeable barrier.

FIG. 12 illustrates another variation where the transmitter is higher than the permeable barrier.

FIG. 13 shows a region selected to predict metal sensitivity for a conventional non-dipole transmitter loop located in free space.

FIG. 14 shows a system in which a dipole magnetic transmitter is located above a magnetically permeable barrier.

FIG. 15 illustrates a variation of a permeable barrier having a peripheral edge that tapers outward from a top surface to a bottom surface.

FIG. 16 shows a cross section of an equipotential profile of a magnetic field extending above and below a reference line generated at points 0,0.

17 shows a graph of a cross section of the same magnetic field equipotential profile shown in FIG. 16 truncated below the reference line by using a magnetically permeable barrier.

FIG. 18 shows a graph of a cross section of the equipotential contour of the same magnetic field shown in FIG. 17, further truncated by adding a conductive plate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 System 2 Transmitter 3 Conductive plate 4 Working space 5 Transmission loop 6 Transmission coil 7 Working space 8 Adjacent space 9 Area 11 Rhombic transmitter 13 Magnetic permeability barrier

Claims (26)

[Claims] 1. A magnetic position measurement system, comprising: means for including a magnetic field used to make a measurement of the position of an object, said means including a magnetic field being adjacent to an area for measuring the position of the object by the magnetic field. A system including a magnetically permeable attenuator, wherein the attenuator is adapted to attenuate the magnetic field on a side of the attenuator opposite the region. 2. The system according to claim 1, wherein said attenuator is flat. 3. The attenuator according to claim 1, wherein said attenuator is 0.25 to 6.35 mm.
3. The system of claim 2, wherein the system has a uniform thickness (0.01 to 0.25 inches). 4. The system of claim 3, wherein said attenuator is manufactured from a material selected from the group consisting of ferrite and mu metal. 5. The system of claim 4, wherein said attenuator has a raised peripheral edge. 6. The system of claim 2, wherein said attenuator has a raised peripheral edge. 7. The system of claim 2, wherein the attenuator has a peripheral edge that tapers outward from a top surface to a bottom surface of the attenuator. 8. The system of claim 4, wherein the attenuator has a peripheral edge that tapers outwardly from a top surface to a bottom surface of the attenuator. 9. The attenuator has a V-shaped cross section.
The system according to claim 1. 10. The attenuator according to claim 1, wherein said attenuator is 0.25 to 6.35 mm
10. The system of claim 9, having a uniform thickness (0.01-0.25 inches). 11. The system of claim 10, wherein said attenuator is manufactured from ferrite. 12. The system of claim 10, wherein said attenuator is manufactured from mu metal. 13. The system of claim 1, further comprising a conductive plate mounted below said attenuator. 14. The plate according to claim 11, wherein said plate has a size of 4.76 mm to 6.35.
mm (0.1875 to 0.25 inch) thickness,
The system according to claim 13. 15. The system of claim 14, wherein said plate is manufactured from a non-ferrous metal. 16. The system of claim 14, wherein said plate is made from a conductive metal. 17. The system of claim 16, wherein said conductive metal is a non-ferrous metal. 18. The three attenuator mounted on top of the attenuator
The system of claim 1, further comprising an axis transmitter. 19. The system of claim 13, wherein said transmitter comprises a printed circuit board with transmitter means etched on top. 20. The printed circuit board as set forth in claim
~ 3.175mm (0.0325-0.125 inch)
20. The system of claim 19, wherein the thickness is: 21. The system of claim 3, wherein said magnetic field is generated by a pulsed DC power supply. 22. A thin magnetically permeable attenuator; b) a thin conductive plate provided below said attenuator; c) a thin transmitter provided above said attenuator; d. A) a magnetic position measuring system, wherein the transmitter, the attenuator and the plate are integrally laminated; 23. The system of claim 22, wherein said transmitter, attenuator, and plate have a combined thickness of about 0.36 to 1625 mm (0.3 to 0.625 inches). 24. The system of claim 22, wherein said transmitter comprises an etched printed circuit board of transmitting means on top. 25. The attenuator is made of a material selected from the group consisting of ferrite and mu metal.
23. The system according to claim 22. 26. The system of claim 22, wherein said plate is manufactured from aluminum.