Position Tracking System Employing External Magnetic Field

Abstract

A common method for providing user-input to an electronic system consists of tracking the position and motion of an object moved by the user and conveying this information to the electronic system. One embodiment of a positional tracking system for an object has an external and stationary magnetic-field emitter, a magnetic-field sensor which moves with the tracked object, and a microprocessor that compares magnetic-field intensity measurements taken by the sensor and compares it to magnetic-field characteristics defined for the external magnetic field emitter. A nonlinear equation solver, particle filter, or other method is used to determine the position of the sensor in the magnetic field. In this way the position of an object can be tracked using a single magnetic field emission source. This positional information can be combined with an inertial tracking system to mitigate drift errors.

Claims

1. A method for tracking motion of an object, comprising: a. providing a magnetic field emission means which is able to emit a predetermined magnetic field, b. providing a magnetic field sensing means to sense said magnetic field, c. providing a computational means for predicting the characteristics of said magnetic field wherein a location in the magnetic field is given as an input and the characteristics of said magnetic field at said location are taken as the output of said computational means, and d. providing a logical means for determining the position of said magnetic sensing means in said emitted magnetic field by varying the positional input of said computational means until the output of said computational means is equivalent to the output of said magnetic field sensing means, whereby the positional input of said computational means is equivalent to the physical location of the magnetic field sensing means.

2. The method for tracking motion of an object of claim 1, additionally providing a computational means for modifying the output of the magnetic field sensing means such that the output is corrected for a change in orientation of the object whose motion is being tracked

3. The method for tracking motion of an object of claim 1, additionally providing computational means for modifying the output of the computational means for predicting the characteristics of the known magnetic field such that the output is corrected for a change in orientation of the object whose motion is being tracked

4. The method for tracking motion of an object of claim 2, additionally comprising: a. providing a plurality of magnetic sensing means arranged with known locations and distances relative to each-other, b. providing a plurality of computational means for correcting the output of the magnetic field sensing means for a given input orientation, and c. providing a logical means for determining the orientation of the object by varying the input orientation of each computational means for orientation correction until the resulting position outputs for each magnetic field measurement means result in relative distances between them being equal to the known distances between them whereby the orientation input that results in agreement between the known and calculated distances is equivalent to the orientation of the objecting being tracked relative to the reference orientation of the magnetic field.

5. The method for tracking motion of an object of claim 1, additionally comprising: a. providing an linear acceleration measurement means, b. providing an angular velocity measurement means, c. providing a computational means for determining object position and orientation using linear acceleration and angular velocity measurements, and d. providing a computational means for combining measurements of object motion using linear acceleration, angular velocity and magnetic positional sensing means using a Kalman filter whereby the position of an object is known to a higher degree of certainty than if it were found using all three measurement means.

Description

A BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 shows an exploded view of one embodiment.

[0016] FIG. 2 shows the block diagram detailing how the system determines a position from a combination of magnetometer readings and a knowledge of a surrounding spatially varying magnetic field.

[0017] FIG. 3 shows an expression for the x-component of the magnetic field surrounding a rectangular permanent magnet, having dimensions having length L, width W, height H, and being magnetized through the height dimension (corresponding to the x-axis) with a magnitude of M

[0018] FIG. 4 shows an expression for the y-component of the magnetic field surrounding a rectangular permanent magnet, having dimensions having length L, width W, height H, and being magnetized through the height dimension (corresponding to the x-axis) with a magnitude of M

[0019] FIG. 5 shows an exploded view of an alternate embodiment having a plurality of magnetometers.

[0020] FIG. 6 shows the block diagram detailing how the system determines a position and orientation from a combination of readings from two or more magnetometers, knowledge of the positioning of the magnetometers relative to each-other, and knowledge of a surrounding spatially varying magnetic field.

[0021]

TABLE-US-00002 Drawings-Reference Numerals 10 permanent magnet 11a enclosure, bottom 11b enclosure, top 12 magnetometer 13 microprocessor 14 accelerometer 15 gyroscope 16 wireless communication unit 17 printed circuit board 19 secondary magnetometer 20 orientation correction operation 21 nonlinear equation solver 22 magnetic field vector generator 23 distance calculation 24 plural orientation correction operation

DETAILED DESCRIPTION—FIGS. 1 THROUGH 4—FIRST EMBODIMENT

[0022] The permanent magnet 10 having a size, shape, and magnetization configuration such that it casts a spatially varying magnetic vector field in the volume surrounding it, with a magnetic field magnitude at a point half of a meter away from the center of the magnet that is several times greater than the earth's magnetic field magnitude at the same point. This can be achieved by a low-grade ceramic permanent magnet being of a rectangular prism shape, having dimensions of 47 mm by 22 mm by 5 mm, and being magnetized in a direction through the smallest dimension. The microprocessor 13 having the ability to generate three components of a magnetic vector field of a permanent magnet based on an input of a position and orientation with respect to the external permanent magnet 22, resulting from a set of equations, lookup tables, or a numerical model of the magnet. The user of the system is instructed to begin operation with the electromechanical assembly in a specific orientation with respect to the permanent magnet, which establishes a baseline orientation.

[0023] The magnetometer 14 measures the three-dimensional magnetic field vector that is produced by the permanent magnet 10 at the point where the magnetometer 14 is located, and is input into the microprocessor 13. This vector is transformed by the microprocessor with information from the gyroscope and acceleration to correct for any deviation between the present orientation of electromechanical assembly and the reference orientation 20. This can be accomplished by means of quaternions, direction-cosine matrices, Euler angles, or other methods. The microprocessor 13 then uses an iterative nonlinear equation solving method (such as Newton's method) 21 or a particle filter to calculate the position of the electromechanical assembly using both the rotated and measured magnetic field vector, and the microprocessor's knowledge of the magnetic field surrounding the permanent magnet 22.

[0024] The motion of the electromechanical assembly is concurrently measured using the accelerometer 14 and gyroscope 13 employing standard inertial measurement methods known by those familiar with the art. The inertial and magnetic measurements are combined using a Kalman filter or an averaging method to obtain an optimal estimate for acceleration, velocity, and/or position of the electromechanical assembly 11a-17 with respect to the permanent magnet 10.

[0025] The Magnetic Field Expression Output 22 is a module of the Microprocessor 13 that can output the three components of the 3D magnetic vector field for a specific magnet design (geometry, material, and magnetization) that is equivalent to the design of the Permanent Magnet 10, at a specific point relative to the Permanent Magnet 10. The source of this output could be a lookup table that derives its data from experimental data or results from a numerical simulation of the magnet. The source of this output could be the direct result of a numerical simulation of the magnet, or an analytic equation. One possible methodology is to derive an analytical expression for the magnetic field surrounding a permanent magnet with a simple geometry such as a rectangular prism. The magnetostatic potential φ.sub.m at point {right arrow over (r)} for a point with a magnetization described by the vector m is given as

[00001]$\stackrel{.Math.}{{\phi }_m}\left(r\right)=\frac{{\mu }_0}{4.Math..Math.\pi }.Math.\stackrel{.Math.}{m}.Math.\stackrel{.Math.}{r}/r^3,$

with μ.sub.0 being the magnetic permeability of free-space. The magnetostatic potential Φ.sub.m at point r for a complete permanent magnet can be found by the integral equation: Φ.sub.m=∫∫∫φ.sub.m dV. The magnetic field at point r can then be found by the equation: {right arrow over (B)}=−∇φ.sub.m(r). For the case of a rectangular prism magnet with length L, width W, and height H, with relatively uniform magnetization M through the length dimension, and the magnetic permeability of free-space μ.sub.0, the magnetic field at a point Sx, Sy, Sz that resides in a plane normal to the height dimension, halfway up the height of the magnet can be computed by the expressions in FIGS. 3 and 4. The expression for Bz is zero in this plane.

[0026] Many other methods can be used for deriving magnetic field expressions for permanent magnets. Gou et. al. (2004) detail a method for calculating magnetic field components for rectangular-prism permanent magnets at any arbitrary point surrounding the magnet. They also extend their expression for use with a stack of rectangular prism permanent magnets.

DETAILED DESCRIPTION—FIGS. 5 AND 6—ALTERNATE EMBODIMENT

[0027] An alternate embodiment includes one or more additional magnetometers 19, arranged on the printed circuit board 17 with a known, constant position relative to one-another. The position for each magnetometer is determined using the method described in the first embodiment. The Distance Calculation 23 is a module of the microprocessor that calculates the difference between any combination of position outputs. The magnetometers are rigidly attached to the printed circuit board, and as such, as the orientation of the device changes relative to a reference orientation, the orientation of each magnetometer will change by the same amount. The output from the Distance Calculation module 23 will be equivalent to the known distance between the sensors when the input reference orientation is correct. Equivalently, the input orientation can be varied until there is sufficient agreement between the calculated and known distances thereby indicating the correct reference orientation.