# US 11,029,369 B1 · Self-leveling magnetic source

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- Number: US 11,029,369 B1
- Document ID: US11029369B1
- Status: Granted
- Kind: B1
- Application number: 16/773,370
- Filed: 2020-01-27
- Granted: 2021-06-08
- Earliest claimed: 2019-01-26
- Assignees: ALKEN Inc.
- Applicants: ALKEN Inc.
- Inventors: Robert F. Higgins; Allan G. Rodgers, Jr.
- Web page: https://patentgrep.com/patent/US11029369B1

## CPC

- G01R 33/0017: {Means for compensating offset magnetic fields or the magnetic flux to be measured; Means for generating calibration magnetic fields}
- G01B 7/003: {for measuring position, not involving coordinate determination (coordinate measuring G01B7/004)}
- G01B 21/042: {Calibration or calibration artifacts (G01B3/30, G01B9/02072 take precedence)}
- G01P 13/00: Indicating or recording presence, absence, or direction, of movement (electric switches H01H; counting moving objects G06M7/00)
- G01P 15/08: with conversion into electric or magnetic values
- G01R 33/02: Measuring direction or magnitude of magnetic fields or magnetic flux (G01R33/20 takes precedence)
- G01R 33/0035: {Calibration of single magnetic sensors, e.g. integrated calibration}
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01P: MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT (measuring or recording blood flow A61B5/02, A61B8/06; monitoring speed or deceleration of electrically-propelled vehicles B60L3/00; vehicle lighting systems adapted to indicate speed B60Q1/54; determining position or course in navigation, measuring ground distance in geodesy or surveying G01C; combined measuring devices for measuring two or more variables of movement G01C23/00; measuring velocity of sound G01H; measuring velocity of light G01J7/00; measuring direction or velocity of solid objects by reception or emission of radiowaves or other waves and based on propagation effects, e.g. Doppler effect, propagation time, direction of propagation, G01S; measuring speed of nuclear radiation G01T; measuring acceleration of gravity G01V; {measuring or recording the speed of trains B61L23/00; speed indicators incorporated in motor vehicles B60K35/00; measuring frequency or phase G01R; traffic control G08G})

## Abstract

A method for calibrating a magnetic source of an electromagnetic tracking system for tracking a position and orientation of an object using an inertial motion unit (IMU) mounted to the magnetic source and aligned with the coil array of magnetic source. Pitch and rolls angles of the coil array are calibrated using the IMU.

## Claims

### Claim 1

A method for calibrating a transmitter circuit of an electromagnetic tracking system for tracking a position and orientation of a first object relative to a second object, wherein the transmitter circuit includes a plurality of transmitter coils aligned at different transmitter axes and an actuating circuit configured to actuate the transmitter coils to generate a time-multiplexed or frequency-multiplexed magnetic field, wherein a receiver circuit including a plurality of sensing elements aligned at different receiver axes is mounted to the first object, and wherein the sensing elements are configured to detect respective components of the magnetic field generated by the plurality of transmitter coils, the method comprising:
aligning an azimuth angle of the transmitter circuit to a reference point so that an azimuth angle of an inertial motion unit (IMU) mounted to the transmitter circuit is aligned within a reference frame of the second object, wherein each of a pitch angle offset and a roll angle offset of the IMU relative to a gravitational force is adjusted so that a reference frame of the IMU is aligned with a reference frame of the transmitter circuit; and
mounting the transmitter circuit to the second object while the azimuth angle of the IMU is aligned within the reference frame of the second object.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the adjustment of each of the pitch angle offset and the roll angle offset of the IMU comprises a mechanical adjustment of the IMU such that the reference frame of the IMU is physically aligned with the reference frame of the transmitter circuit.

### Claim 3 (depends on 1)

The method according to claim 1, wherein the adjustment of each of the pitch angle offset and the roll angle offset of the IMU comprises an adjustment to calibration data stored in the electromagnetic tracking system to counteract the pitch angle offset and the roll angle offset.

### Claim 4 (depends on 1)

The method according to claim 1, further comprising, after mounting the transmitter circuit to the second object:
measuring, by the IMU, each of the pitch angle offset and the roll angle offset of the IMU relative to the gravitational force; and
receiving, by one or more processors of the electromagnetic tracking system, the pitch angle offset and the roll angle offset from the transmitter circuit, wherein the transmitted pitch angle offset and roll angle offset are indicative of an offset between the reference frame of the transmitter circuit and the reference frame of the second object; and
storing, by one or more processors, an indication of the pitch angle offset and the roll angle offset of the IMU in a memory of the electromagnetic tracking system.

### Claim 5 (depends on 4)

The method according to claim 4, further comprising:
determining, by the one or more processors, the position and orientation of the first object relative to the second object based on an analysis of the respective components of the magnetic field detected by the plurality of receiver coils, wherein the analysis is calibrated according to the stored indication of the pitch angle offset and roll angle offset.

### Claim 6 (depends on 1)

The method according to claim 1, wherein the first object is enclosed in an interior space of the second object and capable of moving within the interior space of the second object, and wherein the transmitter circuit is mounted to the second object within the interior space of the second object.

### Claim 7 (depends on 6)

The method according to claim 6, wherein the second object is a vehicle, and wherein the first object is a head mounted display.

### Claim 8 (depends on 1)

The method according to claim 1, wherein each of the first object and the second object are positioned in a common space, wherein the second object is in a fixed position within the common space, and wherein the first object is capable of moving within the common space.

### Claim 9 (depends on 8)

The method according to claim 8, wherein the second object is a platform or a tripod, and wherein the first object is a handheld object or a wearable.

### Claim 10

A method for calibrating an orientation of a first element of an electromagnetic tracking system pair for tracking a position and orientation of a second element of the electromagnetic tracking system pair, wherein the first element includes a plurality of magnetic field sensing or transmitting elements adapted to sense or transmit magnetic field components along three mutually orthogonal magnetic field axes in a first frame of reference, and an inertial motion unit (IMU) adapted to detect at least one of linear acceleration or angular rotation along first, second and third IMU axes in a second frame of reference, the second frame of reference having a known orientation relative to the first frame of reference, the method comprising:
affixing the second element of the electromagnetic tracking system pair to a base having a fixed orientation in a third frame of reference while the base has a known orientation relative to a gravitational force, such that that a first IMU axis is aligned with a reference point of the third frame of reference within a plane having a known angle relative to the gravitational force;
while the second element of the electromagnetic tracking system pair is affixed to the base, measuring, by the inertial motion unit (IMU), the gravitational force along the second and third IMU axes; and
based at least in part on the measured gravitational force along the second and third IMU axes, performing at least one of:
determining at least part of a transform between the second frame of reference and the third frame of reference; or
mechanically adjusting an orientation of the second element of the electromagnetic tracking system pair relative to the base so that the second frame of reference lies in a predetermined orientation relative to the third frame of reference of the platform, and affixing the second element of the electromagnetic tracking system pair to the base in the predetermined orientation.

### Claim 11 (depends on 10)

The method according to claim 10, wherein, while the plane is perpendicular to the gravitational force.

### Claim 12 (depends on 11)

The method according to claim 11, wherein the method includes determining at least part of the transform between the second frame of reference and the third frame of reference, and the transform includes:
a pitch off set angle between the second IMU axis and the plane perpendicular to the downward direction; and
a roll offset angle between the third IMU axis and the plane perpendicular to the downward direction.

### Claim 13 (depends on 10)

The method according to claim 10, wherein the method includes determining at least part of the transform between the second frame of reference and the third frame of reference, and further comprises:
actuating the electromagnetic tracking system pair so that magnetic fields are transmitted by one of the first and second elements of the electromagnetic tracking system pair and sensed by the other one of the first and second elements of the electromagnetic tracking system pair;
deducing a position and an orientation of the first element in the second frame of reference from magnetic fields sensed by the magnetic field sensing elements of the electromagnetic tracking system pair; and
transforming the deduced position and orientation in the second frame of reference to the third frame of reference by applying the transform.

### Claim 14 (depends on 13)

The method according to claim 13, wherein the method further comprises detecting an interaction between a first object affixed to the first element of the electromagnetic tracking system pair and a second object having a known position relative to the second element of the electromagnetic tracking system pair, based at least in part on the transformed position and orientation of the first element.

### Claim 15 (depends on 10)

The method according to claim 10, wherein the second element of the electromagnetic tracking system pair is affixed to a head mounted display.

### Claim 16 (depends on 10)

The method according to claim 10, wherein the second element of the electromagnetic tracking system pair is affixed to a handheld object or to a wearable.