Electronic compass

• US 6,760,678 B1
• Filed: 04/11/2000
• Issued: 07/06/2004
• Est. Priority Date: 04/11/2000
• Status: Expired due to Fees

• Current Assignee: Electronics Tomorrow Limited (Talent Property Group Ltd.)
• Original Assignee: Electronics Tomorrow Limited (Talent Property Group Ltd.)
• Inventors: Wah, Johnny Tam Ping, Hon, Patrick Fong Wing
• Primary Examiner(s): Barlow, John
• Assistant Examiner(s): Cherry, Stephen J.
• Application Number:
  US09/546,516
• Time in Patent Office: 1,547 Days
• Field of Search: 702/92, 73/17.6, 333/56, 333/61
• US Class Current: 702/92
• CPC Class Codes: G01C 17/28 Electromagnetic compasses with north seeking magnetic elements and having electric pick-offs G01C17/26
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses

Abstract

A method and system for continuously and automatically calibrating the settings of an electronic compass, as needed, while a vehicle moves from place to place is shown. The calibration technique of the present invention measures the ocal magnetic field while the vehicle moves from place to place and determines points on a reference Cartesian coordinate system. Each such point defines the endpoint for a vector corresponding with the measured field intensity and orientation of the local magnetic field. Any three such points are used to calculate the center of a circle that extends through all of those points. A vector that begins at the origin of the reference Cartesian coordinate system and terminates at the calculated center of the circle is thereafter subtracted from subsequent field measurement vectors that correspond to the field intensity and direction of the local magnetic field. As a result, the electronic compass compensates for interfering magnetic fields, including those resulting from the varying magnetic field signatures of its associated vehicle.

Claims

1. A method for calibrating an electric compass, comprising the steps of:

measuring a magnetic field while said magnetic field is positioned relative to said compass in a first direction;

identifying a first point that is capable of being represented as A(x₁,y₁) and is indicative of said first direction of said magnetic field;

measuring said magnetic field while said magnetic field is positioned relative to said compass in a second direction;

identifying a second point that is capable of being represented as B(x₂,y₂) and is indicative of said second direction of said magnetic field;

measuring said magnetic field while it is positioned relative to said compass in a third direction;

identifying a third point that is capable of being represented as C(x₃,y₃) and is indicative of said third direction of said magnetic field;

utilizing only said first, second and third points to determine an offset point that can be represented as O Noise (x₀,y₀) and is substantially equidistant from said first, second and third points, the calculation of said offset point being carried out by a set of equations that assumes a first square distance from said first point to said offset point is equal to a second square distance from said second point to said offset point, and said first and second square distances are equal to a third square distance from said third point to said offset point, namely;

2. The method as defined by claim 1 wherein said magnetic field equals a sum of unknown first and second magnetic field components and said offset point permits determination of said first magnetic field component, and said compass is calibrated by subtracting said first magnetic field component from subsequent measurements of said magnetic field to derive said second magnetic field component of said subsequent measurements of said magnetic field.

3. The method as defined by claim 2 wherein said first magnetic field component is an interfering magnetic field and said second magnetic field component is Earth'"'"'s magnetic field.

4. The method as defined by claim 3 wherein said interfering magnetic field substantially results from a magnetic signature of a vehicle associated with said compass.

5. The method as defined by claim 1 wherein said first, second and third points are two dimensional and can be represented on a reference Cartesian coordinate system.

6. The method as defined by claim 5 wherein said first, second and third points are included within a set of points defining a circular locus of points on said reference Cartesian coordinate system.

7. The method as defined by claim 6 wherein said first, second and third points lie within an arc of said circular locus of points and said arc includes a set of points defining substantially less than a semi-circular locus of points.

8. An electronic compass, comprising:

a first data channel circuit that measures a fist magnetic field intensity associated with a first directional component of a local magnetic field and generates a first electrical output signal indicative of said first magnetic field intensity;

a second data channel circuit that measures a second magnetic field intensity associated with a second directional component of said local magnetic field and generates a second electrical output signal indicative of said second magnetic field intensity;

a combiner circuit, electrically coupled to said first and second data channel circuits and responsive to said first and second electrical output signals, that combines said first and second electrical output signals and generates a third electrical output signal indicative of a direction of said local magnetic field and further indicative of a third magnetic field intensity associated with said direction of said local magnetic field;

a processor circuit electrically coupled to said combiner circuit that derives information indicative of said direction from said third electrical output signal, calculates a vehicle heading direction from said derived information and generates a display control signal that corresponds to said vehicle heading direction;

a display electrically coupled to said processor circuit that receives said display control signal and identifies said vehicle heading direction in a visible fashion; and

said processor circuit being programmed to allow for calibration of said compass, said calibration requiring utilization of only three measurements of said third electrical output signal by said processor, provided said three measurements of said third electrical output signal having differing directional values, and said calibration carried out by calculating coordinates for a noise center that can be represented as O_Noise (x_Ø

,y_Ø

) and is positioned on a reference Cartesian coordinate system at a centerpoint of a circle also positioned on said reference Cartesian coordinate system and including a first point corresponding to a first of said three measurements that is capable of being represented as A(x₁,y₁), a second point corresponding to a second of said three measurements that is capable of being represented as B(x₂,y₂). and a third point corresponding to a third of said three measurements that is capable of being represented as C(x₃,y₃), the calculation of said noise center being carried out by a set of equations that assumes a first square distance from said first point to said noise center is equal to a second square distance from said second point to said noise center, and said first and second square distances are equal to a third square distance from said third point to said noise center, namely;

9. The electronic compass as defined by claim 8 wherein said combiner circuit and said processor circuit are separate circuits electrically coupled together.

10. The electronic compass as defined by claim 8 wherein said first directional component of said local magnetic field measured by said first data channel circuit is orthogonal to said second directional component of said local magnetic field measured by said second data channel circuit.

11. The electronic compass as defined by claim 8 wherein said first data channel circuit includes a first magnetic field sensor, said second data channel circuit includes a second magnetic field sensor, and said first and second magnetic field sensors are orthogonally disposed with respect to each other.

12. The electronic compass as defined by claim 11 wherein said compass is adapted to be mounted in a vehicle, first magnetic sensor is disposed along a first magnetic sensor axis, said second magnetic sensor is disposed along a second magnetic sensor axis, a travel direction axis defines a vehicle travel direction, and said first and second magnetic sensor axes are aligned at forty-five degree angles on opposing sides of said travel direction axis.

13. The electronic compass as defined by claim 8 wherein said display is a liquid crystal display.

14. The electronic compass as defined by claim 13 wherein said display identifies a cardinal point indication corresponding to said vehicle heading direction.

15. The electronic compass as defined by claim 14 wherein said display identifies a directional degree indication corresponding to said vehicle heading direction.

16. The electronic compass as defined by claim 15 wherein said display includes a mobile directional compass indicator that emulates a mechanical compass indicator, said mobile directional compass indicator including an arrow indicator and a band indicator of cardinal headings.

17. The electronic compass as defined by claim 8 wherein said display includes a mobile directional compass indicator that emulates a mechanical compass indicator, said mobile directional compass indicator including an arrow indicator and a band indicator of cardinal headings.

18. The electronic compass as defined by claim 17 wherein said arrow indicator remains is displayed in a fixed position on said display and said band indicator is displayed in various formats on said display to provide an indication of various vehicle heading directions.

19. An electronic compass, comprising:

a base portion having an upwardly extending mount and having an energy cell compartment associated therewith;

a display portion pivotally connected to said base portion and having a circuit housing section and a liquid crystal display associated therewith;

an electronic compass circuit contained within said circuit housing section of said display portion and electrically coupled to said energy cell compartment of said base portion, said electronic compass circuit including;

a first data channel circuit that measures a first magnetic field intensity associated with a first directional component of a local magnetic field and generates a first electrical output signal indicative of said first magnetic field intensity;

a second data channel circuit that measures a second magnetic field intensity associated with a second directional component of said local magnetic field and generates a second electrical output signal indicative of said second magnetic field intensity;

a combiner circuit, electrically coupled to said first and second data channel circuits and responsive to said first and second electrical output signals, that combines said first and second electrical output signals and generates a third electrical output signal indicative of a direction of said local magnetic field and further indicative of a third magnetic field intensity associated with said direction of said local magnetic field;

a processor circuit electrically coupled to said combiner circuit and to said liquid crystal display that derives information indicative of said direction from said third electrical output signal, calculates a vehicle heading direction from said derived information and generates a display control signal that corresponds to said vehicle heading direction for controlling the liquid crystal display to display an indication of said vehicle heading direction in a visible fashion; and

said processor circuit being programmed to allow for calibration of said compass, said calibration requiring utilization of only three measurements of said third electrical output signal by said processor, provided said three measurements of said third electrical output signal have differing direction values, and said calibration carried out by calculating coordinates for a noise center that can be represented as O_Noise (x_Ø

,y_Ø

) and is positioned on a reference Cartesian coordinate system at a centerpoint of a circle also positioned on said reference Cartesian coordinate system and including a first point corresponding to a first of said three measurements that is capable of being represented as A(x₁,y₁), a second point corresponding to a second of said three measurements that is capable of being represented as B(x₂,y₂), and a third point corresponding to a third of said three measurements that is capable of being represented as C(x₃,y₃), the calculation of said noise center being carried out by a set of equations that assumes a first square distance from said first point to said noise center is equal to a second square distance from said second point to said noise center, and said first and second square distances are equal to a third square distance from said third point to said noise center, namely;

20. The electronic compass as defined by claim 19 wherein said circuit housing section and said liquid crystal display are secured together by a pair of end caps positioned on opposing sides of said display portion.