System for using a 2-axis magnetic sensor for a 3-axis compass solution

• US 6,836,971 B1
• Filed: 07/30/2003
• Issued: 01/04/2005
• Est. Priority Date: 07/30/2003
• Status: Expired due to Fees

• Current Assignee: Honeywell International Inc.
• Original Assignee: Honeywell International Inc.
• Inventors: Wan, Hong
• Primary Examiner(s): Fulton, Christopher W.
• Application Number:
  US10/630,187
• Time in Patent Office: 524 Days
• Field of Search: 333/56, 702/92, 702/93
• US Class Current: 33/356
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses

Abstract

A tilt-compensated electronic compass can be realized by calculating rather than measuring Earth'"'"'s magnetic field component Z in a direction orthogonal to the two measurement axes of a 2-axis magnetic sensor. The orthogonal component Z can be calculated using a stored value for the Earth'"'"'s magnetic field strength applicable over a wide geographic region. The calculation also requires using measured field values from the 2-axis sensor. Once Z is known, and using input from a 2-axis tilt sensor, compensated orthogonal components X and Y can be calculated by mathematically rotating the measured field strength values from a tilted 2-axis sensor back to the local horizontal plane. Thus, a very flat and compact tilt-compensated electronic compass is possible.

Claims

1. A compass system comprising:

a 2-axis magnetic sensor;

a tilt sensor;

a memory;

a processor;

at least one value for the Earth'"'"'s magnetic field strength stored in the memory; and

a set of instructions stored in the memory and executable by the processor to calculate a magnetic field component, Z, that is orthogonal to measurement axes associated with the 2-axis magnetic sensor using inputs from the 2-axis magnetic sensor and using the at least one stored value for the Earth'"'"'s magnetic field strength.

2. The system of claim 1, wherein the field component Z is calculated as Z=√

{square root over (H²−

X²−

Y²)} where H is the at least one stored value, X is a first measurement from the 2-axis magnetic sensor and Y is a second measurement from the 2-axis magnetic sensor that is orthogonal to X.

3. The system of claim 2, further comprising a set of instructions in memory and executable by the Processor to calculate local horizontal components of the Earth'"'"'s magnetic field X_comp and Y_comp using the calculated value of Z and inputs from the tilt sensor.

4. The system of claim 3, wherein the tilt sensor is a 2-axis tilt sensor that measures pitch (φ

) and roll (θ

) angles and wherein the local horizontal components X_comp and Y_comp are mutually orthogonal and are calculated using the equations;

X_comp=X cos φ

+Y sin² φ

−

Z cos φ

sin φ and

Y_comp=Y cos θ

+Z sin θ

.

5. The system of claim 4, wherein a compensated heading is calculated using the equation

Heading=arcTan(Y_comp/X_comp).

6. A method of compensating for tilt in an electronic compass having a 2-axis magnetic sensor and a tilt sensor, the method comprising:

storing at least one value for the Earth'"'"'s magnetic field strength;

measuring the Earth'"'"'s magnetic field strength with the 2-axis magnetic sensor; and

calculating a magnetic field component, Z, that is orthogonal to the measurement axes associated with the 2-axis magnetic sensor, using the measured field strengths from the 2-axis magnetic sensor and using the at least one stored value for the Earth'"'"'s magnetic field strength.

7. The method of claim 6, further comprising:

calculating the orthogonal field component Z using the equation Z=√

{square root over (H²−

X²−

Y²)} where H is the at least one stored value, X is a first measurement from the 2-axis magnetic sensor and Y is a second measurement from the 2-axis magnetic sensor that is orthogonal to X.

8. The method of claim 7, further comprising:

calculating local horizontal components X_comp and Y_comp of the Earth'"'"'s magnetic field using the calculated value of Z and inputs from the tilt sensor, wherein the tilt sensor is a 2-axis tilt sensor that measures pitch (φ

) and roll (θ

) angles and wherein the local horizontal components X_comp, and Y_comp are mutually orthogonal and are calculated using the equations;

X_comp=X cos φ

+Y sin² φ

−

Z cos θ

sin φ and

Y_comp=Y cos θ

+Z sin θ

.

9. A method of compensating for tilt in an electronic compass having a 2-axis magnetic sensor and a tilt sensor, the method comprising:

storing at least one value for the Earth'"'"'s magnetic field strength;

measuring the Earth'"'"'s magnetic field strength with the 2-axis magnetic sensor;

calculating a magnetic field component, Z, that is orthogonal to the 2-axis magnetic sensor measurement axes using the measured field strengths from the 2-axis magnetic sensor and using the at least one stored value for the Earth'"'"'s magnetic field strength, wherein the orthogonal field component Z is calculated using the equation Z=√

{square root over (H²−

X²−

Y²)} where H is the at least one stored value, X is a first measurement from the 2-axis magnetic sensor and Y is a second measurement from the 2-axis magnetic sensor that is orthogonal to X;

calculating local horizontal components X_comp and Y_comp of the Earth'"'"'s magnetic field using the calculated value of Z and inputs from the tilt sensor, wherein the tilt sensor is a 2-axis tilt sensor that measures pitch (φ

) and roll (θ

) angles and wherein the local horizontal components X_comp and Y_comp are mutually orthogonal and are calculated using the equations;

X_comp=X cos φ

+Y sin² φ

−

Z cos θ

sin φ and

Y_comp=Y cos θ

+Z sin θ

; and

calculating a compensated heading using the equation

Heading=arcTan(Y_comp/X_comp).