Directional Sensor Calibration Under Changing Geomagnetic Fields
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Solution Overview
Problem
Current Measurement While Drilling (MWD) directional sensor calibration methods, such as Total Field Calibration (TFC), are inaccurate when the geomagnetic field is changing, as they rely on stable field conditions.
Innovation Solution
The proposed solution involves a more robust calibration method that uses data from all sensor orientations to determine calibration coefficients, allowing for accurate calibration even during geomagnetic field changes. This method employs a generalized parameter fitting approach, such as the Method of Least Squares (MLS), to simultaneously determine all necessary calibration parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Total Field Calibration (TFC) methods are used to determine calibration coefficients, then calibration can be performed using Earth's magnetic field and gravitational field magnitudes, but calibration accuracy deteriorates when the geomagnetic field is changing
Solution Approach 1:
The patent changes the calibration parameters from relying on Earth's magnetic field magnitude (which varies with location and time) to using sensor output components at multiple known orientations. By collecting data at N different orientations and using the components of magnetic and gravitational fields along sensor axes rather than their magnitudes, the calibration becomes independent of geomagnetic field stability while maintaining ease of calibration through systematic orientation sampling.
2Measurement precision
If calibration relies on stable geomagnetic field conditions, then calibration accuracy is maintained, but the method becomes inapplicable when geomagnetic field changes occur
Solution Approach 1:
The patent introduces dynamics by collecting calibration data at N different orientations rather than relying on a single static measurement. The system adapts to changing geomagnetic conditions by using multiple measurements and solving for calibration coefficients that satisfy all orientation data simultaneously. This dynamic approach allows accurate calibration whether the geomagnetic field is stable or changing, as the multiple orientation measurements provide redundant information that compensates for field variations.
3Measurement precision
If multiple sensor orientations are used to determine calibration coefficients, then calibration accuracy under changing geomagnetic fields is improved, but the calibration process complexity increases
Solution Approach 1:
The patent segments the calibration process into N discrete orientation measurements, where each orientation provides a set of equations relating sensor outputs to known field components. By dividing the calibration into multiple independent orientation steps rather than attempting a single complex measurement, the system achieves accurate calibration under varying geomagnetic conditions. The segmentation allows systematic data collection and simplifies the mathematical solution by providing enough independent equations to solve for all calibration coefficients.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The new calibration method significantly reduces inaccuracies associated with changing geomagnetic fields, providing more accurate directional sensor calibration and maintaining calibration quality regardless of geomagnetic fluctuations.
Implementation Method 1
The magnetometers may be used to measure components of the Earth's magnetic field (i.e., geomagnetic field) along three orthogonal axes related to the drill string section where the MWD directional sensor is located
Implementation Method 2
The three accelerometers measure the Earth's gravity components along the mutually orthogonal X, Y, and Z-directions
Data Source
AI summary
An improved total field calibration system and method is disclosed for reducing the rotational misalignment between magnetic and gravity sensors in a directional sensing system. A method of calibrating a tri-axial directional sensor comprising orthonormal accelerometers and orthonormal magnetometers, comprises measuring Earth's magnetic and gravity fields with said directional sensor in at least 4 sensor orientations; obtaining at least one reference field value of dip drift of Earth's magnetic field from at least one source independent of said directional sensor corresponding to said orientations; and, determining and applying rotational misalignments between said magnetometers and said accelerometers so that measured magnetic dip drifts are substantially equal to said reference values. The calibration process can be performed without monitoring the declination change during the calibration process. Directional sensing systems can be calibrated accurately during a period when the Earth's magnetic field changes rapidly.