Spherical calibration and reference alignment algorithms
First Claim
1. A method for performing a spherical calibration on acquired directional field sensor data to correct directional field sensor measurement errors arising from cross axis gain and offset errors to within a preset limit comprising the steps:
- acquiring directional field sensor data in at least two axes comprising at least four different tilt angles;
determining a set of coefficients from a best fit calculation;
determining a symmetric matrix from the set of coefficients;
solving for a transform matrix;
solving for an offset matrix; and
correcting the acquired directional field sensor data with the transform matrix and the offset matrix such that the corrected directional field sensor data is within the preset limit of the true field being measured.
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Abstract
Inclinometer and directional field sensor readings can have gain, offset, and non-orthogonality errors, as well as reference alignment rotation errors. When a series of readings are taken by a three axis sensor with a variety of different orientations, the resulting dataset looks like a perfect hypothetical sphere in the absence of any errors; with errors as mentioned above the dataset looks like an offset, rotated, ellipsoidal quadratic surface. This invention provides a simple method of removing the above errors from a tilt reference device. A disclosed algorithm is divided into two distinct components: the ellipsoidal quadratic surface component, which covers gain, offset, and axis misalignment; and the rotation component, which covers rotation relative to a set of reference axes. The solution presented here addresses both components combined, or separated and for inclinometers, magnetometers and rate sensors.
41 Citations
22 Claims
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1. A method for performing a spherical calibration on acquired directional field sensor data to correct directional field sensor measurement errors arising from cross axis gain and offset errors to within a preset limit comprising the steps:
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acquiring directional field sensor data in at least two axes comprising at least four different tilt angles; determining a set of coefficients from a best fit calculation; determining a symmetric matrix from the set of coefficients; solving for a transform matrix; solving for an offset matrix; and correcting the acquired directional field sensor data with the transform matrix and the offset matrix such that the corrected directional field sensor data is within the preset limit of the true field being measured. - View Dependent Claims (2, 3, 4, 5)
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6. A method to determine misalignment between a device reference frame and a physical reference frame comprising the steps:
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making a first measurement, Mdevref, with a device in a first physical reference orientation (8110); rotating the device a first angle, phi1, from the first physical reference orientation around a first physical reference frame axis, Vphys1; making a second measurement, Mdev1; rotating the device a second angle, phi2, from the first physical reference orientation around second physical reference frame axis Vphys2; making a third measurement, Mdev2; applying an algorithm to estimate the rotation required to align the device frame to the physical reference frame using the known angles phi1, phi2 and the matching physical axes Vphys1, Vphys2; and outputting a rotation matrix, TRref. - View Dependent Claims (7, 8, 9)
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10. A method to adjust misalignment between a device reference frame and a physical reference frame defined by Urot1_df and Urot2_df to below a preset limit comprising the steps:
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making at least three first sensor field measurements while rotating the device reference frame around a first axis Urot1_ef, wherein Urot1_ef coincides with a physical reference frame axis Rphys1Z_ef; rotating the device reference frame a first angle around a second physical reference frame axis Rphys2X_ef; making at least three second sensor field measurements while rotating the device reference frame around a second axis Urot2_ef, wherein Urot2_ef does not coincide with physical reference frame axis Rphys2Z_ef; and processing the at least three first and the at least three second measurements to obtain a rotation matrix Tr such that Tr rotates the device reference frame to align with the physical reference frame defined by Urot1_df and Urot2_df such that residual errors between the device reference frame and the physical reference frame after rotation by Tr are below a preset limit. - View Dependent Claims (11, 12, 13, 14)
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15. An apparatus for sensing orientation and heading with residual errors below a preset limit comprising:
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a first sensor comprising at least two sensing axes; a second sensor comprising at least two sensing axes; and an algorithm for performing a spherical calibration on acquired first and second sensor data to correct sensor measurement errors arising from cross axis, gain, and offset errors wherein first and second sensor data is acquired in at least two axes comprising at least four different tilt angles such that an offset matrix and a transform matrix is calculated such that the residual errors of the corrected sensor measurements are below the preset limits of the true first and second sensor readings being measured. - View Dependent Claims (16, 17, 18)
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19. An apparatus for sensing orientation and heading comprising:
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a first sensor comprising at least two sensing axes; a second sensor comprising at least two sensing axes; and an algorithm for determining misalignment between a device reference frame and a physical reference frame wherein a series of at least three measurements by both the first and second sensors are made interspersed by at least two rotations about at least two different axes of each first and second sensors such that wherein a rotation matrix, TRref is calculated. - View Dependent Claims (20, 21, 22)
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Specification