Automatic calibration of magnetic field and acceleration sensors using homogeneous coordinates
Date
2013
Authors
Chahl, J.
Mizutani, A.
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Conference paper
Citation
AIAC15 : 15th Australian International Aerospace Congress, 2013, pp.212-220
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15th Australian International Aerospace Congress (AIAC15) (25 Feb 2013 - 28 Feb 2013 : Melbourne, Australia)
Abstract
Handheld devices and sensor networks containing motion and tilt sensors are becoming ubiquitous. Increasingly, cost and accuracy will be traded to achieve commercial requirements. The price of solid state sensors decreases with time, however, any process that might involve human intervention is likely to increase in cost. We have developed techniques for calibration of motion sensors that consider this problem. Although we originally developed the technique for a small UAV autopilot, the approach is valid for many embedded sensor applications including mobile telephones.
The technique performs a nonlinear optimisation through all variables associated with zeroth and first order affects on accuracy of magnetometers and accelerometers. The optimisation is constrained by the known properties of the Earth’s magnetic and gravitational fields. The input data is generated by randomly tumbling the embedded device. The variables found include offset and bias for gyroscopes, accelerometers and magnetometers.
Critically the method includes a solution for static hard and soft iron affects on magnetic field sensors. As sensors become smaller, the challenges presented by magnetic field distortions become overwhelming, particularly in locations where the magnetic field has a high angle of inclination. A unique process for aligning magnetic and acceleration sensor axis triplets is presented that exploits the variation in field directions measured by the sensors. The methods assume that no reference of rate, or position exists other than magnetic North and gravity.
Homogeneous coordinates are introduced as an elegant mathematical solution to unify the effects of all affine transforms caused by sensor distortions into a singlematrix. This application of homogeneous coordinates makes the technique uniquely suitable for applications for mobile telephones that contain embedded coprocessors for accelerating three dimensional graphics.
A miniature autopilot is presented as an example application of automatic of calibration. The deployment concepts and cost sensitivity of potential users is considered. Examples of calibration are presented including data from the calibrated and uncalibrated instrument suite in flight.
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Copyright 2013 Australian International Aerospace Congress