Static Accelerometer Orientation Tumble Calibration Procedures
Static tumble calibration calculates accelerometer bias, scale factor, and cross-axis matrices by optimizing spatial vector residuals against local gravity.
Orthogonal Misalignment
Axis Non-Orthogonality
Geometric deviations from perfect perpendicularity among the axes of a multi-axis sensor array introduce cross-channel measurement errors. Measuring and compensating for orthogonal misalignment is necessary to ensure that acceleration along one axis does not register on the other two axes. This deviation arises from manufacturing and assembly limitations of the sensing element on the silicon die.
The errors are modeled as a non-orthogonal matrix in the calibration algorithm.
Cross-Axis Coupling
When a sensor experiences acceleration along a single axis, any misalignment causes a portion of the signal to project onto the adjacent axes. In a sensor with orthogonal misalignment, this coupling leads to false readings that can distort navigation solutions in inertial measurement units. The crosstalk remains constant over the operating life of the sensor but can vary with extreme temperature changes.
This coupling is characterized during multi-axis calibration runs.
Calibration Rectification
Mathematical correction of the raw output data is performed by multiplying the sensor output vector by the inverse of the alignment matrix. To determine the coefficients of orthogonal misalignment, the sensor is rotated through several known orientations relative to the gravity vector. The calibration algorithm calculates the misalignment angles by fitting the sensor response to the expected gravitational output.
This correction is done in real time by the onboard microcontroller of the inertial sensor. This process ensures that the final output axes are mathematically orthogonal to each other.
Structural Verification
Verification of the correction is completed by applying a test acceleration along one primary axis of a calibrated table. If the orthogonal misalignment is successfully compensated, the outputs of the perpendicular axes remain within the noise floor of the sensor. This verification is performed at the end of the manufacturing process to guarantee the sensor meets its cross-axis sensitivity specifications.