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.
Sensitive Axis Alignment
Directional Coincidence
Spatial alignment between the physical direction of maximum sensor response and the reference datum of the sensor package determines the measurement accuracy. Ensuring precise sensitive axis alignment is necessary to prevent acceleration in orthogonal directions from bleeding into the primary sensor channel. This alignment is defined by the angular deviation between the sensing element on the silicon die and the housing mounting surface.
The deviation must be measured and compensated for during factory calibration.
Angular Discrepancy
Small angular offsets can lead to cross-talk that degrades the accuracy of inertial navigation systems. When sensitive axis alignment is imperfect, a portion of the acceleration along a perpendicular axis is incorrectly measured by the sensor. This error increases in high-g environments where transverse forces are large.
The misalignment angle is typically expressed in milliradians or arcseconds.
Metrological Correction
Multi-position rotation tests are used to determine the exact misalignment angles so that they can be corrected in software. To measure sensitive axis alignment, the sensor housing is mounted on a precision tilt table and rotated through a sequence of positions. The deviation of the sensor output from the expected cosine curve yields the alignment angles.
These calculated values are used to populate the alignment correction matrix of the device. This calibration is verified by applying a known force along the nominal axes and checking that the cross-axis outputs remain below the target threshold.
Mechanical Referencing
High-precision alignment pins on the sensor housing are used to maintain the alignment during installation on the system board. For a sensor requiring precise sensitive axis alignment, these mechanical references must be manufactured to tight tolerances to prevent mounting skew. This physical alignment ensures that the software correction coefficients remain valid after installation.