Tilt correction matrix in accelerometer calibration

[j_perezdelarray]

Hello,

The tilt correction matrix is a rotation matrix to align the pitch and roll angles to the perpendicular plane of gravity so it can be considered as a change of basis or alignment matrix if, for example, the sensor frame is not aligned with the body frame.

I haven't been able to find in the sensor fusion documentation "when" and "where" in the filter this rotation matrix is computed. Does anybody have the answer?

thank you,

[shubhadippaul-b]

Hi,

Please refer to Chapter 3 of Application Note AN4246 available @NXP - Calibrating an eCompass.

The NXP SF Algorithm is based on the method described in the document.

[j_perezdelarray]

Hi Paul,

Thanks for your reply. I'm referring to the tilt compensation matrix in the calibration tab. It's not related to the compass

[shubhadippaul-b]

Hi,

Can you please elaborate and send a snapshot of the Calibration Tab you are referring to?
In NXP SF GUI, the calibration Tab only has options to write and erase calibration parameters to and from FLASH.

I hope your have already referred to Chapter 4 of Application Note AN4248 available @ NXP - Implementing a Tilt-Compensated. Though the documents talk about eCompass (Accelerometer + Magmetometer), you may find the equations for Accelerometer useful.

Now if you are simply looking for mounting offset correction - correction of offsets due to Plane of Sensor not being aligned to gravitational plane, please refer to AN4399 available @ NXP - High-Precision Calibration of a Three-Axis Accelerometer. This offset is corrected in the Accelerometer counts output before using it for SF Algorithms.

[j_perezdelarray]

I'm referring to the "0 flat" button in the "Precision Accelerometer" tab. The funtion of this button is described in section 4 of document AN5021 (Accelrometer tilt orientation matrix). When pressing this button, a tilt correction matrix is stored in memory and applied to orientation data. I would like to know when is this tilt correction matrix applied in the filter.

thank you,

[shubhadippaul-b]

Hi,

Precision Accelerometer Calibration compensates for mounting offsets - correction of offsets due to Plane of Sensor not being aligned to gravitational plane, for details you may refer to chapter 7 in AN4399 available @ NXP - High-Precision Calibration of a Three-Axis Accelerometer. 

In the SF Application, the sequence of steps involved to apply Precision Accelerometer Calibration to Accelerometer Measurements is as follows:

• First, using the Precision Accelerometer tab, 12 point accelerometer data measurements can be carried out and used to compute Calibration parameters.
• Then, using options form Menu item Calibration, the computed calibration parameters can be written to the MCU board Flash.
• Subsequently, from the next board reset, the written parameters are read and used by the SF Application.
• The Calibration parameters are applied to every new accelerometer measurements after it is acquired. This is done inside the processAccelData() function in sensor_fusion.c file in the SF code.
• The resulting measurement is then used in all Filter Algorithms by the SF Application.