You can use the orientation data from the ahrsfilter to compute the gravity vector and derive a linear acceleration, however the filter models linear acceleration as zero-mean Gaussian noise. This is a common assumption when a GPS or similar is not included in the sensor setup. You should plan to operate the filter under those conditions.
However, in general, double integrating the accelerometer measurement (or the linear acceleration you intend to compute) is not a good way to compute position without the aid of some other sensor, like a GPS. MEMs accelerometers have some bias in their measurements. If you were to simply double integrate the measurement, then you would also be double integrating the accelerometer’s constant bias. The position you calculate would have an error that increases quadratically.
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