orientation and movement values in LIS2DW12 accelerometer

[Msaga.1]

Hello We are working with accelerometer LIS2DW12.

we did tests on the different movements of the accelerometer and the graphs we see (the graphs were made with the x_physical y_physical z_physical values). However we see graphs that are not as we expected;

when we look at a 'free fall' movement we expect to see all axis intersect in value 0, however it is not the case.

For example:

1. in this image we see free fall when the orientation bit raised on zh (meaning the fall was on zl), but I do not see intersection of all axis ( Z is not even close) and I do not see intersection on 0 - x&y intersect on ~300 . Please explain this.
2. here we do see intersection , however it is around value ~360 and not 0. Please explain why we see it.

More images :

here we see that the intersection is ~200

here we see that the intersection is ~250 (at 31)

here we don't see intersection and the values are not as expected from free fall.

Please help me understand our results.

[Msaga.1]

Hi @Eleon BORLINI​ ,

I understand that, however I am more interested in the graphs (like the one I have attached in the first message). I do not see the point I am expecting to see.

The threshold is only for saying "Hi I recognize that this is free-fall" .

[Eleon BORLINI]

Hi @Msaga.1​ ,

my suggestion is always offset compensation --> threshold setting around 350mg --> detecting free fall.

Can you please share the graph when the sensor is in steady state, z axis pointing upward, so that we can check whether there is residual offset?

I'm however always afraid that with such a low ODR your data are affected by an average on a time window containing spurious effects (especially at the beginning and at the end of the free fall event), and resulting in an increase of the instantaneous vale of the acceleration on almost all the axis.

-Eleon

[Msaga.1]

[Eleon BORLINI]

Hi @Msaga.1​ ,

the stationary data looks good.

Did you have the possibility to test more samples?

-Eleon

[Msaga.1]

Hi @Eleon BORLINI​ 

I am attaching you the graphs i have made

[Mostafa1]

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@Eleon BORLINI wrote:

Hi @Msaga.1​ ,

please note that the recommended threshold for recognizing a free fall event with the internal interrupt is FF_THS = 0b011 corresponding to ~ 10 x 31.25 mg = 312 mg (see the document), and not strictly zero, since spurious acceleration during the free fall might occur (for example rotations that ends up in linear acceleration, or torsional effect).

For this reason, I suggest you to use a similar threshold.

-Eleon

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Hi, any idea where this 31.25mg came from?