When reading the accels on the LSM6DSL, the magnitude looks correct but the sign does not.

[CK]

For example, placing the chip flat on the table, I would expect to read -1G in the Z-axis.

Instead, I am seeing +1G. Flipping the chip over shows a reading of -1G

Have I configured the registers improperly? Or am I misunderstanding the underlying physics?

Thanks in advance!

[Eleon BORLINI]

Hi @Community member​ ,

well, it can be seen as a kind of convention, and it is defined by the spatial configuration in the datasheet, p.18.

 

In practice, it physically refers to the "constraint reaction" of the surface on which the LSM6DSL is placed: for how its internal structure is made, the internal MEMS moving masses are pulled down towards the stator mass, increasing in this way the "plates" (digits) capacitance and, consequently, the voltage, resulting in an (proportional) increase of the physical acceleration value. If you would leave the LSM6DSL device free falling, you'll experience 0g gravity, while if you accelerate it towards the floor by 1g, you'll read -1g. On the other hand, if you accelerate the device along the positive Z axis of the picture, you'll experience an increasing acceleration (e.g. 2g).

Let me know whether it's clearer now.

-Eleon

[Ozone]

I would agree with the OP.

The image posted suggests positive values in upward direction, so I would expect -1G flat on the table.

At least following the convention used in math and physics diagrams.

[Eleon BORLINI]

Hi @Ozone​ ,

you are right, probably... but since the device is not actually accelerating towards the center of the Earth because there is an equal-magnitude force in the opposite verse (the table), that maintain it in a equilibrium condition, the assumption is to measure a positive acceleration normal to the constraint surface.

We probably can think that -simply for convention- the verses of the arrows indicate the directions of "positive acceleration", and in this way the same configuration can be used for the gyro axis rotational directions too.

-Eleon

[Ozone]

In my opinion, a more descriptive designation of the axes could clarify things.

Especially the sign of the output for an acceleration in the associated axis.

[StroebelJC]

If no forces are acting on an object, it is effectively in freefall (weightlessness). The force acting on the sensor is actually upward from the table it sits on, preventing it from falling freely. Thus, the force applied to the sensor is +1g.

Thank Einstein for discovering that everything is relative. 😉