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HeshanS

How to understand the accelerometer and gyroscope readings from the PmodNav 10DOF?

Question

It seems like I have a problem with understanding the values I am getting from the accelerometer and gyroscope.

First of all,  it is stated that the IMU is using a left-handed coordinate system where Z is directing upwards.  (as mentioned here :www.st.com/web/en/resource/technical/document/datasheet/DM00103319.pdf?_ga=2.91175770.1596273909.1572880735-388278844.1572880735) page number 10

However, I get +1 in the Z direction when the board is standing still ? I thought I should get -1 as the accleration is in the negative direction ? 

I thought that the sensor may be flipped upside down ( so one axis will be flipped while the other stays the same) but both of them are still flipped which does not make sense ?

The gyroscope are following the (right hand rule ) even for a left coordinate system ?

any help would be appreciated. Thanks !

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Hi @HeshanS,

I'm not a physicist, so I don't know the terminology perfectly, but it is my understanding that an accelerometer measures deviations a static (non-moving) environment. Since z-axis is pointing vertical direction for a positive, because when the accelerometer is not moving it is essentially applying 1g upwards (via the normal force from the surface it is sitting on) counteracting the standard 1g in the downward direction from gravity. If the sensor was in freefall, 0g would be measured since it would be in a true "standard" environment. This is explained a little more in section 2.6.2 with their definitions of zero-g and other situations.

Thanks,
JColvin

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Hello @JColvin

 

Thank you for your answer. Excuse me for not fully catching up with what you said as I am a beginner user. 

I think what you are trying to tell me is that I have 1 g because the accelerometer is sensing the counter-force from the surface it is setting on or ?  However, this is not relative to the sensor itself , because the sensor is applying the gravity force downwards to the Surface or ? 

 

Moreover, the reason why I am asking this is that I am using your PmodNav to calculate the Quaternion ( by the means of Kalman Filter). So I have to change the values of accelerometer and gyroscope to match the body coordinate System that the chip is attached to. It is a right handed system with z pointing downwards, x forward and y to the right  .. so I flipped the sign of the Z value, but it is still not really working for me so well.  ( are there any other changes required ? )

Thanks a lot for your help. 

Hesham

 

Edited by HeshanS

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Hi @HeshanS,

I do not know this for certain but I think the chip was setup so that the zero setting for acceleration was while the module is in free fall. So when the module is motionless it senses the standard 1g acceleration due to gravity.

Again, I'm not an expert in the physics side of things regarding the left handedness vs right handedness, but I suspect it won't be as simple as simply inversing the coordinate values. I don't have much more advice for the math side of things unfortunately.

Thanks,
JColvin

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