# How to measure linear acceleration?

1. ## How to measure linear acceleration?

Hi!
We know that accelerometer measure acceleration due to motion and gravity? The question is how we can obtain acceleration measurement due to motion alone?

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2. ## Re: How to measure linear acceleration?

You mount your accelerometer along the axis of motion, or buy a mult-axis accelerometer. If you do the most basic of searches you'll find accelerometers that do what you want. Try searching "accelerometers".

3. ## Re: How to measure linear acceleration?

A quick review of the wiki pages on both motion sensing and accelerometers yields that accelerometers make somewhat poor motion sensing devices, as they measure acceleration only not motion.

e.g. if you have an object that moves at a constant velocity in a direction that is perpendicular to gravity an accelerometer won't sense it's motion as there is no acceleration on the sensor. This is why an accelerometer in free fall measures 0.

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4. ## Re: How to measure linear acceleration?

Originally Posted by barry
You mount your accelerometer along the axis of motion, or buy a mult-axis accelerometer. If you do the most basic of searches you'll find accelerometers that do what you want. Try searching "accelerometers".
Thank you barry for your response. I do have accelerometer (I have 10 DOF IMU from DF robot). But the problem is does it really represents acceleration due to motion? Or acceleration due to motion + gravity acceleration?

5. ## Re: How to measure linear acceleration?

I would suggest you look at the data sheet for both the DF Robot part and the ADXL345 which it's based on. Usually with a 3-axis device like this you orient one axis (say, Z) along the gravity direction. Sometimes the device automatically compensates for gravity so that when it's mounted this way, the x and y axis outputs are zero at rest. If not, then you'll have to calibrate out the gravity term.

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6. ## Re: How to measure linear acceleration?

Originally Posted by barry
I would suggest you look at the data sheet for both the DF Robot part and the ADXL345 which it's based on. Usually with a 3-axis device like this you orient one axis (say, Z) along the gravity direction. Sometimes the device automatically compensates for gravity so that when it's mounted this way, the x and y axis outputs are zero at rest. If not, then you'll have to calibrate out the gravity term.
Thank you barry. I will study on this. For now, this is what i have from the IMU:

X: -0.16 Y: -0.59 Z: 8.67 m/s^2
X: -0.20 Y: -0.55 Z: 8.63 m/s^2
X: -0.12 Y: -0.59 Z: 8.63 m/s^2
X: -0.16 Y: -0.55 Z: 8.63 m/s^2
X: -0.16 Y: -0.55 Z: 8.63 m/s^2
X: -0.16 Y: -0.59 Z: 8.63 m/s^2
X: -0.12 Y: -0.59 Z: 8.63 m/s^2
X: -0.16 Y: -0.59 Z: 8.63 m/s^2
X: -0.08 Y: -0.55 Z: 8.63 m/s^2
X: -0.16 Y: -0.55 Z: 8.59 m/s^2
X: -0.12 Y: -0.59 Z: 8.67 m/s^2
X: -0.16 Y: -0.55 Z: 8.55 m/s^2
X: -0.16 Y: -0.59 Z: 8.59 m/s^2
X: -0.16 Y: -0.59 Z: 8.63 m/s^2
X: -0.12 Y: -0.63 Z: 8.67 m/s^2
X: -0.16 Y: -0.59 Z: 8.59 m/s^2
X: -0.12 Y: -0.63 Z: 8.55 m/s^2
X: -0.20 Y: -0.55 Z: 8.59 m/s^2
X: -0.16 Y: -0.55 Z: 8.55 m/s^2
X: -0.16 Y: -0.55 Z: 8.59 m/s^2
X: -0.12 Y: -0.59 Z: 8.59 m/s^2

But I'm not really sure why the z is not ~9.81 m/s. I will look in this matter also.
Thank you again.

7. ## Re: How to measure linear acceleration?

My first guess would be that you don't have your device mounted exactly perpendicular to the earth. That would explain the values you are seeing.

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8. ## Re: How to measure linear acceleration?

Originally Posted by barry
My first guess would be that you don't have your device mounted exactly perpendicular to the earth. That would explain the values you are seeing.
Yes, you are right..it is due to sensor orientation. Thank you.

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