Calculating Roll & Pitch Angles with Accelerometer Data

[para]

Hi, I'm new to the forum.

I have a 3 axis accelerometer that I am trying to obtain roll and pitch from. Besides the accelerometer itself, I was provided the raw data values of what each axis will report when it has 0g being applied to it and when it has 1g being applied to it. Using this I am able to find and normalize how many gravity forces are currently being applied to each axis.

In the rest of this post assume x, y, and z are normalized; so if x=1 then 1g is applied to the x-axis in the positive direction.

I'm using the following equations:

roll = atan(\frac{z}{x})
pitch = atan(\frac{z}{y})

If the accelerometer is lying flat, with the z-axis facing up, and I rotate it over the y-axis (roll), then I can use the above equation to obtain the roll from 0 to 360 degrees. Same with pitch.

However I run into two problems:

1. When either roll or pitch reach ~\pm90 degrees then the other angle experiences unusual behavior. For instance, if I roll it over to 90 degrees the pitch may suddenly become 45, 92, 120, etc (it's unstable).
2. It also appears that if I both roll and pitch the device at the same time the values are not correct.

Is there a special way to calculate angles based on this type of data, or am I just going about this wrong? It's been a while since I've done any 3d geometric math and I don't remember a whole lot.

Thanks.

 

[TVP45]

How is the accelerometer constructed?

 

[para]

The chip itself is an http://www.analog.com/en/prod/0%2C2877%2CADXL330%2C00.html" .

I think the problem may be that when I roll the chip 90 degrees, the z-axis becomes perpendicular to gravity. The z-axis is used in the calculation for pitch when roll is near 0.

The solution to this is probably somehow incorporating the x-axis into the calculation in order to compensate for the transition from x to z moving towards gravity during roll, however I'm unsure of how to go about doing that. Does that make any sense?

I'm a little surprised I wasn't able to find more about this on Google seeing as how accelerometers have been around forever.

 

[faisalzia84]

Roll= Asin(X-axis) in radian * 180/Pi in degree
Pitch= Asin (Y/axis) in radian * 180/Pi in degree

Try these equations may be this will work for you.

I am Using LIS302DL, both equation are working fine for me.. in order to find the pitch and roll angle from raw acceleration data