Guide to gyro and accelerometer with Arduino including Kalman filtering

Don't you mean the z-axis (yaw)? You need it for my code, but yes it it possible to calculate the angle with just one two values I think, haven't really researched on that, because it hasn't been a problem to me.

  • Lauszus

yes sorry i meant z-axis.

ok, i'll try research into that. Or else i might be able to get an arduino mega.

Worst case scenario i can still work with the values i am getting

Okay. Please upload the code for calculating the angle with only two values from the accelerometer when you get it working :slight_smile:

  • Lauszus

I thought a bit about it, and actually you can calculate the axis with just one value.

In the code it says:

accXangle = acos(accXval/R)*RAD_TO_DEG-90;
accYangle = acos(accYval/R)*RAD_TO_DEG-90;

Instead you could write:

accXangle = asin(accXval)*RAD_TO_DEG;
accYangle = asin(accYval)*RAD_TO_DEG;

Then you should get an angle very close to the first one!! It is not as precise as the first one, but that kind of precision is only important if you are building a balancing robot or something like that.
The resolution will also only be 180 degrees.

That should fix your problem :slight_smile:

  • Lauszus

Guys, I really think I should be getting better yaw values from the Gyro. I found this:

http://code.google.com/p/imumargalgorithm30042010sohm/

This shows some pretty decent output which leads me to believe that my unit could be broken as the Yaw value doesn't change at all when I rotate around that axis. I noticed that on the sparkfun page for this item, someone else had a similar problem. I've emailed the chap from the link above in the hopes that I can figure this out. At the moment, the lack of decent/any yaw (no need to use a magnometer just yet) is halting my project :frowning: Im wondering if there is something else im missing here. I've tried the kalman filter approach as well and that seems worse than just using the raw gyro values :S

You can not use the kalman filter for the yaw value, because the accelerometer can not measure the yaw :slight_smile:
Hope you get it working!

  • Lauszus

Yeah, I've done a little more research and I see what you mean. I guess i just need to integrate over time for yaw off the gyro and then reset when the drift gets too much... until maybe I get a magnetometer as well!

Thanks though! Its been a big help!

Hey guys,

Is it possible to also use the kalman filter for only an accelerometer? It will only use one axis (of the three available). I just need a smooth movement of the servo I'm using

You can not use the kalman filter with only one input. Sorry but you need a gyro too, that is why it called "sensor fusion" :slight_smile:
You could perhaps use some other kind of smoothing algorithm.

  • Lauszus

Hey Lauszus, I finished my final year project in college. I thought I would show it on here so people can see what can be done with the code and sensor. Thanks again for you help!

It look and sounds very cool. Thanks for sharing. So which academic degree do have now?

  • Lauszus

multimedia :slight_smile:

Lauszus:
You can not use the kalman filter with only one input. Sorry but you need a gyro too, that is why it called "sensor fusion" :slight_smile:
You could perhaps use some other kind of smoothing algorithm.

  • Lauszus

Any recommendations? I found these...

http://www.arduino.cc/playground/Main/Smooth

Don't know how smooth they work, didn't get time to test them yet

I haven't got any experience myself, but it look likes what you need :slight_smile:
Or you could just write something like this:

int difference = reading - lastreading;
abs(difference);
if(difference > 10) reading = lastreading;

It will ignore the data if you for example shake the sensor.

The best solution might be combining those two approaches.

  • Lauszus

Hey Lauszus
thanks for providing this great guide!!

I've read your comments to onidaito and I'm not exactly sure of what you mean when you are saying that

the accelerometer can not measure the yaw

I'm trying to understand if for my project I need 6 or 9 degrees of freedom
So let's be straight and have a visual example:

can the sensor you are using (and the code) measure all these 6 variables?
thanks in advance
b.

The accelerometer can measure the roll and the pitch, but not the yaw. The gyro can measure both the roll, pitch, and the yaw, but all these will drift. In total that gives only the roll and the pitch. You need a magnetometer to measure the yaw too.

  • Lauszus

Ok thanks for the clarification...
one last thing: in my project there is actually one movement which I won't need to do: which is the "pitch" on the image I've posted earlier.

I'm guessing that all I need to do is position the sensor perpendicular to ground
hence transforming the yaw (which I wouldn't need) into the pitch
and your code would still work...is that right?
would it work if I shift the frame of reference in such a way or the code measures pitch and roll in reference to the force of gravity?

thanks
b.

No that is not correct. The thing is that the accelerometers can not "see" when rotate the IMU along yaw. So it will not make a difference if you just rotate the sensor. Remember that it measures acceleration, which I use to measure Earth gravitational force to calculate the angle. You can also use a accelerometer in for example a rocket to measure the acceleration of the rocket.

I am not sure that I understand your second question? :slight_smile:

  • Lauszus

ok let's check with an image
I really don't understand why one parameter can't be measured if it's said to be a 6DOF and not a 5DOF!?!?!
moreover gyroscopes measure angular velocity around themselves...not influenced by the force of gravity :~

So here goes the image

I made it very simple :slight_smile:
can you please answer the questions at the bottom simply with a Y or N?
in your setup (I'm deducing) can the following be measured???
1.1 Yaw = N?!?! right?
1.2 Roll = Y
1.3 Pitch = Y
1.4 - 1.5 - 1.6 = x y z = Y Y Y

In my setup - can the following being measured??
2.1 - old Yaw - new Pitch = Y? N?
2.2 - Roll = Y
2.3 - old Pitch - new Yaw = Y? N?
2.4 - 2.5 - 2.6 - x y z = Y Y Y

What I really need for my project is (considering the my sensor positioning pictured right)
to be able to measure
x, y, z, ROLL, and PITCH (old yaw, blue line)

Will it work with Sparkfun's 6DOF Ultra-Thin IMU
or will I need one of the 9DOF? ( 9 Degrees of Freedom - Sensor Stick (Old-School) - SEN-10321 - SparkFun Electronics & 9 Degrees of Freedom - Razor IMU - AHRS compatible - SEN-10125 - SparkFun Electronics)

thanks for the patience :slight_smile:

It is right gyros are not affected by the gravity only accelerometers!

Okay you have to understand that just because you rotate the sensor does not mean that you can measure yaw. So your answer is:
1.1: N
2.1: Y
2.3: N

No matter what you can NOT measure yaw with a accelerometer, unfortunately :frowning: If you only need a estimated angle you could just use the gyro, and then keep calibrating the zeroValue.
If you need to know the exact angle of the yaw, you have to use a 9DOF!!
Hopes that was what you needed to know or just post another comment, and I will gladly help :slight_smile:
Nice hand drawing btw :smiley:

  • Lauszus