Converting MPU raw data for a single axis

[dpatel20]

Hi -

 

I am looking to use the MPU6050 on a self balancing bicycle. I am using a PIC micro so can't take the code directly from i2Cdevlib.

 

I am only interested in the angle in one axis (I'm only interested in roll). I have some questions I was hoping someone could help with:

 

1. If I'm interested in only one axis and only small angles (+/- 10deg), is it OK to just grab the accelerometer and gyro data for that one axis; or should I still get all 3 and combine for some reason?

 

I have seen posts for other IMUs suggesting that you should combine readings to get the required angle. For example, assume I am interested in only the X angle, then it suggests something like this:

//Convert Accelerometer values to degrees
    AccXangle = (float) (atan2(*(acc_raw_y),*(acc_raw_z))+M_PI)*RAD_TO_DEG;

But what is incorrect with doing:

AccXangle = (float) ((acc_raw_x / 9.81) * RAD_TO_DEG);  // For small angles where sin(theta) is approx equal to theta

Since I am not interested in full 360deg locus but just small angles, will the second method not work?

 

I guess I can use the first method but I was curious; and want to know if I really need the overhead of the more complex atan2 function.

 

2. What is the method to 'zero' the gyro? Since the gyro measures angular velocity, the ang_vel*dt will give change in angle, hence needs summing from a known point. So, do I need to put my bike into a 'zero' position and then start the gyro measurements or is there a cleverer way to do it?

 

If I'm using a complimentary filter in the form:

X_angle = 0.98 * (angle + gyroX_vel * dt) + 0.02 * accXAngle

then if I set X_angle = accXangle for the first loop iteration will that be OK?