Thoughts on Calibrating the Compass

A project log for bbot

A 2-wheel balancing robot

Josh ColeJosh Cole 04/10/2021 at 16:490 Comments

I've been giving a lot of thought about how to calibrate the magnetic compass (HMC5883L). There are a few interesting qualities that I've discovered. First - the raw readings, even after self-test, are nonsensical. Technically I'm able to get linearly-scaled values. But they are in unexpected ranges. More importantly however, is that the motors themselves cause significant interference with the operational quality of the HMC5883L readings.

All that to say, I'm thinking of ways I can calibrate this thing. My current plan is very naiive, but I'm going to do some tests and see if it's feasible. At its most basic level, I'd like to discover an offset - and maybe a scale factor - in order to constrain the readings between 0 and 1. For this project, I really only care about the Z axis, as that will represent tilt. Although it would be nice to calibrate all axis so I have more opportunity for advanced functionality in the future.

Calibrating the baseline

So to calibrate, I'm thinking I'll mount the sensor roughly near its final resting place (so it can experience interference from the motors) and then I simply need to make sure bbot stays perfectly level, while I rotate the whole machine 360 degrees collecting data from the sensors all the while. Taking the maximum z value as an offset seems like a reasonable way to collect the baseline. Since it'll have been perfectly level, I know that any reading I get will just be potential noise.

Is that going to be enough? I'm not actually certain. My suspicion is that I'll find it works for some directions, and fails for others. If the values are non-linear, then my approach will need to become infinitely more interesting. I have thoughts on this, but I'll wait until I can prove whether or not they're relevent.

Calibrating the range

Once the baseline is collected, that means the sensor should read some static value when it's perfectly level. Next, I need to make sure that tilting the machine produces similarly consistent values based on the degree of rotation. Ultimately I intend to calculate how many steps per degree are required to actuate the motor in order to return to a level state. In this way I won't be over-correcting.

To calibrate the rotational range, I'm thinking I'll take bbot and rotate forward 90 degrees, then backwards 90 degrees. Repeat this a few times and collecting data all the while. Given these readings I can take the minimum and maximum and derive some kind of range which can then be used to constrain the readings between.

Tilt = ((reading - min) / min)

Or something like that. I haven't had enough coffee yet for math.

Final thoughts

So that's the plan. Calibrating the baseline, and then calibrating the tilt. It'll require a lot of tests to prove this will work, but I'm hopeful that I can find some way to calibrate the readings accordingly. I'll write more updates as I work through the process.