SparkFun Forums

[NleahciM]

Hi - I'm looking at using a 3-axis compass in a project I'm working on. I'm currently considering the MicroMag 3-Axis Magnetometer that Sparkfun sells as well as the AKM AK8970N (http://www.akm.com/datasheets/ak8970N.pdf)

My question is this: how sensitive would modules like this be to small electric motors that would be within about 5-20cm of the module? Thanks.

[m]

Funnily enough, I'd done exactly this. I've used a 3-axis magnetometer in a model aircraft. The magnetometer was about 15 cm from the motor.

The motor is a brushless, drawing about 300W at peak (35 amps at ~ 9V).

Unfortunately, it absolutely stuffs the magnetometer while it's running. The magnet fields from the motor about 20 - 50 times higher than the terrestrial field and utterly drowns the signal.

Worse, the magnet fields from the motor are wildly changing so the magnetometer is totally useless while ithe motor is running.

Sorry for the bad news.

In my app, I only run the motor for 10 - 15 seconds at a time and can reply on the accelerometer for attitude for that short duration.

[NleahciM]

Funnily enough, I'd done exactly this. I've used a 3-axis magnetometer in a model aircraft. The magnetometer was about 15 cm from the motor.

The motor is a brushless, drawing about 300W at peak (35 amps at ~ 9V).

Unfortunately, it absolutely stuffs the magnetometer while it's running. The magnet fields from the motor about 20 - 50 times higher than the terrestrial field and utterly drowns the signal.

Worse, the magnet fields from the motor are wildly changing so the magnetometer is totally useless while ithe motor is running.

Sorry for the bad news.

In my app, I only run the motor for 10 - 15 seconds at a time and can reply on the accelerometer for attitude for that short duration.

I wonder though - my motors consume ~100 ma at 6V when on... quite the fraction of your brushless beast.

[JonChandler]

You might search for "compass compensation." Making compasses work correctly on boats and ships is a big task and I know there are guidelines on metal objects near a compass. The effect of a steel can on an electronic compass will be the same as on a mechanical one.

Jon

[NleahciM]

You might search for "compass compensation." Making compasses work correctly on boats and ships is a big task and I know there are guidelines on metal objects near a compass. The effect of a steel can on an electronic compass will be the same as on a mechanical one.

Jon

I didn't find too much on this.

But I got to thinking - over the summer I worked on a UAV that had an electric compass module about 30cm or so away from the motor which consumed something like 3-5A at 28V (could be off on the current). It seemed to still be fairly accurate...

The only way to really know is to try it out I guess.

Oh one other thing - would a compass module be bothered by a switching DC/DC step-down converter?

[geekything]

You're going about it the wrong way: look at home much EMI the compass can cope with and then work back from that and compensate with EMI combatting techniques.

-marc