AVR Freaks

BigJon,

You do not state what type of magnetometer you are using, or what function it serves. It seems you are using it as a compass, but I'm not sure from your description.

In any event, yes, the DC motor's magnetic field will likely completely overpower the magnetometer. Not to mention the potential for numerous other magnetic fields generated by high currents in your bot's electronics. The Earth's magnetic field is quite weak compared to most man-generated mag fields.

BigJon,

I have experience with traditional magnetometers and metal detectors, but not the IC types. But I can offer the following based on my knowledge of the topic:

1. Yes, distance cannot hurt and will often help. The real question is whether you can get enough distance between the sensor and the interference sources in your application.

2. Magnetic shielding is a very difficult art form to master. Often forgotten is the fundamental empirical fact of physics that there is no known magnetic "insulator" - a magnetic field passes thru every material presently known to man. That's absolutely true for a "DC" (steady) magnetic field, like that of the Earth's terrestrial mag field, but not entirely true for an "AC" (fluctuating) magnetic field as is produced by many electromagnetic devices. AC mag fields can be "insulated" (shielded) to various degrees with various techniques.

3. If the distance remedy does not help you, I would try shielding the sensor itself. Perhaps a combination of shielding and distance may solve your problem.

There are two types of shields - electric and magnetic. The electric type is the easiest to implement. Simply wrap the sensor in some sort of foil (aluminum, copper, etc) and connect this sheild to your DC Common of the bot's power supply. (Copper is better in this regard in that you can solder the connection to the shield for high reliability and resistance to motion issues.)

Magnetic shielding is more difficult because you have to use specific materials and adhere to certain geometric restraints. However, I think it's worth a try in this case. You have to get some "mu metal" foil, then wrap the sensor in it. You might also be able to use some sort of a mu metal preform, like a tube or a channel. You'll have to look on-line for these items. Often, manufacturer's will offer "Engineering Kits" for a modest price. These contain small samples of various foils, tapes and the like. You won't need much, and you will have to experiment to find an optimum arrangement, so such a kit is your best bet to get started.

The mu metal shield will block (to some degree) the changing ("AC") magnetic field from the interfering sources, but pass (to a larger degree) the steady ("DC") mag field of the Earth. (Which, by the way, becomes a varying magnetic field, as you move the magnetometer thru it by virtue of your bot's motion.)

The conductive aluminum or copper field will block interfering electric fields (noise) also produced by your circuitry and which may be interfering with the internal circuitry of the magnetometer.

Sometimes the mu-metal shield is conductive enough that it will also serve as an electric shield, but often you will need both types of shields to get an effective solution.

Good Luck!

Big Jon,

Another thing you might try to evaluate the problem...

Completely isolate the magnetometer from the bot electronics and see if it operates any differenty. In other words, separate power supply, separate driving/reading circuitry, etc. Then place the magnetometer at different locations around the bot as it is operating (perhaps in a restraining test stand so the bot remains stationary which its motros are running). See if you get (more) useable readings in this configuration. If you get improvement, you'll know you have interenece issues besides magnetic coupling alone to deal with.

The earth's magnetic field is indeed quite small.

One thing that you might try is a compass - to see if it is in fact magnetic interference. If a compass won't work correctly, your magnetometer won't work correctly. If the compass works OK, you have other issues.

If it is magnetic:
Moving things helps since the fields fall off as the square of distance. Shielding can work, but you have to shield the source. Basically, you are providing an easier path for the magnetic field from the source, which will lessen it's effect at a distance. (That's why they use terms like concentrators -- the magnetic field is concentrated in the metal. ) The shields will do the same thing to the earth's field, but with careful positioning, it probably will be OK.

Best of luck,

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[not necessarily true from what has been suggested to me. Look up the Wikipedia entry for Mu-metal and decide for yourself. I am not the expert.]