GPS / Autopilot Compass Swings Off 10 Degrees

[RickD]

Originally Posted by highseas

Can I shield the battery cable?It passes by fluxgate in bilge about 1ft apart.

AH HA!!!!

I didn't know what "fluxgate" was so I looked it up.

DUH!!! It's an electromagnetic compass! ( I feel stupid for not realizing that)

That's the problem. If the wire is running close to the compass, the wire will have a strong magnetic field (especially when you place a load on it, more current is drawn, thus more magnetic field will be felt by devices nearby) and that will CERTAINLY interfere with other magnetic fields nearby.

Can you shield the wire?

Without seeing the set up, I don't know for sure. Theoretically and logically, yes. Physically, you will have to decide if that can be done.

Most likely putting a piece of conduit (standard electrical conduit is made of steel) over the length of wire near the electronic equipment would be good (it was certainly prevent stray fields from the wire from getting around the compass).

Cool, I learned something new

Hope this helps. Let me know.

[RickD]

Since there are DC power cables involved, I think you might find the heavy wiring used to feel the inverter is the culprit, rather than the inverter itself.

feel=feed

[Ron_Fran]

I have read that you can shield the magnetic field by twisting the positive and negative wires.

[RickD]

Originally Posted by Ron_Fran

I have read that you can shield the magnetic field by twisting the positive and negative wires.

Generally speaking, yes indeed.

Current is flowing in one direction in one wire, and in the other direction in the other wire.

If the cabling used is such that twisted wires can be used, then yes this will work, though 100% cancellation of the fields doesn't always occur.

Since I don't know what his wiring scheme looks like, I don't know if it this will work in his case.

But you are correct, twisting the wires should do good toward elimination of the stray field from the wire.

[Adventures!]

Originally Posted by highseas

Inverter is Trace Dr 1500,with built in charger.Have recently discovered it only happens if there is a large AC load on inverter.Only Autopilot is affected.Cant move inverter,Fluxgate is in good location as when I swung compass,deviation for align heading came within 3 degrees.Took long time to get it that close,and potential locations are limited.May have to get used to compensating with dev. card.Power supply may be problem,will try switch to seperate 12v source.Wasnt aware of the rough and ripply effect.Thanks

Ok we are getting closer to the problem - the Trace Dr 1500 is the smallest in their range and is only a modified sine wave. What is your large AC load? is it driving the inverter near to maximum? is the input DC struggling? As I said in my previous comment if the inverter is trying to give its max then the ripple will probably be seen on the DC input.

Solutions:
Disconnect the fluxgate from the DC system. Get a good 12V battery (12.8V minimum) and connect to the Fluxgate DC input. Operate the "large AC load" through the inverter. What is the result?

A. No deviation = inverter causing ripple on DC circuits.
B. Still have deviation = magnetic interference.

Here are a choice of Solutions for each.
A.
1- Supply fluxgate by separate battery when you need the "large AC load".
2- Get rid of the "large AC load".
3- Get a new bigger "Sine wave inverter" with a garantee not to cause interference.
4- Use your hand bearing compass when you need to use the "large AC load".

B.
1. Get rid of the "large AC load".
2- Get a new bigger "Sine wave inverter" with a garantee not to cause interference.
3- Use your hand bearing compass when you need to use the "large AC load".
4- Move the items relative to each other till you find they are compatable.

I have known baking foil to work as a shield, ferrite rings on cables, metal conduit, co-axial cables instead of twin, twisting and tying cable in knots in fact you are only limited by imagination. However shielding is only a sticking plaster remedy and is not curing the problem.

One last real gem, make sure your DC and AC do not share a common earth/neutral at any point as electrons will alway look for a short cut back to where they came from!

Good luck
Martin

[Adventures!]

Whats the large AC load?

Please dont say its a HiFi, TV, electric motor device, or similar AC item of equipment close against the Auto Pilot / fluxgate!

Martin

[hellosailor]

"I have read that you can shield the magnetic field by twisting the positive and negative wires." Often done with small wires, not done with battery CABLES.

Simplest most reliable fix would be to buy some heavier cable and route it six feet further away from the fluxgate sensor, or move the fluxgate sensor.

You *might* be able to wrap the cable in layers of mu-metal (which absorbs magnetic fields, but you won't find that at WalMart or Home Depot) but I'd expect conventional conduit would only reduce the problem--not eliminate it. And then the conduit itself might be slightloy magnetized from the manufacturing process, or remagnetized in time as the battery cable's field kept hitting it.

Easier to just MOVE something.

[Wotname]

Shielding cables is a complex subject and shielded cable is used widely in aircraft wiring for both preventing interference INTO the cable and interference FROM the cable.

I have used twisted shielded cable to power HF radios that are carrying currents in excess of 20 amps.

Unlike low level signal wiring, it is best to ground the shield of the cable at both ends to reduce EMI FROM the cable.

You can make your own shielding by running the wiring (preferably twisted) inside that flat braided battery cable.

Hellosailor's advice is best, move the wiring - the effects of EMI reduce at the square of the distance of the conductors; also his comments about mu-metal is spot on.

[hellosailor]

Wotname-
FWIW "currents in excess of 20 amps." I think you'd agree, that's still LOW POWER compared to a battery primary cable, carrying a kilowatt (80+ amps) to even the smallest of starter motors, and often that much comoing back from an alternator.

Although I confess...I probably should change the HF's supply cord to twisted shielded. I just didn't have any around at the time. (Ever heard that one before?[g] )

[Pelagic]

I have seen electrical techs actually use Tinfoil wrap to minimize the electrical spikes in the cable.

[RickD]

Originally Posted by Pelagic

I have seen electrical techs actually use Tinfoil wrap to minimize the electrical spikes in the cable.

Tinfoil can be used to shield electrical spikes, but it won't shield magnetic fields.

In ANY wire with direct current flow you will have a magnetic field.

The heavier the current flow, the bigger the field.

That field will ALWAYS interfere with a compass of any sort if the field is close enough.

Moving the wires a good distance away is probably the BEST option.

[hellosailor]

Tinfoil, more correctly called aluminum foil these days, is a very handy way to create an RFI shield around a cable or wire bundle. And "fully" or "double" shielded data cables often have a layer of it wrapping them under the insulation.

But like RickD says, it won't do squat for magnetic problems.

And it only keeps out alien mind-readers for 24 hours, it breaks down and has to be replaced daily for that purpose. [vbg]

I wonder which is more expensive...real tin foil, or mu-metal??

[RickD]

LOL! I'm not sure it works for 24 hours, according to some.

[bill good]

RickD,

If it were my problem I would relocate the inverter to have the shortest DC leads to it thus reducing any magnetic field caused by the DC current.The AC output is not going to effect the compass. The fact you have DC Hi current 1' from the compass would appear to be the problem.

Regards Bill Goodward

[Wotname]

Originally Posted by hellosailor

Wotname-
FWIW "currents in excess of 20 amps." I think you'd agree, that's still LOW POWER compared to a battery primary cable, carrying a kilowatt (80+ amps) to even the smallest of starter motors, and often that much comoing back from an alternator.
......

Yup, I do agree.

Highseas,

Please et us know what you ended up doing to fix the problem.