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Old 04-03-2013, 05:45   #1
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Shielding autopilot/fluxgate compass

My fluxgate compass reads differently when the engine is running, apparently due to the magnetic field from current flowing from the alternator, as I notice a big difference when the Admiral runs the microwave off the inverter or if it is just battery charging. Makes motorsailing challenging when the autopilot changes course based on whether the microwave is on or not!

The compass itself is a meter from any other electrical, but I know the Seatalk wires are not shielded, nor is the primary wire from the alternator, and the wires are fairly close as they head back to the cockpit/battery bank in the lazarette. I was wondering if anyone has used an external shield (like this 1MX18MM 0 8" Pure Copper Braided Cable Wire Tube Circle Flat Cover Sleeve 112a | eBay ) to run over their wires and if it has helped.
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Old 04-03-2013, 06:05   #2
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Re: Shielding autopilot/fluxgate compass

It will be interesting to hear your answers.

Here is my take....the compass is being effected by the DC draw from the battery. Fields are either electric or magnetic, a compass is effected by the magnetic fields. Copper will do little good.

To be effective you would need to run the wire in a shield that has magnetic properties, mild steel, think electrical conduit. Probably not a good answer for you.

First I would observe the compass while cranking, if it is battery draw, this should be obvious at that time.

Can you mount the inverter close to the battery and run AC lines to the appliance? That might be easier. AC should not effect the compass. Maybe do a temp lash up with an extension cord to test?

Good luck.
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Old 04-03-2013, 06:32   #3
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Re: Shielding autopilot/fluxgate compass

I'm having the same problem with respect to running my inverter. I haven't gotten into solving the problem, but I'm quite certain that shielding the compass wires won't make any difference. That would only help if the interference from the inverter (and alternator in your case) was being picked up by the wires and corrupting the communications to cause your course variation. Because the communications is digital, there is no way that the interference would cause the NMEA sentences to vary the heading by a few degrees while delivering an otherwise intact sentence. You would be getting jibberish instead.

I'm quite confident that the variation you are seeing is due to the magnetic fields created by the alternator, inverter, and the DC wires that serve them, but that it's being picked up by the fluxgate, not its wires. The compass is a very sensitive magnetic field detector, and it doesn't take much to knock it off. The installation manuals for the fluxgate shows how far away you need to be from magnetic sources, but those are just guidelines. In practice, some fields are bigger than others and will require more separation. Also, it's not always obvious where all the sources of interference lie in the boat. DC cable runs, in particular, can be easily overlooked. The other challenge is that it's often difficult to impossible to create the desired separation given the construction of the boat. I'll bet few boats actually achieve the component separation called for by the component manufacturers.

Right now my fluxgate is about 3' away from a bunch of major DC cables including those that serve my inverter. And the inverter is probably about 6' away. I'm planning to experiment with moving the fluxgate around to different locations further away from the cables and inverter. Hopefully that will work.
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Old 04-03-2013, 14:59   #4
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Re: Shielding autopilot/fluxgate compass

Your post motivated me to go try to fix this on my boat today. I tried a number of different positions for the fluxgate, adn although I found one that was slightly better than the original location, it was only slightly better. Other locations were way worse. I was somewhat restricted because I only has about 2' of cord slack, but regardless, I decide to put it back in it's original location. The only movement that improves things was going further forward in the boat. I could go a bunch forward and probably fix the interference, but then the fluxgate would be up in the bow of the boat. It's preferred location is on the center line down at the center of pitch and roll. Moving forward would compromise that more than I think its worth.
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Old 04-03-2013, 15:19   #5
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Re: Shielding autopilot/fluxgate compass

I have the same issue with a Cetrek fluxgate compass. I find that mine is not sensitive at lower inverter draws but moves the compass roughly 20 degrees at higher draws. I can draw about 1200 watts through the inverter without any noticeable effect on the compass. I suppose moving the compass might help but restricting the load on the inverter is my lazy man's solution.
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Old 04-03-2013, 15:49   #6
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Re: Shielding autopilot/fluxgate compass

hpeer, some guy named Maxwell figured out that the magnetic fields which are affecting my fluxgate are due to electrical currents, hence the issue and why copper shielding works. If Mumetal made an external cable jacket that would be optimum, but their foil appears to be cost prohibitive.

The inverter is mounted a good distance away from the compass, as are the batteries; however, the cable from the alternator is much closer and why I think it is suspect.
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Old 04-03-2013, 16:01   #7
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Re: Shielding autopilot/fluxgate compass

Will copper shielding block the magnetic field? I though not, but it's been so long since I've studied this stuff that I'm not sure.
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Old 04-03-2013, 17:33   #8
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Re: Shielding autopilot/fluxgate compass

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Will copper shielding block the magnetic field? I though not, but it's been so long since I've studied this stuff that I'm not sure.
No it won't. Maxwell figured that out too.

Electric currents produce an E (electrostatic) and an H (electromagnetic) field. It is the H field that, as the name suggests, produces the magnetism to swing the compass. While anything can happen this is the most likely cause.

Bonding and grounding can work in many different ways so I don't want to get all fancy here, besides it has been too long for me too and it makes me heads hurt.

But basically you set up a DC current and that creates a magnetic field. The magnetic material, conduit, kinda concentrates the lines inside the metal because of the way the atoms align. Copper won't do that. Nor aluminum. Nor stainless.

If you make the DC cables short, and move them away from the compass that should help.

It is most likely the DC current feeding the inverter that is the culprit. A 1200 watt mwave is drawing 100 amps. That's a lotta current.

P=E x I

1200 = 12 x 100
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Old 04-03-2013, 18:17   #9
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Re: Shielding autopilot/fluxgate compass

The shield will help only by holding the wires close to each other. The Positive and negative wires will cancel each other's magnetic field if close to each other.

One big source could occur if the alternator is grounded thru the engine block and the battery current from the alternator goes some other direction - forming a big loop. Same with distribution panel mains: make sure they are intimate with each other.

Shielding the SeaTalk leads will not affect the heading.

Move the fluxgate compass far away. In a sailboat forward is best as there is less electrical equipment forward. I've put mine forward of the mast.
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Old 04-03-2013, 18:38   #10
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Re: Shielding autopilot/fluxgate compass

Magnetic fields emanate perpendicular to the current flow (which runs along the length of the wire) so maybe repositioning the problem conductors could help.

In industry, it's common practice to apply shielding to wires in to prevent things like crosstalk and even dangerously high voltages from being induced onto signal wires from adjacent hv conductors. The general method of shielding is to ground one end of the shielding only. Grounding both ends could actually result in the opposite effect, apparently- i.e. the shielding itself can re-radiate the unwanted signal.

Since electricity is generated as a result of a moving magnetic field passing through a conductor, I would think that shielding stops crosstalk by reducing the number of lines of magnetic flux from cutting across an adjacent conductor. I can't remember the exact physics of this however, but it may be worth a try to see if basic shielding makes a difference. It probably makes sense to use a ferrous material, but a lot of shielded wire uses aluminium foil so perhaps a simple shielding experiment using aluminium foil could be tried?
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Old 04-03-2013, 19:06   #11
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Re: Shielding autopilot/fluxgate compass

Quote:
Originally Posted by Reefmagnet View Post
Magnetic fields emanate perpendicular to the current flow (which runs along the length of the wire) so maybe repositioning the problem conductors could help.

In industry, it's common practice to apply shielding to wires in to prevent things like crosstalk and even dangerously high voltages from being induced onto signal wires from adjacent hv conductors. The general method of shielding is to ground one end of the shielding only. Grounding both ends could actually result in the opposite effect, apparently- i.e. the shielding itself can re-radiate the unwanted signal.

Since electricity is generated as a result of a moving magnetic field passing through a conductor, I would think that shielding stops crosstalk by reducing the number of lines of magnetic flux from cutting across an adjacent conductor. I can't remember the exact physics of this however, but it may be worth a try to see if basic shielding makes a difference. It probably makes sense to use a ferrous material, but a lot of shielded wire uses aluminium foil so perhaps a simple shielding experiment using aluminium foil could be tried?
Reef, you are on the right track.

Ferrous shielding reduces the number of lines, others do not.

Shielding with other than ferrous material, is very likely a waste of effort.

A copper or aluminum shield reduces the E field by providing a drain path (hence called a drain wire) to ground. This reduces the static charge, which can be expressed through a logintudinal imbalance as noise. This is why it is better to use balanced circuits, coupled through a transformer. Single ended circuits are a plague.

Gounding a shield at both ends allows a current to flow between two different potentials, thus creating a current, thus inducing noise. That can be real fun when working around ubways with 600+ VDC traction power and poor ground systems.

But that all has NOTHING to do with this issue.

Believe me, this is a very poorly understood field and I have seen big professional companies waste many thousands of dollars on just these types of issues.

Take some jumper cables from you battery to your inverter, turn on the mwave. Try this in a few places to see how the placement of the jumpers effects your compass. Just not too close, or you might create enough field to permanently frig your compass.

Good luck. I'll be interested in how you make out.


BTW.... ever worry about those big power transmission lines radiating power? They are not much of an issue as the phases are pretty much balanced and cancel out.

Know what is most likely the biggest source of radiated power you will ever experience? A heated blanket! Single phase, real close to the body.
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Old 04-03-2013, 20:04   #12
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Re: Shielding autopilot/fluxgate compass

Identify the positive and negative wires that are carrying current when you notice the deviation of the fluxgate compass. Try to put these wires right next to each other as close together as possible. It is better to confine the magnetic field using this technique than trying to shield. Magnetic fields are really hard to shield.

Next time you install anything that uses or generates amps of DC current put the positive and negative wires as close together as possible. Don't use any old ground point you can find. Run the return wire all the way back and ground it as close to the positive source as possible keeping both wires close together. Size the wires for the full out and back wire length.

Distance helps. Keep DC wires as far from the fluxgate compass as possible.
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Old 04-03-2013, 21:11   #13
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Re: Shielding autopilot/fluxgate compass

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I have the same issue with a Cetrek fluxgate compass. I find that mine is not sensitive at lower inverter draws but moves the compass roughly 20 degrees at higher draws. I can draw about 1200 watts through the inverter without any noticeable effect on the compass. I suppose moving the compass might help but restricting the load on the inverter is my lazy man's solution.
I have a Cetrek and my compass has issues too. What can I do to fix the problem? My electronics guy is trying to sell me simrad unit but I think it is just the compass and would love to get a few more years out of the current system.
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Old 05-03-2013, 05:52   #14
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Re: Shielding autopilot/fluxgate compass

From this post it's clear that the fluxgates are susceptible to the magnetic fields generated by high current DC. No shocker there, and the solution is to separate the two.

That brings us to the next problem. What are the tradeoffs of having the fluxgate in different locations in the boat? The installation guide calls for the center line as close to the roll and pitch center of the boat as possible. Moving the compass all the way to the bow may solve the magnetic interference problem, but the compass location is now suboptimal.

So which is worse? And in particular, how much difference does it make if you place the compass up in the bow, or overhead in the salon, or back in the laz?

More often than not, you are not solving problems on a boat, but rather trading them off. It certainly appears to be the case in this instance.
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Old 05-03-2013, 06:48   #15
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Re: Shielding autopilot/fluxgate compass

I had the same problem on my last boat when we installed the new electronics... It only happened when the house bank was drained down, like after an over night anchoring... This drove me crazy for a couple of weeks, since when the batteries were not drawn down everything worked great.

I searched around and found that the battery wires running from the engine compartment to the housebank where within about 2 feet of the fluxgate compass. The lightbulb then went off as I realized that when the battery bank was discharged the high output alternator would be pumping 85 amps through the battery cables, creating a magnetic field and of course stepping down as the battery bank began to recharge.

We moved the Fluxgate Compass to the base of the mast and had no further problems.

On our current boat the fluxgate is mounted in the aft cabin, on the port side, about 1 meter off of center line and works fine.

Just remember when you move the compass to re-calibrate your compass. Most systems require you to go out and spin circles clockwise and counter clockwise...
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