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Old 05-03-2013, 08:16   #16
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Re: Shielding autopilot/fluxgate compass

Thanks everyone. Will give it a whirl--no pun intended
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Old 05-03-2013, 08:52   #17
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Re: Shielding autopilot/fluxgate compass

The big deal with DC is when the positive and negative do not travel the same path, then a large magnetic field can be generated. If the positive and negative currents are the same magnitude and next to each other they will cancel each other out (in magnetic field terms). Traditionally to reduce magnetic field size we would use a twisted pair circuit, where the positive and negative wires run together and are twisted to reduce overall magnetic field.

Next best is close, parallel wires, which is what you generally find on most boats for wiring. The Ancor two-conductor cable and similar where both conductors are in a single jacket.

The worst offenders are usually the battery charging cable (alternator is usually grounded to engine, rather than having a separate negative wire following same path to batteries), ignition system (usually just a positive wire runs to the ignition switch, then back down to the engine room, everything on the engine is simply grounded to the block), and alternator field current (again, engine as negative). If you can get your compass as far away as possible from any one-directional DC currents you will be much better off.
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Old 05-03-2013, 09:18   #18
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Re: Shielding autopilot/fluxgate compass

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Originally Posted by jeremiason View Post
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.
That's where I wound up mounting mine and its working fine. Much better than a 30 degree course change when the 1/2 HP DC reefer cranks up.

I used a small handheld compass to find a good location.

John
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Old 05-03-2013, 09:25   #19
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Re: Shielding autopilot/fluxgate compass

You just have to get the fluxgate away from any DC wires that come on and off intermittently. The flugates deviation function cannot compensate for DC generated magnetic fields that occur on a periodic basis.

I had to run my three fluxgates all the way forward in the bilge to get them away from all the DC wires in the engine space.
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Old 05-03-2013, 16:05   #20
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Re: Shielding autopilot/fluxgate compass

A magnetic field crossing a conductor generates a current, but it sounds like trying to shield a wire with the copper won't alleviate the problem, as the magnetic field itself is the issue and not the induced current. Moving the fluxgate further from the induced magnetic fields is the cure, as opposed to trying to shield the conductors.

As I understand the posts, moving the fluxgate to say, the foward cabin, WOULD alleviate the problem. Even though the seatalk cable would run the length of the boat, likely within feet and for quite a length parallel to the offending alternator cable (which yes, is just a positive, grounded through the engine block, does anyone have an alternator with a negative terminal?), this is apparently not an issue.

Did I get this right? I knew there was a reason I didn't become an electrical engineer!
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Old 05-03-2013, 16:16   #21
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Re: Shielding autopilot/fluxgate compass

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Originally Posted by Troubadour52993 View Post
A magnetic field crossing a conductor generates a current, but it sounds like trying to shield a wire with the copper won't alleviate the problem, as the magnetic field itself is the issue and not the induced current. Moving the fluxgate further from the induced magnetic fields is the cure, as opposed to trying to shield the conductors.

As I understand the posts, moving the fluxgate to say, the foward cabin, WOULD alleviate the problem. Even though the seatalk cable would run the length of the boat, likely within feet and for quite a length parallel to the offending alternator cable (which yes, is just a positive, grounded through the engine block, does anyone have an alternator with a negative terminal?), this is apparently not an issue.

Did I get this right? I knew there was a reason I didn't become an electrical engineer!
Pretty much yes. Have to use caution, the bow may move around more than current location and, if the fluxgate is not damped sufficiently may cause problems.

The magnetic field from a one-directional DC cable will cause compass error. You could also get communications (SeaTalk) error from noise in the system, in which case the compass reading might not get where it needs to go. For that problem shielding will help, shield the SeaTalk wires (if they are original equipment they should have come that way) and ground the shield at one end. The longer the communications cable (and the closer to other wires or equipment it runs) the more likely you are to have comms errors.
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Old 06-03-2013, 02:35   #22
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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.
Position in the boat is not so picky. Up forward near the mast will be fine. I think they are concerned about powerboats crashing around. Not cruisers.
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Old 06-03-2013, 05:00   #23
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Re: Shielding autopilot/fluxgate compass

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Position in the boat is not so picky. Up forward near the mast will be fine. I think they are concerned about powerboats crashing around. Not cruisers.
I'm a power boat, though I don't do a lot of crashing around. But that's also what constrains where I can put the sensor. The engine room contain the batteries, lots of large DC power cable runs, generator, and of course the engines. To get far enough forward to clear the magnetic field of the inverter/charger, I'll probably be up around the 1/3 point along the DWL, maybe even a bit forward of it.

It's a tough tradeoff. Compromise AP performance all the time and be immune to the microwave and charger system, or have good AP performance all the time and have to be aware of microwave use and temporarily disengage? The problem is I don't know how much I'd be compromising AP performance with the sensor much further forward than what's recommended, and it's not practical to test it, especially with the boat out of the water as it is now.
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Old 06-03-2013, 05:43   #24
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Re: Shielding autopilot/fluxgate compass

FWIW, twisted wires to components can often resolve magnetic interference, but I don't know how this might be accomplished with current-day Sea Talk, connectors, etc. Might be worth a call to Raymarine tech support; some folks are reporting improved communication from them.
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