Chartplotter/Autopilot Problem

[Bill Balme]

I have a Raymarine C120 chartplotter and an autohelm 6000 autopilot with hydraulic ram and have a strange problem...

I can be using autopilot to follow a set course - say 300deg at let's say 2 knots.

If I increase my speed to perhaps 4 knots, the boat takes a turn to port, and re-establishes a new course on about 280deg and holds constant.

If I increase to say 6 knots, the boat turns again - further to port and holds that new course...

I believe (but can't remember for sure) that if I set the autopilot to follow a waypoint, the boat will follow no matter what - though it might make some turns when the speed is initially changed - and then autocorrect.

Any idea where I should look for the problem?

[belizesailor]

Strange indeed. A couple of SWAGS:

Do you get any dramatic voltage changes when you increase RPMs?

Could some kind of induced current from increased RPM/amps be effecting the fluxgate compass?

What does fluxgate compass read versus accurate magnetic compass during these course changes?

[Bill Balme]

Boat is on the hard right now - so from memory (what's left)...

I don't believe there's a change in voltage - can't think why there would be (this also happens when under sail!)

Anything is possible regarding induced currents and fluxgate compass', but again - when under sail...

Last question has me stumped a bit... I need to review - BUT... If I'm set on a course of 300degress by the chartplotter, then I suppose the fluxgate is maintaining that heading no matter which direction we're actually pointed in - but I'm not completely sure.


I plan to have an expert come aboard once we are launched - hopefully he'll find it quickly!

Raymarine guy suspects the rudder sensor for the autopilot, but I'm not convinced...

[belizesailor]

Voltage change could happen because of an alternator/controller related fault when you increase RPMs, but if this happens under sail only too then that's not it.

The fluxgate compass is simply an electronic compass which the autohelm uses for a heading reference. It only senses the heading and sends it to the AP, it does not maintain it (the AP does that). The fluxgate compass reading will change as boat's heading changes.

Just like a regular magnetic compass, electromagnetic interference can effect the heading sensed by the fluxgate compass. This could be electrically generated or by a chunk of ferrous metal rolling around near your fluxgate compass...don't have anything stored near it do you?

The reason for comparing fluxgate sensed heading versus known accurate magnetic compass is to determine if anything is causing significant deviation in your fluxgate.

The rudder sensor could be another issue, but the AP should still try to adjust back to the set course rather than maintaining a new one. If the rudder sensor were faulty, the AP would still try to maintain course and you would eventually get some other error like "off course" or "drive stop". So, I think your tech is likely heading down the wrong worm hole here.

Parameters like "rudder response" effect behaviour of the the AP relative to speed, but again the AP will still try and maintain proper course...it will just steer more or less actively...not assume a whole new course.

Sounds like something is sending a new/incorrect heading to your AP. Is there a course computer in your configuration?

Make any changes to instrumentation/electronics recently?

[jstevens]

Bill,
I have the same AP and chartplotter setup, just not a hydraulic drive, and have not experienced what your describe. However, I never let an absence of experience prevent me from offering advice.

I assume the two types of course following you describe are (1) AP set on a heading by just hitting the AUTO button on the ST6000 control head; and (2) AP set to a waypoint by activating a route in the C120 and having the C120 control the AP via SeaTalk.

It does sound the like heading used by the AP is changing with boat speed, but why that would happen is a mystery to me. I can see how a rudder position sensor problem might cause the AP to over and under compensate for heading changes, but not set the boat on a new heading.

This problem might happen if deviation on your boat is changing with boat speed, I just can't guess what might be causing that to happen. I don't believe it is a problem with the deviation table calculated by the AP Course Computer in calibration mode. I think this table is used (with variation) to report a true heading on the display, but AP maintains course with the uncompensated heading from the fluxgate.

So I can't offer any solution. My only recommendation is to carefully collect more data on the problem once you are re-launched this spring. Be sure the association with boat speed is real not just a coincidence.

Although I don't think the deviation table in the AP is the problem I would definitely re-calibrate once you have reproduced the problem. Check the location of your fluxgate compass - is it near any thing that moves with boat speed (e.g., free wheeling prop shaft) that could generated some minuscule magnetic field?

If all else fails reset the AP to the factory default (after recording all of the settings) and see if that changes the problem. If it does then one-by-one restore those settings until you find on that is associated with the problem.

After that it is time call in a real expert.

John

[jstevens]

One other quick thought on trouble shooting this problem. See if you can reproduce the problem with the GPS and Chartplotter disconnected or turned off.

John