Autopilot Nav mode with current

colemj

Furuno Fan
We have always had a strange behavior when following a route on our NN3D on autopilot (Simrad). If there is cross current setting the boat so that the COG is different than the heading, every couple of minutes the AP swings off course so that the magnetic heading matches the route heading, then slowly comes back on course so that the COG matches the route heading.

This happens every time the XTE reaches 0. For example, the boat is heading East with a 3kt current flowing North. To maintain a route, the magnetic heading is 20* South of the route bearing and COG. XTE will be 0 with the COG line matching the route line, the AP will then swing off course 20* so that the Heading line matches the route line, the XTE will get much larger (say, 500'), the AP will slowly correct back to course until the XTE is 0 and the whole thing repeats. This causes an "S" shaped track offset from the route line.

This does not happen when the autopilot is receiving its navigation input from a computer charting program - only the NN3D.

Do I have a setting switched on that I can change to prevent this? Does anyone know what is causing this? It could be something to do with the AP, but it only seems to happen with the NN3D as a data source.

Mark
 
How is the pilot interfaced? (0183 or 2000)
What are the sentences NMEA turned on at the NN3D plotter?
What are the sentences you have turned on from your PC plotter?
Are you running the newest software in both the Furuno (V2.13) and the Simrad?
When holding a straight course without cross currents, the heading and the COG match?
 
Thanks. I did not know there was a software update post 2.11. I just updated from 2.11 to 2.15, although I assume nothing has changed regarding navigation data output?

The pilot interface is 2000.

The Simrad software is up to date.

The only PGN output turned on from the NN3D are 129283 (XTE) and 129284 (Nav data).

The PC plotter outputs 0183, which is converted to 2000 by a converter. It outputs a slew of stuff including HDG, HDM, HDT, RMB, RMC, XTE, WPL, RTE, and a bunch of others.

Without current, the heading and COG are exactly the same.

Mark
 
Your PC putting out data in which it really isn't the source of (like heading) could be causing your problem. It is taking data (heading from the heading compass) and regurgitating it back onto the system. This new second heading will now be delayed in time and would be very harmful. I recommend you look at the data flow and not allow data to go back to it's original source. (this is called a data loop and very bad)
All units should be using the heading fresh from the heading compass without anything locking onto redundant regurgitated data. This applies to any other sentences too like GPS. Proper data flow is very important.
 
OK, but the PC isn't the problem. The autopilot works just fine with it.

The NN3D is the problem with the autopilot. The only output PGNs are those two I listed - XTE and Nav Data.

The PC is never connected when the NN3D is active.

Mark
 
I have seen lots of Simrads working fine with the NN3D since it released in 2008.
I would talk to Simrad to see if they have heard of this problem before. If the Simrad accepts NMEA 0183, you could always try using that instead of 2000 to see if it makes a difference. If the pilot is only NMEA 2000, then you might try 0183 to the 2000 interface. Sometimes the NMEA interfaces will give additional info (like the extra Navigation/WP PGN sentence). Our newer TZT & TZT2 added the additional sentence but the NavNet 3D never had or needed it. Maybe the newer Simrad units/software require it. Really odd problem.
 
Back
Top