More heading data problems

[Dockhead]

I have an older RL80CRC+/RL70RC+ Raymarine system which I am hoping to use for a few more years before I have to cough up to replace the whole system. It generally works ok although the radar performance leaves something to be desired. I am having one problem, however, which I can’t figure out.

The plotter sometimes displays erroneous heading data – sometimes as much as 180 degrees off of my real heading. The boat icon on the plotter sometimes appears at a crazy angle to the real heading and radar overlay is ruined.
This is puzzling because apparently the autopilot (hydraulic, using an ST7000+ control head) gets correct heading information – it steers a steady course. If it were getting the same heading info which is being displayed on the plotter, it would spin the boat around, wouldn’t it?

I replaced the fluxgate compass just in case, and this had no effect.
I have an el cheapo Raymarine fluxgate compass (the little black round thing) and the el cheapo non-gyro course computer. I understand from other posts that this will not give me accurate enough data for decent MARPA and maybe not for a decent radar overlay either even while working perfectly.

I might consider upgrading either the course computer and/or the heading sensor. A good gyro stabilized heading sensor like this one: http://www.boatersland.com/mrtssc200-01.html could be used in a future new system, so I wouldn’t mind that investment. On the other hand, maybe the existing course computer is at fault, so maybe I need to concentrate on that.

Anyone have any other ideas? How could the autopilot steer a straight course while the plotter shows a blatantly wrong heading? The leading marine electronics engineers on the Hamble were puzzled by this and proposed to just start ripping out and replacing things one after the other. I’ve been through this diagnostic technique with my Yanmar diesel so politely declined. Maybe someone here has a better idea?

[goboatingnow]

I have an el cheapo Raymarine fluxgate compass

unfortunately therein lies the problem, this device in my experience can output a lot of nonsense at times, upgrading to a quality rate gyro product will help a lot. ( you could add the Ray gyro, unit, but the maretron complete unit is the same price. ( if you are exclsuively seatalk, then feed the compass into the autopilot, this will send out the heading over seatalk to the RL80/70

The reason the pilot doesnt get affected is that there is a large time constant in autopilot software, as they are connected to slow mechanical systems. hence the auto ignores the misleading data. The display however in an attempt to keep overlays under control tends to try to respond quicker and hence you "see" the problem. Its more obvious on RL series units as the redraw is particulary fast.

dave

[Dockhead]

Thanks! I guess it couldn't hurt to put in something like the Maretron heading sensor. I could use it on the new system when I eventually fit it, so the investment won't be wasted.

But I find it hard to believe that the fluxgate compass, working in its normal albeit crappy manner, is the cause of this. I get the wrong heading displayed on the plotter sometimes for a long time, not just a momentary blip. I changed the fluxgate compass with a new one, so it can't be a fault with the compass. I can't believe that the system would work like this out of the box.

[goboatingnow]

whats a "long time", ive seen it happen, for about 2-3 seconds

Dave

[Dockhead]

It can go on like that for an hour.

I'm afraid that if it's a fault in the course computer, putting in an expensive heading sensor won't solve it. I suspect in fact that a new heading sensor won't solve it, because the fault is obviously not in the fluxgate compass.

[Pblais]

The reason the pilot doesnt get affected is that there is a large time constant in autopilot software, as they are connected to slow mechanical systems. hence the auto ignores the misleading data.

There is even more as well. There is adaptive sea state information. It tries to know what will be needed as far as amount of rudder to apply based on a trend. So yes it does not need to react to every blip. Just as it won't react to every bounce the boat makes. It can expect a small amount of variation as "normal". The course computer actually does some of what the rate gyro does just not as well.

At slow speeds the fluxgate will perform quite well unless the environment changes and you get magnetic interference that didn't used to be there (or always had). The rate gyro improves performance but it is far less of a factor at slow speeds. For really fast boats it can make a world of difference.

Real time multi displays just can't smooth out the bumps well because they would need to first integrate the data and sort the anomalies then recompute the displays.

The Kalman filtering technology required to do that just isn't built into a unified display system. You are pretty much getting the data streams as they come at what ever rate they output. Ideally you would need an adaptive display that could change the sample rate only for the important information. Knowing what is important gets to be a bit of magic.

[goboatingnow]

It can go on like that for an hour.

ogh, Thats not a fluxgate issue. ( btw are you happy with the fluxgate mounting).

i wonder is it a wiring problem, ie the data drops out between the course computer and teh RL units ( I presume your using seatalk). Seatalk connectors are in my view, prone to poor connection issues. I suspect the connection between the compass and the course computer is working as a one hour wrong HDG would cause the pilot to do its nut.

Dave

[Dockhead]

Right. And I sometimes get wrong heading data at the dock. So surely it's not the fluxgate.

Could it be the plotter? I am breaking my head over this.

[Pblais]

I suspect in fact that a new heading sensor won't solve it, because the fault is obviously not in the fluxgate compass.

The fluxgate compass can be compromised by interferrence. If you unmount it and have someone watch the display you can manually rotate it in the slip. You want to check in both directions and at the 90 degree borders. The calibration should compensate for small anomalies on the boat as it stores a deviation card internal to the course computer.

The older Raymarine course computer boards were notorious for poor soldering that can eventually go bad and cause data drop outs at the data connection port. Mine has that problem now and I have to replace the course computer. It makes the autopilot take a nap just once in a while.

[goboatingnow]

yeah as I said I suspect its a seatalk connection issue to teh RL from the course computer

Dave

[Dockhead]

That's interesting -- does your bad course computer also give you wacky heading information at your plotter?

Do you happen to know whether the newer course computers are compatible with the older networks?

Maybe I should just put in a new non-gyro course computer and a new good quality heading sensor. I presume that the heading data can be gyro stabilized either in the course computer or at the heading sensor.

[Dockhead]

Originally Posted by goboatingnow

yeah as I said I suspect its a seatalk connection issue to teh RL from the course computer

Dave

Also interesting -- how to check? The cables are short (about a foot) and I have checked the connections -- seem tight and not corroded. Replace the cable for good measure? It's always good to start with the cheap stuff I guess, when doing the "hit or miss, rip them out one by one and replace them" method of fault diagnosis.

[goboatingnow]

Maybe I should just put in a new non-gyro course computer and a new good quality heading sensor. I presume that the heading data can be gyro stabilized either in the course computer or at the heading sensor.

If you stick with Rays course computers the new smart series autopilots are gyro stabilised by the gyro on the course computer board. ( and very good they are too , especially in quartering seas).

The RL series has seatalk so youll be OK. depending on the control head you have that may have to be replaced as well.

Dave

[Dockhead]

Originally Posted by goboatingnow

If you stick with Rays course computers the new smart series autopilots are gyro stabilised by the gyro on the course computer board. ( and very good they are too , especially in quartering seas).

The RL series has seatalk so youll be OK. depending on the control head you have that may have to be replaced as well.

Dave

The X30 "corepack" is about $2,000. I have the ST7000+ control head, which appears not to be suitable. So this will be a pretty expensive upgrade! But will survive the next upgrade, so maybe worth the investment.

[goboatingnow]

Id replace a few of the seatalk cables first before commiting to 3K. !

DAve