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Old 11-03-2014, 08:56   #1
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Autopilot Confusion

Would anybody enlighten me how an electronic autopilot works and precisely where it gets the waypoints from? Is it fed from the navigation software which has to be run all the time during the trip? Or another scenario, is the navigation software loading the waypoints to the GPS which in turn is loading them in real time to the autopilot along with the others NMEA messages? The third option would be - is the autopilot storing the waypoints in itself and receiving only corrections from the GPS while the nav software can be off during the trip?
I need this information to rewire my electronic equipment.
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Old 11-03-2014, 09:15   #2
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Re: Autopilot confusion

Not the answer you were looking for but it would depend on brand and type. More info would be needed. My tiller pilot doesn't accept any input other than a press of a button and sails on that course till I tell it to stop.

Edit; Check out some manufactures sites.
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Old 11-03-2014, 09:37   #3
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Re: Autopilot confusion

There are two Autopilots
- Cetrek 930609
- Nexus NX- series made by ComNav
The manuals for both of them are not very descriptive.
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Old 11-03-2014, 09:42   #4
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Re: Autopilot confusion

I can enter waypoints either manually or via a nav program (I've never tried this one). The weird thing to me on my system is that the chartplotter is at the nav desk but also displays at the helm that is a multi-function unit that is really my radar. Even though the chart chip is in the unit at the nav desk and two communicate the waypoints saved at the helm and not available at the nav desk.
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Old 11-03-2014, 10:07   #5
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Re: Autopilot confusion

Or you can just have the AP steer to its internal compass. That is the way we use our pilot most of the time. Occasionally we steer to a wind course. We almost never have it steer to a waypoint.
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Old 11-03-2014, 10:15   #6
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Re: Autopilot confusion

Quote:
Originally Posted by Paul L View Post
Or you can just have the AP steer to its internal compass. That is the way we use our pilot most of the time. Occasionally we steer to a wind course. We almost never have it steer to a waypoint.
Yes, You're right but to do the permanent wiring I have to assume all of the four modes (NAV, WIND,Manual and Compass ) will be used.
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Old 11-03-2014, 10:32   #7
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Re: Autopilot confusion

Is there any of the OpenCPN guys on line? I'm sure they would know the answer!
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Old 11-03-2014, 13:35   #8
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Re: Autopilot confusion

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Originally Posted by Tristan View Post
Yes, You're right but to do the permanent wiring I have to assume all of the four modes (NAV, WIND,Manual and Compass ) will be used.
If it is NMEA2000 then it is straight forward. Just add the AP to 2K bus. If you are using NMEA0183, then you will want the route source as a talker to the input of the AP. Such as, the chartplotter-->AP or the PC running OpenCPN->AP. The plotter will output the route waypoints and the repeat of the GPS position. I beleive some have dual 0193 inputs, where the GPS could be on one and the CP on the other.
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Old 11-03-2014, 14:00   #9
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Re: Autopilot Confusion

Thanks Paul
Your plan is :

GPS --> OpenCPN --> Autopilot

or

GPS -->
|
v AP
OpenCPN -->

I was hoping for something like:

Planning : OpenCPN --> GPS

Voyage : GPS --> AP
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Old 11-03-2014, 14:10   #10
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Re: Autopilot Confusion

I would ask Otto, our old autopilot but he retired and no longer answers. As a non-electronic guy who has installed and helped to troubleshoot several autopilots, to me it appears that the interface between your navigation systems and your autopilot are critical to a smooth running system. Most modern interfaces have been idiotproofed to just plug in, program and go. On installations, be certain you have mounted your fluxgate compass in an area free from electrical interference like a VHF or UHF radio transmitter. Low and in the center of the vessel is a good spot. More autopilot problems are caused by signal interference than anything else in my experience. Guys like Paul L above, are a great source of knowledge on installation and can, I'm sure, answer technical questions on the inner workings of these amazing 'black boxes'. Had a close friend who purchased a boat nearly 400 miles from where he was going to keep it and he and the former owner had to steer all night and and day to get his new boat home. He asked me to help sort out his autopilot issues which caused the boat to turn in circles to port. no amount of tweeking or adjustment according to the manual would remedy the problem. When I checked the installation, the previous owner had mounted the compass next to the VHF behind the nav station. He had thoughtfully left the connecting cable coiled up and mounted beside the unit. I loosened the coiled cable, duct taped the compass amidships under the floor board and... voila!... the autopilot worked like a dream. Previously, every time the owner had turned on the VHF, there was sufficient interference to drive the compass crazy. He never did figure out the problem and sailed without his autopilot for 3 years! My buddy thought I was an electronic wizard! Actually, I read about the problem in a sailing mag about a week before. A little knowledge can go a long way!! Good luck... Phil
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Old 11-03-2014, 18:52   #11
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Re: Autopilot Confusion

For NAV mode, something guides the AP along a line to a waypoint. That something is typically the chart plotter, but I think other things can do it too including some GPS that are not chart plotters. The "something" is called the Nav device, and all it does is continuously broadcast how far off the boat is to the left or right of the course line to the waypoint. That distance off the line is called the cross track error, or XTE. It's like someone yelling out "too far to the left, too far to the right, you are on course". The AP listens to the XTE reports and steers the boat to bring it back on the line by trying to make the XTE equal to zero.

So, for wiring, you need a way for the Nav device to talk to the AP so it can tell it the XTE. This can be a NMEA 0183 connection from the Nav device to the, or just by placing both on the same NMEA 2000 bus.

Clear as mud?
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Old 11-03-2014, 20:41   #12
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Re: Autopilot Confusion

Autopilots are typical just fed range to waypoint and bearing to waypoint. Using this information. The AP computes how far it is from that line and determined XtE.

It uses XtE to guide itself. Obviously it need a electronic compass feed.

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Old 11-03-2014, 21:12   #13
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Re: Autopilot Confusion

I just use mine to go a certain direction and I will follow the way points. I dont want to be in a position that I am daydreaming and dont notice a waypoint change and my boat suddenly changes course in front of another boat that was not on the chart when I programmed it hours or days ago.
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Old 11-03-2014, 21:36   #14
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Re: Autopilot Confusion

I don't know of any AP that will automatically change course more than 30* without human intervention. Most won't do more than 10*, and they all sound an alarm when doing so.

Simple compass courses can cause more problems because you could be gradually and unknowingly set into a dangerous situation by current or leeway - no daydreaming required.

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Old 11-03-2014, 22:05   #15
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Re: Autopilot Confusion

The APA sentence uses XTE to get on course
The APB sentence uses BTW to get on course, both have advantages and disadvantages
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