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Old 30-01-2015, 15:55   #1
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"Avoid" function with OpenCPN and autopilot?

Is there a way to temporarily change course within OpenCPN while sending steering information to an autopilot? For example, I'm underway and the autopilot is getting info from OpenCPN to maintain track. Suddenly I see a log ahead and want to quickly alter course. A button in OpenCPN to turn left or right would allow me to change course without disabling the autopilot and taking the wheel by hand (what I would normally do) but unfortunately in this particular boat the autopilot controls are not within easy reach of the watch station (admittedly not a good idea but it's what we have). Maybe an idea for a new plugin?
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Old 30-01-2015, 18:12   #2
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Re: "Avoid" function with OpenCPN and autopilot?

It would be useful to integrate with ais also to avoid collisions. It could also avoid obstructions on vector charts. In the case of ais, you might want to give a wide clearance in case the ship turns.

I think it might need wind direction also, so it can choose which way to go around the log or obstruction so that you don't luff the sails. The cases where it can't avoid this would need an alarm, and possibly tacking etc..
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Old 31-01-2015, 05:35   #3
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Re: "Avoid" function with OpenCPN and autopilot?

prof....
On my Autopilot I use its own function for that situations. If I press the +10 or -10 degr. button while navigating towards a track point the AP will first turn 10 degr and then slowly return to track. I suppose that's a "normal" behavior for most APs.
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Old 31-01-2015, 05:49   #4
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Re: "Avoid" function with OpenCPN and autopilot?

On every AP that I have used, the +/- degree buttons simply change the course that amount - they don't change course for a while then return to previous track.

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Old 31-01-2015, 08:35   #5
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Re: "Avoid" function with OpenCPN and autopilot?

How does the autopilot behave when being directed to a waypoint using NMEA instructions from OpenCPN? I haven't tried it yet, but thought if it was getting directions from a chart plotter that the local "avoid" buttons didn't work, or at least using them would disconnect the autopilot - chart plotter connection.

On my boat the controls for the autopilot are aft, next to the wheel and my laptop running OpenCPN are forward at the nav station. Since the nav station is protected by a dodger, that's typically where I sit to keep lookout. If I were to spot a log I'd have to get up, and clamber to the wheel some 10 feet away. I can do it pretty quickly, but wondered if there might be a way to quickly alter course through the OpenCPN interface.
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Old 01-02-2015, 10:27   #6
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Re: "Avoid" function with OpenCPN and autopilot?

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Originally Posted by colemj View Post
On every AP that I have used, the +/- degree buttons simply change the course that amount - they don't change course for a while then return to previous track.

Mark

The autopilot we are using does exactly that. Push the +10(dodge button) and hold it. The course will continue to alter until you release the dodge button and then the boat returns to the original course.

We are using an older model bare bones Wil Hamms "Whale". built in Washington state. Found on many of the commercial fishing boats.

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Old 02-02-2015, 04:36   #7
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Re: "Avoid" function with OpenCPN and autopilot?

Consider a forward looking depth sounder. You may only have seconds to react, so the autopilot would need to know which way to turn because the human is too slow.
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Old 02-02-2015, 06:29   #8
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Re: "Avoid" function with OpenCPN and autopilot?

I don't think OpenCPN is "steering" the boat. OpenCPN is providing something called "cross track error". The autopilot gets the cross track error information and then decides if a course correction needs to be made to reduce the cross track error. The autopilot makes a change to the desired heading and then it steers to that new heading using the electronic compass.

When you want to avoid something just tell the autopilot to change course X degrees. Most pilots have a +/-10 degree button. This will change the course to steer by that many degrees. Once you are safely around the obstacle change the course back by the same -X degrees. The cross track error will automatically guide the autopilot back onto the original track.

I don't think it's reasonable to expect OpenCPN to do this itself since OpenCPN isn't actually steering the course.
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Old 02-02-2015, 07:09   #9
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Re: "Avoid" function with OpenCPN and autopilot?

I wired into the autopilot itself, directly to the same wires that run from the plus or minus buttons. I put a remote control plug up on the bow. Then with the remote plugged in I could con the boat with a clear view of the bottom. This was for exploring the shallows in the Bahamas. So I could steer from up front but I could not control the engine, speed or direction. I hit sand banks a few times. Mac
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Old 02-02-2015, 08:44   #10
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Re: "Avoid" function with OpenCPN and autopilot?

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Originally Posted by transmitterdan View Post
I don't think OpenCPN is "steering" the boat. OpenCPN is providing something called "cross track error"...
Looks like you're right. I looked at the user manuals for several autopilots and they all were similar to this:
Recognized NMEA data
The following table lists the recognized NMEA sentences.

Information NMEA 0183 data
Cross Track Error APB, APA,RMB, XTE, XTR
Bearing to Waypoint APB, BPI, BWR, BWC, BER, BEC, RMB
Distance to Waypoint WDR, WDC, BPI, BWR, BWC, BER, BEC,
RMB
Waypoint Number APA, APB,BPI, BWR, WDR, BWC, WDC,
RMB, BOD, WCV, BER, BEC
Speed Through Water VHW
Apparent Wind Angle and Speed VWR
So no direct wheel controls. Looks like Rock Hard Candy's suggestion (or a remote control) are my options.

Thanks!
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Old 03-02-2015, 18:36   #11
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Re: "Avoid" function with OpenCPN and autopilot?

I only have experience with my own autopilot, an old(ish) Raymarine ST4000+ wheel pilot. Different models may work differently, but there also may be some universal similarities. So I'll offer my own experience here.

I have used OpenCPN to run my autopilot in "Track" mode. (I hope to try "Wind" mode next season when my transducer comes in.) As others have said, O sends various bits of information to the autopilot. It's more than just XTE, though. It's also BTW (bearing to waypoint), DTW (distance to waypoint), and speed (SOW from paddlewheel or SOG from GPS). All these things are used to optimize the AP's response. From what I can tell, BTW is the first thing it uses to set an approximate course, then it monitors change in XTE to tweak the steering over time to compensate for current and other drift factors. Speed and DTW are used to adjust the gain so that the AP does not over- or under- correct. For example, faster speed causes the AP to make smaller adjustments because the boat will respond more sensitively at higher speed. This is probably similar for other autopilots because it is based on generic PID control theory.

When you arrive at a waypoint (check documentation for LOTS of info on this), O sends the next waypoint to the AP. My AP responds to this with a prompt at the control head which you MUST respond to before it will turn. If you don't respond, the boat will maintain its heading forever. I believe that this is an important safety feature that is probably similar in other autopilots. Believe me, you really don't want any machine taking over to such an extent that it could steer you onto a different course without permission. For AP manufacturers, that would seem to be inviting lawsuits, which is why I suspect other APs may be the same. (Others can correct me, though, since I'm speculating here.)

Like others here, I can initiate a manual dodge maneuver while the AP is still in pilot, track, or wind mode. Just press +10 or -10, and it turns. Once the dodge is over, I press the opposite button to turn back. It is possible that if you did not do this, the boat would gradually correct back to head for the original waypoint - I have not tested this, since I consider it more prudent to just "undo" your dodge maneuver.

If you want to control the AP from another part of the boat, try to find a remote control. That's what I bought for my boat, and it is very useful for advancing to the next waypoint or dodging crab pots from the bow. My remote also beeps along with the control head when it's time to go to the next waypoint, so I can respond to it from wherever I am on the boat.

I really think that an automated response to an AIS alarm is a bad idea. Some things are dangerous enough that an alarm with manual operator override is the only prudent response. So many things could go wrong with an automated steering correction that I doubt any chartplotter developer would be willing to accept that liability. Technology is a wonderful thing, but every once in awhile you need to look up from the screen and take control yourself. Even if this were built into O, my AP would still require manual acknowledgement of any change in BTW.
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