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Old 10-07-2008, 11:56   #1
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Strange behaviour on Raymarine Track mode with ECS chart plotter


I just connected my ECS (CM93 maps) chart plotter to my Raymarine SmartPilot S2. Even though the autopilot controller shows the correct course (BTW - Bearing to waypoint, collected via NMEA from the chart plotter), it tends to not steer that way. At least not until it gets substantially out of course. Then it tends to overcompensate, and get out of course on the opposite side. The result is a sinus-like trail around the course. It does actually work, but the deviations in course are large. Are there any parameters I can adjust to help the situation? The BTW on the Raymarine does update every second from the chart plotter, so I guess the autopilot must be the problem?

I also see that the DTW - Distance to waypoint is shown in dashes, indicating that it does not receive this information from the chart plotter. I don't know why, but I guess this information is not necessary to obtain the right course?

Any help appreciated! Thanks in advance

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Old 10-07-2008, 12:33   #2
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I have the same problem with a Comnav autopilot that I use. When I tell it to follow a track line, it will head off to the right until my XTE, cross track error, is about a hundred meters or so. It will then work its way slowly towards the track line until the XTE is close to zero, where it is supposed to be. I found an adjustment to stop the sinusoidal effect in my Nobeltec software. I realize this is not what you have but hopefully it will help to point you in the right direction.

Also, the autopilot needs to learn how much of a heading vector it needs to add or subtract to keep the boat over the trackline....this takes a few minutes usually and is a pain in the rear sometimes I know. After my autopilot "settles in" the XTE is usually no greater than 25 meters around 90% of the time. I would imagine yours should do better since it is newer equipment.

Just a little matter of definitions which do matter when trying to describe this stuff. Heading is the direction your bow is pointed. Course Over Ground is the direction your boat is traveling across the ground regardless of your heading. GPS will not give you heading. GPS gives COG.

GPS only gives heading if you have one of those dual antenna GPS heading compasses which are rarer than hens teeth on smaller yachts.



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Old 10-07-2008, 12:39   #3
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Anders, Unfortunately it can be a number of things causing this. The first thing I would check is the gain setting for the autopilot. The next would be to see if the same thing happens with the chart plotter disconnected. Next you will need to check your settings for the NMEA connection between the plotter and the autopilot. And that will be just for starters. The Raymarine autopilot has several connections on the course computer and you need to be sure you are not connected to a Seatalk connection. Also consider that this is a Raymarine autopilot and those course computers fail all of the time. Also be sure you have the NMEA wires connected correctly. There is not a "standard NMEA" connection between all makers of plotters and autopilots. Talk to the manufacturer for correct connections and settings.
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Old 10-07-2008, 12:56   #4
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Thank you for your tips guys.

It seems to me that the problem is on the autopilot side, since I can see that the current BTW - Bearing to waypoint updates with the one shown on the chart plotter. If the autopilot steered this course (or heading), it would at least hit the next waypoint. Unfortunately it does not correct its bearing until it is way out of course. I guess it should also try to minimize the XTE, in other words try to stay on the course line. It seems to do this, but not until the XTE is 0.04-0.05 NM or higher.

I have tried adjusting the gain on the autopilot, but it did not help.

When it comes to the NMEA connection, the pins 5 (ground) and 3 (data out) from the RS-232 port is connected to the NMEA IN connections on the Smartpilot Controller (not the S2 head unit). It tells me the current BTW, so I guess the connection is working. My logs of the NMEA data sent to the autopilot looks like this:

$INAPB,A,A,0.001,L,N,A,V,60.075,T,WP2,59.189,T,60. 075,T
$INAPB,A,A,0.004,L,N,V,V,60.075,T,WP2,56.553,T,60. 075,T
$INAPB,A,A,0.007,L,N,V,V,60.075,T,WP2,52.617,T,60. 075,T
$INAPB,A,A,0.010,L,N,V,V,60.075,T,WP2,47.806,T,60. 075,T
$INAPB,A,A,0.013,L,N,V,V,60.075,T,WP2,41.923,T,60. 075,T
$INAPB,A,A,0.016,L,N,V,V,60.075,T,WP2,34.705,T,257 .292,T

Googling the APB NMEA sentence, it is to be understood like this:

Field Number:
1) Status
V = LORAN-C Blink or SNR warning
V = general warning flag or other navigation systems when a reliable
fix is not available
2) Status
V = Loran-C Cycle Lock warning flag
A = OK or not used
3) Cross Track Error Magnitude
4) Direction to steer, L or R
5) Cross Track Units, N = Nautical Miles
6) Status
A = Arrival Circle Entered
7) Status
A = Perpendicular passed at waypoint
8) Bearing origin to destination
9) M = Magnetic, T = True
10) Destination Waypoint ID
11) Bearing, present position to Destination
12) M = Magnetic, T = True
13) Heading to steer to destination waypoint
14) M = Magnetic, T = True
15) Checksum
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