Steve and others interested in Arduino based full size boat autopilots. I have been working on a marine autopilot for a 40 foot sailboat for a couple of years. Last summer the autopilot steered my boat 500 miles up the inside NW passage from Seattle to the north end of Vancouver Island and back. I have just finished putting together a YouTube video showing the boat under sail and being steered by the autopilot. The video also does a brief walk through of the hardware and operating modes.
Here is the link to the video. If anyone wants to adapt this project to their needs I'll be glad to help.
GPS can be used to adjust the tiller on a sailboat short term. As long as it's moving about 1 knot or more. The compass heading is constantly changing with each swell, where GPS does not as much. With the appropriate feedback constants and timing you can steer up and down with each swell just like a human navigator would. When the speed is less than 1 knot, this is the best solution. When the seas are more flat, and speed >1 knot, only GPS is needed. I have been sailing and designing my own autopilots for 20 years. Sometimes out in the middle of the Pacific!
When you're cruising for weeks at a time with limited battery power you would not want to use the autopilot to steer up and down each swell. Mechanical wear becomes an issue. With light winds and smooth seas it is difficult to see a consistent heading with GPS, because of the slow speed. You are correct. But then you wouldn't need an update every 1 second. You can write your own code to calculate the heading every 10 seconds instead. There are only a few conditions where GPS alone without a compass is not appropriate, rough sea state with little wind. A compass is needed. But there are also many conditions where an electronic auto pilot fails to work efficiently, regardless of the sensor. During a storm I'd prefer a Windvane.
The absolute error can be 10m. But from one second to the next the error changes by only inches. So even when you're moving slowly, the line is straight when you plot it. Data can be obtained 10x per second with a modern GPS.
coyotewaits:
Steve and others interested in Arduino based full size boat autopilots. I have been working on a marine autopilot for a 40 foot sailboat for a couple of years. Last summer the autopilot steered my boat 500 miles up the inside NW passage from Seattle to the north end of Vancouver Island and back. I have just finished putting together a YouTube video showing the boat under sail and being steered by the autopilot. The video also does a brief walk through of the hardware and operating modes.
Here is the link to the video. If anyone wants to adapt this project to their needs I'll be glad to help.
Many thanks for your work. It has inspired me to start on an autopilot for my old C&C-33-1.
One question about the Pololu:
From github: GitHub - pololu/minimu-9-ahrs-arduino: Arduino program for building an AHRS with a Pololu MinIMU-9
"When the AHRS program first starts running, it takes some readings to establish a baseline orientation, during which it expects both the roll and the pitch of the sensors to be zero. Therefore, it is important to keep the MinIMU-9 level for a few seconds after powering on or resetting the Arduino or connecting to it from a computer."
Won't this be problematic if you power up or reset the autopilot after you're already underway?
michinyon:
My feeling is that using the GPS to determine which way you are going, is bound to fail.
If you take a GPS and walk around with, it will calculate it's position from time to time and calculate an apparent direction you are moving based on those points. If you plot them on a map ( Google earth is the easy way ), you will see a zig-zagging series of straight lines. This would be useless for steering a yacht.
Secondly, I think it would be futile to try steering a boat without knowing which way the boat is facing. A GPS cannot tell you which way the boat is facing. A digital compass which costs about $4 will tell you which way the boat is facing. In simple terms, the boat needs to be pointing in the direction that you want to go. You can then adjust this for leeward sideways drift, currents or whatever else you like. In the short term, to control the tiller to keep the boat pointing in the direction you want to go, you need a digital compass chip, not the GPS. Then, in the medium term, you use the GPS position to compare where you are, to where you want to go, and adjust the course reference that the compass is using. I wouldn't use the GPS course heading information for anything, because it is basically useless IMHO.
