Battle Against The Wind

Yesterday, I brought AMOS back to Woolastook for another test run. Wires and batteries had been moved out of the immediate vicinity of the electronic compass, and some of the control parameters in the software had been tweaked a bit, in the hope that AMOS would more consistently travel in a straight line. Also, I had modified the mounting of the air propeller assembly slightly to help avoid a problem with it sometimes flying off the shaft of the servo motor in high winds. I was optimistic that things would go well...

So I was naturally disappointed when AMOS took off for its destination and promptly ran into the east bank of the inlet. I got in the kayak, paddled a few meters, and then freed it from the grass (more work is required for AMOS to do this by itself). Then AMOS went off in the opposite direction, this time getting stuck on the west bank. I paddled over to free it again, then brought it to shore and connected it to my laptop to check the ship logs to see what might have gone wrong. The logs revealed that AMOS required a long time to get a valid GPS fix, and even when it did get a fix, the initial GPS coordinates were off by quite a lot. For example:

in the above photo the actual GPS track that AMOS thought it was following is given by the thin blue line in the lower-left corner. Although it actually started at the base of the inlet, it thought it was starting about 10 m west of Route 102, and therefore plotted the first interpolated GPS coordinate at a point just west of Route 102. Later, it acquired an accurate GPS position, but by that time it was already headed for an incorrect destination, and kept ramming itself into the west bank accordingly.

After checking the ship logs, the test was repeated, this time with the following result:

This time the interpolated points were plotted correctly, but the GPS location of AMOS seemed to usually be biased about 10 m to the south of where it should have been. So this time AMOS kept crashing repeatedly into the eastern shore. Plotting more frequent GPS waypoints in the center of the inlet probably would have worked a lot better.

At this point, I gave up on the planned GPS course and just tried piloting AMOS around manually. This proved to be more challenging than I expected. I had thought that I would be able to find an azimuth angle for the ship's propeller that would allow AMOS to proceed in a nice straight line, but there was a relatively strong wind coming out of the east that day, with gusts that were probably in excess of 30 km/hour. The gusting wind required constant attention to the azimuth angle of the propeller, and more often than not I would get it wrong, so that AMOS ended up making a lot of circles out in the open water. Eventually, I gave up and just set the angle of the propeller to the zero degree position and observed what kind of a path the boat followed:

The gusting wind proved to be a bit much for AMOS, especially when the boat was headed directly into it. A quick calculation after I got back confirmed that the wind was probably about 30 km/hour:

Assume that the surface area of the boxes and board that are facing into the wind is ~ 0.1 m^2., air density is 1.2 km/m^3, and the wind speed was 30 km/hour or 8.3 m/s:

Force =  0.5 * 1.2km/m^3 * (8.3 m/s)^2 * 0.1 m^2 = 4.1 N = 0.9 lbs.

This wind force is quite close to the maximum thrust of the air propeller on AMOS (~ 1 lb), hence the reason why AMOS appeared to be stalled in the water when the wind picked up.

There are a few ways to combat this wind effect:

1) Reduce the cross-sectional area of material that is facing into the wind.
2) Change the shape of the boxes on AMOS to make them more streamlined, so that the wind flow does not separate as much when it hits them.
3) Increase the thrust of the air propeller, either by upping the pulse frequency on the current setup or by getting a new (larger) propeller and motor altogether. This latter option might also require a beefier battery pack, and a more robust protective cage. I have already had to glue together the existing 3-D printed cage numerous times to repair it after crashes with brush and other shore obstacles.

First I'm going to try #1 above by getting rid of the 3rd largest box (it's almost empty now anyway) and mounting the propeller on the 2nd box instead. The two remaining boxes will then be embedded about 4 or 5 cm into the foam of AMOS. This should hopefully reduce the cross-sectional area down from about 0.1 m^2 to something like about 0.07 m^2. I should also be able to increase the thrust on the current propeller / motor a little bit without too much difficulty or worry of things overheating or drawing too much current. Since the wind force scales as velocity squared though, this would probably only get me up to about 45 km/hour. Perhaps some smart software could be added to drive AMOS at an angle to the wind (say 45 degrees) which could in theory maybe allow for forward progress in winds up to about 60 km/hour. Maybe some future version could also use flatter waterproof boxes so as to get the cross-sectional area down even more.