Resonating Vibrations and Magnetic Noise

I didn't get to Woolastook for any field tests last week, but was still able to learn plenty from some simple tests in the pool before its surface froze over a couple of days ago. The first obvious problem was that the cage / support for the propeller, including the air rudders tended to resonate (quite a bit actually) over a certain range of propeller speeds. The electronic speed controller for the propeller uses pulse width modulation to control the speed, and in my software I have assigned an arbitrary scale of 0 (minimum speed) to 10 (maximum speed).  The resonating vibrations begin at a speed of about 3, reach a maximum at around 4.5, and then taper off over 6, completely disappearing at 7. The shaking was so bad at around 4 or 5 that the tip of the propeller collided with something and broke off. I think probably that the cage should be made from aluminum wires or similar, something sturdier than PLA. So I modified the software to just avoid all speeds between 3 and 6 and that took care of the resonance issue.

Additional pool testing showed that the directional control of the boat using the air rudder was a bit swervy. Partly this might be because there is less of a turning moment (with the air rudders) than the boat had before when it could change the thrusts of the water propellers mounted on the left and right (er port and starboard) sides of the boat. But testing out the compass module output in the software, I noticed that there was more noise in the compass heading data when power was applied to the air propeller. More power = more magnetic noise. The cables supplying power to the air propeller were a few inches away from the magnetometer, but they were close enough to cause random fluctuations of a few degrees, which didn't help with maintaining directional control.

So I did a bit more work on the inertial measurement unit (IMU) that I had picked up a few weeks ago, and finished a new software class for it and just today finished integrating it into AMOS's main program. Just turning the circuit board by hand, you can see that the output orientation angles that it provides are nice and smooth; much better than what was obtained with the compass module output. Magnetic interference from powering the air propeller will still be a minor issue for steady-state magnetic heading measurements, but for real-time control of the boat's direction, the gyros should do a very nice job of measuring the dynamic orientation of the boat, allowing the air rudder control software to maintain a steadier course.

Tomorrow is "Bring Your Kid to Work Day" and Bexie and Kirsten will be joining me in the morning to help work on AMOS. One of the things that needs to be decided upon for the next revision is the shape / design of the hull. So we will be building some toy-sized boats out of cardboard and duct tape and testing them out in the bathtub to see how stable and seaworthy they are. Here is the example boat that I made:

I have no doubt that the girls will be able to do much better!