Paddle Test Take 2

Despite the 3-D printed "paddle fail" of a couple of weeks ago, I had not given up on the idea of 3-D printing something for paddle propulsion. This time around, I made a simple rectangular paddle with a hollow tube along one edge for mounting onto the motor shaft. To test out the concept and see if a pair of paddles would generate sufficient thrust for AMOS, I used a chisel to gouge out some motor mounting holes in the old surfboard, and wired up (this time properly!) the DC motor controller to an Arduino Uno, and wrote a simple program for driving both motors at the same speed. 

It seemed like maybe it was faster than the air propeller version, although to be certain I guess I would need to program both boats for the same GPS course and race them against each other. It still remains to be seen how the paddles would fare in some of the weedy locations that the airboat frequents. This video recorded on the weekend shows the airboat version of AMOS moving through some pretty dense river grass (although the depth transducer and turbidity probe had to be pulled out of the water to minimize drag):

While AMOS was traveling through the river grass, it was also measuring the dissolved oxygen content of the water. From the few tests that I have done in the river near downtown Fredericton and out at Woolastook over the last couple of weeks, it looks like the dissolved oxygen content is dropping.  Here is the latest dissolved oxygen data from downtown Fredericton:

Dissolved oxygen content near downtown Fredericton on Aug. 02, 2020 varied from 5 mg/L (purple) to 9.1 mg/L (red). 

Unfortunately our humid weather over the last few weeks coupled with repeated testing has dissolved the construction adhesive that was bonding the foam pontoons to the aluminum plates. To remedy this, I spent about $13 for a tiny package of two-part "marine" epoxy. It is supposed to offer a bonding strength of 4000 psi, and be waterproof. So we shall see. I re-bonded the delaminated sections a few days ago, and it seems to be holding quite strongly so far!

Zap Pow! Learning About DC Motor Inrush Current The Hard Way

Happy Canada Day! And Happy In Nature Robotics one year anniversary! 

One potential propulsion alternative that I have been considering for AMOS is the use of robotic water paddles instead of the air propeller. The air propeller requires about 60 W of power at top speed, but only provides about 1 pound of forward thrust. Going against a 20 km/hr headwind pretty much cancels that thrust out completely. But paddling with minimal exertion in my kayak is sufficient to move forward against that same wind.

One simple solution being considered was to use two DC motors to drive articulated paddle arms that locked rigidly when moved in one direction for forward thrust, and bent freely inward in the other direction to allow them to move against the flow with minimal drag. Here's a hand sketch of the motors and arms seen from the back of AMOS:

To test this out, I bought a couple of small DC garage door opener motors, rated for 12 volts, 6 A, and 2.2 foot-lbs of torque each. I also searched around on Amazon for a suitable DC motor driver and was a bit confused by what was available. Most of the drivers allowed for motion in both directions, but seemed to have mechanical switches for controlling either the direction or the speed, which wouldn't really be suitable. This led to some reading about H-bridge drivers (really just 4 simple switches) which gave me the bright idea that I could use my favourite 4-channel relay product to function as an H-bridge driver, since it was rated for up to 10 A, and up to 30 V.

Over the weekend I wired up a 4-channel relay to the motor, a 12 V lead-acid battery, an Arduino Uno microcontroller, and two small limit switches. The Uno was programmed to drive the motor continuously, reversing direction whenever one of the limit switches was pressed. 

When everything was ready, I started up the Uno only to hear a loud popping noise, with a small smoke plume, and the acrid odor of electronic failure. The motor didn't budge. Hmmm... maybe I wired something wrong? About 30 minutes of carefully checking the wiring proved this to not be the case. Hmmm... maybe there was some weird starting condition in the Uno program that resulted in too much initial current going through one of the relays? To test this hypothesis, a second (new) relay board was brought to the sacrificial altar. This time, the battery was left disconnected, the Uno program was started, and then the battery was connected. The motor shaft started spinning. I pressed one of the limit switches; it reversed direction and spun the other way. Pressed the other limit switch and it swung back. "Hey Kirsten, come check this out!", I yelled. "It's working!" Five seconds later smoke started to appear, then pop, pop! Dead again. 

At first I thought that the relay boards weren't really capable of 10 A or even 6 A operation. But I don't think this is the case. It turns out that DC motors actually have a large inrush of current for about 200 ms before they start moving and producing back EMF which limits the steady-state current going through them. This inrush current can be 2 to 3 times the specified steady-state current. So yeah, those 10 A relays weren't going to be able to handle 6 x 2 = 12 or 6 x 3 = 18 A of inrush current. Eventually I was able to locate a reasonably priced dual H-bridge driver board on Amazon that is rated up to 30 A. There is no documentation for it though, so I'll be guessing a bit at how to  wire it and use it. Hopefully no more electronics will be sacrificed! 😏