3D design software, multi-battery solar system

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3D Design Software

The core of Black PERL's direction control is a wind vane that will automatically steer her on a given course relative to the wind.  For example, it could keep her bow pointed in a direction that keeps the wind coming from 60 degrees to the right of the bow, what sailors would call a starboard tack beat.  

In order to save power and to minimize the exposure of powered-on electronics to seawater, Black PERL's computing system will be powered off almost all the time, briefly waking up to do a navigation check and any needed adjustments.   The wind vane must be adjustable by an electric motor controlled by the computer, but it must also reliably hold that position while the computer is powered off, no matter what wind and wave forces might hit it.

To do this, I'm planning to use a geneva gear, an old clockworks component.  A gear with a cog alternately turns another gear and locks it in position.  In my design, I want to be able to hold courses at 60 degree increments (i.e. 60, 120 and 180 degrees from the wind), so I need a six-sided geneva gear.

I downloaded SketchUp, a free 3D design app, and in a few hours was able to design the gear pieces that will rotate the wind vane relative to the rudder's position.  One gear is 24mm (1 inch) in diameter, the other is 40mm (1.6 inches), roughly the size of a US quarter and silver dollar.  

The purple gear, in operation, would be shifted left so that the peg enters the slot in the green gear.  When the purple gear is rotated by the motor, the green gear turns in 60 degree increments, and the crescent shape on the purple gear locks against the scalloped curves on the green gear, preventing movement even when power is disconnected.   (The shaft to the steering gear extends down from the green gear, the wind vane is mounted above it.)

After a few failed attempts, I was able to get it into a format acceptable to Shapeways.com, and submitted the trial order in ABS plastic, for $25 including shipping.  If it all works, I'll spend about $80 to have it make in their stainless steel/bronze alloy, possibly having it gold plated for a bit more corrosion resistance.  The plastic prototype should arrive in about 10 days.  (I've had a sample of their stainless/bronze allow sitting in a saltwater tank for many months now, with nothing worse than some surface pitting.)

I also discovered that UPS has a pilot system in six of their locations in the US, providing 3D printing services.  One of them is about five miles from where I live, so I'm having them print the same file.  They're charging $30, but should have it ready in a couple of days.  

Once I get the plastic pieces, I'll build a tiny temporary housing for them, mount it on the vane steering gear, and hook up a tiny gear motor.  With a bit of luck, that will let me steer the boat under Arduino control.

Power System

I've been hoping to be able to run several battery/solar systems in parallel, so that if any one of them survives, it will power the system.   A key to this design is the use of diodes to isolate the power systems, preventing a short in one of them from draining the power from the rest of the system.

On a tip from a fellow MicroTransat competitor (thanks, Robin!), I ordered 10 tiny diodes that only cause a 0.2V loss of potential.  These things were far smaller than I expected, with about a millimeter wide strip of metal on either end, apparently for soldering.  There were no labels to indicate current flow, so I had to solder it up and test which way it blocked current.  (Diodes restrict current flow to one direction only, so three battery systems could be connected to a main positive line without the risk of current flowing back into a failed battery.)  

Here's what the little thing looked like after soldering some leads onto it:

Did I mention it was small?  The diode itself is the small black rectangle between the beads of solder.  Each of them can handle 30 volts at 3 amps, well beyond what I need.

I soldered jumper wires onto three diodes, and used them to make a junction box for the batteries and solar chargers.

Each solar charger/battery system sent a positive line in to the diodes, the diodes shared a common output line that became the positive line for the load...in this case, just a DC motor.

It worked as expected.  One solar charger's output was at 4.5V as it charged its 80% charged battery.  The other charger's output was at 6.7V as its battery was fully charged.   The power output to the motor showed 6.5V, and the current out from the second charger matched the current to the motor, no current was going into the weaker battery bank.  

My next step will be to replace the digital display volt/current monitors with small current monitoring chips, and let the Arduino log current flows on three batteries and the main line simultaneously.  That will complete the electrical system's design.   I'll be keeping these temporary, easy-to-reconfigure connectors for now, but eventually most of it will be soldered onto a single circuit board, and stored in a waterproof container.