Wednesday, January 28, 2015

Hardware Design Choices

Friends on facebook have asked a couple questions about why I chose the hardware I did so I thought I would repeat /and expand on the answers here.

Android Phone vs Arduino or Raspberry Pi

The kids and I had already written some apps using App Inventor and were very impressed with how easy it makes software development.  In the "wifi connect" mode as you change things on your screen your phone is running the new version.  If you want to really install it you just click "build" and have your phone scan the 2D barcode on your screen and it will install.  It is a graphical environment that is great for my 11 and 13 year old boys to learn programming in.

Arduino and Raspberry Pi are better for doing custom electronics hardware but not nearly as good on the software side.   In the end the things I want to do will take a lot of software but we don't need custom electronics.   The phone can control a bunch of relays and has all the sensors and communications we need (GPS, Gyro, Compass, Acceleration, Internet, SMS, wifi, bluetooth).   The phone having a display makes debugging the software much easier.  The phone also has its own battery, so even if all else is down it may still be able to send messages.   You can get a phone with a case for $85 and you don't have to put it together.  The memory, CPU, and communications bandwidth of an $85 phone is far greater than what was used to send men to the moon.   So we should be ok with the phone.

Brushed vs Brushless Motors

I did a calculation with a prop with a 1.4 inch pitch and decided that 3000 RPM is what I wanted. With the solar at 12 volts and the battery at 12 volts, the motor had to be 12 volts. For the 50 watt solar to be able to charge up the battery during the day while running the motors and for the battery to just about make it through the night the motors could only be around the 7 watts of this motor. I only want to get things from Amazon so it is easy for someone else to copy us. With these 4 constraints, this motor was the only one we could find, so I did not have to choose between brushed and brushelss. :-)

Variable Speed Motor Controller vs Just Relay

With a single part with 8 relays we can control both motors, some lights, and electromagnets.  We can do a lot of stuff with this.  By turning relays off and on we can still turn the boat and reduce power usage at night.  It would be nice to have a variable speed motor controller, and to be able to do reverse, but I don't think we need it.   Also, I could not find a bluetooth motor controller on Amazon and did not want it bad enough to order it from someplace else.   

There is some chance that we will later decide that automatic docking without reverse is just not practical; however, I think we can get a sea anchor/brake to work and with one boat using a brake the other will be able to dock with forward thrust only.   In the Caribbean we almost always have the trade winds that are the scale equivalent of hurricane winds for these small boats.  So I think we can count on the wind to push back if they just stop pushing forward.

Weight Limit

We tested the model with sandwich bags full of sand and think that we want try for a weight limit of 6 to 8 lbs.   The solar is 2.5 lbs.  The battery will be either 1.2 or 2.5 lbs.  The two motors together are about 1 lb.    So we might make it.   If we run over we could add flotation to an existing model or build another with more capacity. 


I would like to have the possibility of the boat going on its own for days.   But realistically we may lose a boat or two before this is all done, so keeping costs down is important.   It would be fun if other people built boats that were software compatible and this seems more likely to happen if costs are kept low.  The first set of parts  was bought with Christmas money from grandparents.

My Goals

I want my boys to experience the thrill of doing some cool engineering, learn about programming, test out this design, and develop software that could be used on large solar boats of similar design later.

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