Mike Davis has some land in Arizona that is off the grid. He has generally used a small homemade solar array to keep a small battery bank charged, but he had to move the panels manually every few hours for optimal exposure to the sun. He decided it was time to automate this process, and he did so with an antique television antenna rotator that he got for $15 at a garage sale.
The first thing he had to do was build the frame. Building a wooden frame is pretty easy, but he had to design this one so that it would hold the array at the optimal angle for final deployment on his property (at 34.6 degrees North latitude). The frame itself isn't anything special. It is composed mainly of 2x4 lumber and can be broken down for easy transportation. This was necessary because he was building it in his workshop in Florida and would drive it to his land in Arizona. Davis tells us on his website that he will be using it only during a specific time of the year, so there was no need to make the vertical angle adjustable.
After the physical construction was done, it was time to move on to the electronics. His initial thoughts were that he could salvage the original motor controller and just add an external system for tracking. However, once he dug in, he found that the system probably couldn't handle the abuse he intended to throw at it. The system had been designed for rare rotations for correction, but he planned on rotating for all the daylight hours every day.
He then set out to design a new control system. Having worked with MBED at work, he felt comfortable using that as the brain. He says on his site that the MBED platform is extreme overkill for this, but his familiarity allowed him to build the system quicker. Having that extreme overkill also means that he has plenty of room to expand the system or add features in the future, such as statistics logging.
The MBED reads voltage from two small photovoltaic cells placed at right angles to each other. It rotates until the voltages on the two panels match. However, Davis found that this did not work in the Arizona sun as well as it did on his Florida workbench. The sun was too bright, and the difference between the panels was too minimal at any angle. He remedied this by adding a small piece of metal to shade the center of the two sensors. This occulting bar would cast a shadow that would make the difference in output voltage more extreme based on the angle.
Davis has many more pictures of the project, a full schematic, and the code for the MBED on his website. You can see a video of the final result below.
I know that's what he meant, Caleb, but I was confused by this part:
The MBED reads voltage from two small photovoltaic cells placed at right angles to each other. It rotates until the voltages on the two panels match. However, Davis found that this did not work in the Arizona sun as well as it did on his Florida workbench. The sun was too bright, and the difference between the panels was too minimal at any angle.
That made me wonder, if the Arizona sun was too bright, then why not make it less bright with a filtering lens? Wouldn't an opaque shade cast a longer and longer shadow as it tilts, thereby throwing off the direction more and more? I'm not getting that part.
The entire point was to make them uneven so that the array would shift to put them back in an even state. Without the shade, both panels reached saturation at all times. The shade gives a mechanical means of increasing the disparity between the two.
I wonder, instead of using the shade, wouldn't it have been better to simple put a light-absorbing filter over the photovoltaic cells? The shade would shift in angle, exposing more of one cell than the other, while a filter would keep them even -- replicating Florida in Arizona.