Improved Eppley Solar Tracker
The NOAA Solar Radiation group uses Eppley solar trackers for measuring solar irradiance with pyrheliometer and pyranometer instruments. This requires accurate positioning of the instruments as the sun moves through the day over each monitoring station. Unfortunately, many of them had fallen into disrepair and shown problems with internal cable windup and load-bearing ability. Along with my coworkers Jim Wendell and Emiel Hall, I worked on modifying these old trackers to bring them back to life. We made many hardware, electronic, and software changes to address various problems and improve their abilities overall.
Here is a picture of the modified/improved solar tracker setup and running on the roof at NOAA. The two levels are used with three adjustable-height feet to keep the plane of azimuth rotation level while tracking. The grey cable to the left contains the stepper motor control lines and hall-effect sensor power/comms.
The pyrheliometer is tracking the sun very well, as shown by the dot projected by the sun through a hole on the top of the instrument to a target on the back. This dot stays centered on the target as the sun moves thanks to the highly-accurate tracking.
Many mechanical modifications are made to each tracker. All the screws, washers, bearings, wiring, motors, o-rings, and seals are removed and replaced (using anti-seize and grease liberally). This picture shows one of the new (more powerful) stepper motors on a tracker's azimuth worm gear assembly. The mounting slots are milled larger and a hall-effect sensor is mounted on the side. This sensor is used to zero and count rotations of the tracker, allowing daily resets to a "home" position and preventing internal wire wind-ups.
Shiny New Bearings
This picture shows our new replacement bearings on the azimuth and elevation shaft cups. Some material is removed from the inside of the cups on a lathe before these new bearings are pressed in, eliminating any friction on the bearing inner races.
Electronic and Software Improvements
The electronics for the trackers are completely rebuilt. This box contains the new microcontroller board, stepper motor drivers, power supply, GPS, RTC battery backup, and position offset adjustment joystick/switch. Emiel and I build these boxes, including soldering all parts on the microcontroller boards. I wrote the firmware that runs on the (Atmel AVR) microcontroller. It reads the station position automatically from the GPS, calculates the sun's position, and controls stepper motor movement. Many user settings are available, including the ability to adjust the tracking position with the directional/joystick buttons to correct any inaccuracies in the mounting position. The number of azimuth rotations are counted, and after a certain threshold an unwind is forced to prevent internal wires from snapping (this used to be a problem in places like Barrow, Alaska and the South Pole where the sun stays up for long periods).