Servo - December 2010

VEX Stepper Motor Control Experiments - Part 2

Building the VEX SunBot

By Daniel Ramirez

SunBot II’s mission is to collect critical information eeded from our nearest star — the sun! This insolation data is required to plan for and design the next generation of solar panels and solar cells that will provide environmentally clean power alternatives to fossil fuels now and in the foreseeable future. Reducing or eliminating carbon dioxide (CO2) emissions is important and solar technologies can play a big part in the solution to this.

SunBot II will be a self-powered, VEX-based robot whose mission is to track the sun anywhere on earth under all lighting conditions (during sunny or dark and cloudy days), using astronomical tables to collect critical solar data necessary to produce solar power generation systems by acting as the guide for hundreds of solar panels located within a one mile radius of it. SunBot also demonstrates how VEX starter kits can be used to develop prototype environmentally-friendly products.

Although there are many kinds of solar panels currently sold worldwide by various vendors, the most common configuration is the fixed position panel which is typically positioned facing south and oriented at a 35 to 45 degree angle facing the sun. This position allows solar cells to collect sunlight during most of the day, but misses out on the sun’s direct rays at noon. You see these kinds of panels in solar powered (PV) homes, businesses, and even along

Ancient Babylonians, Egyptians, Aztecs, Mayans, Greeks, and Stonehenge Druids have been following the sun, moon, and star positions for centuries.

These vital details gave them seasonal information about when they could plant and harvest their crops and perform religious ceremonies. It was during the height of the Renaissance that the planetary orbits and the sun and moon positions could be computed using the great discoveries in orbital mechanics and gravitation discovered by Galileo Galilei, Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Sir Isaac Newton, and many other astronomers of that era. These orbital mechanical equations have been refined over the latest centuries to include the effects of Einstein’s Theory of Relativity and Einstein’s general theory of relativity so that now mankind can predict the planetary orbits and the sun’s position with incredible accuracy.

highways.Existing motorized panels are more expensive due to the motors and controllers required, but they can track the sun all day long. These types of panels are usually found in solar energy farms, universities, and energy research facilities worldwide where sunlight is plentiful all year long. Commercially sold solar panels usually use two or more photocells to find the brightest spot in the sky and track it. Bad weather, cloudy, dark, and rainy days can cause some of the tracking mechanisms to hunt for the sun excessively, wasting previously stored battery energy by having the motors move unnecessarily. So, what does this project have to do with stepper motors and VEX? With the Gulf Coast oil disaster now behind us, I believe it’s time to start thinking more about alternative forms of energy.

The prototype SunBot was assembled using VEX components (including standard VEX motors) and is mounted on my VEX-based Gilbert IV Explorer robot shown in Figure 1. The problem with this prototype was that the gearing used did not provide enough pointing accuracy which is why I decided to switch from VEX motors to stepper motors for the azimuth and elevation drive.