Servo - April 2011

VEX Stepper Motor Control Experiments

SunBot III

By Daniel Ramirez

It is said of Archimedes of Syracuse (c. 287 BC – c. 212 BC) — an extraordinary Greek mathematician, physicist, engineer, and astronomer — that he is best known for his mathematical works relating to computing the area under a parabola and also computing the value of pi accurately. Among his many discoveries and inventions were weapons which were used to successfully defend his Sicilian homeland from the Romans. According to historians, Archimedes devised island defenses to repel many Roman attacks and sieges. Legend has it that one of his defenses involved using arrays of parabolic shaped mirrors placed at locations along the shore line that were focused on Roman ships to set them on fire. Kind of like an ancient “heat ray.” While not necessarily the best use of solar power, this is the basic theory behind modern mirror reflector solar power stations (heliostats) that are capable of generating electric power by focusing the mirrors on a steel tower used to super heat water into steam to generate electricity. It is amazing how much of Archimedes’ work influences science, modern architecture, and technology, especially in the area of mathematics.

SunBot III

SunBot II’s mechanical and electrical subsystems and stepper motor drives were described in great detail in the last article that ran in the December ‘ 10 issue of SERVO, leaving the firmware, quadrature encoders, limit switches, and stepper motor calibration details for this installment. I also mentioned deficiencies that I encountered with SunBot II, including backlash from the gears causing pointing errors especially in the azimuth axis. The solution to these problems was to go back to the drawing board and design and build a new SunBot III shown in Figure 1. In this photo, you can see the large stepper motors that we selected. They are powerful enough to drive the azimuth and elevation axes directly, and are mounted to the frame made from VEX components, some scrap metal plates, and a lazy Susan. The lazy Susan supports the weight of the elevation subassembly while providing very smooth motion. In addition, we used two new SparkFun v 4. 3 Easystepper boards which allow these large stepper motors to be micro-stepped.

The steel rail shown in Figure 1 serves two purposes. First, it is a mount for a laser pointer used to indicate how many degrees azimuth the elevation axis has moved. The laser pointer generates a spot below on the setting circle

FIGURE 1. Note the larger stepper motors that are used to drive the azimuth and elevation axes directly. These are are mounted to the frame which is made from VEX components, some scrap metal plates, and a low cost lazy Susan. You can also see the altitude and azimuth setting circles used for calibration.