VEX Stepper Motor Control Experiments
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 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.
that indicates this. Notice that it is placed perpendicular to
the solar panel, so that it would not get in the way and
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