Rated coil current
Current regulated driver
Simple driver with double voltage
Figure 5: Current transient curves showing the benefits of a
regulated current driver
limiting with a transistor. Initially, the transistor allows full
supply voltage across the coil. Then, as current through
the coil increases, the transistor partially turns off to limit
current to the rated value.
The down side to this analog approach is that
significant energy is wasted as heat in the transistor. A
more efficient method is to switch the large supply voltage
all the way on or all the way off. The on time is adjusted
so that the average coil current is appropriately limited to
the rated value. This technique requires a few additional
components to sense and switch the supply current, but it
is one of the best ways to noticeably increase motor torque
and top cutoff speed. And conveniently, we already have a
microcontroller available to monitor the current and control
There are several methods of fine-tuning the control
system to enhance a stepper motor’s performance.
Fortunately, by using a microcontroller like the Propeller
and a few external components, we have tremendous
flexibility to create a control system as simple or complex as
necessary for our application.
Next month, Chris Savage will be back with the final
article in this series to discuss the use of PWM for DC
motor control. Be sure to check out the Parallax web link
for schematics, Spin code, and photos of this project. And
as always, good luck and have fun! SV
Parallax website for this article with example code,
schematics, and photos.
This website provides lots of information and a very
thorough explanation of stepper motors.
Wikipedia article on stepper motors.
50 SERVO 11.2008