by CHRIS SAVAGE
GETTING CONTROL
WITH THE
Propeller
PART 1
Recently, an artist friend asked me how he might go
about automating his latest masterpiece using a
BASIC Stamp®. After looking at what he had, it
turns out he was going to need to control DC motors,
hobby servos, and stepper motors. While there are solutions
for controlling all of these devices it would generally mean
having three separate controllers (one for each device).
A better solution would be to have one controller that
could — ideally — control all three devices. This would
simplify the design considerably; something my artist
friend can really appreciate. A colleague named David
Carrier had recently done some work with servo motor
control for the Parallax QuadRover and another colleague
named Kevin McCullough with DC motor control. This got
me thinking about a cost-effective way to integrate this
type of control into a single design. Since the actual
interface hardware for each device is somewhat different,
the goal would be to focus on the control board itself and
keep the design simple. This would allow the controller
board to focus on providing the correct signals to the
various devices. Each device may or may not require its
42 SERVO 09.2008
own driver circuitry. My colleagues and I decided to work
toward this goal.
Control Methods
Servo motors are generally pretty easy to control. They
can operate from 4.8V to 6V DC and do not require any
special driver circuitry. Servos require a variable pulse
width from about 1 ms to 2 ms at 20 ms intervals on their
signal input pin to control them. The width of the pulse
determines the position the servo will move to within its
range of motion. This pulse must be continuously sent
in order for the servo to maintain its position. There
are a number of servo controllers out there and many
microcontrollers such as the BASIC Stamp that can control
them directly.
Unipolar stepper motors have various voltage ranges
and require only a simple driver circuit using MOSFETs or
power transistors capable of handling the voltage and
current of the stepper motor. You can then control them by
energizing the phase coils in sequence. This control can be
