USING A STEPPER MOTOR
WITHOUT A MICROCONTROLLER
by Samuel Lambert
www.servomagazine.com/index.php?/magazine/article/
december2012_Lambert
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I became involved in a project a while
back that required the precise angular
positioning of a sensor that was to be
accomplished using a stepper motor.
Although this seemed to be a reasonable
approach, upon looking into how one
goes about controlling a stepper motor,
I found some hardware (Figure 1) and in
a short time had the stepper motor
working.
Figure 1. Commercial stepper motor control electronics.
Although I had a solution to the problem I was facing,
the control electronics were complex and rather pricey
($275). I was tied to a computer to control the motion, and
was locked into the vendor’s software for the
microcontroller’s position and velocity commands.
This got me to thinking about another way to use the
stepper motor: Create a simpler and more flexible control
system, and do it cheaply (less than $40).
This article describes a unique technique for
implementing a servo system (Figure 2), making use of a
stepper motor in an unconventional manner without the
need for a microcontroller.
The motivation for this unique design was to build a
simple servo system that used a stepper motor without a
microcontroller. The only digital logic in the design is a
single dual D-type flip-flop to generate the quadrature
Figure 2. Typical servo system.
68 SERVO 12.2012
square waves to drive the stepper motor. The design can be
viewed as a DC servo that just happens to employ a stepper
motor.
Stepper Motor Versus a
DC Servo Motor
Stepper motors are almost always used in an open loop
configuration. If used in a closed loop, they typically
become more expensive than DC servo motor
implementations. The open loop nature of stepper motors
is their main drawback. Position and/or velocity commands
are issued to move the motor, and barring unforeseen
circumstances, the motor moves accordingly. In rare cases,
resonances or unexpected torques can cause a stepper
motor to lose steps or position. Although rare, this is an
ever-present possibility.
DC servo motor resolution depends primarily on the
nature of the feedback device. Since servos are usually
closed loop, they are able to maintain positional accuracy
utilizing a variety of devices, e.g., potentiometers, encoders,
directional antennas, or video devices. The closed loop
nature of the servo also allows such a system to better
utilize peak torque capabilities. By nature, servo motors
have a means of implementing positional feedback, and
may even utilize velocity or acceleration feedback to
improve response. The positional feedback is used to
