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INTRODUCTION
Technological advancement is
changing the performance-cost
ratio between stepper motors and
servo motors for a growing variety
of demanding industrial automation
applications. Thanks to the adoption
of closed-loop technology, less
expensive steppers are making
inroads into applications that have
been considered the exclusive
domain of more expensive servos.
STEPPERS VERSUS
SERVOS
Conventional wisdom states that
servo control systems are superior
in applications requiring speeds
greater than 800 RPM as well
as applications that require high
dynamic response. Stepper motors
are preferable in applications that
run at lower speeds, produce low to
medium acceleration rates and/or
require high holding torque.
Where does the conventional
wisdom concerning steppers and
servos come from?
Construction
A stepper motor rotates in steps,
using magnetic coils to pull a
magnet in steps from one position
to the next. To move the motor 100
positions in a given direction, the
circuit steps the motor 100 times.
The stepper moves incrementally
using pulses and can be precisely
positioned without any feedback
sensor.
The servo’s method of movement is
different. It uses a magnetic rotor
that is connected to a position sensor
- an encoder - which continually
senses the exact position of the
motor. Servos monitor the difference
between the motor’s actual and
commanded positions and adjust
current accordingly. This closed-
loop system enables the motor to
stay on course.
Simplicity and Cost
Not only are steppers less expensive
than servos, but they are also
simpler to commission and maintain.
Steppers are stable at rest and hold
their position, even with dynamic
loads. However, as the demands of
certain applications increase, more
expensive and complex servos must
be applied.
Positioning
A crucial difference between steppers
and servos is in applications that
require knowledge of the precise
position of the machine at every
moment. In an open-loop, stepper-
Stepper Motors Use Closed-Loop Technology
To Step Into Servo Applications
Dr. Markus Erlich, Servotronix
22 l New-Tech Magazine Europe