Stepper & Servo: Basic
The basic difference between a traditional stepper and a
servo-based system is the type of motor and how it is controlled.
Steppers typically use 50 to 100 pole brushless motors while
typical servo motors have only 4 to 12 poles. A pole is an
area of a motor where a North or South magnetic pole is generated
either by a permanet magnet or by passing current through
the coils of a winding.
Steppers don't require encoders since they can accurately
move between their many poles whereas servos, with few poles,
require an encoder to keep track of their position. Steppers
simply move incrementally using pulses [open loop] while servo's
read the difference between the motors encoder and the commanded
position [closed loop], and adjust the current required to
drawing courtesy of National
Stepper & Servo: Pros & Cons
Some performance differences between Stepper and
Servos are the result of their respective motor design. Stepper
motors have many more poles than servo motors. One rotation
of a stepper motor requires many more current exchanges through
the windings than a servo motor. The stepper motor's design
results in torque degradation at higher speeds when compared
to a servo. Using a higher driving bus voltage reduces this
effect by mitigating the electrical time constant of the windings.
Conversely, a high pole count has a beneficial effect at lower
speeds giving the stepper motor a torque advantage over the
same size servo motor.
Another difference is the way each motor type is controlled.
Traditional steppers operate in the open loop constant current
mode. This is a cost savings, since no encoder is necessary
for most positioning applications. However, stepper systems
operating in a constant current mode creates a significant
amount of heat in both the motor and drive, which is a consideration
for some applications. Servo control solves this by only supplying
the motor current required to move or hold the load. It can
also provide a peak torque that is several times higher than
the maximum continous motor torque for acceleration. However,
a stepper motor can also be controlled in this full servo
closed loop mode with the addition of an encoder.
Steppers are simpler to commission and maintain than servos.
They are less expensive, especially in small motor applications.
They don't lose steps or require encoders if operated within
their design limits. Steppers are stable at rest and hold
their position without any fluctuation, especially with dynamic
Servos are excellent in applications requiring speeds greater
than 2,000 RPM and for high torque at high speeds or requiring
high dynamic response. Steppers are excellent at speeds less
than 2,000 RPM and for low to medium acceleration rates and
for high holding torque.
Stepper vs. Servo: The Verdict
Servo control systems are best suited to high speed,
high torque applications that involve dynamic load changes.
Stepper control systems are less expensive and are optimal
for applications that require low-to-medium acceleration,
high holding torque, and the flexibility of open or closed
Who Makes Stepper Controls?
Many manufacturers specialize in low-cost, inexpensive stepper
products that deliver baseline performance at a bargain price.
These companies often source the designs and/or manufacturing
from overseas partners such as China, with limited technical
A small number of U.S. based tech companies design, manufacture
and sell higher performance stepper motor systems that rival
more expensive servo controls. Advanced Micro Controls Inc
(AMCI) is such a company, boasting over 15 years of complete
stepper solution design and manufacturing.
The company's products are listed below for your reference: