velocity and acceleration. Servomotors are controlled by
sending electrical pulses of variable width using a technique
referred to as pulse width modulation (PWM).
Understanding the application requirements by answering
these questions is the first step in making a design selection
between a stepper and a servo motion control system:
• How heavy is the load to be moved, and at what speeds?
• What are the applications’ torque requirements?
• Is torque limiting required?
• Is holding torque required?
• What is the required position resolution?
• At what speeds will your application require the motor to
• Does the load change throughout the move?
Once these questions have been answered, the following
design considerations will help guide the decision.
The table compares some of the leading features of servos
Position Feedback: Servo systems run closed loop, so
position feedback is a requirement. A stepper system runs
open loop, with position commanded to move and hold at one
of its steps without any feedback required. An
incremental or multi-turn absolute encoder can be
added as an option in stepper motor applications
requiring more precise positioning.
Torque versus Speed: The weight of the load
and the speed at which it needs to be moved
help determine the torque requirements. As
shown in Figure 1, servo motors have constant
torque across their speed range, while stepper
motors have higher torque at low speeds and
less at high speeds.
Holding Torque: Holding torque is the amount
of torque needed to move the motor one full step
when the windings are energized but the rotor
is stationary. Stepper motor torque is usually measured in
ounces per inch. Stepper holding torque can be regulated by
the amount of current (idle current) put through the motor at
rest. At 100% idle current, full torque can be expected from a
A servo motor does not have this function, one of the
factors contributing to its lower torque at low speed as
compared to a stepper motor. Positioning applications where
the motor shaft may experience minor rotational force at rest
require holding torque. Typical applications where the holding
torque provided by stepper motors is required include camera,
diverter gate and vertical load positioning.
Torque Control: The current control of a servo motor is
much more complex compared to a stepper motor, with
the regulation of the current going through the servo motor
helping to regulate the torque. This makes servo motors a
good fit for applications where torque must be managed, such
as web control.
Tuning Required: Servo systems require tuning, which can
make control quite complex. For example, if a load is running
at very slow speeds, substantial oscillation may occur without
proper tuning. The straightforward control of a stepper motor
does not require tuning, resulting in simpler control.
Support Dynamic Load: Servo motors can support dynamic
loads, so if the expected load increases, the servo can
respond with peak torque. Stepper motors don’t have this
feature. Dynamic load applications where the weight changes
often and unpredictably benefit from servo motors. Conveyors
moving variable weight products are a good example.
Low Speed Smoothness: Stepper motors have excellent
low speed smoothness, with features such as micro stepping
providing precise speed change without the drift and hunting
often found with servo systems. Applications requiring low
speed smoothness include pumps, video scanners, optical
measuring systems and low-speed conveyors.
Figure 1. Stepper motors have higher torque at low speeds than servomotors
until speeds reach about 1,200 to 1,500 RPM (photo courtesy of AMCI).
Feature Servo Stepper
Position Feedback Required Optional
Torque vs Speed Constant Varies
Holding Torque No Yes
Torque Control Yes No
Tuning Required Yes No
Support Dynamic Loads Yes No
Low Speed Smoothness Good Excellent
Programming Complex Simple
Size Larger Smaller