Servo Motor vs Stepper Motor: Which is right for your application?
It is an engineering truism that there is no perfect solution, just the
best solution for the problem at hand. That holds particularly for servo
motors and stepper motors. Both are broadly used in industry. Neither
is a universal solution. When properly applied, however, both stepper
motor and servo motor can provide effective, reliable power for a highly
successful system. The decision tree for choosing between the two has
many branches but the most important are speed, acceleration, and price
target.
Stepper Motors
Stepper motors consist of a rotor with permanent magnets and a stationary stator that carries
the windings. When current runs through the stator windings, it
generates a magnetic flux distribution that interacts with the magnetic
field distribution of the rotor to apply a turning force. Stepper motors
feature very high pole counts, typically 50 or more. The stepper motor
driver energizes each pole in sequence so that the rotor turns in a
series of increments, or steps. Because of the very high pole count, the
motion appears to be continuous.
Stepper motors have a number of positive attributes. Because they
generate incremental motion, they are generally run open loop,
eliminating the cost and complexity of an encoder or resolver. The high
pole count allows them to generate very high torque at zero speed. They
are compact and generally economical.
Stepper Motor
Figure 1: Stepper motors deliver good performance at an economical price
point for applications requiring low speed, acceleration, and accuracy.
(Courtesy of Kollmorgen)
On the downside, stepper motors have speed limitations. They generally
run best at 1200 RPM or lower. Although they generate high torque at
zero speed, torque falls off as speed increases (see figure 2). A motor
that generates 100 ounce inches at zero speed might only deliver 50
ounce inches at 500 RPM, for example, and just 10 ounce inches at 1000
RPM. In theory, a gearbox could be used to increase torque, but this is
where the low speed of stepper motors becomes a problem. Adding a 10:1
gear reducer to a 1200 RPM stepper motor might boost the torque by an
order of magnitude but it will also drop the speed to 120 RPM. If the
motor is being used to drive a ball-screw actuator or similar, it
probably will not deliver sufficient speed to satisfy the needs of the
application.
Stepper motors generally are not available in frame sizes larger than NEMA 34, with most applications falling in the NEMA 17 stepper motor or NEMA 23 stepper motor sizes. As a result, it is unusual to find stepper motors capable of producing more than 1000 to 2000 ounce inches of torque.
Stepper motors also have performance limitations. You can think of a
stepper motor as a spring-mass system. The motor needs to break friction
to begin turning and move the load, at which point the rotor is not
fully controlled. As a result, a command to advance by five steps may
only result in the motor turning four steps – or six. If the drive
commands a motor to advance 200 steps, however, it will do so to within
just a few steps, which at that point represents an error of a few
percent. Although we command stepper motors with a resolution of
typically between 25,000 and 50,000 counts per revolution, because the
motor is a spring-mass system under load, our typical resolution is 2000
to 6000 counts per revolution. Still, at these resolutions, even a
200-step move corresponds to a fraction of a degree.
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