Techno Linear
Motion Catalog41Stepper or ServoCost ComparisonIn general, stepper motor systemstend to be less expensivethan
servo motor systems. Stepper and servo systems oftenbecome comparable in price when the stepper system usesmotors larger than NEMA23 or when micro-stepping is
used.Servo motorsin the NEMA23 frame size tend to be 10%
to30% more expensive
than similar stepper systems. Brushlessservo
motor systems tend to be 50% to 100% more.Stepper motorsare
brushless. They experience little or nowear, and are virtually maintenance-free.For a given screw pitch, typical four phase stepper motors canproduce
200 full steps, 400 half steps, and up to 25,000 micro stepsper revolution. It is significant to note that since
the stepper motor isopen loop,
it does not necessarily achieve the desired
location,especiallyunderload.Particularlypoorpositionalaccuracycanresultwhen using microstepping, which is primarily useful
for smoothnessof motion.Steppershave very poor torque characteristics athigher speeds. This condition is improvedsomewhat by microstepping, however,unless the stepper is used in a closedloop mode, it does not usuallyperform as well as a servo.Stepper motorsare almost always used in
an open loopconfiguration.
If used in a closed loop, they typically becomeas, if not more, expensive than servo motor systems. Theopen loop nature of stepper motors is their principal
drawback.Commands are issued
to move prescribed amounts, andbarring unforeseen circumstances, the
motor moves theamounts
prescribed. In rare cases, resonances or unexpectedforces can cause a stepper motor to lose steps or stall
out.Although rare, this is an ever
present possibility.Brush-type servomotorsrequireachangeofbrushes,typically,every 5,000 hours. Like steppers, brushless
servo motorshave virtually no
servicing requirements.Servo motorresolution
depends upon the encoder used.Typical encoders produce 2,000 to 4,000 pulses per revolution,and encoders with up to
10,000 pulses per revolution areavailable. Since servos, which are closed loop, can
and doachieve the available
resolution, they are able to maintainpositional accuracy.Servoscan produce
speeds and powers twoto
four times that of similarily sized steppers.This
improvement is a direct result ofthe
closed loop (i.e., constantposition
feedback), whichallows for higher
speed andgreater reliability. The
closedloop nature of the servo alsoallows such a system to betterutilize peak torque capabilities.By nature, servo motors have
constant positional feedback.The
positional feedback is used to correct any discrepancybetween a desired and an actual
position. This constantcorrective action results in faster speeds (up to three
times thethroughput), and increased
power (up to three times thetorque) at high speeds. The closed loop nature
of the servoalso ensures that
stalling cannot occur unless there is
animmovable object in the path.Choosing Between Stepper and Servo
MotorsIn general, we
recommend stepper systems for
cost sensitive applications requiring low-to-moderate
volume productioncapabilities.
Servo systems are recommended for high-speed, high-volume,
high-reliability applications. A typical Techno servogantrysystem sells for about $2,700 more than a stepper system. Servos
can perform high-speed continuous motion reliably,making them particularly superior in
three-dimensional contouring applications. We have found time reductions
of up to 80% onsome applications.
The continuous motion also results in better finish quality without the
fine faceting that is found with steppersystems.
In addition, the servo's reliable high-speed continuous motion can
reduce the possibility of scorching and melting whenworking with woods and plastics.Servo TechnologyStepper TechnologyReliability and MaintenanceResolution and AccuracySpeed and PowerClosed Loop Vs. Open LoopChoosing a System