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| Servo -vs.- Stepper - How are
they different? |
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Stepper motors are permanent magnetic motors that
'step' one increment each time the computer gives its control electronics
one pulse. They don't require position feed back if run within their
limits. When stopped they inherently hold their position.
Servo motors are standard DC or brushless motors
with an encoder feedback loop. The computer reads the position of
the motor and controls the power applied to the motor.
Stepper motors generally are just as accurate as
servos and are simpler and more reliable and maintenance free in
harsh dusty applications. The servomotor's encoder is susceptible
to dirt and vibration causing problems.
Servo's are faster moving point to point and are
better at accelerating very heavy machinery, but their higher maintenance
should be a factor in deciding which to go with. Our stepper motor
system can be just as fast or faster than many servo systems because
of the control's software's algorithms.
Many companies that sell servo controls try to run
steppers down. They don't know how good a properly made stepper
motor system can be! Our stepper systems never loose steps and can
run for days with perfect repeatability . A servo system with dust
on the encoder will loose steps.
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| Motion Characteristics |
Servo Motors |
Stepper Motors |
| High Torque, Low
Speed |
Can be considered
if cost/ complexity is not an issue. |
Continuous duty applications
requiring high torque and low speed. |
| High Torque and
high speed (>2000 rpm) |
Continuous duty applications
requiring high torque and high speed. A DC servomotor can deliver
greater continuous shaft power at high speeds compared to steppers.
High speed up to 12000 rpm is possible. AC servo motors can
handle higher current surges compared to DC servos. You can
get lot stronger AC servo compared to either DC servo or DC
stepper. |
If speeds are less
than 2000 rpm stepper may be economical. Stepper becomes bulky
at high torque. |
| Short, Rapid Repetitive
Moves |
Use servo if you need
high dynamic requirements. |
Stepper will offer
more economic solution when requirements are more modest. |
| Positioning Applications |
Servo can handle effectively
when load is mostly inertia instead of friction. The ability
to overdrive servo motor in intermittent duty allows a smaller
motor to be used. If positioning is critical in micron level
use servo. |
Use stepper motor
if torque is lower than 500 oz-in, less 2000 rpm, low to medium
acceleration rates. |
| Applications in
Hazardous Environments |
Use brushless servo
motor. |
Use step motor. |
| Low Speed, High
Smoothness |
Use DC servo. |
Use microstepping. |
| Control Method |
Closed loop. |
Preferred to be used
in open loop applications. |
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| Stepper motor selection procedure |
| 1. |
Determining the
drive mechanism component |
First determine certain
features of the design, such as mechanism, rough dimensions,
distance moved, and positioning period |
| Determining
the mechanism and required specifications |
| 2. |
Calculate
the required resolution |
From
the required resolution, determine whether a motor only is to
be used whether a geared motor is to be used. |
| Find
the step angle resolution for the motor |
| 3. |
Determine
the operating pattern |
Find
the acceleration (deceleration) period and operating pulse speed
in order to calculate the acceleration torque. |
| Determine
the operating pattern that fulfills the required specifications
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| 4. |
Calculate
the required torque |
Calculate
the load torque and acceleration torque and the required torque
demanded by the motor. |
Calculate
load torque
Calculate acceleration torque
Calculate required torque |
| 5. |
Select
motor |
Select
a motor whose speed -torque characteristics satisfy the requirement. |
Make
a provisional selection of a motor based on required torque
Determine the motor to be used from the speed-torque characteristics |
| 6. |
Check
the selected motor |
Check
the acceleration/ deceleration rate and inertia ratio in order
to determine the suitability of the selection. |
| Confirm
the acceleration/ deceleration rate and inertia ratio |
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| Where are stepper motor being
used? |
| Stepper motors can be found almost anywhere. Most
of us use them everyday without even realizing it. For instance, steppers
power "analog" wristwatches (which are actually digital),
disc drives, printers, robots, cash points, machine tools, CD players,
profile cutters, plotters and much much more. Unlike other electric
motors they do not simply rotate smoothly when switched on. Every
revolution is divided into a number of steps (typically 200) and the
motor must be sent a separate signal for each step. It can only take
one step at a time and each step is the same size, thus step motors
may be considered a digital device. See below for more applications: |
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| Sample stepper motor applications |
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| Sample servo motor applications
for Semiconductor related industries |
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| Forming Machines |
Servo-Press Machines |
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| Sample servo motor applications for
Biotechnology/ Pharmaceutical related industries |
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| Macroarray Staining Machines |
Macroarray Staining Machines |
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| AC Servo Motor Application |
| Labeling Machine |
| Application Description: |
| Bottles on a conveyor run through a labeling
mechanism that applies a label to the bottle. The spacing
of the bottles on the conveyor can slow down, speed up,
or stop at any time. |
| Machine Requirements: |
- Synchronization to conveyor label
to bottle in motion
- Allow for variable conveyor speed
- Allow for inconsistent distance between
bottles
- Pull label web through dispenser
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| Motion Control Requirement: |
- Synchronization to conveyor axis
- Electronic gearbox function
- Registration control
- Open-loop stepper if possible
- High torque to overcome high friction
- High resolution
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| Application Solution: |
| A motion controller that can accept input
from an encoder mounted to the conveyor and reference
all of the speeds and distances of the label roll to the
encoder is required for this application. A servo system
is also required to provide the torque and speed to overcome
the friction of the dispensing head and the inertia of
the large roll of labels. A photosensor connected to a
programmable input on the controller monitors the bottles'
positions on the conveyor. The controller commands the
label motor to accelerate to line speed by the time the
first edge of the label contacts the bottle. The label
motor moves at line speed until the complete label is
applied, and then decelerates to a stop and waits for
the next bottle. |
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| Application Type:
Following Motion:
Linear |
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| AC Servo Motor Application |
| Drive Train (Complete Synchronization) |
In distributed,
multi-axis applications that require electronic gearing and
cam profiles, synchronous, high speed updates of position commands
are essential. NetMotion offers systems which will ensure complete
synchronization between the controllers and all salves at a
high communication rate. Our products offers a set of drivetrain
objects that can be used to electronically link axes together
in geared or mapped relationships. |
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| Sample linear and rotary automation
module |
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NetMotion provides
solutions for your motion control designs. We carry linear and
rotary automation modules that can be found in all phases of
manufacturing, assembly, and handling processes. We offer a
wide range of solutions from compact linear modules to rotary
modules with the capability of withstanding very high axial
and radial loads. Call us at 1-800-790-7837 or e-mail at sales@netmotion.com |
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