|Accuracy considerations in the linear stepper motors|
Linear stepper motors can operate without feedback in open loop which make them a very cost effective direct drive linear actuator.
While some applications require absolute positioning accuracy, others concerned with high degree of repeatability.
Repeatability is defined by linear actuator ability to return to the same point in the same direction.
For example, in "teach" mode linear stepper motor was "taught" a number of predefined positions along the platen. In automatic mode, while making sequence of moves, user will be concerned about forcer returning to same "taught" points repeateably.
In accuracy applications linear motor can be compared to a ruler. For example, in cutting, the user wants to cut piece of material with some tolerance. Accuracy of linear motor positioning will define the tolerance.
Accuracy components in open loop linear stepper applications:
- Cyclic error due to the motor magnetics. Occurs once every pole pitch. Repeatable from pitch to pitch. Can be compensated by mapping
- Unidirectional repeatability measured by repeatable moves to the same point from different distances in the same direction
- Hysteresis is "magnetic backlash" in a motor when changing direction, due to magnetic non-linearity
- Cumulative platen error is linear error of the platen. Can be compensated by correcting for error slope
- Random platen errors is a non-linear errors remaining in the platen when linear error is compensated
- Thermal expansion error caused by a change in temperature expanding or contracting the platen
In closed loop linear stepper motor application accuracy is defined by the accuracy of the linear feedback device and system installation.
When applied with feedback (encoders) linear stepper motors can be used in three modes of operation (in order of increased complexity and cost):
- open loop with stall detection. Monitoring encoder position allows to detect motor stall.
- "pseudo closed loop" with position correction. Motor moves in open loop but system tracks motor position and performs post-move fine correction to position the motor at desired actual position.
- closed loop Linear ServoStep mode when system monitors encoder and adjusts motor position in real time. No post motion correction is required. System automatically adjusts for any disturbances motor may encounter.
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