Linear Stepper Motor – How They Work
Linear stepper motor is essentially rotary stepper motor “unwrapped” to operate in straight line. Motor operates on electromagnetic principle and consists of moving “forcer” and stationary platen. The platen is passive toothed steel bar (stainless is available) extending over desired length of travel. Forcer incorporates electromagnetic modules and bearings and moves bi-directionally along the platen.
linear stepper motors are available with either mechanical roller bearing or air bearings.
Side and bottom mechanical bearings are built into forcer and do not require any adjustments by the user over the lifetime of the motor. They are permanently lubricated and exhibit very little friction.
Air bearing operates by floating the forcer on high pressure air introduced through orifices in the forcer. Air bearing motors can operate continuously at high speed without wear. Air bearing permit smaller air gap resulting in larger motor forces.
Linear stepper motors are micro-stepped by proportioning currents in two phases of the forcer, much same as in rotary stepper motors. When micro-stepper linear stepper motors following
benefits are achieved:
– higher resolution for positioning
– smoothness at slow speeds
– wider speed range
Accuracy considerations in 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 the forcer ability to return to the same point in the same direction.
Liner stepper motor use powerful permanent magnets ( NdFeB magnets ) in the rotor.
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 Servo Step 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.
Linear Steppers and High Lead Ball Screws (accuracy comparison)
High lead ball screws are compared to linear stepper motors. Even though small lead precision ground screws may be somewhat more accurate that high lead screws, they significantly limit linear speed. Ball screw is assumed to be driven by two phase rotary stepper motor with micro-stepper drive to achieve high precision. Thermal expansion errors, equivalent for both systems, were not considered in this analysis.