Linear encoders significantly improve the accuracy and repeatability in stepper or servo-motor positioning stages based on lead or ballscrew drives – they remove drivetrain based play, backlash, and screw pitch errors. But even the best linear encoders are not perfectly linear.
As long as these linearity errors are repeatable, there is a way to reduce them significantly.
What is required is a laser interferometer to measure the errors and a motion controller with the capability to store a dynamic error map.
After the map is stored and enabled, each time when a positioning command is issued, the controller dynamically looks up the new position from the error map correcting the motion profile on the fly.
The longer the travel range, the more effective the error mapping becomes – as shown below with a large LS-270 1 meter travel linear positioning stage.
Positioning accuracy of LS-270 linear positioning stage with 1m travel range, stepper motor / ballscrew drive and linear encoder for position feedback, without error correction.
The LS-270 linear positioning stage is available with travel ranges to 1m (40”). It is driven by servo or stepper motors and high-resolution ball screw, a linear scale for high precision position feedback is optional. A similar stage with linear motor direct drive is also available as the LMS-270. The Hydra TT vector motion controller (right) provides 3000 micro-steps per step (see http://www.pi-usa.us/blog/performance-resolution-and-velocity-control-of-stepper-motor-driven-precision-positioners/ for more details) for very smooth and quiet motion.
Its error mapping capabilities are seen in the graph below.
Same LS-270 stage as tested above, with error mapping. The linearity error is reduced by a factor of 10. Three bi-directional runs show the repeatability of the error mapping.
LS-270 Distance vs. Time plot. The high resolution Hydra stepper motor controller can provide very smooth and constant velocity, even at low speeds in the 10’s of nanometers per second. Measured with laser interferometer.
LS-270 linear stage, distance against time. Consecutive 100nm steps, measured with laser interferometer.
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