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Archived: How does Feedback Work with Closed Loop Contour Moves?

This document has been archived and is no longer updated by National Instruments



Hardware: Motion Control>>Controllers>>PCI-7350, Motion Control>>Controllers>>PCI-7330, Motion Control>>Controllers>>PCI-7340

Problem:
How is feedback used on a NI-Motion controller in closed loop stepper mode performing a standard move or a contour move?

Solution:
Closed loop steppers work differently from closed loop servos. Instead of adjusting the output on each PID iteration like a servo system, closed loop steppers will do a pull-in move at the end of a move to adjust for any difference between the target position and the encoder feedback. By default, it will attempt the pull-in move three times.

When performing a contour move, the adjustment is only made at the final point of the contour move. The feedback will not be used at each buffer point along the way. If the adjustment were made at each point, it's possible that the time interval would not remain constant, which could cause jerky motion.

Related Links:
KnowledgeBase 27QARKOA: Stepper Motors Work in Open Loop Mode, But Do Not in Closed Loop with Encoder Feedback
KnowledgeBase 3DEA4HB1: What is the Minimum Time Interval for a Contour Move?
Motion Control Support Homepage

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Report Date: 03/04/2005
Last Updated: 04/23/2012
Document ID: 3J3D7FKZ

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