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Velocity Filter Settings in NI-Motion

Hardware: Motion Control

Problem:
I am using the NI-Motion driver, and I would like to filter the velocity from my encoder feedback. What do the velocity filter settings mean, and how can they be set?

Solution:
The velocity filter settings determine how often velocity is computed based on encoder feedback from a motion system. Velocity filtering is used to reduce error caused by noise, jittery systems, and feedback quantization.

The velocity filter settings consist of two parameters: Filter Time and Filter Distance. The values of these parameters allow NI-Motion to compute velocity in one of two ways: by averaging the number of encoder counts within a specific time window, or by averaging the time expired within a specific number of encoder counts. For example, if the default settings of 10 ms Filter Time and 100 counts Filter Distance are used, then one of the two following cases would occur:

  • Case 1: Fewer than 100 encoder counts are produced within 10 ms: Filter Time is used
    If 85 encoder counts are produced in 10 ms, the calculated velocity is:
    (number of counts/Filter Time) = (85 counts/.01 s) = 8500 counts/s


  • Case 2: 100 encoder counts are produced in less than 10 ms: Filter Distance is used
    If 100 encoder counts are produced in 8 ms, the calculated velocity is:
    (Filter Distance/elapsed time) = (100 counts/.008 s) = 12,500 counts/s

The velocity filter settings can be specified in Measurement & Automation Explorer (MAX), or they can be set programmatically in LabVIEW.

Velocity Filter Settings in MAX:
The velocity filter settings can be specified for a given axis of an NI-Motion controller on the Trajectory Settings Page.



Velocity Filter Settings in LabVIEW:
The velocity filter settings can be specified with Load Velocity Filter Parameter.flx in LabVIEW. It is found on the functions palette by going to Vision and Motion» NI-Motion»Axis and Resource Configuration.


Related Links:
KnowledgeBase 466HEDS4: Configure and Test a Stepper Motorz

Attachments:


velocity filter settings MAX.JPGvelocity filter settings LV.JPG - velocity filter settings LV.JPG

Report Date: 11/29/2006
Last Updated: 04/24/2009
Document ID: 43SEFNMS

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