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Tuning a Motion Controller for Different Dynamics Forward and Backward

Primary Software: Driver Software>>NI-Motion/FlexMotion
Primary Software Version: 7.0
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Motion Control>>Controllers>>PCI-7340

Problem:
I have a closed-loop system that is driven by a 7340 motion controller. The load moves in the vertical direction. Due to the effects of gravity, I have different loading parameters when traveling downward opposed to traveling upward. How can I tune my motion control card to compensate for the varying dynamics when going forward versus backward?

Solution:
To maintain a stable PID control loop, you can configure a torque offset value which applies an open-loop compensation for systems where the load in one direction is not equal to the load in the opposite direction. Different torque offsets can be loaded for the primary and secondary DAC.

The torque offset can be configured in Measurement & Automation Explorer by selecting  Devices and Interfaces » NI Motion Devices » PCI-73xx » Settings » Axis x » Control Loop Settings and select the Torque Settings tab. 

The torque offset is specifically determined for a system based on the voltage/current conversion (specified by the drive) and the current/torque conversion (specificed by the motor).  Once the torque offset is calculated, you can then manually tune the motor as described in the related link provided below.  




For 7350 Motion Contoller:
This issue can be also resolved by configuring two sets of PID gains (one for each direction).  The 7350 is the only family of motion controllers that provides this feature.  You can use the Switch Feedback parameter in the Load Advanced Control Parameter VI to configure a second set of PID parameters.   This VI determines when the controller will transition from its primary set of PID parameters to the secondary set based on a feedback condition.  This transition takes place in the control loop, therefore no latency occurs between transition time.




Related Links: Developer Zone Tutorialy: Understanding Servo Tune


Attachments:


torque.JPG

Report Date: 05/17/2007
Last Updated: 06/13/2007
Document ID: 49GELRY3

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