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Add Input and Output Scale Factors To Motion Control (NI Motion)

Currently, NI Motion does not allow users to set the proportional gain (or any gain) less than one probably due to the fixed point math on the motion control cards. Many systems, especially those using ultra-high resolution encoders as feedback, are unstable with gains greater than one. Also, any system approaching the need to use Kp close to one will be extremely sensitive to the tuning process and will likely be difficult to optimize. An extremely simple solution to this problem is to add the ability to use Input and Output scale factors to the PID loop. Specifically, scaling the error term input to the PID controller for the input scale factor and the commanded output to the D/A or PWM's for the output scale factors. Even the ability to scale by factors of two would solve most issues. If only one scale factor could be implemented the output scale factor would be more useful as the input scale factor will effectively decrease the feedback resolution on a fixed point system. Other modern controllers utilize this strategy.

 

A quick search of the forums finds several people with this problem.

 

http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/tuning-servo-system/m-p/139507/highlight/tru...

http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/Is-there-any-way-to-drcease-the-open-loop-ga...

http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/MAX-Reducing-Encoder-Resolution/m-p/1030934/...

http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/7344-PID-proportional-gain-1-but-actuator-st...

 

Thanks!