PCI-7342 can't achieve smooth velocity at 1 count/s

[Brian_J]

Hello, my motion setup is a DC brush motor with encoder, linear servo drive, linear regulated DC power supply, and PCI-7342 with UMI breakout board.  In the user manual for the 7342 card, I read that the lower limit of velocity is 1 count/s.  I have used MAX to tune the axis for a nice step response (step size 20 steps) so that velocities greater than approximately 10 counts/s are quite smooth.  At velocities less than 10 count/s however, the motion is quite choppy with backward corrections and a series of rough, discrete step movements. My PID loop rate is set to maximum frequency (88 us).  Does anyone have advice for me on how to achieve smoother velocity profiles at very low speed?  Is 10 count/s simply the practical limit for smooth velocity with the 7342?

[ErikaB]

Hi Brian_J,

I am not aware of any issues with the 7342 in which the functional velocity is different than the range listed in the user manual. This behavior could be stemming from tuning or could be due to something else. Have you tried retuning the device after seeing this behavior? Does that have any effect?

What drive are you using? Does the drive have both a velocity and a torque mode?

Below are some basic guides on tuning servo motors.

Understanding Servo Tune
https://www.ni.com/en/support/documentation/supplemental/12/understanding-servo-tune.html

Hope this gets you moving in the right direction.

[Brian_J]

Hi Erika, the drive is a Trust Automation TA105.  It does have a velocity mode but I haven't tried that.  I only have a position encoder, not a tachometer.  Do you have any literature that might describe when velocity mode may perform better than torque mode?  Also the first two links you sent are broken - could you resend?

[Highland_Controls]

At 1 ct/s, there is no useful feedback for control.  If you need to move that slowly, you will need a higher resolution encoder.