How can I increase velocity setpoint precision, and obtain average motor current using NI 9505

Question

Hello,
I have an application that is roughly based off of the example found
in ...\examples\CompactRIO\Module Specific\NI 9505\Velocity Control
(closed loop)\Velocity Control (closed loop) - NI 9505.lvproj
The "niFPGA Discrete PID.vi" that that this example uses takes an
INT16 in for the velocity set point.  The issue that I am having is that
in our mechanical setup, the difference between setpoint increments
equates to a large changes in mechanical velocity i.e. a setpoint of
1 equates to about 4mm/min of movement to the piece attached to the
screw, and a setpoint of 2 equates to about 11 mm/min.
I would like to have finer control over the motor velocity such that I
can obtain speeds in between integer setpoints.  Any suggestions?
Also, does anyone have any suggestions on how the motor current sense
output of the 9505 can be used to calculate an average current value
(the amount you'd expect to see if you put an ammeter in series with the
motor) and not the current during the "on" phase of the PWM cycle?
Thanks

Answer 1

Burt S,
Thanks for the tip about putting inductors in series with the motors, I ordered some 100uH, 330uH, 470uH and 680uH power inductors (220's were out of stock) and have been doing some experimentation. 
Do you have any suggestions as to how much inductance I should look to add?  Should it be the minimum that gets the inductance to meet the 9505 spec, or are there any ratios that should be adhered to?
I'm seeing some promising results with regards to the 9505 current measurement on the motor that is 0.119mH when I put a 470uH in series with it, however I'm now having some issues with this motor occasionally getting "disabled by drive" when I'm switching velocity between 0 and say 50% of max, and getting its velocity to go all the way to 0 when I command that.
There is another motor in this system that is also undersized at 265uH, and even when I put as little as 100uH in series with this one, I begin having issues with it being "disabled by drive" particularly when changing velocities.
Any ideas what may be causing this behavior?  Could it be the velocity loop PID gains? - if so any general advice on how the to adjust those for the new inductance.
BTW - these are the same motors for that drill project (that you also commented on), figured I'd keep the electrical stuff in this thread.
It's still early on in my experimentation with these inductors, but I also made another observation that when I add the inductors to both of those motors, the signal coming from my load cell gets quieter when both of the motors are on, thou the load cell exhibits a larger offset.  I'm guessing this is an EMI issue, does anyone have any ideas what may be going on?
Thank you for your assistance.
-MKHokie