Stacked PID control

[Martin.D]

Hi,

I have 12 axis most of are brushless servo stages. We decided to use a single NI-control board axis per axis and have the drive units provide the commutation signals. That's generally fine but now I discover that the drive units control loops can not be completely disabled. We have a control loop in the drive units that is addition to the control loop that is on the NI-controller. To reduce the effects of the drive control loop we have reduced the parameters as far as possible (proportional gain of 10% and velocity of 8% etc). But I'm concerned that this is going to give me problems later on (especially with the nano precision control stages).

So an alternative (last resort) may be I could effectively turn off the NI-controllers PID control. Has anyone any ideas on what control parameters would be required for this?

Also does anyone have experience of a good drive for this type of application for my future projects?

Martin

[Jochen]

Martin,

please refer to this discussion forum thread.

Best regards,

Jochen Klier
National Instruments Germany

[Rodj]

Martin,

What is the make and model of the drive you are using? Can you set this drive into torque, speed, or position mode?

If you can set the drive into torque or speed mode, you can control the drive using the analog command outputs of the NI motion control boards. Torque mode should disable the control loop on the drive. Speed mode will interpret our command signal as a velocity set point. (See the link posted by Jochen)

If you can set the drive into position mode, you can use the P-Command mode offered by the NI-733x and NI-735x motion controllers. This special mode uses the step/direction lines to connect to the drive. The drive counts the steps to generate its set point for the control loop. The NI-Motion 6.1.5 update includes the P-Command mode for stepper output. See the ReadMe.txt file for more information.

Rodger S.

[Martin.D]

Thank's for the very fast response.

So it seems that it's OK to cascade the control loops - well time will tell for sure. I hope this works for me because there seems to be no other option other than hardware change (no time for this now).

I'm using Parker brushless servo stages and Compax3 single axis drives "C3 S063 V2 F12 I10 T10 M00" (F12 = feedback for direct drives).

Thank you / Martin

[Martin.D]

Thanks Rodger,

I have just established that we can use any of the modes you mention (Velocity control +/-10V, Torque Control +/-10V and Position control using step and direction or encoder following), but what works best with the NI kit?

Martin

[Rodj]

Martin,

All three modes work and I know of customers using each of them. If your drives/motors have been specifically matched and tuned for you by Parker, then using the Position Mode works well. If not, then I like torque mode because the NI-Motion controller is running the control loop and I can leverage the tuning utilities in MAX.

Rodger S.

[Jochen]

Martin,

I have also worked successfully with drives running in velocity mode but as Rodger mentions, torque mode should be the easiest way from the tuning point of view.

Regards,

Jochen

[Martin.D]

Thanks all,

I'll experiment with position mode and torque mode (starting in about 5 mins). I'll let you know how it goes.

Thanks again.

Martin

[Martin.D]

Hi,

How can I use position mode? do I need to set the axis as stepper even though I have a servo? Or where is this documented, I have looked in the readme file, manuals etc.

Martin

[Jochen]

I can't imagine how you could use position mode. Typically if run in this mode drives accept the position commands via serial protocols like CAN, Profibus etc. I don't think it's a good idea to map the whole travelling range to the +/- 10 V output range of the motion controller and I can't imagine that it is supposed to work like this. This wouldn't result in a good position resolution (DAC resolution of the PCI-7344 is 16 bit).

From my point of view position mode is the only mode I can't recommend. Maybe Rodger knows something else?

Jochen