Cannot tune a PID servo with PXI 7344 card.

[Greece]

Hi all!
I have a 7344 Motion controller installed on a PXI chassis, running real time. I want to control an axis with servo motor (acually it is a hydraulic motor controled by a servo valve. I act on the servovalve through an amplifier).
The card and the UMI are configured right, I can read appropriately the feedback from the encoder, but I cannot tune the PID controller. When I try using the Auto Tune toolkit from the MAX, the drive runs unepxectably at the highest velocity (The controller outputs 10V!! -with the default gains set on the controller).
Manually I used the following settings for the PID: Kp=1, Kd=0, Ki=0. Still, there is no significant change in the reaction. Whenever I enable the axis it runs in a completely umpredictable manner. I have to stop the motion by disableing the axis.
Any idea what could be wrong??
Thanks.

[Jochen]

Please have a look at this document in the NI knowledge base. Make also sure that your encoder is connected correctly (e. g. if phase A and B are swapped you will also see erratical movement).

If the hints in the kb document and the associated links don't help please provide more information about the drive you are using.

Best regards,

Jochen Klier
National Instruments Germany

[AnalogKid2DigitalMan]

Is the +/-10V control signal to the drive wired with correct polarity?

[Greece]

The control signal is conected with the right polarity. Nevertheless, I tried turning the polarity.
The result was that the axis didn't actually move: I used the Step Response window form the MAX. The output voltage was at maximum 20 mVolts, and 2-3 times the axis moved slightly. I tried changing the PID settings but not significant result did I get.
When I used the Auto tuning method, no motion at all was observed.

PS: The attached file is the manual of the amplifier of the servovalve. My signal goes to "Command Position 1" from the circuit diagram shown in page 3 of the manual. (In my project I don't mess at all with the internal PD control of the servoamplifier).

Thanks

[Jochen]

If I understand the datasheet correctly the voltage at the command input of your drive will be proportional to the spool position of your valve.
For a test try to run the system in open loop mode by simply changing the output voltage of the 7344. Here you can find an example how to do that. Please make sure that the axis is configured for stepper mode or that it is disabled. Otherwise the PID loop of the 7344 will override your voltage settings.

If your motor doesn't move with this open loop control use a DMM to measure the output voltage of the 7344. If the DAC is working correctly there is probably a technical problem with the drive.

Jochen

[Greece]

Well I think my problem is solved: I just turned over the two wires "Index A" and "Index B" of the encoder signal and the PID Auto-tunning program from MAX worked decently.
To be honest I cannot understand why this wouldn't happen when I used the reverse polarity from the AO. In any case, the drive is working OK now.

Thank you all for replying,
George

[Jochen]

Probably the command input of your drive is not floating but ground referenced (the same is true for the 7344). Thus reversing the cabling of the command signal will result in shorting the command signal to ground.

Jochen