Open loop control

[Greece]

I want to use a PXI 7344 card as an open loop servo controller.
In fact I just want to send a specific Analog Output profile through the AO port and read the position through the encoder. Any simple idea?

[Dan_Quinn]

Hi Greece,

Thanks for using our discussion forums! Servo control with a 7344 can actually be pretty simple to put together. I'm confused when you mention open loop though, as servos require feedback in order to operate properly. You may have heard of PID control, which is a control system used by our motion boards to move a motor to a given position. Without an encoder on a servo, you are most likely going to run out of control.

The way you can get this going is by installing youor board and the NI-Motion driver from either the CD you may have or for free from our website's Drivers and Updates section. The most current version of the driver is 6.1.5. Once that is installed, open up Measurement & Automation Explorer (MAX) and configure and initialize the axis you want to control. Run a servo tune in MAX which can automatically determine your PID parameters and then you should be good to go with testing out some simple moves in the 1-D interactive section.

Once you have enerything configured in MAX, check out some of the LabVIEW shipping examples and build your program! Let us know if there's anything else you need.

Regards

Dan
National Instruments

[Greece]

Hi Dan,
thank you for replying.

I am actaully aware of what a PID is but still, I don't want to use this kind of control for my aplication. I just want to send a special voltage profile from an AO of the motion control card, and read the velocity of my actuator with an encoder.

The card and appropriate drivers are already installed on my PXI and host PC and everything works perfect.

I know that what I want to do could be done efficiently with a simple DAQ card as well. But maybe later I could use the PID control feature for comparison to my results. So this is the reason I use the motion controller.

What I want to do now is to decorrelate the AO of e.g. Axis 1 from the PID loop, and use it as a simple analogue output without the neeed of a feedback (encoder or analogue).
Do you think this could be done from the MAX? And then would it be o.k. if I use the simple vi's of the example folder (concerning motion control)?

Once again thanks for your response,
George.
Greece

[Jochen]

Greece,

the 7344 can't be used properly for generating arbitrary analog waveforms. Reason: The 7344 doesn't use DMA for board-host-communication so you would only be able to update the analog output based on software timed singel point update operations and these are slow and undeterministic. That means there is a high level of uncertainty how long it takes to update the AOs.

Using the 7344 like this is NOT recommended. If you want to generate analog output waveforms a DAQ device can do a much better job because data is transferred via DMA from the host to the board and the update timing is done by a hardware clock on the board.

Best regards,

Jochen
National Instruments Germany