Configuring the Analog Output of PCI-7358 Motion Controller

[ggenc]

Hi everybody,

I'm working with the PCI-7358 for a period of time and now I need to configure the analog output of the controller for a specific purpose. All I need to do is turning on and off the analog output to a DC voltage level of 5V. I'm using a UMI-7774 as an breakthrough circuit. I connected the MOTION I/O of axis 5-8 to the UMI and I hope to get the voltage out of 1st pin of CONTROL block of the UMI. I'm using Load DAC.vi to turn on and off this pin on UMI. When I first tried the voltage level was 1.5VDC and it was fluctuating so I tried to read the AC voltage on the pin and it was 3.2Vac. It was said in the help of this Load DAC.vi that the offset values or the torque limits set before doesn't affect the voltage level. I can't figure out what is limiting the output voltage level. (I tried both 32000 and -32000 for the input of this .vi)

If someone can help urgently, I will be grateful...

Gencer Genc 

[Jochen]

 Hi Gencer,
 
the most important thing that needs to be considered when using the DACs of a 73xx board directly can be found in the documentation for "Load DAC.flx":
 
"The Load DAC VI is used to send a value directly to an unmapped DAC resource. DACs not mapped as servo axis outputs are available for general-purpose analog output applications.
    
Caution  You must not execute this VI on a DAC mapped to an axis. Doing so causes the DAC output to glitch momentarily before returning to axis control."

The easiest way to unmap an DAC from an axis is to configure the axis as Open Loop Stepper.

I hope this helps,

Jochen Klier

National Instruments

[ggenc]

Thank you Jochen for your instant reply. Time is very important for me now. I can't understand what you mean by configuring the axis as Open Loop Stepper. I tried configuring the axis as Stepper using the MAX interface and selecting the Stepper option in Axis Configuration tab menu but I didn't change the Feedback menu so selecting NONE as Feedback should assign my axis as Open Loop Stepper. And how can I do the same configurations using the Config Axis Resources.vi in LabVIEW? Do I have to set primary and the secondary feedback options of this .vi to None and primary and the secondary outputs to Stepper?

Thanks again four your interest and hoping to get some answers as quickly as possible...

Gencer Genc

[Jochen · Accepted solution]

Hi Gencer,

in MAX you should configure the axis as stepper. You don't have to care about the feedback type. You can leave this setting on "encoder". You can configure the Stepper Loop Mode in the Stepper Settings tab to Open-Loop. But in fact for your purpose, it doesn't matter, if you configure the axis in Open Loop mode or in Closed Loop mode. Any stepper mode will unmap the DAC from the axis.

In LabVIEW it is enough to use "Configure Axis Resources" and map the primary output of your axis to a stepper output or "None". This will also unmap the DAC from the axis. The secondary output should be also mapped to "None".

I hope that helps,

Jochen

[matthewg]

Hello,

I've got a PXI-7358 motion card and successfully performed the procedure described above to unmap the DAC from Axis 4.

Is it possible to then use the analog feedback (AIN4) for that axis as a general-purpose analog input, while the AOUT is being used as a general-purpose analog output? I.e., is it possible to use one axis of the 7358 as a general-purpose analog input and analog output?

Thanks,

Matthew

[Jochen]

Hi Matthew,

as long as you don't use the ADCs of the 7358 as a feedback device, you always can use them as general purpose inputs. Please note, that the feature set of these inputs is not comparable to the rich feature set of a dedicated NI DAQ device. Compared to a DAQ device the 7358 analog inputs provide only non-deterministic and slow software timing, no triggering, no selectable gain factors and limited accuracy.

Within these limitations you can use the 7358's ADCs as general purpose analog inputs but for higher demands in terms of accuracy and performance a dedicated DAQ device is highly recommended.

Regards,

Jochen