PCI-7352 DAC Output

[singlespeed2]

I'm using the PCI-7352 to drive a Physik Instrument C809 amplifier and M230.25 linear acuator.

I've run into some issues with the speed and following error.  I've finally been able to track down that the DAC seems to be operating around 5V as it's 0 velocity point (based on the output from 1-D interactive in NI-MAX and also in the Read DAC.vi during motions).  Basically, if the voltage is >5 then the actuator extends and likewise if <5V it retracts.  Since the DAC can output +/-10V the motor can retract faster than it can extend (extension speed is much less than the rated velocity; 0.4 mm/sec versus 1.5 mm/sec).  This is very odd as the actuator is rated to go the same speed in both directions, and in fact it does on a completely different controller (Newport ESP300).

So, I have been in contact with Physik Instrument as I don't understand the issue.  Their response at this point is:

"At the input of the C-809 it expects 0V for stand still, +10 V for
maximum positive velocity and -10 V for maximum negative speed. The
customer has to achieve this at the output of his DAC. If the customer
doesn't achieve this voltage levels he has a problem inside of his
software and not a problem of our hardware."

Based on the output from 1-D interactive and the Read DAC.vi there is clearly a discrepancy.  Is it possible that the PCI-7352 is really operating around 0 as the zero velocity point, when 1-D and Read DAC seem to indicate otherwise?

At this point it appears as though I'm going to have to manually see what the DAC is outputting, but without a breakout box I'm afraid I'm going to have to purchase a cable as a donor to slice and dice.

Thanks for any help.

-Derek

[Jochen]

Derek,

that's strange behavior. In a closed loop system the controller's output value is defined by the characteristics of the plant. The Read DAC vi returns only the value that is sent to the DAC, but it doesn't measure the real voltage output. One chance is that there is something wrong with the DAC. E. g. I could think of an extremely high offset of -5 V or something like that. Here is an idea how to get a better understanding what's going on:

1. Disconnect the C809 from the 7352.
2. Configure the board for open loop stepper operation. This will unmap the DAC from the axis.
3. Use Load DAC.vi to write a value directly to the DAC.
4. Measure the voltage at the analog output. Depending on the cable that you are using, this might be a little bit tricky. If you can't do the measurement directly at the cable, you could ask your local NI branch if they could loan you an UMI-7764 and an SH68-C68-S cable.

Please let me know how things are going. As the drive and the motor work fine with another controller, I don't think that the C809 has a problem. Stil I have good contacts to PI, so I could talk to them if required. 

Another question: Have you tried the second axis on the 7352? Do you see the same issue with this axis?

Thanks and kind regards,

Jochen Klier

National Instruments 

[Michelle_N]

You also want to check that you have the DAC output set to 0. You can check this in Measurement and Automation Explorer - take a look at the attached image.

[Jochen]

Good idea Michelle!

I didn't think about about that, but that's definitely an option.

Jochen 

[singlespeed2]

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Michelle_B wrote:
You also want to check that you have the DAC output set to 0. You can check this in Measurement and Automation Explorer - take a look at the attached image.

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Thanks for the idea.  We had tried this awhile back and got very strange results.  Basically that value is defaulted to 0 so we were using it as the default value.  When we were having it run at 5V we then tried an offset of -5V.  Guess what?  The input to the DAC output was -5V (on the 1-D interactive plot).  If I remember correctly we tried -2V for the offset and again the input to the DAC was -2V.  So we put it back to 0V offset, the input to the DAC output was again 5V.  

Definitely a strange issue.  We are working with the NI and PI reps to hopefully bring this to a close.

-Derek

[Michelle_N]

Please update us with your results. It would be interesting to hear what the problem is.

[singlespeed2]

Well, it has been a while but we think that the issue has finally been resolved.  It appears as though it was some sort of either wiring or internal set-up issue in the amplifier.  I have never received a very thorough response and based on the lack of literature/manual I'd rethink this purchase if I had to do this again.

At this point they have helped out and gotten us an amplifier that seems to work as it should (0 = stand still, + = extension, - = retraction).

On to developing our Labview code.  (using force feedback to run a creep test).

Thank you for your help.

-Derek