Amplifier for Torque Control

[Varun Agrawal]

I need to control two motors, one in Torque control mode, and another in torque/position control. They are DC graphite brush motors. Since my requirements regarding position control are low (the motion will be very slow), so I guess my DAQ card, PCI 6221 should be sufficient. Can somebody cofirm this. Also can somebody suggest me a good off the shelf current amplifier for the same purpose.

Thanks,
Varun Agrawal

[VuD]

Hello Varun,

To answer your first question, yes, if you are not concerned about feedback or PID control of your motor and if you are operating at a slow speed then you should be able to use your 6221 to control your DC motor (given that you have enough available lines for output/input).  In regards to a current amplifier, I would suggest contacting your motor manufacturer to find an appropriate amplifier/drive and verify that the DAQ card can provide the suitable signal levels.  I hope this helps.  Please let us know if you would like further clarification or assistance regarding this issue. Best Regards,

Vu D

[Varun Agrawal]

Thanks for the reply Vu. I do have enough lines I guess, since I need to voltage measurements, two analog outputs (for motor current) and two encoder readings (using the two counters on 6221).

 And yes the motion will be slow. But I do need to implement a controller. I mentioned about slow motion becuase I thought that the problem with using the DAQ card is, since the interferace is through Windows/LabView I won't be able to use high frequencies for the control. Is there any other potential problem that I might face using PCI 6221. To add to that I do have access to 7831R FPGA card. Will that be a better option. I know that FPGA card will be way too faster, but I have the feeling that interfacing and programming that will be difficult, and I also heard that it takes a lot of time in compiling.

Regards,
Varun

[VuD]

Hello Varun,

In general due to the non-deterministic behavior of Windows you will never be able to create a reliable motion control system using your DAQ card. It is going to be difficult synchronizing your encoder feedback with your motion controller.  This would be the main concern I would present regarding using this setup.  However, if a delayed response is acceptable in your application, then this should work.  This has also been discussed here. 

You are correct that using your FPGA board will be a more intricate set up.  Here is another thread which relates to this issue.  Jochen makes some great points as well as pointing to a useful document as a starting point for this type of architecture which I have linked below:

Build Your Own Motion Control SystemTechnically, the FPGA board would have a more deterministic control, however, for your particular application the DAQ board would require less involved programming.  The most simple solution programming and performance wise would be to go with one of our Motion Boards which integrates perfectly into LabVIEW and runs the control algorithm onboard in a real-time environment.  I hope this helps.

Regards,

Vu D