Dual Loop Feedback via an encoder and a tachometer

[Ron Liou]

I am building a human rotary chair for research purposes and would like to use a dual loop feedback to eliminate backslash. The current system, however, includes only 1 encoder on the load (chair) and 1 tachometer on the motor and due to the geometry of the system, I cannot put an extra encoder on the motor. I would like to know if there is a NI motion controller which can take a encoder signal and a tachometer signal (velocity - analogue signal) to close the dual loop feedback.

Please refer to the attachment for the block diagram of the system.

Thanks for any help.

[The_Chancenator · Accepted solution]

Hi Ron,

Based on your diagram, I don't see a way to do this--are you wanting to do all of the control without pulling the values into LabVIEW?  If so, then I don't see a way to do this.  If you are pulling the tach values into LabVIEW, you do have more options, but none are as comprehensive as having a second encoder.

All of the PCI motion cards do offer analog inputs, so you could potentially read in the tachometer signal.  However, what type of signal is it returning?  If its RPMs, there would be quite a bit of calculation to actually determine the velocity, and I worry with this much legwork in software, you'll kill any 'control loop rates' that you might have.  I don't believe that the system would be as responsive if you have to pull these values into software; and unfortunately there isn't' currently a motion card that reads these signals directly.

I hope this helps,

Marti

[Ron Liou]

Hi Marti,

Thanks for your reply. I think what we will do is to skip the secondary feedback for the moment and see if the backslash is severe. We will then decide if the second encoder is essential.

Thank you very much for your help once again.

Ron Liou