Position Control w. PLC and VFD


Thread Starter


Good day everyone. For a project i'm attempting to control the position of a cart driven by an AC-drive.

I'd like a PLC to generate position and velocity setpoints, and use this as a reference in a cascaded PID-loop. I'm assuming that I can use the autotune option of the VFD/ AC-drive to enable sensorless vector, and use this vector control for the inner velocity loop. The outer loop will then assign a reference velocity to the inner loop, based on linear encoder data and the earlier generated motion profiles. Would this solution be feasible?

Thanks in advance!

Bob Peterson

Instead of reinventing the wheel, why not just buy a VFD that has this built in. If you are using AB Compactlogix or Controllogix the 527 series of VFDs does precise motion control very well with regular AC motors that have an encoder.

Other manufacturers have similar capabilities.

Depending on the level of precision required it is not real hard to generate a trapezoidal profile in the PLC using Newtons laws of linear motion and just figure out how long to enable the VFD to go whatever distance you want it to go. It is just high school algebra and physics. This works remarkably well even in V/Hz mode. Sometimes I had to have a second move to get it within whatever level of accuracy is required. I never tried it in vector mode but it probably would have worked better than in V/Hz mode that worked quite well.