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Flux Vector VFD with feedback; positioning?


May 8, 2003
Buckeye, AZ USA
Can a flux vector VFD with an encoder feedback loop be used to index the motor at the same position when it's stopped as well as for speed control? The application would be for spindle orientation for a tool changer.:scratchchin: If not, any ideas on how it could be adapted?
A vector drive approximates encoder feedback using mathematical algorithms and, often, an analysis of the "back-emf" generated by the motor under load. This approximation is valid until ALMOST zero rpm.

Encoder feedback, which is an option of many vector drives, usually requires an add-on board which interfaces the inverter and the motor/tachometer.

Once the add-on board and the encoder have been properly and completely installed, the drive assumes the function of a true feedback-controlled drive, down to zero rpm.
....but when all is said and done, the structure of the motor as well as the encoder, determines just how accurately it can 'park'.

IOW - if it was really, really good, servomotors and their controls might be less prevalent.

Haven't a clue if it would be 'good enough' for the intended app, (we don't even know 'which' motor or controller or encoder) .... but should be cheap enough to try it instead of going straight to servo.


I think people underestimate what you can do with an ACVector motor . . . we run them in over 75% of our applications up to 200HP using a standard AC Induction motor and adding a quadrature or sin/cos encoder. Nearly all motor manufacturers now have options to put encoders on the non drive end.

The only difference between a brushless permanent magnet servo motor and an AC Induction motor with respect to positioning performance is inertia.

I am running old baldor motors with Nortstar sandwich encoders on my planer mill X and Y axes and can easily hold +/- 0.001 inches.

Typical drives we have done this with include Control Techniques UniDrives, Siemens S120 Drives, Allen Bradley PF755 drives, Yaskawa/EMS/Magnetek drives with encoder boards, etc.

For speed control - depending on the load, it can be superior to servo motors - this is more a tuning issue than anything else and in this case, inertia is your friend.