Positioning Application

[Old No. 7]

We have a project with a couple different fairly simple positioning tasks. Speeds are fairly slow and positioning doesn't need to be super accurate.

One is a mechanism raised and lowered by a ball screw and gearbox. The mechanism needs to be set to multiple positions. The second is a rotating table driven by a gearbox. The only position we need here is the "home" position so the loading mechanism can line up properly with the table.

A couple options we've looked at:

1. Servo - seems like overkill
2. PF527 w/ absolute encoder
3. PF525 w/ absolute encoder
4. PF525 w/ incremental encoder and home switch

#2 seems like maybe the best option, although the one we're least familiar with. Can/should the encoder be wired through the VFD? Or does it need to go to the PLC (Compactlogix)?

Any advantage to #3 or #4?

 

[Ken Roach]

In each of those the encoder would be mounted to the motor tailshaft and wired to the VFD itself. Neither PowerFlex 520 series drive supports a load-mounted encoder.

The positioning in the PowerFlex 525 is basically dead reckoning: take how far you have to go, divide by the target speed, half that for the accel/decel stages = how long to run. Maybe the integral is a little more exact for S-curves, but that's the basic idea. Think of it as a "stop at target" instead of a positioner, which is fine for a very simple indexing table.

It's more sophisticated on the PowerFlex 527: it's truly a position axis in the Logix controller. You need at a minimum one of the CompactLogix with an "-M" part number for CIP Motion.

The first application challenge that comes to mind from your description is the vertical axis. Will there be a shaft or rail brake as well as a brake on the motor, to hold the vertical load ?

Rockwell has a decent application technique document about using Kinetix servo drives with vertical loads:

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/motion-at003_-en-p.pdf

Torque proving for vertical loads is outside of the abilities of those PowerFlex 520-series drives. They just aren't made for crane/hoist applications like the 750-series are.

 

[janner_10]

We have used loads of these TSM Steppers for basic position / repeatability projects.

Pretty decent for basic stuff and in whatever protocol you want.

https://www.applied-motion.com/

 

[Old No. 7]

Thanks Ken. The vertical application isn't a hoist. Just a lift on a table raising 12" or so. The gearmotor would have a brake and I'm pretty sure the jack screws won't back out anyway.

"Positioning" may be overstating what we need a bit. I was initially thinking of just using the encoder more like a limit switch where we raise the lift until we get to x counts then stop. We're traveling at something on the order of 10 inches per minute and only need to get within maybe 1/8" of an inch. We've done some similar in a horizontal application where we just used an incremental encoder wired to the PLC with a PF525 set to coast to stop and a brake on the gearmotor.