Terry Neilson wrote: > Jon > Good to see your upgrade on a Bridgeport, you must have learned a lot while > doing it. I have used these Proto Trak gage wheel type encoders in the shop. > They have wipers and they work surprisingly well. I do not want to include > them on an upgrade though. > I'd never trust them to stay aligned. I sometimes home my machine and then touch off with an edge finder, and work all day without checking the alignment again. > I would be curious if I could replace them with linear scales and integrate > them into an upgrade instead of rotary encoders. I would think that it would > be more accurate due to direct feedback of actual table position. Yes, this is possible. There are a couple caveats. One is that you can easily get a lot finer position resolution with a shaft encoder. For instance, I have 1000 cycle/rev encoders directly on 5 TPI ballscrews. So, the encoders provide 4000 quadrature counts/rev * 5 TPI = 20000 counts/inch, or 0.00005" (50 micro-inch) per count. A typical old-style glass scale is 0.0005", or ten times coarser resolution. Actually, anything made after 1975 or so is really metric, so approximately .0004" = 10 um. You can now get finer resolution scales, but they will cost more.
The other caveat is if there is any backlash in the ballscrew, you will get servo hunting behavior. The servo motor will endlessly hunt back and forth across the backlash distance so it can nudge the table and encoder over one encoder count. You can suppress this so some extent with the deadband setting. So, if your backlash is under .001", it may work out OK. If your backlash is several thousandths of an inch, I think you will be unhappy with the way the machine works. You can suppress the effect of the backlash when settling, but you can never suppress it while the machine is moving. It effectively causes the servo loop to be momentarily in an "open loop" condition every time the motor torque is reversed, and that will make it very difficult to tune the servo response. So, unless your ballscrews are very tight or adjustable, you may find this to be unworkable. What I'm trying to convey here is that anything that introduces a sharp discontinuity in the control loop at any point can make the entire system unstable. But, if your ballscrews are in prime condition, then go for it! EMC certainly can handle linear encoder feedback, and your velocity servo amps will handle the velocity control part of the job. Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by Sprint What will you do first with EVO, the first 4G phone? Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
