Terry Neilson wrote: > Jon, > That was a good explanation of the possible problems that can occur when > using linear scales for position feedback. What I still don't understand > though, is how the knee milling machines in the shop that have linear > scales, continue to run every day with very good accuracy, and no problems > with servo hunting issues. Well, ballscrews can have a very long life. If properly lubed and swarf is kept out, then they can run for decades before developing significant backlash. So, I didn't want to say that only a brand-new $5000 super-precision ballscrew will work. Not at all. But, a lot of machines I hear of being retrofitted do have significant backlash in the ballscrews. > I would think that a rotary encoder reading the > rotary travel of a ball screw would cause a cumulative inaccuracy that > increases with increasing travel of the table. No, certainly NOT cumulative! How could that be? The ballscrew doesn't have a cumulative error, and the encoder reads the rotation fo the screw. if the screw/nut assembly has backlash, then the encoder will not know about that, and there will be "lost motion" when the direction reverses. This "lost motion" will be equal to the amount of backlash, but that is not going to be cumulative, it will be repeatable every time you come back to the same position from the same direction.
Or, are you talking about something completely different than the backlash issue? Then, yes, there is a cumulative error in the leadscrew, but how big is that? It can be quite small on ground ballscrews, or fairly bad on cheap rolled screws. Most commercial machine tools will have ground screws that are pretty accurate. Much better than 0.001" per foot is not usually warranted as the ways aren't straight enough to be able to tell the difference. If you are worrying over drilling two holes to within .001" location and over a foot apart, you really should not be thinking of using a used, retrofitted machine tool. > A linear scale is measuring > the actual movement,so in theory should be more accurate especially at > greater distance. > Can you tune out errors in ball screws using EMC, and get repeatable > accuracy for most of the length of travel? > Yes, EMC has a machine error compensation table. You have to be able to home the machine for this to work. You then map out the position error in each direction, typically using gage blocks and an indicator, and then enter the deviation in a table. But, this doesn't address servo loop control and stability. That is an entirely different issue. Note that while the linear scale is quite accurate measuring linear movement, the machine ways may not be moving in perfectly straight lines, or the axes may not be perfectly orthogonal to each other. Stuart Stevenson has been working on a kinematics module to make these corrections, and I gather he has it working. He has the precision alignment instruments to make the measurements, most of us do not. 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
