> Message: 9 > Date: Thu, 31 Jan 2008 13:36:51 -0800 > From: Dave Engvall <[EMAIL PROTECTED]> > Subject: Re: [Emc-users] DRO input?
> > Hi Dave, > > It all depends on what you want to do. If you just want manual > position then the glass scales will do your job. > > However, if you ever intend to control the axes then encoders mounted > on the end of the ball screws would be my choice. USdigital or > Automation Direct (Koyo) encoders are reasonably priced and should do > the job. With respect to you all, I know about encoders, and I know any CNC control software is going to need positional input many, many times per second. but... sticking an encoder on an axis is not measuring the position of anything except the leadscrew, it takes no account whatsoever of backlash, uneven wear or pitch errors. To a certain extent you can map out backlash and areas of wear on a leadscrew, but leadscrew pitch errors are going to be tough. The DRO is, quite differently, measuring the *actual* X Y Z position, if the DRO says you are 100.000 mm from point A then you can take it to the bank. My leadscrews are ten turns to the inch, I can compensate for backlash etc manually, second nature, and even with less than theoretically perfect accuracy I can still trust feeding a leadscrew in a few thou and know and measure that the error on that movement is very small. We appear to be confusing two things. Encoding leadscrew rotational angles and feeding this back to the human brain or CNC software, and factoring in mental or electronic fudge tables for backlash and wear, can give us fairly good accuracy of MOVEMENT. A DRO with proper glass scales gives us fairly good accuracy of POSITION. 5 times a second is *plenty* for an accurate positional measurement system to update an accurate movement system, and produce a system with true accuracy. While leadscrew encoders and a copy of emc and no stepper or servo motors will indeed give me a system that will display X Y Z co-ordinates on a screen, the accuracy of these readings is going to be just as suspect as it is sans emc and encoders, only the DRO and pukka glass scales will give true positional accuracy. As someone who couldn't code "hello world" (except maybe in BASIC) writing the code isn't an option, so unless emc has this facility on the roadmap then being free as in beer isn't enough. Many thanks to all, hopefully there is some more meat in this subject yet. end ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
