----- Original Message ----- From: "Jon Elson" <[EMAIL PROTECTED]> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> Sent: Monday, August 06, 2007 9:17 AM Subject: Re: [Emc-users] USB Wireless Network & Steppers with encoders?
>> Also, after getting to play with hal configurations a bit, It seems >> like >> it wouldn't be too difficult to get steppers working with encoder >> feedback. If you would set up a pid loop => frequency generator and >> configure velocity feed forward to run the steppers at the commanded >> velocity, then pid gains could be set to take care of position >> deviations. This would be useful in detection and correction of lost >> steps for one. > It will correct for single lost steps, especially at axis > reversals. This should never happen anyway, it is because of a > mismatch between step/direction timing from the controller and > what the step driver requires. It would be better to fix the > step timing rather than cover it up. > > This scheme will not handle motor stalls, however! Once the > motor has stalled, it will just grind and rattle rather than > catch up and get back into lock (until the computer slows down > the commanded velocity.) > On motor stalls, without feedback you normaly don't detect them until you ruin your part or you're milling into your table, vise, clamps, etc. With encoder feedback you can set your following error to kick out, in wich case you can hopefully re-home, adjust your speed/feed/rapid/accel/decel/or whatever caused the error at that part in the program. I would like to use encoder feedback from linear DRO scales and be able to step to the commanded position until it arrives, just like my Anilam mill does (except my mill has servos with tach feedback). A typical machine I might build may use rolled ballscrews with ~.004" per foot accuracy and maybe .002" backlash. Positioning from the linear encoder removes the lead screw error and position errors of the backlash. If following error limits were set tight enough, it could fault when/if backlash increased. If there were no benefit to DRO's using linear scales, we could just put more economical rotary encoders on the handles to give us a digital readout of the handwheel position. Why would manual mills and lathes need linear encoders for their DRO's but they not be needed for CNC equipment, unless you use high precision ground ball screws with 0 backlash? Even still, wouldn't a linear encoder generally be more accurate than even a high $$$ precision ground ball screw? What if you tried to tune a servo and the only feedback was from a linear encoder? I don't predict it would be very stable. If you had an expensive linear scale with 1/2 micron resolution, a half a thou backlash would cause motor oscillations. If you use a feedback directly from the motor, or used stepper motors, or something that was stable by itself, then you could position to the linear scale by moving in the commanded direction at a velocity proportional to the error, and you wouldn't need the 1/2 micron resolution unless you needed to position that close. If you get a chance to get your hands on a DRO scale bargain sometime, you should give it a try and I believe you will find yourself making parts to closer tolerances first time without tweaking. I know you can have excellent repeatability without the feedback, but with it you can have excellent Accruacy AND repeatability. I guess my linear scales on my mill just have me spoiled :-) Roger Neal ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
