On Sat, May 30, 2015, at 03:20 AM, Mark Johnsen wrote: > John, > > Thanks for the response. > > By dither, I am trying to explain how the motor is not stationary at an > idle or rest condition. When stopped at 0 Velocity with the drives and > linuxCNC enabled, the motor constantly seeks a position between linear > scale graduations (0.01mm in this case). I can see end of the ball screw > turning and the DRO readout in Axis will change between say 0.0004" and > .0008". The higher the Pgain the faster the movement. I've tried some > Igain, but doesn't straighten it out. > > I have not tried the deadband equal to one or two, but certainly will do > that in the morning. That was also a a forum suggestion from PCW, however > he also said w/ a coarse scale you might not want to loose the resolution. > This was partially why I thought the 1um scales might be a good idea > because I don't think I'd care about dithering between .00004 and .00008" > or setting a deadband (maybe of 2 or a tad more here?). >
Deadband will probably settle the machine down. Whether you can afford to lose a count or two of accuracy depends on the parts you are trying to make and the rest of the machine. Stuart wrote a very nice response about that topic. > Lastly, to beat a dead horse, I thought the higher resolution linear scales > would be easier to mount because of the existing bracketry. To add the > rotary encoders, I need to add mounts to the end of the ball screws or > backs of the motors. Maybe I'm being dumb here... > Not dumb at all. It's just a matter of priorities. As a hobbyist, I try to avoid putting cash into my machines if I don't have to. If you are running a business, you probably try to avoid putting hours into your machines - because for you, hours are cash. In my case, if I already had linear scales, I wouldn't put cash into getting better ones. If I needed to add encoders for stability and tuning reasons I would do it. US Digital encoders are much cheaper than linear scales. But the cost of mounting them could be much much higher if the motors weren't designed to accept encoders. The need for encoders on the motors really comes down to how much lost motion there is in the axis drive train. If there is lash in the belts or gears, plus more lash in the screw, plus the inertia of all those parts, it might be difficult to tune. By the time the scale sees motion, the motor is already turning fairly fast, and so the system ends up chasing its tail. A sensor (encoder or analog tach) on the motor gives immediate feedback as soon as the motor starts to move, which can make tuning much easier. It might be possible to use the analog tach for velocity feedback combined with position feedback from the scale and again do a composite loop with two PIDs to achieve accurate and stable tuning. I have no experience with that. But it could be cheaper and easier to try compared to mounting encoders. John -- John Kasunich [email protected] ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
