Kirk Wallace wrote: > On Sun, 2009-01-04 at 06:17 +0000, Roger wrote: >> It's unfortunate that there isn't a way to approximate closed loop >> control with steppers and encoders in EMC2. I have read that EMC2 can >> detect a following error using steppers with encoders and trigger a >> fault which is great but this doesn't really allow the use of steppers >> in larger or high performance machines. >> >> It would be great if you could run a stepper like a brushless servo. >> Steppers >> and drivers are cheap and easy to interface. >> >> Roger > > With only step and direction signals on stepper driver inputs, EMC has > no way of modifying the stepper driver behavior. The driver would need > something like a step bias or correction input, which EMC could probably > be configured to work with. If you want to compensate for missed steps, > EMC can send more steps as needed, but you will most likely end up with > just more missed steps until you get a following error, just like when a > servo stalls. Those that have encoders on there stepper machines, may > correct me on this.
Microproto/Taig provide a closed loop stepper drive option, but this only works because the original steeper drive runs off phase signals, so Kurt can keep applying power to the motor until it DOES move. The encoder is then used to stop applying power, or the drive times out if no movement is detected in a reasonable time. The 'step' pulses are buffered and effectively slowed down to a rate the current conditions can support, but once a difference of 200 steps is achieved, the drive faults. The closed loop system is effective and provides a two or three times speed increase over the identical open loop set-up, but when being pushed hard, the position lags behind what the software thinks has been achieved, so on Mach3, a move in another direction will start before the first move has actually finished ( rounded corners ;) ). Bottom line, both software and hardware need to cooperate. The software needs to be reading the same encoder feed as the driver, and slowing down in sync with the driver. The driver needs to be a little more intelligent than step and direction, so that the step pulse triggers a drive cycle similar to the Taig driver. Is this worth the effort - PROBABLY - since at the very least you will get an immediate stop in the case of a jam, or accident, and the problems caused by variable quality material will in most cases be eliminated. But a more powerful driver may be an easier solution anyway? -- Lester Caine - G8HFL ----------------------------- Contact - http://lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
