On Mon, 6 Aug 2007, Dale wrote:
> Date: Mon, 06 Aug 2007 08:40:26 -0400
> From: Dale <[EMAIL PROTECTED]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
> Subject: Re: [Emc-users] USB Wireless Network & Steppers with encoders?
>
> While I can't comment on the wireless network other than it should work.
> I do have some comments on steppers and encoders. Unless you have some
> real high rent stepper drives capable of running your steppers at
> rediculess micro step counts encoder feedback is poimtless. First thing
> a proper stepper setup should NOT lose steps! Second if you add all the
> variables, tolerances, assembly, and missalignments in the machine you
> still probably can't justify using encoder feedback. Unless your machine
> is extremely accurate and near perfectly aligned, you would need drives
> and motors capable of very high microstep counts. Then your steppers
> could be run with encoder feedback and would act more like servos than
> steppers. IMO it would be much more cost effective and practical to just
> switch to actual servo drives. Less burden on the computer (EMC) and
> easier to implement. Glass scales are also expensive at high
> resolutions. To use Glass scales for position feedback requires a very
> tight positioning system with zero or near zero backlash. I'm sure that
> encoder feedback is possible to setup, I just don't see the need or
> benefit of it. Last words, a properly setup stepper system should NOT
> LOSE STEPS! And minor amounts of backlash can probably be ignored. I
> have .002" ~ .003" backlash. It's not really backlash but more of a
> windup due to motor coupling and the extra load of spring loaded anti
> backlash nuts. It doesn't present a problem as the machine repeats
> within .001 or better so the windup issue get's ignored and I have no
> problems with it.
>
> Just my 25 cents, (inflation)
> Dale
Actually we can (and have) done encoder-feedback stepper systems in SoftDMC
pretty well, but its hardly worth the trouble because:
1. Even with a 1000 line (4000 count) line encoder, you only have 20 encoder
counts for the full range of PID output - pretty lumpy
2. This also means encoder - stepper alignment is very fussy, making it hard
to get symmetrical drive,
3. When you are all done you get a stepper that acts just like a brushless
servo, except its a 50 pole 2 phase servo which means the drive frequencies
are much higher than a normal (2 or 4 pole) 3 phase brushless motor. This
is not a good thing...
Peter Wallace
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