Re: Close loop steppers

Hi,

I did understand what Arne is trying to achieve in principle and, in fact,
he is just trying to make an electromagnetic DRO without the readout
however, I can forsee a couple of major problems with just a simple system
as described. The main one of these would be in tying the steps of the
readout motor to the working motor. If you tried to use a 200 step motor as
in a disk drive coupled to a 200 step 'power motor', then ran the system in
half step mode I think it would be difficult to get an output which would be
directly relevant to the power motor steps. Also, as I think Arne has
realised, there would be a problem when the machine reverses and the readout
motor could lose a step - maybe not too good when going round a circle.
Also, if the readout motor doesn't produce its output signal at the same
precise time as the power motor steps there will presumably be the potential
for error. Perhaps the system would work better if there was, say, a 10:1
gearing up between the power motor and the readout motor with suitable
circuitry to round the output to the nearest half step?
I must say I'm not yet totally convinced about the benefits of servo motors.
I can't see how they can reliably stop in an exact position - I can
understand how feedback from the encoders works and even how the software
can ramp down the speed towards the end of a move but how do they get rid of
flywheel effect running the motor on beyond its desired stop position and
what happens if the cutter hits a hard bit near the end of the cut, does the
motor stall early?

Ian
--
Ian W. Wright
Sheffield UK

----- Original Message -----
From: hansw <hansw@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: 15 January 2000 22:58
Subject: Re: [CAD_CAM_EDM_DRO] Close loop steppers


> From: hansw <hansw@...>
>
> Ian and Arne,
>
> If I understand it correctly what Arne wants is a way to feedback data
from the stepper and have any lost steps added.
>
> I think the best way is to use a DRO and feed that data back into the
cutting program. That way if the program instructs a
> cut equal to N steps and the DRO shows N was not accomplished then it can
add pulses until N is reached. Simple servo using DRO
> and steppers...
>
> BTW
> I have plan for several projects, just trying to kill a few alligators
(crocodiles for us British) and then I intend to finish
> the RPM meter for very low RPM and make a Serial (RS-232) stepper
controller... I hate the printer port, can't use it in Windows
> NT as there are no drivers for the DOS based program... Expect I'll end up
write the controller program for it also.
>
> regards
> Hans Wedemeyer
>
>
>
> Ian Wright wrote:
>
> > From: "Ian Wright" <Ian@...>
> >
> > Very interesting Arne - didn't understand a word of it but I'll keep
> > watching until someone posts circuit diagrams I can pretend I know about
;o)
> > I always could understand pictures better than words.
> >
> > Ian
> > --
> > Ian W. Wright
> > Sheffield UK
> >
>
>
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