Re: [CAD_CAM_EDM_DRO] Re: stepper vs servo

Hi, on the small lathe i used to run the spindle motor was 7 1/2 hp and the
ballscrews were driven with 1 1/2 hp brush motors,more than once i shoved
the solid turret right off the base and sheared the 4 bolts holding it on!!!
The new/old lathe they are setting up now has 5 hp motors on both axis
....certainly more than 5000lbs force!!!!!! cul brian f.

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, August 08, 2001 1:11 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: stepper vs servo


> cadcamcenter@... wrote:
>
> > Jon Elson <elson@p...> wrote:
> > > Servos don't stall, unless the physical limits of the machine are
> > reached.
> >
> > Trying to get exact meaning of terms:
> > stall = stop?
> > stall = stop temporily?
> > Servos don't stall = there are no conditions, however severe, that
> > they will stop/stop temporily when they are supposed to be moving?
> > When torque is insufficient, only the feedrate fall below the
> > commanded feedrate?
>
> On many commercial machines, this is literally true. They are so
powerful,
> the will destroy the interfering object, or they will destroy the machine!
> My system is designed to limit at about 1000 Lbs of linear force at the
> table.
> I'm a pretty conservative guy. I think the Bridgeport EZ-trak has a limit
> set
> somewhere around 5000 LBs! That's pretty scary.
>
> I can't imagine any reasonable machining operation on a small mill or
lathe
> that would develop cutting forces anywhere near this level, and these kind
> of forces could easily destroy a mid-sized machine.
>
> So, if the such a servo ever slows down enough to cause a following error,
> or indeed, stalls completely, there is something drastically wrong, and
> causing
> an emergency stop sounds like the most appropriate behavior.
>
> > For steppers:
> > stall = stop?
> > stall = lose step?
>
> When steppers are asked to accelerate too rapidly or run at too high a
speed
>
> under load, they will frequently stall, meaning they stand still and buzz.
> Under
> transient acceleration problems due to ragged step pulse trains, they may
> just lose an occasional step (or 4). This is a real problem, because all
> positions
> will be off by an unknown amount until the machine is re-homed. Stalls
> are pretty obvious, the machine could be inches off the correct position.
> An occasional skipped step will not be obvious, and parts created after
> that point will be off in some manner.
>
> > > They can slow down, however. Some systems (the Allen-bradley 7320
> > > I worked with before, for instance) had a feature called feed-rate
> > > compensation.
> > > Whenever one axis lagged more than some limit behind commanded
> > position,
> > > all axes cut their feedrate in half, hoping that would allow the
> > lagging
> > > axis
> > > to catch up.
> >
> > Meaning servos alway act in unison? When Y slow down, X will slow
> > down simultaneously, and there is no time-lag?
>
> This is a function of the CNC control. Allen-Bradley has it, EMC doesn't.
>
> > My feeble mind: The only way that X will know Y has slowed down is
> > when Y has already slowed down and a signal is issued that this has
> > happened, by which time X may already have proceeded some steps
> > beyond Y? By which time there will already be some deviation, even if
> > minute, from the planned path?
>
> You must understand that the machine is NEVER where it is commanded
> to be. there is ALWAYS some error between commanded position and
> actual position. The job of a servo system is to make smooth motion, the
> CNC control's job is to minimize the error. it is constantly comparing
> actual
> against desired position, and making adjustments in commanded velocity
> to reduce the current error to a smaller value. In my CNC system, the CNC
> control is making this comparison and computing a new velocity 1000 times
> a second. It has a resolution of .00005" (that's 50 micro-inches).
Keeping
>
> following error down to a few counts of the 50 u-In encoder every
> millisecond
> keeps the machine quite close to the desired path at all times, especially
> at
> normal cutting feedrates.
>
> > Is the answer below:
> >
> > > And, it is most likely that the lagging axis will leave extra
> > material in
> > > place,
> > > rather than remove extra material, and a later pass can clean it up.
>
> This helps, too.
>
> Jon
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>
> Post messages: CAD_CAM_EDM_DRO@yahoogroups.com
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@...
> Moderator: jmelson@... timg@... [Moderator]
> URL to this page: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
> bill,
> List Manager
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>