RE: [CAD_CAM_EDM_DRO] Re: breakout boards plus - hard choice

One thing I always try to convey when the "steppers lose steps" discussion
comes up is:
A stepper motor will either lose a step or it won't. Or: If you demand more
torque from a stepper that it can provide, it will miss steps every time. If
you don't demand more torque than it can provide then it will *NEVER* miss a
step. Setting your machine up to ensure this is an easy thing to do. You
just run it faster and faster until it begins to miss steps, then you set
your max feeds and rapids to 80% of the stall speeds.

I reckon its worth making this point because there is often an implication
in these discussions that servo's are more accurate/stable than steppers.

It's a bit like driving a car. You can (and some people do) try to take
every corner at the limit of traction. Or you can take every corner at the
recommended speed. If you do the latter then you will never crash, if you do
the former then you'll have a lot of fun for a short period of time. :-)

I, and many others, have run stepper driven machines for long periods (all
day) without losing a step.
Not saying that servo's aren't any good either. The PID loop (the class or
algorithm used to control them) is an elegant solution to the complexities
of moving a mass in the real world.

In my experience the "Should I buy Steppers or Servo's" argument usually
comes down to "I couldn't get my steppers to go as fast as I wanted because
I picked the wrong ball screw pitch" (another soapbox rant of mine) "So I'll
buy servos and run 'em at 2:1 reduction". Tragically these people are often
the victims of nasty road accidents on the corner just before the CNC
shop....

Just my 2c worth.

Paul Kelly

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of R Rogers
Sent: Friday, 20 July 2007 7:16 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Re: breakout boards plus - hard choice



Mark Vaughan <mark@...> wrote: Hi Ron




Quote "It would be really nice though if we had simple controls for people
to
feedback encoder signal so an allowable error could be mixed with the
federate to control accuracy, you'd then have control over the precision
before you start the job. "
//It's not impossible to do a true closed loop with Mach, it's been
debated several times. Now with the advent of the DLL support, it's quite
do-able from what I understand.
As Art F said, it's not a 100% guarantee for positional control. No system
has that.
The concensus on some of the drawbacks, first and foremost was safety, If
the user energizes the control and has a large discrepancy at boot for some
reason the machine will take off at full speed to get to commanded position.
We did that with the macropump, didn't take long to decide that was
unacceptable. I'm sure some changes to the script could have been made i.e.
If a compare is too great ignore and alert.
A digital system is more of an exact method of motion command. A system
ran within it's capabilities is very reliable and trouble free. There are
probably hundreds of thousands of machines that run in an open loop config.
They rarely lose position. The board I sell is for folks who want to know
when it happens. If a machine is continually losing position, my board won't
fix it, it's going to just keep stopping the machine every time it happens.
It's like hanging a bell on something thats broken. It's just going to keep
ringing.
The main point is that a properly setup and operated digital system will
give very good results ran in an open loop. If it's continually losing
position, something is wrong with it. No amount of software or hardware add
ons is going to fix it. You can make allowances for the problems, but thats
really not a fix.

////Ron




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