Peter,
I *FULLY* understand.  My 8th paragraph states if the drive is poorly tuned
(voltage mode = no tuning) and the poles are low in freq (bingo) the D is a
bandaid.  Voltage mode control doesn't hide/move the plant poles to a
higher freq like current mode can.  Its simpler but lower performance.
Only so much bandwidth a single loop can get if it gets handed all the
multi-order system resonances in one black box.  Much better to handle each
system order at a time (torque,speed,position).  The D adds zero which if
you put ontop of the next systems pole you can help things out.  There are
systems that it works in.  Buts its a bandaid.

Chris,
I didn't say Peters method doesn't work.  LOTS of ways to throw parameters
at the wall and make them stick.  Im simply saying I disagree w/ the order
as its produces a sub-optimal solution.  If you understand why you pick the
parameters to create the poles and zeros the way they do it would be
clear.  If you pick a stable P and then much later increase the I, the zero
you are creating is moving to higher and higher freq.  Eventually it will
move so high that the system will become unstable.  This is not the I's
fault.  Tuning them TOGETHER will allow higher I *AND* P for a much more
'stiff' system.  Put the zero near the first system pole and then crank the
total gain up TOGETHER until the next higher system pole comes into play
and drives the gain/phase margin too low again.

I have velocity mode drives in the Supermax mill.  Though they can be run
as torque mode.  A torque(current) mode drive + velocity loop(either in
drive or linux cnc) + position loop(linux cnc) is optimal.  But AFAIK
linuxcnc can only do the position loop so I leave the drive to handle the
current + velocity loops.

Im a linuxcnc newbie.  But I understand tuning *VERY* well.  We should talk
about state-space control;)

Stephen


On Sat, Dec 22, 2012 at 3:28 PM, Peter C. Wallace <p...@mesanet.com> wrote:

> On Sat, 22 Dec 2012, Stephen Dubovsky wrote:
>
> > Date: Sat, 22 Dec 2012 14:27:39 -0500
> > From: Stephen Dubovsky <smdubov...@gmail.com>
> > 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] Pid saturated, was: Servo error
> >
> > Sorry Peter, I have respect for you (great products!) but I *COMPLETELY*
> > disagree w/ this.  I have my masters degree in power electronics and
> > control and have lots of experience in on and off road traction drives
> (aka
> > electric vehicles.)
>
>
> Sorry, its very clear from you dont understan tuning a voltage mode drive
> This is obvious when you say D is not important. D is critically
> important in a pure voltage mode drive like an HBridge.
>
> Peter wallace
>
>
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