RE: P.I.D.

["Pete Cook" <pete.cook-at-alltracorp.com>]

I'm not sure what units the EMC is actually using in it's equation - the
definition below says it is volts per user unit - but you could check out
the actual results but blocking an axis and then trying to move several
thousanths in the blocked direction and see what the actual change in
voltage is - basically the higher the position error (lag) the higher
voltage.



>
> Pete,
>   Can you give an example of P gain if set at P=50 and there
> is one encoder
> count. What would the math look like to figure the volts per
> user unit? I'm
> not even sure this question is correctly asked. And are user
> units inches?
>                                           Joe
>
>
> >From: "Pete Cook" <pete.cook-at-alltracorp.com>
> >Reply-To: emc-at-nist.gov
> >To: Multiple recipients of list <emc-at-nist.gov>
> >Subject: RE: P.I.D.
> >Date: Thu, 7 Feb 2002 10:54:58 -0500 (EST)
> >
> >
> >As listed here then machine is going to be springy around the desired
> >position - you have to get some error (out of position) and the CNC
> >generates some voltage to counteract the error and corrects
> some portion of
> >the error. For every encoder count there is some error voltage to
> >counteract.
> >
> >The D gain is generally listed as the correction for the
> last error - if
> >you
> >are out of postion by x encoder counts this adds in another
> x volts. If set
> >to high the machine will go into oscilation
> >
> >I gain compensates for the cumulative error over many moves.
> If set to high
> >the machine can get mushy.
> >
> >D Gain will take the springy feeling out of the machine.
> >
> >An example - a servo with a drum and cable with a weight
> hanging on the
> >cable. At standstill the P Gain will cause the servo to come
> to some stable
> >point at some distance out of position (motor torque
> counterbalances the
> >load) Change the load and the system will stablize at a
> different point.
> >
> >The D Gain will add some more compensation to bring the
> servo back into the
> >correct position no matter what the load.
> >
> >These numbers will vary depending on the encoder counts
> (more or less
> >counts
> >per inch) and servo response.
> >
> >Pete
> >
> > >
> > > P = 50
> > > The proportional gain for the axis servo. This value
> > > multiplies the error
> > > between commanded and actual position in user units,
> resulting in a
> > > contribution to the computed voltage for the motor amplifier.
> > > The units on
> > > the P gain are volts per user unit.
> > > I = 0
> > > The integral gain for the axis servo. The value multiplies
> > > the cumulative
> > > error between commanded and actual position in user units,
> > > resulting in a
> > > contribution to the computed voltage for the motor amplifier.
> > > The units on
> > > the I gain are volts per user unit-seconds.
> > > D = 0
> > > The derivative gain for the axis servo. The value multiplies
> > > the difference
> > > between the current and previous errors, resulting in a
> > > contribution to the
> > > computed voltage for the motor amplifier. The units on the D
> > > gain are volts
> > > per user unit per second.
> > >
> > >
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