Hi,

Arnie Grubbs wrote:
>
> I don't understand why you would want to reset the "I" value....
> Maybe someone can clear this up for me...
>
> It seems to me that when a PI loop is stable you will not have any
> proportional contribution to the output, just an integral contribution to
> maintain an output.
> As your system gets close to setpoint, your proportional calculation works
> it way
> down to 'zero'.  As the loop settles down, using  a "P" only, you will have
> an 'offset' from the
> desired setpoint. This is because when SP=PV there is no proportional output.
>  To reach setpoint you need to the "I" term.  This used
> to be called "reset", and can be looked at like the old proportonal only
> controlers, where the offset from setpoint was removed by the operator
> 'resetting' the setpoint  to get the Process Variable at the 'desired
> setpoint'.

> If you are worried about 'low freq' oscillations, and have to clear the "I"
> term for some reason, then I don't think you have your loop tuned correctly.
> If tuned correctly,  you would not normaly have any overshoot, or ringing
> action in the loop..
>
> Maybe I am missing the point.... if you are trying to make this system
> look like the old cook oven controlers, so that you actualy do get a
> constant swing of the PV around the setpoint, and don't really want
> to end up with SP=PV... Then you would not want to use the "I" term
> in your loop equations..  Otherwise  you would... Right????
>

May be this question/idea is related to the behave of the two different
PID
algorithms - the positional PID and velocity PID.

If I've understood this correctely the difference is like below:

Positional PID:         Y =      P + I + D
Velocity   PID:         Y =  Y + P + I + D + M

The velocity  PID has the possibillity to be controlled mannualy ( M )
while
setting the control loop up.

Also the reset of I in the velocity algorithem makes sense when you have
reached
the setpoint. Otherwise your loop will oscillate around the setpoint.

If possible I would appreciate some comments confirming or correcting my
statement's above.

Kind regards,

        Stefan Sczekalla-Waldschmidt
        ssw@oikossw.de