Hi all
The Integral term is a function of 'time that there is an error',
'the magnitude of the error', and 'the gain constant'.
The formula for process control applications is
Gain x Error x Time since error occured x I constant = integral component.
So when there is no error, you'd have to reset the timer but not the
integral component (for process control) because if the
error is zero the integral component is zero.

Remember that the integral has a cumulative effect on the
output. If the integral is zero then the output doesnt move
i.e. The integral ADDS it's calculation to the output (+ve or -ve)
P+I+D.
Justin Grimm


> > 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
>
> 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
>