Hi  Stefan, Justin and Tjaart ;

I guess I was confused as to the application of the PID,  in that I
have not worked with PID for position control.  This is new to me, and
I see what you are driveing at. Thanks for clearning this up for me.

I was assumeing that the loop was controling a process, such as
heating an oven, or running a motor attached to a pump trying to
maintain a setpoint of pressure or a volume of material flow.

These loops normaly don't have an output that goes to zero when
everything is working correctly unless something bad happens
(pipe plugs up, or its hotter outside the oven than inside).  In
other words,  the loops that I am used to working with usualy
need some output just to stay at the setpoint.

I didn't think about an aplication where the object under control,
once placed at the setpoint, would stay there "on its own".   But
rather one where external forces would act upon the setpoint.

I guess an example of this would be if your position control loop
had some force that tended to move the object under control out
of position.. such as gravity or some pressure that the motor has
to continualy work against,  maintaining a torque to stay at setpoint.

If you move something into position and it just sits there with no
outside forces acting on it,  then you clear the "I" term,  this would
indeed make the control less likely to have oscillations..

Thanks again, to all who answered back.

Arnie G


At 09:26 AM 5/20/99 +0100, you wrote:
>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...
>>
>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