Siemens Milltronics BW500, Operating Instructions Manual

Page: 118 / 220

PID Control
9.4 PID Setup and Tuning
BW500 and BW500/L
116
Operating Instructions, 12/2016, A5E33482052-AD
8. Again observe the oscillation of the control output. Compare results to the figures 4,5 and 6
below.
9. The D term is not critical in typical shearing type weigh feeder applications. The purpose of
the D term is to anticipate where the process is heading by looking at thetime, rate, and
direction of change of the process variable. The D term becomes very useful in applications
where the material control point is far away from the measuring point. An example of this
would be a weigh belt or conveyor with a belt scale (constant speed) being fed from a pre-
feeder some distance or more than a few seconds process time from the scale.
A properly set D term will make the initial oscillations around the setpoint smaller, as in figure
6. A D term set too high induces high oscillations, as in figure 4. Omission of the D term, or
set too low, shows no effect on the system.
10.The above closed loop cycling procedure allows ease in start up, but final adjustments may
be necessary in actual process operation.