TABLE OF CONTENTS This exercise will introduce the student to the two major types of OP-limit calculations. The one kind, with OP-limits, allows a saturation value of the OP, based on P-control and D-control, and another kind, does not allow OP values to go into saturation at all.
The controller calculates a VIRTUAL OP value independent of any OP-limit. These may be values far above 100% or far below 0%. Only the real OP, which is the displayed OP value, is limited by OP-limits. The real OP awaits the return of the virtual OP within OP-limits. Then, within the range of the OP-limits, the real OP follows the virtual OP value exactly. Controllers driving field OP normally use this kind of OP-limit handling.
Only real OP values are used for the calculation. If a single calculation results in an OP value beyond OP-limits, the OP value will be set to the value of the OP-limit it would have violated. When the controller calculates the OP value next time (in the next scan), the real OP value (OP = OP-limit) is used. The previous calculation, beyond OP-limits, has been totally forgotten.
Call up the training application General single loop with interactive PID (real form) . In order to operate with virtual OP limit calculation, call display F8 and make sure that the status variable OPCALC is set to VIRTUAL. Then make large changes of SPE...
