GE 90-30/20/Micro manual Pid, Ind

4

PID

The Proportional plus Integral plus Derivative (PID) control function is the best known general purpose algorithm for closed loop process control. The Series 90 PID function block compares a Process Variable feedback with a desired process Set Point and updates a Control Variable output based on the error.

The block uses PID loop gains and other parameters stored in an array of 40 16 bit words (discussed on page 4-173) to solve the PID algorithm at the desired time interval. All parameters are 16 bit integer words for compatibility with 16 bit analog process variables. This allows %AI memory to be used for input Process Variables and %AQ to be used for output Control Variables. The example shown below includes typical inputs.

	_____
%S00007
(enable) —— ——		(ok) Power flow out if OK
	IND
(set point) %R00010	—SP CV— %AQ0001 Control Variable
+21000		+25000
(process variable) %AI0001	—PV
+20950
%M0001
—— ——— MAN
%M0002
—— ——— UP
%M0002	DN
—— ———
	_____
	%R00100	RefArray is 40 %R words

(reference array address)

As the input Set Point and Process Variable and output Control Variable terms are used so frequently, they will be abbreviated as SP, PV and CV. As scaled 16 integer numbers, many parameters must be defined in either PV counts or units or CV counts or units. For example, the SP input must be scaled over the same range as PV as the PID block calculates the error by subtracting these two inputs. The PV and CV Counts may be –32000 or 0 to 32000 matching analog scaling or from 0 to 10000 to display variables as 0.00% to 100.00%. The PV and CV Counts do not have to have the same scaling, in which case there will be scale factors included in the PID gains.

Note

The PID will not execute more often than once every 10 milliseconds. This could change your desired results if you set it up to execute every sweep and the sweep is under 10 milliseconds. In such a case, the PID function will not run until enough sweeps have occurred to accumulate an elapsed time of 10 milliseconds; e.g., if the sweep time is 9 milliseconds, the PID function will execute every other sweep with an elapsed time of 18 milliseconds for every time it executes.

GFK-0467K Chapter 4 Series 90-30/20/Micro Instructions Set 4-171