Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 46)

Programming
Chapter 3
3Ć3
features. The full algorithm flow chart is presented in section titled
Algorithm Flow Chart.
The PID module (rev C or later) uses the ISA algorithm (P, I and D
dependent gains) or the Allen-Bradley algorithm (P, I and D independent
gains). Refer to appendix E for algorithm comparison and selection.
Earlier revision modules use only the Allen-Bradley algorithm.
Figure 3.1
Simplified PID Algorithm
SP
Process
A/D
Hardware
/
Digital
PV
+
S
Error=SPĆPV
P
I
D
V PID
V
BIAS
Lead
Feedforward Input
(FFV)
D/A
Hardware
11093
Variable
Analog
Input
Filter
-
Lag
Analog
Output
S S
/
Control Variable
Block Transfer Data Blocks
Data block files are areas of the PC processor data table used to store loop
control words and loop values. The features of the module are selected by
setting word and bit values in data block files. The data block files are
transferred to the PID module by block transfer instructions.
Three write block transfers are required to load the PID module with data
from three data table block files. The dynamic block DB contains values
which are subject to change for both loops. The loop 1 block LP1 and
loop 2 block LP2 contain loop constants which seldom change.