Cutler-Hammer D320 PLC, Руководство пользователя

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D320 PLC User's Manual
Overview
As small Programmable Controllers gain analog and math capability, the need to perform related
functions has increased. One of these functions is closed-loop control or PID. PID stands for
P roportional, I ntegral, D erivative control, and comes from the error equation used to perform this type
of control:
∆
CV = K
p
E + K
i
ƒEdt + K
d
(
∆
E/
∆
t) + Bias
A closed-loop system is characterized by an ability to compare the actual value of a process variable
(PV) with its desired value (Setpoint SP) and to take the necessary corrective action (Output). The
calculations required to do this smoothly are beyond simple arithmetic and comparison functions.
Figure 1 contains a block diagram of a typical closed-loop system.
Figure 1. Closed Loop System
The PLC must process the input signals for process variable (PV) and setpoint (SP), calculate the error
(E = SP - PV), and change the output, or control value (CV). The PID control function is designed
specifically to do this.
PID Algorithm in the D320CPU320
The D320CPU320 (hereafter referred to as the D320) contains the capability of performing PID
control on up to eight separate closed loop systems. These loops independently operate on their own
process variable, setpoint, and output values.
Each of the PID loops has its own block of 32 register words which defines all of the parameters for
that loop, for a total of 256 words for all eight loops. The first word of the 256 word block is defined
by the value in System Register 8 (SR008). Each loop is also controlled by setting bits in System
Flags F14 and F15. The block list and enable bits are shown in Table 1.
COMPARATOR CONTROLLER
PROCESS
SETPOINT
ERROR
SIGNAL
PROCESS VARIABLE
OUTPUT