Page 248 - Programmable Logic Controllers, Fifth Edition - Mobile version

to produce a difference (the difference is called the
PID
error). PID
Control Block N10:0
• Output from the loop is called the control variable Process Variable N10:28
(CV), which is connected to the controlling part of Control Variable N10:29
the process. Control Block Length 23
• The PID loop takes appropriate action to modify the Setup Screen
process operating point until the control variable Figure 10-47 SLC 500 PID instruction.
and the set-point are very nearly equal.
of the PID instruction parameters can be summarized as
Programmable controllers are either equipped with follows:
PID I/O modules that produce PID control or have suf-
ficient mathematical functions of their own to allow PID • Control Block is the file that stores the data required
control to be carried out. Figure 10-47 shows an SLC 500 to operate the instruction.
PID instruction with typical addresses for the parameters • Process Variable (PV) is an element address that
entered. The PID instruction normally controls a closed stores the process input value.
loop using inputs from an analog input module and pro- • Control Variable (CV) is an element address that
vides an output to an analog output module. Explanation stores the output of the PID instruction.

Data Manipulation Instructions Chapter 10 229

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