Installation manual

1 Publication 1747-RM001G-EN-P - November 2008
Chapter 9
Proportional Integral Derivative Instruction
This chapter describes the Proportional Integral Derivative (PID) instruction.
Overview
This is an output instruction that controls physical properties such as
temperature, pressure, liquid level, or flow rate using process loops.
The PID instruction normally controls a closed loop using inputs from an
analog input module and provides an output to an analog output module. For
temperature control, you can convert the analog output to a time
proportioning on/off output for driving a heater or cooling unit. An example
appears on pages 9-30 through 9-31.
The PID instruction can be operated in the timed mode or the STI mode. In
the timed mode, the instruction updates its output periodically at a
user-selectable rate. In the STI mode, the instruction should be placed in an
STI interrupt subroutine. It then updates its output every time the STI
subroutine is scanned. The STI time interval and the PID loop update rate
must be the same in order for the equation to execute properly.
The PID Concept
PID closed loop control holds a process variable at a desired set point. A flow
rate/fluid level example is shown below.
PID
PID
Control Block N18:0
Process Variable N18:23
Control Variable N18:24
Control Block Length 23
Setup Screen
PID
Output Instruction
Fixed SLC
5/01
SLC
5/02
SLC
5/03
SLC
5/04
SLC
5/05
••••
∑ ∑
Set Point
Flow Rate
Error
Control Valve
PID
Equation
Process
Variable
Level
Detector
Control
Output
Feed Forward
or Bias