5
Elements of process control ....................................................................... 129
5.1 Introduction ....................................................................................... 129
5.2 Basic concepts .................................................................................... 129
5.3 Basic control structures ....................................................................... 131
5.3.1
Feedback control ..................................................................... 131
5.3.2 Feed-forward control ............................................................... 131
5.3.3 Comparative merits of control strategies ................................... 132
5.4 The block diagram .............................................................................. 132
5.5 Input, output and process dynamics .................................................... 133
5.5.1
First order response ................................................................. 133
5.5.2 Second order systems .............................................................. 135
5.6 Control modes (control algorithms) .................................................... 136
5.6.1
On-off (binary) control ............................................................ 136
5.6.2 Proportional (P) control .......................................................... 138
5.6.3 Integral (I) control ................................................................... 139
5.6.4 Proportional-integral (PI) control ............................................. 140
5.6.5 Proportional-integral-differential (PID) control ......................... 140
5.6.6 Optimization of control ........................................................... 141
5.7 The physical elements of the control system ......................................... 142
5.7.1
The sensors (measuring elements) ............................................ 142
5.7.2
The controllers ........................................................................ 149
5.7.3
The actuators .......................................................................... 149
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