Yokogawa EJX930A - Manual (Page 83)

<Appendix 1. Signal Characterizer (SC) Block>
A1-6
IM 01C25T02-01E
A1.5 Application Example
A1.5.1 Input Compensation
The following is an application example of pH
compensation made by performing feedback
control.
The pH is a value representing the degree of acidity
or alkalinity and ranges from 0 to 14. pH 7 indicates
neutral, a value smaller than 7 represents acidity,
and a value larger than 7 denotes alkalinity. It is
very difficult to control pH with a quickly changing
reaction rate at a point near 7.
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Ratio of Reagent to Influent Flow
Small increases in reagent
flow cause large pH shifts
pH
FA0106.ai
Figure A1.6 pH and Reagent Flow
To control this pH, the input is regulated using
line-segment approximation, gain, and input
compensation.
pH Input
PID
Control
Control
Output
Characterizer
GX
X Gain
DM
Input Compensation
FA0107.ai
Figure A1.7 Input Compensation
The following shows the approximation-value graph
of GX Output that is approximation-value output
and GX Input that is pH input. pH with a quickly
changing reaction rate can be controlled at a point
near neutral 7 according to the following graph.
CUR
VE_Y
CURVE_X
0 to 100% = 0 to 14 pH
100
90
80
70
60
50
40
30
20
10
0 10 20 30 40 50 60 70 80 90 100
FA0108.ai
Figure A1.8 Approximation Curve
A1.5.2 Calorie Flow Compensation
AI_1: Inlet temperature, AI_2: Outlet temperature,
AI_3: Flow rate
SC: Corrects the inlet and outlet temperatures.
AR: Calculates a calorie flow rate on the basis of
the difference between the corrected inlet and outlet
temperatures.
AI1
OUT
SC
OUT_2
OUT_1 IN_1
IN_2
SWAP_2=OFF
AR
IN_1
IN_2 OUT
IN
AI2
OUT
AI3
OUT
FA0109.ai
Figure A1.9 Calorie Flow Rate Compensation
(SWAP_2 = Off)