30
WV
x
rAtO
+
bIAS
PID
Control Loop
PID O/P
CSPt
I/P3
PV
I/P1
Manual Output
OP1
Remote Ratio
Process Variable input
I/P2
Wild Variable
rAtO
WV
•1
•1 Template 15 only
L
LR
Local Ratio
Control
Output
Profile Set Point
2.9 Ratio Controller (Templates 14 and 15)
Ratio control enables a controlled process variable to be maintained automatically in definite
proportion to another variable known as the wild variable. The wild variable weighted by ratio (rAtO)
and bias (bIAS) values, forms the control set point for the process variable.
…2 OPERATOR LEVEL
•1 Displayed only if enabled in Level B, Operator Configuration – see Section 5.7.
Actual Ratio
=
Process Variable (PV) – Bias
Wild Variable (WV)
Desired Ratio
Adjustable in Local control only.
Control Set Point = (WV x Ratio) + Bias
Control Output
[0 to 100% (–10 to 110% for analog outputs)]
Adjustable in manual mode only.
Bias
[in engineering units]
Continued…
50.0
55.0
70
PV – CSPt
Deviation
OP1
0.0
bIAS
70
OP1
•1
PV – CSPt
Deviation
Fig. 2.8 Ratio Controller
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