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
L
R
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