SMAR CD600 Plus - Page 54

CD600 Plus - User's Manual
4.34
Q = 0-20 Kg/s
A
A
A
A
A
Q
K
4
B
B
SP
PV
2
Q
=
0
-
80
K
g
/s
AI
001
PID
039
L/R
031
ARTH
051
AI
002
225
)
Fig 4.12.1 - Ratio Control with Fixed Ratio Constant
Lets assume that the control shall maintain QA/QB=8.
As the controller "sees" the signals corresponding to Q
A
and Q
B
as 0-100%, it is necessary to use
an internal factor to show the relation between the two variables:
a) Certify that the two flows are in the same units.
b) Normalize the signals.
[Q
A
] = 0-100% signal, corresponding to Q
A
: 0-80 kg/s.
[Q
B
] = 0-100% signal, corresponding to Q
B
: 0-20 kg/s.
][
100
80
AA QQ ⋅=
(1) (2)
][
100
20
BB QQ ⋅=
Dividing (1) per (2):
][
[
20
80 ]
B
A
B
A
Q
Q
Q
Q
⋅=
(3)
As Q
A
/Q
B
=8 (4 ),
Substituting it in (3):
][
][
48
B
A
Q
Q
⋅=
∴ [Q
B
] = 0.5 [Q
A
] = SP (5)
That means: when the process has the right ratio, the signal corresponding to the Setpoint of flow
Q
B
is the half of the signal, corresponding to flow Q
A
.
c) Calculate the Arithmetic Block as follows:
32
2
11
)(
BiasDG
BiasC
BiasBAG
OUTPUT ++
+
+
⋅
⋅
= ⋅
(6
[Q
A
] connected to input A makes A=[Q
A
]. The output is the Setpoint for Q
B
. Making (5) = (6).
32
2
11
0
0
)0(][
BiasG
Bias
BiasQG
SP
A
++
+
+
⋅
= ⋅
⋅
Bias = 100%
Bias
2
= 100%
Bias
3
= 0 SP = 0.5[Q
A
]
G
1
= 0.5
G
2
= 0