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SC100/200 Series Function Block Application Manual EM-6460-C
4.3.9 PROPORTIONAL CONTROL
Proportional-only control without integral and derivative terms is widely used. The control method stabilizes the control loop
without a risk of cycling as far as a certain deviation in control result is tolerated.
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LOOP CONFIGURATION EXAMPLE: TANK LEVEL CONTROL
The following graph is an example showing correlation between setpoint (set to 50%), manual reset value (set to 50%) and
MV output with the proportional band set to 50%.
The MV output provided when the deviation (SP − PV) equals 0 in a proportional control system is called ‘manual reset value.’
In the above process, the tank level stays at a constant value when the outflow equals the inflow. The deviation (SP − PV)
detected at this point is called ‘offset,’ where the control is settled out.
The MV output is expressed with the following equation:
100
MV
= –––—
e
+
MR
PB
where
e
= Deviation
MR
= Manual reset value
Manual reset value is preset in the Control function block.
level
PV
LIC
control valve
(reverse action)
MV
SP
MV output 0% (valve opening 0%)
MV output = manual reset value (50%) (valve opening 50%)
MV output 100% (valve opening 100%)
level 75%
50%
(= SP)
25%
inflow
outflow
50
proportional
band 50%
100
%
0
0
25
50
75
100
%
MV output
PV input
SP = 50, manual reset = 50
(reverse action)