YOKOGAWA SC450G - Page 46

IM 12D08N05-01E
5-8
<5. MENU STRUCTURE COMMISSIONING>
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Integral control
Integral control is used to eliminate the steady state error and any future process changes.
Itwillaccumulatesetpointandprocess(load)changesbycontinuingtoadjusttheoutput
untiltheerroriseliminated.Smallvaluesofintegralterm(I-timeinseconds)providequick
compensation, but increase overshoot. Usually, the integral term is set to a maximum
valuethatprovidesacompromisebetweenthethreesystemcharacteristicsof:overshoot,
settlingtime,andthetimenecessarytocanceltheeffectsofstaticloading(process
changes).Theintegraltermisprovidedwithanantiwindupfunction.WhentheoutputofPI
portionofthecontrollerisoutsidethecontrolrange(lessthan-5%orgreaterthan105%),
the I-part is frozen.
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Derivative control
Thecontrolactsontheslope(rateofchange)oftheprocessvalue,therebyminimizing
overshoot. It provides “rate” feedback, resulting in more damping. High derivative gains
can increase the rising time and settling time. It is difficult to realize in practice because
differentiation leads to “noisy” signals.
SP
PV
e
+
-
+
+
+
+
+
-
e
Range
∫e dt
1
T
i
T
d
dPV
dt
z
Process
Controller
Actuator
Process
Figure 5.1 Control Diagram
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 Expiretime
Iftheoutputisover100%forlongerthantheexpiretime,theoutputwillreturnto0%.
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Damping time
The response to a step input change reaches approximately 90 percent of its final value
within the damping time.
100%
0%
set
point
process
value
Direct
100%
0%
set
point
process
value
Reverse
range
range
manual
reset
manual
reset
Figure 5.2 Direct/Reverse action