basic process control - idc-online process control. basic control principles level (controlled...
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Open vs Closed Loop
! Closed loop ! Automatic control
! Open Loop! Manual control ! Person takes the place of the controller
Feedback Control
Comparator Amplifier Process (Tank)Error
(SP - M)
Manipulated Output
(ControlledVariable - Level)
Disturbances
Variable
ProcessSensor
SignalMeasurement
Set Point
Controller
Feedback vs. Feedforward
! Feedback! Control action after an error exists
! Feedforward! Reacting to the disturbance before the
error occurs
Typical ON/OFF Control System
P1
L1
L2
V
QinSP
Qout
1
P1 closes @ L1P1 opens @ L2
Power SupplySolenoid
Solenoid valveElectrically Operated
Inflow
llllll
Controller Action
Qin
Qout
SP
LT
20 - 100 kPa
A/C
LICllllll
Qin
Qout
SP
LT
20 - 100 kPa
A/O
LICllllll
Direct
Reverse
Proportional Band and Gain
Proportional Band� the input change required to change the output 100%
PBgain %100
=
inputoutputgain∆∆
=
Proportional Control Response Curve
Offset
Outflow
Loss in VolumeNew mass balanceoccurs here
New levelbelow setpoint
timet 1
1t0t
0t
time
at setpointLevel originally
Input/Output
Level
t
Inflow
Proportional Response with Narrower PB
Level originally Offset
tt 0time
1Level
at setpointbelow setpointNew level
New mass balanceoccurs hereLoss in Volume
InflowInput/Output
0 t 1t
t
Outflow
time
Response Versus PB, Proportional Control Only
time
Step Disturbance
Load
Cha
nge
Offset
Offset
time
Syst
em R
espo
nse
SP
SP
SP
"Wide" PB
"Moderate" PB
"Narrow" PB
Additional Control Signal Restores Process to Setpoint
Setpoint
Initial mass balance
Inflow
Outflow
Final mass balance
Reset Action
timeOffsetRemoved
Units! Minutes per repeat
! MPR! The length of time that it will take the
integrator to add an amount equal to the proportional response
! Repeats per minute! RPM! The number of times the proportional
response is repeated in one minute
Proportional Plus Reset, Open Loop Response
time
e
Fast ResetNormal Reset
Slow Reset
error
keProportionalResponse}
Con
trol S
igna
l
A problemOutput initially 50%, Gain = 2, reset = 2 minutes per repeatA direct acting controller control is subjected to a sustained error of 5%What is the output after 4 minutes?
Proportional Response = ke = 2x5=10%Integral Action- in 4 minutes the control will go through 2 repeats.Integral action = 2 x 10 = 20%Total output change is proportional + integral = 30%
Proportional and Derivative � Open Loop Pressure
time
Inpu
tO
utpu
t
Derivative
Derivative ceases aserror stops changingProportional
Action
Derivative Control
+-
k�eSetpoint
Measurement
Error
Bias
++
Output
∫ edtTR1
dtdeTD
++
+
biasdtdekTedt
Tkem D
R
++
+= ∫1
The open Loop Response of Proportional Plus Derivative (PD) Action to Rapidly Changing Error Signals
0t 1t 2t
0t 1t 2t
time
time
A
B
C
Rate action due to cessation of increase in eRate actionB -C
RateactionA - B
Proportional actionA - B
Proportional action B - C
Rate action due to cessation of increase in e
Control signal at end of excursionControlSignal
%
Proc
ess
Large System Under Proportional and Proportional Plus Derivative Control
time
Control Signal
Setpoint
Level
LoadDisturbance
Applied
Prop. + DerivativeProp. Only
Multiple Control Modes
! Virtually all controls have a proportional response
! Integral and derivative are added to improve performance
! Majority have proportional and integral! Some, typically heat exchangers have
derivative added