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Basic Process Control
Basic Control Principles
Level(Controlled Variable)
OverflowPump
InflowSupply
llllll
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
Typical ON/OFF Response
L2
Periodic TimeL1
SPtime
Proportional Control
Level Control of Open Tank
Qout
LT
SPQin
V1
20 - 100 kPa
A/Cllllll
Simple Proportional System
Pivot
Outflow
Float
Valve
Inflow
lll lll
Open Tank Control
Qin
Qout
SP
LT
20 - 100 kPa
A/O
LIC
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 Control
+-
k�eSetpoint
Measurement
Error
Bias
++
Output
m= k(SP-M) + bias
Proportional Band and Gain
Proportional Band� the input change required to change the output 100%
PBgain %100
=
inputoutputgain∆∆
=
Narrow, Wide, High & Low
Wide PB � Low Gain
Narrow PB � High Gain
OutIn
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
¼ Decay Response Curve
A
A/4A/16
Syst
em R
espo
nse
time
For You To Do
! Read pp. 89-105! Answer Questions pp. 121-122, #1-19
Reset or Integral
Response Curve: Proportional Control Only
Syst
em R
esp o
nse
SP
Step Disturbance
time
Offset
Additional Control Signal Restores Process to Setpoint
Setpoint
Initial mass balance
Inflow
Outflow
Final mass balance
Reset Action
timeOffsetRemoved
Integral Action
biasedtTR
kem +
+= ∫
1
+-
k�eSetpoint
Measurement
Error
Bias
++
Output
∫ edtTR1
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%
A Couple More Things
! Reset Windup! Instability because of lag
For You to Do
! Read over text pp. 89 �110! Answer questions pp. 121-122, 1-24
Rate or Derivative
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
Simple Flow Control System
lllFT
FC
A/C
√_
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
Open Tank Level Control With Valve In Inflow
QiA/O lll
lllQo
SP
LIC LT
Typical Flow Control Loop
llFT
FIC
A/O
√_
SP
Pressure Control � Constant Bleed
PIC PT
A/CFeed
SP
Pressure Vessel PressureBleed
Split Ranged Feed and Bleed Pressure Control
PIC PT
A/CFeed
SP
Pressure Vessel Bleed
A/O
Representative Hot Bleed/Cold Service Water Heat Exchanger
TT TC
A/C
SPCooled Bleed
Hot Bleed
Cold
For You To Do
! Read pp. 106-120! Answer Questions pp. 122-123, #20-38