Basic Of Process Control
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Basic Process Control
Basic Control Principles Inflow Supply
l ll
l ll
Level (Controlled Variable) Pump
Overflow
Open vs Closed Loop !
Closed loop !
!
Automatic control
Open Loop ! !
Manual control Person takes the place of the controller
Feedback Control Disturbances
Controller Set Point
Comparator
Error
Amplifier
(SP - M)
Manipulated Variable
Process (Tank)
Output (Controlled Variable - Level)
Measurement Signal
Process Sensor
Feedback vs. Feedforward !
Feedback !
!
Control action after an error exists
Feedforward !
Reacting to the disturbance before the error occurs
Typical ON/OFF Control System V1
SP
l ll
Qout
Qin
l ll
L2 L1
P1
P1 closes @ L1 P1 opens @ L2
Inflow
Electrically Operated Solenoid valve Solenoid Power Supply
Typical ON/OFF Response
L2
SP
time
L1
Periodic Time
Proportional Control
Level Control of Open Tank V1
SP
l ll
Qout
Qin
l ll
A/C
LT
20 - 100 kPa
Simple Proportional System Inflow Valve
Pivot
l l
Float
ll l
Outflow
Open Tank Control A/O 20 - 100 kPa
Qin SP
LIC
LT Qout
Controller Action A/C
SP
l ll
Qout
Qin
l ll
Direct
20 - 100 kPa LIC
LT A/O
SP
Reverse
l ll
Qout
Qin
l ll
20 - 100 kPa LIC
LT
Proportional Control Setpoint
+
Error -
k•e
+
+
Bias
Measurement
m= k(SP-M) + bias
Output
Proportional Band and Gain Proportional Band – the input change required to change the output 100%
∆output gain = ∆input
100% gain = PB
Narrow, Wide, High & Low In Wide PB – Low Gain
Narrow PB – High Gain
Out
Proportional Control Response Curve New mass balance occurs here
Loss in Volume
Outflow Inflow
Input/Output t0
time
t1 t
Level originally at setpoint Offset
Level t0
t1
time
New level below setpoint
Proportional Response with Narrower PB New mass balance occurs here
Loss in Volume
Outflow Input/Output
Inflow t0
time
t1 t
Level originally at setpoint Level
Offset
t0
New level below setpoint
t1 time
Load Change
Response Versus PB, Proportional Control Only
SP
Step Disturbance
time
"Wide" PB
System Response
Offset
SP
SP
"Moderate" PB Offset
"Narrow" PB
time
System Response
¼ Decay Response Curve
A A/16 A/4
time
For You To Do ! !
Read pp. 89-105 Answer Questions pp. 121-122, #1-19
Reset or Integral
Response Curve: Proportional Control Only
System Response
Step Disturbance
time
SP Offset
Additional Control Signal Restores Process to Setpoint Initial mass balance
Final mass balance
Outflow Reset Action Inflow Setpoint Offset Removed
time
Integral Action 1 edt ∫ TR
Setpoint
+
Error -
Measurement
k•e
+
+
Bias
1 m = ke + edt + bias ∫ TR
Output
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 error
Control Signal
e
Fa
}
st
se e R
t No
ke Proportional Response
R es l a rm
et
Slow Reset
time
A problem Output initially 50%, Gain = 2, reset = 2 minutes per repeat A 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
Output
Input
Proportional and Derivative – Open Loop Pressure
Derivative ceases as error stops changing
Proportional Action Derivative
time
Derivative Control 1 edt ∫ TR
Setpoint
+
Error
k•e
-
+ + +
+
Output +
Bias Measurement
TD
de dt
1 de + bias m = ke + ∫ edt + kTD dt TR
Simple Flow Control System
_l l l √ FT
A/C
FC
Process
The open Loop Response of Proportional Plus Derivative (PD) Action to Rapidly Changing Error Signals
Control Signal %
B A
C
t0
t1 t2
time
Proportional action A-B Rate action due to cessation of increase in e Rate Rate action action B -C A-B Control signal at end of excursion Rate action due to cessation of increase in e
Proportional action B - C t0
t1 t2
time
Large System Under Proportional and Proportional Plus Derivative Control Control Signal
Load Disturbance Applied Prop. + Derivative Prop. Only Setpoint
Level
time
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
A/O
LIC SP
LT
l ll
Qi
Qo
l ll
Typical Flow Control Loop
_l l √ FT
A/O
FIC SP
Pressure Control – Constant Bleed SP PIC
Feed
PT
Pressure Vessel A/C
Pressure Bleed
Split Ranged Feed and Bleed Pressure Control SP PIC
PT
A/O Feed
Pressure Vessel A/C
Bleed
Representative Hot Bleed/Cold Service Water Heat Exchanger Hot Bleed Cold A/C
TT
Cooled Bleed
TC SP
For You To Do ! !
Read pp. 106-120 Answer Questions pp. 122-123, #20-38
Basic Control Principles Inflow Supply
l ll
l ll
Level (Controlled Variable) Pump
Overflow
Open vs Closed Loop !
Closed loop !
!
Automatic control
Open Loop ! !
Manual control Person takes the place of the controller
Feedback Control Disturbances
Controller Set Point
Comparator
Error
Amplifier
(SP - M)
Manipulated Variable
Process (Tank)
Output (Controlled Variable - Level)
Measurement Signal
Process Sensor
Feedback vs. Feedforward !
Feedback !
!
Control action after an error exists
Feedforward !
Reacting to the disturbance before the error occurs
Typical ON/OFF Control System V1
SP
l ll
Qout
Qin
l ll
L2 L1
P1
P1 closes @ L1 P1 opens @ L2
Inflow
Electrically Operated Solenoid valve Solenoid Power Supply
Typical ON/OFF Response
L2
SP
time
L1
Periodic Time
Proportional Control
Level Control of Open Tank V1
SP
l ll
Qout
Qin
l ll
A/C
LT
20 - 100 kPa
Simple Proportional System Inflow Valve
Pivot
l l
Float
ll l
Outflow
Open Tank Control A/O 20 - 100 kPa
Qin SP
LIC
LT Qout
Controller Action A/C
SP
l ll
Qout
Qin
l ll
Direct
20 - 100 kPa LIC
LT A/O
SP
Reverse
l ll
Qout
Qin
l ll
20 - 100 kPa LIC
LT
Proportional Control Setpoint
+
Error -
k•e
+
+
Bias
Measurement
m= k(SP-M) + bias
Output
Proportional Band and Gain Proportional Band – the input change required to change the output 100%
∆output gain = ∆input
100% gain = PB
Narrow, Wide, High & Low In Wide PB – Low Gain
Narrow PB – High Gain
Out
Proportional Control Response Curve New mass balance occurs here
Loss in Volume
Outflow Inflow
Input/Output t0
time
t1 t
Level originally at setpoint Offset
Level t0
t1
time
New level below setpoint
Proportional Response with Narrower PB New mass balance occurs here
Loss in Volume
Outflow Input/Output
Inflow t0
time
t1 t
Level originally at setpoint Level
Offset
t0
New level below setpoint
t1 time
Load Change
Response Versus PB, Proportional Control Only
SP
Step Disturbance
time
"Wide" PB
System Response
Offset
SP
SP
"Moderate" PB Offset
"Narrow" PB
time
System Response
¼ Decay Response Curve
A A/16 A/4
time
For You To Do ! !
Read pp. 89-105 Answer Questions pp. 121-122, #1-19
Reset or Integral
Response Curve: Proportional Control Only
System Response
Step Disturbance
time
SP Offset
Additional Control Signal Restores Process to Setpoint Initial mass balance
Final mass balance
Outflow Reset Action Inflow Setpoint Offset Removed
time
Integral Action 1 edt ∫ TR
Setpoint
+
Error -
Measurement
k•e
+
+
Bias
1 m = ke + edt + bias ∫ TR
Output
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 error
Control Signal
e
Fa
}
st
se e R
t No
ke Proportional Response
R es l a rm
et
Slow Reset
time
A problem Output initially 50%, Gain = 2, reset = 2 minutes per repeat A 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
Output
Input
Proportional and Derivative – Open Loop Pressure
Derivative ceases as error stops changing
Proportional Action Derivative
time
Derivative Control 1 edt ∫ TR
Setpoint
+
Error
k•e
-
+ + +
+
Output +
Bias Measurement
TD
de dt
1 de + bias m = ke + ∫ edt + kTD dt TR
Simple Flow Control System
_l l l √ FT
A/C
FC
Process
The open Loop Response of Proportional Plus Derivative (PD) Action to Rapidly Changing Error Signals
Control Signal %
B A
C
t0
t1 t2
time
Proportional action A-B Rate action due to cessation of increase in e Rate Rate action action B -C A-B Control signal at end of excursion Rate action due to cessation of increase in e
Proportional action B - C t0
t1 t2
time
Large System Under Proportional and Proportional Plus Derivative Control Control Signal
Load Disturbance Applied Prop. + Derivative Prop. Only Setpoint
Level
time
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
A/O
LIC SP
LT
l ll
Qi
Qo
l ll
Typical Flow Control Loop
_l l √ FT
A/O
FIC SP
Pressure Control – Constant Bleed SP PIC
Feed
PT
Pressure Vessel A/C
Pressure Bleed
Split Ranged Feed and Bleed Pressure Control SP PIC
PT
A/O Feed
Pressure Vessel A/C
Bleed
Representative Hot Bleed/Cold Service Water Heat Exchanger Hot Bleed Cold A/C
TT
Cooled Bleed
TC SP
For You To Do ! !
Read pp. 106-120 Answer Questions pp. 122-123, #20-38
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