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Fundamental Principles of Process Control
Class 2
1. Motivation for Process Control
• Safety First:– people, environment, equipment
• The Profit Motive:– meeting final product specs– minimizing waste production – minimizing environmental impact – minimizing energy use– maximizing overall production rate
“Loose” Control Costs Money
• It takes more material to make a product thicker, so greatest profit is to operate as close to the minimum thickness constraint as possible without going under
• It takes more processing to remove impurities, so greatest profit is to operate as close to the maximum impurities constraint as you can without going over
3.6
3.8
4.0
4.2
4.4
60 80 100 120 140
Process: Gravity Drained Tank Controller: Manual Mode
Mor
e P
rofit
able
Ope
ratio
n
Time (mins)
30
40
50
60
70
Mor
e P
rofit
able
Ope
ratio
n
operating constraint
poor control means large variability, sothe process must beoperated in a lessprofitable regionprocess variable
Examples
Tight Control = Most Profitable Operation
• A well controlled process has less variability in the measured process variable, so the process can be operated close to the profitable constraint
3.4
3.6
3.8
4.0
4.2
80 100 120 140 160
Process: Gravity Drained Tank Controller: Manual Mode
Mor
e P
rofit
able
Ope
ratio
n
Time (mins)
30
40
50
60
70M
ore
Pro
fitab
le O
pera
tion
operating constraint
process variable
tight control permitsoperation near theconstraint, whichmeans more profit
Poorly Maintained Control Loops Can Compromise Product Yield,
Quality, and Throughput
Cont
roll
oop
perf
orm
ance
Time
Opt imizat ion pot ent ial
Of f cont rolPoorly t unedcont rol
Idealcont rol
Profitability of the Process Can Be Improved by
Reducing Variability in the Control Loop
Loop variabilit y Loop t uned Increased product ionor qualit y
$
Standard Deviation Provides a Benchmark for Controller Performance
0.6 0.8 1 1.2 1.4 1.6
set point
st dev = 0 .025
st dev = 0.1
-3 +3
8
Terminology for Home Heating Control
– Control Objective– Measured Process Variable (PV)– Set Point (SP)– Controller Output (OP)– Manipulated Variable (MV)– Disturbances (D)
furnace
temperaturesensor/transmitter
set pointheat loss
(disturbance)
thermostatcontroller
valve
TTTC
controlsignal
fuel flow
Copyright © 2007 by Control Station, Inc. All Rights Reserved
Automatic Control isMeasurement Computation Action
• Is house cooler than set point? ( TSetpoint Thouse > 0 )
Action open fuel valve
• Is house warmer than set point? ( TSetpoint THouse < 0 )
Action close fuel valve
Components of a Control LoopHome heating control block diagram
Set PointThermostat Home Heating
ProcessFuel Valve
TemperatureSensor/Transmitter
Heat LossDisturbance
+-
house temperaturemeasurement
signal
controllererror
manipulated fuel flow to
furnace
house temperature
controlleroutputsignal
General Control Loop Block Diagram
Set PointController Process
FinalControlElement
MeasurementSensor/Transmitter
Disturbance
feedbacksignal
controllererror
manipulated process variable
measured processvariable
controlleroutput
+-
Example: Heated Tank
Ti
wi
To
woHeater
Q
• Controlled variable:– To
• Feedback– Measure T0
– Control voltage to heater
TT
VC
Example: Heated Tank
Ti
wi
To
woHeater
Q
• Controlled variable:– To
• Feedback– Measure T0
– Control voltage to heater
• Feedforward– Measure Ti
– Control voltage to heater
TT
VC
In-Class Activity
Other Definitions
• Final Control Element– Usually a valve or pump– Electricity (to heat or cool)– Solenoid valve (open or shut)
• SISO– Single input, single output
• NomenclatureFC
Flow (to becontrolled)
Controller
LT
Level (of liquid)Transmitted (or sensor)
Application
• First Order Plus Time Delay (FOPTD)
time-DeadGain
constant Time
)(
K
tKuxtx
Modeling and Control
• x = 2.2 m/s – Desired Velocity • m = 400 kg (cart and donkey)• b = 45 N-s/m (resistive coefficient)• c = 100 N/(12 switches per minute)• u = ? (switches/min)• Time delay = 2 seconds
Force Balance
)2( tucxb
txm
time-DeadGain
constant Time
)(
K
tKuxtx
Import donkey_cart.cs4
Control Station Exercise
