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OVERVIEW OF CONTROL SYSTEM DESIGN
LECTURE 02
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Objectives Students should be able to
◦ask necessary questions for developing a control system.
◦determine possible control objectives, input and output variables, and constraints.
◦assess the importance of process control from safety, environmental and economics point of view
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Although the specific control objectives vary from plant to plant, there are a number of general requirements:
◦ Safety ◦ Stable plant operation ◦ Product specifications and production rate
◦ Environmental regulations
◦ Economic plant operation
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Steps of Control System Design
◦ The most widely used process control strategy is multi-loop control.
◦ A multi-loop control system consists of a set of PI, PID controllers, one for each controlled variable.
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Steps of Control System Design The key design decision for multi-loop control is to determine an ………………….., that is, to find a ………………… of controlled and manipulated variables.
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The first step of developing a control strategy is to
Formulate control objective (s)
What are the control objectives ?
…………………..
…………………..
………………….
Typically they are dependent on the process boundary.
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Coal-fired power plant
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Steps of Control System Design
After the control objectives have been formulated, the control system can be designed. The first step is to identify:
Select input variables: …………………. and …………………… variables
Select output variables: ……………….. and ………………….. variables
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Classification of the Variables in Chemical Process Control System
Input Variables :- effect of the surroundings on the chemical process
Output Variables:- effect of the process on the surroundings
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Input Variables :- Effect of the surroundings on the chemical process
Manipulated (or adjustable) variables • they can be adjusted freely by the human operator or a control mechanism.
Disturbances • their values are not the result of adjustment by an operator or a control system.
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Output Variables :- Effect of the process on the surrounding
Measured output variables • their values are known by directly measuring them.
Unmeasured output variables • they are not or cannot be measured directly.
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Process: A tank with level controller
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LT LC
Fi
Fo
SP
h
Process variables are:
h, Fi, F0, A, r
0FFdt
dhA i rrr
What are the input and output variables?
Process: A tank with level controller
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LT LC
Fi
F0
SP
h
Process variables are: h, F
i, F0, A, r
0FFdt
dhA i rrr
Input variables are: ………….
Process: A tank with level controller
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LT LC
Fi
F0
SP
h
Process parameters are:
A, r
0FFdt
dhA i rrr
Output variables are: ……
Steps of Control System Design
After that: we have to realize
operating constraints such as: ◦ hard constraints: max or min flow rate ◦ soft constraints: product composition
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Steps of Control System Design
What are the operating characteristics of the considered process?
◦ Continuous
◦ Batch ◦ Semibatch
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Steps of Control System Design
Safety, environmental, and economic considerations are all important.
◦Economics is the driving force
◦Unsafe and environmentally hazardous process will cost more to operate
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Steps of Control System Design
Then, control structure or control configuration will have to select and implement to control the process.
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Control configurations
A control configuration is the ……………………………………….. that is used to connect the available ……………………. to the available …………………… variables.
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Control configurations
Feedback control
Feedforward control
Feedback-Feedforward control
Cascade control
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Feedback control
CONFIGURATION
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Feedback control
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We want to keep the
liquid level at a certain
value.
Fi
Fo
What measurement should be performed?
Feedback control
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Fi
Fo
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0FFdt
dhA i rrr
LC SP
LT
Fi
Fo
),(0 SPhhfF
SPc hhKF 0
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SP
Fi
Fo
Controller provides
linkage between flow
out and liquid level.
Feedback control System
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Which variable do we want to control?
Which variable can we monitor?
Which variable can we adjust?
Feedback control
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LC SP
LT
Fi
Fo
Another alternative, if Fi is free from upstream process.
Feedback control
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Another
alternative, if Fi is
free from upstream process.
LC SP
LT
Fi
Fo
0FhhKdt
dhA SPc rrr
Feedforward control
CONFIGURATION
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Feedforward control
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We also want to
keep the liquid level at a certain value.
Fi
F
Feedforward control
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We also want to
keep the liquid
level at a certain value.
What is feed-forward control configuration?
Fi
F
Feedforward control
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What question should be asked?
What make the level change?
liquid feed
Fi
F
Feedforward control
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What make the level changed?
Fi
F
Feedforward control
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How to take the action?
Which flow should be changed?
There are two alternatives.
Fi
F
Feedforward control
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First alternative
Fi
F
Feedforward control
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Second alternative
Fi
F
Feedforward control
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Water bath Heater
We want to keep T inside the heater constant.
Fi, Ti
Steam
condensate
F, T
Fs, Ts
Feedforward control
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Water bath Heater
Fi, Ti
Steam
condensate
F, T
Revisit: A Blending Process
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m1, x1
m, x
Mt m2, x2=1
x1 is varied
One solution
CT
CC
M
What control configuration is in this set-up?
Revisit: A Blending Process
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m1, x1
m, x
Mt m2, x2=1
x1 is varied
Another solution
CT CC
M
What control configuration is in this set-up?
Consider both configurations
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What are the advantages of feedback control?
What are the advantages of feedforward control?
How about disadvantages?
Consider both configurations
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Apply a new configuration
Use both feedback and feedforward control
Feedback-Feedforward control
CONFIGURATION
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Feedforward control
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Feedback-
FT
FFW
+
LT LC
SP
+
Two control signals
combine together.
What are major differences between FB and FF control?
Monitor ……………….. variable (FB)
Monitor …………………. (FFW)
Adjust the ……………… input variable to compensate the change
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Cascade control
CONFIGURATION
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Cascade control Cascade control is a multi-loop feedback control configuration.
Typical there is one manipulated variable, but several controlled variables, (SIMO).
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Cascade control
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FT
LT
LC SP
FC SP
Develop a model for an isothermal CSTR
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AAi CCF 0 VCek A
RT
Eq
0dt
dCV A
For first-order elementary reaction
with Arrhenius eqn.
F CA
Fi CA0
A --> B CA
RT
EkTk aexp)( 0
What are the input and output variables?
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Select the control configuration How do we decide on the control configuration?
◦Accuracy
◦Speed of response
◦Fluctuation
◦Budget
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Steps of Control System Design After selecting the control structure, control algorithm or controller type has to choose for taking action during the controller correction.
The most widely used process control strategy is multi-loop control.
A multi-loop control system consists of a set of PI, PID controllers, one for each controlled variable.
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Steps of Control System Design Typical types of control algorithm or controller are
◦ ON/OFF controller
◦ PID controller
◦ Fuzzy logic controller
◦ Model based controller
etc.
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Revisit
If the unit of (1.0) is m3, what is the unit of (0.7)?
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FCFCdt
dCii 7.05.20.15.00.1
Steps of Control System Design
The most popular control algorithm or controller in industries are:
ON/OFF controller
PID controller ◦Proportional controller
◦Proportional integral controller
◦Proportional integral derivative controller
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Controller algorithm / PID controller
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dtde
D
t
c edteKCOi
0
1
msp yye
CO = Controller output
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Controller algorithm / PID controller
dtde
D
t
c edteKCOi
0
1
msp yye
Proportional Controller
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Time
Controlled Variable
Setpoint
0 t
{ysp
(t)-y(t)}
Proportional action
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PID controller
dtde
D
t
c edteKCOi
0
1
msp yye
Integral Controller
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Time
Controlled Variable
Setpoint
0 t
t
dtte0
)( = shade area
Integral action
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dtde
D
t
c edteKCOi
0
1
msp yye
Derivative Controller
PID controller
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Time
Controlled Variable
Setpoint
0 t
{ysp
(t)-ym
(t)}
Magnify
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t tdt
dy
Derivative action
Data for each mode Proportional controller uses current data for decision making
Integral controller uses past til present data for decision making
Derivative controller uses future data for decision making
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Design control objective Which variable will be kept constant?
Level
Temperature
Pressure
Concentration
Flow
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Selection of Controlled Variables
Guideline 1
All variables that are not self regulating must be controlled.
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Which one is self regulating system?
F is the function of the liquid height
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Fi
F
Which one is self regulating system?
F has constant flow rate
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Fi
F
Selection of Controlled Variables
Guideline 1
Choose output variables that must be kept within equipment and operating constraints
Select output variables that represent a direct measure of product quality
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Selection of Controlled Variables
Guideline 2
Choose output variables that seriously interact with other controlled variables such as steam header pressure is affected the pressure of downstream units
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Selection of Controlled Variables
Guideline 2
Choose output variables that have favorable dynamic and static characteristics.
Variable that has large time delay or insensitive to manipulated variables are poor choices.
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Selection of Manipulated Variables
Guideline 3
Choose input variables that have the large impact on controlled variables.
Choose input variables that rapidly affect the controlled variables.
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Selection of Manipulated Variables
Guideline 3
The manipulated variables should affect the controlled variables directly rather than indirectly.
Avoid recycling of disturbances
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Selection of Measured Variables
Guideline 4
Reliable, accurate measurements are essential for good control
Select measurement points that have an adequate degree of sensitivity
Select measurement points that minimize time delays and time constants.
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Select the measurements Besides the guideline 4, the following issue has to be considered as well:
The availability of the sensor
Its operating range
Its response time
Its cost
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Type of Pressure Sensors
1. Gage pressure sensors
2. Vacuum pressure sensors
3. Differential pressure sensors
4. Absolute pressure sensors
5. Barometric pressure sensors
5 2 3
1
Pressure sensor
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Temperature sensor
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Resistance Temperature Detector, RTD
Thermocouple
Level sensor
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Float-type level sensors
Flow sensor
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Concentration sensor
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Steps of Control System Design
The key design decision for multi-loop control is to determine an appropriate control structure, that is, to find a suitable pairing of controlled and manipulated variables.
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Select control configuration Feedback control system
Feedforward control system
Feedback- Feedforward control system
Cascade control system
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Select a controller Finally, select a controller
◦ON/OFF controller
◦PID controller
◦Fuzzy logic controller
◦Model based controller
◦etc.
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Process: A tank with level controller
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LT LC
Fi
F0
SP
h
0FFdt
dhA i rrr
At the controller:
hm is sent by a sensor.
hm is compared with hsp.
e = (hm – hsp)
F0 = F0s+ Kce
F0 = F0s + Kc(hm – hsp)
Process: A tank with level controller
)(0 spmcsi hhKFFdt
dhA rrr
)(0 spmcsi hhKFFdt
dhA
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LT LC
Fi
Fo
SP
h
0FFdt
dhA i rrr
For liquid
Control systems present in the form of Block diagram
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The Feedback control loop can be simplified in a block diagram.
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Converter
Controller Final
Control Element
Process
Measuring devices
+ -
Set
Point
y
ym
e
The Feedforward control loop can be simplified in a block diagram.
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Converter
Controller Final
Control Element
Process
Measuring devices
+ -
Set
Point
y
ym
e
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LC
LT
LC
LT CT CC
TT TC
FT FC
Distillation column and its control system
Hierarchy of process control activities
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Process
1. Measurement & actuation
2. Safety and Environmental Equipment Protection
3. Regulatory control
4. Multivariable and constraint Control
5. Real-time Optimization
6. Planning and Scheduling
< 1 sec.
Days-months
Hrs-Days
Minutes-Hrs
Sec.-Minutes
< 1 sec.
Advantages of Automatic Control Systems Less human manual effort
Lower labor costs
Better uniformity and possibility of products
Increased production
Saving in Raw Material
Saving in Energy
Saving in Plant Equipment
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The development of a control strategy consists of formulating or identifying the following.
Control objectives
Input variables
Output variables
Constraints
Operating characteristics: batch, continuous, semibatch,
Safety, environmental, and economic considerations
Control structure / controller
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Tutorial 2.2 Design a control system in order to
obtain the desired concentration of the solution at 20% wt
produce the solution at a specified flow rate
maintain the level in the dilution tank
reduce the temperature of the solution before sending it to the dilution tank
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Three general classes of needs that a control system is called on to satisfy:
Suppressing the influence of external disturbances
Ensuring the stability of a chemical process
Optimizing the performance of a chemical process
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A Process to be controlled
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End of Slide set 02
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