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This lecture was delivered by Dr. Sakal Japendu for Process Control course at Ambedkar University, Delhi. It includes: Feedback, Loop, Elements, Variables, Control, Manipulate, Performance, Data, Measures, Dynamic
Typology: Slides
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When I complete this chapter, I want to be
able to do the following.
Identify the major elements in thefeedback loop
-^
Select appropriate candidate variables tobe controlled and manipulated
-^
Evaluate the control performance datausing standard measures of dynamicperformance
Outline of the lesson.
Typical loop elements
-^
Relating variables to control objectives- Examples
-^
Typical control performance measures
-^
Five approaches to feedback control
v1 T A
v
The Reality: Many elements in the loop affect the safety,reliability, accuracy, dynamics and cost. Engineers needto understand the details!
4-20 mA
4-20 mA
3-15 psi
Sensor &transmitter
electronictransmission
input and A/Dconversion
computingnetwork
electronictransmission
I/P conversion
pneumatictransmission
finalelement
Make a step(without feedback control)
What affects the response to thecomputer?
outputand D/A
Class exercise: Giventhe following dynamics,sketch the responses fora step in manual stationto the displayed value.What you see (from thedisplay) is not alwayswhat is occurring!!
The engineer must decide what measurement to controland what valve to adjust (and provide the equipment tosupport the decisions).
The engineer must decide what measurement to controland what valve to adjust (and provide the equipment tosupport the decisions).
Causal relationship
2.^
Automated
3.^
Fast dynamics
4.^
Compensate for large disturbances
5.^
Can adjust quickly with little adverse affect on processperformance
The engineer must decide what measurement to controland what valve to adjust (and provide the equipment tosupport the decisions).
Input
variables
that
affect
the measured variable
Selected adjustable flow
Manipulated valve
Feed temperatureSolvent flow rateFeed composition, before mix disturbances
Coolant inlet temperatureFlow of pure A
Flow of pure A
v^ A
adjustable
Flow of coolant
We could use either valve.We will revisit this choice
later (Chapter13)
- FT^1 FT - PT Music: āI cannot define good music, but I know what I like.āControl Performance: We must be able to define what we desire, so that we can design
equipment and controls to achieve our objectives.
Set pointentered byperson
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1.5^1 0.5^0
Time
Controlled Variable^0
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(^2) 1.5 (^1) 0.5 0
Time
Manipulated Variable
Controlled variable, value from a sensorManipulated variable, usually a valve
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1.5^1 0.5^0
Time
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(^2) 1.5 (^1) 0.5 0
Time
|SP(t)-CV(t)| dt
Set point Change
Return to set point,āzero offset
Rise time
D
B
B/A = Decay ratio C/D = Maximum overshoot of manipulated variable
C
A
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Disturbance Response
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0.8 0.6 0.4 0.2^0 -0.
Time
0
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(^0) -0.5 -1 -1.
|SP(t)-CV(t)| dt Maximum CV deviation from set point
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Class exercise: For each of the performance measuresbelow, determine a good value, i.e., large/small,positive/negative, etc.
Offset
-^
Decay ratio
-^
Rise time
-^
Settling time
MV overshoot
-^
Maximum CVdeviation
-^
CV variance
-^
MV variance
Can we achieve good values for all at the same time?
What are the tradeoffs?
-^ To reduce the variability in the CV,we increase the variability in the MV. ā¢^ We must design plant with MVāsthat can be adjusted at low cost.
(^20100) -10 -20^0 100 200 300 400 500 600 700 800 900 1000
Time
Controlled Variable
(^20100) -10 -20^0 100 200 300 400 500 600 700 800 900 1000
Time
Manipulated Variable