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APCO/TSD/PPMSB
RMPCT for Process Operators Control Algorithms 1-1
Control Algorithms
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RMPCT for Process Operators Control Algorithms 1-2
Control Algorithms
• Feedback control
• Proportional control
• Integral control• Derivative control
• Controller tuning
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APCO/TSD/PPMSB
RMPCT for Process Operators Control Algorithms 1-3
Concept of Feedback
Water
Temperature
Error =
Actual-
Desired
Move valve
to left
No action
Move valve
to right
Control
Algorithm
Temperature
Feedback
Desired
Temperature
PV
E
H C
SP ?
I’m too
hot!
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RMPCT for Process Operators Control Algorithms 1-4
Feedback Control
• Advantages : Measures the final product or
variable and so does not need an exact knowledge
of the process to work effectively
• Disadvantages : Deviation from the target musthave occurred before feedback control system will
function
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RMPCT for Process Operators Control Algorithms 1-5
Definitions
• Process Variable
– The measured value of the variable to be controlled
e.g. obtained from orifice plate or thermo-couple
– Given in engineering units – Abbreviated as “PV”
• Setpoint
– The required value of the process variable may be set
by the operator or by the output of another controller(cascaded)
– Abbreviated as “SP”
– Same units as PV
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RMPCT for Process Operators Control Algorithms 1-6
Definitions (continued)
• Error
– Measure of how far the process variable is from the
setpoint
– Abbreviated as “E”
– Defined as PV-SP
• Manipulated Variable (or Output)
– The variable that is adjusted to correct any error
– Abbreviated as “MV”, Output is abbreviated as “OP”
– Output to valve or secondary controller (cascade)
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RMPCT for Process Operators Control Algorithms 1-7
Proportional control• Change in controller output is proportional to change in
error – MV = Kc.E + C
– Where MV= manipulated variable
– E = error – Kc = controller gain
– C = valve position at zero error (0.5 or 50%)
• Proportional band = 100/Kc – % change in error required to move valve full scale
• Proportional only control results in offset – Never reaches SP
• Action is instantaneous (proportional kick)
– As soon as error is produced, control action is taken
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RMPCT for Process Operators Control Algorithms 1-8
Integral Control
• Change in controller output is proportional to sum
of past errors,
• K C = gain as used in proportional only control
• TI = reset/integral time (mins)
• 1/TI sometimes used (repeats/min)
• As long as error exists, integral action will work to
eliminate the error• Used to Remove Offset
MV K T Edt C I
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RMPCT for Process Operators Control Algorithms 1-9
Derivative control
• Change in controller output is proportional to rate
of change of error
– MV = K c.Td.(dE/dt)
– Where Td = derivative time
• Derivative action amplifies measurement noise
• Very effective if deadtime present and often
under-rated
– But problems if noisy signal
– Sensitive to tuning errors
– Provides an additional “pulse” to push the control in
the right direction
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RMPCT for Process Operators Control Algorithms 1-10
Controller Tuning
• Open loop method
– introduce step change to process in open loop
– derive controller tuning from process response
• Closed loop method
– adjust controller tuning to obtain specifc process
behavior
– derive controller tuning based on process response
• Trial and error method
– always needed for fine tuning
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RMPCT for Process Operators Control Algorithms 1-11
Tuning Objective
• Flow control
– Constant flow required
– Very fast response to setpoint or load disturbance
– Tune as fast as possible without causing excessiveoscillation or valve wear
• Level control
– Keep feed to next unit as constant as possible
– Tune as slowly as possible
– Do not tune so slow as to empty or fill vessel!
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RMPCT for Process Operators Control Algorithms 1-12
Tuning objective (Temperature to flow
cascade)• Flow controller
– Fast response required
– Small lag – Fast tuning possible
• Temperature controller
– Steady temperature required
– Larger process lag and often deadtime
– Tune fast but not so as to cause oscillationFC
TCSP
SP
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RMPCT for Process Operators Control Algorithms 1-13
Manipulated Variable Overshoot
• Most of the tuning methods do not take account of
effect on manipulated variable
– e.g. fuel valve position
• Excessive control valve movement – causes wear and accelerates failure
• Controller tuning becomes a compromise between
fast approach to setpoint and minimum MV
overshoot
– Max 15% recommended
Recommended