topic 5 enhanced regulatory control strategies. in the previous lecture… plant test interacting...
TRANSCRIPT
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Topic 5
Enhanced Regulatory Control Strategies
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In the previous lecture…
Plant test
Interacting loops
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What We Will Cover
Topic 1
Introduction To Process Control
Topic 2
Introduction To Process Dynamics
Topic 3
Plant Testing And Data Analysis
Topic 5Enhanced
Regulatory Control Strategies
Topic 7
Process Control Hardware Systems
Topic 4
Controller Actions And Tuning
Topic 8
Control Valves
Topic 9
Process Control Troubleshooting
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In This Lecture…
Cascade Control– What is cascade control– Advantages of cascade control– Features of cascade control– Testing & tuning cascade control loops
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Furnace Example
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Controlling FOT with FG
TIC
FG Source
PI
SP
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Controlling FOT with FG
In the diagram, the CV is controlled using the fuel gas valve opening as an MV
From our plant test, we obtain values for Kp
– Kp tells us how much to open our valve by if we want the FOT to increase by a certain amount
– Using this Kp value together with θ and τ, we calculate PID tuning parameters
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Controlling FOT with FG
TIC
FG Source
PI
SP
•Take a situation where TIC is in AUTO and the PV is at SP.
•Suddenly, the FG source pressure increases.
•What do you think will happen to the FG flow?
•What do you think will happen to the FOT?
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Controlling FOT with FG If the FG source pressure increases, for a constant valve
opening, the FG flow will increase
FOT increases due to the increase in FG flow. This happens after a while, because of deadtime and lag
When will the controller start to take action?– The controller starts to reduce the valve opening only after the
FOT starts to increase
Can we prevent the FOT from increasing by reducing the valve opening early?
Yes, control the FG flow rate instead.– If the FG flow rate increases due to increased pressure, the flow
controller will close the valve to keep the flow rate to SP.– Because flow dynamics is much faster than FOT dynamics, FG
flow rate will be brought back to SP before FOT gets disturbed
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Controlling FOT with FG
TI
FG Source
PIFIC
SP
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Controlling FOT with FG
Now, the control valve is the MV for controlling the FG flow rate
To control the FOT, the flow SP will need to be adjusted either manually or automatically
To do this automatically, we use a FOT controller and we link its OP to the SP of the flow controller
This is called a Cascade Controller
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TIC
FG Source
PIFIC
OP
SP
SP
FOT Cascade Control
Master or Primary controller
Slave or Secondary controller
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Controlling FOT with FG The FOT PID controller calculates what FG flow rate is required
to maintain FOT to SP
The flow PID controller calculates what valve opening is required to maintain flow rate to SP
Because the flow dynamics are faster than the FOT dynamics– Disturbances to the flow rate are detected much faster than
disturbances to the FOT– Disturbances to the flow rate are rejected much faster than
disturbances to the FOT
Flow controller can take action to restore flow rate to SP before it disturbs the FOT
FOT controller to control valve, on the other hand, requires the FOT to be disturbed before it can take action
Cascade control allows the FOT to be isolated from disturbances to the FG flow
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Different dynamics at different P Consider the single-loop case again: CV-FOT, MV-Valve
opening
At a normal pressures,– Increasing the valve opening results in a corresponding increase
in FOT– SS Gain Kp obtained during plant test
At higher pressures,– Increasing the valve opening results in a greater increase in FOT
due to the larger increase in FG flow– Higher SS Gain, Kp, at higher pressures– Ideally, PID tuning should be changed because of different
dynamics
Cascade control allows the primary controller to be isolated from changes in dynamics in the secondary loop
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Cascade Control
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Features Of Cascade Control At least two controllers are involved
– A primary (master) controller– A secondary (slave) controller– More complex cascade controls have one or more secondary
loops and one ultimate primary and one ultimate secondary
The primary controller is the one whose PV is the important one
In the Furnace example, the FOT controller is the primary controller because we care about the FOT, not the FG flow rate
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Features Of Cascade Control
The OP of the primary controller becomes the SP of the secondary controller
They are configured such that OP (in %) is changed into SP (in other engineering units like kg/hr etc…)– E.g. span of secondary is 100~300 kg/hr– Pri.OP = 0% Sec.SP = 100 kg/hr– Pri.OP = 100% Sec.SP = 300 kg/hr– Pri.OP = 50% Sec.SP = 200 kg/hr
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Features of Cascade Control Important!
Cascade control will only work if the secondary loop dynamics is much faster than the primary loop– Secondary loop has shorter deadtime and shorter lag– This means secondary loop is tuned much faster, i.e. smaller τI
This allows the secondary to bring its CV to SP before the primary demands a new SP change
If primary is faster than secondary,– Primary controller will adjust the secondary’s SP faster than the
secondary can bring its PV to SP– Primary controller does not know the secondary has not reached SP
yet– All it knows is that its own PV has not reached SP and it will adjust the
secondary’s SP by larger amounts subsequently
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Advantages Of Cascade Control
2 advantages:
Disturbances arising within the secondary loop are corrected by the secondary controller, often before they can influence the primary variable
SS Gain variations in the secondary part of the process are overcome within its own loop
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Testing Cascade Control Loops
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Testing Cascade Control Loops
Remember that there are two loops to test– Secondary loop (fuel gas flow)– Primary loop (furnace outlet temperature)
Which one to test first?
The secondary must work well first, before the primary can do its job– Imagine a good boss with loads of vision and insight
instructing a lazy employee! No matter how good the instructions are, will work get done?
– Slacker students will never do well even if the lecturer is excellent! Good thing that all present here are dilligent...
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TIC
FG Source
PIFIC
OP
SP
SP
Tuning Cascade Control Loops
Tune primary after tuning secondary
Tune secondary first
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Testing The Secondary Loop
Always start with the secondary loop first– Put it in manual– Step it a few times– Identify its dynamics– Tune the flow controller– Test it to ensure that it works
Test it as per single-loop controller (Topic 3)
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Testing The Primary Loop
Primary loop in what mode? – Manual
Secondary loop in what mode?– Cascade
Step the primary’s OP and observe its CV response
TIC
FIC
OP
SP
OPMAN
(Manual)
CAS(Cascade)
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Tuning Of Cascade Loops
Important reminder:
It is critical that the secondary loop must be working properly on its own if it is to act as a secondary loop
The secondary loop must be a much faster loop compared to the primary loop
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In This Lecture…
Cascade Control– What is cascade control– Advantages of cascade control– Testing cascade control loops– Tuning cascade control loops
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In The Next Lecture…
Feedforward Control– Measured Vs Unmeasured Loads– Purpose of feedforward control– Feedforward gain– Deadtime compensation