Topic 5

Enhanced Regulatory Control Strategies

In the previous lecture…

Plant test

Interacting loops

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

In This Lecture…

Cascade Control– What is cascade control– Advantages of cascade control– Features of cascade control– Testing & tuning cascade control loops

Furnace Example

Controlling FOT with FG

TIC

FG Source

PI

SP

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

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?

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

Controlling FOT with FG

TI

FG Source

PIFIC

SP

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

TIC

FG Source

PIFIC

OP

SP

SP

FOT Cascade Control

Master or Primary controller

Slave or Secondary controller

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

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

Cascade Control

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

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

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

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

Testing Cascade Control Loops

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...

TIC

FG Source

PIFIC

OP

SP

SP

Tuning Cascade Control Loops

Tune primary after tuning secondary

Tune secondary first

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)

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)

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

In This Lecture…

Cascade Control– What is cascade control– Advantages of cascade control– Testing cascade control loops– Tuning cascade control loops

In The Next Lecture…

Feedforward Control– Measured Vs Unmeasured Loads– Purpose of feedforward control– Feedforward gain– Deadtime compensation