Topic 5

Enhanced Regulatory Control Strategies

In The Last Lecture

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

Feedforward Control– Measured Vs Unmeasured Loads– Purpose of feedforward control– Feedforward gain– Deadtime compensation– Lead-lag compensation– Testing Feedforward loops

Process Control

Feedback Control Feedforward Control

PID controller

Proportional Gain, Kp

Integral Time, τI

Derivative Time, τD

SS Gain withlead/lag & deadtime compensation

Feedforward Gain, KFF

Deadtime Compensation, θFF

Lag time, τLag

Lead time, τLead

Feedback control PID control is a type of feedback control

– To control the CV, we measure the CV– PV is “fed back” to controller for comparison with SP– Controller decides and executes new OP– CV is measured...

No knowledge of process disturbances are required– Loads need not be measured (ULOAD)

Controller takes action after CV is disturbed so perfect control is not possible

Good for short deadtime and lag time processes Not as good for long deadtime and lag time processes but still

ok If we have knowledge of disturbances, can we do anything to

improve control?

Feedback Response

Measured Load

Temperature with only feedback control

Desired temperature response

Feedforward control Knowledge on how loads affect the CV is required

– Loads must be measured (MLOAD)

When a disturbance is detected, the feedforward controller takes synchronized action before the CV is disturbed so potential for perfect control exists

Used to supplement PID controller in a long deadtime, lag time process

Requires tuning for perfect synchronization:– Feedforward gain, KFF

– Deadtime compensation, θFF

– Lag time, τLag

– Lead time, τLead

Feedforward control

TIC

FG Source

FIC

SP

+

FI

FeedforwardController

If the flow rate increases, the feedforward controller will increase the flow of FG before the FOT is disturbed.

All other unmeasured disturbances are taken care of by the PID controller (TIC).

Process dynamics

t

t

TIC.OP

TIC.PVKp , θp , τp

Process dynamics

t

t

FI.PV

TIC.PV KCV/MLOAD , θCV/MLOAD ,

τCV/MLOAD

Feedforward Gain, KFF

Whenever you see the feed flow rate increase by 1%, you want the FG flow rate to increase by 2%

Whenever you see the feed flow rate decrease by 2%, you want the FG flow rate to decrease by 4% and so on….

This relationship is the feedforward gain

Feedforward Gain, KFF

Assume you have the following information: FI.Span = 100 – 0 = 100 m3/h TIC.Span = 100 – 0 = 100C You observe that if FI.PV rises by 1 m3/h, TIC.PV

decreases by 2C Also, if TIC.OP increases by 1%, TIC.PV increases by 1C Therefore, if FI.PV rises by 1 m3/h, we can prevent TIC.PV

from changing by increasing TIC.OP by 2% We can instruct the feedforward controller to increase

TIC.OP accordingly by setting the correct FF Gain

Feedforward Gain, KFF

FI.Span = 100 – 0 = 100 m3/h

TIC.Span = 100 – 0 = 100C

You observe that if FI.PV rises by 1 m3/h, TIC.PV decreases by 2C

Also, if TIC.OP increases by 1%, TIC.PV increases by 1C

SS Gain between FI.PV (MLOAD) and TIC.PV (CV)–

SS Gain between TIC.OP (MV) and TIC.PV (CV)– %

%1%0%100

%10100

1

pK

%%2

01001

01002

/

MLOADCVK

Feedforward Gain, KFF

SS Gain between FI.PV (MLOAD) and TIC.PV (CV)–

SS Gain between TIC.OP (MV) and TIC.PV (CV)–

Feedforward Gain, KFF

– –

%%1

%0%100%1

01001

pK

%%2

01001

01002

/

MLOADCVK

p

MLOADCVFF K

KK /

212

FFK

Deadtime Compensation, θFF

While KFF deals with the disturbance and process at steady-state, it does not take into account the fact that deadtime may exist– Deadtime between FI.PV (MLOAD) and TIC.PV (CV)– Deadtime between TIC.OP (MV) and TIC.PV (CV)

Suppose θCV/MLOAD = 10 min and θp = 3 min When FI.PV changes, TIC.PV will start to change 10 min later When TIC.OP is adjusted, TIC.PV will start to change 3 min later For perfect control, when FI.PV changes, we need to adjust TIC.OP

after a fixed time (10 – 3 = 7 min)

pMLOADCVFF /

min 7310 FF

Lag Time, τLAG

Some lag time will certainly exist between FI.PV and TIC.PV

When FI.PV increases, we do not want to increase TIC.OP by a step change

Instead, we want to synchronize the increase in TIC.OP with the way TIC.PV responds to MLOAD (FI.PV)

To do this, we want slow down the change in TIC.OP. We do this by applying a lag of τCV/MLOAD to the change in TIC.OP

τLAG = τCV/MLOAD

Lead Time, τLEAD

Some lag time will also exist between TIC.OP and TIC.PV

To compensate for this lag, TIC.OP will need to be speeded up

We do this by applying a lead time of τp to TIC.OP

τLEAD = τp

Summary of FF parameters Feedforward Gain,

Deadtime compensation,

Lag time,

Lead time, pLEAD

MLOADCVLAG /

pMLOADCVFF /

p

MLOADCVFF K

KK /

FF parameters may be implemented in a single block

TIC

FG Source

FIC

SP

+

FI

FeedforwardController

FF parameters may be implemented in separate blocks

TIC

FG Source

FIC

SP

+

FI

FeedforwardGain

DeadtimeCompensation

Lead/LagCompensation

Plant Testing For FF Control There are two sets of dynamics we need

to know in order to configure a feedforward control scheme– Between OP and PV– Between MLOAD and PV

Take a furnace example where we already have a FOT controller and we wish to implement a FF controller to guard against disturbances to the feed flow rate

How do we perform our test?

TIC

FG Source

PIFIC

OP

SP

SP

FI

How do we perform our test? To get dynamics between TIC.OP (MV) and TIC.PV

(CV), perform step change test as per single feedback loop– Put loop to MAN, make step change to OP and collect data on

PV response

To get dynamics between FI.PV (MLOAD) and TIC.PV (CV),– Ensure TIC.MODE is in MAN. We do not want the PID controller

to interfere with the response of TIC.PV wrt FI.PV– Make a step change in FI.PV (assuming you can control FI.PV)

and collect data on TIC.PV response– If you cannot change FI.PV at all, and you cannot obtain

appropriate dynamic data, FF control cannot be implemented

When not to use FF control When there is no MLOADS (all loads are ULOADS) When θCV/MLOAD is less than θp (i.e. θFF < 0)

– However, you may still be able to approximate the ideal FF controller by setting θFF = 0 and trying to adjust τLAG and τLEAD

When it takes too long to get one value of MLOAD– Some analyzers may take minutes to make a measurement, by

which time the CV is already showing signs of disturbance When dynamic models are poor

– Your FF control is only as good as your underlying models– Harder to tune FF controllers by trial and error than PID

controllers

In This Lecture…

Feedforward Control– Measured Vs Unmeasured Loads– Purpose of feedforward control– Feedforward gain– Deadtime compensation– Lead-lag compensation– Testing feedforward loops– When feedforward control cannot be used

In The Next Lecture…

Split-range control

Override control

Ratio control