Advanced Control Seborg Chapter 15 16

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Advanced Control Techniques

Dr. U. D Dwivedi

(Assistant Professor)

Rajiv Gandhi Institute of Petroleum Technology,

Raebareli


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AT Analyzer (Composition) Transmitter FT Flow Transmitter

LT Level Transmitter PT Pressure Transmitter TT Temperature Transmitter FC Flow Controller LC Level Controller

TC Temperature Controller PC Pressure Controller LL Liquid Level LI Level Indicator TI Temperature Indicator PV Pressure Valve PI Pressure Indicator I/P Current to Pressure transducer

I ns t r um en t Abbr ev ia t ion s


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Feedforward and Ratio Control

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Introduction : Feedforward Control

Control Objective: Maintain Y at its set point, Ysp, despite

disturbances.

Feedback Control: Measure Y, compare it to Ysp, adjust U so as to maintain Y at Ysp. Widely used (e.g., PID controllers)

Feedback is a fundamental concept

Feedforward Control: Measure D, adjust U so as to maintain Y at Ysp. Note that the controlled variable Y is not measured.

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Comparison of Feedback and Feedforward Control

1) Feedback (FB) Control

Advantages:Corrective action occurs regardless of the source and type

of disturbances.Requires little knowledge about the process (For example,a process model is not necessary).

Versatile and robust (Conditions change? May have tore-tune controller).

Disadvantages:FB control takes no corrective action until a deviation in thecontrolled variable occurs.

FB control is incapable of correcting a deviation from set point atthe time of its detection.Theoretically not capable of achieving perfect control.For frequent and severe disturbances, process may not settleout.

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2) Feedforward (FF) Control

Advantages:Takes corrective action before the process is upset (cf. FB

control.)Theoretically capable of "perfect control"Does not affect system stability

Disadvantages:Disturbance must be measured (capital, operating costs)

Requires more knowledge of the process to be controlled(process model)Ideal controllers that result in "perfect control: may be physicallyunrealizable. Use practical controllers such as lead-lag units

3) Feedforward Plus Feedback Control

FF ControlAttempts to eliminate the effects of measurable disturbances.FB ControlCorrects for unmeasurable disturbances, modeling errors, etc.

(FB trim)

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4) Historical Perspective :

1925: 3 element boiler level control1960's: FF control applied to other processes

EXAMPLE : Heat Exchanger

retemperatuliquidExitT

retemperatuliquidInletT

rateflowSteamw

rateflowLiquidw

2

1

s

=

=

=

=

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Control Objective:

Maintain T2 at the desired value (or set-point), Tsp,despite variations in the inlet flow rate, w. Do this bymanipulating ws.

Feedback Control Scheme:

Measure T2, compare T2 to Tsp, adjust ws.

Feedforward Control Scheme:Measure w, adjust ws (knowing Tsp), to control exit

temperature,T2.


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Feedback Control

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Feedforward Control


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Feedforward/Feedback Control of a Heat Exchanger

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Ratio Control

Objective: maintain the ratio of two process

variables at a specified value

where, u and d are physical variables,

Used in mixing systems where an uncontrolled flow of

material (wild flow) is monitored and used to control the

second material which is controlled according to the desired

ratio between the two components.


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Method I


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Method II


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Cascade Control A cascade control system is a multiple-loop system. Desirable

when Single-loop performance unacceptable and a measuredvariable is available.

Cascade control systems use a second feedback loopwith a separate sensor and controller. Cascade reduces the effect of specific types of disturbances.

objective in cascade control is to divide a difficult processcontrol into two portions

a secondary control loop is formed around a majordisturbances

thus leaving only minor disturbances to be controlled by the

primary controller Better control of the primary variable

Primary variable less affected by disturbances

Faster recovery from disturbances


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Cascade Control (multi-loop):


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Cascade Control (multi-loop):


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Analysis of Cascade Example

Without a cascade controller, changes in thepressure of fuel gas supply will disturb the hotoil temperature. Sluggish response

With cascade controller, changes in gaspressure will be corrected by the pressurecontroller (PC) before it can significantly affecthot oil temperature because the PC responds

faster to this disturbance than TC (the primarycontroller).


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Cascade Control (multi-loop)

Distinguishing features:

1. Two FB controllers but only a single control valve (orother -final control element).2. Output signal of the "master" controller is the set-

point for slave" controller.3. Two FB control loops are "nested" with the "slave"

(or "secondary") control loop inside the "master" (or"primary") control loop.

Terminology

slave vs. mastersecondary vs. primary

inner vs. outer

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SELECTIVE CONTROL SYSTEMS (Overrides)

For every controlled variable, there must be at least one manipulatedvariable.

In some applications

# of controlledvariables

# of manipulatedvariables

Low selector:

High selector:

MC NN

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multiple measurementsone controllerone final control element

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Inferential Control

Problem: Controlled variable cannot be measured or haslarge sampling period.

Possible solutions:

1. Control a related variable (e.g., temperature instead

of composition).2. Inferential control: Control is based on an estimate

of the controlled variable.

The estimate is based on available measurements.

Examples: empirical relation, Kalman filter, Artificial

Intelligence (AI), Neural Network Modern term: soft sensor


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Adaptive Control

A general control strategy for control problems where the process or

operating conditions can change significantly and unpredictably.

Example: Catalyst decay, equipment fouling

Many different types of adaptive control strategies have been proposed.

Self-Tuning Control (STC): A very well-known strategy and probably the most widely used adaptive

control strategy.

Basic idea: STC is a model-based approach. As process conditions change,

update the model parameters by using least squares estimation and recent u &

y data. Note: For predictable or measurable changes, use gain scheduling

instead of adaptive control

Reason: Gain scheduling is much easier to implement and less trouble

prone.


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Time Delay Compensation

Model-based feedback controller that improvesclosed-loop performance when time delays are present

Effect of added time delay on PI controller performancefor a second order process (1 = 3, 2 = 5) shown below


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No model error:

(sensitive to model errors > +/- 20%)

* *s sG G e G e

= =

( )*

*

* *

11 1

(16 22)1 1

C CC ssp CC

s

C C

sp C C

G G GYG Y G GG G e

G G e G GY

Y G G G G

= =++

= = + +

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Gain Scheduling


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Gain Scheduling

Objective: Make the closed-loop system as linear as possible.

Basic Idea: Adjust the controller gain based on current measurements of

a scheduling variable, e.g., u, y, or some other variable.

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Note: Requires knowledge about how the process gain changes with this

measured variable.

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Advanced Control Seborg Chapter 15 16

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