Fundamental Principles of Process Control

Motivation for Process Control

• Safety First:• people, environment, equipment

• The Profit Motive:• meeting final product specs

• minimizing waste production

• minimizing environmental impact

• minimizing energy use

• maximizing overall production rate

“Loose” Control Costs Money

• It takes more material to make a product thicker, so greatest profit is to operate as close to the minimum thickness constraint as possible without going under

• It takes more processing to remove impurities, so greatest profit is to operate as close to the maximum impurities constraint as you can without going over

3 .6

3 .8

4 .0

4 .2

4 .4

6 0 8 0 1 0 0 1 2 0 1 4 0

P ro ces s : G rav i ty D rai n ed T an k C o n t ro l l er: M an u al M o d e

Mo

re

P

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fit

ab

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Op

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T im e (m in s)

30

40

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3 .4

3 .6

3 .8

4 .0

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8 0 1 0 0 1 2 0 1 4 0 1 6 0

P ro ces s : G rav i ty D rai n ed T an k C o n t ro l l er: M an u al M o d e

Mo

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fit

ab

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Op

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T im e (m in s)

operating constraint

poor control means

large variability, so

the process must be

operated in a less

profitable regionprocess variable

Examples

Tight Control = Most Profitable Operation

• A well controlled process has less variability in the measured process variable, so the process can be operated close to the profitable constraint

3 .4

3 .6

3 .8

4 .0

4 .2

8 0 1 0 0 1 2 0 1 4 0 1 6 0

P ro ce s s : G ra v i t y D ra i n ed T a n k C o n t ro l l e r : M a n u a l M o d e

Mo

re

P

ro

fit

ab

le

Op

er

at

ion

T im e ( m in s)

30

40

50

60

70

3 .4

3 .6

3 .8

4 .0

4 .2

8 0 1 0 0 1 2 0 1 4 0 1 6 0

P ro c e s s : G ra v i t y D ra i n e d T a n k C o n t ro l l e r : M a n u a l M o d e

Mo

re

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ro

fit

ab

le

Op

er

at

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T im e (m in s)

operating constraint

process variable

tight control permits

operation near the

constraint, which

means more profit

Profitability of the Process Can Be Improved by Reducing Variability in the Control Loop

Loop variabilit y Loop t uned Increased product ion

or qualit y

$

Standard Deviation: Controller Performance Benchmark

0.6 0.8 1 1.2 1.4 1.6

set point

st dev = 0 .025

st dev = 0.1

-3 +3

7

Terminology for Home Heating Control

• Control Objective = Set Point (SP)• System Output (Y)

• PID: Measured Process Variable (PV)• MPC: Controlled Variable (CV)

• System Input (U)• PID: Controller Output (CO or OP)• MPC: Manipulated Variable (MV)

• Disturbances (D)

furnace

temperature

sensor/transmitter

set point

heat loss

(disturbance)

thermostat

controller

valve

TTTC

control

signal

fuel flow

Copyright © 2007 by Control Station, Inc. All Rights Reserved

Automatic Control is Measurement Computation Action

• Is house cooler than set point? ( TSetpoint Thouse > 0 )

Action open fuel valve

• Is house warmer than set point? ( TSetpoint THouse < 0 )

Action close fuel valve

Components of a Control Loop

Home heating control block diagram

Set PointThermostat Home Heating

ProcessFuel Valve

Temperature

Sensor/Transmitter

Heat Loss

Disturbance

+-

house temperature

measurement

signal

controller

error

manipulated

fuel flow to

furnace

house

temperature

controller

output

signal

General Control Loop Block Diagram

Set PointController Process

Final

Control

Element

Measurement

Sensor/Transmitter

Disturbance

feedback

signal

controller

error

manipulated

process

variable

measured

process

variable

controller

output

+-

11

Thought Experiment: Cruise Control in a Car• Control Objective:

• maintain car velocity

• Measured Process Variable (PV): • car velocity (“click rate” from transmission rotation)

• Manipulated Variable: • pedal angle, flow of gas to engine

• Controller Output (CO): • signal to actuator that adjusts gas flow

• Set point (SP): • desired car velocity

• Disturbances (D): • hills, wind, curves, passing trucks....

12

Cruise Control Block Diagram

Magnet and Coil Sensor

set point, SP

(desired speed)

Gas Pedal

Control

Element

disturbances

(hills, wind, etc.)measured “click rate”

PV signal

controller error

e(t) = (SP– PV)

manipulated

gas flow rate

to car engine car speed

controller output, CO

signal to gas pedal

+-+-Car Speed

ProcessCruise

Controller

Copyright © 2007 by Control Station, Inc.

All Rights Reserved

The PID Controller

• is in denominator so smaller values provide a larger weighting to the integral term

• and have units of time and are always positive

where:

OP = controller output signal (also seen as CO in PPC)

OPbias = controller bias or null value

PV = measured process variable

SP = set point

e(t) = controller error = SP – PV

Kc = controller gain (a tuning parameter)

= controller reset time (a tuning parameter)

= controller derivative action (a tuning parameter)

t

PVKdtte

KteKOPOP Dc

I

ccbias

)()(

DI

I

I D

Example: Heated Tank

Ti

wi

To

wo

Heater

Q

• Controlled variable:• To

• Feedback• Measure T0

• Control voltage to heater

TT

VC

Example: Heated Tank

Ti

wi

To

wo

Heater

Q

• Controlled variable:• To

• Feedback• Measure T0

• Control voltage to heater

• Feedforward• Measure Ti

• Control voltage to heater

TT

VC

Other Definitions

• Final Control Element• Usually a valve or pump

• Electricity (to heat or cool)

• Solenoid valve (open or shut)

• SISO• Single input, single output

• Nomenclature

FC

Flow (to be

controlled)

Controller

LT

Level (of liquid)Transmitted (or sensor)

In-Class Activity