Basic Process Control

Basic Control Principles

Level(Controlled Variable)

OverflowPump

InflowSupply

llllll

Open vs Closed Loop

! Closed loop ! Automatic control

! Open Loop! Manual control ! Person takes the place of the controller

Feedback Control

Comparator Amplifier Process (Tank)Error

(SP - M)

Manipulated Output

(ControlledVariable - Level)

Disturbances

Variable

ProcessSensor

SignalMeasurement

Set Point

Controller

Feedback vs. Feedforward

! Feedback! Control action after an error exists

! Feedforward! Reacting to the disturbance before the

error occurs

Typical ON/OFF Control System

P1

L1

L2

V

QinSP

Qout

1

P1 closes @ L1P1 opens @ L2

Power SupplySolenoid

Solenoid valveElectrically Operated

Inflow

llllll

Typical ON/OFF Response

L2

Periodic TimeL1

SPtime

Proportional Control

Level Control of Open Tank

Qout

LT

SPQin

V1

20 - 100 kPa

A/Cllllll

Simple Proportional System

Pivot

Outflow

Float

Valve

Inflow

lll lll

Open Tank Control

Qin

Qout

SP

LT

20 - 100 kPa

A/O

LIC

Controller Action

Qin

Qout

SP

LT

20 - 100 kPa

A/C

LICllllll

Qin

Qout

SP

LT

20 - 100 kPa

A/O

LICllllll

Direct

Reverse

Proportional Control

+-

k�eSetpoint

Measurement

Error

Bias

++

Output

m= k(SP-M) + bias

Proportional Band and Gain

Proportional Band� the input change required to change the output 100%

PBgain %100

=

inputoutputgain∆∆

=

Narrow, Wide, High & Low

Wide PB � Low Gain

Narrow PB � High Gain

OutIn

Proportional Control Response Curve

Offset

Outflow

Loss in VolumeNew mass balanceoccurs here

New levelbelow setpoint

timet 1

1t0t

0t

time

at setpointLevel originally

Input/Output

Level

t

Inflow

Proportional Response with Narrower PB

Level originally Offset

tt 0time

1Level

at setpointbelow setpointNew level

New mass balanceoccurs hereLoss in Volume

InflowInput/Output

0 t 1t

t

Outflow

time

Response Versus PB, Proportional Control Only

time

Step Disturbance

Load

Cha

nge

Offset

Offset

time

Syst

em R

espo

nse

SP

SP

SP

"Wide" PB

"Moderate" PB

"Narrow" PB

¼ Decay Response Curve

A

A/4A/16

Syst

em R

espo

nse

time

For You To Do

! Read pp. 89-105! Answer Questions pp. 121-122, #1-19

Reset or Integral

Response Curve: Proportional Control Only

Syst

em R

esp o

nse

SP

Step Disturbance

time

Offset

Additional Control Signal Restores Process to Setpoint

Setpoint

Initial mass balance

Inflow

Outflow

Final mass balance

Reset Action

timeOffsetRemoved

Integral Action

biasedtTR

kem +

+= ∫

1

+-

k�eSetpoint

Measurement

Error

Bias

++

Output

∫ edtTR1

Units! Minutes per repeat

! MPR! The length of time that it will take the

integrator to add an amount equal to the proportional response

! Repeats per minute! RPM! The number of times the proportional

response is repeated in one minute

Proportional Plus Reset, Open Loop Response

time

e

Fast ResetNormal Reset

Slow Reset

error

keProportionalResponse}

Con

trol S

igna

l

A problemOutput initially 50%, Gain = 2, reset = 2 minutes per repeatA direct acting controller control is subjected to a sustained error of 5%What is the output after 4 minutes?

Proportional Response = ke = 2x5=10%Integral Action- in 4 minutes the control will go through 2 repeats.Integral action = 2 x 10 = 20%Total output change is proportional + integral = 30%

A Couple More Things

! Reset Windup! Instability because of lag

For You to Do

! Read over text pp. 89 �110! Answer questions pp. 121-122, 1-24

Rate or Derivative

Proportional and Derivative � Open Loop Pressure

time

Inpu

tO

utpu

t

Derivative

Derivative ceases aserror stops changingProportional

Action

Derivative Control

+-

k�eSetpoint

Measurement

Error

Bias

++

Output

∫ edtTR1

dtdeTD

++

+

biasdtdekTedt

Tkem D

R

++

+= ∫1

Simple Flow Control System

lllFT

FC

A/C

√_

The open Loop Response of Proportional Plus Derivative (PD) Action to Rapidly Changing Error Signals

0t 1t 2t

0t 1t 2t

time

time

A

B

C

Rate action due to cessation of increase in eRate actionB -C

RateactionA - B

Proportional actionA - B

Proportional action B - C

Rate action due to cessation of increase in e

Control signal at end of excursionControlSignal

%

Proc

ess

Large System Under Proportional and Proportional Plus Derivative Control

time

Control Signal

Setpoint

Level

LoadDisturbance

Applied

Prop. + DerivativeProp. Only

Multiple Control Modes

! Virtually all controls have a proportional response

! Integral and derivative are added to improve performance

! Majority have proportional and integral! Some, typically heat exchangers have

derivative added

Open Tank Level Control With Valve In Inflow

QiA/O lll

lllQo

SP

LIC LT

Typical Flow Control Loop

llFT

FIC

A/O

√_

SP

Pressure Control � Constant Bleed

PIC PT

A/CFeed

SP

Pressure Vessel PressureBleed

Split Ranged Feed and Bleed Pressure Control

PIC PT

A/CFeed

SP

Pressure Vessel Bleed

A/O

Representative Hot Bleed/Cold Service Water Heat Exchanger

TT TC

A/C

SPCooled Bleed

Hot Bleed

Cold

For You To Do

! Read pp. 106-120! Answer Questions pp. 122-123, #20-38