type of controller

Type of Controller

Regulatory and servo control

• For process that the controlled variable

deviates from set point because of disturbance

– regulatory control

• For process that the most important

disturbance is the set point itself – servo

control

Unit 1: Terminology 3:25 PM

Controlled

variable

Sym: c, PV

“The parameters

that indicate

product quality or

the operating

condition of the

process.” (L.M.

Gordon)

Manipulated

variable

Sym: m, CO, MV

A process being

adjusted (controlled)

by a final control

element (such as

control valve,

damper, motor

speed).

Load variable

Sym:

“All other

affecting

variable, other

than the one

being

manipulated.”

(L.M. Gordon)

Set point

Sym: r, SP, SV

The desired

operating

condition.

TC

TT

controlled variable

(temperature of

sterilised saline) Load variable

Manipulated variable

(saturated steam)

(Flow, temperature

of saline solution) Uncontrolled variable

(Saturated steam)

Heat exchanger

30°C

100 L/min

95°C

80°C

I/P

100°C

Terminology

© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

Unit 1: Control strategies

TC

TT

controlled variable

(temperature of

sterilised saline)

What is feedback control? A method of control that the action taken is

based on past result.

Merit? Simple design. Easy to tune (for linear, fast &

short deadtime processes: flow, level &

pressure).

Demerits? Slow recovery for slow process such as

temperature control upon load disturbance or

change in setpoint. A process with long deadtime

may exhibit oscillatory process response with

long settling time.

Load variable


(saturated steam)

(Flow of saline solution)


(Saturated steam)

Heat exchanger

30°C

100 L/min

95°C

80°C

The Process Saline solution at 30°C is sterilized by

heating the solution in a heat-exchanger

system to 80°C. The temperature of the

sterilized saline solution is maintained by

regulating the control valve of steam input

to the heat-exchanger.

Feedback control

I/P

100°C

3:25 PM



Cascade control

30°C

100 L/min

95°C

80°C

I/P FC

100°C

What is cascade control? A method of control of which the inner loop

controller obtains its setpoint from an outer loop

controller.

Merit? Faster recovery time as compared to SISO

feedback control.

Demerits? Controllability will be worst than SISO if both

controllers were not properly tuned.

Characteristic of cacade control: Two transmitters, two controllers and one

control valve (2-level cascade).

Application: Transform from SISO to

cascade strategy if faster recovery time is

desired.

Heat exchanger FT

TC

TT


controlled variable

(temperature of

sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

TSP

FSP


Feedforward control

30°C

100 L/min

95°C

80°C

I/P FFC

What is feedforward control? A method of control based on process model:

mass and energy balances.

Merit? Faster recovery than cascade or SISO.

Compensate for disturbances before they effect

the controlled variable.

Demerits? Cannot work if any of the sensors fail esp. load

variable.

Requires process model in order to design

feedforward system.

Characteristic of feedforward

control: Measurements of load variables online and

Utilization of computational blocks.

Heat exchanger TT

12 TTWH

CW p

s

ps


controlled variable

(temperature of

sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

FT


Feedforward/ Feedback control

30°C

100 L/min

95°C

80°C

I/P

Heat exchanger TT

12 TTWH

CW p

s

ps

TC

TT


controlled variable

(temperature of sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

FT

What is feedforward/feedback control? Feedback compensation added to feedforward control

Merit? Feedforward control compensates for major disturbances, while feedback control compensates for all other disturbances.

FFC +

+


Ratio control

Wild flow 30°C

100 L/min

95°C

80°C

I/P FC

What is ratio control? Keeps a flowrate in proportion to another

flowrate.

Merit? Keep two flow in proportion.

Demerits? Inherits PID characteristics

Heat exchanger

FT

FT

X


controlled variable

(temperature of

sterilised saline)


(saturated steam)



(Saturated steam)

Ratio Control

• Type of feedforward control

• The objective is to maintain the ratio of two variables at

a specified value

• One variable is manipulated to keep it as ratio of

another.

• Implemented in two basic schemes.

multiplier

divider

• Required to blend 2 liquid streams, A and B

in some ratio

R=FB/FA

•Measuring the flow in steam A

•Multiplying it with the desired ratio

to obtain the required flow rate in

stream B (set point to the flow

controller of stream B)

•FBset = R x FA

•If the flow of stream A varies, the

set point of the controller of stream

B will vary.

Multiplier : set the ratio

The flow of stream B must vary, as the flow rate of

stream A is varies

(Wild flow)

•Measuring both stream

•Divide them in FY16 to obtain the

actual ratio flowing through the

system

•R=FB/FA

•The calculated ratio is the sent to

the controller which manipulated

the flow B to maintain set point.

•The set point to this controller is

the required ratio.

Divider

• Feedback control

Feedback

controller Process

Y(s) Y

D

•Feedforward control

Feedforward

controller Process

Y(s) Y

D

•Cascade control

Primary

controller Process II

Y(s) Y

Process I Secondary

controller

BLOCK DIAGRAM

• Question

- How to ensure that the level in the tank remains at

or near the set point? Propose the following

control strategy:

- Feedback

- Feedforward

- Feedforward/Feedback

type of controller

Documents