Block Diagram

and

Transfer function

FUNDAMENTAL OF FEEDBACK

CONTROL

Unit 1: Process Control Loop

Process control loop

I/P

Process

Sensor

Transmitter

Controller

Transducer

Control valve

4-20 mA

1-5 Vdc

PID

Fuzzy logic

4-20 mA

3-15 psig

dP cell

Capacitance

Radar, Sonic

Magnetic

Resistance

IR/Laser

Pressure

Flow

Level

Temperature

pH

Linear

Equal percentage

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

Unit 1: Process Control Loop

Process

Sensor

Transmitter

Controller

Transducer

Control valve

Pressure

Flow

Level

Temperature

pH

dP cell

Capacitance

Radar, Sonic

Magnetic

Resistance

IR/Laser

4-20 mA

1-5 Vdc

Field/profibus

PID

Fuzzy logic

4-20 mA

3-15 psig

Linear

Equal percentage

CONTROLLER

CONTROL

VALVE

PROCESS

TRANSMITTER

PV

SP

Process control loop

PV

SP

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

*SP = set point

*PV = process value

Unit 1: Process Control Loop

CO

NT

RO

LL

ER

CO

NT

RO

L

VA

LV

E

PR

OC

ES

S

TRANSMITTER

PV SP

Simulation mode

Process control loop: The Block Diagram

CONTROLLER

CONTROL

VALVE

PROCESS

TRANSMITTER

PV

SP

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

Block Diagram and Signal Flow

• Rectangular block diagram-represents the cause-and-effect

relationship between the input and output of a physical

system.

• Control element-represents any of the system component

such as controller, process, sensor, transmitter, transducer,

control valve, metering pump, solenoid valve, etc.

• Arrow-represents the direction of signal flow

Heat Exchanger

Fluid in

Steam out

Ti T desired

Fluid out

TT

TC Steam in

•Example SP

Signal operation • Circle block diagram represents the signals operation. A

negative or positive sign is placed inside the circle to

represent deduction or addition of the respective signals.

Takeoff point • Takeoff point is a point where the signal is diverted into

multiple flows which has similar values to various

operators or control elements.

Heat Exchanger T desired

Fluid out

TT

TC Steam in

SP

• Example 1 Draw a block diagram for the following equation:

Step 1

a. Identify control elements: a1 and a2

b. Identify signals: x1, x2 and 5

Step 2

Identify summing junction and arrows

Step 3

Draw and label complete block diagram.

• Assosiative and commutative properties:

y(s)=G1(s)G2(s)x(s)

y(s)=G1(s) x1(s)- G1(s) x2(s)

*More info in Principles and Practice of Automatic Process Control 3rd

edition pg 80

• Draw a block diagram for the following equation:

• Determine the equation that relating y(s)

and x(s)

• Feedback loop

• Example

Reduce the following block diagram to canonical form,

isolating block K in the forward loop. Then, calculate C/R.

Reduce the block diagram below to canonical form.