Feedback Control Systems (FCS)

Dr. Imtiaz Hussainemail: imtiaz.hussain@faculty.muet.edu.pk

URL :http://imtiazhussainkalwar.weebly.com/

Lecture-8-9Block Diagram Representation of Control Systems

Introduction• A Block Diagram is a shorthand pictorial representation of

the cause-and-effect relationship of a system.

• The interior of the rectangle representing the block usually contains a description of or the name of the element, gain, or the symbol for the mathematical operation to be performed on the input to yield the output.

• The arrows represent the direction of information or signal flow.

dt

dx y

Introduction• The operations of addition and subtraction have a special

representation.

• The block becomes a small circle, called a summing point, with the appropriate plus or minus sign associated with the arrows entering the circle.

• The output is the algebraic sum of the inputs.

• Any number of inputs may enter a summing point.

• Some books put a cross in the circle.

Introduction• In order to have the same signal or variable be an input

to more than one block or summing point, a takeoff (or pickoff) point is used.

• This permits the signal to proceed unaltered along several different paths to several destinations.

Example-1• Consider the following equations in which , , , are variables, and ,

are general coefficients or mathematical operators.

522113 xaxax

Example-1

522113 xaxax

Example-2• Draw the Block Diagrams of the following equations.

11

22

2

13

11

12

32

11

bxdt

dx

dt

xdax

dtxbdt

dxax

)(

)(

Canonical Form of A Feedback Control System

Characteristic Equation• The control ratio is the closed loop transfer function of the system.

• The denominator of closed loop transfer function determines the characteristic equation of the system.

• Which is usually determined as:

)()()(

)()(

sHsG

sG

sR

sC

1

01 )()( sHsG

Example-31. Open loop transfer function

2. Feed Forward Transfer function

3. control ratio

4. feedback ratio

5. error ratio

6. closed loop transfer function

7. characteristic equation

8. Open loop poles and zeros if 9. closed loop poles and zeros if K=10.

)()()()(

sHsGsE

sB

)()(

)(sG

sE

sC

)()()(

)()(

sHsG

sG

sR

sC

1

)()()()(

)()(

sHsG

sHsG

sR

sB

1

)()()()(

sHsGsR

sE

1

1

)()()(

)()(

sHsG

sG

sR

sC

1

01 )()( sHsG

)(sG

)(sH

Reduction techniques

2G1G 21GG

1. Combining blocks in cascade

1G

2G21 GG

2. Combining blocks in parallel

3. Eliminating a feedback loop

G

HGH

G

1

G

1H

G

G

1

Example-4: Reduce the Block Diagram to Canonical Form.

Example-4: Continue.

Example-5• For the system represented by the following block diagram

determine:1. Open loop transfer function2. Feed Forward Transfer function3. control ratio4. feedback ratio5. error ratio6. closed loop transfer function7. characteristic equation 8. closed loop poles and zeros if K=10.

Example-5– First we will reduce the given block diagram to canonical form

1sK

Example-5

1sK

ss

Ks

K

GH

G

11

11

Example-5 (see example-3)1. Open loop transfer function

2. Feed Forward Transfer function

3. control ratio

4. feedback ratio

5. error ratio

6. closed loop transfer function

7. characteristic equation

8. closed loop poles and zeros if K=10.

)()()()(

sHsGsE

sB

)()()(

sGsE

sC

)()()(

)()(

sHsG

sG

sR

sC

1

)()()()(

)()(

sHsG

sHsG

sR

sB

1

)()()()(

sHsGsR

sE

1

1

)()()(

)()(

sHsG

sG

sR

sC

1

01 )()( sHsG

)(sG

)(sH

Example-6• For the system represented by the following block diagram

determine:1. Open loop transfer function2. Feed Forward Transfer function3. control ratio4. feedback ratio5. error ratio6. closed loop transfer function7. characteristic equation 8. closed loop poles and zeros if K=100.

Reduction techniques

4. Moving a summing point behind a block

G G

G

G G

G

1

5. Moving a summing point ahead a block

7. Moving a pickoff point ahead of a block

G G

G G

G

1

G

6. Moving a pickoff point behind a block

8. Swap with two neighboring summing points

A B AB

Example-7

R_+

_+

1G 2G 3G

1H

2H

+ +

C

• Reduce the following block diagram to canonical form.

Example-7

R_+

_+

1G 2G 3G

1H

1

2

G

H

+ +

C

Example-7

R_+

_+

21GG 3G

1H

1

2

G

H

+ +

C

Example-7

R_+

_+

21GG 3G

1H

1

2

G

H

+ +

C

R_+

_+

121

21

1 HGG

GG

3G

1

2

G

H

C

Example-7

R_+

_+

121

321

1 HGG

GGG

1

2

G

H

C

Example-7

R_+

232121

321

1 HGGHGG

GGG

C

Example-7

Example 8

Find the transfer function of the following block diagram

2G 3G1G

4G

1H

2H

)(sY)(sR

1. Moving pickoff point A ahead of block2G

2. Eliminate loop I & simplify

324 GGG B

1G

2H

)(sY4G

2G

1H

AB3G

2G

)(sR

I

Solution:

3. Moving pickoff point B behind block324 GGG

1GB)(sR

21GH 2H

)(sY

)/(1 324 GGG

II

1GB)(sR C

324 GGG

2H

)(sY

21GH

4G

2GA

3G 324 GGG

4. Eliminate loop III

)(sR

)(1

)(

3242121

3241

GGGHHGG

GGGG

)(sY

)()()(

)()(

32413242121

3241

1 GGGGGGGHHGG

GGGG

sR

sY

)(sR1G

C

324

12

GGG

HG

)(sY324 GGG

2H

C

)(1 3242

324

GGGH

GGG

2G 4G1G

4H

2H

3H

)(sY)(sR

3G

1H

Example 9

Find the transfer function of the following block diagrams

Solution:

2G 4G1G

4H)(sY

3G

1H

2H

)(sRA B

3H4

1

G

4

1

G

I1. Moving pickoff point A behind block

4G

4

3

G

H

4

2

G

H

2. Eliminate loop I and Simplify

II

III

443

432

1 HGG

GGG

1G)(sY

1H

B

4

2

G

H

)(sR

4

3

G

H

II

332443

432

1 HGGHGG

GGG

III

4

142

G

HGH

Not feedbackfeedback

)(sR )(sY

4

142

G

HGH

332443

4321

1 HGGHGG

GGGG

3. Eliminate loop II & IIII

143212321443332

4321

1 HGGGGHGGGHGGHGG

GGGG

sR

sY

)()(

Example-10: Reduce the Block Diagram.

Example-10: Continue.

Example-11: Simplify the block diagram then obtain the close-loop transfer function C(S)/R(S). (from Ogata: Page-47)

Example-11: Continue.

Superposition of Multiple Inputs

Example-12: Multiple Input System. Determine the output C due to inputs R and U using the Superposition Method.

Example-12: Continue.

Example-12: Continue.

Example-13: Multiple-Input System. Determine the output C due to inputs R, U1 and U2 using the Superposition Method.

Example-13: Continue.

Example-13: Continue.

Example-14: Multi-Input Multi-Output System. Determine C1 and C2 due to R1 and R2.

Example-14: Continue.

Example-14: Continue.

When R1 = 0,

When R2 = 0,

Block Diagram of Armature Controlled D.C Motor

Va

iaT

Ra La

J

c

eb

V f=constant

(s)IK(s)cJs

(s)V(s)K(s)IRsL

am

abaaa

Block Diagram of Armature Controlled D.C Motor

(s)V(s)K(s)IRsL abaaa

Block Diagram of Armature Controlled D.C Motor

(s)IK(s)cJs ama

Block Diagram of Armature Controlled D.C Motor

END OF LECTURES-8-9

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