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NATIONAL UNIVERSITY OF SINGAPORE

EXAMINATION FOR

(Semester I: 2010/2011)

EE3302/EE3302E INDUSTRIAL CONTROL SYSTEMS

November/December 2010 - Time Allowed: 2Hours

INSTRUCTIONS TO CANDIDATES:

1. This paper contains FOUR (4)questions and comprises FIVE (5)pages.

2. Answer all questions.

3. All questions carry equal marks.

3. This is a CLOSED BOOK examination. However, each student may bring ONE (1) A4

size crib sheet into the examination hall

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Q.1 In a production factory, a simple elevator is to be used to transfer materials from a

lower to the upper floor as shown in Fig 1. The elevator will travel between two

extreme positions corresponding to the lower floor and upper floor. S1 and S2 are

proximity sensors which will turn on when the elevator is at the lower and upper levelrespectively.

There is a control panel containing a pushbutton (G) at the lower floor to command

the elevator to proceed to move up after the loading of materials is done. The elevator

will then move to the upper floor for the materials to be unloaded. There is a control

panel containing a pushbutton (R) to signal that the unloading has been completed

and to command the elevator to return to the home position at the lower floor. The

elevator is actuated by a motor to move in the up or down direction via the UP and

DOWN inputs to the motor.

Fig. 1

(a)

Provide a diagram to show the physical connections of the PLC.

(5 marks)

(b) Provide a Sequential Function Chart for the sequence control.

(8 marks)

(c) Provide a ladder program to implement the logic.

(8 marks)

Finally, include a non-repeat locking feature to disallow commands due to the R

and G push buttons continuously actuated (i.e., the buttons have to be released andactuated to issue the next command).

(4 marks)

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Q2

(a) Explain the main characteristics which can be used to identify a cascade control

system.

(3 marks)

Provide two block diagrams to show the differences between a series and a parallel

cascade control system.

(4 marks)

(b) Fig. 2 shows the block diagram of a process with input u, primary output y and a

secondary output v(both vand y).

If v is known to be affected by a disturbance wwhich cannot be measured, draw a

block diagram of an advanced control scheme for the process.

(4 marks)

Explain why it can be effective.

(5 marks)

Explain how this controller should be designed/tuned. If a plain proportional gain is to

be used for any controller needed, design any control gain to give closed-loop

stability.

(9 marks)

2.01)(2+= s

sG

v yu

23

1

)( 21 ++= sssG

Fig. 2

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Q.3 Consider the unit step response of a process in Fig. 3a.

Time (s)

y(t)

0 1 2 3 4 5 6 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Fig. 3a

(a) Using the Ziegler-Nichols step response method, find cK and iT of the PI controller

+==

i

ccsT

KsGsE

sU 11)(

)(

)(

(5 marks)

(b) Using the area method, findpK , iT and L for the first-order plus dead-time model

given as

1)(

)(

)(

+==

sT

eKsG

sU

sY sL

p

p

(10 marks)

(c) The controller )(sGc and the process )(sGp found in Parts (a) and (b) respectively are

connected as shown in Fig. 3b. For a unit step change in )(tr , give the values of the

control signal, )(tu , at (i) += 0t and (ii) =t . Assume that all initial conditions are at

zeros and there is no need to show working.

(10 marks)

Fig. 3b

)(sGc

)(sGp

)(sY )(sU)(sE )(sR

+_

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Q.4 The output of the continuous time PID controller is given as

)()()()( tDtItPtu ++=

where

)()( teKtPc=

= t

i

c dtteT

KtI

0)()(

dt

tdyTK

dt

tdD

N

TtD dc

d )()()( =

The error signal and process output are given by )(te and )(ty respectively. cK , iT , dT

and N are controller parameters. The digital implementation of the proportional

controller, )(tP , is given as

)()(kck

teKtP =

wherekt denotes the sampling instant.

(a) Derive the digital implementation of the integral controller )( ktI .

(10 marks)

(b) Derive the digital implementation of the derivative controller )(ktD .

(10 marks)

(c) Give the computer code for the digital implementation of the PID controller

(5 marks)

END OF PAPER