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Should the pressure supply fail then the valve will reset. However the pressure

stored in the volume will only bleed slowly away at a rate determined by thevariable port setting. Therefore pressure will be supplied via the valve/shuttle

to cause a retract of the cylinder!

[4 marks]

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Q3 a(i) Factors considered when selecting the a PLC

1) Speed of operationThe speed of the PLC is important because it has to befast enough to respond to the process which is being controlled. This is

especially important when controlling DC and AC drive controls and rapidly

varying control signals.

2) Number and type of inputs and outputs requiredWhether the I/O is analogue

or digital, plus the quantity and additional capacity for expansion.

3) Type of construction - Whether a block type PLC would be suitable orcard/slot system is needed.

4) Cost

5) Is Networking requiredWhether the PLCs can be or are required to be

networked

[6 marks]

Q3a (ii) Factors considered when selecting a PLC network

Key design issues are normally speed of operation, distance between nodes, number

of nodes required, criticality of application

Most likely to be a Bus or Ring configuration

1) Data consideration - What data is going to be shared and communicated

b t PLC d t SCADA t

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3c)

i) Star : Large number of wires required : Expensive and confusing[2 marks]

Ring : As star (lesser cost) but also potential ground loop problems

[2 marks]

ii) Use a Tree structure by connecting bus structures to each other via

repeaters to boost signal strength

[3 marks]

Q4

a)

[8 k ]

I:0/0

O:2/0

T4:0 DN

I:0/1

1 scan cycle

O:2/150 seconds

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c) CBAX .

CBACBACBACXY ...... [5 marks]

Q5.

a)

I1Card

24V DCInput

0

1

2

34

5

6

7

COM

24V DC

Output

Cd

0

1

2

34

5

6

7

COM

24V+

DC

-

StartStop

Sensor 1

Process

O2 Card

+24V

-DC

Enable

Conv.Heater

R 1

R 2

3Phase

415 volt

R2To Gas

Heater

R1 To MCC

PLC

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[13 marks]