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DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Question Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
1
Examination
Sitting
Page 1 of 4
Semester 1
2005/2006
(i) Explain the terms open loop and closed loop as applied to a control system. Illustrate your
answer by reference to a particular example of each system and sketch its relevant block diagram.
[8]
(ii) What is meant by the term integral control and why is it often used in control systems? What is
meant by the term ‘integral windup’?
[8]
(iii) Describe the ultimate cycle and process reaction curve methods of tuning a PID control loop.
Name one disadvantage of using the ultimate cycle method.
[8]
(iv) Explain each of the four steps which make up the programme scan cycle of a PLC.
[8]
(v) Write a ladder logic program to correctly implement Start/Stop control of a fixed speed motor
by a PLC. What types of push buttons are normally used for Start and Stop and why are they used?
[8]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Question Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
2
Examination
Sitting
Page 2 of 4
Semester 1
2005/2006
(a) The contents of a batch reactor are heated by passing steam through a jacket surrounding the
reactor. The relationship between the reactor temperature, TR, and the temperature of the steam
supplied to the jacket, TJ, is described by the first-order differential equation:

where
dTR
 TR  TJ
dt
 is the time constant
TR is the temperature of the reactor contents
TJ is the jacket temperature
(i) Use Laplace transforms to convert this equation to the frequency domain and hence determine
the transfer function for the system
[5]
(ii) What is the frequency domain equation for the output from the system if a step change in the
input of magnitude a is applied?
[5]
(iii) Apply the inverse Laplace transform to provide the output from the system in the time domain
in response to a step change in the input of magnitude a.
[5]
(b) Write a fully commented program to configure a Mitsubishi FX-4AD analogue input card for the
following situation:
The card is installed in the first slot next to the PLC
Channels 4, 3 and 2 are off (no field devices connected)
Channel 1 is connected to a 4 to 20 mA temperature transmitter
The number of averages for channel 1 is set to 100
The averaged value from channel 1 is read into address D0.
[10]
How can the effect of noise be reduced in the analogue input card configuration?
[5]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Question Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
3
Page 3 of 4
Examination
Sitting
Semester 1
2005/2006
The block diagram for a control system with a proportional controller acting on a first order process
with unity feedback is shown in Figure Q3.
i(s) +
Kp
K
s+1
o(s)
-
Figure Q3
(i) For the overall system determine the following
the transfer function
the static gain
the time constant
[4]
[4]
[4]
(ii) Given a static gain of 3 for the process, determine the proportional gain required to make the
time constant of the system equal to 1/10th the time constant of the process. In this case, what is the
steady state error?
[10]
(iii) Explain what is meant by proportional offset and describe how it can be removed.
[4]
(iv) Is there any danger in making Kp larger and larger in an effort to achieve zero steady state
error? Briefly explain your answer.
[4]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Question Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
4
Examination
Sitting
Page 4 of 4
Semester 1
2005/2006
The temperature of a hot water tank is controlled by a Programmable Logic Controller (PLC). The
tank is heated by an electric element situated at the base of the tank. An RTD based temperature
measurement system is used to monitor the temperature of the water in the tank. The RTD is
connected to the PLC via a signal conditioning circuit. An On-Off control algorithm is used in the
PLC to switch the output to the electric heater.
(i) Draw a block diagram indicating all the components and variables in the system. Draw a dashed
line around those items that reside in the PLC.
[9]
(ii) Explain how On-Off control operates in this application.
[4]
(iii) What are the advantages and disadvantages of On-Off control?
[4]
(iii) What improvement could be made to the On-Off control algorithm to prolong the life of the
electric heater?
[4]
(iv) Sketch the time response of the following system variables:
Power to the electric heater
Tank temperature
Error
[9]