DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
1
Examination
Sitting
Page 1 of 8
Semester 1
Winter
2007/2008
(a) Types of pushbuttons: Start is normally open or press to close switch – done this way so that if
wire breaks motor can’t start. Stop is normally closed or press to open switch – done this way so
that if wire breaks the motor stops (it’s the same as pressing stop button).
[4]
Ladder logic – two ways of doing this.
Option 1 is as follows:
Option 2:
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Examination
Sitting
Page 2 of 8
Semester 1
Winter
2007/2008
Option 1 or 2
[4]
(b) A proportional plus integral plus derivative controller is usually referred to as a three term
controller. Equation is:
Output  Ke 
K
de
edt  KTd

Ti
dt
[2]
The proportional term acts in proportion to the error. It acts on the present state of the error.
[2]
The integral term eliminates the proportional offset or steady state error. It acts on the past history
of the error.
[2]
The derivative mode reduces the tendency towards oscillations and provides a control action which
anticipates changes in the error signal. The derivative mode is useful if the process has sudden load
changes. It anticipates the future direction of the error.
[2]
(c) Open loop - simple, inexpensive. Result of output not measured. Good knowledge of system
required
[2]
Suitable example
[2]
Closed loop - more complicated but better. Output is measured and compared against setpoint to
give an error.
[2]
Suitable example
[2]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Examination
Sitting
Page 3 of 8
Semester 1
Winter
2007/2008
(d) Ultimate cycle - set controller to P control only; increase gain until oscillations develop; stop
increasing gain when system becomes unstable
[3]
Process reaction curve - place controller in manual mode; set output to a constant value and allow
system to reach a steady state; make a step change in the output and record the process variable
response with time
[3]
Disadvantage of ultimate cycle is that it operates on the borderline of instability - this could cause
damage to plant and/or personnel
[2]
(e) Step 1 is Self Test – check for errors in hardware/software
Step 2 is Input Scan – Reads input values from input cards to the memory
Step 3 is Execute Logic – each ladder in program is executed and outputs are updated
Step 4 is Output Scan – Writes output values from memory to output cards
[8]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
(a)
2
Examination
Sitting
Page 4 of 8
Semester 1
Winter
2007/2008
Channel configuration
[4]
Averages for channels
[4]
Read averages for channels
[4]
Comments
[4]
(Note – above code has a length of read/write of 3 on the second and third lines. This can also be
done using individual lines of code.)
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Examination
Sitting
V+
I+
VIFG
Channel 1
Input Terminals
V+
I+
VIFG
Channel 2
Input Terminals
V+
I+
VIFG
Channel 3
Input Terminals
V+
I+
VIFG
Channel 4
Input Terminals
Page 5 of 8
Semester 1
Winter
2007/2008
[6]
(b) Shield is a metal foil covering wire to prevent electrostatic coupling or electric field effects and
is grounded at one point only
[4]
Twisting is to prevent inductive coupling due to magnetic field effects. Voltages are still induced
but in opposite directions.
[4]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
3
Page 6 of 8
Examination
Sitting
Semester 1
Winter
2007/2008
(i) Convert the system into a single block to give the equation below:
KpK
KpK
KpK
1 K pK
 o s 
 s  1 

K p K s  1  K p K

 i s 
1 s
1
1 K pK
s  1
the overall output over input = transfer function
the static gain is above the line
the time constant is the constant in the s term
(ii)
 new 
[4]
[4]
[4]
1


10
1 K pK
 10  1  K p K
 K pK  9
 Kp 
9
3
3
i.e. proportional gain = 3
[5]
At steady state we have:
KpK
 o s 
9


 0.9
 i s  1  K p K 1  9
  o s   0.9 i s 
E ( s) steadystate   i s    o s    i s   0.9 i s   0.1 i s 
Therefore, the error is 10% of the input.
[5]
(iii) Proportional offset is the steady state error observed when using proportional control.
[2]
It can be removed by adding integral action. The integral term acts on the integral of the error and
removes any steady state error.
[2]
(iv) Yes. A high gain may result in overshoot and undershoot oscillations which may not die off.
This causes instability.
[4]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Question No.
4
(i) P&ID
TIC-01
Examination
Sitting
Page 7 of 8
Semester 1
Winter
2007/2008
[6]
HS-01
TET-01
TK-01
Hot Water Supply Tank
(ii) Error = SP – PV (PV = Temp in this case).
[2]
When error is positive, heater is On. When error is negative, heater is Off.
[2]
Without deadband – can get rapid switching of output (chattering) if error is hovering around zero
or traveling slowly through zero. PV stays as close to SP as possible.
[2]
If deadband is added, frequency of switching is reduced but PV fluctuates more.
[2]
DUBLIN INSTITUTE OF TECHNOLOGY
Kevin Street, Dublin 8.
SCHOOL OF CONTROL SYSTEMS AND ELECTRICAL ENGINEERING
Solution Sheet
Course/Year
Subject
Examiner
DT009 Year 3
Control and Automation Minor
Gavin Duffy
Examination
Sitting
Page 8 of 8
Semester 1
Winter
2007/2008
(iii)
PLC
Error SPPV
i
SP +
o Temp
Controller
On-Off
Process
Water Tank
-
A/D card
Transmitter
RTD
[8]
(iv)
Temperature
Temp
off
67ºC
65ºC
63ºC
off
on
on
Power
Time
[6]
Error
+ve
0
-ve
Time
[2]