MSc EEE
Advanced Instrumentation and Control
MODEL ANSWER
Q. 1
((a) What is this in LabVIEW? Label all the tools in following diagram.
This is Common Tools Palette.
(2 marks)
The names of the tools in the diagram are:
Automatic Tool Selector
Operating Tool
Positioning Tool
Labeling Tool
Wiring Tool
Object Shortcut Menu Tool
Scrolling Tool
Breakpoint Tool
Probe Tool
Get Color Tool
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
(1 marks)
Coloring Tool
(1 marks)
(b) What types of Data Structures available in LabVIEW?
String Data Type
Numeric Data type
Boolean Data Type
Dynamic Data Type
Arrays
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(2 marks)
(2 marks)
(2 marks)
(2 marks)
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Clusters
Enums
(2 marks)
(2 marks)
Total (25 marks)
Q. 2
(a) What is the difference between while loop and For loop? What is the purpose of
the following LabVIEW block diagram? Explain what you would expect to see on
the Waveform Chart?
The While Loop executes the code it contains until the conditional terminal, an input
terminal, receives a specific Boolean value. The For Loop differs from the While
Loop in that the For Loop executes a set number of times. A While Loop stops
executing only if the value at the conditional terminal exists.
(3 marks)
The programme uses a random number generator to generate numbers between 0 to 1,
and calculate the Y=0.2*R + 0.8*Y’, where R is the current random value, and Y’
is the previous Y value.
(2 marks)
It runs in a while loop, with 100ms rest between each loop, it only stops when stop
button is pushed. The chart will display a random value R chart and attenuated vale
Y chart which is trying to follow R chart trend.
(2 marks)
(b) What does following LabVIEW programme do? Explain the output results on the
Front Panel.
The answer should give an explanation through each function block, including
Two simulated signals
(2 marks)
Low pass filter/Spectral Measurements
(2 marks)
Amplitude and Level Measurements/Greater or Equal
(2 marks)
Time Delay
(1 marks)
Write to Measurement File
(2 marks)
Waveform displays
(3 marks)
(c) What is case structure? What is the output of the following LabVIEW block
diagram?
A Case Structure is a branching control mechanism that allows different executions
depending on the value of the label.
(2 marks)
The following Case Structure takes in a numeric input value. The case structure will
output 2 (1 + 1) if the input is 1,
(2 marks)
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MSc EEE
10 (2 x 5) if the input is 2, and 0 if the input is anything else (the 0 case and Default
case).
(2 marks)
Total (25 marks)
Q.3
(a) The following block diagram represents which common type of VI architecture?
Explain what will happen if it is executed.
This is State Machine.
When executed, it will read status value from a subVI called “START”,
(3 marks)
(2 marks)
depends on the status value, if it is true, case “Shutdown” will be passed to next round
of the while loop through shift register,
(2 marks)
otherwise, case “Idle” will be passed to next round.
(2 marks)
The while loop rests for 10 ms between loops.
(1 marks)
(b) Explain the operation of following block diagram. What do you expect to see in
the Waveform Graph?
The For loop will increment the index variable
by 1 each round,
is starting
from zero, then multiply by 2,
(3 marks)
these values will build into an Array after the loop running for 5 times.
(2 marks)
The “Array Subset” function will take a subset of the array, index 2,3,4, and display in
the Waveform Graph, see following.
(2 marks)
(3 marks)
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MSc EEE
(c) What will be displayed in the “resulting string”?
In the “resulting string” is will display My Name is Hans-Petter,
My phone is 911
(3 marks)
(2 marks)
Total (25 marks)
Q.4
(a) Describe the terms of Sampling Theorem and Anti-Aliasing Filtering in the
context of digital signal processing.
The sampling Theorem, also known as the Nyquist (or Nyquist–Shannon) sampling
theorem, is a principle that engineers follow in the digitization of analog signals.
(2 marks)
For analog-to-digital conversion (ADC) to result in a faithful reproduction of the
signal, slices, called samples, of the analog waveform must be taken frequently.
The number of samples per second is called the sampling rate or sampling
frequency. According to the Nyquist sampling Theorem, the sampling rate must be
at least twice the highest analog frequency component.
(3 marks)
In signal processing and related disciplines, aliasing is an effect that causes different
signals to become indistinguishable (or aliases of one another) when sampled. An
anti-aliasing filter is a filter used before a signal sampler, to restrict the bandwidth
of a signal to approximately satisfy the sampling theorem.
(3 marks)
(b) In the context of Control Theory, what is a PID controller? Use a suitable diagram
to illustrate how PID controllers work. Describe the stability and limitations of PID
controllers.
A proportional-integral-derivative controller (PID controller) is a control loop
feedback mechanism (controller) widely used in industrial control systems.
(3 marks)
A PID controller calculates an error value as the difference between a measured
process variable and a desired setpoint. The controller attempts to minimize the
error by adjusting the process through use of a manipulated variable.
(3 marks)
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MSc EEE
(3 marks)
The PID controller algorithm involves three separate constant parameters, and is
accordingly sometimes called three-term control: the proportional, the integral and
derivative values, denoted P, I, and D. Simply put, these values can be interpreted
in terms of time: P depends on the present error, I on the accumulation of past
errors, and D is a prediction of future errors, based on current rate of change.[1]
The weighted sum of these three actions is used to adjust the process via a control
element such as the position of a control valve, a damper, or the power supplied to
a heating element.
(2 marks)
If the PID controller parameters (the gains of the proportional, integral and derivative
terms) are chosen incorrectly, the controlled process input can be unstable, i.e., its
output diverges, with or without oscillation, and is limited only by saturation or
mechanical breakage. Instability is caused by excess gain, particularly in the
presence of significant lag. Generally, stabilization of response is required and the
process must not oscillate for any combination of process conditions and setpoints,
though sometimes marginal stability (bounded oscillation) is acceptable or desired.
(3 marks)
PID controllers, when used alone, can give poor performance when the PID loop
gains must be reduced so that the control system does not overshoot, oscillate or
hunt about the control setpoint value. They also have difficulties in the presence of
non-linearities, may trade-off regulation versus response time, do not react to
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MSc EEE
changing process behavior (say, the process changes after it has warmed up), and
have lag in responding to large disturbances.
(3 marks)
Total (25 marks)
Q.5
(a) What are open-loop controllers and closed-loop controllers? Describe the
advantages of closed-loop controllers over open-loop controllers.
In a open-loop controller, no measurement of the system output is used to alter the
control. To overcome the limitations of the open-loop controller, control theory
introduces feedback.
(3 marks)
Closed-loop controllers have the following advantages over open-loop controllers:
disturbance rejection (such as hills in the cruise control example above)
guaranteed performance even with model uncertainties, when the model structure
does not match perfectly the real process and the model parameters are not exact
unstable processes can be stabilized
reduced sensitivity to parameter variations
improved reference tracking performance
(5 x 1 marks)
(b) Use a suitable diagram to illustrate what is closed-loop transfer function.
A closed-loop transfer function in control theory is a mathematical expression
(algorithm) describing the net result of the effects of a closed (feedback) loop on the
input signal to the circuits enclosed by the loop.
(3 marks)
If we use following diagram to represent a closed-loop system,
(3 marks)
We have,
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MSc EEE
(3 marks)
(3 marks)
The closed-loop transfer function can be expressed as:
(3 marks)
Total (25 marks)
End of Paper
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