Physics:%20Principles%20and%20Applications

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Lecture 1
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Purpose and outline of the Course

• To combine the principles of EE and Physics to
  gain an understanding of the fundamentals and
  applications of sensors for the measurement of
  physical properties such as, for example,
  temperature, pressure, light, stress, chemical
  composition, fatigue etc, etc
• At the end of the course, students should be able
  to design a solution to a particular sensing
  problem.
• Some of the sensors to be covered
• Electrical
• Mechanical
• Chemical
• Optical

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Assessment

• End of Semester Examination 50
• Tutorial Assignment 30
• Laboratory Reports 20

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What is a sensor?

• A sensor is a device that
• responds to an applied stimulus
• in response to that stimulus produces an
  electrical signal
• the electrical signal must correspond in a
  predictable way to the stimulus

A biologically based sensor system
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Transducers versus Sensors

• Transducers convert energy from one form to
  another.
• Are the following transducers or sensors or both?
• Microphone
• An electrocardiograph
• A loudspeaker

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Sensors are usually part of larger control systems
Non-contact sensor
Passive
Active
Internal
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Passive versus Active Sensors

• Active Sensors
• Require an external power supply and driving
  circuit
• Eg infrared or ultrasonic motion sensor
• Passive Sensors
• generate own electrical signal based on the
  stimulus.
• Eg Thermocouple.

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Direct, Indirect and Inferential Measurements

• Direct
• Measurement made directly on a parameter, eg
  measuring mass with an electronic balance
• Indirect
• Requires interpretation, calculation or
  interpolation, eg rotor speed to measure fluid
  flow
• Inferential
• measurement cant be made directly or indirectly
  on a parameter, so requires a chain of
  interpolation and/or interpretation eg measuring
  blood flow through the heart by use of a
  thermistor.

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Sensor Classification

• What does it measure (ie what is the stimulus)
• Eg acoustic, biological, chemical, electric,
  magnetic, optical, mechnanical, radiation,
  thermal.
• Specifications
• Eg, sensitivity, stability, linearity (more on
  this later).
• Means of detection
• Eg, biological, chemical, electrical, heat,
  temperature, radioactivity
• Conversion phenomena
• eg thermolectric, piezoelectric, electrochemical
• Material from which it is constructed.
• Field of applications.

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Sensor Selection

• There is often a wide choice of sensors to
  monitor a particular stimulus.
• The choice of the right sensor must take into
  account
• availability
• cost
• power consumption
• environmental conditions
• Reliability and lifetime.
• Therefore the choice is often not black and white
  and it is prudent to retain a few alternatives.

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Sensor Characteristics The Transfer function

• The transfer function converts from the stimulus,
  s, to the electrical output signal, S, ie. S
  fn(s)
• Many functions are possible
• Linear S a bs (b slope or sensitivity)
• Logarithmic S a bln(s)
• Power S a bsk
• For nonlinear transfer functions b dS/ds
• Sensitivity can also be defined as the minimum
  input (or change) in the physical stimulus
  parameter which will create a detectable output
  change

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Transfer function

• Span
• Full Scale Output
• Accuracy
• May be specified as a of full scale or in
  absolute terms
• Eg a pressure sensor has 100kPa input full scale
  and 10 ohms FSO. We can specify the inaccuracy as
  0.5 or 500 Pa or 0.05ohms

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Transfer function

• Span
• Full Scale Output
• Accuracy
• May be specified as a of full scale or in
  absolute terms
• Eg a pressure sensor has 100kPa input full scale
  and 10 ohms FSO. We can specify the inaccuracy as
  0.5 or 500 Pa or 0.05ohms

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Transfer Function Calibration Error

• This is inaccuracy permitted by the manufacturer
  when the sensor is calibrated in the factory
• Systematic in nature, affects all future
  measurements

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Hysteresis

• Deviation in sensor output when it is approached
  in opposite directions

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Non-linearity
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Saturation

• Even if the transfer function is linear, at some
  level of input stimulus, its output will no
  longer be responsive
• There may be the risk of physical damage the
  sensor

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Dead Band

• Dead band is the insensitivity of the sensor to a
  range of input signals.

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Repeatability

• Repeatability error is caused by the inability of
  the sensor to represent the same value under
  identical conditions.
• Causes include thermal noise, temperature drift,
  build up of charge, material plasticity

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Dynamic Characteristics

• A sensor does not change its output state
  immediately when an input parameter change
  occurs.
• The response time is the time it takes for the
  sensor output to reach a final settled state
  (within a tolerance band)
• S Sm(1-exp(-t/t)) Sm steady state output, t
  is time, t is the time constant

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Types of Dynamic Response

• A unlimited upper and lower frequencies
• B Limited upper cut-off frequency
• C Limited lower cut-off frequency
• D first order upper and lower cutoff frequency
• E Narrow bandwidth response

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Damping Eg Temperature Controller
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Example A GaN based UV detector
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Response Function of UV detector
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Environmental Factors

• Storage Conditions
• Eg, lowest and highest storage temperatures
• Short and long term drift
• short (minutes, days) usually environment
• long( months years) usually materials related
• Temperature
• Specified range over which specifications are
  met sometimes compensated for by internal
  sensors
• Self-heating error.
• Eg thermistors.

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Summary of Sources of Error or Uncertainty

• Characterisation Errors
• Eg DC offset, calibration errors,
• Dynamic Errors
• Eg a static sensor used in a dynamic environment
• Environmental errors
• eg self heating
• Insertion errors
• the sensor disturbs the system being measured
• Application errors
• incorrectly placing sensors, eg blood pressure
  monitor, ECG monitor.

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Case Study SNUPA

• Basic Physics
• When a neutron hits a nucleus it can cause it to
  decay and emit a gamma ray
• The Gamma ray is characteristic of the type of
  atom hit. When 14N is struck a characteristic
  g-ray is emitted at about 10MeV

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Neutron impacts on 14N nucleus
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Intermediate unstable 15N forms
15N
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15N decays emitting an energetic g- ray
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Protein measurement unit
Gamma-ray detectors
Now in operation at Monash Medical Centre
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Principle of Operation

• Explosives contain the element NITROGEN

TNT ? 20 RDX ? 40
We detect the nitrogen using nuclear techniques
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low
S N U P A
eutron
niversal
arcel
nalyser
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SNUPA Prototype
or SUSPECT
SAFE
Completely automated 30 second scan
Operator friendly
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Proof-of-principle anti-tank-landmine detector
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Proposed anti-personnel landmine detector
Portable neutron generator
Neutron beam
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Its not as easy at it looks at first
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Summary You should know

• Definition of Sensors
• Sensor Classification
• The Transfer Function
• Span Full scale output Accuracy
• Calibration Error
  Hysteresis
• Non-linearity
  Saturation
• Repeatability
  Dead band
• Dynamic Characteristics
• Response time, frequency response
• Damping.
• Sources or error and uncertainty
• which are likely to degrade sensor reliability
  and performance.