Lecture 1

1. Lecture 1

2. 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

3. Assessment • End of Semester Examination: 50% • Tutorial Assignment: 30% • Laboratory Reports 20%

4. 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

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

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

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. 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

13. 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

14. 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

15. Hysteresis • Deviation in sensor output when it is approached in opposite directions

16. Non-linearity

17. 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

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

19. 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

20. 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

21. 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

22. Damping : Eg Temperature Controller

23. 5m Example: A GaN based UV detector

24. Response Function of UV detector

25. 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.

26. 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.

27. 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

28. 14N n Neutron impacts on 14N nucleus

29. Intermediate unstable 15N forms 15N

30. 15N 10.8 MeV  ray 15N decays emitting an energetic g- ray

31. Neutron source under Protein measurement unit Gamma-ray detectors Now in operation at Monash Medical Centre

33. Principle of Operation • Explosives contain the elementNITROGEN • TNT  20% • RDX  40% We detect the nitrogen using nuclear techniques

34. low S N U P A eutron niversal arcel nalyser

35. SNUPA Prototype Computer output Sample size 80 x 60 x 40 cm orSUSPECT SAFE Completely automated 30 second scan Operator friendly

36. Proof-of-principle anti-tank-landmine detector Neutronsource Gamma-ray detector M19 landmine 200 mm below

37. Proposed anti-personnel landmine detector Large array of gamma-ray detectors Land mine Portable neutron generator Neutron beam

38. Its not as easy at it looks at first

39. 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.