Lecture 1 Purpose and outline of the

Lecture 1

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

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

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

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

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

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.

• 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.

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.

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.

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 = d. S/ds • Sensitivity can also be defined as the minimum input (or change) in the physical stimulus parameter which will create a detectable output change

Transfer function • Span • Full Scale Output • Accuracy – May be specified as a % of full scale or in absolute terms – Eg a pressure sensor has 100 k. Pa input full scale and 10 ohms FSO. We can specify the inaccuracy as 0. 5% or 500 Pa or 0. 05 ohms

Transfer function • Span • Full Scale Output • Accuracy – May be specified as a % of full scale or in absolute terms – Eg a pressure sensor has 100 k. Pa input full scale and 10 ohms FSO. We can specify the inaccuracy as 0. 5% or 500 Pa or 0. 05 ohms

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

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

Non-linearity

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

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

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

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

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

Damping : Eg Temperature Controller

Example: A Ga. N based UV detector 5 m

Response Function of UV detector

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.

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.

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 14 N is struck a characteristic g-ray is emitted at about 10 Me. V

Neutron impacts on 14 N n nucleus

Intermediate unstable 15 N forms

15 N decays emitting an energetic - ray 15 N V Me. 8 10 y ra

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

Principle of Operation • Explosives contain the element NITROGEN TNT RDX 20% 40% We detect the nitrogen using nuclear techniques

S low eutron N Universal Parcel A nalyser

SNUPA Prototype Computer output SAFE Operator friendly or SUSPECT Completely automated 30 second scan Sample size 80 x 60 x 40 cm

Proof-of-principle Neutron source anti-tank-landmine detector Gamma-ray detector M 19 landmine 200 mm below

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

Its not as easy at it looks at first

• • • 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.