Instrumentation (and Sensor Technologies Process Control) Fall 1393 Bonab University
Sensor Introduction Technologies • Range of sensors available for measuring various physical quantities • A wide range of different physical principles are involved • capacitance change, resistance change, magnetic phenomena (inductance, reluctance, and eddy currents) • Hall effect, properties of piezoelectric materials, resistance change in stretched/ strained wires (strain gauges), properties of piezoresistive materials, light transmission (along an air path - along a fiber-optic cable) • Properties of ultrasound, transmission of radiation, and properties of micro-machined structures (micro-sensors) • Physical principles on which they operate is often an important factor in choosing a sensor for a given application (a sensor using a particular principle may perform much better) 2
Sensor Capacitive Sensors Technologies • Consist of two parallel metal plates • Dielectric: air • Other medium • Distance between the plates is fixed or not? • No: displacement sensors • Directly • Indirectly pressure, sound, acceleration • Yes: dielectric changes • Dielectric: air humidity sensor • Dielectric: air+Liquid Liquid level sensor 3
Sensor Resistive Sensors Technologies • Resistive sensors: measured variable is applied the resistance of a material varies • This principle is applied most commonly: • Temperature measurement (using resistance thermometers or thermistors) • Displacement measurement (using strain gauges or piezoresistive sensors) • Moisture meters 4
Sensor Magnetic Sensors Technologies • Utilize the magnetic phenomena of • Inductance • Reluctance • Eddy currents • To indicate the value of the measured quantity (usually some form of displacement) • Inductive sensors: movement change in the mutual inductance (between magnetically coupled parts, Fig) • the central limb of an “E” -shaped ferromagnetic body is excited (AC) • The displacement to be measured is applied to a ferromagnetic plate (close to “E”) • Movements of the plate alter the flux paths and hence cause a change in the current • Ohm’s law: current : I=V/ω L For fixed w and V I=1/KL (Non-linear relation, constant K) • The inductance principle is also used in differential transformers 5
Sensor Magnetic Sensors Technologies • Variable reluctance: a coil is wound on a permanent magnet (not an iron core) • As the tip of each tooth moves toward and away from the pick-up unit, the changing magnetic flux in the pickup coil causes a voltage to be induced in the coil (magnitude is proportional to the rate of change of flux) • The output is a sequence of positive and negative pulses whose frequency is proportional to the rotational velocity 6
Sensor Magnetic Sensors Technologies • Eddy Current Sensor: consist of a probe containing a coil (Fig) • Excited at a high frequency (typically 1 MHz) • measures displacement (probe to a moving metal target) • high frequency of excitation eddy currents are induced only in the surface of the target • the current magnitude reduces to almost zero a short distance inside the target • sensor works with very thin targets (steel diaphragm of a pressure sensor) • The eddy currents alter the inductance of the probe coil (this change can be translated into a d.c. voltage output, proportional to distance) • Measurement resolution as high as 0.1 mm can be achieved • Non-conductive target a piece of aluminum tape is fastened to it 7
Sensor Hall-Effect Sensors Technologies • Hall-effect sensor: a device used to measure the magnitude of a magnetic field • Consists of a conductor carrying a current that is aligned orthogonally with the magnetic field (Fig) • Produces a transverse voltage difference • Excitation current: I • Magnetic field strength: B • Output voltage: V = KIB (K = Hall constant) • Conductor: usually a semiconductor larger Output voltage • Example: • Proximity sensor (a permanent magnet) • The magnitude of field changes when the device comes close to any ferrous metal object • Computer keyboard push buttons • Operate at high frequencies without contact bounce 8
Sensor Piezoelectric Transducers Technologies • Piezoelectric Transducers • Produce an output voltage when a force is applied • And reverse • Used as: • Ultrasonic transmitters and receivers • Displacement transducers (particularly as part of devices measuring acceleration, force, and pressure) • Asymmetrical lattice of molecules: a mechanical force lattice distorts a reorientation of electric charges inside relative displacement of positive and negative charges induces surface charges on the material of opposite polarity between the two sides • By implanting electrodes into the surface of the material, these surface charges can be measured 9
Sensor Piezoelectric Transducers Technologies • Piezoelectric Transducers • The polarity of the induced voltage: material compressed or stretched • Input impedance of the instrument used to measure the induced voltage must be very high : provides a path for the induced charge to leak away • Materials exhibiting piezoelectric behavior: • Natural: quartz • Synthetic: lithium sulphate • Ferroelectric ceramics: barium titanate • Piezoelectric constant (k) 2.3 for quartz (e.g. force = 1 g , crystal area = 100 mm2, thickness = 1 mm output of 23 µV) • • 140 for barium titanate (1.4 mv) • Certain polymeric films such as polyvinylidine: • Higher voltage • Lower mechanical strength (not good if resonance happens) • piezoelectric principle is invertible: Ultrasonic transmitter sound wave 10
Sensor Strain Gauges Technologies • Experience resistance change if stretched / strained • Detect very small displacements (usually in the range of 0 - 50 µm) • Part of other transducers • for example: diaphragm pressure sensors (convert pressure changes to displacements) • Inaccuracies: as low as ±0.15% FSD • Life expectancy is usually three million reversals • nominal values: 120, 350, and 1000 O are very commontypical • maximum change of resistance in a 120-O device would be 5 O (max deflection) • length of metal resistance wire formed into a zigzag pattern and mounted onto a flexible backing sheet • Recently, largely been replaced • Metal-foil types • Semiconductor types • piezoresistive elements: gauge factor (x100) • Temperature co-efficient: worse • Mettalic: usually, copper – nickel – manganese alloy 11
Sensor Piezoresistive Sensors Technologies Materials that under pressure/force change resistance Usually semiconductors (Silicon + impurities) 1 ρ = 𝑓𝑂µ ρ : specific resistance Schematic cross-section of the basic elements of a e : charge (electron) silicon n-well piezoresistor N : # of charge carriers (depends on impurities) µ : charge carrier mobility (depends on the strain) Resistance : 30,000 greater than copper Pressure can be applied in 3-directions on cristal Very high sensitivity (~100), 50 times greater than strain gauge So, can measure tiny force/pressure 12
Sensor Optical Sensors Technologies • Source + Detector • Air path • Fiber optic • immunity to electromagnetically induced noise • Greater safety (in hazardous environment) • Air path: • Proximity • Translational motion • Rotational motion • Gas concentration • Sources: • Tungsten-filament lamps (visible spectrum prone to interferences from Sun, etc.) • So, infrared LEDs, or infrared laser diodes • Laser diodes, and light-emitting diodes (LEDs) 13
Sensor Optical Sensors - Air path Technologies • Detectors: • Photoconductors (photoresistors) • Changes in incident light changes in resistance • Photovoltaic devices (photocells) • Light intensity Voltage magnitude • Phototransistors • Light base-collector junction • Output current (like photodiode) • Internal gain • Photodiodes • Amount of light output current • Faster response 14
Sensor Optical Sensors – Fiber Optic Technologies • Fiber-optic cable to transmit light • Plastic • inexpensive, large diameter 0.5-1mm • Not good in harsh environment • Glass (fragile) • Combination • Cost? • Sensor cost is dominated by the cost of the transmitter and receiver • Main difficulty? • Maximizing proportion of light entering the cable • Major classes of fiber-optic sensors: • Intrinsic • Fiber-optic cable itself is the sensor • Extrinsic • Cable is only used to guide light to/from a conventional sensor 15
Sensor Optical Sensors – Fiber Optic - Intrinsic Technologies • Measurand physical quantity causes: measurable change in characteristics of transmitted light: • Intensity (Use multi-mode fibers, simplest) • Phase • Polarization Single mode • Wavelength • Transit time • Useful feature: • Provide distributed sensing over distances (of up to 1 meter, if required) • Example of manipulating intensity: • Various form of switches • Light path is simply blocked 16
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