Introduction Technologies Range of sensors available for measuring - - PowerPoint PPT Presentation

Instrumentation (and

Process Control)

Fall 1393 Bonab University

Sensor Technologies

Introduction

• 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)

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Sensor Technologies

Capacitive Sensors

• 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

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Sensor Technologies

Resistive Sensors

• 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

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Sensor Technologies

Magnetic Sensors

• 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/ωL  For fixed w and V  I=1/KL (Non-linear relation,

constant K)

• The inductance principle is also used in differential transformers

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Sensor Technologies

Magnetic Sensors

• 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

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Sensor Technologies

Magnetic Sensors

• 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
• nly 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

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Sensor Technologies

Hall-Effect Sensors

• 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

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Sensor Technologies

Piezoelectric Transducers

• 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

• pposite polarity between the two sides
• By implanting electrodes into the surface of the material, these surface charges

can be measured

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Sensor Technologies

Piezoelectric Transducers

• 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

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Sensor Technologies

Strain Gauges

• 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

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Sensor Technologies

Piezoresistive Sensors

Materials that under pressure/force change resistance Usually semiconductors (Silicon + impurities) ρ = 1 𝑓𝑂µ ρ : specific resistance e : charge (electron) 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

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Sensor Technologies

Schematic cross-section of the basic elements of a silicon n-well piezoresistor

Optical Sensors

• 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)

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Sensor Technologies

Optical Sensors - Air path

• 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

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Sensor Technologies

Optical Sensors – Fiber Optic

• 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

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Sensor Technologies

Optical Sensors – Fiber Optic - Intrinsic

• Measurand physical quantity causes:

measurable change in characteristics of transmitted light:

• Intensity (Use multi-mode fibers, simplest)
• Phase
• Polarization
• 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

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Sensor Technologies

Single mode

Optical Sensors – Fiber Optic - Intrinsic

• Also possible:
• Modulation of the intensity of transmitted light
• Takes place in:
• Proximity
• Displacement
• Pressure: deformation  refractive index  intensity
• pH (pH-dependent color)
• Smoke sensors (intensity reduction)
• A simple accelerometer:
• Placing a mass on a multimode fiber
• Acceleration  force exerted on the fiber  a change in

intensity of light transmitted

• Very high accuracy

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Sensor Technologies

Reflected light changes

Optical Sensors – Fiber Optic - Intrinsic

• Slightly more complicated:
• Method of affecting light intensity modulation:

Variable shutter sensor

• Two fixed fibers
• Variable shutter
• Application?
• Measure the displacement
• Bourdon tubes
• Diaphragms
• Bimetallic thermometers
• Temperature Sensor:
• Refractive index is close:
• Core
• Cladding
• Temperature rise  index even closer together  losses from the core increases 

reducing the quantity of light transmitted

• Can be used in cryogenic leak detection

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Sensor Technologies

Optical Sensors – Fiber Optic - Extrinsic

• Fiber-optic cable (normally multimode) to:
• Transmit modulated light from a conventional sensor (say, resistance thermometer)
• A major advantage:

Ability to reach places that are otherwise inaccessible

• Example:
• Insertion of fiber-optic cables into the jet engines

Transmitting radiation into a radiation pyrometer located remotely  measure temperature

• Internal temperature of electrical transformers (presence

Of extreme electromagnetic fields

• Advantage: excellent protection against noise
• Disadvantage: many sensors’ output can’t easily transmitted by a fiber-optic cable
• Piezoelectric sensors : good fit because the modulated frequency of a quartz crystal can be

transmitted readily into a fiber-optic cable

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Sensor Technologies

Ultrasonic Transducers

• Used in many fields of measurement:
• Fluid flow rates
• Liquid levels
• Translational displacements
• Ultrasound: a band above 20 kHz (above the sonic=range

that humans can hear)

• Ultrasound transmitter & device that receives the wave
• Changes in measured variable 
• Change in time taken for the ultrasound wave to travel between the transmitter and receiver
• Change in phase or frequency of Wave
• most common (ultrasonic element): a piezoelectric crystal
• Can act both as Transmit/Receiver
• Operating frequencies: 20KHz-15MHz

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Sensor Technologies

Ultrasonic Transducers - Transmission Speed

• Speed varies according to the medium
• through air: the speed is affected by:

environmental factors such as:

• Temperature
• 0 to 20oC  331.6 to 343.6 m/s
• Humidity
• 20%  331.6 to 331.8 m/s (at 0oC)
• Air turbulence

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Sensor Technologies

Ultrasonic Transducers - Directionality of Ultrasound Waves

• An ultrasound element emits a spherical wave of energy
• peak energy: always in a particular direction
• along a line that is normal to the transmitting face (direction of travel)
• Attenuation increases with angle
• For many purposes:
• Better to treat the wave as a conical volume of energy:
• Transmission angle where energy is half
• At 40KHz  ±50o
• At 400KHz  ±3o
• Air currents can deflect ultrasonic waves
• 10 km/h deflects an ultrasound wave by 8 mm
• ver a distance of 1 m
• Frequency - wavelength of ultrasound waves:

Depends on the velocity temperature of medium

• Ultrasound as a Range Sensor (care of Temp.)

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Sensor Technologies

Nuclear Sensors

• Nuclear sensors are uncommon measurement devices because:
• Strict safety regulations
• They are usually expensive
• Very low-level radiation sources are now available
• Operation: very similar to optical sensors:
• Radiation is transmitted (transmit/receiver)
• Magnitude attenuate according to the value of the

measured variable

• Caesium-137 is used commonly: as a 𝜹-ray source
• Sodium iodide device is used commonly as a 𝜹-ray detector
• A common application:

noninvasive technique for measuring the level of liquid in storage tanks

• Also used in mass flow meters & medical scanners

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Sensor Technologies

Microsensors

• Millimeter-sized 2-D / 3-D micro-machined structures
• Have smaller size
• Improved performance
• Better reliability
• Lower production costs (Compared to alternative forms of sensors)
• Devices currently in use:
• Measure temperature
• Pressure
• Force
• Acceleration
• Humidity
• magnetic fields
• Radiation
• chemical parameters

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Sensor Technologies

Microsensors

• Construction:
• Usually: from a silicon semiconductor (excellent mechanical properties)
• but other materials such as:
• metals, plastics, polymers, glasses, and ceramics deposited on a silicon base
• Micro-engineering techniques are an essential enabling technology:

(designed so that their electromechanical properties change in response to a change in the measured parameter)

• Many of the techniques used for integrated circuit (IC) manufacture are also

used in sensor fabrication:

• Crystal growing
• Polishing
• Thin film deposition
• Ion implantation
• Wet and dry chemical and laser etching
• Photolithography

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Sensor Technologies