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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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 5

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

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

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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 opposite 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 wave10

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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Piezoresistive Sensors

Materials that under pressure/force change resistance

Usually semiconductors (Silicon + impurities)

ρ =1

𝑒𝑁µρ : 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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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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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 detection18

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

over 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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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 𝜸-ray source

• Sodium iodide device is used commonly as a 𝜸-ray detector

• A common application:

noninvasive technique for measuring the level of liquid

in storage tanks

• Also used in mass flow meters & medical scanners23

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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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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Technologies