Negotiation Dr. Moazami Maryam Abdolmaleki Somayeh Shaabani 1393.
Instrumentation (and Sensor Process Control)...
Transcript of Instrumentation (and Sensor Process Control)...
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 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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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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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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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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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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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
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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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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