final sensor 2024 c3 c4 Flashcards
other names for potentiometer sensor
adjustable resistor
simplest type of displacement sensor involves..
the action of displacement in moving the wiper of a potentiometer
how potentiometric sensor works (2)
1- move the wiper of the potentiometer
2- the device then converts linear or angular motion into a changing resistance that may be converted directly to voltage and current signals
problems using potentiometric device (4)
1- mechanical wear (haus)
2-friction in the wiper action
3- limited resolution in the wire-wound units
4- high electronic noise
capacitive
the basic operation of a capacitive sensor can be seen from the familiar equation for a parallel-plate capacitor:
C= K(عo) (A/d)
K= dielectric constant
عo = permittivity = 8.85pF/m
A= plate common area
d= plate separation
How to change the capacity ? (3)
1- variation of the distance between the plates (d)
2- variation of the shared area of the plates (A)
3- variation of the dielectric constant (K)
Facts about capacitive sensor (2)
- can detect any material with or without contact
- can adjust the sensitivity to detect liquids or solids even through non-metallic tanks or vessels
A capacitor consists of ?
two plates generates an electrical between the plates when supplied with power.
inductive sensor(2)
- an electronic device that can detect ferrous metal targets (iron metal based) like aluminium, brass and copper.
- using non-ferrous metal target decreases an inductive sensors’ sensing range.
4 level sensors
- mechanical
- pressure
- electrical
- ultrasonic
tensile stress and tensile strain
stress = the effect of applied force
strain = the resulting deformation
5 formula
Tensile stress
Tensile strain
Compressional stress
Shear stress
Shear strain
tensile stress = F/A
F= applied force in N
A= cross- sectional area of the sample in m^2
tensile strain = Al/l
Al= change in length in m (in.)
l= original length in m (in.)
compressional stress = F/A
shear stress = F/A
shear strain = Ax/l
Ax= deformation in m
l= width of a sample in m
difference between compressional and tensile stress
the direction of the applied force and the polarity of the change in length.
tensile = tarik bagi panjang
compressional = penyek
the basic technique of strain gauge measurement
involves attaching (gluing using electrical glue wihich is not an insulator) a metal wire or foil to the element whose strain is to be measured
materials that are commonly used for an electrical glue
gold- expensive
silver/ copper- easy to oxidize
formula for gauge factor (GF)
GF = (AR/R)/strain
AR/R = fractional change in gauge resistance because of strain
strain = Al/l = fractional in length
GF semiconductor = (-)
GF metal = (+)
why high gauge factor is desirable
indicates a larger change in a resistance for a given strain and is easier to measure
construction for a metal strain gauge (5)
1- used in two forms, wire and foil (foil is more accurate)
2-very long
3-often made unidirectional (so it responds to strain in only one direction)
4- folding the material back and forth so we achieve a long length to provide high resistance
5- usually mounted on a paper backing that is bonded (using epoxy) to the element whose strain is to be measured. the most common value for SG resistance is 120 ohm
2 effects are critical in the signal-conditioning techniques used for SGs
1- fractional changes in resistance that require carefully designed resistance measurement circuits
2- the need to provide some compensation for temperature effects to eliminate masking changes in strain
solution to the effects of signal conditioning
bridge circuit (to avoid power dissipation)
- the sensitivity of the bridge circuit for detecting small changes in resistance is well known
advantages of dummy gauge / one-arm bridge (3)
-will not affect the output
-can provide the required temperature compensation
-to help active gauge with temperature problem
how dummy is mounted (3)
-in an insensitive orientation but in the same proximity as the active SG.
-both gauge change in resistance from temperature effects, but the dummy does not respond to a change in both strain gauges
-only the active SG responds to strain effects
Common application for strain gauges
measure deflections cantilever beam ( a beam that is supported at only one end and deflects when a load is applied)
how strain gauge measure the deflection of the cantilever beams
use a two arm bridge
how two arm bridge works in measuring the deflection of a cantilever beam (5)
- one pair of active A1 and dummy D1 gauge is mounted on the top surface
- the active gauge will experience tension with downward deflection of the beam, and its resistance will increase
- the second pair, A2 and D2, are mounted on the bottom surface
- the active gauge will experience compression with downward deflection and its resistance will decrease
A1 D1 - tensile
A2 D2 - compress
principle of semiconductor strain gauge (5)
- the basic effect is a change of resistance with strain
- in the case of a semiconductor, the resistivity also changes with strain, along with the physical dimensions
- this is due to changes in electron and hole mobility with changes in crystal structure as strain is applied.
- the net result is a much larger gauge factor than is possible with metal gauge
-For semiconductor strain gauge, the GF is often negative (the resistance decreases when a tensile (stretching) stress is applied.
construction of semiconductor strain gauge (3)
- physically appears as a band or strip of material with electrical connection
- the gauge is either bonded directly onto the test element or id encapsulated, is attached by the encapsulation material.
- these SGs also appear as IC assemblies in configurations used for other measurements
motion sensor
to measure the rate of change of position, location, or displacement of an object is occurring
4 types of motion
- shock (accident)
- rectilinear (pergerakan biasa)
-angular (rotation) - vibration
rectilinear motion
- characterized by velocity and acceleration which id composed of straight-line segment
- example : car key
angular motion (3)
- designed to measure only rotations about some axia
- such devices cannot be used to measure the physical displacement of the whole shaft but only in rotation
- example : the angular motion motion of the shaft of a motor
vibration motion
-random in both the frequency of periodic motion and the magnitude of displacements from equilibrium
shock motion
- a special type of acceleration occurs when an object that may be in uniform motion or modestly accelerating is suddenly brought to rest, as in collision
cause of shock motion (2)
- such phenomena are the result of very large accelerations or actually decelerations as when an object is dropped from some height onto a hard surface
-typically on the order of milliseconds with peak accelerations over 500g
types of accelerometer
- potentiometric
-LVDT
-variable reluctance
-piezoelectric
accelerometer : potentiometric (3)
- measures mass motion by attaching the spring mass to the wiper arm of the potentiometer
- the mass position conveyed as a changing resistance
- natural frequency is less than 30 Hz (limiting their application to steady state acceleration or low frequency vibration measurement)
accelerometer : LVDT (4)
- measure mass displacement
- the core itself is the seismic mass
- displacements of the core are converted directly into a linearly proportional ac voltage
- natural frequency of less than 80 Hz and commonly used for steady-state and low frequency vibration
accelerometer : variable reluctance (5)
- the test mass is usually a permanent magnet
- the measurement is made from the voltage induced in a surrounding coil as the magnetic mass move under the influence of an acceleration
- used in vibration and shock studies only (has an output only when the mass is in motion)
-natural frequency is typically less than 100Hz
-often used in oil exploration to pick up the vibrations reflected from underground rock strata
Accelerometer : Piezoelectric (3)
- When exposed to an acceleration, the test mass stresses the crystal by a force , resulting in a voltage generated across the crystal.
- A measure of this voltage is then a measure of the acceleration. The crystal per se is a very high-impedance source, and thus requires a high-input impedance, low noise detector.
- natural frequency may exceed 5 kHz, so
that they can be used for vibration and shock
measurements.
example of steady-state acceleration
stop-go motion of an automobile
steady state acceleration application (3)
- measure of acceleration that vary in time but nonperiodic.
- a good sensor
1. adequate range to cover expected acceleration magnitudes
2. a natural frequency sufficiently high that its period is shorter than the characteristic time span over which the measured acceleration changes. - by using electronic integrators, the basic accelerometer can provide both velocity and position information
Pressure Principles (3)
-the force per unit area that a fluid exerts on its surroundings.
- If every wall in the gas must have the same pressure in order for the gas to be fully enclosed.
- In a liquid, the pressure will vary, need not be enclosed
Static pressure (2)
- For a fluid that is not moving in space, that is not being pumped through pipes or flowing through a channel.
- The pressure in cases where no motion is occurring
Dynamic pressure
- If a fluid is in motion, the pressure that it exerts on its surroundings depends on the motion.
- Pressure can depend on flow, compressibility of the fluid and external force
types of pressure
- static pressure
- dynamic pressure
- gauge pressure
- head pressure
gauge pressure
pressure in a relative
sense—that is, compared to
atmospheric pressure
Pg = Pabs-Pat
Pg= gauge pressure
Pabs= absolute pressure
Pat= atmospheric pressure
Head pressure
- describe the pressure of the liquid in a tank or pipe.
p = pgh
p= pressure in Pa
p= density in kg/m^3
g= acceleration due to gravity (9.8m/s^2)
h= depth of liquid in m
types of pressure sensors for (p>1 atm)
- diaphragm
- bellow
- bourdon tube
- solid state pressure sensor
bellow (3)
- Converts a pressure differential into a physical displacement ( a straight-line expansion. )
- The accordion-shaped sides of the bellows are made from thin metal. When there is a pressure difference, a net force will exist on the flat, front surface of the bellows.
- The bellows assembly will then collapse like an accordion if p2 is greater than p1 or expand if is p2 less than p1 .
bourdon tube (5)
- A hard metal tube, usually a type of
bronze or brass, is flattened, and one
end is closed off - The tube is then bent into a curve or arc, sometimes even a spiral.
- The open end is attached to a header
by which a pressure can be introduced to the inside of the tube. - The tube will deflect when the inside
applied pressure is different from the
outside pressure. - The tube will tend to straighten out if the inside pressure > the outside pressure, and to curve more if the pressure inside < outside
solid state pressure sensor (5)
- pressure ranges of 0 to 100 kPa (0 to 14.7 psi).
- 3 components - dc power, ground, and the sensor output.
- The basic sensing element is a small wafer of silicon acting as a diaphragm that, as usual, deflects in response to a pressure difference.
- This signal conditioning includes temperature compensation and circuitry that provides an output voltage
- application is in the
commercial field, in home appliances such
as dishwashers and washing machines.
how to modify SS pressure sensor so it can measure differential pressure?
For differential measurement, facilities are provided to allow application of independent pressures p1 and p2 and to the two sides of the diaphragm.
characteristics of SS pressure sensor
1- sensitivities in the range of 10 to 100mV/kPa
2- response times on the order of 10 ms
3- linear voltage versus pressure within the specified operating range
4- easy of use, only 3 connections: dc power (typically 5V), ground and the sensor output voltage
3 types of pressure sensors for (p<1 atm)
1- pirani gauge
2- thermocouple
3- ionization gauge
pirani gauge (3)
-determines the filament temperature through a measure of filament resistance in accordance with the principles
* Filament excitation and resistance measurement are both performed with a bridge circuit.
* The response of resistance versus pressure is highly nonlinear.
thermocouple (2)
- A second pressure transducer or gauge measures filament temperature using a
thermocouple directly attached to the heated filament.
*Ambient room temperature serves as a reference for the thermocouple, and the
voltage output, which is proportional to pressure, is highly nonlinear.
ionization gauge (3)
- employs electrons, usually from a heated filament, to ionize the gas whose pressure is to be measured, and then measures the current flowing between two electrodes in the ionized environment.
- The number of ions per unit volume and current depends on gas pressure.
- This current is then monitored as an approximately linear indication of pressure.
how to measure solid flow ?
conveyor flow
what is conveyor flow (2)
- mass or weight per unit time that is being transported by the conveyor system.
- To make a measurement of
flow, it is only necessary to weigh the quantity of material on some fixed length of the conveyor system.
how conveyor flow works ? (4)
- A mechanical valve controls the rate from
the hopper onto the conveyor belt - The belt is driven by a motor system
- The conveyor belt slides over the platform, which deflects slightly due to the weight of material
- A load cell measures this deflection as an indication of weight
flow rate (conveyor belt) formula
- flow rate can be calculated using Q= (WR)/L
Q=flow (kg/min or lb/min)
W = weight of material on section of length, L (kg or lb)
R= conveyor speed (m/min or ft/min)
L= length of weighing platform (m or ft)
components to measure liquid flow
1- mass or weight flow rate
F=pQ
F= mass or weight flow rate
p = mass density or weight density
Q= volume flow rate
2- flow velocity = distance in liquid travels in the carrier per unit time (m/min or ft/min)
V= Q/A
V= flow velocity
Q= volume liquid flow
A= cross sectional area of flow carrier
3- volume flow rate (gals/min)
Restriction flow sensors
- One of the most
common methods of
measuring the flow of
liquids in pipes is by
introducing a restriction
in the pipe and measuring the pressure
drop that results across
the restriction. - When such a restriction
is placed in the pipe, the
velocity of the fluid
through the restriction
increases, and the
pressure in the
restriction decreases. - there is a
relationship between
the pressure drop and
the rate of flow such
that, as the flow
increases, the pressure
drops.
2 fundamental of electromagnetic radiation
1- nature of EM
2- frequency and wavelength
nature of EM (3)
- EM radiation is a form of energy that is always in motion—that is, it propagates through space.
- An object that releases, or emits, such radiation loses energy.
- One that absorbs radiation gains energy.
frequency and wavelength of the fundamental of em radiation
- The frequency represents the oscillation per second as the radiation passes some fixed point in space.
- The wavelength is the spatial distance between two successive maxima or minima of the wave in the direction of propagation
electromagnetic spectrum for UV (ultraviolet radiation)
100nm-400nm
electromagnetic spectrum for VIS (visible radiation)
400nm-800nm
electromagnetic spectrum for IR (infrared Radiation)
800 nm - 3 (mikro)m
8 characteristics of light
1- photon
2- energy
3- coherency
4- power
5- spectrum
6- divergence
7- intensity
8- power
formula for the characteristics of light
Wp= hf= hc/lambda
Wp= photon energy (J)
h= planck constant (Js)
f= frequency (s^-1)
lambda = wavelength (m)
formula for intensity of light
I= P/A
I= intensity in W/m^2
P=power in W
A= beam cross-sectional area in m^2
photometry
the science of measurement of light, in terms of its perceived brightness to the human eye.
radiometry
the science of measurement of radiant energy in terms of absolute power
Photodetector Characteristics
the spectral sensitivity.
most common photoconductive detectors/ photodetectors
based on the change in conductivity of a semiconductor material with radiation intensity.
photoresistive cells
-The change in conductivity appears as
a change in resistance,
- Intensity increase, resistance will decrease exponentially
principle of semiconductor photodetector (3)
- a photon is absorbed and thereby excites an electron from the valence to the conduction band.
- As many electrons are excited into the conduction band, the semiconductor resistance decreases, making the resistance an inverse function of radiation intensity.
- Lambda max= hc/ Wg
Lambda max= max detectable radiation wavelength
h= planck constant
Wg = semiconductor energy gap (J)
How to prevent
the photoconductor
from showing similar thermal effects ?(2)
1- operate the devices at a controlled temperature
2- make the gap too large for thermal effects to produce conduction electrons
Two common photoconductive semiconductor materials
- Cadmium Sulfide (CdS), with a band gap of 2.42 Ev
- Cadmium Selenide (CdSe), with a 1.74-eV gap.
what is the effect of having large gap energies
both materials have a very high resistivity at room temperature.
How to overcome high resistivity
- minimize resistance and provide .aximum surface area for the detector
- R= pl/A
p= resistivity - The area presented to light, A= WL, is large for maximum exposure,
whereas the electrical length, l = L , is small, which reduces the nominal resistance.
characteristics of photoconductive
detectors
-Both the PbS and PbSe cells are able to detect infrared radiation, but they are therefore also susceptible to thermal
resistance changes.
-They are usually used in temperature-controlled enclosures
to prevent thermal resistance changes
from masking radiation effects.
-The CdS cell is the most common photoconductive cell in commercial use. Its
spectral response is similar to that of the human eye
signal conditioning photoconductive cell
-exhibits a resistance
that decreases nonlinearly with radiation intensity.
- the change in resistance is
pronounced where a resistance can change by
several hundred orders of magnitude from
dark to normal daylight.
photovoltaic detectors
- generates a voltage that is proportional to
incident EM radiation intensity - convert the EM energy into electrical energy.
-Applications: EM radiation detectors and power sources converting solar radiation into electrical power
principle of photovoltaic cell
1- Photons striking the cell pass through the thin p-doped upper layer
2- Photons are absorbed by electrons in the n layer → formation of
conduction electrons and holes
3- The depletion-zone potential of the pn junction separates these
conduction electrons and holes
4- Causes a difference of potential to
develop across the junction
5- The upper terminal is positive, and the
lower negative
construction of photovoltaic cell
- The cell is actually a giant diode that is constructed using a
pn junction between appropriately doped semiconductors - first layer: p-doped semiconductor
- sec layer: pn junction
- third layer : n-doped semiconductor
- fourth layer: conductor base
IV Curves (a photosensitive diode as a function of
light intensity)
- When the junction is illuminated → a
voltage is generated across the diode →
shown by the IV curve crossing the zero
current axis with a nonzero voltage. This is
the photovoltaic voltage. - the reverse
current through the diode, when it is
reverse biased, also increases with
radiation intensity – Basic photodiode - when voltage is zero, but intensity is a non zero, so a short-circuit current will occur
short circuit current
- the cell voltage divided by its internal resistance, Vc/Rsc, varies linearly
with light intensity.
-short-circuit current
as the values of current when the IV
curve crosses the zero-voltage line.
relationship between current and Isc
the current is proportional to the number of
charge carriers, which is proportional to
the number of photons striking the cell
and thus the radiation intensity
signal conditioning of photovoltaic cell
- Since the current to the op amp input must be zero, the feedback
current through R must equal . - Vout= -RIsc
characteristics of photovoltaic cell
- used only at low temperatures to prevent thermal effects from obscuring radiation detection.
-The silicon photovoltaic cell is probably the most common.
photodiode detectors
- photons impinging on the pn junction also alter the reverse current-versus-voltage
characteristic of the diode.
-the reverse current will
be increased almost linearly with light
intensity. Thus, the photodiode is operated in the reverse-bias mode.
function of load line
to determine the voltage
changes across the diode as a function of light intensity.
dark current response
the basic diode reverse current, with no illumination of the pn
junction
-
advantages of photodiode
- fast time response
- very small
- a lens must be used
to focus light on the pn junction.
spectral response of photodiodes
typically peaked in the infrared, but with
usable response in the visible band.
extension concept of photodiode
phototransistor
phototransistor
the intensity of EM radiation impinging on the collector-base junction of the transistor acts much like a base current in producing an amplified collector-emitter current
constructions of photoemissive
- The cathode is maintained at some negative voltage with respect to the wire anode that is grounded through resistor R.
- The inner surface of the cathode has been coated with a photoemissive agent.
- This material is a metal for which electrons are easily detached from the metal surface.
principles of photoemissive device
1- A photon can strike the surface
2- Impart sufficient energy to an electron to eject it from this coating
3- Electron will be driven from cathode to anode by potential and resistor
4- The current that depends on the intensity of light striking the cathode
how photomultiplier works
1- A photoelectron from the cathode strikes the first dynode to eject several electrons
2- electrons are accelerated to the second dynode
3- eject several more electrons
4- process is repeated until the electrons are greatly multiplied in number
5- photomultiplier (has a gain associated with its
detection
construction of photomultiplier tube
*many following electrodes, called
dynodes, maintained at successively more positive voltages.
*The final electrode is the anode, which is grounded through a resistor, R
specifications of photomultiplier tube
1- the number of dynodes and the material
-determine the
amplification or current gain
2- the spectral response ( **The time constant for photomultiplier tubes ranges from 20µs down to 0.1µs.)
pyrometry
- a type of remote sensing thermometer
used to measure the
temperature of distant objects in the noncontact measurement
blackbody radiation
- An idealized object absorbs all radiation
impinging on it, regardless of wavelength, and therefore becomes an
ideal absorber. - This object also emits radiation without
regard to any special peculiarities of
particular wavelength and therefore becomes an ideal emitter
explanation on ideal curves of EM radiation as a function of temperature
- as temperature increased, wavelength decreased as blue shifting occurred and the peak shifted to the left as the temperature increased
- the object will glow as the temperature increase because the wavelength is near the visible band
-As the temperature is increased, the maximum
emitted radiation is in the shorter wavelengths, and finally, at very high temperatures, the maximum emitted radiation is near the visible band. - Because of this shift in emission peak with
temperature, an object begins to glow as its
temperature increases.
blackbody approximation
- Most materials emit and absorb radiation at preferred wavelengths,
giving rise to color, for example. Thus, these objects cannot display
a radiation energy versus wavelength curve like that of an ideal
blackbody. - Correction factors are applied to relate the radiation curves of real
objects to that of an ideal blackbody. - For calibration purposes, the radiation emitted from a small hole in a metal
enclosure is close to an ideal blackbody.
broadband pyrometer
uses the relation between total emitted radiation energy and given temperature
types of broadband pyrometer
- total radiation pyrometer
- IR pyrometer
total radiation pyrometer
- Collect radiation from the visible through the infrared wavelengths
- the detector is often a series of
microthermocouples attached to a blackened platinum disc - Advantage: responds to visible and IR radiation with little regard to
wavelength
IR pyrometer
- Uses a lens formed from silicon or germanium to focus the IR radiation on a suitable detector
- Handheld, pistol-shaped devices that
read the temperature of an object toward which they are pointed
characteristics of IR pyrometer
- have a readout directly in temperature, either analog or digital
- switchable range is from 0 to 1000ºC to
with an accuracy of ±5ºC to ±0.5ºC
applications of IR pyrometer
1- metal production facilities
2- glass industries
3- semiconductor process
- used to regulate induction heating equipment, crystal pull rates, and other related parameters.
narrow band pyrometer
- depends
on the variation in monochromatic
radiation energy emission with
temperature. - the intensity of a heated platinum filament is varied
until it matches an object whose
temperature is to be determined.
another name for narrow band pyrometer and why
optical pyrometers because they generally
involve wavelengths only in the visible part of the spectrum.
construction of narrow band pyrometer/ optical pyrometer
focused on the object
whose temperature is to be determined, and the filter picks out only the desired wavelength, which is usually in the red.
how narrow band differentiate between high temperature and low temperature
- At low heating, the filament appears dark
against the background object, - As the filament is heated, it eventually appears as a bright filament against the background
object
types of conventional light sources
1- incandescent sources
2- atomic sources
characteristics of an incandescent light source
1- not monochromatic/ polychromatic bcs the light is distributes in a very broad wavelength spectrum
2- incoherent bcs such light results from molecular vibrations induced by heat , and light from one section of the wire is not associated with the light from another section
atomic sources
1- If one of these electrons is excited from its normal energy level to a different position (a→b), energy must
be provided to the atom
2- This electron returns to its normal level in a very short time
3- It gives up energy in the form of emitted EM radiation (b’→ a’)
Laser sources
- created when electrons
in the atoms in optical materials like glass, crystal, or gas absorb
the energy from an electrical current or a light. - That extra energy “excites” the electrons enough to move from a
lower-energy orbit to a higher-energy orbit around the atom’s
nucleus
properties of laser
1- monochromatic
- comes predominantly from a particular energy level transition
2- collimated/ coherent
- it emerges from the laser output mirror and remains so for a certain distance from the laser
3- diverged
- the laser light emerges perpendicular to the output mirror, the beam has very little divergence. Typical divergency may be 0.001 rad
