pe-4030 ch 2 sensors and transducers part 1 oct 1 2013
TRANSCRIPT
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Sensors
and
Transducers
Prof. Charlton S. InaoProfessor Mechatronics System Design
Defence Engineering CollegeBishoftu, Ethiopia
PE-4030Chapter 2/a
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Sensors
• Sensor is used for an element which produces
a signal relating to the quantity being
measured
Example: Resistance Temperature
Element/(RTD), the quantity being measured
is temperature and the sensor transforms
and input of temperature into a change inresistance.
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Sensors
• Sensor is used for an input device that provides ausable output in response to a specified physicalinput. For example, a thermocouple is a sensorthat converts a temperature difference into an
electrical output.The term transducer is generally used to refer to adevice that converts a signal from one form to adifferent physical form. Thus sensors are often
transducers, but also other devices can betransducers, such as a motor that converts anelectrical input into rotation.
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Transducers
• Transducer is often used in place of the termsensor. They are elements that when subjectto some physical change experience a re-
lated change.• Sensors are transducers.
• A measurement may use transducers, in
addition to the sensor, in other parts of thesystem to convert signals in one form toanother form.
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Transducers
• A transducer is a device that converts one type of energy to another. Energy types include (but are not limited to) electrical, mechanical,electromagnetic (including light), chemical,
acoustic or thermal energy. While the termtransducer commonly implies the use of asensor/detector, any device which convertsenergy can be considered a transducer.
Transducers are widely used in measuringinstruments.
• Transduce means converts.
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Transducers
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Terminology
Signal Conditioning - a front-end preprocessing,which generally includes functions such assignal amplification, filtering, electrical
isolation, and multiplexing.In addition, many transducers requireexcitation currents or voltages, bridgecompletion, linearization, or highamplification for proper and accurateoperation.
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Signal Conditioning Devices
Charge amplifiers, lock-in amplifiers, power
amplifiers, switching amplifiers, linear
amplifiers, tracking filters, low-pass filters,
high-pass filters, and notch filters are some of
the signal-conditioning devices used in
mechatronic systems.
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Signal Conditioning Requirements of
Common Transducers
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Terminology
• Range- it is the limits between which the
input can vary. eg – load cell for the
measurement of forces might have a range of
0-50 kN.
• Error- the difference between the result of
the measurement and the true value of the
quantity being measured.
Error= measured value- true value
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Terminology
• Accuracy- is the extent to which the valueindicated by a measurement system might bewrong. It is the summation of all possible errorsthat are likely to occur, as well as the accuracy to
which the transducer has been calibrated.• Accuracy is often expressed as the full range
output or full scale deflection. Eg. A sensor mightbe specified as having an accuracy of +
5% of full range output. Given: Temp Range 0-200 deg Centigrade. Reading could be within +or -10 deg centigrade of the true reading.
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Terminology
• Sensitivity- the sensitivity is the relationship
indicating how much output you get per unit
input, ie. Output/input.
Example: A resistance thermometer may havesensitivity of 0.5 ohms/deg Centigrade.
-A transducer for the measurement of pressure
might be quoted as having a temperaturesensitivity of + 0.1% of the reading per degree
Centigrade change in temperature.
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Terminology
• The sensitivity is defined as the ratio between
output signal and measured property. For
example, if a sensor measures temperature
and has a voltage output, the sensitivity is aconstant with the unit [V/K]; this sensor is
linear because the ratio is constant at all
points of measurement.
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Terminology
• Hysteresis error- Transducers can give
different outputs from the same value of
quantity being measured according to
whether that value has been reached by acontinuously increasing change or a
continuously decreasing change. The
hysteresis error is the maximum difference inoutput for increasing and decreasing values.
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Hysteresis
• Hysteresis is an error
caused by when the
measured property reverses
direction, but there is somefinite lag in time for the
sensor to respond, creating
a different offset error in
one direction than in the
other.
Hysteresis Error
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Nonlinearity and hysteresis
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Terminology• The error of a measurement is the difference between the result of
the measurement and the true value of the quantity beingmeasured.
• Nonlinearity error is used to describe the error that occurs as a
result of assuming a linear relationship between the input andoutput over the working range, that is, a graph of output plotted
against input is assumed to give a straight line.
• Non linearity error- The error associated in the deviation fromlinearity between the input and the output. The error is quoted asthe percentage of the full range output.
•
Three methods: 1) Draw the straight line joining the output valuesat the end points of the range 2) Find the straight line by using themethod of least squares to determine the best fit line. 3) Find thestraight line by using the method of least squares to determine thebest fit line which passes through zero point.
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Non Linearity Error
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Terminology
• Repeatability- The repeatability of the transducer is itsability to give the same output for repeatedapplications of the same input value. Example: Angularvelocity => repeatability + 0.01% of the full range at a
particular angular velocity.• Reproducibility- the ability to give the same output
when used to measure a constant and is measured ona number of occasions. Between the measurements
the transducer is disconnected and reinstalled. Theerror is usually expressed as a percentage of the fullrange output.
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Terminology
• Stability- The stability of a transducer is itsability to give the same output when used tomeasure a constant input over a period of
time. The term drift is often used to describethe change in output that occurs over time.
• The drift may be expressed a s a percentage of the full range output.
• The term zero drift is used for the changesthat occur in output when there is zero input.
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Terminology
• Deadband- the deadband or dead space of a
transducer is the range of input values for
which there is no output. For example bearing
friction in a flowmeter using a rotor mightmean that there is no output until the input
has reached a particular velocity threshold.
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Terminology
• Resolution- The resolution of a sensor is thesmallest change it can detect in the quantity thatit is measuring. Often in a digital display, the least
significant digit will fluctuate, indicating thatchanges of that magnitude are only just resolved.The resolution is related to the precision withwhich the measurement is made. For example, a
scanning tunneling probe (a fine tip near asurface collects an electron tunnelling current)can resolve atoms and molecules.
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Example: Strain Gauge Transducer
•Thermal Sensitivity :0.03 % full range output /deg Centigrade
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Interpretation for Strain Gauge
Transducer Specs
• The range indicates that the transducer can be
used to measure pressures between 70 and
1000 kPa or 2000 and 70000kPa.
• It requires a supply of 10 Vdc or ac rms for its
operation
• It will give an output of 40mV when the
pressure on the lower range is 1000 kPa and
on the upper range of 70 000kPa
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Interpretation for Strain Gauge
Transducer Specs
• Nonlinearity and hysteresis will lead to errors of +.5% of 1000, i.e + 5kPa on the lower range and +.5% of 70 000 , that is i.e + 350kPa on the upper
range.• The transducer can be used between the
temperature range -54 deg C and +120 deg C.
• When the temperature changes by 1 deg C the
output of the transducer for zero input willchange by 0.03% of 1000=0.3kPa on the lowerrange and 0.03% of 70 000=21 kPa on the upperrange.
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Interpretation for Strain Gauge
Transducer Specs
• When the temperature changes by 1deg C, the
sensitivity of the transducer will change by 0.3
kPa on the lower range and 21kPa on the
upper range.
• This means that readings will change by these
amounts when such temperature change
occurs.
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Example Velocity Sensor Specs
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Example : MX100AP Pressure Sensor
Specs
• Supply voltage: 3 V (6 V max)
• Supply current: 6 mA
• Full-scale span: 60 mV
• Range: 0 to 100 kPa
• Sensitivity: 0.6 mV/kPa
• Nonlinearity error: 0.05% of fullrange
• Temperature hysteresis: 0.5% of full
scale• Input resistance: 400 to 550 O
• Response time: 1 ms (10% to 90%)
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Displacement/Position Sensors
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Displacement/Position Sensors
The term position sensor is used for a
sensor that gives a measure of the
distance between a reference point and
the current location of the target, while adisplacement sensor gives a measure of
the distance between the present position
of the target and the previously recordedposition.
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1.Displacement
/Position Sensors
• Two Groups
– Linear
– Angular
Linear displacement sensors might be used to
monitor the thickness or other dimensions of
sheet materials, separation of rollers, the position
or presence of a part, the size of a part. Angular displacement methods might be used to
monitor the angular displacement of shafts.
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Application of Displacement
Sensor
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Application: Location and position of
object on a conveyor
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1.1 Displacement Sensors
• Displacement Sensors
– Potentiometer
– Strain Gauge element
– Capacitive Element
– Differential Transformer
– Optical Encoders
• Absolute
• Incremental
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Potentiometer
A potentiometer consists
of a resistance element
with a sliding contact
which can be moved over
the length of the element.
Such element can be used
for linear or rotarydisplacements.
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Potentiometer
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Strain Gauge
•
The electrical resistance strain gauge is a metal wire,metal foil strip, or a strip of semiconductor materialwhich is waferlike and can be stucked into surfaces likea postage stamp.
• When subject to strain, its resistance R changes, thefractional change in resistance Δ R/R being proportionalto the strain E, that is delta(Δ ) R/R=G x E
Where G, the constant of proportionality, is termed a sthe gauged factor.
The resistance change of a strain gauge is a measurementof the change in length of the element to which thestrain gauge is attached.
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Example: An electrical
resistance of 100 ohms
and a gauge factor of
2.0. What is the changeof resistance of the
gauge when it is subject
to a strain of 0.001.
• Answer=Fractional
Change in R= G x strain
x R=2.0X0.001x100=0.2
ohms
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Linear Variable Differential
Transformer.Linear variable differential transformer is a mechanical displacement
transducer.
It gives an a.c. voltage output proportional to the distance of the transformercore to the windings.
The LVDT is a mutual-inductance device with three coils and a core Anexternal a.c. power source energizes the central coil and the two- identical
secondary coils connected in series in such a way that their outputsoppose each other.
The net result is zero output.
A magnetic core is moved through the central tube as a result of displacement being monitored. However when the core is displaced fromthe central position there is a greater amount of magnetic core in one coil
than the other.A greater displacement means even more core in one coil than the other, the
output, the difference between the emf increases, the greater thedisplacement being monitored.
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LVDT
A greater displacement means even more core in one coil than the other, the
output, the difference between the emf increases, the greater the
displacement being monitored.
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LVDT
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Optical Encoder
• An encoder is a device that provides digital
output as a result of linear and angular
displacement.
• Position encoders are of two types:
1)Incremental
2) Absolute
Incremental encoders detect changes in rotationfrom some datum while the absolute encoders
give the actual angular position.
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Absolute
encoderIncremental
encoder
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Optical Encoder
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Optical Encoder
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1.2 Proximity Sensors
Proximity switches are used to detect the presence of an item without making contact with it.
Proximity Sensors
-eddy
-reed
-capacitive
-inductive
There are a number of forms of such switches, some being suitable only formetallic objects.
The eddy current type of proximity switch has a coil that is energized by aconstant
alternating current and produces a constant alternating magnetic field. When a
metallic objectis close to it, eddy currents are induced in it .
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Proximity Sensors
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Photoelectric Sensors
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Capacitive
A proximity switch that can be used with metallicand nonmetallic objects is the capacitive proximity switch. The capacitance of a pair of plates separated by some distance depends on
the separation; the smaller the separation, thehigher the capacitance. The sensor of acapacitive proximity switch is just one of theplates of the capacitor, the other plate being themetal object for which the proximity is to bedetected .Thus the proximity of the object isdetected by a change in capacitance.
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Inductive Sensor
• The inductive proximity switch, consists of a coil wound around a ferrous metallic core. When one end of this core isplaced near a ferrous metal object, there is effectively achange in the amount of metallic core associated with thecoil and so a change in its inductance.
• This change can be monitored using a resonant circuit, thepresence of the ferrous metal object thus changing thecurrent in that circuit.
• The current can be used to activate an electronic switchcircuit and so create an on/off device. The range over which
such objects can be detected is typically about 2 mm to 15mm. An example of the use of such a sensor is to detectwhether bottles passing along a conveyor belt have metalcaps on.
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Velocity Sensor
• Incremental Encoder-This can be used for measuringangular velocity, number of pulses produced persecond being determined.
• Tachogenerator -Used to measure angular velocity. It
is essentially a small electric generator, consisting of coil mounted in magnetic field .when the coil rotatesan alternating emf is induced in the coil, the size of themaximum emf being a measure of the angular velocity.
when used with a commutator a dc output can beaobtained which is a measure of the angular velocity.
Motion Sensor
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Motion Sensor
Note that each variable is the time derivative of the preceding one. Motion
measurements are extremely useful in controlling mechanical responses and
interactions in mechatronic systems. Numerous examples can be cited:
The rotating speed of a work piece and the feed rate of a tool are measured
in controlling machining operations.Displacements and speeds (both angular and translatory) at joints (revolute
and prismatic) of robotic manipulators or kinematic linkages are used in
controlling manipulator trajectory In high-speed ground transit vehicles,
acceleration and jerk measurements can be used for active suspension
control to obtain improved ride
quality.
By motion, we mean the four kinematic variables:
• Displacement (including position, distance, proximity, and size or gage)
• Velocity
• Acceleration
• Jerk
• Angular speed is a crucial measurement that is
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• Angular speed is a crucial measurement that is
used in the control of rotating machinery, such as
turbines, pumps, compressors, motors, and
generators in power-generating plants.
Proximity sensors (to measure displacement) and accelerometers (to measure
acceleration) are the two most common types of measuring devices used in
machine protection systems for condition monitoring, fault detection, diagnostic,
and on-line (often real-time) control of large and complex machinery
The accelerometer is often the only measuring
device used in controlling dynamic test rigs. Displacement
measurements are used for valve control in process
applications. Plate thickness (or gage) is continuously monitored
by the automatic gage control (AGC) system
in steel rolling mills.
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Force SensorStrain Gauge Load Cell – a very commonly used
form of force measuring transducer on the use of electrical resistance strain gauges to monitor the
strain produced in some member when stretched,
compressed or bent by the application of the
force.The arrangement is generally referred to as
load cell.
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St i G
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Strain Gauges a)metal wire b)metal foil c)semiconductor
Strain Gauge
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Strain Gauge
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Strain Gauge
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Pressure Sensors
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Fluid Pressure Sensors
• A pressure sensor measures pressure,
typically of gases or liquids. Pressure is an
expression of the force required to stop a fluid
from expanding, and is usually stated in termsof force per unit area.
• A pressure sensor usually acts as a
transducer; it generates a signal as a function of the pressure imposed. For the purposes of
this article, such a signal is electrical.
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• Pressure sensors are used for control and
monitoring in thousands of everydayapplications. Pressure sensors can also be
used to indirectly measure other variables
such as fluid/gas flow, speed, water level, and
altitude. Pressure sensors can alternatively be
called pressure transducers, pressure
transmitters, pressure senders, pressure
indicators and piezometers, manometers,among other names.
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Fluid Pressure
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Diaphragms a) Flat
b) Corrugated
Diaphragms c) Capsule
d) Bellows
Tube Pressure Sensorse) helical tube
d) Bourdon tube, C-type
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Man of the de ices sed to monitor fl id
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Many of the devices used to monitor fluid
pressure in industrial processes involve the
monitoring of the elastic deformation of diaphragms, capsules, bellows and tubes.
For a diaphragm, when there is a difference in
pressure between the two sides then thecenter of the diaphragm becomes displaced.
Corrugations in the diaphragm result in
greater sensitivity. This movement can bemonitored by some form of displacement
sensor like strain gauge, which measures the
strain in circumferential and radial direction.
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Capsules(c) can be considered to be just two
corrugated diaphragms combined and give
even greater sensitivity.
A stack of capsule is just a bellows and even
more sensitive. Bellows can be combined with
LVDT to give a pressure with an electrical
output.
Pressures in the range of about 103 to 10 8
Pascal can be monitor with such sensors.
Materials for these sensors are SS, phosphor
bronze, nickel , rubber and nylon.
A different form of deformation is obtained
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A different form of deformation is obtained
using a tube with an elliptical cross section.
Increasing the pressure in such a tube causes
it to tend to a more circular cross section.
When the tube is formed in the shape of a C
shaped tube, the C opens up to some extent
when the pressure in the tube increases.
A helical form of tube gives a greater
sensitivity.
Materials of these tubes are made of stainless
steel and phosphor bronze and are used for
pressures in the range of about 103 to 10 8 Pa.
O ifi Pl
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Orifice PlateAn orifice plate is a device used for
measuring flow rate. Either a
volumetric or mass flow rate may
be determined, depending on
the calculation associated with
the orifice plate. It uses the sameprinciple as a Venturi nozzle,
namely Bernoulli's principle
which states that there is a
relationship between thepressure of the fluid and the
velocity of the fluid. When the
velocity increases, the pressure
decreases and vice versa.
Venturi Meter
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Venturi Meter
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The End
Thank You