PE-4030 Ch 2 Sensors and Transducers Part 1 Oct 1 2013

7/27/2019 PE-4030 Ch 2 Sensors and Transducers Part 1 Oct 1 2013

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Sensors

and

Transducers

Prof. Charlton S. InaoProfessor Mechatronics System Design

Defence Engineering CollegeBishoftu, Ethiopia

PE-4030Chapter 2/a



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.



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.



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.



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.

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Energy



http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Electricity

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Mechanics



http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Acoustics

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Electromagnetism

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Heat


http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Light

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Chemistry





http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Chemistry

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Light

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Electromagnetism






Transducers



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.



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.



Signal Conditioning Requirements of

Common Transducers



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



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.



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.



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.

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Sensitivity_(electronics)

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Sensitivity_(electronics)



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.



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

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Hysteresis






Nonlinearity and hysteresis



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.



Non Linearity Error



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.



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.



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.



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.

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Digital_display

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Accuracy_and_precision

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Scanning_tunneling_microscope

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Atom

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Molecule

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Molecule

http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Atom






http://localhost/var/www/apps/conversion/tmp/scratch_1/wiki/Accuracy_and_precision






Example: Strain Gauge Transducer

•Thermal Sensitivity :0.03 % full range output /deg Centigrade



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




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.




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.



Example Velocity Sensor Specs



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%)



Displacement/Position Sensors



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.



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.



Application of Displacement

Sensor



Application: Location and position of

object on a conveyor



1.1 Displacement Sensors

• Displacement Sensors

– Potentiometer

– Strain Gauge element

– Capacitive Element

– Differential Transformer

– Optical Encoders

• Absolute

• Incremental



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.



Potentiometer



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.



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



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.



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.



LVDT



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.



Absolute

encoderIncremental

encoder



Optical Encoder



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 .



Proximity Sensors



Photoelectric Sensors



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.



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.



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



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



• 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.



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.



St i G



Strain Gauges a)metal wire b)metal foil c)semiconductor

Strain Gauge



Strain Gauge



Pressure Sensors



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.

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Gas

http://en.wikipedia.org/wiki/Liquids

http://en.wikipedia.org/wiki/Transducer

http://en.wikipedia.org/wiki/Function_(mathematics)

http://en.wikipedia.org/wiki/Function_(mathematics)

http://en.wikipedia.org/wiki/Transducer

http://en.wikipedia.org/wiki/Liquids

http://en.wikipedia.org/wiki/Gas

http://en.wikipedia.org/wiki/Pressure



• 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.



Fluid Pressure



Diaphragms a) Flat

b) Corrugated

Diaphragms c) Capsule

d) Bellows

Tube Pressure Sensorse) helical tube

d) Bourdon tube, C-type



Man of the de ices sed to monitor fl id



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.



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



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



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

http://en.wikipedia.org/wiki/Venturi_effect

http://en.wikipedia.org/wiki/Bernoulli's_principle




http://en.wikipedia.org/wiki/Venturi_effect



Venturi Meter



The End

Thank You

PE-4030 Ch 2 Sensors and Transducers Part 1 Oct 1 2013

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Transcript of PE-4030 Ch 2 Sensors and Transducers Part 1 Oct 1 2013