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TRANSDUCERS

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Why we’re here today?

• To learn a technical subject ( the content) in English (the language)

• Because there is no other way to learn a language than listening and talking

• Because 90% of technical literature is in English• Because we’d like you to be competitive with

European students of your age

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Some friendly reminders

• From now till the end of this presentation we will talk in English only!

• I will speak in plain English and slowly but I won’t speak in Italian/English and neither in English/Italian!

• Questions are most welcome … if they are simple!

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What is a transducer ?(a fairly good definition)

A transducer is a device which transforms a non-electrical

physical quantity (i.e. temperature, sound or light) into an

electrical signal (i.e. voltage, current, capacity…)

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Where are they used and what for?• Antenna: is the most basic transducer and can be made from a simple

piece of wire. It converts electromagnetic energy into electricity when it receives signals and does the opposite when it transmits

• Strain gauges: have a long thin wire attached to a foil backing which is glued to an object. When the object changes shape, the strain gauge also changes shape and its resistance changes. The amount of stress or strain in the object is calculated from this change in resistance

• Accelerometer: which converts the change in position of mass into an electrical signal. Accelerometers measure the force of acceleration and deceleration. They are used in car airbags, stability control, hard drives, and many electronic gadgets.

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…more• Geiger counter: detects radiation levels by using a transducer called a

Geiger-Muller tube

• Microphone and Speaker. Microphones convert sound pressure waves into electrical current, while speaker convert electrical current into sound pressure waves.

• And many many others

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electrical signal

non-electrical physical quantity

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What is its structureA transducer is made of three blocks:o Input I/Fo Sensoro Output I/F

InputI/F sensor Output

I/F

Transducer

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Where does it fit in the DAQ

Transducer Conditioning +filtering A/DSampling P

Control logic

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Important parameters of a transducer

• Static response: how does it respond to slowly variable signals, is it precise and accurate

• Dynamic response: how does it respond to quickly variable signals (bandwidth of control system, tr, ts !!!)

• Environmental factors: how these factors are affecting transducer performance

• Reliability: MTBF

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Precision Accuracy

Precise

not accurate

Not precise

accuratePrecise

+ accurate

Not precise

not accurate

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TRANSDUCERS

• Temperature transducers▫ Thermocouples▫ Resistance - Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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TRANSDUCERS

• Temperature transducers▫ Thermocouples ▫ Resistance - Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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What is a Thermocouple It’s a temperature sensitive device which works thanks to the Seebeck effect:“a voltage is generated in a circuit containing two different metals by keeping the junctions between them at different temperatures”

Estonian physician Thomas Seebeck (1770–1831)

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Pros and Cons in temperature measuring using Thermocouples• Pros▫ They are inexpensive. ▫ They are rugged and reliable. ▫ They can be used over a wide temperature range.

• Cons▫ low output voltage▫ low sensitivity▫ non-linearity▫ electrical connections.

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How does a thermocouple look like ?

Here it is!

please note the two wires (of two different metals) joined in the

junction.

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How does a thermocouple work ?

High impedance voltmeter !

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In normal operation, cold junction is placed in an ice bath

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What types of thermocouples can we have ?

temp. range (°C)

• Type K : Chromel - Alumel -270 / 1370• Type J : Iron-Constantan -210 / 1050• Type E : Chromel -Constantan -270 / 790• Type N : Nicros -Nisil -260 / 1300• Type T : Copper-Constantan -270 / 400

It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature

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More features:• Type K ‘General Purpose' and low cost thermocouple, very popular

• Type J Limited range (-40 to +750°C), less popular than type K.

• Type E High output (68 mV/°C) well suited to low temperature (cryogenic) use

• Type N High stability and resistance to high temperature oxidation, designed as an 'improved' type K, it’s becoming more popular.

• Type T They are used for moist or sub-zero temperature monitoring applications because of superior corrosion resistance

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Alloys used:

• Constantan: 55% Copper and 45% Nickel• Cromel: 90% Ni + 10% Cr• Alumel: 95% Ni + 2% Mn + 2% Al + 1% Silicon• Nicrosil: 14.4%Cr +1.4 Silicon + 0.1% Mn + Ni• Nisil: same as Nicrosil but different %

How much are thermocouples ?• Type K Thermocouple (Exposed wire, fiberglass insulated) Tip

Diameter: 1.5 mm Tip Temperature: -60 to +350 °C Price $9.90

• Type K Thermocouple (Insertion Probe) Tip Diameter: 3.3 mm Tip Temperature: -50 to +250 °C Price $39.60

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Magnitude of thermal EMFThe temperature is usually expressed as a polynomial function ofthe measured voltage. Sometimes it is possible to get a decent linear approximation over a limited temperature range.

wherec and k = constants of the thermocouple materialsT1 = the temperature of the ‘hot’ junctionT2 = the temperature of the ‘cold’ or ‘reference’ junction

)()( 22

2121 TTkTTcE

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Measurement circuit: issuesWe would like DVM to read only V1, but the voltmeter created two more metallic junctions: J2 and J3 voltmeter reading V will be proportional to the temperature difference between J1 and J2 we cannot find the temperature at J1 unless we first find the temperature of J2.

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The circuit will provide accurate readings, but it is desirable to eliminate the ice bath

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30One way to do this is to replace the ice bath with another isothermal block

directly measures the temperature of the isothermal block (the reference junction) and use that informationto compute the unknown temperature

Cu Fe C CCu

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Last but not least: reference junction compensation circuit

The AD580 is a three-terminal, low cost, temperature compensated,voltage reference which provides a fixed 2.5 V output for inputs between 4.5 V and 30 V.

AD590 is an integrated-circuit temperature transducer which produces an output current proportional to absolute temperature.The device acts as a high impedance constant current regulator, passing 1µA/oK for supply voltages between +4V and +30V

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Thermocouple - applications

• Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K.

• They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, Thermistors and RTD’s are more suitable.

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TRANSDUCERS

• Temperature transducers▫ Thermocouples ▫ Resistance - Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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Resistance temperature detectors (RTDs)

RTDs are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature.

Temperature Metal Resistance

The resistance ideally changes linearly with temp.

Principle of operationRTDs are manufactured from metals whose

resistance increases with temperature.Within a limited temperature range, this

resistivity increases linearly with temperature:

where:Rt = resistance at temperature tR0= resistance at a standard temperature t0

= temperature coefficient of resistance (°C-1)

PRTs ( platinum RTDs )

• They are the most popular RTD type• Nearly linear over a wide range of temperatures• Small enough to have response times of a

fraction of a second• They are among the most precise temperature

sensors available with resolution and measurement uncertainties of ±0.1 °C or better

Why Platinum

• Chemical stability

• Availability in a pure form

• Highly reproducible electrical properties

• Platinum probes will read 100Ω at 0°C and at 100°C the

DIN grade (i.e. pure platinum intentionally contaminated with other platinum group

metals) platinum RTD will read 138.5Ω

• Only platinum RTDs have an international standard ( =

0.00385 Ω/Ω/°C )

How does an RTD look like ?

• Usually they are provided encapsulated in probes

• They have an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function (i.e. temperature controller, precision thermostat... )

…more

• price $ 65.00 • Temperature Range: -200 to 260°C • High-Accuracy• Platinum Elements •3-Wire Construction Standard, 2 and 4-Wire Constructions Available

• Wall mount transducer• Temperature, Temperature/Humidity and Barometric Pressure Transmitter Models• Low-Cost Miniature DesignStylish Design Blends in well with Your Office, Computer Room or Laboratory Décor• $46.00 Wall mount RTD sensor

Pros and Cons in temperature measuring using RTDsPros• stable output for long period of time• ease of recalibration• accurate readings over relatively narrow

temperature ranges

…more

Cons (compared to thermocouples)• Smaller overall temperature range• Higher initial cost • Less rugged in high vibration environments• Active devices requiring an electrical current to

produce a voltage drop across the sensor that can be then measured by a calibrated read-out device

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Platinum

Copper

TungstenNickel

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Construction of a PRTs

• The coiled element sensor, made by inserting the helical sensing wires into a packed powder-filled insulating mandrel, provides a strain-free sensing element.

• All work is done manually under a microscope.• Strain-free elements required for industrial measurements below –200°C.•They also insure superior interchangeability and stability to the highest temp.

…more

•The thin film sensing element is made by depositing a thin layer of platinum in a resistance pattern on a ceramic substrate.•A glassy layer is applied for seal and protection.

Specifications

• Wiring configuration (2, 3, or 4-wire)• Self-heating• Stability• Repeatability• Response time

A. Wiring ConfigurationSerious lead-wire resistance errors for2/wire RTD especially in a 100Ω sensor.

• If lead wire resistance remains constant, it can be offset and not affect the temperature measurement.

• If ambient temperature conditions change, the wire resistance will also change, introducing errors. If the wire is very long, this source of error could be significant.

• Two-wire RTDs are typically used only with very short lead wires, or with a 1000Ω element.

B. Wiring ConfigurationIn a 3-wire RTD, there are 3 leads coming fromthe RTD instead of 2. L1 and L3 carry themeasuring current, while L2 acts only as apotential lead. Ideally, the resistances of L1 and L3are equal and therefore canceled. The resistance inR3 is equal to the resistance of the sensor Rt at agiven temperature—usually the beginning of thetemperature range. At this point, V out = zero. Asthe temperature of the sensor increases, theresistance of the sensor increases, causing theresistance to be out of balance and indicated at Vout. Resistances L1 and L3 in leads up to tens offeet long usually match well enough for 100 ohmthree-wire RTDs.

(L1+L2+Rt)xR2=R1x(L2+L3+R3)

But: L1=L3 therefore:

(L3+L2+Rt)xR2=R1x(L3+L2+R3)

Self-Heating• To measure resistance, it is necessary to pass a current

through the RTD• The resultant voltage drop across the resistor heats the

device ( Joule heating )• The sensor's indicated temperature is therefore slightly

higher than the actual temperature• The amount of self-heating also depends heavily on the

medium in which the RTD is immersed. An RTD can self-heat up to 100x higher in still air than in moving water, so self-heating specifications are just a conservative guide.

Stability• Sensor’s ability to maintain a consistent output when a

constant input is applied.• Unintended physical or thermal shocks can cause small,

one-time shifts. The material that the platinum is adhered to, when wound on a mandrel or deposited on a substrate, can expand and contract differentially to cause strain incorporated in normal performance but not cause shifts.

• The stability of platinum RTDs is exceptional, with most experiencing drift rates < 0.05°C over five years.

Repeatability

• Sensor’s ability to give the same output or reading under repeated identical conditions

• Absolute accuracy is not necessary in most applications. The focus should be on the stability and repeatability of the sensor (i.e. if an RTD in a 100.00°C bath consistently reads 100.06°C, the electronics can easily compensate for this error)

Response Time• Sensor's ability to react to a change in temperature, and

depends on the sensor's thermal mass and heat transfer from the material being tested

• Surface RTDs respond quickly to surface temperature change

• RTD specifications will list the sensor's time constant, which is the time it takes for an RTD to respond to a step change in temperature and come to 63% of its final equilibrium value.

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TRANSDUCERS

• Temperature transducers▫ Thermocouples ▫ Resistance - Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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Milestones of ThermistorsTHERMal resISTORS

• 1833: negative temperature coefficient of silver sulphide was first observed by M. Faraday

• Before 2003 only ceramic materials (a mix of different metal oxides) were used for production of Thermistors

• After 2003 AdSem started manufacturing of Si and Ge high temperature NTC Thermistors with better performance than any ceramic NTC Thermistors

Types• NTC - Negative Temperature Coefficient, used

mostly in temperature sensing

Temperature semiconductor resistance

• PTC - Positive Temperature Coefficient, used

mostly in electric current control

• Symbol

Key features• the change in electrical resistance when subjected to a

corresponding change in body temperature is▫ Predictable▫ Precise ▫ Stable

• extremely high temperature coefficient of resistance• typical temperature range of -100° to over +600° F• Thermistors are generally accepted to be the most advantageous

sensor for many applications including temperature measurement and control.

THERMISTOR APPLICATIONS

And…

And…

And…

And

How they look like

How they look like

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Thermistor

Semiconductor Temp. Sensors• The ordinary semiconductor diode may be used as a

temperature sensor • The forward biased voltage across a diode has a

temperature coefficient of about 2.3mV/°C and is reasonably linear

• The bias current should be held as constant as possible using constant current source, or a resistor from a stable voltage source

…more• If two identical transistors are operated at different but

constant collector current densities, then the difference in their base-emitter voltages is proportional to the absolute temperature of the transistors

• The transistor sensor is used in diode mode by connecting the base and collector together.

• VBE changes by approx. -2.2 mV/C°• The result is a sensor

very low power sensitive linear but electrically and mechanically delicate

IC temperature sensors• The AD590 and the LM35 have traditionally

been the most popular devices, but over the last few years better alternatives have become available.

• They provide a current or voltage output signal with relatively low output impedance

• Require an excitation power source and are essentially linear

AD590:Product Description•The AD590 is a two-terminal integrated circuit temperature transducer• produces an output current proportional to absolute temperature(1µA/K which means 298.2 µA 298.2K (25°C)

•The AD590 should be used in any temperature sensing application below 150°C• low cost • Linearization circuitry, precision voltage amplifiers, resistance measuring circuitry and cold junction compensation are not needed in applying the AD590•The AD590 is particularly useful in remote sensing applications. The device is insensitive to voltage drops over long lines due to its high impedance current output.

LM 35:Product Description• Calibrated directly in ° Celsius +10.0 mV/°C

• Rated for full -55° to +150°C range

• Suitable for remote applications

• Low cost due to wafer-level trimming

• Operates from 4 to 30 volts

• Low self-heating, 0.08°C in still air

• Nonlinearity only ±¼°C typical

• Low impedance output, 0.1 Ohm for 1 mA load

Price: Qty 1K+ $0.73

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TRANSDUCER

• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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Resistive position transducersDistance

Electrical signal

Principle of operation of an RPT• The principle of the resistive position transducer is that

the measured quantity (i.e. the position of an object, or the distance it has moved) causes a resistance change in the sensing element.

• One type of displacement transducer uses a resistance element with a sliding contact linked to the object being monitored.

• Thus the resistance between the slider and one end of the resistance element depends on the position of the object.

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Resistive position transducers

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2

RRR

EEout

…more • The output voltage depends on the wiper

position and therefore is a function of the shaft position.

• The output voltage Eout is a fraction of E, depending on the position of the wiper.

• The element is considered perfectly linear if the resistance of the transducer is distributed uniformly along the length of travel of wiper.

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TRANSDUCER

• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

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Displacement transducers

• Capacitive transducer

• Inductive transducer

• Variable inductance transducer

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Capacitive transducers• The capacitance of a parallel-plate capacitor is

given by

• ε = dielectric constantεo = 8.854 x 1o-12, in farad per meterA = the area of the plate, in square meterd = the plate spacing in meters

• Since C is inversely proportional to d, any change in d will cause a change in C.

dAC o

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Capacitive transducers – physical design

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Inductive transducers (IT)

• Inductive Transducers may be either

▫ Self - generating type transducers

▫ Passive type transducers.

Self-Generating IT• Self-Generating IT utilises the basic electrical

generator principle that: when there is relative motion between conductor and magnetic field, a voltage is induced in the conductor.

• An example of this is Tachometer that directly converts speeds or velocity into an electrical signal.

Tachometers• Examples of a Common Tachometer

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IT– tachometer with a PM stator

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IT– tachometer with a PM rotor

Passive type IT (LVDT)• Passive inductive transducers require an

external source of power.• The Differential transformer is a passive

inductive transformer, well known as Linear Variable Differential Transformer (LVDT).

• It consists basically of a primary winding and two secondary windings, wound over a hollow tube and positioned so that the primary is between two of its secondaries.

Linear Variable Differential Transformer (LVDT)• Some examples of LVDTs.

Linear Variable Differential Transformer • An example of LVDT electrical wiring.

Linear Variable Differential Transformer

• An iron core slides within the tube and therefore affects the magnetic coupling between the primary and two secondaries.

• When the core is in the centre , the voltage induced in the two secondaries is equal.

• When the core is moved in one direction of centre, the voltage induced in one winding is increased and that in the other is decreased. Movement in the opposite direction reverses this effects.

Linear Variable Differential Transformer• In next figure, the winding

is connected ‘series opposing’-that is the polarities of V1 and V2 oppose each other

• Consequently, when the core is in the center so that V1=V2, there is no voltage output, Vo = 0V.

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Linear Variable Differential Transformer

• When the core is moved in one direction from the center, the voltage induced in one winding is increased and that in the others is decreased.

• Movement in the opposite direction reverse the effect.

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Variable Inductance Transducers –operation

Core at the centerV1 = V2

Vo = 0

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Variable Inductance Transducers –operation

Core moves towards S1V1 > V2

Vo increase

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Variable Inductance Transducers –operation

Core moves towards S2V2 > V1

Vo decrease

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Variable Inductance Transducers –operationThus, the amplitude of Vo is

a function of distance the core has moved. If the core is attached to a moving object, the LVDT output voltage can be a measure of the position of the object. The farther the core moves from the centre, the greater the difference in value between V1 and V2, and consequently the greater the value of Vo.

Advantages of LVDT• It produces a high output voltages for small

changes in core position.• Low cost • Solid and robust - capable of working in a wide

variety of environments. • No permanent damage to the LVDT if

measurements exceed the designed range.

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TRANSDUCER

• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors

• Resistive position transducers• Displacement transducers• Strain gauge

Strain gauge• The Strain Gauge is an example of a passive

transducer that uses electrical resistance variation in wires to sense the strain produced by a force on the wire.

• It is a very versatile detector and transducer for measuring weight, pressure, mechanical force or displacement.

Strain gauge: how they look likeThe construction of a bonded strain gauge shows a fine wire looped back and forth on a mounting plate, which is usually cemented to the element that undergoing stress.

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Strain gauge

From the equation of resistance,

R = resistanceρ = specific resistance of the conductor materialL = the length of the conductor in metersA = the area of the conductor in square metersWhen a strain produced by a force is applied on

the wires, L increase and A decrease.

ALR

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Strain gauge

L increase A decrease

From the equation of resistance,

R increaseALR

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Strain gauge – the gauge factor

LLRRK

//

K = the gauge factorR = the initial resistance in ohms (without strain)ΔR = the change of initial resistance in ohmsL = the initial length in meters (without strain)ΔL = the change of initial length in meters

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Strain gauge – the gauge factor

LLRRK

//

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Strain gauge – the gauge factor

GRRK /

For many common materials, there is a constant ratio between stress and strain. Stress is defined as the internal force per unit area.

S = the stress in kilograms per square meterF = the force in kilogramsA = the area in square meters

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Stress equation

AFS

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Young’s modulus

The constant of proportionality between stress and strain for the curve is known as the modulus of elasticity of the materials

E = Young’s modulus in kilograms per square meter

S = the stress in kilograms per square meterG = the strain (no unit)

GSE

Measuring Circuits

Measuring Circuits

Application & Installation• The output of a strain gage circuit is a very low-level voltage signal

requiring a sensitivity of 100 microvolts or better.• The low level of the signal makes it particularly susceptible to

unwanted noise from other electrical devices.• Capacitive coupling caused by the lead wires' running too close to

AC power cables or ground currents are potential error sources in strain measurement.

• Other error sources may include magnetically induced voltages when the lead wires pass through variable magnetic fields, parasitic (unwanted) contact resistances of lead wires, insulation failure, and thermocouple effects at the junction of dissimilar metals. The sum of such interferences can result in significant signal degradation.

Evaluation test on Transducers Name………………………………………………………….. 5a CE Thursday, 5th March 2009

1. where does a transducer fit in the DAQ (i.e. Data AcQuisition)

2. two important parameters of a transducer are Static and Dynamic response. Can you

explain the difference between these two?

3. explain the principle of operation of a thermocouple

4. are thermocouples suitable for heavy industrial applications? Why?

5. why should you use an ice bath when measuring temperature with a thermocouple?

6. explain the principle of operation of RTD’s

7. are RTD’s and specifically PRT’s precise temperature sensors?

8. how can you minimize lead - wire resistance errors for RTD

9. an ordinary semiconductor diode may be used as a temperature sensor, can you explain

how?

10. LM35 is by far the most popular temperature sensor among students. Can you remember

why?

11. explain the principle of operation of a resistive position transducer

12. LVDT’s (Linear Variable Differential Transformer) are passive type inductive displacement

transducers. Can you explain how they work?

13. what can you measure with a strain gauge?

14. what is the equation that explains the operation of a strain gauge

15. the gauge factor is indicated as K in the following formula. Can you tell the meaning of the

terms which appear in the formula?

LLRRK

//

ΔΔ

= Please note that:

• all answers must be in English, answers in Italian will not be evaluated • you are free to use drawings to better explain your answers • all questions have the same weight, so in order to have a sufficient score you have to

provide correct answers to 9 questions • you are not allowed to use any book or material with related information, you are allowed to

use English dictionary