industrial analog sensors

8/12/2019 Industrial Analog Sensors

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MME 486 Fall 2006Lecture Industrial Sensors 3 1 of 53

Industrial Analog Sensors

Industrial Control Systems

Fall 2006


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Analog Sensors Are used to measure continuous variables

Temperature

Liquid level Force

Pressure

Flow

Position etc.


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Analog Sensors

Provide much more information about aprocess than digital sensors do

Transducers:

A transducer converts one physical parameter intoanother

Example: A fuel level sensing unit in anautomobile fuel tank. Essentially a thick filmhybrid circuit variable resistor, it converts a liquidlevel to a variable voltage or current through themovement of the wiper.


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Analog Sensor Properties Range

The upper and lower limits of measurement

Accuracy

The agreement between the actual and measuredvalue.

Resolution The smallest difference the sensor will respond to.

Repeatability Does the sensor give the same reading for thesame measurement value?


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Analog Sensor Types

Temperature

Thermocouples

Resistance temperature device (RTD) Thermistor

Semiconductor (Diodes, TransistorsIntegrated circuit) temperature sensors


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Analog Sensors Types Strain gauges Measure strain to determine force, stress, pressure

Linear Variable Displacement Transformer (LVDT) Linear displacement

Resolvers Rotation angle

Variable Resistors Linear Displacement

Magnetostrictive

Linear Displacement Pressure Sensors Use strain gauges or LVDT


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Thermocouples 2 wires composed of dissimilar metals

are joined at one end

When heated current will flow throughthe circuit a thermoelectric current(Thomas Seebeck, 1821)

MeasurementCircuitryMeasurement

Point

Symbol


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Thermocouples Contd

Current implies a differential voltagewhich can be measured as a function of

temperature (linearly proportional)Voltage is linearly proportional to

temperature V= T


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Consider the following scenario:

Thermocouples Contd

Measurement

Circuitry Measurement

Point

Copper

Terminals

Copper

Constantan

'T' Thermocouple


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Thermocouples Contd

Measurement

Circuitry Measurement

Point

Copper

Terminals

Copper

Constantan

'T' Thermocouple

The addition of the copper terminals introduces another voltage in opposition to the voltage

measured at the measuring point.

This additional voltage is compensated for through the addition of compensating circuitry that

senses the ambient temperature, and will add a voltage in opposition to V2 to cancel it out.

This is called the "cold junction compensation technique".

V1

V2


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Thermocouples Contd

Thermocouples are color-coded forpolarity and for type

The negative terminal is RED

The positive terminal is a different color


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Thermocouples Contd


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Thermocouples Contd

750Temp. (C)

Seebeck

Voltage (mV)

0

40

The Seebeck voltage can be non-linear

with certain thermocouples.

In order to obtain the temperature vs. voltage

relationship, look-up tables, or polynomial fittingare used.

e.g., T = A0 + A1 V +A2 V2 +....+An V

n


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Thermocouples Wiring It is not possible to connect a

thermocouple directly to a PLC analogmodule because Seebeck voltage is too

low (less than 50 millivolts) Electronic devices are used to amplify,

condition, and linearize thethermocouple output.

Most manufacturers offer thermocoupleinput modules for direct connection


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Resolution of Thermocouples The resolution is determined by the

device that takes the output from the

thermocouple When using an analog PLC module with

12-bit resolution and measuring a rangeof 1200, the resolution would be:

1200/4096 = 0.29296875 degrees/bit


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Thermocouple Advantages and Disadvantages

Advantages:

Self Powered (doesnot require a currentor voltage source)

Rugged

Inexpensive

Simple

Disadvantages:

Extremely LowVoltage output (mV)

Not very stable inpresence of EMI

Needs a reference

point


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Thermocouples Improving Operation Use the largest possible wire that will not sink

heat away from the measurement area. Avoid mechanical stress and vibration that

could cause the wires to break. Use shielded wire to shield the wire from

stray EM fields. Avoid steep temperature gradients (thermal

time constant). Use thermocouples well within the

temperature rating.


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Resistance Temperature Device (RTD)

All metals exhibit a positive resistancetemperature coefficient if we can

measure resistance we can measuretemperature

An RTD is a precision resistor whoseresistance changes with temperature

RTDs are made form a pure wire-woundmetal.

Symbol


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Resistance Temperature Device (RTD)

Typical metals include copper, nickel,and Platinum

The most commonly used metal inRTDs is platinum which can withstandhigher temperatures

Platinum has a temperature coefficientof = + 0.00385

100 RTD means 100 @ 0C


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RTD Advantages and Disadvantages

Advantages: Stable

Very accurate

Change in resistanceis linear

Disadvantages: Expensive

Current sourcerequired

Small change inresistance

Self heating Less rugged thanthermocouples.


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Thermistor Same principle as RTD

except it uses asemiconductor insteadof a metal

A thermistor can have anegative or positivetemperature coefficient

Greater change in

resistance withtemperature moresensitive

Symbol

T


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Thermistor Contd

Excellent for temperatures near roomtemp useless for temps above 50C

Not linear resistance decreases

exponentially with temperature increase

Temp. (C)

Resistance

0


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Thermistor Advantages and

Disadvantages Advantages:

Very sensitive (hasthe largest output

change from inputtemperature)

Quick response

More accurate than

RTD andThermocouples

Disadvantages:

Output is a non-linear function

Limited temperaturerange.

Self heating

Fragile


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Semiconductors

Forward voltage of a diode can be usedto measure temperature (NTC ~ 4.0mv/K)

Transistors can be used to measuretemperature in the same manner.

Dedicated IC can also be used


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Strain Gauges

The resistance of a metal wire is givenby:

As the wire is stretched, L increasesand A decreases.

As a result, R increases.

These effects are exploited in straingauges

LR

=


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Strain Gages

When applying force (stress), thematerial undergoes temporarilydeformation (strain)


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Strain Gages Thin wires change

resistance whenstretched

This can be used toobtain a measurementproportional to strain

Strain is related tostress


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Strain Gauges

Typical conditioning circuit

+

V+

VO

R

R

R

R(1+x)


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Linear Variable Displacement Transformer

(LVDT) An LVDT is a

transformer with amoveable core

As the position(displacement) ofthe core varies, theoutput voltagevaries


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LVDT

www.macrosensors.com


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LVDT

Moveable Iron Core

VO

VO

Larger Displacement

Smaller Displacement

Time


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LVDT


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When to use an LVDT High accuracy

Linear operation

Harsh environment

Analog position control

Embedding (in cylinder for example)


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Resolvers A resolver is a transformer with two

fixed and one rotating winding

The output voltage varies with therotation angle of the rotor


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Resolvers In a resolver, a sine wave is applied to one stationary

coil. A cosine wave is applied to the other winding.

Frequency is typically 2 kHz.

The rotor contains the third winding. This winding

produces the output. The rotor winding has one half the turns of the stator

windings.

If the stator windings produce 12 V, the rotor willhave a 6 V peak voltage appearing at its terminals.

Depending on the rotor position, the phase of therotor voltage will vary with the position of the rotor.


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Resolvers

12 V, 0deg

12 V, 90deg

6 V, 225 deg

Rotor Angle = 45 deg

The phase difference between the stator and rotor voltages is usedto measure displacement


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Magnetostrictive Sensor

In some applications, linear displacementmust be measured to a high degree ofaccuracy (10ths of a mm).

In these applications, a linear variable resistoris not desired due to drift and issues withlinearity over temperature.

In these applications, a magetorestrictivesensor should be used.


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Magnetostrictive Sensor Sensor operates based on 2 principles

i) If a current is passed through a conductor in a magnetic field, atorque is produced.

ii) The speed of sound travels through a solid material at a definedspeed.

Control

Electronics

WaveguideHousing Waveguide

Toroidal Magnet


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Magnetostrictive Sensor

An interrogation pulse is applied to the waveguide (1-3 s).

The magnetic field produced by the waveguide interacts with that ofthe magnet.

A short mechanical pulse is produced on the waveguide.

The control electronics measure the torsional motion of the waveguideand converts it to an electric pulse. The time is measured between the

interrogation pulse and the mechanical pulse. The time is converted to a voltage between 0 and 10 V corresponding

to a distance of 0 to the length of the tube.

Applications Include

Clamp control for injection molding machines.

Hydraulic cylinder positioning.


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Flow Sensors

Some of the more widely used onesinclude:

Paddle wheel flow sensors

Turbine flow sensors

These two sensors use a proximity sensor to sense the speedof the impeller blades.

Pitot tubes


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Pressure Sensors Pressure sensors are used to measure

and control fluids

Some operate through:A change in resistance

A change in capacitance

A change in inductance


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Pressure Sensors Pressure can be

measured severalways:

Using strain gauges tomeasure stress or forcerelated to pressure

Using an LVDT to

measure displacementcaused by pressure


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Bellows-style Pressure SensorsAny change in pressure affects the

bellows and moves the core in the

LVDT


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Installation Considerations Electrical

Sensor must be wired properly so that

allowable load current is not exceeded

If high-voltage wiring is run close proximityto sensor cable, the cable should be run

through a metal conduit


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Installation Considerations Mechanical

Mount horizontally so that sensor doesnt

collect chips and debris Be sure sensor doesnt sense its own

mount

Dont damage the sensor when mounting


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Choosing Sensors Size of object

Material of object

Environmental conditions (dust, heat,fluid, etc.)

Required accuracy

Available space Etc.


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Temperature and Flow Application


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Fill Level Sensing


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Seven Inductive Sensors Being used to

Identify Pallet Types


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Pressure and Level Control


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Small Parts Sensing


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Pressure Sensing

industrial analog sensors

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