Lecture Notes Set 5 -Measurement Systems - 2008-09€¦ · · 2014-07-21Measurement Systems Dr...

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Page 4.1MFE3004 Mechatronics IC. Pace

MFE 3004Mechatronics I

Measurement SystemsDr Conrad Pace


Introduction to Measurement Systems• Role of Measurement Systems

– Detection – receive an external stimulus (ex. Displacement)

– Selection – measurement of one property of that stimulus (ex. direction of displacement, filtering our disturbances)

– Signal Management – transform signal that represents the measured property in a form legible by the information processor (ex. amplification, linearisation, digital conversion)

– Communication – communicate signal to the observer/ information processor

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Introduction to Measurement Systems• A sensor is a device capable of detecting a physical parameter

– Receives energy from the measured medium (stimulus)– It produces a signal output which depends on the stimulus– Will always extract energy from the measured medium

• Transducers are devices which convert a physical parameter into another (often being a conversion from a physical parameter into an electrical quantity).– Not all transducers are sensors but all sensors are transducers

• The measurand is the quantity, property, or condition that is measured (i.e. sensed and converted into a usable electrical output), by a transducer

Transducer(Thermistor)

Change intemperature

Change inResistance


Transducer Classification

• Transducer classification – measurandbased or physical effect based.

Classification by Measurand

• Displacement• Velocity• Acceleration• Angle• Angular Velocity• Torque• Pressure• Flow rate• Time• Temperature• Radiation• Magnetic Flux, etc..

Classification by Physical Effect

• Variable Resistance• Variable Capacitance• Variable Inductance• Deformation of elastic materials• Seismic masses• Piezo-electric effect• Optical interference• Electro-magnetic induction• Hall effect• Thermo-resistivity• Thermo-expansion• Thermo-electric (Seebeck) effect• Photo-electric effect, etc..

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Measurand – Sensor Output Relation

• The relation between the measurand and the sensor output is generally clearly identifiable and linear.

SensorOutput

Measurand

O/P = Const x Measurand


Overview of Sensor Technologies• Displacement Contact Type Sensors

• Analogue Type

• Digital Type – Encoders

• Resistance (Potentiometric, Strain Gauge)

• Capacitive• Inductive

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.7MFE3004 Mechatronics IC. Pace

Overview of Sensor Technologies• Displacement – Contact Type Sensors

(k)

(1-k)RVref

RL

Vout

Cantilever beam

Strain α deflection(for measuring smalldistances

Potentiometer Application

Strain Gauge Application


Overview of Sensor Technologies• Other Strain Gauge Sensor Applications

Force Force

Strain gaugeLoad cell

Note: rounded corners toavoid stress concentration

Force Force

Strain gaugeLoad cell

Note: rounded corners toavoid stress concentration

Torque Torque

Strain gauges (mounted at 45°to measure shear strain)

Torque Torque

Strain gauges (mounted at 45°to measure shear strain)

Diaphragm

Cavity

Strain gauge

Diaphragm

Cavity

Strain gauge

Seismic massStrain gauges

Beam

Support

Seismic massStrain gauges

Beam

Support

FORCE

TORQUE

PRESSURE

ACCELERATION

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Capacitive Application

x

g

x

g

x

l

ε1

ε2

Core displacement

Secondary 1 Primary Secondary 2

Inductive Application



LEDLightSensor

Disc

Digital Rotary Encoder

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Overview of Sensor Technologies• Displacement – Non-Contact Type Sensors

• Detect proximity of object to sensor

• Various technologies are used for proximity sensors including Capacitive and Inductive.

Transducer(primary sensing

element)

SignalConditioning

stage

Triggeringstage

AmplificationStage


Overview of Sensor Technologies• Force Sensors

• Measuring small displacements caused by the force.• Strain Gauge Load Cells• Piezo-Electric Load Cells

LOAD CELLS using Strain Gauges Piezo-Electric Application

+ + + + + + +

- - - - - - - - - -

Force

Surfacesbecomecharged

Piezo-Electric effect

Piezo-ElectricMaterial

+ + + + + + +

- - - - - - - - - -

Force

Surfacesbecomecharged

Piezo-Electric effect

Piezo-ElectricMaterial

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Overview of Sensor Technologies• Temperature Sensors

– Use of expansion/ contraction of solids or liquids– Measurement of gas pressures– Change in electrical resistance– Thermoelectric e.m.f.– Radiation measurement


Overview of Sensor Technologies• Temperature Sensors

Tem peratureR ange

M easuringInstrum ent

M ethod o f M easurem ent

0.65 K to 5 K G as Therm om eter M easurem ent of vapour pressures ofH e (he lium ) using specified equations.

3 K to 24.6 K G as Therm om eter M easurem ent from a constant vo lum egas therm om eter

14 K to 303 K P la tinum resistancetherm om eter (PRT)

Specified reference function togetherw ith a devia tion equation whosecoeffic ien ts are determ ined in theca libration against the fixed po in ts.

0 .01°C to962°C

Platinum resistancetherm om eter (PRT)

Specified reference function and adevia tion equation

962°C andabove

R adia tion pyrom eter D efined by P lanck’s law taking therad ia tion em itted from the body.

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Basic Measurement System Components

Primary sensing Element

Power supply

Measurand

Measured Medium

Signal Conditioning

Element

Signal Processing

Element

Data Transmission

Element

Data Presentation

Element Observer

Data storage/ playback Element

Presented Data


Basic Measurement System Components

• Example of a Measurement System

Pillar Load Cell

Strain Gauge

Bridge Circuit

Amplifier Circuit

Visual Display

Unit

Microprocessor (linearisation and

error compensation)

A/D Converter

Signal Processing

mV

Resistance

V

Strain Input

True Weight (Measurand) Primary Secondary

Sensing Signal Conditioning

Output

Measured Weight

Data Presentation

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Application Areas of Measurement Systems

• Monitoring of Processes and Operations • Control of Processes and Operations

– Most common application in mechatronic products and processes.

• Experimental Engineering Analysis– In solving engineering problems, two general methods are

available: theoretical and experimental. – Measurement systems are a fundamental component of

experimental work.


Sensory Characteristics

• Measurand Characteristics• Electrical Characteristics• Mechanical Characteristics• Performance Characteristics

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Sensor Measurand Characteristics• Sensor Type is often defined by the Measurand

– Sensors can be used to measure other parameters indirectly due to a known relation between the parameter of interest and the measurand.

• The Range of the sensor is given by the upper and lower limits of measurand values to which the sensor will respond to within specified performance tolerances

• The Span is the algebraic difference between the two limits of the range.

• Example : Force Sensor Range 10 to 50kNSpan 40kN


Sensor Electrical Characteristics

• Sensor Output Types– Analogue – Resistance Change, Capacitance Change,

Inductance Change, Voltage Change, Charge build-up, frequency output

– Digital – discrete function of the measurand

Transducer

Zin

Zout

Power Supply

Output

ZL Load

Measurand

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Sensor Mechanical Characteristics• Mechanical Characteristics define the Physical

interface of the sensor– Mode of mounting– Sensor Orientation– Environmental Conditions to which the sensor is

exposed (ex. vibration and mechanical stress)


Sensor Performance Characteristics

• Classification of Performance Characteristics– Static– Dynamic– Environmental

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Sensor Performance Characteristics• Static (steady state) Performance Characteristics

– Steady-state relation

0 10 20 30 40 50 60 70 80 90 100

0 100 200 300 400 500 600 700 800 900 1000100

90

80

70

60

50

40

30

20

10

0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Out

put (

% F

SO

)

Out

put (

Vol

ts d

c)

Pressure (kPa)

Measurand (% Range)

Sensor output given as a % of the Full Scale Output (%FSO)



– Steady-state relation

0 10 20 30 40 50 60 70 80 90 100

0 100 200 300 400 500 600 700 800 900 1000100

90

80

70

60

50

40

30

20

10

0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Out

put (

% F

SO

)

Out

put (

Vol

ts d

c)

Pressure (kPa)

Measurand (% Range)

Relationship is derived from either

(a) By calculation from a known theoretical response

(b) By calibration

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0 10 20 30 40 50 60 70 80 90 100

0 100 200 300 400 500 600 700 800 900 1000100

90

80

70

60

50

40

30

20

10

0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Out

put (

% F

SO

)

Out

put (

Vol

ts d

c)

Pressure (kPa)

Measurand (% Range)

• Static (steady state) Performance Characteristics– Steady-state relation

Sensitivity = Change in Output

Change in Measurand 0.5V

100kPa

Sensitivity = 5mV/kPa



– Resolution• The smallest change in measurand value that can be

detected. • For an Ideal ANALOGUE Sensor – an infinitesimally

small change in measurand will result in an equivalent change in output

• In practice the smallest detectable change is limited due to various reasons (electrical noise, friction, inertia, etc..)

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– Resolution• Analogue Sensors – Limited by noise• Digital Sensors – Limited by the value of the least significant bit of

the digital output signal

• Resolution is often defined as a percentage of the full-scale output (% FSO)

– Example - Angular velocity sensor • minimum change in speed detected 2 rad/s• Maximum measurable input 200 rad/s• Resolution = 1.0 %FSO



– Accuracy and Errors (uncertainty)• Errors distort the expected relationship between the measurand

and sensor output

Error = Measured Value – True Measurand Value

• The Accuracy of a sensor is the ability to give an indication equivalent to the true value of the measurand (it is a reflection of the maximum error to be expected from the sensory device).

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– Accuracy • Dependent on the Errors to which the sensor is subjected• Reflects the precision of calibration of the sensor• Is stipulated as a %FSO

– Example : Temperature Sensor• Range 0 to 200°C• Sensor Max. Error of ± 10°C• Sensor Accuracy = ± 5% FSO



– Error Types – REPEATABILITY• The ability of the sensor to give the same output for

repeated applications of the same input measurandvalue

Repeatability = Maximum – Minimum Values Given x 100%

FSO

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– Error Types – HYSTERESIS• The maximum difference in sensor output for a specific measurand

value when the value is approached, first with increasing and then with decreasing measurand.

0 10 20 30 40 50 60 70 80 90 100

100

90

80

70

60

50

40

30

20

10

0

Out

put (

% F

SO

)

Measurand (% Range)

Decreasing Measurand

Increasing Measurand

Hysteresis Error



– Error Types – NON-LINEARITY• Deviation from an idealised linear relationship• Reasons for non-linearity

– Techniques/ Phenomena used for sensing the measurand

– Non-linear characteristics of certain parameters arising from manufacturing variations (ex. Diaphragm properties)

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– Error Types – NON-LINEARITY• Interpretations of Non-Linearity Errors

– Terminal non-Linearity– Independent non-Linearity

Maximum deviation gives the non linearity error

Ideal

Measurand

Measured Output

Actual

Maximum deviation gives the non linearity error

Line of best fit

Measurand

Measured Output

Actual

(a) Terminal non-linearity (b) Independent non-linearity



– Error Types – Offset and Gain Errors• Offset Error (Zero Shift)

– Shift of the straight line relationship from the origin• Gain Error (Sensitivity Shift)

– Change in the straight line slope

– Reasons for zero shift/ sensitivity shift• Environmental conditions (temperature fluctuations)• Wear in sensor components, etc..

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– Error Types – Offset and Gain Errors

(a) Offset ErrorMeasurand

(b) Gain Error

Offset error

ActualIdeal

Measurand

Measured Output

Gain error

Actual

Measured Output

Ideal



– Error Types – Stability• The ability to give the same output when

subjected to a constant measurand input over a period of time.

• Commonly referred to as the sensor drift or creep.

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– Error Types – Dead Band

• The measurand range for which there is no output

Dead-band

Measured Output

Measurand

Actual



• Dynamic Performance Characteristics– Define the transient behaviour of the sensor

– The importance of dynamic characteristics depends on the rapidity of the sensor response required (compared to the system under control)

– Typical dynamic responses of interest• Step Response (step change in measurand)• Frequency Response (sinusoidal frequency change in

measurand)

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• Dynamic Performance Characteristics– Frequency Response

• Amplitude Ratio = Sensor Output Magnitude/ Measurand Magnitude

• Phase Shift = Phase lag between Sensor output and Measurand



• Dynamic Performance Characteristics– Frequency Response

Frequency range AFrequency range B

Response Curve A

Response Curve B

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• Dynamic Performance Characteristics– Step Response

Time

Dead Time

0

100

90

80

70

60

50

40

30

20

10

0

Per

cent

of o

utpu

t cha

nge

Application of Measurand Change

Rise Time

63.2%

95%

5%

Time Constant

Response Time

Transient Response Steady-State Response



• Dynamic Performance Characteristics– Step Response

0

160

140

120

100

80

60

40

20

0

Per

cent

of o

utpu

t cha

nge

Application of Measurand Change

Maximum Overshoot

Steady state output value

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• Environment Characteristics– Describe the environmental effects on the sensor

performance– Example

• Temperature Offset and Gain Errors (amongst the most common environmental effects on sensors)

– %FSO per °C change in temperature

• Temperature Offset Error is often given as a Temperature Sensitivity – change in output which is solely due to changes in temperature



• Environment Characteristics– Reducing Temperature Effects

• Controlling the sensor’s ambient temperature

• Compensating for temperature effects– Using dummy sensors (a dummy sensor is subjected only to

temperature effects and not the measurand change)

– Active temperature measurement and compensation

• Using low temperature coefficient materials/ low power dissipation components

• Repeated calibration in smart sensors

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• Environment Characteristics– Other environmental factors that can influence the

sensor output• Humidity• Pressure• Mechanical Stress/ Strain• Vibrations• Electro-magnetic interference• Electrostatics


Designing Measurement Systems• Measurement Systems form a principal component

within the design concept of a Mechatronic System

Establish Information

Requirements

Design Measurement

Systems

What information is required to be gathered and

managed by the system

Identify Appropriate Sensory Technology and

Signal Processing, Manipulation and communication requirements

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Designing Measurement Systems• Consideration of the following factors

– The information required and the identification of the system physical parameters that must be measured in order to provide this information.

– The nature, quality and performance of the measurement in terms of parameters such as linearity, accuracy and resolution.

– A determination of the most inaccurate measurementthat would be acceptable (required accuracy)

– The effect on the system performance of any drift in the measurement circuit (zero or sensitivity)


Designing Measurement Systems• Consideration of the following factors (continued)

– The environmental conditions under which the sensors are expected to operate

– The cost targets to be met

– The nature and form of the information transfer required

– The reliability of the system

– The form of the interface to adjacent modules in the system.

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Designing Measurement Systems

• When designing Measurement Systems care should be taken not to provide too much or too little information

– Too Much Information• Added cost• Added Processing burden

– Too Little Information• Inadequate Accuracy• Insufficient Update Rate• Lack of Desired Performance

Lecture Notes Set 5 -Measurement Systems - 2008-09€¦ · · 2014-07-21Measurement Systems Dr...

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Transcript of Lecture Notes Set 5 -Measurement Systems - 2008-09€¦ · · 2014-07-21Measurement Systems Dr...