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  • 8/9/2019 4.Sensors Mecatrónica

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    1540 Introduction To Mechatronics: Slide 1Stefan B. Williams

    Introduction to Mechatronics

    Mech-1540

    Sensors 1

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    1540 Introduction To Mechatronics: Slide 2Stefan B. Williams

    Schedule of Events

    Q 2 of Assign 2MID SEMESTER BREAK 

    Spare19/614

    Due 13/6Major Assignment12/613

    Case Study : Unmanned Air/Land/Sea Vehicles5/612

    Case Study : Formula SAE29/511

    Active Sensor Systems (Graham Brooker)22/510

    Due 16/5Assignment 2 - Control and Modelling15/59

    Q 4 of Assign 2System Modelling and Control8/58

    Q 3 of Assign 2Computer – Software and Design Tools1/57

    6

    5

    4

    3

    2

    1

    Week 

    17/4

    10/4

    3/4

    27/3

    20/3

    13/3

    Date

    Computer – Hardware

    Assignment 1 – Design Exercise

    Sensors

    Actuators

    Design Process

    Introduction

    Content

    Q 1 of Assign 2

    Due 11/4

    Q 3 of Assign 1

    Q 2 of Assign 1

    Q 1 of Assign. 1

    Assignment Notes

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    1540 Introduction To Mechatronics: Slide 3Stefan B. Williams

    Industrial Sensors

    • Proximity

     – Mechanical

     – Optical

     – Inductive/Capacitive

    • Position/Velocity – Potentiometer 

     – LVDT

     – Encoders

     – Tachogenerator • Force/Pressure

    • Vibration/acceleration

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    1540 Introduction To Mechatronics: Slide 4Stefan B. Williams

    Definitions

    • Accuracy: The agreement between the actual value and

    the measured value

    • Resolution: The change in measured variable to which

    the sensor will respond• Repeatability: Variation of sensor measurements when

    the same quantity is measured several times

    • Range: Upper and lower limits of the variable that can be

    measured• Sensitivity and Linearity

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    1540 Introduction To Mechatronics: Slide 5Stefan B. Williams

    Proximity Sensors

    • Widely used in general industrial automation

     – Conveyor lines (counting,jam detection, etc)

     – Machine tools (safety interlock, sequencing)

    • Usually digital (on/off) sensors detecting thepresence or absence of an object

    • Consist of:

     – Sensor head: optical, inductive, capacitive

     – Detector circuit

     – Amplifier 

     – Output circuit: TTL, solid state relay

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    1540 Introduction To Mechatronics: Slide 6Stefan B. Williams

    Mechanical Proximity Switches

    • Essentially amechanical switch

    • On/off operation only• Two general modes

     – Normally Open (NO)

     – Normally Closed (NC)

    • Come in a wide variety

    of mechanical forms

    • For a wide range of uses

    Actuator 

    Common

     Normally

    Closed

     Normally

    Open

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    1540 Introduction To Mechatronics: Slide 7Stefan B. Williams

    Example Mechanical Proximity

    Switches

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    1540 Introduction To Mechatronics: Slide 8Stefan B. Williams

    When to Use Mechanical

    Proximity Switches

    • Where physical contact is possible

    • Where definitive position is required• In operation-critical or safety-critical

    situations

    • Where environment conditions preclude the

    use of optical or inductive sensors

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    1540 Introduction To Mechatronics: Slide 9Stefan B. Williams

    Applications and Use of

    Mechanical Proximity Switches

    • Easy to integrate into machinery of all types

    • Requires contact (thus wear)

    • Range of voltages: DC 0-1000V, AC, etc.• Very robust (explosion proof if required)

    • Usually used as:

     – Limit switch

     – Presence/absence indicator 

     – Door closed/open

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    1540 Introduction To Mechatronics: Slide 10Stefan B. Williams

    Places You Find Mechanical

    Proximity Switches !

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    1540 Introduction To Mechatronics: Slide 11Stefan B. Williams

    Optical Proximity Sensors

    • Consist of a light source (LED) and light detector(phototransistor)

    • Modulation of signal to minimize ambient lightingconditions

    • Various models: 12-30V DC, 24-240V AC, power 

    • Output: TTL 5V, Solid-state relay, etc.

    Demodulator 

    Amplifier 

    Modulator Power  

    Output

    Mixer Signal

    Power Supply

    Load

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    1540 Introduction To Mechatronics: Slide 12Stefan B. Williams

    Operational Modes

    • Through Beam:

     – Long range (20m)

     – Alignment is critical !

    • Retro-reflective – Range 1-3m

     – Popular and cheap

    • Diffuse-reflective – Range 12-300mm – Cheap and easy to use

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    1540 Introduction To Mechatronics: Slide 13Stefan B. Williams

    Example Optical Proximity I

    Optical Fibre

    Delivery System

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    1540 Introduction To Mechatronics: Slide 14Stefan B. Williams

    Example Optical Proximity II

    Slot Beam

    Systems

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    1540 Introduction To Mechatronics: Slide 17Stefan B. Williams

    Other Optical Devices

    Light

    Curtain

    Collision Detection

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    1540 Introduction To Mechatronics: Slide 18Stefan B. Williams

    Ultrasonic Proximity Sensors

    • Use sound pulses• Measures amplitude and time

    of flight

    • Range provides more than

    on/off information• Frequencies 40KHz-2MHz

    Pulse

    Echo

    Vibrating Membrane

    (metal or ceramic)

    Sensor  Object

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    1540 Introduction To Mechatronics: Slide 19Stefan B. Williams

    When to use Ultrasonic Sensors

    • Provide range data directly:

    • Level monitoring of solid and liquids

    • Approach warning (collisions)

    • Can (usually) work in heavy dust and water 

    • Ambient noise is potentially an issue

    http://www.automationsensors.com/

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    1540 Introduction To Mechatronics: Slide 20Stefan B. Williams

    Example Applications

    Car Wash

    Application

    Paper roll

    Thickness Monitor 

    Waste water

    flow volume

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    1540 Introduction To Mechatronics: Slide 21Stefan B. Williams

    Inductive and Capacitive

    Proximity Sensors

    • Inductive sensors use change in local

    magnetic field to detect presence of metal

    target

    • Capacitive Sensors use change in local

    capacitance caused by non-metallic objects

    • Generally short ranges only

    • Regarded as very robust and reliable

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    1540 Introduction To Mechatronics: Slide 22Stefan B. Williams

    Example Inductive Sensors I

    Detection of

    open/close functions

    Detection of

    rotation

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    1540 Introduction To Mechatronics: Slide 23Stefan B. Williams

    Example Inductive Sensors II

    Bulk mounted inductive

    sensors. Detect presence of 

    object without contact.

    Range 3mm +/- 10%

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    1540 Introduction To Mechatronics: Slide 24Stefan B. Williams

    Example Capacitive Sensors

    Panel Mounted Capacitive

    Sensor. Can detect wood,

     plastic and metal.Range 3mm-25mm

    Flat mounted Capacitive

    Sensor. Used for detecting

     panels of glass.

    Range=10mm +/- 10%

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    1540 Introduction To Mechatronics: Slide 25Stefan B. Williams

    Position and Velocity

    Measurement

    • Position and velocity measurementis often required in feedback loops

    • For positioning, and velocity control

    • Position measurement: – Potentiometers

     – LVDT

     – Encoders

    • Velocity Measurement:

    • Tachometer 

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    1540 Introduction To Mechatronics: Slide 26Stefan B. Williams

    Potentiometers

    R Vin

    Vout

    An analog sensor 

    Works as a voltage divider 

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    1540 Introduction To Mechatronics: Slide 27Stefan B. Williams

    Types of Potentiometer 

    • Wirewound

     – Wiper slides along coil of Ni-chrome wire

     – Wire tends to fail, temperature variations

    • Cermet – Wiper slides on conductive ceramic track

     – Better than wire inmost respects

    • Plastic film

     – High resolution

     – Long life and good temperature stability

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    1540 Introduction To Mechatronics: Slide 29Stefan B. Williams

    When to use a Potentiometer 

    • Require analog signal for control

    • Require absolute positional information

    • Low cost

    • Temperature and wear variations

    • Not in dusty or wet environments

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    1540 Introduction To Mechatronics: Slide 31Stefan B. Williams

    LVDT Signal Conditioning

    • Uses AC modulation,

    demodulation and

    phase comparison

    • Available in a singlemonolithic package

    Power Supply

    Carrier Oscillator 

    AmplitudeControl

    LVDT

    CurrentAmplifier 

    Demodulator 

    Phase

    Shifter 

    Zero

    Set

    AC Power 

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    1540 Introduction To Mechatronics: Slide 32Stefan B. Williams

    Example LVDTs

    Spring-loaded

    Standard for use

    In hydraulic cylinders

    Free core LVDTs for

    use in hostile environments

    And total emersion

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    1540 Introduction To Mechatronics: Slide 33Stefan B. Williams

    When to use an LVDT

    • High accuracy

    • Linear operation (synchro resolver is

    equivalent rotary LVDT)

    • Harsh environment

    • Analog position control

    • Embedding (in cylinder for example)

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    1540 Introduction To Mechatronics: Slide 34Stefan B. Williams

    Optical Encoders

    • Encoders are digital Sensors commonly used toprovide position feedback for actuators

    • Consist of a glass or plastic disc that rotates

    between a light source (LED) and a pair of photo-

    detectors• Disk is encoded with alternate light and dark sectors

    so pulses are produced as disk rotates

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    1540 Introduction To Mechatronics: Slide 35Stefan B. Williams

    Encoder Internal Structure

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    1540 Introduction To Mechatronics: Slide 36Stefan B. Williams

    Incremental Encoders

    • Pulses from leds are

    counted to provide

    rotary position

    • Two detectors are usedto determine direction

    (quadrature)

    • Index pulse used to

    denote start point• Otherwise pulses are

    not unique

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    1540 Introduction To Mechatronics: Slide 37Stefan B. Williams

    Absolute Encoders

    • Absolute encoders havea unique code that can

    be detected for every

    angular position

    • Often in the form of a“grey code”; a binary

    code of minimal change

    • Absolute encoders are

    much more complexand expensive than

    incremental encoders

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    1540 Introduction To Mechatronics: Slide 38Stefan B. Williams

    Encoder processing

    • Need a squaring circuit

    to digitise signal

    • A counter and index

    monitor • Generally available in

    monolithic form

    • Often with algorithms

    for control externallyprogrammable

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    1540 Introduction To Mechatronics: Slide 39Stefan B. Williams

    When to Use an Encoder 

    • Require accurateposition information:

     – 10,000 line incremental

     – 360 line absolute

    • Digital feed-back loop

    • Compact and

    reasonably rugged (not

    as good as inductive)

    • Linear encoders also

    available

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    1540 Introduction To Mechatronics: Slide 40Stefan B. Williams

    Tachometers

    • Measurement of rotaryspeed using a DCgenerator 

    • Essentially a motorrunning in reverse

    • Used to be common tohave these attached tomotors to enable directanalog feedback

    • Much less common nowwith digital control (useincremental encoders)

    Tacho generator for large

    industrial plant (GE)

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    1540 Introduction To Mechatronics: Slide 41Stefan B. Williams

    Force and Pressure

    • Force and Pressure generally measuredindirectly through deflection of an alternate

    surface

    • Mechanism include: – Physical motion and measurement using (eg) an

    LVDT

     – Strain gauges (metal that changes resistance when

    stressed) – Piezo electric materials that generate a current

    when deformed

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    1540 Introduction To Mechatronics: Slide 42Stefan B. Williams

    LVDT Load Cell

    Table ForceSpring or Piston

    LVDT

    Outer 

    Platform

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    1540 Introduction To Mechatronics: Slide 43Stefan B. Williams

    Strain Gauge Bridge

     R R R RGF 

     L L

     R R GF 

    ε

    ε

    ∆ ∆= =∆

    ∆ = ⋅ ⋅Tension

    Strain Gauges

    ( )

    3 2

    3 4 1 2

    1 2 4

    3

    , ,

    4

    2

    meas

    exc

    G

    G

    meas

    meas exc

    V R RV R R R R

    assume R R R R

     R R RV 

    thenGF V V  

    ε

    = −  + +

    = =

    = + ∆−=

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    1540 Introduction To Mechatronics: Slide 44Stefan B. Williams

    Example Load Cells

    Reaction torque

    load cell

    Axial load cell

    http://www.entran.com/ltoc.htm

    Subminature

    Load cells

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    1540 Introduction To Mechatronics: Slide 45Stefan B. Williams

    Sub-miniature Load cells

    All signal conditioning

    and amplification integrated

    with the sensor Load cell bridge structure

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    1540 Introduction To Mechatronics: Slide 46Stefan B. Williams

    Piezo Load Cells

    • Distortion of crystal,

    either quartz or BaTiO3

    • Used for accuratemeasurement of small

    loads

    • Come in the form of:

    • single axis load washers• or multiple axis load

    washers and tables

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    1540 Introduction To Mechatronics: Slide 47Stefan B. Williams

    Pressure

    • Pressure measured by: – Pitot tube and

     – Deformation of fixed

    membrane

    • Deformation measuredusing same methods as

    for force:

    • Spring (manometer)

    • Piezo distortion• Strain gauges

    Miniature

    Industry IP69

    High Temperature

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    1540 Introduction To Mechatronics: Slide 48Stefan B. Williams

    Acceleration

    • Acceleration is alsomeasured via the force

    exerted by an

    accelerating mass

    • Distortion of a piezo• Motion of a cantilever 

    • Strain on mass

    restraints

    • Accelerometers mainlyused to measure

    vibration

    Single Axis,

    10,000g

    Shielded forSevere

    environment

    EMI

    shielded

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    1540 Introduction To Mechatronics: Slide 49Stefan B. Williams

    Tri-axial Accelerometers

    • Triaxial accelerometers

    used in mobile systems

     – In high-performance cars

     – Inside rotating elementsof turbines

     – In aircraft elements

    • Provide vibration

    information• Provide short-term

    position data

    Triple axis

    Accelerometer

    For racing cars

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    1540 Introduction To Mechatronics: Slide 50Stefan B. Williams

    Silicon Machined

    Accelerometers

    Cantilever 

     beams

    Used in eg air-bags

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    1540 Introduction To Mechatronics: Slide 51Stefan B. Williams

    Silicon Gyroscopes

    • Structural arrangementof silicon which records

    centrifugal acceleration

    and thus angular speed

    • Use strain-gaugebridges and/or piezo

    structure to record

    deformations

    • Multiple componentelements to calibrate

    other accelrations

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    1540 Introduction To Mechatronics: Slide 52Stefan B. Williams

    Inertial Systems

    • Many different types of accelerometer andgyroscope systems

    • Mechanical bodies, fibre optic, etc

    • Together in an orthogonal arrangement ofaccelerometers and gyroscopes, thesecomprise an inertial measurement unit (IMU)

    • An IMU that is used for navigation is called aninertial navigation system (INS)

    • These are widely used in aircraft and missilenavigation and guidance

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    Aerospace INS

    http://www.littongcs.com/products/2guidance/space/overview.html

    Aircraft

    Ballistic

    Missile