Sensors - PowerPoint Presentation

Slide1

Sensors

Efrain

Teran

Carol Young

Brian

O’SabenSlide2

Optical Encoders

Efrain

TeranSlide3

What are Optical Encoders ?

An Optical

R

otary Encoder is an electro-mechanical device that converts the angular position

of a shaft

to a digital code.

Provide information

on

angular position, speed, and direction.The information is used for system control (e.g. motor velocity feedback control). It is the most popular type of encoder.

What are they used for?Slide4

How do they work?

Use light

and photo detectors to

produce

a digital

code

As

the encoder shaft rotates, output signals are produced proportional to the angle of rotation. The signal may be a square wave (for an incremental encoder) or an absolute measure of position (for an absolute encoder).Slide5

Optical Encoder parts

Code disk:

has one

or more

tracks

with

slits (windows) to allow light to pass through.

Photodetector: electronic sensor that reacts to light. Usually a phototransistor or photodiode.

Light source: produces the light that will “trigger” the photodetectors during motion. Usually LEDs or IR LEDs Mask: collimates the beams of lightSlide6

Optical Encoder parts

Shaft:

mechanically attached to the system we want to measure; usually a motor.

Housing:

protection from the environment.

Electronic board:

filters signal into square wave used by microcontroller.Slide7

Types of Optical Encoders

Absolute Optical Encoders

Incremental Optical Encoders:

Single channel

Dual channel

Dual channel with Z indexSlide8

Incremental Encoders

Generate a series of pulses as the shaft moves and provide relative position information.

They are typically simpler and cheaper than absolute encoders.

Need external processing of signals.

TYPESSlide9

Incremental Optical Encoder: Single channel

Has only one output channel for encoding information.

Used in unidirectional systems or where you don’t need to know direction.

Lo Hi Lo Hi Lo

0 1 0 1 0

Voltage

BinarySlide10

Incremental Optical Encoder: Dual channel

The output has two lines of pulses (“A” and “B” channel)

They are 90° offset in order to determine rotation direction.

This phasing between the two signals is called

quadrature

.

Lo Hi

Hi

LoChannel A

Lo

Lo

Hi

Hi

Channel B

Repetitive sequenceSlide11

Incremental Optical Encoder: Dual channelSlide12

Incremental Optical Encoder:

Dual channel with Z index

Some quadrature encoders include a third channel (Z or Index)

It supplies a single pulse per revolution used for precise determination of a reference position.

Need to do “homing” for it to work. Doesn’t hold after power down.

ZSlide13

Absolute Encoders

Provides a unique digital output for each shaft position

The code disk has many tracks. The number determines resolution.

Upon a loss of power it keeps the correct position value.

Uses binary or “grey” code.Slide14

VIDEO: https://www.youtube.com/watch?v=cn83jR2mchwSlide15

Absolute encoders:

Binary vs. Gray code

000

111

001

010

011

100

101

110

Transition possible results: 011 -

010 - 001 - 011- 111

- 100Slide16

Absolute encoders:

Binary vs. Gray code

000

100

001

011

010

110

111

101

Transition possible results: 010 - 110Slide17

Encoder Resolution

Resolution can be given in

number of bits

or

degrees

Depends on the number of tracks on the code disk. Each track requires an output signal, also known as an “encoder bit”.

Absolute Optical

Encoder

Resolution = 360°/(2N)N = number of encoder bits (number of tracks)

Example:

An absolute encoder has 8 tracks on the disc. What is its angular resolution in degrees?

Resolution

= 360°

/(2

N

) =

360

°

/(

2

8) = 1.4° Slide18

Encoder Resolution

Resolution essentially depends on the

number

of windows on

the code disk

Incremental

Optical

Encoder

Resolution = 360/NN = number of windows on code disk

BUT, we can increase resolution by using channels A and B

Example:

What number of

windows

are needed on the code disk of

an incremental

optical encoder to

measure displacements

of

1.5°?

Resolution

=360° /N =1.5 ° → N = 240 windowsSlide19

Encoder Resolution

Incremental

Optical

Encoder

X4 Resolution

=

360/4N

N = number of windows (slits or lines) on the

code disk

Today’s standard

We may count

rising and falling edges

in both channel’s signalsSlide20

(

Sabri

Centinkunt, page 236)

Example:

Consider an incremental encoder that produces 2500-pulses/revolution. Assume

that the

photo detectors in the decoder circuit can handle signals up to 1 MHz frequency.

Determine the maximum shaft speed (RPM) the encoder and decoder circuit can handle.

 Slide21

Absolute Encoder

Incremental Single channel

Incremental Dual channel

Incremental with Z index

ApplicationsSlide22

Mechatronics

,

Sabri

Cetinkunt, Wiley, 2007. Section 6.4.3

http

://

en.wikipedia.org/wiki/Rotary_encoder

http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041235/Incremental-Versus-Absolute-Encoders.htmlhttp://www.ni.com/white-paper/7109/en/http://www.digikey.com/PTM/IndividualPTM.page?site=us&lang=en&ptm=2420

REFERENCES:Slide23

Noise cancellationSlide24

Laser Interferometer

Carol YoungSlide25

What is a Laser Interferometer ?

Laser- single frequency light wave

Interferometry- Family of techniques where waves are super imposed in order to extract information about the waves

Uses the interference

patterns from lasers to

produce high precision

measurementsSlide26

Physics Background

Waves

Light is an Electrometric wave and therefore has wave properties.

http://

en.wikipedia.org

/wiki/

File:Light-wave.svgSlide27

Physics Background

Diffraction and Interference

Diffraction

Light spreads after passing a narrow point

Interference

superposition of two waves to form new wave with different amplitude

Constructive or Destructive

http://

en.wikipedia.org/wiki/File:Doubleslit3Dspectrum.gifSlide28

Types of Laser Interferometers

Homodyne

Homo (same) + dyne (power)

Uses a single frequency to obtain measurements

Heterodyne

Hetero (different) + dyne (power)

Uses two different (but close) frequencies to obtain measurements.Slide29

Homodyne Interferometer

(Michelson)

Laser

Mirror Reference

Mirror Moveable

(Sample)

Beam Splitter

ScreenSlide30

Homodyne

Interferometer

Analysis

Photograph of the interference fringes produced by a Michelson interferometer.

λ

is the wavelength of the light

L

ref

is the distance to the reference mirror

L is the distance to the moveable mirror

n is the number of fringesSlide31

Homodyne Interferometer

Uses

Absolute distance

O

ptical testing

Refractive index

Angles

Flatness StraightnessSpeed

VibrationsSlide32

Physics Background

Doppler Effect

Point creating a wave and movement

Wave ahead of point has higher frequency

Wave behind point has lower frequency

Frequency change corresponds to velocity

http://

en.wikipedia.org/wiki/File:Dopplereffectsourcemovingrightatmach0.7.gifSlide33

Physics Background

Beat Frequency

Rate of constructive and destructive interferenceSlide34

Heterodyne Interferometer

Produces two close but not equal frequencies (Creating a Beat Frequency)

Doppler effect from moving reflector shifts the frequency proportional to the velocitySlide35

Heterodyne

/ Homodyne

Interferometer

ComparisonComparing with a Homodyne Interferometer

Can determine movement direction (but limited range)

More useful when direction of movement is importantSlide36

Heterodyne / Homodyne

Interferometer Comparison

Homodyne

Smooth surfaces onlyHeterodyne

Can be used for

Distance to rough surfaces

Surface roughness measurementsSlide37

Resolution

XL-80 Laser Measurement System

Xiaoyu DingSlide38

References

http://www.aerotech.com/products/engref/intexe.html

http://www.renishaw.com/en/interferometry-explained--7854

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

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

Interferometry

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

www.ljmu.ac.uk/GERI/GERI_Docs/interferometry_presentation(1).

ppt

http://www.olympus-controls.com/documents/GEN-NEW-0117.

pdf

http://www.lambdasys.com/product/LEOI-20.

htm

http://www.intechopen.com/books/advances-in-solid-state-lasers-development-and-applications/precision-dimensional-metrology-based-on-a-femtosecond-pulse-

laser

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

http://www.gitam.edu/eresource/Engg_Phys/semester_1/optics/

intro_polari.htm

A. F.

Fercher

, H. Z. Hu, and U.

Vry, “Rough surface interferometry with a two-wavelength heterodyne speckle

interferometer”,

Applied OpticsSlide39

Linear Variable Differential Transformer (LVDT)

Brian O’SabenSlide40

Outline

What is a LVDT?

How LVDTs Works

LVDT PropertiesLVDT Support Electronics

Types of LVDTs

LVDT ApplicationsSlide41

What is a LVDT?

Linear variable

d

ifferential transformerElectromechanical transducer measuring linear displacement Slide42

What is a LVDT?

Primary coil

Energized with constant A/C

Two identical secondary coils

Symmetrically distributed

Connected in opposition

Ferromagnetic coreSlide43

How LVDT works

If core is centered between S1 and S2

Equal flux from each secondary coil

Voltage E1 = E2Slide44

How LVDT works

If core is closer to S1

Greater flux at S1

Voltage E1 increases, Voltage E2 decreasesE

out

=E1 – E2 Slide45

How LVDT works

If core is closer to S2

Greater flux at S2

Voltage E2 increases, Voltage E1

decreases

E

out=E2 – E1 Slide46

How LVDT worksSlide47

LVDT properties

Friction-free operation

Unlimited mechanical life

Infinite resolution Separable coil and core

Environmentally robust

Fast

dynamic response

Absolute output Slide48

LVDT support electronics

LVDT signal conditioning equipment

Supply excitation power for the LVDT

Typically 3 Vrms at 3 kHz

Convert low level A/C output to high level DC signals

Gives directional information based on phase shiftSlide49

Types of LVDTs

DC LVDT

Signal conditioning equipment built in

Pre-calibrated analog and/or digital output

Lower

overall system cost

AC

LVDTWide operating environments Shock and vibrationTemperatureSmaller package sizeSlide50

Types of

LVDTs

Separate

core

Core is completely separable from the transducer body

Well-suited for short-range (1 to 50mm), high speed applications (high-frequency vibration)

Guided core

Core is restrained and guided by a low-friction assembly

Both static and dynamic applicationsworking range (up to 500mm)

Spring

-loaded

Core is restrained and guided by a low-friction assembly

Internal spring to continuously push the core to its fullest possible extension

Best suited for static or slow-moving applications

Lower range than guided core(10 to 70mm)Slide51

LVDT applications

Industrial gaging systems

Electronic dial indicators

Weighing

systems

Crankshaft balancer

Final product inspection (checking dimensions)

Octane

analyzer (provides displacement feedback for Waukesha engine)Valve position sensingSlide52

References

http://www.macrosensors.com/lvdt_tutorial.html

http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm

http://www.directindustry.com/industrial-manufacturer/lvdt-73930.html

http://macrosensors.com/blog/view-entry/Why-Use-an-AC-LVDT-versus-a-DC-LVDT-Linear-Positio/31/

http://www.meas-spec.com/downloads/LVDT_Selection,_Handling_and_Installation_Guidelines.pdf

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

http://www.transtekinc.com/support/applications/LVDT-applications.htmlLei Yang’s student lectureSlide53

Thank You!