Sensors

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ME4447/6405
ME 4447/6405
Microprocessor Control of Manufacturing Systems
and
Introduction to Mechatronics
Sensors
Optical Encoder: Ryder Winck
Laser Interferometer: Aaron Scott
LVDT: Alexandre Lenoble
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Presentation Outline
• Optical Encoders
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Introduction
Optical Encoder Components
Types of Optical Encoders
Encoder Discs and Digital Codes
Encoder Reliability and Errors
Applications
• Laser Interferometer
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What is a Laser Interferometer
Types of Laser Interferometer
How Do they Work
Resolutions and Sampling Rate
Applications
• Linear Variable Displacement Transducer (LVDT)
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What is a LVDT
Types of LVDTs
How Do they Work
Resolutions and Sampling Rate
Applications
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
What is an Encoder?
• Any transducer that changes a signal into a
coded (digital signal)
• Optical Encoders
– Use light & photosensors to produce digital code
(ie. Lab 3 encoder).
– Most popular type of encoder.
• Can be linear or rotary.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Types of Optical Encoders
• 2 types of Optical Encoders:
– 1. Incremental (Lab 3 encoder)
• Measure displacement relative to a reference point.
– 2. Absolute
• Measure absolute position.
• Advantages – A missed reading does not affect the next
reading. Only needs power on when taking a reading.
• Disadvantages – More expensive/complex.
Cost/complexity proportional to resolution/accuracy.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Fundamental Components
• Light source(s)
– LEDs or IR LEDs provide light source.
– Light is collimated using a lens to make the beams
parallel.
• Photosensor(s)
– Either Photodiode or Phototransistor.
• Opaque disk (Code Disk)
– One or more “tracks” with slits to allow light to
pass through.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Optical Encoder Components
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Other Components
• Stationary “masking” disk
– Identical track(s) to Code Disk
– Eliminates error due to the diameter of the light beam being
greater than the code disk window length.
• Signal amplifiers and pulse shape circuitry.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Quadrature
• Two tracks (A & B) at
90 degrees offset.
• Provide direction
information.
• Provides up to 4 times
resolution.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Incremental Disk
Ryder Winck
Encoder Disks
Absolute Disks
Binary
Gray Code
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Absolute Disk Codes
• Example: 3 bit binary code
Bit 0
Bit 1
Bit 2
Bit 0
Bit 1
Bit 2
Angle
Binary
Decimal
0-45
000
0
45-90
001
1
90-135
010
2
135-180
011
3
180-225
100
4
225-270
101
5
270-315
110
6
315-360
111
7
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Ryder Winck
ME4447/6405
Problem with Binary Code
• One angle shift results
in multiple bit changes.
• Example: 1 => 2
– 001
– 000
– 010
(start at 1)
(turn off bit 0)
(turn on bit 1)
Angle
Binary
Decimal
0-45
000
0
45-90
001
1
90-135
010
2
135-180
011
3
180-225
100
4
225-270
101
5
270-315
110
6
315-360
111
7
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Ryder Winck
ME4447/6405
Problem with Binary Code
• One degree shift results
in multiple bit changes.
• Example: 1 => 2
– 001
– 000
– 010
(start at 1)
(turn off bit 0)
(turn on bit 1)
• It looks like we went
from 1 => 0 => 2
Angle
Binary
Decimal
0-45
000
0
45-90
001
1
90-135
010
2
135-180
011
3
180-225
100
4
225-270
101
5
270-315
110
6
315-360
111
7
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Gray Code
• One bit change per angle change.
Angle
Binary
Decimal
Bit 0
0-45
000
0
Bit 1
Bit 2
45-90
001
1
90-135
011
2
135-180
010
3
180-225
110
4
225-270
111
5
270-315
101
6
315-360
100
7
Bit 0
Bit 1
Bit 2
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Converting from Gray Code to Binary Code
1. Copy MSB.
2. If MSB is 1, write 1s until next 1 is met.
If MSB is 0, write 0s until next 1 is met.
3. When 1 is met, logically switch what you are
writing (1=>0 or 0=>1).
4. Continue writing the same logical until next
1 is met.
5. Loop back to step 3.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Example: Convert 0010 to Binary
Code
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Copy MSB: 0_ _ _
Write 0s until next 1 is met: 00_ _
Switch to writing 1s: 001_
Write 1s: 0011
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Example: Convert 1110 to Binary
Code
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Copy MSB: 1_ _ _
Write 1s until next 1 is met: 1_ _ _
Switch to writing 0s until next 1 is met: 10_ _
Switch to writing 1s until next 1 is met: 1011
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Encoder Reliability and Errors
• Resolution
360
– Incremental  
where N=# of windows.
N
• Resolution can be increased by reading both rising and
360
360
falling edges (   2 N ) and by using quadrature (   4 N ).
360
– Absolute   n where n=# of tracks.
2
 
George W. Woodruff School of Mechanical Engineering, Georgia Tech
360
 90
4
ME4447/6405
Ryder Winck
Encoder Reliability and Errors
•
Encoder errors
1. Quantization Error – Dependent on digital word
size.
2. Assembly Error – Dependent on eccentricity of
rotation (is track center of rotation=center of
rotation of disk)
3. Manufacturing tolerances – Code printing
accuracy, sensor position, and irregularities in
signal generation.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Encoder Reliability and Errors
• Comment on pulse irregularity
– It is a result of noise in signal generation,
variations in light intensity, and imperfect edges.
– It can be mitigated using a Schmidt Trigger, but
this can lead to hysteresis.
– Using 2 adjacent sensor will negate this problem.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Encoder Reliability and Errors
•
More encoder errors
4. Structural Limitations – Disk Deformation,
physical loads on shaft.
5. Coupling Error – Gear backlash, belt slippage,
etc…
6. Ambient Effects – Vibration, temperature, light
noise, humidity, etc…
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Ryder Winck
Applications
• Any linear/rotary position/velocity sensing
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DC Motor control – robotics/automation
Mechanical computer mouse
Digital readouts for measurement gauges
Tachometers – planes, trains and automobiles
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
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Ryder Winck
References
http://hades.mech.northwestern.edu/wiki/index.php/Image:Maxon-small2.jpg
http://www.designworldonline.com/Uploads/Leadership/Encoder_Montage1.jpg
http://www.gpi-encoders.com/06_Technical_Articles.htm
http://books.google.com/books?id=CjB2ygeR95cC&pg=PA630&lpg=PA630&dq=
optical+encoder+mechatronics&source=bl&ots=uPB9nyu0AP&sig=PJYTMIG1dJ
6UOPzj6uNhvYx1xSE&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA
639,M1
http://books.google.com/books?id=gUbQ9_weg88C&pg=PA97&lpg=PA97&dq=o
ptical+encoders&source=web&ots=X2AbRCs5bL&sig=dotsCBPIq7KGQodesPx3QJ_qos&hl=en&sa=X&oi=book_result&resnum=3&ct=re
sult#PPA98,M1
http://books.google.com/books?id=uG7aqgal65YC&pg=RA1-PA163&lpg=RA1PA163&dq=optical+encoders&source=web&ots=6-NhfhYb-F&sig=ufVtBwSPRNUaCfujxu0gFbxqY&hl=en&sa=X&oi=book_result&resnum=5&ct=result#PRA1-PA163,M1
http://mechatronics.mech.northwestern.edu/design_ref/sensors/encoders.html
http://books.google.com/books?id=9e4Omibz3L4C&pg=PA395&lpg=PA395&dq=
optical+encoders&source=web&ots=5bTXzKDiWG&sig=cGa9IdHuxw3Zq49SyV
CJbzjGQnc&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA410,M1
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Laser Interferometers
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What is a Laser Interferometer?
Types of Laser Interferometers
How Do they Work?
Resolutions and Sampling Rate
Applications
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
What is a Laser Interferometer?
• Interferometry = “interference” + “measurement”
• Basic application: hi-res measurement of distances
• Basic principle: superposition of light waves
Constructive interference
Destructive interference
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
What is a Laser Interferometer?
• The Michelson Interferometer
– Difference in path length results
in phase difference
– Phase difference causes
interference
– Interference determined by
analysis of fringe patterns
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
What is a Laser Interferometer?
• Brief historical background
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First American Nobel Prize in Sciences 1907
Optical precision instruments
Invented the interferometer
Most accurate measurement of c in his time
Disproved existence of ether with famous
Michelson-Morley experiment
Albert Michelson
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
What is a Laser Interferometer?
Why “lasers” ?
• High coherence
• Collimated
• Predictable
– Frequency known
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Types of Laser Interferometers
• Homodyne detection (standard interferometry)
– DC output signal from photodiode related to intensity of
light from interference
– Both beams have same frequency
• Heterodyne detection
– One beam is frequency modulated prior to detection
– AC output signal of interference at the beat frequency (see
board)
– Phase determined by signal analysis
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Types of Laser Interferometers
• Advantages of Heterodyne Detection
– AC signal frequency can be greatly reduced
• AC frequency = fbeat = fmod – fsignal
– Detection at low frequency reduces effect of high
frequency noise
– Insensitive to ambient light and signal intensity
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
How Do They Work?
• Homodyne – already discussed (Michelson interferometer)
• Heterodyne
– Dual frequency,
polarized
laser source
– Polarizing
beam splitter
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Resolutions and Sampling Rate
• Representative values
• Resolution
– 10 nm digital resolution
– sub-angstrom analog resolution
achieved by “external interpolation”
• Angstrom, Å = 1  10-10 m
• Sampling Rate
– 20 MHz
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• Michelson used his interferometer to
measure the rotation rate of the Earth
– Perimeter of his ring was 1.9 km
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• 3 axis ring laser gyro
– Many winds of optic fibers achieve 1 km path
– Sensitive enough to measure
Earth’s rotation despite small
size
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• Distance measurement
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–
–
–
Profilometer to measure nanoscale surface features
Nanopatterning Lithography
Precision machining calibration
High-precision linear feedback encoder
• Velocity measurement
– Doppler shift along measurement path changes beat
frequency
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• Other measurements made possible by rearrangements of the light paths. We can
measure
– angle
– straightness
– flatness
– parallelism
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• LIGO
Laser Interferometer Gravitational-Wave Observatory
– Gravity waves, predicted by Gen. Relativity, could be
detected by sensing changes in length in perpendicular
directions
– Light bounces 75 times before returning to be combined
– Each arm 4 km
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Aaron Scott
Applications
• LISA
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Laser Interferometer Space Antenna
NASA/ESA expected 2018-2020
Similar to LIGO but MUCH larger
5 gigameter arm length
3 interferometers in 1
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
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References
Aaron Scott
http://en.wikipedia.org/wiki/Interferometry
http://en.wikipedia.org/wiki/Albert_Abraham_Michelson
http://encarta.msn.com/encyclopedia_761555191/Albert_Michelson.html
http://www.renishaw.com/UserFiles/acrobat/UKEnglish/GEN-NEW0117.pdf
http://www.ligo-la.caltech.edu/contents/overviewsci.htm
http://lisa.nasa.gov/
http://www.maxvalue.co.th/download/Excel.PDF
DVD: “Albert A. Michelson Laboratory, History and Heritage” Public
Release, NAWCWD, China Lake
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
LVDT
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
What is a LVDT ?
- Linear Variable Displacement Transducer
- Electrical transformer used to measure linear
displacement
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Construction
Secondary #1 Primary Secondary #2
Lead wires
Displacement
Moveable core
-
Primary coil and 2 symmetric secondary coils
Coils are encapsulated in metal/Epoxy
Ferromagnetic core
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
LVDT Types
- Distinction by :
- Power supply :
- DC
- AC
- Type of armature :
- Unguided
- Captive (guided)
- Spring-extended
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
DC LVDTs
- Easy to install
- Signal conditioning easier (equipment part of
LVDT)
- Can operate from dry cell batteries
- High unit cost
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
AC LVDTs
- Small size
- Very accurate – Excellent resolution (0.1 µm)
- Can operate with a wide temperature range
(-65° F to +221° F) (30°F to 120°F for
DC)
- Lower unit cost than DC LVDTs
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Cost per unit
- Unguided armature :
- DC : $485
- AC : $330
- Spring-extended armature
- DC : $1359
- AC : $1156
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Unguided armature
- Simplest mechanical configuration, armature
fits loosely on the bore of the LVDT, being
attached to the moving point by a male thread.
- Armature completely separable from the
transducer body.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Unguided armature : applications
Well-suited for short-range (1 to
50mm), high speed applications (highfrequency vibration)
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Captive (guided) armature
- Both static and dynamic applications
- Armature restrained and guided by a lowfriction assembly
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Captive (guided) armature
-Advantages compared to unguided armature :
- Better for longer working range (up to 500mm)
- Preferred when misalignment may occur
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Spring-extended armature
- Armature restrained and guided by a lowfriction assembly (as for captive armature)
- Internal spring to continuously push the
armature to its fullest possible extension
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Spring-extended armature
- Best suited for static or slow-moving
applications
- Lower range than captive armature (10 to
70mm)
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
LVDT Function
Primary coil
Secondary coil
#1
Primary coil
Secondary coil
#2
Input to primary
Secondary coil
#1
Secondary coil
#2
Input to primary
Output from secondary
coils
Secondary coil #1 output (V1)
Output from secondary
coils
Secondary coil #1 output (V1)
Secondary coil #2 output (V2)
V1 - V2
Secondary coil #2 output (V2)
V1 - V2
Demodulated
output
George
W.
Demodulated
Woodruff School of Mechanical Engineering,
Georgia output
Tech
Alexandre Lenoble
ME4447/6405
Summary
• LVDTs are robust equipment
measuring displacement
for
• AC LVDTs require separate signal
conditioning equipment, while DC
LVDTs include signal conditioning
equipment on the device.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Summary
• There are three types of LVDT: unguided
armature, captive armature, and springextended armature.
• AC LVDT’s cost less than DC, but the entire
measurement system must be considered.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Applications
LVDTs find lots of applications in :
- automation machinery
- civil engineering
- power generation
- manufacturing
- metal stamping
- OEM (Original Equipment Manufacturer)
- aeronautics
- R&D
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Applications
Examples for OEM :
- Measure displacement of thermostat valve stem for diesel truck
engine monitoring system.
- Blood-testing device measuring the displacement of blood cells
as they contract. Clinical usage, diagnosis of blood disorders.
- Measuring displacement of diamond tip to determine material
hardness.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
Applications
Examples for civil engineering :
- Displacement measurement of imbedded concrete anchors tested
for tensile, compression, bending strength and crack growth in
concrete
- Deformation and creep of concrete wall used for retaining wall
in large gas pipe installation.
- Dynamic measurement of fatigue in large structural components
used in suspension bridges.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Alexandre Lenoble
ME4447/6405
References
•
www.dankuchma.com/cee498/presentations/LVDT%20Jason%20Hart.ppt
•
Pr. Kurfess’s lecture
•
http://www.daytronic.com/products/trans/lvdt/default.htm
•
http://www.macrosensors.com
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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