ME 4447/6405

1
ME 4447/6405

• Microprocessor Control of Manufacturing Systems
• and
• Introduction to Mechatronics
• Sensors
• Optical Encoder Ryder Winck
• Laser Interferometer Aaron Scott
• LVDT Alexandre Lenoble

2
Presentation Outline
Ryder Winck

• Optical Encoders
• Introduction
• Optical Encoder Components
• Types of Optical Encoders
• Encoder Discs and Digital Codes
• Encoder Reliability and Errors
• Applications
• Laser Interferometer
• What is a Laser Interferometer
• Types of Laser Interferometer
• How Do they Work
• Resolutions and Sampling Rate
• Applications
• Linear Variable Displacement Transducer (LVDT)
• What is a LVDT
• Types of LVDTs
• How Do they Work
• Resolutions and Sampling Rate
• Applications

3
What is an Encoder?
Ryder Winck

• 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.

4
Types of Optical Encoders
Ryder Winck

• 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.

5
Fundamental Components
Ryder Winck

• 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.

6
Optical Encoder Components
Ryder Winck
7
Other Components
Ryder Winck

• 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.

8
Quadrature
Ryder Winck

• Two tracks (A B) at 90 degrees offset.
• Provide direction information.
• Provides up to 4 times resolution.

9
Encoder Disks
Ryder Winck
Incremental Disk
Absolute Disks
Binary
Gray Code
10
Absolute Disk Codes
Ryder Winck

• Example 3 bit binary code

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
11
Problem with Binary Code
Ryder Winck
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

• One angle shift results in multiple bit changes.
• Example 1 gt 2
• 001 (start at 1)
• 000 (turn off bit 0)
• 010 (turn on bit 1)

12
Problem with Binary Code
Ryder Winck
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

• One degree shift results in multiple bit changes.
• Example 1 gt 2
• 001 (start at 1)
• 000 (turn off bit 0)
• 010 (turn on bit 1)
• It looks like we went from 1 gt 0 gt 2

13
Gray Code
Ryder Winck

• One bit change per angle change.

Angle Binary Decimal
0-45 000 0
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
14
Converting from Gray Code to Binary Code
Ryder Winck

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 (1gt0 or 0gt1).
4. Continue writing the same logical until next 1 is
   met.
5. Loop back to step 3.

15
Example Convert 0010 to Binary Code
Ryder Winck

• Copy MSB 0_ _ _
• Write 0s until next 1 is met 00_ _
• Switch to writing 1s 001_
• Write 1s 0011

16
Example Convert 1110 to Binary Code
Ryder Winck

• 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

17
Encoder Reliability and Errors
Ryder Winck

• Resolution
• Incremental where N of windows.
• Resolution can be increased by reading both
  rising and falling edges ( ) and by
  using quadrature ( ).
• Absolute where n of tracks.

18
Encoder Reliability and Errors
Ryder Winck

• Encoder errors
• Quantization Error Dependent on digital word
  size.
• Assembly Error Dependent on eccentricity of
  rotation (is track center of rotationcenter of
  rotation of disk)
• Manufacturing tolerances Code printing
  accuracy, sensor position, and irregularities in
  signal generation.

19
Encoder Reliability and Errors
Ryder Winck

• 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.

20
Encoder Reliability and Errors
Ryder Winck

• More encoder errors
• Structural Limitations Disk Deformation,
  physical loads on shaft.
• Coupling Error Gear backlash, belt slippage,
  etc
• Ambient Effects Vibration, temperature, light
  noise, humidity, etc

21
Applications
Ryder Winck

• Any linear/rotary position/velocity sensing
• DC Motor control robotics/automation
• Mechanical computer mouse
• Digital readouts for measurement gauges
• Tachometers planes, trains and automobiles

22
References
Ryder Winck

• http//hades.mech.northwestern.edu/wiki/index.php/
  ImageMaxon-small2.jpg
• http//www.designworldonline.com/Uploads/Leadershi
  p/Encoder_Montage1.jpg
• http//www.gpi-encoders.com/06_Technical_Articles.
  htm
• http//books.google.com/books?idCjB2ygeR95cCpgP
  A630lpgPA630dqopticalencodermechatronicssou
  rceblotsuPB9nyu0APsigPJYTMIG1dJ6UOPzj6uNhvYx1
  xSEhlensaXoibook_resultresnum4ctresultP
  PA639,M1
• http//books.google.com/books?idgUbQ9_weg88CpgP
  A97lpgPA97dqopticalencoderssourcewebotsX2
  AbRCs5bLsigd-otsCBPIq7KGQodesPx3QJ_qoshlensa
  Xoibook_resultresnum3ctresultPPA98,M1
• http//books.google.com/books?iduG7aqgal65YCpgR
  A1-PA163lpgRA1-PA163dqopticalencoderssource
  webots6-NhfhYb-Fsiguf-VtBwSPRNUaCfujxu0gFb-xqY
  hlensaXoibook_resultresnum5ctresultPRA1
  -PA163,M1
• http//mechatronics.mech.northwestern.edu/design_r
  ef/sensors/encoders.html
• http//books.google.com/books?id9e4Omibz3L4CpgP
  A395lpgPA395dqopticalencoderssourcewebots
  5bTXzKDiWGsigcGa9IdHuxw3Zq49SyVCJbzjGQnchlens
  aXoibook_resultresnum10ctresultPPA410,M1

23
Laser Interferometers
Aaron Scott

• What is a Laser Interferometer?
• Types of Laser Interferometers
• How Do they Work?
• Resolutions and Sampling Rate
• Applications

24
What is a Laser Interferometer?
Aaron Scott

• Interferometry interference measurement
• Basic application hi-res measurement of
  distances
• Basic principle superposition of light waves
• Constructive interference
• Destructive interference

25
What is a Laser Interferometer?
Aaron Scott

• The Michelson Interferometer

• Difference in path length results in phase
  difference
• Phase difference causes interference
• Interference determined by analysis of fringe
  patterns

26
What is a Laser Interferometer?
Aaron Scott

• Brief historical background
• 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
27
What is a Laser Interferometer?
Aaron Scott

• Why lasers ?
• High coherence
• Collimated
• Predictable
• Frequency known

28
Types of Laser Interferometers
Aaron Scott

• 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

29
Types of Laser Interferometers
Aaron Scott

• 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

30
How Do They Work?
Aaron Scott

• Homodyne already discussed (Michelson
  interferometer)
• Heterodyne
• Dual frequency,
• polarized
• laser source
• Polarizing
• beam splitter

31
Resolutions and Sampling Rate
Aaron Scott

• 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

32
Applications
Aaron Scott

• Michelson used his interferometer to measure the
  rotation rate of the Earth
• Perimeter of his ring was 1.9 km

33
Applications
Aaron Scott

• 3 axis ring laser gyro
• Many winds of optic fibers achieve 1 km path
• Sensitive enough to measure
• Earths rotation despite small
• size

34
Applications
Aaron Scott

• Distance measurement
• 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

35
Applications
Aaron Scott

• Other measurements made possible by
  re-arrangements of the light paths. We can
  measure
• angle
• straightness
• flatness
• parallelism

36
Applications
Aaron Scott

• 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

37
Applications
Aaron Scott

• LISA Laser Interferometer Space Antenna
• NASA/ESA expected 2018-2020
• Similar to LIGO but MUCH larger
• 5 gigameter arm length
• 3 interferometers in 1

38
References
Aaron Scott

• http//en.wikipedia.org/wiki/Interferometry
• http//en.wikipedia.org/wiki/Albert_Abraham_Michel
  son
• http//encarta.msn.com/encyclopedia_761555191/Albe
  rt_Michelson.html
• http//www.renishaw.com/UserFiles/acrobat/UKEnglis
  h/GEN-NEW-0117.pdf
• http//www.ligo-la.caltech.edu/contents/overviewsc
  i.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

39
Alexandre Lenoble

• LVDT

40
What is a LVDT ?
Alexandre Lenoble

• Linear Variable Displacement Transducer
• - Electrical transformer used to measure linear
  displacement

41
Construction
Alexandre Lenoble
Primary
Secondary 1
Secondary 2

• Primary coil and 2 symmetric secondary coils
• Coils are encapsulated in metal/Epoxy
• - Ferromagnetic core

Lead wires
Displacement
Moveable core
42
LVDT Types
Alexandre Lenoble

• - Distinction by
• - Power supply
• - DC
• - AC
• Type of armature
• - Unguided
• - Captive (guided)
• - Spring-extended

43
DC LVDTs
Alexandre Lenoble

• Easy to install
• Signal conditioning easier (equipment part of
  LVDT)
• Can operate from dry cell batteries
• - High unit cost

44
AC LVDTs
Alexandre Lenoble

• Small size
• Very accurate Excellent resolution (0.1 µm)
• Can operate with a wide temperature range
• (-65 F to 221 F) (30F to 120F for DC)
• - Lower unit cost than DC LVDTs

45
Cost per unit
Alexandre Lenoble

• - Unguided armature
• - DC 485
• - AC 330
• - Spring-extended armature
• - DC 1359
• - AC 1156

46
Unguided armature
Alexandre Lenoble

• 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.

47
Unguided armature applications
Alexandre Lenoble

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

48
Captive (guided) armature
Alexandre Lenoble

• - Both static and dynamic applications
• Armature restrained and guided by a low-friction
  assembly

49
Captive (guided) armature
Alexandre Lenoble

• Advantages compared to unguided armature
• - Better for longer working range (up to 500mm)
• - Preferred when misalignment may occur

50
Spring-extended armature
Alexandre Lenoble

• - Armature restrained and guided by a
  low-friction assembly (as for captive armature)
• - Internal spring to continuously push the
  armature to its fullest possible extension

51
Spring-extended armature
Alexandre Lenoble

• Best suited for static or slow-moving
  applications
• - Lower range than captive armature (10 to 70mm)

52
LVDT Function
Alexandre Lenoble
Primary coil

Primary coil
Secondary coil 2
Secondary coil 1
Secondary coil 1
Secondary coil 2
Input to primary
Input to primary
Output from secondary coils
Output from secondary coils
Secondary coil 1 output (V1) Secondary coil 2
output (V2) V1 - V2
Secondary coil 1 output (V1) Secondary coil 2
output (V2) V1 - V2
Demodulated output
Demodulated output
53
Summary
Alexandre Lenoble

• LVDTs are robust equipment for measuring
  displacement
• AC LVDTs require separate signal conditioning
  equipment, while DC LVDTs include signal
  conditioning equipment on the device.

54
Summary
Alexandre Lenoble

• There are three types of LVDT unguided armature,
  captive armature, and spring-extended armature.
• AC LVDTs cost less than DC, but the entire
  measurement system must be considered.

55
Applications
Alexandre Lenoble

• LVDTs find lots of applications in
• - automation machinery
• - civil engineering
• - power generation
• - manufacturing
• - metal stamping
• - OEM (Original Equipment Manufacturer)
• - aeronautics
• - RD

56
Applications
Alexandre Lenoble

• 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.

57
Applications
Alexandre Lenoble

• 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.

58
References
Alexandre Lenoble

• www.dankuchma.com/cee498/presentations/LVDT20Jaso
  n20Hart.ppt
• Pr. Kurfesss lecture
• http//www.daytronic.com/products/trans/lvdt/defau
  lt.htm
• http//www.macrosensors.com