1. ME 4447/6405 Sensors Microprocessor Control of Manufacturing Systems
and
Introduction to Mechatronics
Sensors
Optical Encoder: Ryder Winck
Laser Interferometer: Aaron Scott
LVDT: Alexandre Lenoble

2. Presentation Outline Optical Encoders Laser Interferometer
Ryder Winck
Presentation Outline
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
Linear Variable Displacement Transducer (LVDT)
What is a LVDT
Types of LVDTs

3. 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.

4. Types of Optical Encoders
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.

5. Fundamental Components
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.
Code disk is only moving part.

6. Optical Encoder Components
Ryder Winck
Optical Encoder Components

7. Other Components Stationary “masking” disk
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.

8. Quadrature Two tracks (A & B) at 90 degrees offset.
Ryder Winck
Quadrature
Two tracks (A & B) at 90 degrees offset.
Provide direction information.
Provides up to 4 times resolution.

9. Encoder Disks Incremental Disk Absolute Disks Binary Gray Code
Ryder Winck
Encoder Disks
Incremental Disk
Absolute Disks
Binary
Gray Code

10. Absolute Disk Codes Example: 3 bit binary code Bit 0 Bit 1 Bit 2
Ryder Winck
Absolute Disk Codes
Example: 3 bit binary code
Bit 0
Bit 1
Bit 2
Angle
Binary
Decimal
0-45
000
45-90
001 1
90-135
010 2
011 3
100 4
101 5
110 6
111 7

11. Problem with Binary Code
Ryder Winck
Problem with Binary Code
Angle
Binary
Decimal
0-45
000
45-90
001 1
90-135
010 2
011 3
100 4
101 5
110 6
111 7
One angle shift results in multiple bit changes.
Example: 1 => 2
001 (start at 1)
000 (turn off bit 0)
010 (turn on bit 1)

12. Problem with Binary Code
Ryder Winck
Problem with Binary Code
Angle
Binary
Decimal
0-45
000
45-90
001 1
90-135
010 2
011 3
100 4
101 5
110 6
111 7
One degree shift results in multiple bit changes.
Example: 1 => 2
001 (start at 1)
000 (turn off bit 0)
010 (turn on bit 1)
It looks like we went from 1 => 0 => 2

13. Gray Code One bit change per angle change. Bit 0 Bit 1 Bit 2
Ryder Winck
Gray Code
One bit change per angle change.
Angle
Binary
Decimal
0-45
000
45-90
001 1
90-135
011 2
010 3
110 4
111 5
101 6
100 7
Bit 0
Bit 1
Bit 2

14. Converting from Gray Code to Binary Code
Ryder Winck
Converting from Gray Code to Binary Code
Copy MSB.
If MSB is 1, write 1s until next 1 is met If MSB is 0, write 0s until next 1 is met.
When 1 is met, logically switch what you are writing (1=>0 or 0=>1).
Continue writing the same logical until next 1 is met.
Loop back to step 3.

15. Example: Convert 0010 to Binary Code
Ryder Winck
Example: Convert 0010 to Binary Code
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
Example: Convert 1110 to Binary Code
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
Encoder Reliability and Errors
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 Reliability and Errors
Encoder errors
Quantization Error – Dependent on digital word size.
Assembly Error – Dependent on eccentricity of rotation (is track center of rotation=center of rotation of disk)
Manufacturing tolerances – Code printing accuracy, sensor position, and irregularities in signal generation.

19. Encoder Reliability and Errors
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.

20. Encoder Reliability and Errors
Ryder Winck
Encoder Reliability and Errors
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 Any linear/rotary position/velocity sensing
Ryder Winck
Applications
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. Ryder Winck
References

23. Laser Interferometers
Aaron Scott
Laser Interferometers
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
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

25. What is a Laser Interferometer?
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

26. What is a Laser Interferometer?
Aaron Scott
What is a Laser Interferometer?
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
What is a Laser Interferometer?
Why “lasers” ?
High coherence
Collimated
Predictable
Frequency known

28. Types of Laser Interferometers
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

29. Types of Laser Interferometers
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

30. Aaron Scott
How Do They Work?
Homodyne – already discussed (Michelson interferometer)
Heterodyne
Dual frequency,
polarized
laser source
Polarizing
beam splitter

31. Resolutions and Sampling Rate
Aaron Scott
Resolutions and Sampling Rate
Representative values
Resolution
10 nm digital resolution
sub-angstrom analog resolution achieved by “external interpolation”
Angstrom, Å = 1  m
Sampling Rate
20 MHz

32. Aaron Scott
Applications
Michelson used his interferometer to measure the rotation rate of the Earth
Perimeter of his ring was 1.9 km

33. Applications 3 axis ring laser gyro
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

34. Applications Distance measurement Velocity measurement
Aaron Scott
Applications
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. Aaron Scott
Applications
Other measurements made possible by re-arrangements of the light paths. We can measure
angle
straightness
flatness
parallelism

36. Applications LIGO Laser Interferometer Gravitational-Wave Observatory
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

37. Applications LISA Laser Interferometer Space Antenna
Aaron Scott
Applications
LISA Laser Interferometer Space Antenna
NASA/ESA expected
Similar to LIGO but MUCH larger
5 gigameter arm length
3 interferometers in 1

38. References http://en.wikipedia.org/wiki/Interferometry
Aaron Scott
References
DVD: “Albert A. Michelson Laboratory, History and Heritage” Public Release, NAWCWD, China Lake

39. Alexandre Lenoble
LVDT

40. What is a LVDT ? Linear Variable Displacement Transducer
Alexandre Lenoble
What is a LVDT ?
Linear Variable Displacement Transducer
- Electrical transformer used to measure linear displacement

41. Construction Primary coil and 2 symmetric secondary coils
Alexandre Lenoble
Construction
Secondary #1
Primary
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 - Distinction by : - Power supply : Type of armature :
Alexandre Lenoble
LVDT Types
- Distinction by :
- Power supply :
- DC
- AC
Type of armature :
- Unguided
- Captive (guided)
- Spring-extended

43. DC LVDTs Easy to install
Alexandre Lenoble
DC LVDTs
Easy to install
Signal conditioning easier (equipment part of LVDT)
Can operate from dry cell batteries
- High unit cost

44. AC LVDTs Small size Very accurate – Excellent resolution (0.1 µm)
Alexandre Lenoble
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

45. Cost per unit - Unguided armature : - Spring-extended armature
Alexandre Lenoble
Cost per unit
- Unguided armature :
- DC : $485
- AC : $330
- Spring-extended armature
- DC : $1359
- AC : $1156

46. Alexandre Lenoble
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.

47. Unguided armature : applications
Alexandre Lenoble
Unguided armature : applications
Well-suited for short-range (1 to 50mm), high speed applications (high-frequency vibration)

48. Captive (guided) armature
Alexandre Lenoble
Captive (guided) armature
- Both static and dynamic applications
Armature restrained and guided by a low-friction assembly

49. Captive (guided) armature
Alexandre Lenoble
Captive (guided) armature
Advantages compared to unguided armature :
- Better for longer working range (up to 500mm)
- Preferred when misalignment may occur

50. Spring-extended armature
Alexandre Lenoble
Spring-extended armature
- 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
Spring-extended armature
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 #1
Secondary coil #2
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 LVDTs are robust equipment for measuring displacement
Alexandre Lenoble
Summary
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. Alexandre Lenoble
Summary
There are three types of LVDT: unguided armature, captive armature, and spring-extended armature.
AC LVDT’s cost less than DC, but the entire measurement system must be considered.

55. Applications LVDTs find lots of applications in :
Alexandre Lenoble
Applications
LVDTs find lots of applications in :
- automation machinery
- civil engineering
- power generation
- manufacturing
- metal stamping
- OEM (Original Equipment Manufacturer)
- aeronautics
- R&D

56. Applications Examples for OEM :
Alexandre Lenoble
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.

57. Applications Examples for civil engineering :
Alexandre Lenoble
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.

58. Alexandre Lenoble
References
Pr. Kurfess’s lecture