Optical Encoders, Laser
Interferometer, LVDT
Rushi Vyas
Xiaoyu Ding
Lei Yang
Rushi Vyas
Outline
• Optical Encoders: Theory and applications
– Fundamental Components
– Theory
– Types of optical encoders
– Quadrature
– Errors
– Applications
Rushi Vyas
What are Encoders
• An accessory to a mechanical device that translates
mechanical motion into a measurable electrical
signal Digital or Analog (preferably digital).
• Optical Encoders
– Use light & photosensors to produce digital code
– Most popular type of encoder.
• Can be linear or rotary.
Rushi Vyas
Optical Encoders: Components
• Code Disk: Used to produce
different light patterns on a photo
detector assembly from a
stationary light source.
• Code Disk: Determines the Optical
Encoder type.
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Optical Encoders: Components
• Light source(s)
– LEDs or IR LEDs provide light source.
– Light is collimated using a lens to make the beams
parallel.
• Photodetector(s)
– Either Photodiodes or Phototransistors.
• Opaque disk (Code Disk)
– One or more “tracks” with slits to allow light to
pass through.
Rushi Vyas
Optical Encoders: Theory
LED
Code
Disk
Photosensor
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Optical Encoder Types
• Incremental Encoders: Mechanical motion computed by
measuring consecutive “on” states.
• Absolute Encoders: Digital data produced by code disk, which
carries position information.
Incremental Encoder
code Disk
Absolute Encoder
code Disk
Lab 3
Rushi Vyas
Standard Binary Encoding
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
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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
Rushi Vyas
Gray Encoding
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
Notice only 1 bit has to be
changed for all transitions.
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Quadrature
• ❖ Quadrature describes two signals 90° out of phase
• ❖ Used to determine direction of measurement
• ❖ Only two directions possible, A leads B or B leads
A
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Quadrature
An incremental rotary encoder, also known as a quadrature encoder or a relative
rotary encoder, has two outputs called quadrature outputs that are 90 deg out of
phase. Direction of rotation can be determined from output sequence.
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Encoder Resolution:
• Absolute Optical Encoder
– Resolution = 360º/(2n)
– n = number of encoder bits
– Measures the rotational displacement that can be
measured per bit change.
• Incremental Optical Encoder
– Resolution = 360/n
– N = number of windows on code disk
– Resolution can be increased by reading both rising and
falling edges (
) and by using quadrature
(
).
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Examples
Number of bits on encoder code
disk n = 3
Resolution = 360º/23 = 45º
Number of bits on encoder code
disk n = 4
Resolution = 360º/24 = 22.5º
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Example:
• What resolution absolute optical encoder is
needed to be able to measure rotational
displacements of 1.5 degrees?
– N=?
– Resolution = 1.5 degrees
For absolute optical encoder:
Resolution=360/2N =1.5 → N = 7.91 ≈ 8 bits
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Example:
• What number of slits (windows) are needed
on the code disk of an incremental optical
encoder to be able to measure rotational
displacements of 1.5 degrees?
– N=?
– Resolution = 1.5 degrees
For incremental optical encoder
Resolution=360/N =1.5 → N = 240 windows
Rushi Vyas
Optical Encoders: Reliability
• Encoder errors
1.Quantization Error – Dependent on digital word
size.
2.Assembly Error – Due to instability in rotational
motion of code disk
3.Manufacturing tolerances – Code printing
accuracy, sensor position, and irregularities in
signal generation.
Rushi Vyas
Optical Encoders: Reliability
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…
7. Diffraction of light: occurs due to edge of codes
disk windows. Fixed in newer encoders by using
mask and minimizing distance to photodetector.
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Applications
• Primarily used in motors for
monitoring velocity and position.
– Robotics
– Conveyor belts
– Locomotives: Automobiles,
planes..
– Tachometers
Rushi Vyas
References
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Kawasaki Industries Optical Encoders: www.khi.co.jp
Compumotors: www.compumotor.com
ME class notes: Dr. Kurfess, Georgia Tech
www.motioncontrol-info.com
Sensors: Fall 08. ME6405
Wikipedia
Computer Optical Products: http://www.opticalencoder.com/
Laser interferometer
Xiaoyu Ding
Xiaoyu Ding
Laser Interferometer
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What’s laser interferometer?
The principle of standard interferometer
Types of interferometers
Applications
Xiaoyu Ding
What’s a Laser Interferometer
• Laser Interferometer:
– the instrument used for high precision
measurements (distance, angles…. etc.)
– it uses interferometry as the basis for
measurement.
– it uses the very small, stable and accurately
defined wavelength of laser as a unit of measure.
Xiaoyu Ding
Physics Review
• Diffraction
Diffraction is a sure
sign that whatever is
passing through the
hole is a wave.
Diffraction of Water Waves
Xiaoyu Ding
Physics Review
• Diffraction of Light
Light, just like a water
wave, does spread out
behind a hole is the hole
is sufficiently small.
Light is a electromagnetic
wave.
Diffraction of light Wave
Xiaoyu Ding
Physics Review
• A Double-Slit Interference Experiment
Interference of Light
Xiaoyu Ding
Principle of Michelson Interferometer
• Albert Michelson (1852~1931)
– the first American scientist to receive
a Nobel prize, invented the optical
interferometer.
– The Michelson interferometer has
been widely used for over a century
to make precise measurements of
wavelengths and distances.
Albert Michelson
Xiaoyu Ding
Principle of Michelson Interferometer
• Michelson Interferometer
1) Separation
2) Recombination
3) Interference
A Michelson Interferometer for use on an optical table
Xiaoyu Ding
Principle of Michelson Interferometer
• Analyzing Michelson Interferometer
– The central spot in the fringe pattern alternates
between bright and dark when Mirror M2 moves.
If we can know the spacing distance
of M2 between two sequent central
bright spots and the number of
central bright spots appeared, then
we can calculate how long M2
moved.
Photograph of the interference fringes
produced by a Michelson interferometer.
Xiaoyu Ding
Principle of Michelson Interferometer
• Analyzing Michelson Interferometer
– Spacing distance of M2 is  2 .
laser has very small, stable
and accurately defined
wavelength which can help
us get high precision
measurement.
Xiaoyu Ding
Types of Laser Interferometers
• Homodyne Laser Interferometer (Standard)
– It is based on interference of laser waves
(Michelson interferometer)
• Heterodyne Laser interferometer
– It is based on Doppler Effect.
Xiaoyu Ding
Principle of Heterodyne Laser
interferometer
• Doppler Effect
Doppler Effect:
The change of frequency
when a source moves relative
to an observer.
f0
f0
f 
, f 
1  vs v
1  vs v
f
f0
f
We can get the velocity of an object by
measure the frequency change between
incident laser wave and reflected laser
wave.
Xiaoyu Ding
Applications
• Measurement of Distance
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1) frequency stabilized He-Ne laser tube
2) combination of beam-splitter and retroreflector
3) a moving retroreflector
4) detection electronics
Aerotech’s LZR3000 Series Laser Interferometer System
Xiaoyu Ding
Applications
• Other Applications
– Measure angles, flatness, straightness, velocity
and vibrations, etc.
Rearrangements
of the light paths
Xiaoyu Ding
Resolution
• XL-80 Laser Measurement System
Xiaoyu Ding
References
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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
PHYSICS FOR SCIENTISTS AND ENGINEERS, Randall D. Knight, 2003.
Linear Variable Differential
Transformer（LVDT）
Lei Yang
Lei Yang
LVDT
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What is LVDT?
Construction of LVDT
How LVDT works
Support electronics of LVDT
Properties of LVDT
Types of LVDT
Applications of LVDT
Lei Yang
What is a LVDT
• Linear variable differential transformer
• Electrical transformer measuring linear
displacement
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Construction of LVDT
• One Primary coil
• Two symmetric secondary coils
• Ferromagnetic core
Primary coil
•The primary coil is energized with a A.C.
•The two secondary coils are identical,
symmetrically distributed.
•The two secondary coils are connected in
opposition
Ferromagnetic
core
Secondary
coils
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Recall of conventional transformer
• Mutual induction
• the secondary voltage proportional to the primary
voltage
• The transformer core is fixed
• Energy transferred is high
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How LVDT works
• If the core is located midway
between S1 and S2
• Equal flux is coupled to each
secondary.
• Voltage E1 and E2 are equal.
• The differential voltage output, (E1 E2 ), is zero.
• This core position is called null
point.
Lei Yang
How LVDT works
• If the core is moved closer to
S1 than to S2
• More flux is coupled to S1 than
S2 .
• The induced voltage E1 is
increased while E2 is
decreased.
• The differential voltage is (E1 E2).
Lei Yang
How LVDT works
• If the core is moved closer to
S2 than to S1
• More flux is coupled to S2 than
to S1 .
• The induced E2 is increased as
E1 is decreased.
• The differential voltage is (E2 E1).
Lei Yang
How LVDT works
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Support electronics of LVDT
• LVDT signal conditioning equipment
• Supplying excitation power for an LVDT
• typically 3 V rms at 3 kHz
• Converting AC output into DC signals with
directional information from the 180 degree
output phase shift
External electronics
Self-contained electronics
e.g. DC-LVDT
Lei Yang
Properties of LVDT
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Friction-Free Operation
Infinite Resolution
Unlimited Mechanical Life
Single Axis Sensitivity
Environmentally Robust
Null Point Repeatability
Fast Dynamic Response
Absolute Output
Lei Yang
Types of LVDT
• DC LVDT
• Signal conditioning easier
• Can operate from dry cell batteries
• High unit cost
• AC LVDT
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Small size
Very accurate – Excellent resolution (0.1 µm)
Can operate with a wide temperature range
Lower unit cost
Lei Yang
• Free core
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Types of LVDT
Core is completely separable from the transducer body
Well-suited for short-range (1 to 50mm), high speed
applications (high-frequency vibration)
• Guided core
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Core is restrained and guided by a low-friction assembly
Both static and dynamic applications
working range (up to 500mm)
• Spring-extended core
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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)
Lei Yang
Example of commercial LVDT
• SE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors
Lei Yang
Applications of LVDT
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For power generation
1.
2.
Conditioning valves for large and medium steam turbines.
Reheat and stop valves for large and medium steam
turbines.
Feed water boiler pump valve positioning.
Natural gas fuel valve position for gas turbines for throttle
control.
Monitoring hydraulic fluid level in reservoir of feed water
pumps in nuclear reactor core.
3.
4.
5.
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Applications of LVDT
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For manufacturing
1.
Measuring final height placement for automotive wheel
trim
Measuring injector height for diesel engines Feed water
boiler pump valve positioning.
Thickness measuring in multiple locations of fly-wheel to
insure balance.
Controlling depth of hole during machining operations in a
rotary transfer machine.
Providing indication and feedback position of rocket engine
nozzle actuators during testing.
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4.
5.
Lei Yang
Other Applications
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Automation Machinery
Civil / Structural Engineering
Metal Stamping / Forming
OEM
Pulp and Paper
Industrial Valves
R&D and Test
Automotive Racing
Lei Yang
References
• http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/index.h
tml#automation
• http://en.wikipedia.org/wiki/Linear_variable_differential_transformer
• http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm
• http://www.directindustry.com/industrial-manufacturer/lvdt-73930.html
• http://www.macrosensors.com/lvdt_macro_sensors/lvdt_products/lvdt_p
osition_sensors/dc_lvdt/free_core_dc/se_750_single_ended.html
• Alexandre Lenoble’s lecture
Lei Yang
Thank you!