Sensors
Efrain Teran
Carol Young
Brian O’Saben
Optical Encoders
Efrain Teran
What are Optical Encoders ?
An Optical Rotary Encoder is an electro-mechanical device that
converts the angular position of a shaft to a digital code.
What are they used for?
 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.
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).
Optical Encoder parts
Light source: produces the light that
will “trigger” the photodetectors
during motion. Usually LEDs or IR
LEDs
Photodetector: electronic sensor
that reacts to light. Usually a
phototransistor or photodiode.
Code disk: has one or more tracks
with slits (windows) to allow light to
pass through.
Mask: collimates the beams of light
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.
Types of Optical Encoders
Incremental Optical Encoders:
• Single channel
• Dual channel
• Dual channel with Z index
Absolute Optical Encoders
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.
TYPES
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.
Voltage Lo
Hi
Lo
Binary
1
0
0
Hi Lo
1
0
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.
Channel A
Channel B
Lo
Lo
Hi
Lo
Hi
Hi
Lo
Hi
Repetitive sequence
Incremental Optical Encoder: Dual channel
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.
Z
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.
VIDEO:
https://www.youtube.com/watch?v=cn83jR2mchw
Absolute encoders:
Binary vs. Gray code
010
011
000
100
111
101
Transition possible results:
001
110
011 - 010 - 001 - 011- 111 - 100
Absolute encoders:
Binary vs. Gray code
011
001
010
000
110
100
111
Transition possible results:
101
010 - 110
Encoder Resolution
Absolute Optical Encoder
• 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”.
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°/(2N) = 360°/(28) = 1.4°
Encoder Resolution
Incremental Optical Encoder
• Resolution essentially depends on the number of
windows on the code disk
Resolution = 360/N
N = number of windows on code disk
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 windows
• BUT, we can increase resolution by using channels A and B
Encoder Resolution
Incremental Optical Encoder
• We may count rising and
falling edges in both
channel’s signals
Today’s standard
X4 Resolution = 360/4N
N = number of windows (slits or lines) on the code disk
Example:
(Sabri Centinkunt, page 236)
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.
𝑤𝑚𝑎𝑥
1,000,000 𝑝𝑢𝑙𝑠𝑒/𝑠𝑒𝑐
𝑟𝑒𝑣
=
= 400
= 24,000 𝑅𝑃𝑀
2,500 𝑝𝑢𝑙𝑠𝑒/𝑟𝑒𝑣
𝑠𝑒𝑐
Applications
Incremental Single channel
Incremental with Z index
Incremental Dual channel
Absolute Encoder
REFERENCES:
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/Increme
ntal-Versus-Absolute-Encoders.html
http://www.ni.com/white-paper/7109/en/
http://www.digikey.com/PTM/IndividualPTM.page?site=us&lang=en&ptm=2420
Laser Interferometer
Carol Young
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
measurements
Physics Background
Waves
• Light is an Electrometric wave and therefore
has wave properties.
http://en.wikipedia.org/wiki/File:Light-wave.svg
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.gif
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.
Homodyne Interferometer
(Michelson)
Mirror Reference
Mirror Moveable
(Sample)
Laser
Screen
Homodyne Interferometer
Analysis
• λ is the wavelength
of the light
• Lref is the distance to
the reference mirror
• L is the distance to
the moveable mirror
• n is the number of
fringes
Photograph of the interference fringes produced by a
Michelson interferometer.
Homodyne Interferometer
Uses
•
•
•
•
•
•
•
•
Absolute distance
Optical testing
Refractive index
Angles
Flatness
Straightness
Speed
Vibrations
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:Dopplereffectsourcemovingrig
htatmach0.7.gif
Physics Background
Beat Frequency
• Rate of constructive
and destructive
interference
Heterodyne Interferometer
• Produces two close
but not equal
frequencies
(Creating a Beat
Frequency)
• Doppler effect from
moving reflector
shifts the frequency
proportional to the
velocity
Heterodyne / Homodyne
Interferometer Comparison
• Comparing with a Homodyne Interferometer
– Can determine movement direction (but limited
range)
– More useful when direction of movement is
important
Heterodyne / Homodyne
Interferometer Comparison
• Homodyne
– Smooth surfaces
only
• Heterodyne
– Can be used for
• Distance to rough
surfaces
• Surface roughness
measurements
Xiaoyu Ding
Resolution
• XL-80 Laser Measurement System
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-lasersdevelopment-and-applications/precision-dimensional-metrology-based-on-afemtosecond-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 twowavelength heterodyne speckle interferometer”, Applied Optics
Linear Variable Differential
Transformer (LVDT)
Brian O’Saben
Outline
•
•
•
•
•
•
What is a LVDT?
How LVDTs Works
LVDT Properties
LVDT Support Electronics
Types of LVDTs
LVDT Applications
What is a LVDT?
• Linear variable differential transformer
• Electromechanical transducer measuring
linear displacement
What is a LVDT?
• Primary coil
– Energized with constant A/C
• Two identical secondary coils
– Symmetrically distributed
– Connected in opposition
• Ferromagnetic core
How LVDT works
• If core is centered
between S1 and S2
– Equal flux from each
secondary coil
– Voltage E1 = E2
How LVDT works
• If core is closer to S1
– Greater flux at S1
– Voltage E1 increases,
Voltage E2 decreases
– Eout=E1 – E2
How LVDT works
• If core is closer to S2
– Greater flux at S2
– Voltage E2 increases,
Voltage E1 decreases
– Eout=E2 – E1
How LVDT works
LVDT properties
•
•
•
•
•
•
•
Friction-free operation
Unlimited mechanical life
Infinite resolution
Separable coil and core
Environmentally robust
Fast dynamic response
Absolute output
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 shift
Types of LVDTs
• DC LVDT
–Signal conditioning equipment built in
–Pre-calibrated analog and/or digital
output
–Lower overall system cost
• AC LVDT
–Wide operating environments
• Shock and vibration
• Temperature
–Smaller package size
• Separate core
Types of LVDTs
• 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 lowfriction assembly
• Both static and dynamic applications
• working range (up to 500mm)
• Spring-loaded
• Core is restrained and guided by a lowfriction 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)
LVDT applications
•
•
•
•
•
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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 sensing
References
• http://www.macrosensors.com/lvdt_tutorial.html
• http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm
• http://www.directindustry.com/industrial-manufacturer/lvdt73930.html
• http://macrosensors.com/blog/view-entry/Why-Use-an-AC-LVDTversus-a-DC-LVDT-Linear-Positio/31/
• http://www.measspec.com/downloads/LVDT_Selection,_Handling_and_Installation_
Guidelines.pdf
• http://en.wikipedia.org/wiki/Linear_variable_differential_transform
er
• http://www.transtekinc.com/support/applications/LVDTapplications.html
• Lei Yang’s student lecture
Thank You!