optical encoders, laser interferometer, lvdt rushi vyas xiaoyu ding lei yang
TRANSCRIPT
Optical Encoders, Laser Interferometer, LVDT
Rushi VyasXiaoyu Ding
Lei Yang
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.
Rushi Vyas
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: TheoryLED Code
DiskPhoto-sensor
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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 EncodingAngle 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
Problem with Binary Code
• 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)
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 EncodingAngle 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.
Rushi Vyas
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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QuadratureAn 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 ( ).
Rushi Vyas
ExamplesNumber 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
Rushi Vyas
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
Rushi Vyas
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 errors1.Quantization Error – Dependent on digital word
size.2.Assembly Error – Due to instability in rotational
motion of code disk3.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• 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/
Rushi Vyas
Laser interferometer
Xiaoyu Ding
Laser Interferometer
• 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 of Water Waves
Diffraction is a sure sign that whatever is passing through the hole is a wave.
Xiaoyu Ding
Physics Review
• Diffraction of LightLight, just like a water wave, does spread out behind a hole is the hole is sufficiently small.
Light is a electromagnetic wave.
Xiaoyu Ding
Diffraction of light Wave
Physics Review
• A Double-Slit Interference Experiment
Xiaoyu Ding
Interference of Light
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 Interferometer1) Separation2) Recombination3) 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.
Photograph of the interference fringes produced by a Michelson interferometer.
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.
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 EffectDoppler Effect: The change of frequency when a source moves relative to an observer.
0 0,1 1s s
f ff f
v v v v
ff 0f
Xiaoyu Ding
We can get the velocity of an object by measure the frequency change between incident laser wave and reflected laser wave.
Applications
• Measurement of Distance– 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.
Xiaoyu Ding
Rearrangements of the light paths
Resolution
• XL-80 Laser Measurement System
Xiaoyu Ding
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• PHYSICS FOR SCIENTISTS AND ENGINEERS, Randall D. Knight, 2003.
Xiaoyu Ding
Linear Variable Differential Transformer ( LVDT)
Lei Yang
LVDT
• 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
Lei Yang
Construction of LVDT
• One Primary coil• Two symmetric secondary coils• Ferromagnetic core
•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
Primary coil
Secondary coils
Ferromagnetic core
Lei Yang
Recall of conventional transformer
• Mutual induction• the secondary voltage proportional to the primary
voltage• The transformer core is fixed• Energy transferred is high
Lei Yang
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 worksLei Yang
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• 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• Small size• Very accurate – Excellent resolution (0.1 µm)• Can operate with a wide temperature range• Lower unit cost
Lei Yang
Types of LVDT• Free core• 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 low-friction assembly• Both static and dynamic applications• working range (up to 500mm)
• Spring-extended core• 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
• For power generation1. Conditioning valves for large and medium steam turbines. 2. Reheat and stop valves for large and medium steam
turbines. 3. Feed water boiler pump valve positioning. 4. Natural gas fuel valve position for gas turbines for throttle
control. 5. Monitoring hydraulic fluid level in reservoir of feed water
pumps in nuclear reactor core.
Lei Yang
Applications of LVDT
• For manufacturing1. Measuring final height placement for automotive wheel
trim 2. Measuring injector height for diesel engines Feed water
boiler pump valve positioning. 3. Thickness measuring in multiple locations of fly-wheel to
insure balance. 4. Controlling depth of hole during machining operations in a
rotary transfer machine. 5. Providing indication and feedback position of rocket engine
nozzle actuators during testing.
Lei Yang
Other Applications
• 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.html#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_position_sensors/dc_lvdt/free_core_dc/se_750_single_ended.html• Alexandre Lenoble’s lecture
Lei Yang
Thank you!
Lei Yang