Efrain Teran Carol Young Brian O’Saben 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 Hi Lo Binary 0 1 0 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 Lo Hi Hi Lo Repetitive sequence Channel B Lo Lo Hi Hi

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 010 001 011 000 100 111 101 110 Transition possible results: 011 - 010 - 001 - 011- 111 - 100

Absolute encoders: Binary vs. Gray code 011 001 010 000 110 100 111 101 Transition possible results: 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

(Sabri Centinkunt, page 236) Example: 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. 𝑤 𝑚𝑎𝑥 = 1,000,000 𝑝𝑢𝑙𝑠𝑒/𝑠𝑒𝑐 2,500 𝑝𝑢𝑙𝑠𝑒/𝑟𝑒𝑣 =400 𝑟𝑒𝑣 𝑠𝑒𝑐 =24,000 𝑅𝑃𝑀

Applications Incremental Single channel Incremental Dual channel Incremental with Z index 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/Incremental-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

Noise cancellation

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 Crest and trough

Physics Background Diffraction and Interference http://en.wikipedia.org/wiki/File:Doubleslit3Dspectrum.gif Diffraction Light spreads after passing a narrow point Interference superposition of two waves to form new wave with different amplitude Constructive or Destructive Young’s double-slit experiment

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 Laser Mirror Moveable (Sample) Beam Splitter 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 Since lambda changes with the refractive index of air can use to find lambda too. 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:Dopplereffectsourcemovingrightatmach0.7.gif

Physics Background Beat Frequency Rate of constructive and destructive interference Magenta + Cyan = blue

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 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-lasers-development-and-applications/precision-dimensional-metrology-based-on-a-femtosecond-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 two-wavelength 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 Two identical secondary coils 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 Inductance! The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown in the middle graph. This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. This is shown in the bottom graph, where the polarity of the output signal represents the core's positional relationship to the null point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity. The output characteristics of an LVDT vary with different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.

How LVDT works If core is closer to S1 Greater flux at S1 Voltage E1 increases, Voltage E2 decreases Eout=E1 – E2 The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown in the middle graph.

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 LVDTs have certain significant features and benefits, most of which derive from its none contact construction. Friction-Free Operation One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems. Infinite Resolution Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, its resolution is very high. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its outstanding repeatability. Unlimited Mechanical Life Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It means that LVDT is very reliable. It is also highly desirable in many industrial process control and factory automation systems. Single Axis Sensitivity An LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn't travel in a precisely straight line. Null Point Repeatability The location of an LVDT's intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and high-performance servo balance instruments. Fast Dynamic Response The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core's slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner. Absolute Output An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT's output value will be the same as it was before the power failure occurred.

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 Although an LVDT is an electrical transformer, it requires AC power of an amplitude and frequency quite different from ordinary power lines to operate properly (typically 3 V rms at 3 kHz). Supplying this excitation power for an LVDT is one of several functions of LVDT support electronics, which is also sometimes known as LVDT signal conditioning equipment. Other functions include converting the LVDT's low level AC voltage output into high level DC signals that are more convenient to use, decoding directional information from the 180 degree output phase shift as an LVDT's core moves through the null point, and providing an electrically adjustable output zero level. A variety of LVDT signal conditioning electronics is available, including chip-level and board-level products for OEM applications as well as modules and complete laboratory instruments for users. The support electronics can also be self-contained, as in the DC-LVDT shown here. These easy-to-use position transducers offer practically all of the LVDT's benefits with the simplicity of DC-in, DC-out operation. Of course, LVDTs with integral electronics may not be suitable for some applications, or might not be packaged appropriately for some installation environments.

Types of LVDTs DC LVDT AC 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

Types of LVDTs Separate core Guided core Spring-loaded 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-loaded 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) Internal spring to continuously push the armature to its fullest possible extension Lower range than captive armature (10 to 70mm)

LVDT applications 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 Talk about

References http://www.macrosensors.com/lvdt_tutorial.html http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm http://www.directindustry.com/industrial-manufacturer/lvdt-73930.html http://macrosensors.com/blog/view-entry/Why-Use-an-AC-LVDT-versus-a-DC-LVDT-Linear-Positio/31/ http://www.meas-spec.com/downloads/LVDT_Selection,_Handling_and_Installation_Guidelines.pdf http://en.wikipedia.org/wiki/Linear_variable_differential_transformer http://www.transtekinc.com/support/applications/LVDT-applications.html Lei Yang’s student lecture

Thank You!