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 • • • • • • 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!