Sensors

SensorsEfrain TeranCarol YoungBrian OSaben

Optical EncodersEfrain 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.

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.

What are they used for?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 Code disk: has one or more tracks with slits (windows) to allow light to pass through.

Photodetector: electronic sensor that reacts to light. Usually a phototransistor or photodiode. Light source: produces the light that will trigger the photodetectors during motion. Usually LEDs or IR LEDs 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 EncodersAbsolute Optical EncodersIncremental Optical Encoders:Single channelDual channelDual channel with Z index

Incremental EncodersGenerate 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.

TYPESIncremental Optical Encoder: Single channel

Has only one output channel for encoding information.Used in unidirectional systems or where you dont need to know direction.

Lo Hi Lo Hi Lo0 1 0 1 0VoltageBinaryIncremental Optical Encoder: Dual channelThe 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.

Lo Hi Hi LoChannel ALo Lo Hi HiChannel BRepetitive sequenceIncremental Optical Encoder: Dual channel

Incremental Optical Encoder: Dual channel with Z indexSome 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. Doesnt hold after power down.

ZAbsolute EncodersProvides 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

000111001010011100101110Transition possible results:011 - 010 - 001 - 011- 111 - 100

Absolute encoders:Binary vs. Gray code000100001011010110111101Transition possible results:010 - 110

Encoder ResolutionResolution can be given in number of bits or degreesDepends on the number of tracks on the code disk. Each track requires an output signal, also known as an encoder bit.Absolute Optical EncoderResolution = 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 ResolutionResolution essentially depends on the number of windows on the code diskIncremental Optical EncoderResolution = 360/NN = number of windows on code diskBUT, we can increase resolution by using channels A and BExample: 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

Encoder ResolutionIncremental Optical Encoder

X4 Resolution = 360/4NN = number of windows (slits or lines) on the code diskTodays standardWe may count rising and falling edges in both channels signals (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.Absolute EncoderIncremental Single channelIncremental Dual channelIncremental with Z indexApplications

Mechatronics, Sabri Cetinkunt, Wiley, 2007. Section 6.4.3http://en.wikipedia.org/wiki/Rotary_encoderhttp://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041235/Incremental-Versus-Absolute-Encoders.htmlhttp://www.ni.com/white-paper/7109/en/http://www.digikey.com/PTM/IndividualPTM.page?site=us&lang=en&ptm=2420

REFERENCES:

Noise cancellationLaser InterferometerCarol YoungWhat 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 BackgroundWavesLight is an Electrometric wave and therefore has wave properties.

http://en.wikipedia.org/wiki/File:Light-wave.svgCrest and trough26Physics BackgroundDiffraction and InterferenceDiffractionLight spreads after passing a narrow pointInterferencesuperposition of two waves to form new wave with different amplitude Constructive or Destructive

http://en.wikipedia.org/wiki/File:Doubleslit3Dspectrum.gif

Youngs double-slit experiment27Types 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)

LaserMirror ReferenceMirror Moveable(Sample)Beam SplitterScreen

Homodyne InterferometerAnalysis

Photograph of the interference fringes produced by a Michelson interferometer. is the wavelength of the lightLref is the distance to the reference mirrorL is the distance to the moveable mirrorn is the number of fringes

Since lambda changes with the refractive index of air can use to find lambda too.30Homodyne InterferometerUsesAbsolute distanceOptical testingRefractive indexAngles Flatness StraightnessSpeedVibrations

Physics BackgroundDoppler EffectPoint creating a wave and movementWave ahead of point has higher frequencyWave behind point has lower frequencyFrequency change corresponds to velocityhttp://en.wikipedia.org/wiki/File:Dopplereffectsourcemovingrightatmach0.7.gif

Physics BackgroundBeat FrequencyRate of constructive and destructive interference

Magenta + Cyan = blue33Heterodyne InterferometerProduces two close but not equal frequencies (Creating a Beat Frequency)Doppler effect from moving reflector shifts the frequency proportional to the velocity

Heterodyne / HomodyneInterferometer ComparisonComparing with a Homodyne Interferometer

Can determine movement direction (but limited range)More useful when direction of movement is important

Heterodyne / HomodyneInterferometer ComparisonHomodyneSmooth surfaces onlyHeterodyneCan be used forDistance to rough surfaces Surface roughness measurements

ResolutionXL-80 Laser Measurement System

Xiaoyu DingReferenceshttp://www.aerotech.com/products/engref/intexe.htmlhttp://www.renishaw.com/en/interferometry-explained--7854http://en.wikipedia.org/wiki/Michelson_interferometerhttp://en.wikipedia.org/wiki/Interferometryhttp://en.wikipedia.org/wiki/Doppler_effectwww.ljmu.ac.uk/GERI/GERI_Docs/interferometry_presentation(1).ppthttp://www.olympus-controls.com/documents/GEN-NEW-0117.pdfhttp://www.lambdasys.com/product/LEOI-20.htmhttp://www.intechopen.com/books/advances-in-solid-state-lasers-development-and-applications/precision-dimensional-metrology-based-on-a-femtosecond-pulse-laserhttp://en.wikipedia.org/wiki/Fringe_shifthttp://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 OSabenOutlineWhat is a LVDT?How LVDTs WorksLVDT PropertiesLVDT Support ElectronicsTypes of LVDTsLVDT Applications

What is a LVDT? Linear variable differential transformerElectromechanical transducer measuring linear displacement

What is a LVDT?Primary coilEnergized with constant A/CTwo identical secondary coilsSymmetrically distributedConnected in oppositionFerromagnetic core

How LVDT worksIf core is centered between S1 and S2Equal flux from each secondary coilVoltage 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.

43How LVDT worksIf core is closer to S1Greater flux at S1Voltage E1 increases, Voltage E2 decreasesEout=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.

44How LVDT worksIf core is closer to S2Greater flux at S2Voltage E2 increases, Voltage E1 decreasesEout=E2 E1

How LVDT works

LVDT propertiesFriction-free operationUnlimited mechanical lifeInfinite resolution Separable coil and coreEnvironmentally robustFast dynamic responseAbsolute output

LVDTs have certain significant features and benefits, most of which derive from its none contact construction.

Friction-Free OperationOne 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 LifeBecause 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 RepeatabilityThe 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 ResponseThe 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.

47LVDT support electronicsLVDT signal conditioning equipmentSupply excitation power for the LVDTTypically 3 Vrms at 3 kHzConvert low level A/C output to high level DC signalsGives 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.

48Types of LVDTsDC LVDTSignal conditioning equipment built inPre-calibrated analog and/or digital outputLower overall system costAC LVDTWide operating environments Shock and vibrationTemperatureSmaller package size

Types of LVDTsSeparate coreCore is completely separable from the transducer bodyWell-suited for short-range (1 to 50mm), high speed applications (high-frequency vibration)Guided coreCore is restrained and guided by a low-friction assemblyBoth static and dynamic applicationsworking range (up to 500mm)Spring-loadedCore is restrained and guided by a low-friction assemblyInternal spring to continuously push the core to its fullest possible extensionBest suited for static or slow-moving applicationsLower range than guided core(10 to 70mm)

Internal spring to continuously push the armature to its fullest possible extensionLower range than captive armature (10 to 70mm)

LVDT applicationsIndustrial gaging systemsElectronic dial indicatorsWeighing systemsCrankshaft balancerFinal product inspection (checking dimensions)Octane analyzer (provides displacement feedback for Waukesha engine)Valve position sensing

Talk about 51References http://www.macrosensors.com/lvdt_tutorial.htmlhttp://www.rdpe.com/displacement/lvdt/lvdt-principles.htmhttp://www.directindustry.com/industrial-manufacturer/lvdt-73930.htmlhttp://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.pdfhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://www.transtekinc.com/support/applications/LVDT-applications.htmlLei Yangs student lecture

Thank You!