Sensors F09

Optical Encoders, Laser Interferometer, LVDTRushi VyasXiaoyu DingLei Yang

OutlineOptical Encoders: Theory and applicationsFundamental ComponentsTheoryTypes of optical encodersQuadratureErrorsApplications

Rushi Vyas

What are EncodersAn accessory to a mechanical device that translates mechanical motion into a measurable electrical signal Digital or Analog (preferably digital).Optical EncodersUse light & photosensors to produce digital code Most popular type of encoder.Can be linear or rotary.

Rushi Vyas

Optical Encoders: ComponentsCode 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

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Optical Encoders: ComponentsLight 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: TheoryLEDCode DiskPhoto-sensorRushi Vyas

Optical Encoder TypesIncremental Encoders: Mechanical motion computed by measuring consecutive on states. Absolute Encoders: Digital data produced by code disk, which carries position information. Incremental Encoder code DiskAbsolute Encoder code DiskLab 3Rushi Vyas

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Standard Binary EncodingRushi Vyas

AngleBinaryDecimal0-45000045-90001190-1350102135-1800113180-2251004225-2701015270-3151106315-3601117

Problem with Binary CodeOne angle shift results in multiple bit changes.Example: 1 => 2001(start at 1)000(turn off bit 0)010(turn on bit 1)Rushi Vyas


Gray EncodingNotice 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 ARushi Vyas

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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. Rushi Vyas

Encoder Resolution:Absolute Optical EncoderResolution = 360/(2n)n = number of encoder bitsMeasures the rotational displacement that can be measured per bit change.Incremental Optical EncoderResolution = 360/nN = number of windows on code diskResolution can be increased by reading both rising and falling edges ( ) and by using quadrature ( ). Rushi Vyas

ExamplesNumber of bits on encoder code disk n = 3Resolution = 360/23 = 45Number of bits on encoder code disk n = 4Resolution = 360/24 = 22.5Rushi 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 bitsRushi 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 encoderResolution=360/N =1.5 N = 240 windowsRushi Vyas

Optical Encoders: ReliabilityEncoder errorsQuantization Error Dependent on digital word size.Assembly Error Due to instability in rotational motion of code diskManufacturing tolerances Code printing accuracy, sensor position, and irregularities in signal generation.

Rushi Vyas

Optical Encoders: ReliabilityStructural Limitations Disk Deformation, physical loads on shaft.Coupling Error Gear backlash, belt slippage, etcAmbient Effects Vibration, temperature, light noise, humidity, etcDiffraction of light: occurs due to edge of codes disk windows. Fixed in newer encoders by using mask and minimizing distance to photodetector.

Rushi Vyas

ApplicationsPrimarily used in motors for monitoring velocity and position.RoboticsConveyor beltsLocomotives: Automobiles, planes..Tachometers

Rushi Vyas

ReferencesKawasaki Industries Optical Encoders: www.khi.co.jpCompumotors: www.compumotor.comME class notes: Dr. Kurfess, Georgia Techwww.motioncontrol-info.comSensors: Fall 08. ME6405WikipediaComputer Optical Products: http://www.opticalencoder.com/

Rushi Vyas

Laser interferometerXiaoyu Ding

Laser InterferometerWhats laser interferometer?The principle of standard interferometerTypes of interferometersApplicationsXiaoyu Ding

Whats a Laser InterferometerLaser 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 ReviewDiffraction

Diffraction of Water WavesDiffraction is a sure sign that whatever is passing through the hole is a wave.Xiaoyu Ding

Physics ReviewDiffraction 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 DingDiffraction of light Wave

Physics ReviewA Double-Slit Interference ExperimentXiaoyu DingInterference of Light

Principle of Michelson InterferometerAlbert 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 MichelsonXiaoyu Ding

Principle of Michelson InterferometerMichelson InterferometerSeparationRecombinationInterferenceA Michelson Interferometer for use on an optical tableXiaoyu Ding

Principle of Michelson InterferometerAnalyzing Michelson InterferometerThe 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 InterferometerAnalyzing Michelson InterferometerSpacing distance of M2 is .

laser has very small, stable and accurately defined wavelength which can help us get high precision measurement.Xiaoyu Ding

Types of Laser InterferometersHomodyne Laser Interferometer (Standard)It is based on interference of laser waves (Michelson interferometer)

Heterodyne Laser interferometerIt is based on Doppler Effect.Xiaoyu Ding

Principle of Heterodyne Laser interferometerDoppler EffectDoppler Effect: The change of frequency when a source moves relative to an observer.

Xiaoyu DingWe can get the velocity of an object by measure the frequency change between incident laser wave and reflected laser wave.

ApplicationsMeasurement of Distance1) frequency stabilized He-Ne laser tube2) combination of beam-splitter and retroreflector3) a moving retroreflector 4) detection electronicsAerotechs LZR3000 Series Laser Interferometer SystemXiaoyu Ding

ApplicationsOther ApplicationsMeasure angles, flatness, straightness, velocity and vibrations, etc.Xiaoyu DingRearrangements of the light paths

ResolutionXL-80 Laser Measurement SystemXiaoyu Ding

Referenceshttp://www.aerotech.com/products/engref/intexe.htmlhttp://www.renishaw.com/en/interferometry-explained--7854http://en.wikipedia.org/wiki/Michelson_interferometerhttp://en.wikipedia.org/wiki/InterferometryPHYSICS FOR SCIENTISTS AND ENGINEERS, Randall D. Knight, 2003.

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Linear Variable Differential TransformerLVDTLei Yang

LVDTWhat is LVDT?Construction of LVDTHow LVDT worksSupport electronics of LVDTProperties of LVDTTypes of LVDTApplications of LVDT

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What is a LVDTLinear variable differential transformerElectrical transformer measuring linear displacementLei Yang

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Construction of LVDTOne Primary coilTwo symmetric secondary coilsFerromagnetic 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 coilSecondary coilsFerromagnetic core Lei Yang

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Recall of conventional transformer

Mutual inductionthe secondary voltage proportional to the primary voltageThe transformer core is fixedEnergy transferred is high

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How LVDT worksIf 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

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How LVDT worksIf 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

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How LVDT worksIf 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

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How LVDT worksLei Yang

Support electronics of LVDTLVDT signal conditioning equipmentSupplying 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 LVDTFriction-Free OperationInfinite Resolution Unlimited Mechanical LifeSingle Axis Sensitivity Environmentally Robust Null Point Repeatability Fast Dynamic Response Absolute Output Lei Yang

Types of LVDTDC LVDTSignal conditioning easierCan operate from dry cell batteriesHigh unit costAC LVDTSmall sizeVery accurate Excellent resolution (0.1 m)Can operate with a wide temperature rangeLower unit cost

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Types of LVDTFree 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-extended coreCore 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)

Lei Yang

Lei Yang

Example of commercial LVDTSE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors Lei Yang

Applications of LVDTFor power generationConditioning valves for large and medium steam turbines. Reheat and stop valves for large and medium steam turbines. Feed water boiler pump valve positioning. Natural gas fuel valve position for gas turbines for throttle control. Monitoring hydraulic fluid level in reservoir of feed water pumps in nuclear reactor core. Lei Yang

Applications of LVDTFor manufacturingMeasuring final height placement for automotive wheel trim Measuring injector height for diesel engines Feed water boiler pump valve positioning. Thickness measuring in multiple locations of fly-wheel to insure balance. Controlling depth of hole during machining operations in a rotary transfer machine. Providing indication and feedback position of rocket engine nozzle actuators during testing. Lei Yang

Other ApplicationsAutomation MachineryCivil / Structural EngineeringMetal Stamping / Forming OEMPulp and PaperIndustrial ValvesR&D and TestAutomotive Racing Lei Yang

Referenceshttp://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/index.html#automationhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://www.rdpe.com/displacement/lvdt/lvdt-principles.htmhttp://www.directindustry.com/industrial-manufacturer/lvdt-73930.htmlhttp://www.macrosensors.com/lvdt_macro_sensors/lvdt_products/lvdt_position_sensors/dc_lvdt/free_core_dc/se_750_single_ended.htmlAlexandre Lenobles lecture

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Thank you!Lei Yang

****Before we talk about how LVDT works. First we will recall some knowledge of conventional transformer. Here is a picture showing it. An alternating current is driven in the primary coils . Then it will create a varying magnetic flux in the transformers core, and thus a varying magnetic field through the secondary coil. This varying magnetic field then induces a varying voltage in the secondary coil. This effect is called mutual induction. LVDT is also a kind of transformer. However, there is some differences between LVDT and conventional transformers. The first difference, conventional transformer is used to transform energy level electricity while LVDT is used to process signal level electricity. The second difference is that for conventional transformer, the magnetic core is fixed while for LVDT , the magnetic core is moving. Now we will talk about how LVDT works. When the LVDT is used ,the core is always moving along the axis.

Here S1 and S2 are secondary coils and P is the primary coil.

The LVDT's primary coil, P, is energized by a constant amplitude AC source.

The magnetic flux thus developed is coupled by the core to the adjacent secondary coils, S1 and S2 .

At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2 ), is essentially zero.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.

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.

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.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.According to different power supply, LVDT is divided to DC LVDT and AC LVDT.According to the types of magnetic cores

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

data sheet

Parameter

typesLVDT finds its applications in wide areas.

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Sensors F09