Optical Encoders, Laser Interferometer, LVDT Rushi Vyas Xiaoyu Ding Lei Yang

Outline Optical Encoders: Theory and applications Rushi Vyas 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.

Optical Encoders: Components 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.

Optical Encoders: Components 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.

Optical Encoders: Theory Rushi Vyas Optical Encoders: Theory Code Disk LED Photo-sensor

Optical Encoder Types Lab 3 Rushi Vyas 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

Standard Binary Encoding Rushi Vyas Standard Binary Encoding Angle Binary Decimal 0-45 000 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

Problem with Binary Code Rushi Vyas Problem with Binary Code Angle Binary Decimal 0-45 000 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 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)

Gray Encoding Notice only 1 bit has to be changed for all transitions. Rushi Vyas Gray Encoding Angle Binary Decimal 0-45 000 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.

Quadrature ❖ Quadrature describes two signals 90° out of phase 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

Rushi Vyas Quadrature An 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.

Encoder Resolution: Absolute Optical Encoder Rushi Vyas 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 ( ).

Examples Number of bits on encoder code disk n = 3 Rushi Vyas Examples Number 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

Optical Encoders: Reliability Rushi Vyas Optical Encoders: Reliability Encoder errors Quantization Error – Dependent on digital word size. Assembly Error – Due to instability in rotational motion of code disk Manufacturing tolerances – Code printing accuracy, sensor position, and irregularities in signal generation.

Optical Encoders: Reliability Rushi Vyas Optical Encoders: Reliability Structural Limitations – Disk Deformation, physical loads on shaft. Coupling Error – Gear backlash, belt slippage, etc… Ambient Effects – Vibration, temperature, light noise, humidity, etc… Diffraction of light: occurs due to edge of codes disk windows. Fixed in newer encoders by using mask and minimizing distance to photodetector.

Rushi Vyas Applications Primarily used in motors for monitoring velocity and position. Robotics Conveyor belts Locomotives: Automobiles, planes.. Tachometers

References Kawasaki Industries Optical Encoders: www.khi.co.jp 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/

Laser interferometer Xiaoyu Ding

Laser Interferometer What’s laser interferometer? Xiaoyu Ding Laser Interferometer What’s laser interferometer? The principle of standard interferometer Types of interferometers Applications

What’s a Laser Interferometer 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.

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

Physics Review Diffraction of Light Xiaoyu Ding Physics Review Diffraction of Light Light, just like a water wave, does spread out behind a hole is the hole is sufficiently small. Light is a electromagnetic wave. Diffraction of light Wave

Physics Review A Double-Slit Interference Experiment Xiaoyu Ding Interference of Light

Principle of Michelson Interferometer Xiaoyu Ding 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

Principle of Michelson Interferometer Xiaoyu Ding Principle of Michelson Interferometer Michelson Interferometer Separation Recombination Interference A Michelson Interferometer for use on an optical table

Principle of Michelson Interferometer 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. 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. Photograph of the interference fringes produced by a Michelson interferometer.

Principle of Michelson Interferometer Xiaoyu Ding Principle of Michelson Interferometer Analyzing Michelson Interferometer Spacing distance of M2 is . laser has very small, stable and accurately defined wavelength which can help us get high precision measurement.

Types of Laser Interferometers 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.

Principle of Heterodyne Laser interferometer Xiaoyu Ding Principle of Heterodyne Laser interferometer Doppler Effect Doppler Effect: The change of frequency when a source moves relative to an observer. 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 Xiaoyu Ding 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

Applications Other Applications Xiaoyu Ding Applications Other Applications Measure angles, flatness, straightness, velocity and vibrations, etc. Rearrangements of the light paths

Xiaoyu Ding Resolution XL-80 Laser Measurement System

References http://www.aerotech.com/products/engref/intexe.html 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.

Linear Variable Differential Transformer（LVDT） Lei Yang

LVDT What is LVDT? Construction of LVDT How LVDT works 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

What is a LVDT Linear variable differential transformer Lei Yang What is a LVDT Linear variable differential transformer Electrical transformer measuring linear displacement

Construction of LVDT One Primary coil Two symmetric secondary coils Lei Yang Construction of LVDT One Primary coil Two symmetric secondary coils Ferromagnetic core Primary coil 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 Ferromagnetic core Secondary coils

Recall of conventional transformer 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 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 transformer’s 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.

How LVDT works If the core is located midway between S1 and S2 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. 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.

How LVDT works If the core is moved closer to S1 than to S2 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). 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 the core is moved closer to S2 than to S1 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). 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.

Lei Yang How LVDT works

Support electronics of LVDT Lei 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 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. External electronics Self-contained electronics e.g. DC-LVDT

Properties of LVDT Friction-Free Operation Infinite Resolution 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 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.

Types of LVDT DC LVDT AC LVDT Signal conditioning easier 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 According to different power supply, LVDT is divided to DC LVDT and AC LVDT.

Types of LVDT Free core Guided core Spring-extended core Lei Yang 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 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) According to the types of magnetic cores Internal spring to continuously push the armature to its fullest possible extension Lower range than captive armature (10 to 70mm)

Example of commercial LVDT Lei Yang Example of commercial LVDT SE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors data sheet Parameter types

Applications of LVDT For power generation Lei Yang Applications of LVDT For power generation Conditioning 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. LVDT finds its applications in wide areas.

Applications of LVDT For manufacturing Lei Yang Applications of LVDT For manufacturing Measuring 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.

Other Applications Automation Machinery Civil / Structural Engineering 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!