1. Efrain Teran Carol Young Brian O’Saben
Sensors
Efrain Teran
Carol Young
Brian O’Saben

2. Optical Encoders
Efrain Teran

3. 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.

4. 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).

5. 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

6. 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.

7. Types of Optical Encoders
Incremental Optical Encoders:
Single channel
Dual channel
Dual channel with Z index
Absolute Optical Encoders

8. 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

9. 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

10. 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

11. Incremental Optical Encoder: Dual channel

12. 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

13. 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.

14. VIDEO: https://www.youtube.com/watch?v=cn83jR2mchw

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

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

17. 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°

18. 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

19. 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

20. (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

21. Applications Incremental Single channel Incremental Dual channel
Incremental with Z index
Absolute Encoder

22. REFERENCES:
Mechatronics, Sabri Cetinkunt, Wiley, Section 6.4.3

23. Noise cancellation

24. Laser Interferometer
Carol Young

25. 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

26. Physics Background Waves
Light is an Electrometric wave and therefore has wave properties.
Crest and trough

27. 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
Young’s double-slit experiment

28. 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.

29. Homodyne Interferometer (Michelson)
Mirror Reference
Laser
Mirror Moveable
(Sample)
Beam Splitter
Screen

30. 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.

31. Homodyne Interferometer Uses
Absolute distance
Optical testing
Refractive index
Angles
Flatness
Straightness
Speed
Vibrations

32. 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

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

34. 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

35. Heterodyne / Homodyne Interferometer Comparison
Comparing with a Homodyne Interferometer
Can determine movement direction (but limited range)
More useful when direction of movement is important

36. Heterodyne / Homodyne Interferometer Comparison
Smooth surfaces only
Heterodyne
Can be used for
Distance to rough surfaces
Surface roughness measurements

37. Xiaoyu Ding
Resolution
XL-80 Laser Measurement System

38. References http://www.aerotech.com/products/engref/intexe.html
A. F. Fercher, H. Z. Hu, and U. Vry, “Rough surface interferometry with a two-wavelength heterodyne speckle interferometer”, Applied Optics

39. Linear Variable Differential Transformer (LVDT)
Brian O’Saben

40. Outline What is a LVDT? How LVDTs Works LVDT Properties
LVDT Support Electronics
Types of LVDTs
LVDT Applications

41. What is a LVDT? Linear variable differential transformer
Electromechanical transducer measuring linear displacement

42. 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

43. 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.

44. 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.

45. How LVDT works If core is closer to S2 Greater flux at S2
Voltage E2 increases, Voltage E1 decreases
Eout=E2 – E1

46. How LVDT works

47. 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.

48. 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.

49. 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

50. 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)

51. 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

52. References http://www.macrosensors.com/lvdt_tutorial.html
Lei Yang’s student lecture

53. Thank You!