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Sensors. Bryson Cook James Wyler Hao Phan. Bryson Cook. Outline. Optical Encoders: Theory and applications –Types of encoders –Fundamental Components –Quadrature –Errors –Applications 

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sensors

Sensors

Bryson Cook

James Wyler

Hao Phan

outline

Bryson Cook

Outline
  • Optical Encoders: Theory and applications –Types of encoders –Fundamental Components –Quadrature –Errors –Applications 
  • LVDT (Linear Variable Differential Transformer)–What is a LVDT –Types of LVDT –How do they work? –Applications
what are encoders

Bryson Cook

What are Encoders
  • For our class, an encoder is a device that senses position or orientation for use as a reference or active feedback to control position.
    • Most are either:
      • Rotary: converts rotary position to an analog or electronic signal.
      • Linear: converts linear position to an electronic signal.
    • Are also either absolute or incremental.
      • Absolute gives the absolute position and knowledge of the previous position is not needed.
      • Incremental encoders is more ambiguous and requires counting of cycles to determine absolute position.
optical encoders

Bryson Cook

Optical Encoders
  • Use light & photosensors to produce digital code
  • Most popular type of encoder.
  • Can be linear or rotary.
optical encoders components

Bryson Cook

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

Bryson Cook

Optical Encoders: Theory

Code Disk

Photo-sensor

LED

rotary optical encoder types

Bryson Cook

Rotary 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

binary and gray encoding

Bryson Cook

Binary and Gray Encoding
  • In some devices, Binary Encoding is used to keep track of the various positions. The areas of the disk are named counting in binary.
    • This can cause problems since multiple bits can change from one successive area to the next, such as in 011 to 100 all three bits change.
  • Gray Encoding is a binary system where the adjacent areas only differ in one bit.
standard binary encoding

Bryson Cook

Standard Binary Encoding

*Note: Extremely simplified encoder

gray encoding

Bryson Cook

Gray Encoding

Notice only 1 bit has to be changed for all transitions.

quadrature

James Wyler

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
slide12
Standard Encoder Track

Gives velocity and position but not direction

Quadrature Encoder Track

Gives velocity, position AND direction

James Wyler

Quadrature
quadrature how it works

James Wyler

Quadrature – How It Works
  • Grey Encoding
  • Identical tracks
    • Phase offset of 90º
  • Two sensors
  • Current state vs. next state
quadrature rotary encoders

James Wyler

Quadrature – Rotary Encoders
  • Examples of Quadrature Rotary Encoders

2 Bit Wheel 64 Bit Wheel

optical encoder errors
Quantization Error – Dependent on resolution of sensor

Assembly Error – Disk not positioned correctly with respect to sensor

Manufacturing Error – Tolerances of sensor positioning and code printing lead to inaccurate signals

James Wyler

Optical Encoder Errors
optical encoder errors cont

James Wyler

Optical Encoder Errors – Cont.
  • Structural Limitations – Loading on shaft or disk deformation
  • Coupling Error – Gear backlash, belt slippage, etc…
  • Ambient Effects – Vibration, temperature, light noise, humidity, etc…
optical encoder applications

James Wyler

Optical Encoder Applications
  • Coordinate Measuring Machine (CMM)
  • Digital Calipers
  • CNC Machining
  • Electric Motors
  • Robotics
slide18

James Wyler

LVDT
  • What is a LVDT
  • Types of LVDT
  • How do they work?
  • Applications
what is a lvdt

James Wyler

What is a LVDT
  • Linear Variable Differential Transformer
  • Electrical transformer used to measure linear displacement
construction of lvdt

James Wyler

Construction of LVDT
  • One Primary coil
  • Two symmetric secondary

coils

  • Ferromagnetic core
types of lvdt

Hao Phan

Types of LVDT
  • Power supply :
    • DC
    • AC
  • Type of armature :
    • Free (Unguided)
    • Captive (Guided)
    • Spring-extended
power supply dc lvdt

Hao Phan

Power supply : DC LVDT
  • Easy to install
  • Signal conditioning easier
  • Can operate from dry cell batteries
  • High unit cost
power supply ac lvdt

Hao Phan

Power supply : AC LVDT
  • Small size
  • Very accurate –Excellent resolution (0.1 μm)
  • Can operate with a wide temperature range
  • Lower unit cost
armature free core unguided

Hao Phan

Armature : Free Core (Unguided)
  • Core is completely separable from the transducer body
  • Well-suited for short-range applications
  • high speed applications (high-frequency vibration)
captive core guided

Hao Phan

Captive Core (Guided)
  • Core is restrained and guided by a low-friction assembly
  • Both static and dynamic applications
  • Long range applications
  • Preferred when misalignment may occur
spring extended core

Hao Phan

Spring-Extended Core
  • Core is restrained and guided by a low-friction assembly
  • Internal spring to continuously push the core to its fullest possible extension
  • Best suited for static or slow-moving applications
  • Medium range

applications

how do they work

Hao Phan

How do they work?
  • An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.
how do they work1

Hao Phan

How do they work?
  • The coils are connected in reverse series
  • The output voltage is the difference (differential) between the two secondary voltages
null position

Hao Phan

Null Position
  • When the core is in its central position, it is placed equal distance between the two secondary coils.
  • Equal but opposite voltages are induced in these two coils, so the differential voltage output is zero.
in phase voltage

Hao Phan

In Phase Voltage
  • Displacing the core to the left causes the first secondary to be more strongly coupled to the primary than the second secondary.
  • The higher voltage of the first secondary in relation to the second secondary causes an output voltage that is in phase with

the primary voltage.

  • The phase of the voltage

indicates the direction of

the displacement.

out of phase voltage

Hao Phan

Out of Phase Voltage
  • Displacing the core to the right causes the second secondary to be more strongly coupled to the primary than the first secondary.
  • The greater voltage of the second secondary causes an output voltage to be out of phase with the primary voltage.
how do they work2

Hao Phan

How do they work?
  • The magnitude of the output voltage is proportional to the distance moved by the core, which is why the device is described as "linear".
  • Note that the output is not linear as the core travels near the boundaries of its range.
lvdt applications

Hao Phan

LVDT Applications
  • Crankshaft Balancing
  • Testing Soil Strength
  • Automated Part Inspection
  • Automotive Damper Velocity
references
References
  • http://www.macrosensors.com/lvdt_tutorial.html
  • http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3
  • http://en.wikipedia.org/wiki/Linear_variable_differential_transformer
  • http://prototalk.net/forums/showthread.php?t=78\
  • http://www.transtekinc.com/support/applications/LVDT-applications.html
  • http://www.sensorsmag.com/sensors/position-presence-proximity/modern-lvdts-new-applications-air-ground-and-sea-7508
  • http://www.macrosensors.com/lvdt_tutorial.html
  • http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3
  • http://en.wikipedia.org/wiki/Linear_variable_differential_transformer
  • Sensors Lecture: Fall ME6405 2009
  • http://electricly.com/absolute-optical-encoders-rotary-encoders
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