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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


Sensors

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


Sensors

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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