Laser-Based Displacement Transducers

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Laser-Based Displacement Transducers

• Joe Wilkey
• CEE 398

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Overview

• Types of Laser-Based Displacement Transducers
• Operation
• Specifications
• Applications
• Advantages/Disadvantages
• Additional Information and References

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Types of Laser-Based Displacement Transducers

• Optical Triangulation

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Operation (Optical Triangulation)

• Laser shines onto surface of material which
  reflects to receiver
• Location of reflected beam allows calculation of
  reflection angle and consequently distance can be
  calculated

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Specifications

• Ranges 0.5 400 mm
• Frequency Limits 10 kHz, 25 kHz, 500 kHz
• Resolution 0.5 600 µm
• Typically Class 3B laser used with wavelength of
  675 nm

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Applications (Optical Triangulation)

• Flaw Detection

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Applications (Optical Triangulation)

• Displacement or Vibration Amplitude

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Types of Laser-Based Displacement Transducers

• D Type Fiber Optic

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Operation (D Type Fiber Optic)

• D Type Fiber Optic sensors directly detect motion
  of target using intensity of reflected light

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Operation (D Type Fiber Optic)

• D Type Fiber Optic sensors directly detect motion
  of target
• Output voltage follows general curve as shown
• Slopes of near side and far side portion of
  curve given for each unique sensor

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Specifications

• Ranges 20 µm 7 mm
• Frequency Limit 200 kHz
• Resolutions up to 0.002 µm

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Applications

• Measuring small displacements in axis of sensor
  including
• Vibration (including ultra-sonic vibration)
• Simple Displacement Measurements for solids or
  fluids
• Displacement or Vibration in mechanisms

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Types of Laser-Based Displacement Transducers

• RC Type Fiber Optic

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Operation (RC Type Fiber Optic)

• RC Type Fiber Optic sensors also directly detect
  motion of target

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Operation (RC Type Fiber Optic)

• RC Type Fiber Optic sensors also directly detect
  motion of target
• Output voltage follows general curve as shown
• Only one output voltage for each position, so no
  near and far side slopes

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Specifications

• Ranges 50 µm 2.5 mm
• Frequency Limit 200 kHz
• Resolutions up to 0.01 µm

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Applications

• Measuring small displacements in axis of sensor
• Also measures rotation and movement past sensor
  including
• Thermal Growth
• Deformation Studies
• Bearing/Rotor Dynamics

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Advantages/Disadvantages (Optical Triangulation)

• Advantages
• Gives accurate measurements regardless of
  reflectivity of object being measured
• Relatively long range (up to 400 mm)
• Faster sampling rate

• Disadvantages
• Rather bulky, can only measure exterior parts
• Low resolution (0.5 µm)
• Accuracy may be affected by surrounding conditions

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Advantages/Disadvantages (D Type Fiber Optic)

• Advantages
• Small and sturdy, can take measurements in
  mechanisms, fluids, extreme pressures and extreme
  temperatures
• Fiber optic principle allows measurements to be
  taken in magnetic/electric fields
• Extremely high resolutions give more accurate
  measurements

• Disadvantages
• Only measures distance in direction of laser beam
• Surfaces with high, changing, or low reflectivity
  affect readings
• Range only goes to 7 mm
• One output voltage corresponds to two different
  displacements

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Advantages/Disadvantages (RC Type Fiber Optic)

• Advantages
• Gives accurate measurements regardless of
  reflectivity of object being measured, even if
  reflectivity of object changes
• Offers same versatility as D Type sensor
• Only one displacement value for each output
  voltage
• Measures rotations and transverse displacements
  as well as axial displacements

• Disadvantages
• Range less than 2.5 mm
• Resolution 5x lower than that of D Type

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Advantages/Disadvantages (General)

• Advantages
• Very accurate non-contact measurement device
• Easier to transport, set-up, and operate than
  video systems
• Use of multiple sensors increases number of
  applications (thickness measurements, etc.)

• Disadvantages
• Do not use targets like camera system, so must
  ensure system is in line with point of interest
  and perpendicular to sensor
• Some limited in use (i.e. can only be used in
  axial direction)

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References/Additional Info

• http//www.sensors-uk-transducers-instruments.com/
  
• http//www.scantron-net.co.uk/laserdistance.htm