Laser Bar Code Scanner

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Laser Bar Code Scanner

• Chunyu Zhao

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OPTI696D requirement

• System overview
• Describe the class of systems, stating the key
  metrics
• Explain the principles of how the system works
• Identify key subsystems, relate system
  performance to subsystem requirements.
• Summarize the current state of technology for
  this type of system
• Analysis
• Disassemble the system and show key components
  and subsystems
• Discuss features of this system and surmise
  design decisions

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Outline

• Barcode basics and laser scanners
• How a laser scanner works
• Scan engine
• Optical design and analysis
• Manufacturing laser scanners in volume
• State of the art scanning technology

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Barcode basicsType of barcodes

• 1D barcode
• UPC
• Code 39
• Code 128
• etc
• 2D barcode
• PDF 417
• MaxiCode
• etc

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Barcode basicsBenefit of using barcodes

• For retailers
• Quickly identifying fast and slow selling items
  to help stocking decisions
• Repositioning a given product within a store to
  move more profitable items to occupy the best
  space,
• Historical data can be used to predict seasonal
  fluctuations very accurately.
• For shipping companies
• Keep track of packages from start to destination

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1D laser scanners
Handheld, single line
Hand-free, multi-line
Scan engine
Fixed mount, multi-line
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Key metrics

• Size
• Working range
• Poor quality barcode reading capability
• Width of field/scan angle
• Pitch, roll and yaw angles
• Barcode contrast
• Ambient light level

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How a laser scanner works

• A laser spot is scanned across the bar code
  symbol that is to be read.
• The light reflected from the symbol is directed
  to a photodiode where it is converted from
  optical energy to electrical current.
• The signal is processed through both hardware and
  software, and the information it carries is
  extracted.

Signal Processing Hardware and Software
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Scanning the Bar Code

• When the laser is scanned across the bar code,
  the reflected signal is the convolution of the
  laser spot and bar code symbol. Simply stated,
  the convolution is the area of the overlap of the
  two waveforms.
• In order to make finding the transition point
  from bar to space more easy to find, the signal
  is differentiated.

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Noise Corrupted Signal
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Sources of Noise

• Internal Sources
• Thermal noise of the electronic components.
• Intrinsic noise of the preamplifier.
• External Sources
• Printed noise on the symbol.
• Speckle noise created by the laser.
• Sunlight.
• Fluctuating ambient light.
• EMI, RFI and power supply noise.

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System performance defining factors

• The characteristics that define how well a
  scanner will decode are
• Depth of Modulation
• laser focus
• other optical components
• Signal Amplitude
• optical alignment
• signal blockage
• laser focus
• optical AGC
• Noise Amplitude
• field of view
• optical alignment
• laser focus

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Retro or non-retro system

• Non-retro system Scan mirror is NOT part of the
  collection optics, so the FOV is fixed and cover
  the whole scanning field, and its BIG.
• More ambient light noise.
• Need a big photodiode, thus the noise is huge, so
  the working range is reduced.
• Alignment is easier.

• Retro system Scan mirror is part of the
  collection optics, so the FOV follows the laser
  spot.
• Small FOV, therefore less ambient light noise
• Small photodiode, so noise from PD is small.
• Need better alignment.

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The Design Process

• To design a scanner you need to do the following
  things
• extract size and performance information from the
  customer or marketing spec
• develop an optomechanical configuration
• calculate optical field of view and photodiode
  size
• develop a laser profile to meet the performance
  requirements
• perform sensitivity study and tolerance analysis
• develop inspection criteria for manufacturing

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The most important part laser beam profile

• Laser Profile
• Ideally, the cross section of the laser beam
  should be a Delta function.
• In reality, the beam size is finite and expands
  as it propagates due to diffraction.
• A small spot diameter is required to read high
  density bar codes. A large spot area is needed
  to minimize speckle noise and poorly printed
  symbols. Trade-off needs to be made.

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Convolution ProcessSmall Spot
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Convolution ProcessLarge Spot
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Convolution ProcessVery Large Spot
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The Laser Beam Profile General Requirement

• Basic requirement - the spot diameter must be no
  greater than some fixed multiple of the bar code
  symbol narrow element width over the entire
  working range. This multiple can range between
  2.8 and 3.3 depending on the type and sensitivity
  of digitizer used.
• Secondary requirement - ellipticity should be as
  large as possible to improve speckle noise
  characteristics and poorly printed symbol
  readability, and beam pedestal and ripple should
  be kept to a minimum.

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The Laser Beam Profile

• The characteristics of the laser beam can be
  controlled and manipulated using the following
  parameters
• position and focal length of laser focusing lens
• aperture size, shape and aspect ratio
• laser divergence angle and astigmatism
• rotation of laser (high or low divergence in x
  axis)
• external beam shaping optics

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Laser Beam Profile
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Scanning Optics

• Flatness of mirrors controls accuracy of laser
  profile.
• Curvature will shift waist size and location.
• Random aberrations will distort overall beam
  shape.
• Curvature can be used to add desired ellipticity,
  if applied to the Y axis.

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Tolerance analysis and error budget

• Decenter Lens Barrel vs. Laser
• Decenter Lens vs. Barrel
• Decenter Phase Plate vs. Barrel
• Tilt Lens Barrel vs. Laser

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Error Budget
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Tolerance analysis/sensitivity study Example 1
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Tolerance analysis/sensitivity study Example 2
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Collection Optics

• Collect as much of the laser light reflected from
  the bar code as possible.
• Track the position of the laser spot, and keep it
  in the center of the receiver field of view.
• Define the size of the optical FOV to be as small
  as possible.

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Optical Collection Area

• A large collection area increases the signal
  received (improving signal to noise ratio) and
  reduces the effect of speckle noise, but makes
  the scanner physically bigger and collects more
  ambient light interference.

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Optical Field of View

• Alignment has to be maintained between what the
  laser illuminates and what the photodiode is
  looking at. A large FOV makes this alignment
  less critical, but increases the amount of
  ambient light collected and requires a larger
  photodiode to do the collecting, both degrading
  noise performance. A small FOV requires active
  alignment of the optics or higher tolerance
  parts, and it may move out of alignment with time
  or drop and vibration.

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Optical filter

• Right in front of the photo-detector
• Let the laser reach the detector and block most
  of the ambient light

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Manufacturing a scanner

• Step 1 Focusing the laser module to obtain the
  desired beam profile
• Step 2 Install collection and scanning optics,
  and detector
• Step 3 Align the collection FOV with scanning
  beam

Quality and yield!!
Inspection DURING and AFTER production!!!
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Laser focusing
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Focusing spec
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Optical alignment

• Align the collection FOV with the flying laser
  spot
• For Non-retro system, adjust the center position
  of the flying spot to the axis of the collection
  optics
• For Retro system, if the collection optics and
  scan optics are separated, then adjust the scan
  mirror to maximize the signal otherwise
  alignment relies on tight mechanical tolerance

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Final Acceptance Test (FAT)
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State of the art

• Extended working range
• Double scan beams
• Diffraction-free laser beam by using axicon

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Any questions?