how to startpage 1. how to start how to specify the task how to get a good image
TRANSCRIPT
How to start page 1
How to start
How to specify the task
How to get a good image
How to start page 3
How to specify the inspection task
• Expectations
• How to specify the requirements
• Control and absolute measurements
• High yield and/or zero faults
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Too high expectations?
• Focus on key issues – the basis for the investment• Don’t include all “nice to have” inspections in the
beginning• It is easy to make a system so complex that it
never will work according to your expectations• “Keep it simple”• Implement “nice to have” issues over time and
after acceptance of the initial system
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How to set the requirements
• A vision system perform measurements– Dimensions, form, area, angles
• Requirements must be a figure or a set of figures with tolerances
• Define the measurement procedures
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Tolerance and Accuracy
• Tolerances:– The specified max and min limits for the
measurement– Used as inspection criteria (Pass, Fault)
• Accuracy:– The uncertainty of the measurement performed
by the inspection system
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Control or Absolute Measurements
• Absolute measurements– The result is absolute
correct within the measurement accuracy
– Must be verified by alternative methods
– Require complex calibration
– Measurement accuracy1/10 of tolerance
– Time consuming
• Control– Relative measurements
(compare with one or a set of master objects)
– Standard deviation (Repeatability) within measurement accuracy
– Measurement accuracy1/3 – 1/5 of tolerances
– Most vision systems and the subject for this lecture
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The ideal inspection system
X(1) ppmDefect Parts
FinishedProducts
Operation
No. 1
At CustomersSite
StationNo. 1
StationNo. N
Operation
No. N
X(N) ppmDefect Parts
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The real life
+A(1) ppm
Parts due toInspection
Failed
+A(1) ppm
Parts due toInspection
Failed
Y ppmDefect Parts
X(1) ppmDefect Parts
+Z(1) ppmOK Parts
FinishedProducts
Operation
No. 1
At CustomersSite
StationNo. 1
StationNo. N
Operation
No. N
X(N) ppmDefect Parts
+Z(N) ppmOK Parts
Inspection Conditions: Defect, OK, Inspection Failed
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Yield problems
• Yield often drop due to:– Larger variation in the products than expected– Varying contrast due uncontrolled light– Production working close to the limits
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45
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55
60
0 10 2040
45
50
55
60
0 10 20
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How to get a good image
• Camera – sufficient resolution and quality
• Lens – according to distance to and size of object
• Light – sufficient contrast to see the details
• External light protection – secure constant light and contrast
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The optical system
• Camera– Technology:
Digital using CCD chips
• Lens– Adapted to the individual
applications
• Camera types• Area
• Line scan(high speedapplications)
Y mm
X mm
Field of View (FOW)for an area camera
A Line scanning overthe area - Field of View (FOW)
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Impact of the lens
• Gives the FOW with a given distance between object and camera
• Lens distortion and perspective
Camera
Lens
Distance
Grid used forminimise
lens distortionand perspective
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Very high precision gauging
• With a normal lens perspective and shadows may cause it impossible to measure with required accuracy
• With a Tele centric lens this problem is minimised
• High cost lens, must be bigger than the object
Camera
Tele centricLens
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Field of View (FOW)
Piksels in X direction
Pix
els
in Y
dire
ctio
n
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Grey scale pictures
Original
Greyscale
Pixel grid
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Pixel resolution
• VGA Camera– 640 x 480 pixels
• FOV: 5mm x 5mm– Resolution ca. 0,01mm
• FOV: 50mm x 50 mm– Resolution ca. 0,1mm
• FOV: 500mm x 500mm– Resolution ca. 1 mm
• XGA Res. Camera– 1024 x 768 pixels
• FOW: 5mm x 5mm– Resolution ca. 0,005mm
• FOW: 50mm x 50 mm– Resolution ca. 0,05mm
• FOW: 500mm x 500mm– Resolution ca. 0,5 mm
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How to select camera
• Pixel resolution– Requirement are set by the
size of object and measurement tolerances
– In many situation we can measure in the sub pixel range
• Quality– Normally means the ability
to separate greyscale values between individual pixels
Measure distance between two linesin stead of two points
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To big object
• Select a camera with higher resolution
• Use two cameras calibrated together
• Use an array of cameras
or
• Use Robot Vision– The robot moves the object
in different positions
50 cm
Camera 1
Camera 2
CameraArray
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Multi camera example
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Camera in perspective
• Advanced four point calibration for correction of perspective
Camera
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The purpose of lighting
• To illuminate the object and highlight contrasts of details
• To secure constant light environment, 24 hour a day, 360 days a year
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Moving object during inspection
• Shutter time must be adjusted to match speed and resolution requirements
• In the example a shutter time of max 0,1 ms is required
• Shorter shutter time require more light intensity
Camera
Moving object
Speed: 6 m/min100 mm/sec0,1 mm/ms
Required pixelresolution: 0,1 mm
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Front light
• Direct light– Experiment with
number of lamps and light angle
• Diffuse light– Must be used when
reflections is a problem
– Gives no shadow
Camera
Light sourceLight source
Lightangle
Camera
Light sourceLight source
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Special front light
• Ring light– For circular object
• Cloudy day
• On Axis– To avoid shadows in holes
• Fibre optics– For small objects and very
light intensive applications
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Back light
• Create a silhouette of the object
• Ideal for inspecting holes, edges, measuring areas
• May be difficult to implement in a production line
Camera
Light source
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Protection from external light
• External light is not suitable as illumination for optical inspection system
• All system should be protected against sunlight and external light
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Final comments
• The key to get a good image is the combination of camera(s) and light
• Spend time to find the best solution• The final solution is often different from your
starting point• Sometimes an optical inspections can not be done
according to your initial requirements– Revise requirements– Or better find it out early than after weeks of efforts after
system implementation