Pietro Craighero

Laboratory of Optoelectronics, University of Brescia

2D Vision Course

Telecentric Measurement System

Pietro Craighero

Problem definition

Measurement of the inner and outer radius

• Set up for inner radius • Set up for outer radius

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System Engineers vision have to face these problems:

1. Alteration of the magnification (due to object displacement)

2. Image distortion

3. Perspective errors

4. Low image resolution

5. Uncertainty of the position of the boundary (due to lighting geometry)

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Hardware available

Telecentric technology addresses the need to have high-contrast images and almost no distortion

Telecentric lenses reduce or even cancel most of these problem, and for this reason have become a key component for any high accuracy gauging applications

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Alteration of the magnification(telecentric lenses)

dH = ds * theta = 1 * 0.0017 mm = 0,0017 mm

With telecentric lenses the image size is left unchanged with object displacement, provided the object stays within a certain range often referred to as “depth of field” or “telecentric range”

In a telecentric lens the magnification change is

determined by the “telecentric slope”

theta = 0,0017 rad ds = 1 mm

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Perspective errors

Telecentric lenses eliminate the perspective effect

This effect is due to the specific path of the rays

Non telecentric lens Telecentric lens

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Good image resolution

The resolution provided by telecentric lenses is compatible with very small pixel sizes and high resolution cameras thus increasing the measurement resolution.

• Frame rate : 11.2 fps• Sensor type : CMOS• Pixel size : 3.2 um• Optical size : 6.554 x

4.915 mm• Resolution : 2048 x 1536• Optical class : ½”• Resolution class : 3.15

Mpix

RSx = FOVx/(#pixel) = 46.5 mm / 2048 pixel = 0.022 mm/pixelRSy = FOVy/(#pixel) = 34.9 mm / 1536 pixel = 0.022 mm/pixel

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Imaging system set up

Working Distance : 182,3 (+/-) 5 [mm] Field of View (with ½” detector) : 46,5 x 34,9 [mm]

WD

46.5 mm

34.9mm

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Software structure

State machine

User choises

Init / Idle /Calibrazione offline /Calibrazione online / Acquisizione online / Acquisizione offline / Misura Raggi InterniMisura Raggi Esterni

Calibration & Acquisition

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Inner radius

ROI selection?

• Default mode • Manual mode

ROI Cluster

Compute inner radius and display

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Outer radius

While(stop == false){..

If(new ROI drawn == false) {..

}}

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Outer radius

While(stop == false){..

If(new ROI drawn == false) {..

}}

30,33 mm

30,33 mm

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Conclusion

• set and choose the right hardware• friendly interface design

• Thank you for your attention