Optimized Design of 3D Laser Triangulation Systems

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2 The Scan Principle of 3D Laser Triangulation Triangulation Geometry Example of Setup Z Y X Target as seen from the Camera Sensor Image of Laser Line

3 The Scan Principle of 3D Laser Triangulation Detektion of Laser Line in the Sensor Image: MAX TRSH COG FIR-PEAK Smoothing filter di ( y) dy ZCD FIR Low Height Resolution High Height Resolution

4 The Scan Principle of 3D Laser Triangulation Camera Output: 3D Scan Transportation of Target Object by means of: - Linear Stage - Conveyor Belt - Turntable - Robot Scan Direction

5 Which Components? - Kamera Model - Triangulation Angle - Laser?? - Bandpass Filter? - Camera Objective Lens - Calibration

6 The Specification of the Application -Object Size (Width, Height, Length) -Precision in X, Y und Z Axis or Size of smallest Defect -Transportation Speed mm/s -Type of Application -Material H W Z L Y X The scan precision must be 5-10 times higher than the defect size in order to be detectable

7 Computation of the Resolution Lateral Resolution (X Axis): dx = FOV / Number of Pixels per Sensor Row Height Resolution (Z Axis): dz = dx / sin(α) α = Triangulation Angle Height Resolution with Subpixeling (e.g. 6 Subpixels, Factor 2 6 = 64): dz_subpix = dz / 64 Resolution in the Transportation Direction (Y Axis): dy = Transportation Speed / Profile Frequency

8 Determination of Scan Precision Lateral Precision (X Axis) = Lateral Resolution dx Precision in the Transportation Direction (Y Axis) = Resolution in the Transportation Direction dy Height Precision (Z Axis) Height Resolution LASER SPECKLE EFFECT! LASER SPECKLE EFFECT: An interference pattern on the sensor image of laser line causes a virtual noise of the height profile data

9 Determination of Scan Precision Noise of Height Profile Data Standard Deviation (σ)= Factor * Height Resolution with 6 Subpixels, where Factor = The higher the resolution the bigger the Factor, e.g.. Resolution dx = 5 µm Factor = 30 Resolution dx = 500 µm Factor = 10 The shorter the laser wavelength the smaller is the Factor, e.g. Resolution dx = 20 µm and Laser 660 nm Factor = 20 Resolution dx = 20 µm and Laser 405 nm Factor = 15

10 Selection of Camera Model Computation Tool Lateral Resolution / Precision depends on the FOV and Camera Model

11 Min WD Nominal WD Max WD Optimized Design of 3D Laser Triangulation Systems The Z-Range Z-Range Z-Range = Range of Height Measurement Min FOV Z-Range Max FOV FOV (nominal)

12 The Triangulation Angle Triangulation Angle α Step Height The bigger the triangulation angle: - the higher the height resolution - the bigger the occlusion - the bigger the sensor AOI, which reduces the camera profile speed Occlusion: Step Height * tan(α)

13 Die Height Resolution Height Precision σ = x dz_subpix6

14 The Profile Frequency

15 Selection of Camera Model

16 Selection of Laser Criteria: Line Thickness & Depth Of Field The Laser line thickness must be smaller than : - the required precision in transportation direction (Y axis) - the size of the smallest objects or defects (e.g. holes, cracks etc.) D 1 The Laser DOF is defined as the length along the laser beam in which the line thickness is not greater than 2 times of the line thickness at the focus distance : D 1 < 2*D 0, D 2 < 2*D 0 DOF D 0 D 2 The DOF must be greater than the Z-Range of the application!

17 Line Thickness DOF Optimized Design of 3D Laser Triangulation Systems Selection of Laser Focus Distance The line thickness and DOF depend on the focus distance of the Laser! They define the focus distance required by the application Laser Fan Angle = 2* Arctan (0,5 * FOV / Focus Distance) Focus Distance FOV (nominal)

18 Selection of Laser Power The essential Laser power depends on the following parameters: - Profile Frequency - Line Length - Working Distance - Surface Reflectivity - Wavelength and Camera Sensor Sensitivity Typical Examples : 1) Scan white surface at 10kHz with Laser 660nm, line length 160 mm at working distance 200 mm Required Laser power 20mW 2) Scan black surface at 2kHz with Laser 660nm, line length 1000 mm at working distance 800 mm Required Laser power 120mW

19 Focus distance of Laser Optimized Design of 3D Laser Triangulation Systems Selection of Camera Objective Lens Criteria Focus distance of camera lens - Adequate resolution depending on the pixel size of the camera sensor chip (MTF curves) - Adequate image circle diameter depending on the size of the camera sensor chip α - The working distance of the objective lens can be estimated as: focus distance of Laser / cos (α) Determination of focal length of the lens

20 Selection of Bandpass Filter Bandpass Filter: Essential component for suppressing ambient light effects - Laser wavelength Criteria - The filter bandwith depends on the application and variation of the laser wevelength (narrow or broad band filter) - In order to reduce the Blue Shift effect (a known issue of interference filters) it is recommended to install the filter between the objective lens and the sensor (e.g. using filter with C-Mount outer thread) Mechanical stop for fixing the bandpass filter

21 Scheimpflug Adapter Depth of Field Line out of focus Limited Camera Depth of Field caused by the Triangulation Geometry

22 Scheimpflug Adapter Scheimpflug angle A special adapter inside the camera housing allows to tilt the sensor chip according to the Scheimpflug optical principle Line is everywhere in focus Enhanced Camera Depth of Field by means of a Scheimpflug Adapter

23 Scheimpflug Angle The Scheimpflug tilt angle depends on the following parameters: - Triangulation angle - Focus distance of the objective lens - Focal length of the objective lens - Length of the lens housing Distance of Lens Principal Plane to the Sensor α - Length of extension tubes (if any used for focussing) Distance of Lens Principal Plane to the Target

24 Calibration The calibration of a triangulation setup occurs by means of the scan of reference object (calibration target). Different calibration methods are available: Static Target Linear Target Zig Zag Target Multiple Zig Zag Target

25 Calibration Comparison of Calibration Methods Calibration Method Perspectivity Lens Distortion Line Curvature Precision/ Quality Static Target X Linear Target X Zig Zag Target X X X +++ Multiple Zig Zag Target X X X ++++

26 Establishing the Components of the Setup Camera Model Triangulation Angle Laser Bandpass Filter Camera Objektive Lens Scheimpflug Adapter Calibration

27 An Alternative to the Discrete Setup of a Triangulation Camera with Laser The Compact Sensors Factory calibrated 3D Laser triangulation scanners integrated in a ruggedized protective enclosure (IP67) Broad range of models with different FOV, resolution, Z-Range and working distance Already optimized triangulation designs featuring selected components such as Laser, camera objective lens and bandpass filter Enhanced depth of field by means of Scheimpflug optical setup

28 An Alternative to the Discrete Setup of a Triangulation Camera with Laser The Compact Sensors Main Advantages No focusing or calibration is required The 3D scan data can be acquired very quickly User friendly devices during commissioning and maintenance cases They accelerate the application development and shorten the time to market They offer the highest possible 3D scan precision thanks to the selected components and high accuracy factory calibration

29 An Alternative to the Discrete Setup of a Triangulation Camera with Laser The Compact Sensors Model 5

30 An Alternative to the Discrete Setup of a Triangulation Camera with Laser The Compact Sensors The Trend The compact sensors are preferred more and more in the development of 3D laser triangulation applications

31 Thank you for your attention!