Improving the 3D Scan Precision of Laser Triangulation

Transcription

1 Improving the 3D Scan Precision of Laser Triangulation

2 The Principle of Laser Triangulation Triangulation Geometry Example Z Y X Image of Target Object Sensor Image of Laser Line

3 3D Laser Triangulation Example of an AT 3D camera Image Acquisition by Means of High Speed CMOS Sensor Integrated Image Processing using FPGA technology Output of Profile Data Over GigE-Vision ADVANTAGE Profile Frequency up to Hz!

4 3D Scan Acquisition 3D Scan Scan Direction Conveyance of Target Object - Linear Stage - Conveyor Belt - Turntable

5 Common Line Detection Algorithms Reflexion of laser light on the object surface causes a Gauss shaped intensity distribution on sensor image Intensity distribution along a column of the sensor image MAX TRSH COG Finds Position of Maximum Intensity Finds Threshold Points Finds Center of Gravity

6 Parameters affecting 3D Scan Precision: Image Resolution Resolution depends on imaging scale and triangulation angle Lateral Resolution: dx=fov / Pixels per row Height Resolution: dz=dx/sin(α) COG Height Resolution with 6 Subpixels (factor 2 6 =64): dz_cog=dz/64 Example with AT Camera C GigE and α=30 FoV: 50mm dx=24µm dz=48µm dz_cog=0.75µm (with Output of 6 Subpixels)

7 Parameters affecting 3D Scan Precision: Noise due to Laser Speckles Phenomenon Laser Speckles The waves of laser light reflected from different parts of the target surface impinge on the sensor with different phase, which is caused by the different optical path. The interference of these waves on the sensor causes intensity variations of the laser line image Laser light reflection impinging on the sensor with different phase

8 Parameters affecting 3D Scan Precision: Noise due to Laser Speckles Phenomenon Typical Sensor Image of 660nm Laser Line. Intensity variations caused by Speckles Maximum Intensity per Sensor Image Column Distorted Intensity Distribution along a column due to Speckles

9 Parameters affecting 3D Scan Precision: Noise due to Laser Speckles Phenomenon Example of height profile detected by COG algorithm (laser 660nm) Standard deviation of noise: ca. 14 µm 3D Setup: AT C GigE 2048 Pixels/Profile, FoV=50mm, α=30, dx=24µm, dz_cog=0.75µm

10 Improving the 3D Scan Precision with the Finite Impulse Response Filter (FIR) FIR-Filter (Savitzky-Golay) Selection 5, 7 or 9 Coefficients Smoothed intensity data Raw intensity data Smoothing Filter Numerical Average Gauss Average COG

11 Improving the 3D Scan Precision with the Finite Impulse Response Filter (FIR) Intensity distribution along a column of sensor image after application of FIR filter in smoothing mode Standard deviation of noise: ca. 11 µm 20% improvement Height profile detected by COG algorithm with FIR smoothing filter (Laser 660nm) 3D Setup: AT C GigE 2048 Pixels/Profile, FoV=50mm, α=30, dx=24µm, dz_cog=0.75µm

12 Improving the 3D Scan Precision with the FIR-PEAK algorithm FIR-Filter (Savitzky-Golay) FIR-PEAK Selection 5, 7 or 9 Coefficients Differential Filter Raw intensity data Detection of Zero Crossing outputs the position of Gauss curve at a resolution of 6 subpixels (1/64 pixel)

13 Improving the 3D Scan Precision with the Finite Impulse Response Filter (FIR) Comparison between COG with FIR smoothing filter and FIR-PEAK Standard deviation of height values in the profile (µm) Surface Material COG with FIR FIR-PEAK Anodized Aluminum Machined Aluminum 10 9 Galvanized Steel 9 9 White Plastic 8 10 Black Plastic Both algorithms give almost the same precision. The COG with FIR filter works better with thick lines. The FIR-PEAK is more precise in applications with thin lines. 3D Setup: AT C GigE 2048 Pixels/Profile, FoV=50mm, α=30, dx=24µm, dz_cog=0.75µm

14 Improving the 3D Scan Precision by means of Laser with Shorter Wavelength Image of 660nm Laser Line The Speckle effect depends on the laser wavelength. Image of 405nm Laser Line Shorter laser wavelength causes less speckles and reduces the noise.

15 Improving the 3D Scan Precision by means of Laser with Shorter Wavelength Height profile of white plastic surface acquired using COG algorithm with FIR smoothing filter Laser 660nm Standard deviation of noise: ca. 6 µm Laser 405nm Standard deviation of noise: ca.4 µm 30% improvement 3D Setup: AT C GigE 2048 Pixels/Profile, FoV=50mm, α=30, dx=24µm, dz_cog=0.75µm

16 Improving the 3D Scan Precision of Laser Triangulation Summary The 3D scan precision of Laser triangulation depends on: Image scaling / resolution Laser Speckle effect The precision can be improved by means of: sophisticated algorithms such as the Finite Impulse Response smoothing Filter (FIR) and the FIR-PEAK detection method Lasers with shorter wavelength such as 405nm

17 Thank you for your attention!