On the Design, Construction and Operation of a Diffraction Rangefinder MS Thesis Presentation Gino Lopes A Thesis submitted to the Graduate Faculty of.

On the Design, Construction and Operation of a Diffraction

Rangefinder

MS Thesis Presentation

Gino LopesA Thesis submitted to the Graduate Faculty of Fairfield University

in partial fulfillment of the requirements for the degree of a Master of Science in the Electrical and Computer Engineering.

Outline

• Problem• Approach• Motivation• Rangefinding• Design and Testing• Performance and Comparison• Conclusion• Future Work

Problem

• Design a diffraction rangefinder, subject to the following constraints:– Fit on a desktop, – Digitize and display objects,– Be affordable,– Be easy to use,– Not suffer from occlusion issues, characteristic of

triangulation rangefinders,– Characterize the performance of the rangefinder

Approach

• Design a Prototype for testing.– Hardware• Diffraction grating.• Network Camera instead of USB camera.• Laser line generator.• Motion control hardware.

– Software• JAVA was used for everything.

– Layout of 3D Scanner• Dependent on hardware parameters.

Motivation

• Diffraction rangefinders represent a new class of rangefinder for digitizing object.

• Verify predicted performance.

Rangefinding

• Types of Rangefinders:– Shape to shading:• Process of computing the shape of a three-dimensional

surface by looking at the brightness of one image of the surface.• Shape to shading is difficult to implement.

Rangefinding Continued

– Triangulation:• Finds the range-to-target by using two different views

(angles) of the target, or by making use of off-access illumination. • Transmitter and receiver are separated.• Subject to shadows.

Rangefinding Continued

– Light Detection and Ranging (LIDAR) system:• Uses laser pulse time of flight.• Receiver and transmitter can be co-axial and shadows

and occlusion limitation are minimized.• For surface scanning the laser source or target would

need to be moved in both the x-axis and y-axis.– To collect enough data points to reproduce the surface detail.

Rangefinding Continued

– Diffraction Rangefinders:• Measures the distance to a target by reading the

curvature of the wave front.• Work with (active illumination) using a laser.• Less susceptibility to occlusion.• Receiver and transmitter can be co-axial.• Limitation in range of measurement due to size of the

grating.

Design

• Scanner Design:– Illumination Source:• Off the shelf red laser line generator

– Vision System:• Network Camera• 1000 line/mm Diffraction Grating

– Motion System:• Motor and controller.• Rotary Turntable.

2D View of Scanner Layout

2D View Of Scanner Layout Cont.

3D Scanner Prototype

Testing

• Testing of scanner performance.– Calibration wedge used as a resolution target. • Target with known dimensions.• Verification of operation

Scanner Test Configuration

Testing Continued

• Calibration wedge was positioned at 49mm, 92mm, and 135mm from grating.

Wedge at 49mm

After Processing at 49mm

Wedge at 92mm

After Processing at 92mm

Wedge at 135mm

After Processing at 135mm

Scanner Comparison

• Scanner characteristics was compared against two other scanners on the market.

– One from VXTechnologies.

– One from Cyberware.

Scanner Comparison Continued

3D ScannerVXTechnologies

StarCam Cyberware

Field of View 12" X 7" (310mmX178mm) 21" X 16" (533mmX406mm) 14" X 17" (350mmX440mm)

Resolution 0.017" (0.44mm) 0.019" (0.48mm) 0.015" (0.38mm)

Width 11.5" 16.375” (416mm) 188.2 cm (74.1")

Height 14" 11.000” (280mm) 205.3 cm (80.8")

Length 30" 9.250” (235mm) Not Given 

3D Image of Chess Piece

3D Image of Chess Piece Cont.

3D Image of Chess Piece Cont.

Conclusion

Average resolution of the 3D Scanner was between 0.43mm and 0.44mm.

(Comparable to other rangefinders on the market)

Future Work

• Replacing the turntable with an improved model.

• Replacing the Lego motor and RXTX controller with a stepper motor and controller.

• Increasing the laser fan angle from 60.

Future Work Continued

• Replacing the camera with one that allows for turning automatic gain off.– Reduce noise and blooming.

• Improve image acquisition and processing software user interface.

• Verify repeatability of scanner.

Data Analysis

• Using Grating equation to calculate dispersion angle of 1000 line per mm grating.

Number of slits per mm (q): 1000

One mm in meters: 0.001

Center to center distance between slits (p) in meters (1mm/q): 0.000001

Wavelength of light source (lambda) in meters: 0.000000629

Diffraction Order (n): 1

Dispersion Angle (sin(a)=(n*lambda)p) in degrees: 39

Data Analysis Cont.

• Using trigonometry to calculate mm per pixels from acquired data.

Calculated dispersion Angle of grating: 39

Distance from grating to target (D) in mm: 135

Tan(b): 0.806

Distance between zero-order and first-order fringes in mm: 108.865

Experimental Data

• Average Number of Pixels:

Distance to Target Between Zero Order and First Order on Right Side

Between Zero Order and First Order on Left Side

49mm 301.029 323.206

92mm 260.559 266.059

135mm 201.735 207.088

Experimental Data Continued

• Pixels per mm.

Distance to TargetBetween Zero Order and First Order on Right

Side (pixels/mm)Between Zero Order and First Order on Left

Side (pixels/mm)

49mm 2.765 2.969

92mm 2.393 2.444

135mm 1.853 1.902

Experimental Data Continued

• Average distance resolvable.

Distance to TargetBetween Zero Order and First Order on Right

Side (mm)Between Zero Order and First Order on Left

Side (mm)

49mm 0.36 0.34

92mm 0.42 0.41

135mm 0.54 0.53

Average 0.44 0.43

Standard Deviation 0.091 0.096