A LowCost 3D Laser Imaging System

1
A Low-Cost 3-D Laser Imaging System

• B.P. Horana, S. Nahavandia
• a Intelligent Systems Research Lab, Deakin
  University, Victoria, Australia

2
Introduction

• As technology is rapidly advancing so does the
  need for further research into machine vision
• 3-D Laser Imaging technology provides the ability
  for a system to scan a particular surface to
  provide accurate measurement
• There are many commercially available laser
  imaging systems however, they are expensive
  prohibiting their use from otherwise suitable
  applications

3
Aims

• To investigate available technology to determine
  suitable system components
• To design and develop a 3-D Laser imaging system
  which exhibits a relatively low development cost
• To achieve the highest possible accuracy and
  minimum possible acquisition time

4
Advantages of 3-D Laser Imaging

• Capable of measuring extremely long distances
• Provides non-contact measurement
• Capable of very high accuracy
• Ability to measure hard to reach surfaces
• Faster than manual measurement

Reality
3-D Point Cloud
3-D CAD Model
Figure 1. Operation of commercial 3-D laser
imaging system
5
The Required Operation

• The laser imaging system is required to scan the
  object to provide the required information to
  construct an accurate 3-D surface model

Acquisition of 3-D data points
Data meshed into computer
Object
3-D point cloud created
Surface reconstruction
Reconstructed 3-D surface model
Figure 2. High level diagram of system operation
6
Methodology

• The interactions within the 3-D Laser Imaging
  System

Micro Controller Based control system
PC Interface Software
Angle Data
Deflection of laser beam
Control of 1-D rangefinder
Measured range value
Rangefinder
Figure 3. Overview of system interaction
7
1-D Laser Rangefinder
The rangefinder implemented in this application
is a commercial laser device.

• This rangefinder met most requirements
• Relatively low cost
• Sufficiently accurate
• Appropriate scanning range

Figure 4. Implemented Rangefinder
Table 1. Rangefinder specifications
8
Interfacing with the Rangefinder

• This rangefinder was not intended for this
  purpose.
• This rangefinder does not provide a serial
  interface.
• It was therefore necessary to develop a method
  to
• (1) Control the operation of the rangefinder
• (2) Extract the range data from the laser
  device

9
Control of the Rangefinder

• Relays were used to activate the buttons of the
  device
• The relays are controlled by the Micro Controller
• The open loop control of the rangefinder is
  achieved

Buttons Activated
Signal
Relay
Control System
Figure 5. Control of the rangefinder
10
Extraction of Range Data

• The determination of the shown range value was
  achieved by the following steps
• Obtain live video of the rangefinders screen
• Specify the locations of each segment of the
  range value
• Analyse the RGB values of each segment
• Determine whether each segment is ON or OFF
• Use algorithm to determine the digits displayed

Figure 6. Segments of displayed range value
11
Twin-Axis Tilting Mirror
A twin axis tilting mirror was designed and
developed to deflect the laser beam about Axis A
and Axis B Important considerations of this
design were

• Co-alignment of both axes
• Light weight design
• Accurate movement
• Simplistic construction
• Low development cost

Figure 7. Twin-axis tilting mirror
12
Drive Power

• Stepper motors were chosen to drive each axis of
  the mirror assembly.
• Advantages of using stepper motors
• Relatively low cost
• Simple to control
• Distinct steps allow accurate positioning

Figure 8. Stepper motor
Table 2. Specifications of stepper motors
13
Position Feedback
The system is provided with feedback of positions
at the upper and lower limit of each axis of
rotation. Advantages of optical sensors

• Achieved by optical sensors

• No limit to rotation of either axis
• Detected position remains consistent
• Suitability to mirror design
• Far cheaper than encoders

Figure 9. Optical Switch
14
Control System

• The control system is based around the BasicAtom
  Pro-24M Micro Controller.
• Uses the BASIC programming language

• Advantages of the BasicAtom Pro
• Support of floating point numbers
• Facilitates serial data transfer
• Large program memory
• Short development time

Figure 10. Control system electronics
15
Mounting of the Rangefinder

• The rangefinder was mounted above the mirror
  assembly.
• Allows effective calibration
• Allows a larger scanning range
• Scanning range
• 330 degrees in the horizontal motion
• - 45 degrees in the vertical motion

Figure 11. Physical orientation of system
16
The System Prototype
USB Camera
Light capsule
Mounted rangefinder
Light source
Tilting mirror assembly
Control system
Figure 12. The constructed system
17
Software Design
Start
The main requirements of our interface software
are to
Receive serial angle data

• Receive the serial angle data from our control
  system
• Determine the shown range value
• Calculate the 3-D surface co-ordinates
• Write the 3-D co-ordinates to a text file

Determine the shown range value
Calculate 3-D co-ordinates
Output 3-D co-ordinates to text file
Figure 13. Flowchart of Interface Software
18
Software Design

• The software was developed in Visual Basic,
• providing
• A graphical user interface
• Simple programming language
• Support for all our required components
• Short development time

19
Interface Software
 

Figure 14. Software design
20
Results

• The operation of the Laser Imaging system
• The ball is being scanned, and the 3-D surface
  co-ordinates are determined

Figure 15. System performing a scan
21
Results
3-D Scanning

• This object was scanned by the system
• Scanning distance of 1.0 m

Figure 16. The scanned ball
22
Results

• The 3-D point cloud of the scanned region

Figure 17. The point cloud of the ball

• The constructed 3-D mesh

Figure 18. The constructed mesh
23
Results- Accuracy

• The accuracy of an imaging system is an extremely
  important consideration
• The accuracy of the system was determined by the
  following

• A series of 3-D co-ordinates were obtained by the
  system
• These co-ordinates were then compared to known
  values
• The error relative to each axis can be observed

Figure 19. Accuracy in the X- direction
24
Results
Figure 20. Accuracy in the Y- direction
Figure 21. Accuracy in the Z- direction
25
Results

• Performance of the developed system
• Achieved an accuracy of approximately 2cm with
  respects to each of the X,Y and Z axes
• Spatial resolution is proportional to the
  scanning distance
• Acquisition time is proportional to scanning
  range

Table 3. Specifications of completed system
26
Conclusion

• Achieved the desired operation of the system
• Maintained low development cost
• Obtained the desired output
• Identified areas needing improvement

27
References

•  
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