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PACMAN
Path-Following Autonomous Convoy with
Multiple Asynchronous Nodes
Preliminary Design Review
ECE 4007, L01 DK2
March 17, 2010
Kyle Lemons, Heather Macfie,
Tri Pho, G. M. Ewout van Bekkum
Georgia Institute of Technology
School of Electrical and Computer Engineering
Project Overview
• Proof-of-concept prototype
• Two or more Autonomous
Convoy Vehicles (ACVs)
• Path-follow algorithm
• Cost: $300
• Military convoy applications
• Reduction of human
requirements
• Supplement to existing
navigation systems
• Alternative to complex intervehicle communications
Design Objectives
• Path-following
o Follow distance: 100 cm
o Max deviation from path: 10 cm
• Autonomous operation
o Speed: 60 cm/s
o Turning radius: 50 cm
• Passive operation
• Modular ACVs
Project Schedule
• Completed on or ahead of schedule
o Parts ordered
o Wheel encoders and infrared cameras mounted, wired,
and communicating with FPGA
o PWM implemented to control steering and acceleration
o FPGA powered and equipped with basic track and follow
• Ultrasonic range finders may be unnecessary
• Upcoming work
o Custom PCB development begins March 22
o Algorithm determination and optimization begin March 29
o Sensor bar mounting begins April 2
System Module Interaction
Current Prototype
Component Protocols and Standards
Component Protocols and Standards
Component Protocols and Standards
Component Protocols and Standards
Robot Vision
• Options for optically tracking preceding vehicle:
o Vision sensor: CMUcam
o Laser rangefinder: Neato Robotics’ Revo LDS
o Infrared sensor: Nintendo’s Wii Remote
CMUcam
Revo LDS
Wii Camera
Images: http://www.cmucam.org; http://www.hizook.com/blog/2009/12/20/ultra-low-cost-laser-rangefindersactualized-neato-robotics; http://www.gadgetspage.com/toys-games/how-does-the-wii-remote-work.html
Wii Remote's PixArt Infrared Camera
• Sensitive to any bright light source
o IR pass filter isolates IR wavelengths
• Fast embedded blob tracking
o Up to four blobs at once
o Refresh rate of 100 Hz
• Communication over I²C protocol
o Supports fast mode (400 kbit/s)
• Field of view of 40°
o Initially troublesome
Resolved Problem:
Configuration of Camera and IR LEDs
• Initially, the sensor bar (the IR LEDs) was to be placed
horizontally
o The issue arose for discerning
the difference between turning
car and distant car
o Stereoscopic vision for
depth perception was the fix
Limited Field of View
• In order to meet design specifications, more than two
cameras were needed
• Stereoscopic vision required an overlap of camera field of
views
Thinking in the Wrong Direction
• Attempting to compensate for poor initial configuration
• Reorient the sensor bar vertically
o Resolves turning ambiguity
o Simplifies relative location calculation
• Requires three cameras instead of seven for a 120° FOV
Relative Location: Direction
Relative Location: Distance
PWM Control: Steering and Throttle
• Pulse Width Modulation (PWM)
o Controlled by signal duty cycle
o Variable duty cycle
o Constant frequency
o Simple to implement in hardware
• RC platform includes PWM control of steering and throttle
o Experimentally determined timings
o 50 Hz frequency, limited to 5% to 10% duty cycle
Steering Control
• Steering PWM control signal
o Calibration required per RC unit
o Control module uses 1024 steps
• Servo motor
o Servo motor controller has unknown PWM resolution
o Experimentally determined behaviors
 Symmetrical "left" and "right" sensitivity
 Centered around 7.1%
Throttle Control
• Throttle PWM control signal
o Large "idle" dead-band for throttle
o Higher "forward" sensitivity than "reverse"
o Brake mode function problems
• Drive motor
o Requires higher throttle to "kick-start" movement
 Will prove problematic when trying to move slowly
o Higher top forward speed than reverse
Odometry
• Where are we on the measured path
• Estimate our location with odometry
o Optical Flow
 Optical mouse
 Web-cam
o Rotary Encoder
 Magnetic
 Photo-interrupters
 Photo-reflector
Image: http://www.mccoop.de/images/street-video.png
Dead Reckoning with Differential Drive
• Measure rotation of rear wheels
• Calculate change in heading and position
• Implement in the FPGA as a module in VHDL
Quadrature Wheel Encoding
• Pattern inside rear wheels
• Quadrature encoding
o Gray code output
o Double resolution
o Simpler acceptance testing
Electrical Design of Wheel Encoders
• HLC1395-002 Photo Reflector from Honeywell
o Infrared LED and photo-transistor in the same package
o 100 mW power dissipation
o 0.6 mA photo-transistor on current
• Inverting Schmitt Trigger
o Debounce and convert analog to discrete output
Physical Design of Wheel Encoders
Problems with Wheel Encoding
• What if the wheels slips?
o Remove power to the rear wheel
• Maximum resolution?
o No optical specifications other than “unfocused”
o Testing showed ~36-48 steps per revolution
• Duty cycle in the gray code exactly 50%?
o Photo-reflector measures reflectance, but what if it sees
more than one step?
o Optimize duty cycle of the light areas on the pattern
Path Generation
• Markers denote lead
vehicle position
• "Visual Snakes"
• Options
o Regression
o Cluster averaging
• Issues for consideration
o Computational complexity
o Path accuracy
Path Traversal
• Comparing vehicle location with
path generated
• Options
o Absolute reference frame
o Relative reference frame
o Moving reference frame
Absolute reference
• Issues for consideration
o Numeric overflow
o Pre-/Post-processing
o Compounding rounding error
Relative reference
Moving reference
PACMAN
• Final Prototype
o More cameras
o Two ACVs
o Path-following
• Current Prototype
o One camera
o Limited field of view
o Basic point-and-steer follow
o Coasts to a stop
Questions
But first! A demo...
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