Path-Following Autonomous Convoy with Multiple Asynchronous Nodes

Kyle Lemons, Heather Macfie,Tri Pho, G. M. Ewout van Bekkum

PACMAN

Georgia Institute of TechnologySchool of Electrical and Computer Engineering

Preliminary Design ReviewECE 4007, L01 DK2

March 17, 2010

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 inter-

vehicle communications

Design Objectives

• Path-followingo Follow distance: 100 cmo Max deviation from path: 10 cm

• Autonomous operationo Speed: 60 cm/so Turning radius: 50 cm

• Passive operation

• Modular ACVs

Project Schedule

• Completed on or ahead of scheduleo Parts orderedo Wheel encoders and infrared cameras mounted, wired,

and communicating with FPGAo PWM implemented to control steering and accelerationo FPGA powered and equipped with basic track and follow

• Ultrasonic range finders may be unnecessary

• Upcoming worko Custom PCB development begins March 22 o Algorithm determination and optimization begin March 29o 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: CMUcamo Laser rangefinder: Neato Robotics’ Revo LDSo Infrared sensor: Nintendo’s Wii Remote

Images: http://www.cmucam.org; http://www.hizook.com/blog/2009/12/20/ultra-low-cost-laser-rangefinders-actualized-neato-robotics; http://www.gadgetspage.com/toys-games/how-does-the-wii-remote-work.html

CMUcam Revo LDS Wii Camera

Wii Remote's PixArt Infrared Camera

• Sensitive to any bright light sourceo IR pass filter isolates IR wavelengths

• Fast embedded blob trackingo Up to four blobs at onceo Refresh rate of 100 Hz

• Communication over I²C protocolo 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 caro 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 verticallyo Resolves turning ambiguityo 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 cycleo Variable duty cycleo Constant frequencyo Simple to implement in hardware

• RC platform includes PWM control of steering and throttleo Experimentally determined timingso 50 Hz frequency, limited to 5% to 10% duty cycle

Steering Control• Steering PWM control signal

o Calibration required per RC unito Control module uses 1024 steps

• Servo motoro Servo motor controller has unknown PWM resolutiono Experimentally determined behaviors

Symmetrical "left" and "right" sensitivity Centered around 7.1%

Throttle Control• Throttle PWM control signal

o Large "idle" dead-band for throttleo Higher "forward" sensitivity than "reverse"o Brake mode function problems

• Drive motoro Requires higher throttle to "kick-start" movement

Will prove problematic when trying to move slowlyo 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

• Measure rotation of rear wheels• Calculate change in heading and position• Implement in the FPGA as a module in VHDL

 

Dead Reckoning with Differential Drive

Quadrature Wheel Encoding

• Pattern inside rear wheels  • Quadrature encoding

o Gray code output  o Double resolutiono Simpler acceptance testing 

     

• HLC1395-002 Photo Reflector from Honeywell o Infrared LED and photo-transistor in the same package o 100 mW power dissipationo 0.6 mA photo-transistor on current

• Inverting Schmitt Triggero Debounce and convert analog to discrete output

   

 

Electrical Design of Wheel Encoders

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"

• Optionso Regressiono Cluster averaging

• Issues for considerationo Computational complexityo Path accuracy

Path Traversal• Comparing vehicle location with

path generated

• Optionso Absolute reference frameo Relative reference frameo Moving reference frame

• Issues for considerationo Numeric overflowo Pre-/Post-processingo Compounding rounding error

Absolute reference

Relative reference

Moving reference

PACMAN

• Final Prototypeo More cameraso Two ACVso Path-following

• Current Prototypeo One camerao Limited field of viewo Basic point-and-steer followo Coasts to a stop

QuestionsBut first!  A demo...