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Senior Project – Computer Engineering - 2012
Advanced Educational Robot
Calder Phillips-Grafflin and Erik Skorina (ME)
Advisors – Prof. John Spinelli and David Hodgson (ME)
Objectives
Customers in the Computer Science department identified a number of critical features for a
research and teaching robot:
• The robot must have an equivalent level of maneuverability as an adult human (2 degrees of
linear freedom, 1 degree of angular freedom, speed of 1.5 m/s) without incurring undue
mechanical complexity.
• The robot must be capable of carrying a meaningful payload (~10 kg) and supporting it with
the necessary power, data connections, and processing power.
• The robot must provide an accessible software platform that is both accessible and powerful,
posing only limited obstacles to use for novice student users without limiting the capabilities
available to more advanced users.
• The robot must be as inexpensive and flexible as possible, using off-the-shelf components
whenever possible to limit costs and ease fabrication. In addition, it must be easy to modify
without the need of special tools.
Performance
Mechanical Design
• The mechanical design of the robot achieves
maximum maneuverability with minimal
mechanical complexity through the use of
mecanum wheels that provide holonomic drive
without the need for steering axles.
The robot largely meets or exceeds the project objectives:
• Software design is simple and lightweight while maintaining complete user-level abstraction
of the mechanical and electrical characteristics of the robot.
• ROS driver provides a simple interface to all robot actuators and sensors following standard
ROS conventions and accepted practices.
• All parts can be easily fabricated with the tools
available in a college machine shop.
• Robot achieves maneuverability goals with minimal mechanical complexity
(complexity is handled in software, not hardware).
• Complete assembly of the robot can be
completed in less than a day.
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• Vehicle design is mechanically simple, straightforward to assemble, and cost-effective (less
than $3500) in comparison to similar robotics platforms (such as the Pioneer 3).
System Design
The control system of the robot is designed to
maintain complete abstraction of the inherent
performance complexities of the mecanum wheel
platform. It achieves this through a 3-tier system:
• The individual motors are controlled by serialconnected motor controllers that provide braking,
over-current monitoring, and fault detection.
• The motor controllers are commanded by an
ARM Cortex-M3 microcontroller that converts float
speed commands to byte control commands,
automatic stopping (1 second timeout), and speed
calculation via Hall-effect encoders.
• The microcontroller is commanded over a USB
serial link by an ROS driver running on the
onboard computer (a 64-bit Linux PC).
Problems
During the construction of our robot, we encountered a number of problems due to the unexpected
operating characteristics of the mecanum wheels and the complexity of the control system:
• Wheel slippage is the primary cause of performance problems
(not driving in a straight line, inability to maintain a constant-radius curve)
• Wheel slippage is almost always caused by irregularities and differences in the floor
(the robot behaves very differently on concrete than it does on carpet)
• Consumer-grade accelerometers and gyroscopes are not accurate enough to allow active
correction of vehicle motion (<10°/s course deviation at 1.5 m/s)
• Feedback from wheel encoders is enough to identify wheel slip but not enough to correct for it
Future Work
Software Design
The ROS driver provides three basic software
configurations for users:
Further improvements on the robot itself are almost entirely focused on the drive system and vehicle
sensors:
• A core driver that provides sensor interfacing,
drive control, and battery monitoring. The driver
responds to the standard ROS “cmd_vel” velocity
command messages.
(Run with “./DriverCore.sh”)
• Design and implement a suspension system
(this should address the main causes of wheel slippage)
• Implement an inertial measurement system for self-contained indoor navigation
• Implement a closed-loop motion control system
• A standard teleoperation driver that provides all
features of the core driver combined with user
control via a USB game controller.
(Run with “./Teleop.sh”)
• A stripped-down teleoperation driver that
provides the minimum functionality needed to
control the vehicle in the event of software failure.
(Run with “./Rescue.sh”)
Acknowledgements
John Spinelli
David Hodgson
John Rieffel
Rhonda Becker
Lisa Bremigen
Paul Tompkins
Nick Webb
Stan Gorski