Ballbot

The Ballbot is a mobile robot, an attempt to solve the robotic unicycle problem, and is designed to balance itself on its single spherical wheel while travelling about. It is the focus of the Ballbot Research Platform, a project conducted at Carnegie Mellon University, made possible by grants from the National Science Foundation. The robot is being developed by Ralph Hollis and George Kantor, with help from the graduate students Tom Lauwers, Anish Mampetta and Eric Shearer. The purpose of the Ballbot project is to discover how robots may maintain "dynamic stability" (that is, reliable balance), to enable designs with narrower bases for improved navigability (such as in a crowded room). This is a departure from the current paradigm in robot design, which relies on the static stability provided by having three or more wheels (and a much wider base). The project is titled "Dynamically-Stable Mobile Robots in Human Environments".

Ballbot balances with the aid of on-board sensors and computers and uses an "actuator mechanism based on an inverse mouse-ball drive" to move and change direction without needing to turn first [cite web|url=http://www.msl.ri.cmu.edu/publications/pdfs/isrr05.pdf |title=One is Enough! |accessdate=2006-08-14 |author=Tom Lauwers |coauthors=George Kantor, Ralph Hollis |date=2005-10-12 |format=PDF |work=12th International Symposium on Robotics Research |publisher=The Robotics Institute at Carnegie Mellon University |pages=10 |language=English] .

Background

Historically, mobile robots have been made to be "statically stable", which means that when stopped, the robot does not need to expend energy to remain still. This is typically achieved through the use of three or more wheels combined on a base. Unfortunately, robots built on this model are frequently "unstable" when moving unless equipped with a very wide base and low center of gravity. This severely limits their usefulness in normal human environments as the pathways are typically too narrow and often have many obstacles (like humans) that will impair the robot's movement.

To solve this problem, the team at Carnegie Mellon decided to investigate robots that are safe, agile and capable of graceful motion, slender enough to easily maneuver in people-cluttered environments, and which readily yield when pushed around [cite web|url=http://www.msl.ri.cmu.edu/publications/pdfs/isrr05.pdf |title=One is Enough! |accessdate=2006-08-14 |author=Tom Lauwers |coauthors=George Kantor, Ralph Hollis |date=2005-10-12 |format=PDF |work=12th International Symposium on Robotics Research |publisher=The Robotics Institute at Carnegie Mellon University |pages=10 |language=English] .

The first "dynamically stable" robot demonstrated for the public was built in Japan and shown in 1994. This design had two wheels and used an inverted pendulum for control. Later, that same research team introduced another machine that used a single prolate ellipsoid (somewhat like a rugby ball) on an axle combined with a hinge to provide stability both forward/backward through wheel torque and side-to-side by leaning on the hinge. Since then, the Segway Human Transporter has been released along with a number of robots based on its self-balancing concepts [cite web|url=http://www.msl.ri.cmu.edu/publications/pdfs/isrr05.pdf |title=One is Enough! |accessdate=2006-08-14 |author=Tom Lauwers |coauthors=George Kantor, Ralph Hollis |date=2005-10-12 |format=PDF |work=12th International Symposium on Robotics Research |publisher=The Robotics Institute at Carnegie Mellon University |pages=10 |language=English] .

These systems have limitations that make them poorly suited to a constantly changing human environment. They cannot immediately roll in any direction, nor can they turn in place, or open a door without knowing exactly where the hinges are located (to form the correct arc). Ballbot addresses those problems by replacing the ellipsoidal wheel and hinge with a single spherical wheel and actuators to roll it.

The System

The Ballbot prototype is a human-height cylinder balanced on top of a single ball about 20 centimeters in diameter. The cylinder contains the battery, sensors, and control computer in the top with the drive system in the bottom. There are also three legs that can be extended to keep the system upright after it is turned off. The whole system weighs about 45 kilograms and has a relatively high center of gravity.

The drive mechanism is an inversion of the principle used in computer mice to sense movement. Instead of the ball moving rollers as in a mouse, it consists of one drive roller on the back of the ball and two "idler rollers" on the sides that move the ball. The rollers are turned using drive belts attached to independently controlled electric servomotors. This setup led to some interesting results in testing where it was noticed that the prototype could move backward more easily than forward since having the drive roller on only one side created a force imbalance [cite web|url=http://www.msl.ri.cmu.edu/publications/pdfs/ballbot_ICRA06_web.pdf |title=A Dynamically Stable Single-Wheeled Mobile Robot with Inverse Mouse-Ball Drive |accessdate=2006-08-14 |author=Tom Lauwers |coauthors=George Kantor, Ralph Hollis |date=2006-05-16 |format=PDF |work=IEEE International Conference on Robotics and Automation |publisher=The Robotics Institute at Carnegie Mellon University |pages=6 |language=English] . This imbalance causes the ball to be pushed up into the body when rolling forward, thus increasing the static friction that must be overcome to move the robot forward. Future designs may address this by including a second drive rotor on the opposite side of the ball.

See also

* Autonomous robot
* BigDog (dynamically stable quadruped robot)
* Electronic Stability Control
* Humanoid robot (balance is required to stand and walk)
** ASIMO (can traverse stairs, and can run)
* Inverted pendulum
* Robot locomotion
* Robotic unicycle

External links

[http://www.msl.ri.cmu.edu/projects/ballbot/ Project web site]

References