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A ballbot is a dynamically-stable mobile robot designed to balance on a single spherical wheel i. Through its single contact point with the ground, a ballbot is omnidirectional and thus exceptionally agile, maneuverable and organic in motion compared to other ground vehicles. Its dynamic stability enables improved navigability in narrow, crowded and dynamic environments.
The ballbot works on the same principle as that of an inverted pendulum. The first successful ballbot was developed in    by Prof. Hollis and his group at CMU demonstrated that the ballbot can be robust to disturbances including kicks and shoves, and can also handle collisions with furniture and walls.
In , around the same time when CMU Ballbot  was introduced, a group of researchers at University of Tokyo independently presented the design for a human-ridable ballbot wheelchair that balances on a basketball named "B.
Since the introduction of CMU Ballbot  in , several other groups around the world have developed ballbots. Kumagai and his group demonstrated the capability of ballbots to carry loads and be used for cooperative transportation. Ballbots have also appeared in the science fiction world.
Syfy 's TV series Caprica featured "Serge", a ballbot butler robot. Historically, mobile robots have been designed to be statically stable, which results in the robot not needing to expend energy while standing still. This is typically achieved through the use of three or more wheels on a base. In order to avoid tipping, these statically-stable mobile robots have a wide base for a large polygon of support, and a lot of dead weight in the base to lower the center of gravity.
They also tend to have low acceleration or deceleration to avoid tipping. The wide base makes it difficult for statically-stable mobile robots to navigate cluttered human environments.
Moreover, these robots have several other limitations that make them poorly suited to a constantly changing human environment. They can neither roll in any direction, nor can they turn in place. The desire to build tall and narrow mobile robots that do not tip over led to development of balancing mobile robots like the ballbot. A ballbot generally has a body that balances on top of a single spherical wheel ball.
It forms an underactuated system, i. The ball is directly controlled using actuators , whereas the body has no direct control. The body is kept upright about its unstable equilibrium point by controlling the ball, much like the control of an inverted pendulum. The counter-intuitive aspect of the ballbot motion is that in order to move forward, the body has to lean forward and in order to lean forward, the ball must roll backwards.
All these characteristics make planning to achieve desired motions for the ballbot a challenging task. In order to achieve a forward straight line motion, the ballbot has to lean forward to accelerate and lean backward to decelerate. Unlike two-wheeled balancing mobile robots like Segway that balance in one direction but cannot move in the lateral direction, the ballbot is omni-directional and hence, can roll in any direction.
It has no minimal turning radius and does not have to yaw in order to change direction. The most fundamental design parameters of a ballbot are its height, mass, its center of gravity and the maximum torque its actuators can provide. The choice of those parameters determine the robot's moment of inertia, the maximum pitch angle and thus its dynamic and acceleration performance and agility.
The maximum velocity is a function of actuator power and its characteristics. Beside the maximum torque, the pitch angle is additionally upper bounded by the maximum force which can be transmitted from the actuators to the ground. Therefore, friction coefficients of all parts involved in force transmission also play a major role in system design.
Also, close attention has to be paid to the ratio of the moment of inertia of the robot body and its ball in order to prevent undesired ball spin, especially while yawing. The ball is the core element of a ballbot, it has to transmit and bear all arising forces and withstand mechanical wear caused by rough contact surfaces.
A high friction coefficient of its surface and a low inertia are essential. Rider  used a basketball, while BallIP  and Adelaide Ballbot  used bowling balls coated with a thin layer of rubber. In order to solve the rather complex problem of actuating a sphere, a variety of different actuation mechanisms have been introduced. The CMU Ballbot  used an inverse mouse-ball drive mechanism.
Unlike the traditional mouse ball that drives the mouse rollers to provide computer input, the inverse mouse-ball drive uses rollers to drive the ball producing motion. The inverse mouse-ball drive uses four rollers to drive the ball and each roller is actuated by an independent electric motor. In order to achieve yaw motion, the CMU Ballbot uses a bearing, slip-ring assembly and a separate motor to spin the body on top of the ball. This drive mechanism does not require a separate yaw drive mechanism and allows direct control of the yaw rotation of the ball.
Unlike CMU Ballbot  that uses four motors for driving the ball and one motor for yaw rotation, BallIP  and Rezero  use only three motors for both the operations. Moreover, they only have three force transmission points compared to four points on CMU Ballbot.
Since the contact between an omni-wheel and the ball should be reduced to a single point, most available omni-wheels are not properly suitable for this task because of the gaps between the individual smaller wheels that result in unsteady rolling motion.
Therefore, the BallIP  project introduced a more complex omni-wheel with a continuous circumferential contact line. The Rezero  team equipped this omni-wheel design with roller bearings and a high-friction coating. Masaaki Kumagai,  who developed BallIP  introduced another ball drive mechanism that uses partially sliding rollers .
In order to actively control the position and body orientation of a ballbot by a sensor-computer-actuator framework, beside a suitable microprocessor or some sort of other computing unit to run the necessary control loops, a ballbot fundamentally requires a series of sensors which allow to measure the orientation of the ball and the ballbot body as a function of time.
To keep track of the motions of the ball, rotary encoders CMU Ballbot,. The CMU Ballbot  is the first and currently, the only ballbot to have arms. The arms are hollow aluminium tubes with a provision to add dummy weights at their ends. In their present state, the arms cannot be used for any significant manipulation, but are used to study their effects on the dynamics of ballbot. The mathematical MIMO-model which is needed in order to simulate a ballbot and to design a sufficient controller which stabilizes the system, is very similar to an inverted pendulum on a cart.
Initially, CMU Ballbot  used two 2D planar models in perpendicular planes to model the ballbot   and at present, uses 3D models without yaw motion for both the ballbot without arms    and the ballbot with arms. Rezero  uses a full 3D model that includes yaw motion too. The CMU Ballbot  uses an inner balancing control loop that maintains the body at desired body angles and an outer loop controller that achieves desired ball motions by commanding body angles to the balancing controller.
It uses the laser range finder to actively detect and avoid both static and dynamic obstacles in its environment. The biggest concern with a ballbot is its safety in case of a system failure. There have been several attempts in addressing this concern. The CMU Ballbot  introduced three retractable landing legs that allow the robot to remain standing statically-stable after being powered down.
It is also capable of automatically transitioning from this statically-stable state to the dynamically-stable, balancing state and vice versa.
Due to its dynamic stability, a ballbot can be tall and narrow, and can also be physically interactive, making it an ideal candidate for a personal mobile robot. The present day ballbots    are restricted to smooth surfaces. The concept of the ballbot has attracted a lot of media attention,      and several ballbot characters have appeared in Hollywood movies. From Wikipedia, the free encyclopedia. International Conference on Control. Application of passive motion to transport".
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