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maxon1110cover story

An interdisciplinary team of students has an impressive way into reality, which sounds a little crazy at first, and actually contradict basic laws of physics seem. Her Ballbot "Rezero" not only creates a balancing act on the ball at it's finest, but it is also impressive with convincing technical solutions, unprecedented agility and aesthetic design.

In a focus project of the Swiss Federal Institute of Technology Zurich (ETHZ) to give students the Autonomous System Lab (ASL) with colleagues at the Zurich University of Applied Sciences (ZHAW) and the Zurich University of the Arts (ZHdK) coalesced into a collaboration of a special kind . The result of this collaboration is called Rezero and is the first Ballbot who not only mastered the complex art of balancing. (Ballbot is the name of a robot class, which balance out independently on a ball and is able to move it.) He is also able to exploit its movement potential fully and clearly demonstrated that a robot can look very appealing and opposite him hardly need to pass inhibitions. Whether as a tourist guide, servant, or help everyday toys, Rezero feels in tight spaces or busy environment in the element. He is a movement artist and entertainer, he impressed, stirs emotions and interact around an impressive way with the people around him.Maxon Motor2

The principle Ballbot

A striking feature of a Ballbot is the "ball" on which he stands. Its contact area with the ground is really just a point, which makes it fundamentally unstable and highly vulnerable to fall. The other hand, this circumstance permits the free movement in either direction and the rotation about its own axis.

Another aspect: The Roles in a certain direction causes a counter moment - tilting of the robot's direction of motion - which would take him to actually turn to the case. This fact is much more but a Ballbot to benefit by slightly before driving leaning in the opposite direction to nearly back down behind his focus. To further accelerate the Ballbot can easily fall a little more in the direction of travel. To delay it must however only further accelerate to "overtake" his focus before he, along with the synchronized erection can come to a halt. But utter standstill "Ballbots of death." Even though he seems to stand fully upright, he keeps his balance by constantly under his emphasis balanced by movement of the ball.

A fundamental hurdle that must be overcome in the drive of the ball: A "normal" wheel rotates so radially around its own axis, either forward or backward. However, a movement in a direction parallel to its axis without overcoming the friction is not possible. Applied to the drive of a ball, this means that during its movement, at least inhibits one of the drive wheels in the axial direction. This fact makes the use of a multi drive necessary. So-called "Omniwheels" take over the power transmission in one direction at the same time freewheel in the other direction. This multi-directional wheels work in the radial direction as "normal" wheels and thereby transferred corresponding forces. In the axial direction, they have rotating universal elements that move freely under axial load.

The "ball"

The ball is a project-house development, which clearly stands out from other solutions. Their construction and the requisite process for preparing established together with an industrial partner.

It is constructed from a high-precision geometric aluminum hollow body and a 4 mm thick rubber, wear-resistant surface that has a very high coefficient of static friction. The exact spherical shape and the high friction ensure high running smoothness and extremely dynamic acceleration and high speeds of up to 3.5 m / s (nearly 13 km / h!). Taking Ballbots known as reference, shows that many of them often move slightly "out of round" and more noticeable by inertia and slowness.

Power Type

Maxon Motor3Also the three-degree angle in 120 arranged around the ball Omniwheels are proprietary development. Because available solutions on the market could not meet the demand for a completely closed outer contour. Their elaborate geometry, low-loss design of individual parts and the optimized power transmission to surface resulting in an exceptional, high-quality construction. The wheels are driven by engine / transmission combinations Maxon Motor from Sachseln in Switzerland. They each consist of a Maxon EC 4pole 30, One Planetary Gearhead GP 42 C and Angle Encoder HEDL 5540. The suspension of the drive is designed so that the ball of the three best and always lies Omniwheels adhesion exists. The drives are controlled via EPOS positioning 70 / 10 which are addressed in the CAN network as a slave in the current control mode. The coordination of the three drives takes the real-time low-level computer.

The propulsion system provides an exceptionally high dynamic performance on acceleration and deceleration, high speed and highly accurate positioning and smooth running.

Control, position control and sensor

The control is performed by two computer systems, a real-time low-level calculator for quick, precise control of balance and position, and a high-level computer running Linux with the robot operating system, ROS (Robot Operating System), which takes over the interaction with the environment . Rezero can be controlled by various input methods, such as joysticks or trajectory using "Matlab". Likewise has Rezero a "game mode", in which he is within defined parameters (such as maximum speed, acceleration, range of motion) can steer clear of a group of people by pushing and pressing.Maxon Motor 4

The Inertial Measurement Unit (IMU, Inertial Measurement Unit) is one of the vital components of the robot. The system measures translational and rotational accelerations and the inclination angle in the room and has a compass. The measuring values ​​are smoothed by an internal Kalman filter and transmitted to the low-level computer. The unit operates at a frequency of Hz and is 160 the clocking element in the chain.

To fulfill its surroundings has Rezero about various laser and ultrasonic sensors. Their arrangement allows 360 ° surround detect objects up to a distance of about 6,5 m. Depending on the mode, detected objects seen as obstacles or they can be incorporated in the robot's behavior with. Thus Rezero example, receive the "persecution" of people covered. Complementing the sensory capabilities by ambient microphones whose signals are involved in the behavior.


Children (both big and small!) Can hardly resist the temptation to touch the robot to play with him or call him to himself. This is not just because he moves very organic, but also because his appearance is so very personable and welcoming. The nature of its movement is also reflected in its outer packaging; shaped round, beautiful, full of character, dynamic and elegant.

Looking to the future

Rezero Ballbot is the first, in which not only balancing but also to the achieving of the incomparable movement potential of a Ballbots in the foreground. The natural development of mechatronic elements is very well done, the high quality of the execution and the discipline's overall total solution Rezero make eye-catching. Not so surprised that Rezero has attracted attention from big names in the film and entertainment industry itself. Certainly Rezero is to be admired in the near future by a wide audience.

Technical data:

Total weight (including casing): 19,5 kg
Weight (ready to drive, without casing): 14,5 kg
Power: 3 x 200W
max. Speed: 3,5 m / s
max. Acceleration: 3 m / s2
max. Angle: 20 °
Autonomy: 5 h

The author Martin RĂĽegg, maxon motor AG.
Copyright: All photos and renderings © 2010 Péter Fankhauser

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