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Rezero's balancing act is all a matter of motor control

21 May 2012

With the assistance of maxon motor engineers, a team of Swiss students has developed what is believed to be the world’s first high agility, high stability ballbot. Balancing on a single sphere, the robot, ‘Rezero’, can instantly accelerate in any direction, taking ball mobility to a new level in terms of both balance and speed.

‘Rezero’ is able to balance on a single ball foot
‘Rezero’ is able to balance on a single ball foot

Rezero achieves this remarkable act by making continuous, minute adjustments to keep its centre of gravity perfectly positioned. To move, the robot performs a controlled fall in the desired direction of travel, and then applies the exact amount of drive to the ball to hold it at a constant angle. It stops by outrunning itself, counteracting the forward momentum and bringing its body upright.

These precise movements require Rezero to be ready to move in any direction. Drive to the ball is delivered via three specially-developed omniwheel assemblies, allowing a complete range of axial and radial rotation.


To achieve the required performance, the academic team enlisted engineers from maxon motor to help specify a drive system capable of delivering the speed, torque and lightning responses necessary to maintain balance, within the limited space of the omniwheel housing.

Together, the teams devised a combination of maxon’s flagship EC-4pole dc brushless motor, 42mm planetary gearhead and encoder, offering high torque and responsiveness in a compact unit. Further space was saved by using maxon EPOS 70/10 positioning controllers working as slaves in a CAN network.

Drive to the ball is delivered via three specially-developed omniwheel assemblies
Drive to the ball is delivered via three specially-developed omniwheel assemblies

The end result is a versatile robot that can respond in thousandths of a second, with an acceleration of 3m/s², a top speed of 8mph, and a maximum lean angle of 20º. Indeed, Rezero is so responsive that its creators have incorporated a game mode, inviting bystanders to ‘play’ by jostling and pushing it around.

Ian Bell, maxon motor senior sales engineer said: “The potential for such a device is huge. The developers have proposed it could be used as medical transportation, an automated photographer, a tour guide, or even a toy. One thing is clear – while Rezero is truly one of a kind, it is likely the first of many.”


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