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The Wheelbot: A symmetric unicycle with jumping reaction wheels


Credit: Geist et al.

Researchers at RWTH Aachen University within the staff of Prof. Sebastian Trimpe and the Max Planck Institute for Clever Techniques (MPI-IS) Stuttgart have not too long ago developed the Wheelbot a symmetric response wheel unicycle that may autonomously bounce onto its wheels from any preliminary place. This distinctive robotic, launched in a paper revealed within the IEEE Robotics and Automation Letters was fabricated utilizing a mix of off-the-shelf and 3D printed elements.

“Our research group works at the intersection between data science and engineering. One particular direction of our research focuses on combining insights from management principle with machine learning,” René Geist, member in Trimpe’s staff in Aachen and lead researcher behind the Wheelbot, informed Tech Xplore. “Testbeds such as pendulums, robot arms, and quadcopters, help us to check if the theoretical assumptions underlying an algorithm are practical in reality. Ideally, these testbeds are simple to use while being challenging to control with state-of-the-art algorithms, forcing us to think outside the box.”

Two different examples of such testbeds are the so-called “Balancing Cube” and its descendant the “Cubli”. These two techniques have usually helped to guage the effectiveness of community management techniques and data-driven strategies for reaching non-linear management.

The current work by Trimpe, Geist and their colleagues builds on these earlier efforts within the area. Their aim was to develop a minimalistic unicycle robot that might be used as a testbed by roboticists and laptop scientists worldwide. To do that, Trimpe and his analysis group at RWTH Aachen University joined forces with Jonathan Fiene, head of the ZWE Robotics laboratory on the MPI-IS.

“Early on in the project, we opted for using brushless motors, as the ZWE robotics has plenty of experience using these in the open dynamic robot initiative, while prices for light-weight brushless motors dropped due to the widespread adaption of quadcopters in the consumer market,” Geist defined. “When you look at a motor, arguably the simplest actuator one can come up with is to attach a wheel to it. If such a wheel touches the ground, we call it a ‘rolling wheel’, if it does not, we refer to it as a ‘reaction wheel’.”

Balancing a single-body, non-flying robotic with the flexibility to drive and carry out maneuvers typically requires a minimal of two wheels. These can both be rolling wheels, leading to a Segway-like robotic or a single rolling wheel and a response wheel, leading to what is named a “reaction wheel”—or “second change’—unicycle robotic.

Unicycle robots integrating response wheels have quite simple designs and might simply be assembled by each knowledgeable roboticists and college students. Regardless of their simplicity, response wheel unicycles are fairly tough to manage. This makes them notably favorable testbeds for analysis on robotic networks and learning-based management strategies.

“Previously proposed unicycle robots are designed to solely balance closely around their upright equilibrium position which considerably limits what you can do with these systems,” Geist mentioned. “To maximize the utility of a reaction wheel unicycle robot, we decided that the Wheelbot must be able to recover from rather large disturbances, has an onboard power supply to prevent cables limiting its maneuverability, and in addition must be able to self-erect after toppling.”

Just like the wheels of unicycles, the Wheelbot has a rolling wheel that forestalls it from toppling whereas transferring longitudinally. In distinction with unicycles, nonetheless, the robotic additionally integrates a response wheel that forestalls it from toppling within the lateral route.

“To grasp the inner workings of a reaction wheel, you can do a simple experiment at home,” Geist mentioned. “All you need is a swivel chair and a moderately heavy object, such as a cat. Sit with your knees on the chair and straighten your arms while holding the cat, then rotate your upper body clockwise. While the cat succumbs to its fate, you will notice that your knees rotate anti-clockwise. In this analogy, your upper body denotes the motor’s rotor while your lower body denotes the motor’s stator.”

Primarily, in response wheels, when a motor’s rotor (hooked up to the wheel) rotates clockwise, the motor’s stator (hooked up to the remainder of the Wheelbot) will rotate anti-clockwise. Robots that steadiness utilizing a response wheel differ from robots that resort to gyroscopes for balancing. In a gyroscope, a fast-spinning wheel is rotated orthogonally in relation to its route of rotation creating balancing torques on account of a conservation of angular momentum.

In a response wheel these results additionally happen, however are miniscule in comparison with the response torques. Response torques are created straight contained in the wheel and level parallel to the wheel’s route of rotation.

The Wheelbot: A symmetric unicycle with jumping reaction wheels
Credit: Geist et al.

“During self-erection, reaction torques must rotate the Wheelbot by 90 degrees,” Geist defined. “During the maneuver, the Wheelbot’s motor draws 16 Ampere at 24 Volts. For a 22 cm (8.7 inch) large robot, the motors actually pack quite a punch, forcing us to use a custom designed motor controller, as commercially available motor controllers were either too big or could not handle enough current at the given voltage.”

At a present draw of 16 Ampere, the researchers discovered that the motor transferring the Wheelbot reached its charge restrict in simply 0.25 seconds. As a consequence of this limitation and different challenges usually encountered when constructing response wheel-based unicycles, Geist and his colleagues determined to plan a completely new design for his or her robotic.

“First, we decided that the robot should be symmetric, effectively reducing the number of different parts that one needs to print and allowing the Wheelbot to use any of its wheels as rolling wheel,” Geist mentioned. “Symmetry has the additional advantage that the upper wheel must be considerably smaller compared to existing unicycle robots which reduces its rotational inertia in yaw direction. Second, we designed the robot’s dimensions to minimize the required torque for self-erection.”

In preliminary assessments, the researchers discovered that the usual model of their robotic may bounce onto its wheels from any preliminary positions in two steps. This enables the robotic to decelerate its response wheel earlier than finishing the second and closing step (i.e., pushing itself again up). ‘

Along with making a prototype of their robotic, Geist and his colleagues additionally created a custom-made state estimator, an algorithm that may estimate the robotic’s roll and pitch angles. This method derives its estimations from measurements of the robotic’s 4 inertial measurement items (IMUs) and wheel encoders, that are solely based mostly on prior and accessible data in regards to the robotic’s so-called kinematic mannequin (i.e., a mathematical description of the place of the robotic’s middle of mass).

“We think that in the case of wheeled robots (including Ballbots), the proposed estimator forms an interesting alternative to other estimation algorithms such as Kalman filtering,” Geist mentioned. “The Wheelbot demonstrates that a concise choice of a reaction wheel unicycle robot’s dimensions and hardware yields a versatile testbed for robotics control.”

The Wheelbot: A symmetric unicycle with jumping reaction wheels
Credit: Geist et al.

Geist and his colleagues had been the primary to create a unicycle robotic that may effectively bounce on its wheels from any preliminary place. Their paper is thus an necessary contribution to the sphere of robotics, because it solves the digital and mechanical challenges usually encountered when creating this new kind of response wheel-based unicycles.

“Besides being a challenging testbed for robot control, we see big potential for the Wheelbot as an educational platform introducing students to robotics,” Geist mentioned. “In this regard, the Wheelbot is a typical example for demonstrating the interdisciplinary nature of robotics. Besides its mechanical design requiring modeling and simulating the robot’s quite interesting dynamics, the high-current draw of its motors pose significant challenges on its electronics design.”

Sooner or later, the Wheelbot might be utilized in each instructional and analysis settings to check robotic management networks, machine studying algorithms and different fashions. A key benefit of the robotic is that its operation solely requires a fundamental data of software program engineering, which makes it a great experimental platform for college students and engineers making their first steps in robotics.

Researchers in Trimpe’s staff at the moment are engaged on a brand new model of their robotic, known as Wheelbot v3. A core aim is to make the robotic much more accessible, so that’s simpler to construct, use, and experiment with.

“The next version of the Wheelbot will be a bit smaller than the current version, uses a more powerful microcontroller, and in term of its firmware design is easier to operate,” Geist added. “In addition, we currently work on a control algorithm that steers the Wheelbot along a predefined path. After building the first jumping reaction wheel unicycle robot, we are excited to demonstrate that the Wheelbot is also able to perform agile driving maneuvers.”


A wheeled car, quadruped and humanoid robot: Swiss-Mile Robot from ETH Zurich


Extra data:
A. Rene Geist et al, The Wheelbot: A Leaping Response Wheel Unicycle, IEEE Robotics and Automation Letters (2022). DOI: 10.1109/LRA.2022.3192654

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