Enhancing Stability via Adaptive Torque Control for a Four-Wheeled Omniwheel Ballbot

Traditional omniwheel-based ballbots typically use three omniwheels for locomotion, offering high maneuverability but limited stability and load capacity. Introducing a fourth wheel can address these limitations; however, it also makes torque distribution underdetermined, requiring an additional constraint to distribute motor torques effectively. This paper presents a normal force based torque distribution strategy for a four-wheeled omniwheel ballbot. The proposed method leverages real-time normal force measurements to dynamically adjust torque limits, reducing wheel slippage and improving stability. The effectiveness of the proposed strategy is evaluated through recovery angle and braking experiments. Compared to the baseline symmetric torque distribution strategy, incorporating normal force feedback reduces wheel slippage by 43-74% during recovery and by 76% during braking. These improvements demonstrate the efficacy of the normal force based torque distribution strategy in enhancing stability.