Design of a 3-DOF Hopping Robot with an Optimized Gearbox: An Intermediate Platform Toward Bipedal Robots

TL;DR

This paper introduces a 3-DOF hopping robot with a custom-designed gearbox and reinforcement learning control, demonstrating stable jumping motions and serving as a platform for future bipedal robots.

Contribution

It presents a novel compact gearbox design and a custom-built robot platform with reinforcement learning control for dynamic hopping.

Findings

Successful stable and repetitive hopping demonstrated

Custom gearbox achieves high torque and large hollow shaft

Platform provides a foundation for future bipedal robots

Abstract

This paper presents a 3-DOF hopping robot with a human-like lower-limb joint configuration and a flat foot, capable of performing dynamic and repetitive jumping motions. To achieve both high torque output and a large hollow shaft diameter for efficient cable routing, a compact 3K compound planetary gearbox was designed using mixed-integer nonlinear programming for gear tooth optimization. To meet performance requirements within the constrained joint geometry, all major components-including the actuator, motor driver, and communication interface-were custom-designed. The robot weighs 12.45 kg, including a dummy mass, and measures 840 mm in length when the knee joint is fully extended. A reinforcement learning-based controller was employed, and robot's performance was validated through hardware experiments, demonstrating stable and repetitive hopping motions in response to user inputs. These experimental results indicate that the platform serves as a solid foundation for future bipedal robot development.