Researchers develop a four-legged robot capable of walking on a balance beam

Researchers use a reaction wheel actuator system to make a quadruped robot walk on a narrow balance beam.
Kavita Verma
Quadruped robot to walking on a narrow balance beam
Carnegie Mellon University researchers use a reaction wheel actuator system to make a quadruped robot walk on a narrow balance beam.

Carnegie Mellon University 

A team of researchers at Carnegie Mellon University's Robotics Institute (RI) has created a method that enables a quadruped robot to walk on a narrow balance beam.

Their solution involves implementing a Reaction Wheel Actuator (RWA) system, which is mounted on the back of the quadruped robot. Through a novel control technique, the RWA system enables the robot to balance independently, irrespective of the position of its feet. To enhance the robot's balancing capabilities, the team leveraged hardware that is commonly used to control satellites in space.

By improving the stabilizing capabilities of a quadruped robot, it could become a commercially available product similar to drones about a decade ago. The next big thing in robotics is quadrupeds, and the team anticipates that they will soon move from being primarily used as research platforms in labs to widely used products in the field.

The robot possesses cat-like abilities 

The research was conducted using a Unitree A1 robot that had two RWAs attached to a pitch and roll axis. The robot's ability to maintain balance was tested by walking it on a narrow 6-centimeter-wide balance beam without falling over. Furthermore, the researchers simulated the falling-cat problem by dropping the robot from a height of approximately half a meter while it was upside down, and the RWAs enabled the robot to turn itself mid-air and land on its feet. 

The team's research paper, titled "Enhanced Balance for Legged Robots Using Reaction Wheels," has been approved for presentation at the 2023 International Conference on Robotics and Automation, which will take place in London from May 29 to June 2.

Reaction Wheel Actuator system improves this robot's balance

The standard elements of most modern quadruped robots include a torso and four legs that each end in a rounded foot, allowing the robot to traverse basic, flat surfaces and even climb stairs. However, quadruped robots do not have instinctive agility like four-legged animals. If only one or two feet are on the ground, the robot can't easily correct for disturbances and has a much higher risk of falling.

To address this, the team employs a Reaction Wheel Actuator (RWA) system that mounts to the back of a quadruped robot. With the help of a novel control technique, the RWA allows the robot to balance independently of the positions of its feet. RWAs are widely used in the aerospace industry to control satellites by manipulating the angular momentum of the spacecraft.

The team prototyped their approach by mounting two RWAs on a commercial Unitree A1 robot, providing control over the robot's angular momentum. The hardware can be modeled like a gyrostat and integrated into a standard model-predictive control algorithm.

The team tested their system with a series of successful experiments that demonstrated the robot's enhanced ability to recover from sudden impacts. With continued work to enhance a quadruped robot's stabilizing capabilities to match the instinctual four-legged animals that inspired their design, they could be used in high-stakes scenarios like search-and-rescue in the future.

Study Abstract

We introduce a reaction wheel system that enhances the balancing capabilities and stability of quadrupedal robots during challenging locomotion tasks. Inspired by both the standard centroidal dynamics model common in legged robotics and models of spacecraft commonly used in the aerospace community, we model the coupled quadrupedreaction-wheel system as a gyrostat, and simplify the dynamics to formulate the problem as a linear discrete-time trajectory optimization problem. Modifications are made to a standard centroidal model-predictive control (MPC) algorithm to solve for both stance foot ground reaction forces and reaction wheel torques simultaneously. The MPC problem is posed as a quadratic program and solved online at 1000 Hz. We demonstrate improved attitude stabilization both in simulation and on hardware compared to a quadruped without reaction wheels, and perform a challenging traversal of a narrow balance beam that would be impossible for a standard quadruped.

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