Not Quite Legs, Not Quite a Wheel

As technology continues to advance, robots are becoming increasingly capable of performing a wide range of tasks. One important aspect of robot design is the ability to move and navigate their environment. There are several methods of robot locomotion that have been developed, each with their own unique advantages and limitations.

One common method of robot locomotion is wheeled motion. This is the most common type of locomotion used by robots, as it allows for relatively simple, efficient, and rapid movement on more or less flat surfaces. Wheeled robots can be designed to move in a variety of ways, including by using traditional wheels, tracks, or even by using multiple smaller wheels for improved maneuverability.

Another popular method of robot locomotion is legged motion. This type of locomotion allows robots to move more like animals, with the ability to walk, run, and even climb over obstacles. Legged robots can be designed to have a variety of different leg configurations, such as two legs, four legs, or more. Legged robots allow for greater versatility and mobility, especially on rough or complex terrain, but can also be more complicated and difficult to control.

For robotics to move beyond the current paradigm in which a custom design is created for each application, and move towards more general purpose robots that can operate under any conditions, new forms of locomotion are needed. A team of engineers at ShanghaiTech University has put forth a proposal that takes advantage of the best aspects of wheeled and legged locomotion by combining the two into a transformable wheel. They have implemented this technology in a robot called OmniWheg that makes use of omnidirectional wheels that can transform into legs when the situation calls for it.

This is not the first attempt to create a wheel-legged robot, but previous attempts have not proven to be entirely practical due to problems with stability. When such a robot approaches an obstacle that requires deployment of the legs, if it is not properly aligned, some of the legs may not deploy, or they may not make contact with the ground, which can cause the robot to either fail in the climb attempt, or worse yet, flip itself over. Since most wheeled robots steer by differentially driving their wheels, misalignments are very difficult, or impossible, to correct.

OmniWheg solves this problem by using a Mecanum-inspired omnidirectional wheel that can move or rotate a robot in virtually any direction. A set of sixteen rollers on the surface of each wheel allow it to move in arbitrary directions, contrary to the direction in which it is pointing. These omnidirectional wheels make it possible to achieve perfect alignment, perpendicular to an obstacle, before transforming the wheels to legs by shifting the wheel surface into claw-like appendages using a four-bar linkage mechanism. Accordingly, obstacles can be climbed with all legs simultaneously, and the robot will not be prone to instability issues that can cause it to flip over.

A set of experiments were conducted to test OmniWheg’s ability to climb over obstacles of various heights, and also to assess the robot’s stability. It was found that the system could overcome common indoor obstacles and move flexibly on flat surfaces. The team is presently looking to build on their early successes by incorporating more powerful motors into the design to enhance the robot’s climbing abilities. They are also exploring the possibility of adding depth or ultrasonic sensors to give OmniWheg new navigational capabilities.