Four-legged robot can reproduce animal movement with fewer actuators

Many of the robotic systems developed in the past decades are inspired by four-legged (i.e., quadruped) animals, such as dogs, cheetahs and horses. By replicating the agile movements of these animals, quadruped robots could move swiftly on the ground, crossing long distances on various terrains and rapidly completing missions.

Yet realistically and robustly replicating the fluid motions observed in animals using robotic systems can be very challenging. While some existing four-legged robots were found to be very agile and responsive to changes in their environment, these systems typically integrate advanced actuators and computational components that consume a lot of energy.

Researchers at EPFL’s CREATE Lab and Delft University of Technology (TU Delft) recently developed a new four-legged robot called PAWS (Passive Automata With Synergies), which could reproduce the fluid and adaptive movements of animals using fewer actuators. This robot, introduced in a paper in Nature Machine Intelligence, leverages so-called motor synergies, which are coordinated patterns of muscle activation that allow animals to perform agile motions consuming less energy.

“Robotic design is strongly inspired by biological examples,” Francesco Stella and Mickael Achkar, first authors of the paper, told Tech Xplore. “However, the process of extracting design features from nature is often strongly dependent on human intuition and expertise. We wanted to find a more structured and methodological way of doing this.”

In contrast with other quadruped robots that execute their movements via active control strategies, PAWS also moves by leveraging the flexible connections between its joints, known as compliant mechanical couplings. This allows it to realistically perform dynamic movements that resemble those of animals, while requiring minimal actuation.

“Animals control their body using motion synergies—a biological phenomenon where the central nervous system coordinates patterns of activation across different muscle groups (rather than individual muscles) to enable efficient movement,” explained Stella and Achkar.

“In our article, we started with real-world data of dogs to capture their principal motion synergies, and then we computationally translated them into a tendon-driven system with fewer motors than joints. Leveraging this reduced actuation set, we computationally designed PAWS (Passive Automata with Synergies).”

To assess their robot’s ability to move passively (i.e., without relying on active actuation), the researchers placed it on a treadmill, without connecting any motors to its tendons. They found that the robot could effectively run on the treadmill even without the motors, while also avoiding any obstacles in its path.

Moreover, by integrating only four independently controlled actuators, the researchers successfully allowed PAWS to execute different animal-like gaits (i.e., styles of moving), including crouching, standing, walking, galloping and jumping. Their quadruped robot could soon be improved and tested further in other experiments, to validate its potential for tackling various real-world problems.

“Our design demonstrates the level of robustness achievable in passive bodies through purposeful coupling and compliance, and that with minimal actuation, it is possible to maintain the advantages of natural dynamics while achieving diverse behaviors and interactions with the environment,” added Stella and Achkar.

“We are now actively continuing this research by looking at how minimal sensing and simple feedback loops could further increase the stability of our system. We want to find ways in which we can continue to leverage the passive dynamics of the system while increasing its diversity in behaviors.”

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