Researchers have created a robotic leg that mimics artificial muscles, allowing it to jump across various surfaces quickly and with minimal energy, drawing inspiration from living beings.
For nearly 70 years, inventors and scientists have been designing robots. Most of the devices produced, whether for industrial use or otherwise, share a commonality: they rely on motors, a technology that has been around for 200 years. Even robots that exhibit walking movements use motor-powered limbs, differing significantly from the muscle power seen in animals and humans, which may partly explain their limited mobility and flexibility compared to living organisms.
Unlike standard robotic legs, this novel leg harnesses muscle power for enhanced energy efficiency. It can execute high jumps and rapid movements while detecting and avoiding obstacles without the need for complex sensor systems. The development team, based at ETH Zurich and the Max Planck Institute for Intelligent Systems (MPI-IS), collaborated through the Max Planck ETH Center for Learning Systems (CLS). Robert Katzschmann from ETH Zurich and Christoph Keplinger from MPI-IS led the project, and their doctoral students, Thomas Buchner and Toshihiko Fukushima, co-authored the publication in Nature Communications about their animal-inspired musculoskeletal robotic leg.
Balloon-like electrification
Similar to the muscular structure in living beings, this robotic leg employs an extensor and a flexor muscle to facilitate movement in both directions. Researchers have termed these electro-hydraulic actuators HASELs, which are connected to the leg’s skeleton via tendons.
These actuators are essentially plastic bags filled with oil, resembling ice cube trays. Each bag is lined with conductive electrodes on either side. Buchner highlights that “when a voltage is applied to the electrodes, they attract each other due to static electricity. It’s akin to how static causes a balloon to stick to your hair.” By increasing the voltage, the electrodes get closer together and displace the oil within the bag, effectively shortening it.
Pairs of these actuators replicate the dual muscle actions found in living organisms: as one muscle contracts, the opposing one lengthens. The team employs custom software that interacts with high-voltage amplifiers to manage which actuators contract and which relax.
More efficient than traditional motors
The researchers evaluated how energy-efficient their robotic leg was compared to a typical motor-driven robotic leg. They particularly looked at the energy wasted as heat. According to Buchner, infrared imaging clearly shows that a motorized leg uses more energy when maintaining a bent position. In contrast, the electro-hydraulic leg maintains a consistent temperature, as it relies on electrostatic principles. “It’s similar to how a balloon can hold onto hair for a long time,” he notes. “Robots powered by electric motors typically require thermal management, necessitating additional components to dissipate the heat, whereas our design does not.” says Fukushima.
Spry movement on uneven surfaces
The robotic leg’s jumping capability stems from its power to lift its own weight in an explosive manner. The research also demonstrated the leg’s exceptional adaptability, a crucial aspect in soft robotics. The musculoskeletal system must have adequate elasticity to navigate diverse terrains effectively. “Living creatures operate the same way. If our knees couldn’t bend, traversing an uneven surface would be significantly more challenging,” Katzschmann explains. “Consider stepping down from a sidewalk to the street.”
Unlike traditional electric motors that rely on sensors to monitor the leg’s position, the electro-hydraulic muscle adjusts its position through environmental interaction. It operates on just two commands: one for bending and one for extending. Fukushima elaborates: “Terrain adaptability is vital. When a person lands after a jump, they instinctively adjust their knees without overthinking the exact angle.” This concept applies to the robotic leg’s design: upon landing, the joint will instinctively adjust its angle based on the surface texture beneath it.
Emerging technology paves the way for innovation
The area of electro-hydraulic actuators has only been explored for six years. “While robotics is advancing rapidly with cutting-edge controls and machine learning, innovations in robotic hardware have lagged behind. This study emphasizes how introducing new hardware concepts, like artificial muscles, can lead to groundbreaking developments,” Keplinger remarks. Katzschmann suggests that while electro-hydraulic actuators might not replace heavy machinery on construction sites, they have specific advantages in applications like grippers, where precise movements are crucial depending on whether they are handling a ball, an egg, or a tomato.
However, Katzschmann raises a concern: “Currently, our system is restricted compared to walking robots that use electric motors. The leg is fixed to a rod, jumps in circular patterns, and cannot move freely at this stage.” Future improvements aim to overcome these restrictions, enabling the creation of genuine walking robots powered by artificial muscles. He also envisions: “If we integrate this leg into a quadruped or bipedal robot, there may come a day when it can function as a battery-operated rescue robot.”
