Robot leg powered by artificial muscles outperforms conventional designs

Inventors and researchers have been developing robots for nearly 70 years. So far, all the machines they have built—whether for factories or elsewhere—have one thing in common: They are powered by motors, a 200-year-old technology. Even walking robots have arms and legs powered by motors, not muscles like humans and animals. That partly explains why they lack the mobility and adaptability of living creatures.

A new muscle-powered robotic leg is not only more energy-efficient than a conventional leg, it can also perform high jumps and rapid movements, as well as detect and react to obstacles – all without the need for complex sensors. The new leg was developed by researchers at ETH Zurich and the Max Planck Institute for Intelligent Systems (MPI-IS) as part of a research partnership called the Max Planck ETH Center for Learning Systems, better known as CLS.

The CLS team was led by Robert Katzschmann of ETH Zurich and Christoph Keplinger of MPI-IS. Their doctoral students Thomas Buchner and Toshihiko Fukushima are co-first authors of the team’s publication on an animal-inspired musculoskeletal robotic leg. Nature Communications.

Electrically charged like a balloon

As in humans and animals, an extensor muscle and a flexor muscle provide the bidirectional movement of the robotic leg. These electrohydraulic actuators, which the researchers call HASEL, are connected to the skeleton by tendons.

The actuators are plastic bags filled with oil, similar to those used to make ice cubes. About half of each bag is covered on each side with a black electrode made of a conductive material.

“As soon as we apply a voltage to the electrodes, they are attracted to each other by static electricity,” Buchner explains. “Similarly, when I rub a balloon against my head, my hair sticks to the balloon because of the same static electricity.”

As the voltage increases, the electrodes move closer together and push the oil in the bag to one side, making the bag shorter overall.

Pairs of these actuators attached to a skeleton produce the same paired muscle movements as in living creatures. When one muscle shortens, its counterpart lengthens. The researchers used computer code that communicates with high-voltage amplifiers to control which actuators contract and which extend.

More efficient than electric motors

The researchers compared the energy efficiency of their robotic leg with that of a conventional robotic leg powered by an electric motor. In particular, they analyzed the amount of energy unnecessarily converted into heat.

“In the infrared image, it is easy to see that the motorized leg consumes much more energy if, for example, it has to maintain a bent position,” explains Buchner.

In contrast, the temperature in the electrohydraulic leg remains the same, because the artificial muscle is electrostatic.

“It’s like the balloon and hair example, where the hair stays stuck to the balloon for quite a long time,” Buchner adds.

“Typically, electric-powered robots need heat management, which requires additional heat sinks or fans to dissipate heat into the air. Our system does not need this,” Fukushima says.

Agile movement on rough terrain

The robotic leg is able to jump thanks to its ability to lift its own weight explosively. The researchers also showed that the robotic leg has a high adaptability, which is particularly important for soft robotics. Only if the musculoskeletal system has sufficient elasticity can it flexibly adapt to the terrain in question.

“The same goes for living things. If we can’t bend our knees, for example, walking on an uneven surface becomes much more difficult,” Katzschmann says. “Just imagine stepping off the sidewalk onto the road.”

Unlike electric motors that require sensors to constantly indicate the angle of the robotic leg, the artificial muscle adapts to an appropriate position through interaction with the environment. It is driven by only two input signals: one to bend the joint and the other to extend it.

“Adaptation to the terrain is a key aspect,” Fukushima explains. “When a person lands after a jump, they don’t need to think in advance about whether they should bend their knees to a 90-degree or 70-degree angle.” The same principle applies to the robotic leg’s musculoskeletal system: When landing, the leg joint adaptively moves to an appropriate angle depending on whether the surface is hard or soft.

Emerging technologies open up new possibilities

The research field of electrohydraulic actuators is still young, having only emerged about six years ago.

“The field of robotics is making rapid progress with advanced controls and machine learning; however, progress has been much slower in the equally important field of robotic hardware. This publication is a powerful reminder of the potential for disruptive innovation that can be found in the introduction of new hardware concepts, such as the use of artificial muscles,” says Keplinger.

Katzschmann adds that electrohydraulic actuators are not likely to be used in heavy machinery on construction sites, but they offer specific advantages over standard electric motors. This is particularly evident in applications such as grippers, where movements must be highly customized depending on whether the object being gripped is, for example, a ball, an egg or a tomato.

Katzschmann, however, has a reservation: “Compared to walking robots with electric motors, our system is still limited. The leg is currently attached to a rod, jumps in circles and cannot yet move freely.”

Future work should help overcome these limitations and pave the way for the development of true walking robots with artificial muscles. He continues: “If we combine the robotic leg with a quadruped robot or a two-legged humanoid robot, maybe one day, when it’s battery-powered, we can deploy it as a rescue robot.”