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Artificial muscles power a new robotic leg developed by researchers at ETH Zurich and the Max Planck Institute for Intelligent Systems (MPI-IS). Inspired by living organisms, the robotic leg can traverse a variety of terrains in an agile and energy-efficient manner.
As in humans and animals, the extensor and flexor muscles ensure that the robotic legs can move in both directions. These electrohydraulic actuators, which the researchers call HASELs, are attached to the skeleton by tendons.
The actuator is a plastic bag filled with oil, similar to those used to make ice cubes. About half of each bag is coated on both sides with black electrodes made of a conductive material. “As soon as you apply voltage to the electrodes, the static electricity attracts them to each other,” explains Thomas Buchner, a PhD student at ETH Zurich. “Similarly, if you rub a balloon against your head, the same static electricity will cause your hair to stick to the balloon.” As you increase the voltage, the electrodes come closer together, pushing the oil in the bag to one side, shortening the entire bag.
These pairs of actuators attached to the skeleton form muscle-like pairs in living organisms, meaning that when one muscle shortens, the opposite muscle lengthens. The researchers use computer code to communicate with a high-voltage amplifier to control which actuators contract and which extend.
More efficient than electric motors
The researchers compared the energy efficiency of the robotic leg to that of a conventional robotic leg powered by electric motors. Most importantly, they analyzed how much energy was being wasted and converted to heat.
“In the infrared images, it is easy to see that the motorized leg consumes much more energy if it has to maintain a bent posture,” says Buchner. In contrast, the temperature of the electrohydraulic leg remains the same. This is because the artificial muscles are electrostatic. “It’s like the example of a balloon and a hair. The hair sticks to the balloon for quite a long time,” adds Buchner. “Usually, robots driven by electric motors require thermal management, which requires additional heat sinks or fans to disperse the heat into the air. Our system does not require that,” says Toshihiko Fukushima, a PhD student at ETH Zurich.
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Robot legs move nimbly even on uneven terrain
The jumping ability of the robotic leg is based on its ability to explosively lift its own weight. The researchers also showed that the robotic leg has a high degree of adaptability, which is especially important for soft robots. The musculoskeletal system must have sufficient elasticity to flexibly adapt to the terrain.
“Living animals are no different. If you can’t bend your knees, for example, it becomes much harder to walk on uneven surfaces,” says Robert Katzschmann, who founded and runs the Soft Robotics Lab at ETH Zurich. “Think about taking a step from a paved road to a road.”
Unlike electric motors that require sensors to constantly tell the angle of the robot’s leg, artificial muscles adapt to the appropriate position through interaction with the environment. This is driven by two input signals: one to bend the joint, and one to extend the joint.
“Adapting to the terrain is a key aspect. When a person jumps into the air and lands, they don’t have to think in advance whether their knees should be bent at a 90-degree angle or a 70-degree angle,” says Fukushima. The same principle applies to the musculoskeletal system of the robot’s legs. When landing, the leg joints adaptively move to the appropriate angle depending on whether the surface is hard or soft.
When a robot leg has to hold a certain position for a long time, a lot of current flows through the DC motor (left) that drives it. Over time, the energy is lost in the form of heat. In contrast, an efficient artificial muscle (right), which operates on the principle of electrostatics, remains cool because no current flows under a constant load. | Source: ETH Zurich and MPI-IS
Emerging technologies open up new possibilities
The field of electro-hydraulic actuator research is still young, having emerged only about six years ago. “Robotics is advancing rapidly with advanced control and machine learning, while equally important robotic hardware is much less advanced.”
Katzschmann added that while electro-hydraulic actuators are unlikely to be used in heavy construction equipment, they do offer certain advantages over standard electric motors. This is especially true in applications such as grippers, where the motion must be highly customized depending on whether the object being gripped is, for example, a ball, an egg or a tomato.
Katshuman has one concern: “Compared to walking robots with electric motors, our system is still limited. The legs are currently attached to bars, and they can only move in circles and jump, not yet freely.”
Future work could open the door to overcoming these limitations and developing real walking robots with artificial muscles. He elaborates: “By combining robotic legs with quadruped robots or bipedal humanoid robots, they could one day be deployed as battery-powered rescue robots.”
Editor’s note: This article was republished from ETH Zurich.
