Artificial muscle could replace electric motors in robots
An electro-hydraulic actuator as an artificial muscle for robots has many advantages over electric motor actuators.
The robot leg with its electro-hydraulic actuator bounces in a circle and adapts to the ground.
(Image: Thomas Buchner, Toshihiko Fukushima)
A research team from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) and the Max Planck Institute for Intelligent Systems (MPI-IS) have developed a robotic leg with an artificial muscle based on electrohydraulic actuators as part of the Max Planck ETH Center for Learning Systems (CLS) research partnership. The artificial muscle could later be used in robots, where it could replace electric motor-driven actuators.
Electric motor actuators are not particularly energy-efficient. When used in a robot leg, for example, they require a lot of energy to maintain the position when the leg is bent. This generates heat that needs to be dissipated. This requires additional heat sinks and fans, which also consume energy.
The research team therefore uses electrohydraulic actuators, which they describe in the study "Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion", published in Nature Communications. These are essentially plastic bags filled with oil. Around half of the bags are coated with a conductive material and each forms an electrode. When a voltage is applied, the electrodes are attracted to each other due to the static electricity. The oil in the bag is pressed to one side, causing the bag to shorten.
The actuators are connected to the skeleton in pairs via artificial tendons. They simulate the muscle movements of an animal or human. If one muscle shortens, its counterpart automatically lengthens. To create this artificially, the scientists use high-voltage amplifiers that are controlled by a computer using a corresponding algorithm. The algorithm coordinates which actuator contracts and which expands.
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Adaptive adjustment
The researchers found that their system in a robot leg has a number of advantages over electric motor-driven robot actuators. For example, the heat generated by the electrohydraulic muscles – remains the same regardless of the load-bearing position of the artificial muscles –. This means that no heat management system is required. The researchers also showed that the leg can lift its own weight almost explosively. However, the robotic leg remains elastic so that it can adapt flexibly to the terrain.
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"It's no different with living beings. If we can't bend our knees, for example, walking on uneven ground becomes much more difficult," says Robert Katzschmann from ETH and one of the two heads of the CLS. "Just think about taking a step from the sidewalk onto the street."
The artificial muscle adapts automatically. Only two input signals are required, one to flex the joint and the other to extend it. This enables the robot leg to adapt to the terrain when jumping, for example. Depending on the hardness of the ground, the leg joint moves adaptively to a suitable angle to cushion the weight. The situation is different with electromotive actuators: They have to constantly monitor the angle of the leg via sensors and adapt accordingly.
However, the scientists admit that there is still some development work to be done on the electro-hydraulic actuators and their control before it can be used in a robot. So far, the leg is still attached to a rod on which it can jump in a circle. The leg can not yet move freely.
(olb)
