A soft robot created by German and Swiss scientists performs movements similar to those we observe in humans and animals. The electrohydraulic actuators connected to the skeleton with tendons act like real muscles. The device, compared to those powered by engines, is much more energy efficient and copes better in non-standard environments, without the need for any sensors. This electrohydraulic leg will likely find applications in devices such as grippers.
“The robotic leg we’ve developed is different in several ways from those used so far. One of the most noticeable differences is that a motor with a drive system is used in traditional legs. Basically, this is a gearbox, and by engine I mean a system that has magnets and wires and rotates. We have one joint and another that form a joint. In our leg we use ‘muscles’, which are connected from the outside to a joint, like the one we have in our bodies. Our whole skeletal system has joints and they do not have motors. We achieve a similar effect by connecting artificial muscles with artificial tendons to the external part of the joints and around them,” says Robert Katzschmann of the Federal Polytechnic in Zurich in an interview for Newseria Innovations agency.
The device was developed jointly with scientists from the Max Planck Institute for Intelligent Systems (MPI-IS). The leg, like in humans and animals, is equipped with flexor and extensor muscle equivalents. The electrohydraulic actuators responsible for movement are attached to the skeleton via tendons. The actuators themselves are oil-filled plastic bags, covered with an electrode on both sides to about halfway. When voltage is applied to the electrodes, they attract each other, pushing the oil in the bag to one side, causing it to shorten. This is similar to what happens in the case of natural pair muscles – when one contracts, the other stretches, resulting in movement.
“The main advantage of the system is that it meets the definition of a compliant mechanism. If I strike, fall to the ground, the forces generated pass through the muscles, which also serve as a shock absorber, also being an element of the compliant mechanism. This is reminiscent of the way animals and humans move – we use muscles to soften the effects of a blow, to take a hit. When we step on something with the front of the foot, we tighten the calf muscles to accept the force of the fall. Thanks to muscles that can contract and change the level of tension and behavior, we can provide more natural movements. This allows us, for example, to jump on a rocky surface with different positions of boulders, grass and stones. The leg can overcome these obstacles thanks to the same approach, while the same will be true on other surfaces using the same type of propulsion,” explains Robert Katzschmann.
A key aspect is adapting to the terrain. Unlike electric motors that require sensors to constantly indicate the angle at which the robot’s leg is, the artificial muscle adjusts to the correct position through interaction with the environment. It is powered by only two input signals: one for bending the joint and one for straightening it.
“In the future, we will be able to think about building robots in a different way, that is, building robots using ‘muscles’ as engines. We can start thinking about robots that, in the way they are designed, and in their behavior, are naturally more suited to a human-dominated environment. If we want robots to help us at home, to move around the factory floor and perform given tasks, we won’t need a metal machine equipped with motors and joints, but we can use soft materials from plastics to create muscles and a carbon fiber skeleton to build light robots that are not heavy machines but lightweight, biology-inspired and more gentle beings that can interact with humans,” predicts the scientist.
An additional advantage of the solution is energy saving. The researchers compared the energy efficiency of their robotic leg to the efficiency of a conventional robotic leg driven by an electric motor. They analyzed, among others, how much energy is unnecessarily converted into heat. It turned out that a regular mechanized leg consumes significantly more energy, for example, if it has to maintain a bent position, while the temperature in the electrohydraulic leg remains the same. The device, although in its current form it already provides huge possibilities, will still be refined.
“There are still a few things we want to improve. We are working not only on the ability to contract muscles but also on their stretching. In the first construction covered by the study, they can already contract quite well, but when it comes to the stretching phase, the muscles do not stretch properly. This limits their efficiency. We are working on other mechanisms that allow us to create muscles that are soft and have the ability to stretch, because in the movement of antagonistic muscles, if we want to achieve a really wide range of movement, we must be able to greatly stretch the opposing muscle to achieve a large contraction of the second muscle. This is one area for improvement. The second area that needs refinement is the creation of power supply systems, i.e., the electronics that provide the current and voltage supplied to our muscles. We place all of them in the robot, in this mobile unit. This is another important stage of our work we are currently working on,” emphasizes the researcher.
According to Business Research Insights, the worldwide soft robot market was valued at $1.2 billion in 2023. By 2032, its value is expected to rise to $45 billion. One of the factors driving the development of the market will be the growing demand for automation in various industries. Soft robots are useful for handling delicate items, assembling and packing goods in warehouses, and working in other situations where typical rigid robots are useless.
