Recently, researchers at ETH Zurich published a breakthrough in developing a robot leg based on electro-hydraulic artificial muscles, which has shown amazing performance in terms of flexibility, adaptability and energy efficiency, opening up a new direction for future robot design.
The study, published in Nature Communications, was done by Thomas J. K. Buchner, Toshihiko Fukushima, et al. The research team proposes a biologically inspired musculoskeletal leg structure driven by antagonistic electro-hydraulic artificial muscles. They mounted the leg on a cantilever that proved to be adaptively jumping on a variety of terrains in an energy-efficient and flexible way. In addition, it detects obstacles by capacitive self-induction.
So, what's so special about this robotic leg, called PELE (Peano-HASEL driven leg)? Let's find out.
▍Subvert the traditional design, the bionic structure brings new possibilities
For a long time, robot legs have relied mainly on rigid electromagnetic motors and sensorized drive systems to adapt to the environment. While this design has been able to achieve some impressive movements, it is still far from the flexible and relaxed movements that animals exhibit in their natural environment.
Researchers at ETH Zurich have taken a different approach and proposed a bionic musculoskeletal leg structure. They replaced traditional electromagnetic motors and rotary joint encoders with antagonistic electro-hydraulic artificial muscles. The gearing in the joints is replaced by a bionic transarticular tendon path, a design that creates a nonlinear moment arm capable of producing the appropriate torque output for the contracted muscles.
Specifically, PELE consists of a carbon fiber skeleton, 3D-printed joints (hip and knee), and two sets of electro-hydraulic artificial muscles connected by tendons. Each set of muscles contains electrohydraulic muscles stacked in parallel with a tendon attached at the end. The whole leg has 4 sets of muscle packs, which are controlled by up to 4 independent high-voltage amplifiers via computer.
Each electrohydraulic muscle is a Peano-HASEL (Hydraulically Amplified Self-Healing Electrostatic Actuator) consisting of actuator bags stacked in series. The individual bag is a polymer shell filled with a liquid dielectric and covered with electrodes on both sides. When different potentials are applied on the two electrodes, the charge moves to the electrodes and the electrostatic force causes the bag shape to change. This change in shape creates a linear contraction along the actuator bags stacked in series. When the electrode is discharged, the contraction resumes.
This design brings several advantages: First, electro-hydraulic actuators are usually locked and do not require additional energy to hold their position even under load, only to compensate for small charge leaks through the dielectric layer. This means that PELE consumes very little power when holding the position, even when applying considerable joint torque.
Second, unlike electromagnetic motors, the voltage applied to an electro-hydraulic actuator is related to the actuator force output, not to the current. This controllable actuator force output, combined with the antagonistic muscle pair and the force-strain characteristics of the actuator, allows the leg to move in open-loop force control mode without the need for a joint angle encoder. This gives PELE its inherent adaptability.
Finally, each tendon has a nonlinear moment arm drive that provides a suitable angle-torque curve for the joint. This design is closer to biological systems and helps to improve movement efficiency and flexibility.
Overall, this biomimetic design not only enhances the versatility and adaptability of the robot, but also helps to simplify the control of systems with high degrees of freedom, which is expected to lead to more flexible and efficient robotic systems.
▍Flexible and agile movement ability, challenging the limits of traditional robots
PELE shows amazing athleticism, reaching a high level in terms of jump height, frequency, and adaptability. The research team conducted a series of experiments to test the performance limits of PELE.
In the jump experiment, PELE achieved a jump height of 128 mm while maintaining a short support time of 91 milliseconds. This short support time reflects the rapid response of the muscles and the low moment of inertia of the system. PELE's vertical jump agility reaches 0.75 m/s and jumps at a frequency of 5.8 Hz, demonstrating extremely high agility.
Considering the short support time, the research team further explored PELE's ability to make quick consecutive jumps. Since gravitational acceleration is not able to bring the legs back to the ground fast enough, it limits the maximum frequency of movement that can be achieved. To overcome this limitation, the researchers utilized antagonistic muscle pairs in the legs to achieve higher frequency movements. With an open-loop force controller, PELE has successfully achieved a jump frequency of 3 Hz on a variety of terrains, demonstrating its agility and versatility. At a frequency of 3 Hz, PELE reaches a jump height of 80 mm and remains dynamically stable during vertical jumps.
The research team also studied high-frequency motion patterns without ground contact. They employ an open-loop force controller that applies a phase-shifted sinusoidal signal to each muscle, enabling higher frequency gait motion. In the absence of ground contact, PELE demonstrated a 5 Hz running motion and a 10 Hz linear movement. This high-frequency movement fully demonstrates PELE's ability to perform rapid agile gait cycles.
Notably, the researchers observed that the range of motion of the foot changed with the frequency of operation, reaching a maximum at 3 Hz, which matched the natural frequency of the leg system. This finding suggests that antagonistic muscle pairs can inherently produce human-like gait locats.
PELE has been successful in several experiments such as jumping, continuous jumping, and aerial movement. Together, these results demonstrate PELE's versatility and agility capabilities in high jumps, multiple rapid jumps, and fast gait cycles. Compared to conventional rigid motor-driven robots, PELE exhibits greater flexibility and adaptability.
This high degree of flexibility and agility stems from several innovative designs of PELE: first, the rapid response and high power-to-weight ratio of the electro-hydraulic muscles provide the basis for agile movement; Second, the biomimetic musculoskeletal structure provides dynamics that are closer to those of biological systems; Finally, the open-loop force control simplifies the control system while ensuring a high degree of adaptability. These innovations enable PELE to demonstrate superior locomotion capabilities in a variety of complex environments, opening up new possibilities for the application of future robots in unstructured environments.
▍Inherent adaptability and energy efficiency are equally important to promote the development of leg and foot technology
In addition to its excellent athletic performance, PELE also exhibits impressive adaptability and energy efficiency. These two characteristics are essential for future robots to work for long periods of time in complex and changing environments.
In terms of adaptability, PELE has shown an amazing performance. The research team designed a feedforward force controller that periodically generates a single activation pattern of applied voltage. With this simple control mode alone, PELE is able to jump on a variety of natural terrains such as rock, sand, gravel, and grass. This experiment fully demonstrates PELE's ability to adapt to different terrains.
WHAT'S EVEN MORE SURPRISING IS THAT PELE DEMONSTRATES THE ABILITY TO SEAMLESSLY TRANSITION BETWEEN DIFFERENT TERRAINS. The researchers had PELE switch between a soft sponge surface and a hard tabletop. PELE exhibits a periodic jumping gait on the surface of the sponge, forming a stable limit ring. When the sponge is removed, PELE goes through a brief chaotic period and then smoothly transitions from a steady jump gait on the sponge to a new steady jump gait on the table. It is important to note that this conversion occurs inherently without changing the control inputs.
PELE also demonstrated the ability to achieve a soft landing through reversible drive of muscles. The researchers recorded the angle of the joints when PELE fell to the surface from the same height, while applying different constant activation voltages. The results show that PELE can achieve a soft landing by simply maintaining a fixed voltage. Experiments have also revealed the adjustable stiffness of PELE, with lower activation voltages leading to softer landings, greater variation in joint angles, and greater compression rebound. This inherent soft-landing characteristic stems from the muscle's inherent adjustable reversible driveability, eliminating the need for a complex computational controller to adjust leg stiffness.
When it comes to energy efficiency, PELE also excels. The research team evaluated the net cost of transportation (COT) during exercise and the results showed that PELE had a COT between 0.73 and 1.79, depending on the type of exercise. This value is more energy-efficient than most electromagnetic motor-based systems. What's even more impressive is that PELE only needs about 1.2% of the energy equivalent to a conventional DC motor drive leg when performing a squat experiment. This high efficiency is due to the unique properties of electro-hydraulic actuators, which consume almost no energy while holding their position and only need to compensate for small charge leaks.
While traditional rigid motor drive systems require complex controllers to simulate adaptability, PELE achieves a high degree of adaptability through its inherent physical properties. At the same time, PELE's energy-efficient nature paves the way for long-running robotic applications.
This research not only demonstrates the excellent performance of a new type of robot leg, but more importantly, it creates a completely new design concept. By mimicking the design principles of biological systems, the researchers succeeded in unifying the three goals of flexibility, adaptability, and energy efficiency. This approach may lead to a new direction in the design of future robots, enabling robots to better adapt to complex and changing natural environments while maintaining high efficiency and long battery life.
Paper Links:
https://www.nature.com/articles/s41467-024-51568-3