Researchers have constructed a robotic leg that, thanks to its ratite-inspired foot-leg coupling, requires little motor and control technology. This makes the “BirdBot system” very energy-efficient and sure-footed, as tests on the treadmill show. The scientists say that the concept could even one day set walking robots weighing several tons in motion with little effort. However, the results are not only important for robotics, but also shed light on the biological basis of the anatomical concept.
They move nimbly on two legs at up to 55 kilometers per hour: the ostriches, which can weigh up to 100 kilograms, illustrate the efficiency of the ratites’ locomotion system. In contrast to our upright gait, their anatomical principle has deep roots in the history of development: Tyrannosaurus and Co already had a similar leg structure and gait as today’s birds. Apparently, it is therefore a concept that has proven itself due to its energetic and functional advantages. But what is the secret of the bird’s leg and to what extent can its mechanisms be transferred to robotic systems? The scientists led by Alexander Badri-Spröwitz from the Max Planck Institute for Intelligent Systems in Stuttgart are investigating this question.
Effective pairing is key
As they explain, the concept of the bird’s leg has an aspect that is quite different from our mechanism: when people walk, they raise their leg and bend their knee, but their foot and toes point forward almost unchanged. Birds, on the other hand, fold their feet backwards during the swing phase. The research of Badri-Spröwitz and his colleagues suggested that this movement is the passive result of a mechanical coupling: it is not caused by the nervous system and muscle activity. “We see that the network of muscles and tendons that spans multiple joints enables this coupling. The multi-jointed muscle-tendon cables cause the foot to fold in during the swing phase,” says Badri-Spöwitz. It is suspected that this may have something to do with a favorable basic mechanism.
To test their assumption, the researchers built a robotic leg modeled on that of ratites. Using cables and springs, they constructed it in such a way that the foot does not require a separate motor, but only one joint that is mechanically coupled to the rest of the leg joints. A motor on the hip joint ensures that the leg moves forwards and backwards. Another motor on the knee joint also causes knee flexion to pull up the artificial limb. Finally, the researchers built a two-legged prototype to test the system.
The investigations on the treadmill made it clear that the robot actually folds its feet in and out when walking, similar to the natural models. The analyzes also showed that the coupled mechanism in the leg and foot enables the “BirdBot” to run in a very energy-efficient and robust manner and also literally comes into play when standing. Because the ankle and leg joints do not need motors in the stance phase. “The power comes from the spring and the coordination from the multi-jointed cable pull mechanism. When you pull your leg up in the swing phase, the foot then switches off the leg spring – the muscle,” explains Badri-Spröwitz. “In the past, however, our robots had to work against the spring or with a motor either when standing or when raising their legs so that the leg did not collide with the ground during the swing phase. This energy input is no longer necessary with BirdBot”. This has a positive effect, emphasizes his colleague Aghamaleki Sarvestani: “All in all, only a quarter of the energy is required compared to previous walking robots.”
Energy efficient, stable and robust
In addition, a comparatively complex control technology was previously required, which can now be saved. With most robots, switching between standing and walking has so far been ensured by a motor on the joint, which is switched on and off via a sensor system. “With the BirdBot, the foot does this automatically for the running machine. We only need the motor on the hip joint and one to bend the knee in the swing phase – the rest is done by the leg itself. We leave the engagement and disengagement of the leg springs to the bird-inspired mechanics. It’s robust, fast and energy-efficient,” sums up Badri-Spröwitz.
As the researchers emphasize, the replica now also retrospectively sheds light on the biological significance of the concept. The coupling of the leg and ankle joints and the forces involved explain why a large animal like an ostrich can not only run so fast, but can also stand for a long time without any effort. In addition, the system appears to be a factor in the ratites’ low susceptibility to tripping, which co-author Monica Daley of the University of California at Irvine has shown through previous research. As the physical model of the BirdBot now makes clear, the time-consuming perception and transmission of stimuli in birds is replaced by pure mechanics.
However, the focus of the scientists is apparently on the potential for the technology: “The design presented now is scalable and enables the development of robots with large legs,” write Badri-Spröwitz and his colleagues. In concrete terms, according to them, this means that, in theory, it is possible to construct meter-high legs based on the principle of ratites, with which robots weighing tons could one day walk around in an energy-saving and stable manner.
Source: Max Planck Institute for Intelligent Systems, specialist article: Science Robotics, doi: 10.1126/scirobotics.abg4055
Video: © DLG MPl-lS and UC Irvine