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BirdBot saves energy by using nature as a model


If a Tyrannosaurus Rex living 66 million years ago has a leg structure comparable to an ostrich galloping on the savanna today, we may presume bird legs withstood the test of time – a good example of evolutionary selection.

Graceful, beautiful, and strong, flightless creatures such as the ostrich are mechanical marvels. Ostriches, some weighing more than 100kg, can travel at speeds of up to 55km/h across the savanna. The extraordinary locomotor capability of ostriches is assumed to be possible by the animal’s leg anatomy. Birds, unlike humans, fold their feet back as they lift their legs up towards their body. Why do the animals behave in this manner? Why is this foot movement pattern for walking and running so energy efficient? Is it possible to transfer the bird’s leg anatomy, including all of its bones, muscles, and tendons, to walking robots?

Alexander Badri-Spröwitz has worked on these issues for almost five years. He is the director of the Dynamic Locomotion Group at the Max Planck Institute for Intelligent Systems (MPI-IS). In the fields of biomechanics and neurocontrol, his team works at the crossroads of biology and robotics. The group’s major emphasis is on dynamic movement in animals and robotics.

Badri-Spröwitz and his PhD student Alborz Aghamaleki Sarvestani have built a robot leg that, like its natural analog, is energy-efficient: BirdBot requires fewer motors than previous machines and, in theory, may expand to gigantic sizes. Badri-Spröwitz, Aghamaleki Sarvestani, Metin Sitti, a director at MPI-IS, and biology professor Monica A. Daley of the University of California, Irvine published their findings in the journal Science Robotics on March 16th.

Muscles and tendons form a spring-tendon network

Humans walk by pulling their feet up and bending their knees, yet their feet and toes continue to point forward essentially unaltered. Birds are considered to be unique in that they curl their feet backward during the swing phase. But what is the purpose of this movement? This movement is attributed to a mechanical linkage by Badri-Spröwitz and his colleagues. “It’s not the neurological system, it’s not electrical impulses, it’s not muscular action,” adds Badri-Spröwitz. “We proposed a novel role of the foot-leg linkage through a network of muscles and tendons spanning several joints.” During the swing phase, these multi-joint muscle-tendon pairs coordinate foot folding. We built linked mechanics in the leg and foot of our robot, allowing for energy-efficient and durable robot walking. “Our findings proving this process in a robot led us to infer that comparable efficiency improvements also apply to birds,” he says.

According to the researchers, the coupling of the leg and foot joints, as well as the pressures and motions involved, might explain why a huge animal like an ostrich can not only run rapidly but also stand without tiring. A person weighing more than 100kg can also stand properly and for a prolonged period of time, but only with the knees ‘locked’ in an extended posture. It gets laborious after a few minutes if the individual squats slightly. The bird, on the other hand, does not seem to mind its bent leg structure; in fact, many birds remain erect when sleeping. A robotic bird’s leg should be able to perform the same thing: no motor power is required to maintain the construction erect.

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The robot walks on a treadmill

The researchers created a robotic limb based like the leg of a flightless bird to test their idea. They designed their prosthetic bird leg such that its foot does not have a motor, but rather a joint with a spring and cable system. Through cables and pulleys, the foot is mechanically connected to the rest of the leg’s joints. Each leg has just two motors: one for the hip joint, which swings the leg back and forth, and another for the knee joint, which flexes to lift the leg up. After assembling BirdBot, the researchers walked it on a treadmill to see the robot’s foot fold and unfold. “In the stance phase, the foot and leg joints do not need actuation,” adds Aghamaleki Sarvestani. “These joints are powered by springs, and the multi-joint spring-tendon system coordinates joint motions.” When the leg is dragged into swing phase, the foot disengages the leg’s spring – or, as we think in animals, the muscle-tendon spring,” Badri-Spröwitz says.

When standing and flexing the leg and knee, there is no effort required

The leg expends no energy while standing. “Previously, whether standing or lifting the leg up, our robots had to struggle against the spring or using a motor to keep the leg from colliding with the ground during leg swing.” “This energy input is not required in BirdBot’s legs,” Badri-Spröwitz explains, and Aghamaleki Sarvestani adds, “Overall, the new robot consumes a fourth of the energy of its predecessor.”

The treadmill is now turned back on, the robot begins to run, and the foot disengages the leg’s spring with each leg swing. The significant foot movement slacks the wire, allowing the other leg joints to swing freely. In most robots, a motor at the joint provides the transition between standing and leg swing. In addition, a sensor transmits a signal to a controller, which controls the robot’s motors. “Previously, motors were swapped based on whether the leg was in swing or stance.” In the walking machine, the foot now takes up this job, automatically transitioning between stance and swing. In the swing phase, we only require one motor at the hip joint and one motor to bend the knee. We leave leg spring engagement and disengagement to the mechanics inspired by birds. “It’s strong, swift, and energy-efficient,” Badri-Spröwitz explains.

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Monica Daley discovered in previous biology research that the bird’s leg structure not only saves energy when walking and standing, but it is also built by nature such that the animal seldom slips and injures itself. She assessed the birds’ exceptional locomotor resilience in studies with guineafowls running over disguised potholes. The technology includes morphological intelligence, which enables the animal to behave fast – without having to think about it. Daley demonstrated that animals control their legs during movement via mechanisms other than the nervous system. When an impediment suddenly appears in the path, the animal’s sense of touch or sight does not necessarily come into play.

“The structure, with its multi-jointed muscle-tendons and unusual foot action, may explain why even enormous, heavy birds run so fast, robustly, and efficiently.” If I suppose that everything in the bird is dependent on sense and action, and the animal encounters an unanticipated impediment, the animal may not be able to respond fast enough. “Perception and sensing, as well as the transmission of stimuli and response time, all cost time,” Daley explains.

Nonetheless, Daley’s work on running birds over the last 20 years shows that birds react faster than the neurological system permits, suggesting mechanical contributions to control. It all makes sense now that the team has created BirdBot, a real model that plainly displays how these processes work: the leg shifts physically if there is a bump in the ground. The changeover occurs instantly and without any time lag. The robot, like birds, has a high locomotion robustness.

Whether it’s a Tyrannosaurus Rex, a little quail, or a small or giant robotic limb. In theory, meter-high legs may now be used to transport robots weighing several tons and walking about with minimum power input.

The expertise gathered via BirdBot, which was created at the Dynamic Locomotion Group and the University of California, Irvine, leads to fresh insights into species that have evolved through evolution. Robots enable for the testing and occasionally confirmation of biological theories, furthering both professions.

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