A team of scientists at the Max Planck Institute for Intelligent Systems (MPI-IS) led by Alexander Badri-Spröwitz has constructed a robot leg that is based on the natural model of flightless birds. The new BirdBot was created with fewer motors than other machines, and theoretically, it could scale to large size.
Badri-Spröwitz leads the Dynamic Locomotion Group, which does crossover work in the fields of biology and robotics. The team included Alborz Aghamaleki Sarvestani, Badri-Spröwitz’s doctoral student; Metin Sitti, roboticist and a director at MPI-IS; and Monica A. Daley, biology professor at the University of California, Irvine.
The research was published in the journal Science Robotics.
Looking to Birds in Nature
Birds rely on a motion that involves folding their feet backward when they are in the swing phase, and the team attributed this movement to a mechanical coupling.
“It’s not the nervous system, it’s no electrical impulses, it’s not muscle activity,” Badri-Spröwitz says. “We hypothesized a new function of the foot-leg coupling through a network of muscles and tendons that extends across multiple joints. These multi-joint muscle-tendon coordinate foot folding in the swing phase. In our robot, we have implemented the coupled mechanics in the leg and foot, which enables energy-efficient and robust robot walking. Our results demonstrating this mechanism in a robot lead us to believe that similar efficiency benefits also hold true for birds,” he explains.
The team tested their hypothesis by building a robotic leg modeled after the leg of a flightless bird. The artificial bird leg was constructed with no motor. It instead relied on a joint that was equipped with a spring and cable mechanism. The researchers then used cables and pulleys to mechanically couple the foot to the leg’s joints. Each one of these legs has a hip joint motor, which is responsible for swinging the leg back and forth, and a small motor that pulls the leg up by flexing the knee.
BirdBot was tested on a treadmill as the team observed its food folding and unfolding.
“The foot and leg joints don’t need actuation in the stance phase,” Aghamaleki Sarvestani says.
“Springs power these joints, and the multi-joint spring-tendon mechanism coordinates joint movements. When the leg is pulled into swing phase, the foot disengages the leg’s spring — or the muscle-tendon spring, as we believe it happens in animals,” Badri-Spröwitz continues.
Zero Energy When Standing
The robot leg uses zero energy when standing, meaning it only requires a quarter of the energy of its predecessor.
When the robot is running, the foot disengages the leg’s spring with each leg swing. In order to disengage, the large foot movement slacks the cable while the remaining leg joints swing loosely. This creates a transition of states between standing and leg swing, which is usually provided to robots by a motor at the joint. A sensor also normally sends a signal to a controller that switches the robot’s motors on and off.
“Previously, motors were switched depending on whether the leg was in the swing or stance phase. Now the foot takes over this function in the walking machine, mechanically switching between stance and swing. We only need one motor at the hip joint and one motor to bend the knee in the swing phase. We leave leg spring engagement and disengagement to the bird-inspired mechanics. This is robust, fast, and energy-efficient,” Badri-Spröwitz says.
BirdBot is a physical model demonstrating many of the amazing mechanisms of birds in nature, such as those that help them act quickly without having to think. If there is a bump in the ground, BirdBot’s leg switches mechanically and without any time delay. Just like birds in nature, the robot has high locomotion robustness.
This new research could result in meter-high legs that can be implemented to carry robots with several tons of weight, all with little power input. The construction of BirdBot could also provide deep new insights into biology, leading to advancements in the field.