At Japan’s Yamashita Pier, about 25 miles south of Tokyo, the world’s largest humanoid robot has taken shape. Modeled on the RX-78-2 Gundam—a fictional robot that has been the subject of some 50 namesake TV series and manga since 1979—the giant towers nearly 60 feet tall and features 24 degrees of freedom, meaning that it can move in as many directions.
This beast of a bot appears to be the world’s largest bipedal walking robot, and has become an iconic fixture along the Yokohama skyline. Fans began touring the exhibit, which includes an on-site museum and cafe, on December 19 last year.
But there is dissent among faculty from some of the most prominent robotics departments in the U.S. about whether it qualifies as a walking robot at all. Because this Gundam appears to use a supporting structure to help it move, they consider it to be a kinetic sculpture, or an art installation that relies on motion to create some affect in the viewer.
Gundam Factory Yokohama, the organization that built the robot, did not return multiple interview requests for this story.
These U.S.-based roboticists believe that it would be nothing short of an engineering marvel for a robot of this size to really walk, run, and wreak havoc; the laws of physics would be pushed to their logical extremes. Specifically, scaling rules would dictate a whole slew of changes to the actuators (or motors) that allow the Gundam to raise its legs and take strides.
“By scaling rules, it means that if you make something bigger, then different aspects of it get bigger or smaller in different ways,” explains Andy Ruina, Ph.D., a professor of mechanical engineering at Cornell University’s Sibley School of Mechanical and Aerospace Engineering.
Scaling rules are not just the stuff of robotics. Moore’s law predicts that the number of transistors in a silicone computer chip will double every two years as the technology advances. Allometry, the biological study of the scaling relationship between the size of a body part and the size of the entire body, describes why ants can haul roughly 100 times their weight and humans can’t, Ruina says.
In the case of a mammoth Gundam, the motors that allow the hulking robot to move must become drastically stronger, especially if the frame is made from a heavy metal, like steel. But if the motors are larger, they’ll also become heavier and weaker relative to the torques, or rotational forces, necessary. In this way, the scaling is “unfortunate,” according to Chris Atkeson, Ph.D., a professor at Carnegie Mellon University’s Robotics Institute.
To get around this paradigm, he says, engineers could attempt to create a whole new kind of motor. “The scaling laws assume that technology is always the same…but you can change the technology so that it works,” Atkeson says.
The Math of Scaling Laws ⚙️
Basic rules in geometry and physics—plus the strength restraints of materials—are one reason it’s difficult to create robots that can walk.
As linear dimensions increase, two-dimensional quantities, like how much “skin” you need to cover the robot, scale up by a power of two. Three-dimensional quantities, like mass, increase by a power of three. Forces due to gravity scale up by a power of three, and forces due to acceleration scale up by a power of four. Torques due to gravity scale up by a power of four, and torques due to angular acceleration scale up by a power of five.
Therefore, small escalations in size lead to slower movements, and the need for those massive motors.
Electric motors like the ones used in Gundam are composed of two kinds of magnets to impart motion. The first is a permanent magnet, which is often made of naturally occurring materials, like rare earth metals. These magnets retain their magnetic properties, even in the absence of an electric current or an inducing field. Then there are electromagnets, which rely on coils of wire to act like a magnet when an electric current passes through.
Motors rely on the interaction between the permanent magnet and electromagnet to create mechanical energy. As the electromagnet’s polarity is manipulated by electricity, it spins, rotating an axle that can drive the Gundam’s leg, for instance.
If that spinning motion becomes stronger, so will the motor. To make it happen, an engineer would introduce a larger magnetic field, Atkeson explains. Theoretically, he says, you could create an electromagnet as large as a neutron star—the collapsed core of a massive supergiant star—but there are practical limitations on Earth, as there is a limit to how much an object can be magnetized.
Magnetic resonance imaging machines, or MRIs, push those boundaries, as some of the strongest man-made magnets. So, if engineers could create motors with the power of an MRI machine, they could almost certainly get the colossal Gundam to walk. Of course, new issues arise; namely, the robot’s mass ends up being dominated by the 24 actuators required to create 24 degrees of freedom.
“An engineering black hole ensues, where no matter how big the actuators, the robot is still too weak to move at the desired speed,” Atkeson says. This is partly why legged robots—and the exacting locomotion that they demand—are difficult to successfully engineer, even for the world’s top robotics experts.
So maybe this whale of a Gundam can’t really walk. It’s still a feat of engineering that it’s stiff enough to move the way it does without substantial vibration, Atkeson says. And between its sheer size, illuminated eyes, and ability to wave, it’s the kind of ambassador that any country would be proud to have.
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