[Popular Science] Why are many quadruped robots designed to look like dogs?

If you’re at all interested in robots—and frankly, who isn’t?—you’ve no doubt heard the term “robot dog” a lot. They seem to be everywhere, which begs the question: Why are so many quadrupedal robots designed to look like dogs?

To answer this question, we (IEEE Transmitter) interviewed two experts in the field of robotics, IEEE Senior Member Liu Ming and IEEE Member Jayakrishnan Thrivikraman Nair.

Liu said the robot dog is part of a class of robots called "quadrupeds," which obviously means "four legs." The reason most quadruped robots are designed as dogs, rather than horses or rabbits, is because of their unique knee structure.

"Unlike horses, whose front knees point forward and rear knees point backward, dogs' knees point backward all the time. This design helps dog-shaped robots climb stairs, with more room for legs to move around and avoid collisions," Liu said. "For example, Boston Dynamics' Spot, MIT's Cheetah3, and Unitree's Go1 all use a backward knee design, which is consistent with the shape and appearance of dogs."

"When the quadruped robot performs the task of going up stairs, the backward elbow design provides a larger collision-free working space for the legs," Liu said. "When going down stairs, the situation is reversed. The design with the knee forward becomes more advantageous, so the robot mentioned above tends to walk down stairs backward. In addition, there are quadruped robots that can switch the direction of their elbows."

Dog's legs are simpler

Nair said dogs tend to have a lower center of gravity, which makes them more stable.

"A dog's front legs are significantly different from those of a taller animal, and the length of the last link is quite short in comparison," he said. "This also ensures that the front and hind legs are of the same shape and proportion, simplifying hardware replication and gait control."

Advantages of quadruped robots

One of the most prominent uses of robot dogs is monitoring and inspection.

They can be used in search and rescue operations or in places that are too small or dangerous for humans.

"This is the simplest form of statically stable locomotion that can traverse challenging terrain," Nair said. "It offers the best balance of cost, controllability and the ability to walk over rough terrain. The design has four legs and can move in three different gaits: walk, trot and gallop."

Comparison of two-legged and four-legged robots

"Although bipedal robots can also perform similar tasks, the motion stability of bipeds lags far behind that of quadrupeds," Liu said, "In addition, the cost of building and maintaining a bipedal robot is several times that of a quadrupedal robot."

Disadvantages of quadruped robots

However, quadrupeds also have some disadvantages, Liu said. "Their energy efficiency is often low, and most quadruped robots can only work continuously for an hour. In addition, the robot's gait is noisy, which some people may find unbearable. These shortcomings will prevent quadruped robots from being more widely used in daily life."