December 1, 2022   |   Updated: December 5, 2022

If you are a fan of robots (and let’s be frank, who isn’t?), you’ve almost certainly seen one of those four-legged specimens jaunting around obstacle courses or serving as a goalie in soccer practice.

It’s common to refer to them as “robot dogs” and they seem to be everywhere, which begs the question: Why are so many robots shaped like dogs? 

To answer that we checked in with two experts in the field of robotics, IEEE Senior Member Ming Liu and IEEE Member Jayakrishnan Thrivikraman Nair. 

Liu says that the robot dogs are a subset of robots referred to as “quadrupeds,” which means “four-legged.” The reason they resemble dogs, rather than horses or rabbits, has to do with the knees – the big joints connecting the two largest segments of the legs. 

“The knee of the dog’s limbs bend backward (toward the rear), while the horse’s knee bends forward,” Liu said. “Most quadruped robots, like Boston Dynamics’ Spot, MIT’s cheetah3 and Unitree’s Go1 adopt the backward knee design, which is consistent with the shape and appearance of a dog.”

“When the quadruped robot performs the task of going upstairs, the backward elbow design provides legs with a larger collision-free workspace,” Liu said. “When going downstairs, the situation would be reversed. The knee-forward design becomes more advantageous, so the robots mentioned above tend to walk backward to go downstairs. Also, there are quadruped robots that can switch the direction of the elbow.” 

Dogs’ legs are simpler

Nair said that the dogs tend to have lower centers of gravity, and that makes them more stable.  

“Dogs’ front legs differ significantly from those of tall animals, and the last link length is quite short in comparison. This also ensures that the form and proportion of the front and back legs are the same, making it simple to replicate hardware and control gait,” he said. 

Advantages of four-legged robots

One of the most prominent uses of dog-shaped robots involves monitoring and inspection. They can be used in search-and-rescue operations or in places that are too small or too dangerous for humans. 

“It is the simplest form of statically stable locomotion that can traverse challenging terrain,” Nair said. “It offers the best balance of cost, controllability, and ability to walk on rough terrains. With four legs, the design can move in three different gaits: the walk, trot and gallop.”

Comparing two-legged and four-legged robots

“Although biped robots can also perform similar tasks, the stability of the locomotion of bipeds lags far behind that of quadrupeds,” Liu said. “Besides, the cost of building and maintaining a biped robot is several times higher than that of a quadruped robot.”

Disadvantages of four-legged robots

There are a few drawbacks of quadrupeds,” Liu said. “They tend to have low energy efficiency. Most can only work for one hour before they need to be recharged. They can also be noisy because of their gait. Some people aren’t able to tolerate the noise, which might prevent them from becoming a part of our daily life.” 

In-Depth: What are quadruped robots used for, and how does that impact their design? Those questions are answered in this article from the IEEE Robotics and Automation Society magazine.

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