This three-legged robot walks like nothing you've seen before


STriDER is a three-legged robot that walks by ... uh, well, the best way to understand its patent-pending "tripedal gait" is to watch the videos below. Basically the robot steps forward by swinging one leg between the other two while flipping over its top body and then doing the same with another leg and so forth. Wicked!


The robot generated quite some buzz at this year's ICAR in Korea [read the excellent paper here, in pdf] and more recently in the robotics blogosphere.

To learn more about the robot, Automaton went straight to the source: Dennis Hong [right], director of the Robotics & Mechanisms Laboratory (RoMeLa) at Virginia Tech, in Blacksburg, who leads the team developing STriDER (if you're wondering, that's short for Self-excited Tripedal Dynamic Experimental Robot).

Next, a Q&A with Hong on STriDER's development and applications, and also videos showing the robot's first step and hypotrochoid-based joint mechanism.

Automaton: I see STriDER is taking its first steps -- it looks very cool. When we'll see the robot walking around?

Hong: We now have two working prototypes. STriDER 1.0 was designed to only take a single step -- it was a prototype to study the passive dynamic motion of its single step gait. STriDER 2.0 is designed to be walking around, and it does. The body of STriDER 1.0 was too heavy with too many motors. The three abductor joints which align the two stance legs so that the swing leg can swing between them were controlled by three DC motors. This motion really is a single degree of freedom motion, thus theoretically they could be replaced with a single motor. In STriDER 2.0, we have developed an ingenious mechanism that utilizes an internal gear set with three pin-in-slot joints to replace the three motors with a single one, using a mathematical curve called the hypotrochoid curve. We are in the process of redesigning and remaking it using advanced carbon fiber composite materials.

Automaton: I think many people will wonder about this: Why three legs?

Hong: One of the problems of legged robots in general is the complexity of the mechanical design and the control of its many motors. Six legged hexapods or four legged quadrupeds have so many active joints (motors), controlling them and coordinating their motion is a challenge. On the other hand, two legged bipedal robots have less number of active joints (motors), but they are inherently unstable with only two legs. If you look at a camera tripod, it has three legs -- a statically stable structure with the minimum number of legs!

However, the problem of trying to walk with three legs is that, every time it takes a step, the legs will be entangled up. STriDER prevents this from happening by flipping its body 180 degrees every time it takes a step ("tripedal gait," patent pending).

The main benefits are:

* very energy efficient with its actuated passive dynamic motion

* tall by nature -- great for deploying sensors such as cameras. It can see above bushes etc.

* statically stable with three legs, unlike unstable bipedal robots -- think camera tripod

* Kinematically simpler and needs minimal control compared to other type of legged robots

* can change directions easily, simply by changing the sequence of the swing leg (a very unique property of STriDER)

Automaton: And what about applications?

Hong: STriDER is not being developed to perform manipulation tasks or for carrying payloads. Its purpose is for deploying sensors to difficult to access areas. Since STriDER is tall by nature (STriDER 1.0 is about 1.8 meters tall) it is great for deploying cameras, whereas most other autonomous robots (which often resemble a small remote controlled car) cannot see above bushes etc. STriDER is as tall as a human being. This is one of its main advantages over other type of robots.

Automaton: Now, I don't recall seeing anything that moves like that. What's the inspiration?

Hong: Interestingly, STriDER's walking gait is more closer to that of a human walking than most bipedal humanoid robots you see today which actually resemble humans. STriDER uses the concept of "actuated passive dynamics," which means that the swing leg simply swings between the two stance legs, not actively controlled nor actuated. The "control" is automatically done by design, utilizing the built-in dynamics of the system (with the right mass properties and dimensions). This is how we humans walk -- we do not actively control our knees when walking, we just let it swing.

So even though STriDER does not resemble any kind of biological organisms on earth, it is still truly a "biologically inspired" robot, where all the principles and physics of human walking can be applied. "Bio-inspiration" is not the same thing as "bio-mimicry." Bio-inspired robots does not have to resemble its biological counterpart in appearance, as long as it shares the same guiding principles of how it operates. STriDER is an excellent example of this.

STriDER's first step:

An overview of the project:

STriDER's joint aligning mechanism:



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