An engineering team has published a paper revealing that by giving a robot the right tail, it can always land on its feet – just like a cat.

The University of Pennsylvania's Kod*lab appears to have found a way to gracefully circumvent a robot's embarrassing tendency to scrabble helplessly at the air, like a bug on its back, when it lands awkwardly. In experiments sending the six-legged X-RHex Lite running off a 62cm-high cliff edge and then into a nosedive from eight times its standing height, the team discovered the robot always righted itself mid-air in time for impact using the "dynamic locomotion" (tail swinging) employed by some in the animal kingdom.

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"By swinging their tails, geckos can self-right in less than a body length after a fall, or reorient through zero net angular momentum maneuvers," explains the paper, published in the Proceedings of the International Conference on Climbing and Walking Robots, July 2012, and primarily funded by the U.S. Army Research Laboratory.

"The effectiveness of this mechanism inspired Tailbot, a robot with an active tail which enabled disturbance regulation and other dynamic behaviors, including air-righting and traversing rough terrain. The stabilizing function of tails appears to operate effectively over a wide range of size scales in natural systems, from one gram geckos to 10 kg lemurs and possibly beyond."

The team decided to see if the same could be said of robots of different sizes, so fit the extra appendage to a model 60 times bigger than the tiny Talibot (a toy car device with a lizard-like tail attached). In keeping with the Talibot's design, the brass tail weighed about a tenth of the robot's body mass and was attached to a carbon fiber tube the length of its body. Airborne self-righting was achieved by the 8.1kg hexapedal X-RHex Lite by ensuring that the "power density of the tail's actuator" increased with size "in order to achieve the same maneuver in the same relative time". This was made possible by using a "point mass" tail – a tail that carries the bulk of its mass at the tip. Using its inertial navigation sensors, the robot detects its rotation or the upcoming cliff edge and activates the tail to realign the body pitch.

The paper concludes that adding a tail is an ideal solution to helping create autonomous robots, since mechanisms that are already developed can simply have the appropriately-sized appendage attached, rather than undergo a total structure amendment to achieve the same effect.