Linear Actuators and Robotic Tails Help Balance

One of the most difficult problems in robotic science is how to design a robot that can land on its feet if it falls or is dropped from a height. Usually a robot will land in a heap and then struggle like an overturned insect to right itself. However, by taking a lesson from the animal kingdom, a robot has been fitted with a tail which can aid a robot in righting itself. The tail uses a linear actuator to control the robot’s horizontal position. Mini actuators, usually under two feet in overall length, are ideal for robotics.

Animals ranging in size from geckos to lemurs use their tails to orient themselves in space, and two robots of very different sizes have been taught to do the same. The Tailbot, weighing six ounces, and about 5 inches long, wuses a stiff, but hinged tail. However, the X-RHex Lite (XRL) robot is a hefty 18 pounds, and 20 inches in length, and it has successfully demonstrated the same ability. The robots contain actuators in their tails. The action is similar to that of a tightrope walker using a balancing pole to counteract small inconsistencies in his center of balance.

The robot tail is simply a stiff rod. But at the end of the tail is a relatively heavy mass containing inertial navigation sensors. Feedback from the sensors goes to the robot’s control center, which sends directions to the linear actuator connecting the robot’s tail to the body. The actuator then changes the angle between the two until the robot’s feet are pointing downward.

XRL is a much more practical size for a robot. Potential applications are navigating earthquake sites as part of search and rescue, or to traverse difficult surfaces, such as deserts. In both cases, navigation over uneven or broken terrain is critical for performance. Both robots use electro-mechanical mini actuators.

One key factor in the two design sizes was that the power density of the tail actuator had to increase as the size of the robot increased. The mini actuators had to be sized to function within the manoeuverability task space. The response time and power of the linear actuator, and its gearing, had to be mathematically integrated with the elapsed time of the fall and the possible range of body angles. This allowed the various-sized robots to make equal corrections in an equal amount of time.

When the XRL is dropped from a height of 4 feet (only 2.7 times its body length) without the tail it lands in an awkward heap. With the tail activated, its linear actuator corrects the body to a horizontal plane and the robot lands on its feet, still within that short distance.

Each of these robots have full 90 degree body correction capability. At present, Tailbot and the XRL tail actuators are integrated with their leg actuators. However, the research team believes that separating them could result in maximizing the performance of the tail. Certainly, talented and useful robots are on the near horizon.

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