Squirrels Make It Look So Easy

Researchers at the University of Pennsylvania Department of Electrical and Systems Engineering in conjunction with members of Boston Dynamics have developed a rather unique robot that can perform some pretty amazing tasks. In a jointly authored paper, “Rapid Pole Climbing with a Quadrupedal Robot,” they explain the workings and capabilities of RiSE Version 3, a prototype dynamic four-legged robot that can quickly scamper over uneven terrain, but more importantly, rapidly ascend a vertical structure. Potential applications of RiSE include search-and-rescue operations along with military or industrial surveillance.

This climbing machine has a mass of 5.4 kg, a body length of 70 cm, and a 28 cm tail. Looking much like a large four-legged rodent, RiSE Version 3 has successfully scaled a telephone pole in 21 cm/s. This may not seem speedy, but it's believed to be the fastest recorded ascent to date for a robot of this type. Previous versions only achieved 1cm/s and 5 cm/s, respectively, and those speeds were on a tree trunk, not a telephone pole.

As stated in the paper, “The key innovation of this design is a variable transmission leg linkage that allows climbing-targeted leg motions—high torque during stance, high speed during leg recirculation—with near-constant motor velocity, while maintaining the general purpose morphology of a functional quadruped.” Additionally, a variable transmission actuator provides the capability of the feet to grip surfaces during climbing sequences. The front legs of the robot are positioned so they wrap slightly around the pole to help keep the robot from falling backwards.

Control electronics are located near the tail while the power electronics are situated in the front and mid sections of the robot. Communications are via a wireless link to a laptop while the robot itself handles all joint commands and controls. On-board electronics include a CPU carrier with a PC, a wireless card, a flash module containing the operating system, and appropriate I/O. Internal communications are via the CAN bus that ties the CPU to the motor drives.

As you might expect, special considerations had to be given to developing a suitable gait for the robot, enabling it to climb the pole quickly while maintaining positive contact with the surface. Two types of gaits come into play: a crawling gait and a bounding gait. Legs are recirculated individually in a crawl gait, which helps to stabilize the robot as it climbs up the pole. However, its speed becomes slower. The bounding gait provides faster climb rates, but the robot had a tendency to pitch back from the surface.

The final resolution for RiSE 3 was a cross between the crawl and the bound. The back legs work together to propel the robot forward for speed while the front legs move individually so one is in contact with the pole at all times. This gait arrangement accomplished the speed/stability goal.

Ongoing work on the robot will center on the use of bounding gaits to obtain more vertical speed and the incorporation of surface attachment and detachment techniques to limit pitch back. Special sensors will be implemented to control slipping and correct for movement errors as the robot is climbing. Automating the process of smoothly converting from horizontal travel to vertical ascent also is on the list of future tasks. Achieving this seemingly simple maneuver may prove to be the most difficult of all.

Paul Milo
Editorial Director
[email protected]

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