University of Toronto engineers have given a one-legged hopping robot a flexible foot pad that helps it handle sand, ice and other difficult terrain. The robot, called the Hopcopter, is essentially a quadcopter-style drone perched on a single large, spring-loaded foot.The team designed it for search and rescue, remote infrastructure inspection and, perhaps, interplanetary exploration. The work appears in Advanced Intelligent Systems, with Professor Pakpong Chirarattananon of mechanical and industrial engineering as senior author.A drone on springsChirarattananon said drones are cheap, can fly over any terrain and excel at photography and videography. But because they must be light, they carry little cargo, and their range is limited by the fuel they can carry.Wheeled and legged robots can carry more and travel farther, but they are much more expensive and cannot always handle rough terrain. He sees room for a new kind of robot in between.His lab, the Convergent Locomotion, Intelligent Mechanisms and Bio-inspired robotics lab (CLIMB), looks to nature for designs that balance range, efficiency, cost and cargo capacity. The Hopcopter takes cues from birds, which do not stay airborne constantly but flit between spots, rest for long stretches and sometimes hop along the ground.PhD student Song Li of the City University of Hong Kong, on academic exchange at Toronto, played a key role in developing the robot. Its long leg keeps the center of mass high above the ground, and pre-stretched rubber bands counteract gravity and other forces. That makes it easier to model and control than other hopping robots, said lead author Jeeraphat (Owen) Kledkruemas, a PhD student in mechanical and industrial engineering.Hopping lets the robot bound over obstacles that would stop most wheeled or legged robots, while conserving energy and carrying more weight than a standard drone.Fixing the footThe original Hopcopter worked well on flat concrete or fine gravel but struggled elsewhere. If it landed at a steep angle or on something slippery like ice, the foot slipped and the robot lost balance. On sand or other soft surfaces, the foot sank and got stuck.Kledkruemas designed a foot pad that spreads the robot’s weight over a larger area, much as a snowshoe keeps its wearer from sinking into snow. A spherical joint lets the pad slant in any direction to match the surface, and rubber bands snap it back into position after each hop.The bottom of the pad is covered in short spikes that add traction, like the crampons ice climbers use.Measured gains on terrainAccording to the researchers, the new pad reduces take-off error by up to 66% and improves efficiency by up to 64%. It also cuts performance variation across terrains by 84%, so the Hopcopter performs almost as well on difficult ground as on smooth surfaces. Kledkruemas said the improvements make it possible to hop on ice and sand.From lab to MarsThe team plans to develop autonomous navigation for the Hopcopter and take on bigger challenges. Chirarattananon said the improvement lets them “bring this robot out of the lab and into the real world.”He imagines using it for search and rescue after natural disasters or for inspecting remote infrastructure such as pipelines and mining sites. The team also dreams of exploring places like Mars, where the atmosphere is too thin for most kinds of drones.