MUWA or "Multi-ﬁeld Universal Wheel for Air-land Vehicle" (and also Japanese for "Dream Ring") is one of these designs: it's a quadrotor surrounded by a circular piece of foam that makes it capable of (among many other things) balancing itself sideways like a wheel and rolling along the ground.
MUWA's big foam ring makes contact with obstacles, water, or the ground not much of an issue, but the secret to the balancing tricks and wheel motion is that unlike most quadrotors, MUWA is equipped with independently controllable variable pitch propellers (+/- 20 degrees of pitch) that can direct thrust in two opposite directions, allowing it to stand up into a vertical position from the ground:
MUWA can also roll, rotate, and even roll while maintaining an arbitrary angle relative to the ground.
There are a few reasons why these capabilities are relevant. First, it gives the quadrotor ways to move around without always having to expend energy flying. Second, by rolling, MUWA can squeeze through vertical gaps that it wouldn't be able to while flying horizontally. And by getting MUWA to do things like rotate around a point on the ground while changing its angle ("tornado motion," the researchers call it), a Kinect sensor on the robot can rapidly build up a complete 3D map of its surroundings.
The next generation of MUWA promises to be able to use its wheel motion on water, and somehow, it'll also be able to demonstrate "vertical attitude flight."
"MUWA: Multi-ﬁeld Universal Wheel for Air-land Vehicle with Quad Variable-pitch Propellers," by Koji Kawasaki, Moju Zhao, Kei Okada, and Masayuki Inaba from the University of Tokyo, was presented yesterday at IROS 2013 in Japan.