Bipedal robots, such as Boston Dynamics' Atlas may be able to balance on one leg, but Disney Research has gone one better and built a one-legged hopping robot. This unidexter automaton isn't the first hopping robot, but it's the first to not rely on a tether or external power source to keep bouncing.

Hopping robots have been around since at least the early 1980s and though a robotic pogo stick may seem as pointless as an ejection seat on a helicopter, they have helped engineers in developing and refining robotic locomotion. While robots with two or more legs can handle rough terrain and use different gaits for different tasks, single-legged robots have a much simpler topology and are limited to hopping to move and stay upright.

According to the Disney Research team, this makes them ideal for testing robot leg mechanics and algorithms. A single-legged robot needs to respond at high speeds and with much higher forces causing much higher stresses than on multi-legged robots if they're going to stay upright and not trip over things. Meeting these requirements is much more difficult than making a two-legged robot walk and provides valuable information to designers on how to overcome problems.

Early hopping robots, such as those built by Mark Raibert at MIT in 1983, were heavy affairs with massive metal crash cages. Relying on pulleys, cams, and electric solenoids, it and later hopbots required tethers and external power sources to work without damaging themselves. Building such robots was so hard that the Disney team says that no real progress has been made on increasing the portability of hopping robots since 2007.

The Disney approach for its hopping robot was to make a much lighter, simpler design. In this case, the robot is built around a Linear Elastic Actuator in Parallel (LEAP) for greater agility. This mechanism uses two parallel compression springs to steady the central leg, which provides thrust by means of a voice coil. As the name suggests, the voice coil was originally designed for certain types of speakers, but the coil's ability to provide an easy to control proportional thrust without direct contact between coil and shaft made it ideal for this purpose.

The robot has a special hip made out of a gimbal joint incorporating a pair of servo motors. In addition, there's an onboard microcontroller that controls thrust, orientation, and foot placement. Put them altogether and it adds up to a robot that can maintain its balance for up to 19 hops over seven seconds on its own. Though the robot did have a safety tether, this was slack during the tests and was mainly a telemetry link to record data.

The Disney Research team says that there's still room for improvement on the robot. The software assumes the foot is static when it lands, causing it to slip and the sensors could be more reliable and accurate. Also they are working on a better way to model the robot's velocity while in mid-hop, and they hope to make the mechanism more modular and compact, so it can be adapted to multi-legged robots.

The Disney Research team's findings can be found in this paper (PDF).

The video below shows the Disney Research robot going for a quick hop.

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