Editor's note: Legged robots have the ability to follow troops on long journeys across extremely difficult terrain. In our series on legged robotics, Scientific American Online explores the challenges such technology poses as well as two DARPA projects—BigDog and LittleDog—that have shown great promise.

NASA has robots that can wheel around on the surface of Mars, hundreds of millions of miles away, meanwhile mine rescue workers deploy models that climb through caverns hundreds of feet below this planet's surface. The U.S. military, however, is looking for electromechanical companionship that's a bit more practical, if much more difficult to deliver: a four-legged robot that can walk alongside troops, carrying supplies across any terrain without help, and all without toppling over.

With large U.S. military deployments across the craggy landscapes of Iraq and Afghanistan (and no sign of them being withdrawn soon), the Defense Advanced Research Projects Agency (DARPA) is hoping to have a version of its much anticipated BigDog, an autonomous robot being developed by Boston Dynamics, by the end of the year.

Legged robots spare the military from having to build roads and can travel across rugged areas just like pack mules, says Robert Mandelbaum, a program manager in DARPA's Information Processing Techniques and Tactical Technology offices who is in charge of the agency's biorobotics program, which includes BigDog. "Some of the wars that we're engaged in now happen to have that kind of terrain," he notes.

Powered by a gasoline engine that drives a hydraulic actuation system (which uses fluids and pressure to move the robot's legs), BigDog is a 3.3- by 2.3-foot (one- by 0.7-meter), 165-pound (75-kilogram) robot that relies on four legs designed like those of a quadrupedal animal. Its design allows it to absorb shock and recycle energy from one step to the next. The robot, which at times resembles two people walking while carrying an overturned raft over their heads, has an onboard computer that controls locomotion and small servo motors that manage the position of the legs using data they receive from a variety of sensors. Those sensors tell the device where its joints are and what they are doing, where it is in contact with the ground, and also allow for stereo vision.

The robot also has a gyroscope that uses a laser to precisely measure changes in its orientation. Other sensors monitor its hydraulic pressure, oil temperature, engine temperature, rpm (revolutions per minute) and battery charge. "BigDog has a fantastic control mechanism to not only balance but move on some pretty rugged terrain," says Sanjiv Singh, a research professor with Carnegie Mellon University's Robotics Institute who in 2005 and 2006 worked with Boston Dynamics to develop BigDog's computer vision system.

Next steps

Work on BigDog began in 2002 as part of DARPA's biorobotics program, which launched that same year and was at the time called "biodynotics" (for biologically inspired multifunctional dynamic robotics). "The idea is to look to nature and the way nature has solved different robotics problems," Mandelbaum says. In 2005 the agency funded the development of a "mini-me" version of BigDog called LittleDog, a smaller, autonomous quadruped robot that measures 11.8 by 7.1 inches (30 by 18 centimeters), stands 5.5 inches (14 centimeters) and weighs 4.9 pounds (2.2 kilograms). LittleDog is part of the agency's Learning Locomotion Program to develop learning algorithms that help robots operate in unfamiliar environments. In addition to their size difference, the robots differ in that BigDog ambles along as if it were blind, whereas LittleDog is programmed to recognize its surrounding terrain. (For more on LittleDog, read "DARPA Pushes Machine Learning with Legged LittleDog Robot.")

BigDog is now in its third phase of development, which will last through the end of 2008. Boston Dynamics is trying to demonstrate to DARPA that the robot can carry military payloads across a terrain that mimics a five-mile (eight-kilometer) trail at the Marine Corps base in Quantico, Va. This trail features holes in the ground, steep inclines and other hazards, and a successful crossing—while loaded with an assortment of equipment typically carried by Marines—will prove to the military that the robot is on the right track. Mandelbaum expects to see a demonstration of BigDog's progress (and the value of his agency's $24-million investment in the technology so far) in these areas by September.

BigDog has become something of a celebrity online thanks to several videos available on Boston Dynamics' Web site as well as YouTube. It even spawned a spoof video of two people in black leggings pretending to be the robot. "BigDog has such a realistic-looking gait, you can't tell it's not BigDog at first," Singh says. "You know that you have something big when someone takes so much effort to spoof a robot."