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Beyond Breadcrumbs: Leader-Follower Shows the Way January 2009
If you happen to see a 2004 Ford Explorer being followed by two odd-looking tracked vehicles called Centaurs, you may be seeing an experiment in leader-follower robotics. You'll know for sure if the Explorer's roof bristles with antennas, lasers, and video cameras, and if the tracked vehicles have no drivers. MITRE is using these autonomous (driverless) vehicles to develop leader-follower technology that will eventually help our military save lives. By putting teams of leader-follower robots to work in supply convoys, for example, the U.S. Army could remove its soldiers from supply vehicles that must travel long distances through dangerous territory. MITRE recently investigated the requirements for the command and control of large unmanned ground vehicle convoys. "We used our experience in systems engineering and systems integration of components into large scale systems," says Bob Grabowski, a lead artificial intelligence engineer in MITRE's robot systems engineering group. "The result is a leader-follower team of autonomous vehicles that will use route following, speed control, and obstacle avoidance to support upcoming work." The lead vehicle already had experience guiding itself in the 2005 Grand Challenge sponsored by the Defense Advanced Research Projects Agency. (For the leader-follower project, a safety officer rode along during experiments in case human intervention was required.) The researchers chose the two Centaurs, which are normally used for off-road utility work, because they are good surrogates for military ground vehicles. The Centaurs use caterpillar-like tracks instead of wheels and are skid-steered, which allows them to do zero-radius, "pirouette" turning for extreme maneuverability. "The skid-steering Centaurs are good models for future combat system robots. Their turning dynamics are similar and they have low-contact pressure, which allows them to carry heavy loads over soft surfaces," says Grabowski. Outfitted with a Suite of Sensors To modify the Centaur as a follower, the team added a suite of sensors: a horizontal laser for obstacle avoidance; a vertical laser for terrain modeling; a pan-tilt camera for object detection; a global positioning system (GPS) for localization; and sensors for speed control. Depending on the environment, the researchers use different tracking modes: breadcrumb, laser tracking, vision tracking, and range-based speed adjustment. The breadcrumb mode is used in open terrain when the robot vehicles are far apart and the lead vehicle sends GPS points ("breadcrumbs") to the following vehicles. When vehicles get close together, they switch to a laser tracking mode; a following robot adjusts its speed based on the speed of the vehicle in front of it. Position is calibrated using relative laser measurements to compensate for differences in individual positions. "But urban driving creates a number of problems," says Grabowski. "Sometimes you can't get an accurate GPS reading or any at all. If a large robot vehicle tries to follow another robot through a crowd, you have issues of avoiding people and other obstacles. The vision tracking mode will be used here. This mode augments the laser tracking mode. When a specific distance must be maintained between vehicles, we use range-based speed adjustments." In developing the autonomous skills for the robot vehicles, the researchers used computer-based simulation techniques to tune the robots before putting them on the road. "The simulation told us what we needed to develop our leader-follower capabilities," explains Grabowski. "There was as much development on the software side as there was on the robot side." The team developed the software architecture so that it could test and simulate multiple robots that appear to be connected to a communication network. For Civilian Emergency Response, too In addition to military operations, leader-follower teams could be used for civil emergency response situations. "For example, a future scenario could be a train carrying toxic chemicals that derails," says Grabowski. "We'll want to get emergency tools and supplies to that train site quickly. For fast deployment, large unmanned leader-follower vehicles could be staged around the country in tactical locations like fire houses. From the nearest fire house, the autonomous vehicles would follow a fire truck to the site. If the chemical spill prevents people from dealing with it safely, the robotic vehicles could go beyond the safety line to put out fires or dampen toxic fumes. It could also provide critical situational awareness of the event." But the military will probably be the first user of leader-follower systems once they've been proven. As MITRE continues to develop robots, its future work will involve multi-robot coordination in urban environments. The robots will also allow MITRE to examine the operational performance of existing tactical radios in various environments. "Our development of leader-follower software and the addition of the Centaur robot assets puts MITRE in a position to rapidly test and assess future robotic mission capabilities," Grabowski says.
—by David A. Van Cleave Related Information Articles and News
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