Surround and Diversion by a Squad of Mobile Robotic Vehicles
Cooperating squads of robotic vehicles may some day be employed for tasks such as fighting forest fires, cleaning up oil spills, delivering and distributing supplies to remote field operations and conducting a variety of military missions. Control and coordination of groups or squads of robots has thus far been largely science fiction; however Sandia's Intelligent Systems and Robotics Center (ISRC) is working to develop just such capabilities. In a Tactical Mobile Robotics (TMR) program for the Defense Advanced Research Projects Agency, the ISRC demonstrated the capability to remotely surround a facility with a squad of mobile robotic vehicles. This proof-of-concept scenario involved a squad of six RATLER™ mobile robotic vehicles. The RATLERs™ and their remote base-station began the vignette two hundred meters away from the target facility. To complete the mission, the vehicles had to autonomously navigate terrain typical of the desert Southwest, including crossing a deep ravine, negotiating tumbleweeds and other thick brush, and following man-made tracks. Several of the vehicles then surrounded the perimeter near the rear exit of the facility while other vehicles entered the main entrance to create a diversion.
A single operator was able to plan and execute the entire six vehicle mission using the remote command station. The mobile command station consists of an off-the-shelf laptop computer connected to a suite of radio transceivers. A custom PC-based application provides the interface. A command and control channel is used to download path and obstacle information to the vehicles, issue commands to the vehicles and to relay status information about each vehicle to the laptop. Another radio link is connected to a Differential GPS (DGPS) transceiver. This sub-system transmits corrections to the GPS receivers on the vehicles so they can accurately determine their location. A separate RF link is used to transmit a video signal from the vehicles. This allows the operator to view the camera view from any one of the vehicles in real time.
In spite of their small size, the RATLERs™ have impressive off road capability. These carbon composite electric all-wheel drive vehicles are approximately the size of two bread boxes. A passive central pivot connecting the two halves provides a simple but effective suspension. Software on the vehicles accepts and processes path, obstacle and goal perimeter information and executes the mission autonomously. By monitoring its own and the positions of the other vehicles, a vehicle is able to autonomously navigate to the goal while avoiding the pre-defined obstacles and other vehicles. An automated path-planning algorithm accomplishes this using artificial force fields to repel the vehicle away from obstacles and other vehicles and attractive forces to direct the vehicle toward its goal. Tilt sensors provide a rudimentary obstacle recovery capability for unexpected obstacles.
To perform the mission, first, the base-station's location was ascertained by correlating the operator's position to that on an aerial photograph or GIS map. The DGPS transceiver was then initialized. The operator used the interface to draw paths for the individual vehicles and to outline obstacles and goal perimeters. A simulation capability allowed the operator to preview the plan prior to execution. Then, the plan was downloaded to the vehicles and the mission began. Real time updates and a video link allowed the operator to monitor the progress of each of the vehicles.
The mission plan for the demonstration was to have the vehicles first traverse the deep ravine by following carefully selected paths that offered the easiest possible transit routes. Then, using their on-board path planning software, some of the vehicles would approach and surround the facility while others would enter the front gate to create a diversion. On the aerial photograph four different paths were selected for six vehicles to follow. An attractive goal was chosen to be the perimeter of the chosen facility. In addition, obstacles such as ditches were outlined with repulsive perimeters.
In the Demonstration the vehicles were able to traverse the ravine by following their designated paths. Then, they autonomously navigated toward the goal perimeter while avoiding obstacles and automatically spreading out. Once they reached the goal perimeter, one vehicle was given the task of entering the front gate. The successful completion of this proof-of-concept scenario demonstrated a unique capability of the ISRC RATLER™ squad. Using a robotic vehicle in place of a human in these potentially dangerous situations can reduce the risk to human lives. Because a single operator can plan a set of tasks for a squad of robotic vehicles, the RATLER™ system can significantly increase the effectiveness of a single war-fighter in the field.
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