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		    Docking Station for Autonomous Robot Recharging
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		| Motivation |  
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		    Autonomous mobile robots are being developed to perform
		    numerous tasks from planetary exploration and mine
		    sweeping, to entertainment.  Each application requires the
		    robot to survive in its environment, whether it be open or
		    closed, indoors or outdoors.  Operating time is limited for
		    many of these robots due to their on-board power supplies.
		    As a result, long-term autonomy is not possible.  If this
		    limitation could be overcome, then mobile robots could be
		    used for new and more meaningful applications on a regular
		    basis. In addition, new avenues of mobile robotics research
		    could be explored.  Currently, rechargeable batteries are
		    typically used that provide power for only a few hours.
		    Once depleted, the robot/batteries must be connected to a
		    recharger via human intervention.  This results in a
		    non-continuous robot task cycle as shown in the following
		    figure, thereby preventing long-term autonomy.
		     
		     
			  
			Figure 1: Robot task cycle comparison
		    
		    
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		| Approach |  
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		    A recharging station that allows the Pioneer robot to
		    recharge autonomously has been developed. The recharging
		    station physical design is described in two parts: the
		    docking station and the robot docking mechanism.
		     
		     
		    
		        
			     
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			| (a) Docking station | 
			(b) Robot docking mechanism | 
			 
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			    Figure 2: Docking station components
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		    The docking station is a stationary fixture that provides a
		    connection point for the robot docking mechanism.  A power
		    charger is connected to the docking station providing the
		    necessary power. The docking station is designed with 2
		    passive DOF, providing compliance for numerous robot
		    docking angles and conditions. The robot docking mechanism
		    is mounted to the back of a Pioneer robot as shown Figure
		    5. The robot docking mechanism was designed with one
		    passive DOF. A contact switch is attached to the robot
		    docking mechanism, controlling the power connection to the
		    batteries.
		     
		     
			 
			 
			Figure 3: Recharging station setup
		    
		    
		    Refer to relavant papers and the video clips for more details.
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		| Experiments |  
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		    To test the stability of the docking capability, a robot
		    was directed to follow a continuous cycle of wandering and
		    docking for 100 trials.  Results of the 100 trials showed a
		    99% success rate for mechanical docking, and a 97% success
		    rate for electrical docking.  We tracked both mechanical
		    and electrical docking separately due to the differences
		    involved to complete each operation.
		     
		    To test if the robot could survive without human
		    intervention over an extended period of time, a robot was
		    programmed as a sentry, monitoring our lab entrance.  The
		    robot wanders while avoiding obstacles until the main doors
		    of the lab are detected, at which time it stops and begins
		    monitoring them.  Whenever someone opens a door, the robot
		    takes a picture every second the door is detected open
		    using a camera.  Once the robot's battery voltage level
		    reached our preset lower limit, the robot began its Docking
		    Module sequence - looking for the docking station.
		     
		     
			 
			 
			Figure 4: Voltage level vs. Time for the monitoring task
		    
		    
		     
		    Refer to relavant papers for more details.
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		| Video Clips |  
		
		    
		        
			     
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			    Autonomous Docking (AVI, 22.5MB) 
			    The clip shows the docking stratey. Vision is
			    initially used to find the docking station using a
			    camera. When the robot moves close enough to the
			    docking station, the laser rangefinder on the robot
			    scans for the laser beacon and determines its angle
			    to the wall. At a proper distance from the docking
			    station, the robot executes the blind docking
			    sequence based on the odometry.
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			    Door Monitoring (AVI, 59.4MB) 
			    The clip shows a robot performing a door-monitoring
			    task. The robot wanders while avoiding obstacles
			    until the main doors of the lab are detected, at
			    which time it stops and begins monitoring them.
			    Whenever someone opens a door, the robot takes a
			    picture every second the door is detected open
			    using a camera. Once the battery voltage level
			    reaches a preset lower limit, the robot begins the
			    docking sequence for recharging, and goes to
			    suspension mode until the battery is fully charged. 
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		| Acknowledgements |  
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		    This research was supported in part by DARPA under grant
		    DABT63-99-1-0015 and NSF under grant ANI-0082498. The first
		    author (Milo C. Silverman) was supported by a fellowship
		    from Raytheon Electronic Systems, Raytheon Company.
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