The design criterion for the disk acquisition subsystem was simplicity;
we wished to involve few moving parts and only simple binary sensors.
The disk acquisition mechanism weighs approximately 
kg.
The landing gear, which supports the disk retrieval system, consists of a
in. circular aluminum rim supported by four castering rubber
wheels. This arrangement allows the helicopter to maneuver on the ground
within the pickup-ring using cyclic and yaw controls (Figure 3). The
circular rim of the broad-based undercarriage protects the vehicle from
dynamic rollover.
Figure 3: Landing Gear and Disk Retrieval Mechanism
The disk acquisition system consists of two parts: a ``scoop'' and a
``grabber.'' The scoop, a spiral one-inch high sheet of aluminum/lexan
with a maximum radius of 
in., is mounted under the helicopter at
the center of gravity.
When the helicopter is rotated clockwise using tail rotor control,
any disks in the path of the scoop will be drawn to the grabber
after several rotations. The grabber, at the center of the scoop,
detects the presence of a disk, secures it, and signals the flight
computer that a disk has been acquired (Figure 4).
Tests indicate that, on the ground, the helicopter will not rotate
around its center of gravity. The resulting incidental translation of
the spinning helicopter is sufficient to cover the entire surface
area of the pickup-ring in several rotations. This fact obviates the need
to explicitly locate and move towards a disk - the helicopter will
simply spin about the ring until ``random'' motion brings a disk into
the scoop.
Figure 4: Top View of the Scoop and Grabber
Figure 5: Side View of the Scoop and Grabber
When the scoop's rotation guides a disk into the grabber, closing the ``ready'' microswitch, the servo lowers a permanent magnet into contact with the disk surface (Figure 5). A microswitch attached to the magnet detects contact, and indicates if the disk is lost at any time. When the ``magnet down'' and ``disk acquired'' switches are closed, the helicopter briefly rotates counterclockwise, then lifts off and proceeds to the drop-off ring. Once over the drop-off area, the servo retracts the magnet, forcing the disk to drop.
To accommodate the chance that the scoop may land on one or more disks, it is attached so that it may flex up approximately one inch from its normal position. As the helicopter rotates, the scoop will drop back into its normal position once it is clear of obstacles. Only a single disk will fit into the grabber. The brief counterclockwise rotation before lift off serves to dislodge any extra disks which have been gathered by the scoop. As a precaution, a timeout safeguards against unanticipated situations. If the grabber fails to acquire a disk within a fixed time interval, the helicopter will take off, reposition itself over the ring, land, and repeat the process of collecting a disk.