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DISK RETRIEVAL MECHANISM

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 tex2html_wrap_inline367 kg. The landing gear, which supports the disk retrieval system, consists of a tex2html_wrap_inline385 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.

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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 tex2html_wrap_inline257 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.

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Figure 4: Top View of the Scoop and Grabber

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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.


next up previous
Next: FLIGHT TEST PROCEDURES Up: The MIT / Draper Previous: FLIGHT CONTROL

Bill Hall
Fri Jan 31 14:15:17 EST 1997