From ARVP
2009 Treasure Grabber
Objective/Problems · Need to design a treasure grabber to pick up the treasure from above and constrain it in at least 3 degrees of freedom · Unfortunately, the 2009 rules have not been released yet and so the shape of the treasure is unknown. · We know that the treasure will probable be made from 1” PVC and previous treasure geometries from 2008 and 2007 (12” cube and 12” cross) · Other Teams Grabbers: are mostly passive with mechanical bump switch with array of claws. We observed that these grabbers have trouble with knocking over the stand as well as misfiring or not firing at all.
OBJECTIVE: To design a modular treasure grabber that can be easily adapted to previous treasures and that can be actively controlled using actuators and sensors.
SOLUTION: The key design feature needed in the treasure grabber was flexibility because at the time of the design, the final competition specifications had not been released yet. However, it was known that the treasure would probably be made of 1” PVC and would need to be constrained in at least three degrees. From observing other teams at the 2008 competition, it was noticed that most treasure grabbers relied on an array of claws with mechanical bump switches to trigger them. However, a lot of these arms knocked over the treasure or did not fire at all. Based on this, the final design was settled on being a relative simple two claw system. The claws were made so that they could accommodate various misalignments between the submersible and the treasure. Each of the claws has two servos on it for each arm of the claw. After some searching, the Traxxas 2075 underwater servo was selected as the best choice for this amplification. By giving each arm its own servo, the need for a gear system was eliminated. The servos were then linked up to existing servo controllers inside the submersible. However, custom servo shafts were needed to accommodate the shaft for the arm. These servo shafts can screw directly into the servo motor and are lock tightened into place. On the other side of the arm, the shafts are supported by insert friction bearings (McMaster Part No. 6362K201). To trigger the claw, a water-proofed momentary bump switch was placed under the main housing of the arm mechanism. A trigger plate was then placed above this switch to give it a wider area of sensitivity. The bump switches relay information back to the onboard embedded computer to help assess whether or not the treasure has been grabbed correctly and to prevent the claws from misfiring in other stages of the competition. These two claws are then mounted onto the 80/20 beams already present on the submersible. Both the 3D-models (treasure_grabber_asm.asm) as well as the drawings of the treasure grabber can be found in the provided CAD and drawing packages, respectively.
