Difference between revisions of "DEWBOT III Arm"

Revision as of 12:07, 12 August 2009

Al Koenig, Ben and Kevin test the Arm prototype

Dewbot III ended up with an effective arm for scoring, but this arm had a difficult birth.

The Arm used on Dewbot III has three articulations:

1. A hinge shoulder driven by a pneumatic cylinder
2. A hinge elbow driven by motors and monitored via potentiometer
3. A gripper driven by pneumatic cylinder

Original Arm Mechanics

The Shoulder articulation was used to deploy the arm for play from the robot's initial staring position. The pneumatic cylinder (Parker 1.50DPSR8.0 -1½" diam x 8" stroke) was retracted and the shoulder folded back for the start of competition (this kept the robot within the FRC starting envelope dimensions). At the start of Autonomous Play, DEWBOT III's compressor came on and the Shoulder deployed to the forward position. The Shoulder could also be actuated during the game to allow bi-directional play (picking up ringers on one side of the robot and scoring on the other) or to manage center-of-mass when climbing ramps at the match end for bonus points. Bi-directional play was seldom used in practice.

Dewbot III's elbow was driven by a pair of Nisso-Denso motors (L & R) with a 4:1 "gear" reduction via 10 & 40-tooth timing belt pulleys. A 270° potentiometer provides elbow angle information.

The original Gripper was a simple pneumatically-actuated vertically-oriented claw which grabbed the ringers center-to-outside.

Both arm sections were fabricated from Aluminum tubing, braized/welded with Zinc (like the frame). This produced a strong, lightweight arm. This joining method was satisfactory.

The first-generation Gripper proved to be awkward. It was difficult to hang the ringers on the Rack and especially difficult to successfully disengage afterwards. This Gripper was used only during DEWBOT III's first competition (Pittsburgh) before being replaced with an improved design.

Improved Gripper

Following Pittsburgh, an improved gripper was designed by Ed Faller. The improved Gripper had two long fingers which spread horizontally and it gripped the OD of the Ringer torus, rather than from inside to outside.

Gripping just the Ringer OD leaves the Ringer hole clear of robot parts. This makes scoring easier. It especially avoids interference with the Rack while disengaging after hanging a Ringer. DEWBOT III's scoring performance and competition ranking improved dramatically with this improvement.

Thanks Ed!

Alternative Arm Design

One of the Team's "process" problems in 2007 was our failure to make a decision early in the build decision between two competing Arm designs. As a result, an Alternative Arm was developed throughout the build season and (briefly) mounted on DEWBOT III.