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Re: Robotic Arms
In theory you only need a maximum of 6 degrees of freedom to control the motion of an object and once a degree is consumed it is redundant to use it again. So considering a typical robot design from 2011, and really any year, the robot's drive train consumes all of the translation on the X,Y plane (parallel to the field) and rotation about the Z axis. This leaves three axes to be consumed, translation on Z and rotation about X and Y. The jointed arm or elevator allows access to your Z translation and due to a need to go from horizontal while the tubes were on the ground to vertical to be placed on a peg you needed some way to control rotation about the X axis (perpendicular to arm of robot). This motion was achieved either by having a simple wrist (1114) or an active roller claw that could spin the body of the tube relative to the claw (67 -2007, 48 - 2011). The latter design even had redundancy as it had two control conditions about the X axis, the rotation of the arm and the rotation of the tube. The addition of a rotating wrist (about the Y axis) is questionable due to the orientation needed to hang a disc. The addition of a turreted base only adds redundancy about your Z axis rotation.
In the end you want to simplify the controllable axes your drivers have to deal with during match play. There are even cases of simplifying the number of degrees of freedom a robot has by the creative use of linkages. I suppose with the appropriate level of vision tracking one could implement small Z axis rotations as the drive base approaches the rack, but that is beside the point.
In the end you only want to control a necessary amount of axes and lock down the rest in an effort to make control of the robot by the operator more intuitive. I.e in 2011 our driver had to fight having control over two rotations about the X. As the arm rotated the tube stayed relative to it and then to adjust to the current peg the operator had to adjust the rotation of the tube in the claw. These redundancies tend to slow down the operation of the robot and siphon off offensive potential.
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