I’m currently in a program at my school called “Senior Year Project,” in which we get out of classes during the fourth quarter of our senior year in order to work on a special research project. Since I’ve been on my city’s FRC team (2877 - the LigerBots) for four years and we have never been able to create a really successful robotic arm, I decided I wanted to create a robotic arm.
I’m specifically designing my arm so that it would have been able to be used in the game two years ago - Logomotion. So, if any of you have any suggestions for cool designs, or have CAD models that you think might be useful to me, I would greatly appreciate them! Any models or designs for robotic arms would be useful - not just those designed for Logomotion.
I have a few ideas milling around my head for the design. For example, I want to put the arm on a Lazy Susan, to have one joint controlled by a piston, one by a motor, and then have the last part of the arm be telescoping. But I’m open to new ideas or suggestions.
Really a turreted arm is overkill and your central bearing/bushing assembly will be prone to failure due to high loading (not to mention large expense). Trading the complexity of a rotating arm for a faster, simpler, better implemented fixed arm will usually result in a more competitive robot. The most common types of arms that were effective in 2011 were:
Should you want to continue to pursue the rotating arm assembly you may want to ask 118 about their rotating elevator assembly (https://www.youtube.com/watch?v=h3l6Xqts4to) though arms and elevators will induce different kinds of loads they may be able to help you with bearing selection. Best of luck!
After looking at the CAD for 610 I don’t see how the final stage moves. It doesn’t appear to be connected to anything. I assume it was connected in cascading fashion to the other two stages. Anybody know for sure?
There is a flat nylon strap that “weaves” through the inside of the stages. The strap causes the stages to cascade. The strap is similar to what you would find on a backpack. When you extend the 1st stage then next one extends as well. I believe you can see the mounting brackets and roller assemblies for this strap in the CAD if you look closely at the ends of the stages.
The 2011 arm is pneumatically extended. There are two pistons, back-to-back, attached to only the 1st stage. The nylon strap then cascades the rest of the arm. The back-to-back pistons allowed for 3 different extended positions.
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.
The CAD files for some custom plastic pulleys that we made are here on our website. They are designed for use with pneumatic tubing bands but I would assume that you could use polycord as well if your team prefers that.
We found that this pulley assembly works well for picking up tubes, foam basketballs, and frisbees and can be about as versatile as you want it to be as long as you are willing to be a little creative with how you mount it.