Im Back for more wisdom from the FRC and CD community
Ive been intrested in knowing what is the most simple arm to build, an elevator or telescopic arm and how to prevent it from over extending, another question is do yall use Bearings in the arms to allow it to travel free?
Also, how do yall run the pnuematics and electrionc wires in this type of arms and keep them from getting pinched or ripped apart
What motors are also good to use with this arms?
If you have any pictures it greatly appreciated. I know im probably asking alot, but i really want to improve my knowledge and better and to improve our team. Thanks again for taking your time, and giving me yall advice and recommendations
For the wiring throughout the arm we used ribbon cable through the main tubing parts. The ribbon cable was attached to the plastic chain they supply in the KoP that is meant for securing wire to. It keeps things very neat.
I know many teams have also used coiled wire on elevators. I however do not have experience using those.
As far as whether an elevator or telescoping arm is better, it really depends. In 2007, a telescoping arm seemed to be more effective because it helped to have more reach outside the robot frame. In 2011 however, the elevators were more effective because it was easier to control the height and placement of the tubes and a large reach out of the frame wasn’t as necessary.
In 2011 our mentor had an idea that didn’t quite make it to the final design. It was to use the telescoping pole that diesel drivers use to clean there stacks and rig. Im pretty sure you can pick one up at your nearest diesel dealership.
Pneumatic tubing can be purchased that is already coiled or you train it around a large diameter pipe (sewer pipe works for this) with a bit of work. I’ve even seen teams just create a bundle, occasionally wrapping it in some sort of protective material no slip mat seems popular, and using rubber bands or other elastic material to attach it at several points.
The plastic chain from the KOP is provided by Igus and is called “energy chain.”
I suggest that, for rotationally driven trave mechanisms, people always consider using a mechanical, adjustable torque, slip clutch combined with a hard mechanical stop to prevent over travel.
In 2011 we used a single axis, chain driven (by way geared down dual wiper motors) swing arm with rheostat positioning control.
The arm was very long and could not take a lot of abuse, so we needed to make sure if it was moving and hit too much resistance, the motion would stop while the slip clutch just spun, with no further motion to cause damage.
This also worked great for re-synching the rheostat, since with the chain stopped, we could always still swing the arm to any position by pushing the arm enough to overpower the set torque limit of the slip clutch.
Even with micro switches and other programmed means for limiting travel, stuff happens, and having a slip clutch can be a godsend for preventing damage. Combining a rubber bumper travel stop with the slip clutch is all it takes for rotary actuated motion limiting…
The best arm is the one your team has the capability of building. If you don’t know how to do it, prototype first.
To prevent overextending you can:
put in hard stops (don’t stall your motors !!)
Put in an encoder (use software to stop the travel)
Put in limit switches (can be used for software stops or hardware -Jags- stops)
Using bearings is, usually, preferrable.
Create a travel conduit with a looped back wire/tube set. Use rubber bands / surgical tubing to retract the excess wire when the arm retracts.
Any motor will move an arm with the proper gear reduction. What you need to ask yourself is ‘how long do I want it to take to extend/retract the arm under worst case conditions’ and size the motor accordingly. You may want to look at assisting the motor in a particular direction (gas shock?) if the load in 1 direction is far more than in the reverse direction.
My team has been around since 1995 and we have done quite a bit of refining and improving the basic design of the telescoping arm in that time. Our eveolutionary changes reflect the KOP changes that have taken place over time as well as rule changes.
1996: 177 introduces the first telescoping arm on the Hexorcist. It was a monocoque sheetmetal construction with the “inner arm” being stationary and attached to the shoulder. The outer arm glides over the inner arm on custom linear slides made from HDPE. The slides start out at the top and bottom of the arm and move closer together as the arm extends. The extension is driven by window tape, a plastic that appears like the edge of a piece of film. The shoulder was powered by a van door motor driving a 45 chain sprocket on a solid steel axle. This was a high shoulder robot using the super structure to create a reduction with chain to drive the shoulder.
2005: Same as 2001 & 2003 but a high shoulder and with a COTS automotive winch gearbox powered by to FPs.
2007: The first year expansion rules were in place which caused us to go with a vertical extension instead of extendeing the arm. We changed the slide material to delrin for the first time. We ditched the window tape for extension because it no longer in the kit and used #25 chain. The extension was a banebots motor and so was the shoulder which was a COTS worm drive gearbox. http://www.chiefdelphi.com/media/photos/28221 http://www.chiefdelphi.com/media/photos/28222
2008: We reversed which part of the arm was stationary and which part moved by making the outer arm stationary. We also went to a hollow, large OD tube of 4130 for the shoulder to save weight and improve strength. http://www.chiefdelphi.com/media/photos/30533
Great synopsis Peter. I always figured that team 178 would feel tremendous pressure to demonstrate a flawless telescoping arm (since they’re sponsored by UTC sister division Otis Elevators); it looks like you’ve done pretty well yourself.
For Logomotion we built a two stage timing belt elevator that was fast and had sub inch accuracy on the set points. We started with one Fisher Price motor but added a second one when we re-geared the transmission to make it scary fast. I was surprised to see how easy the elevator was to build and how reliable it was through the season. Rivet construction. External fan cooled motors, Jag speed controllers. The timing belt was the key to our elevator. We didn’t need a counter spring to lower. We caught a peg in a match and the elevator lifted our 140 lbs bot no problem. FYI don’t use the KOP limit switches use a industrial strength limit switches to prevent the over travel.
Here is how 971 ran their bearings on the elevator.
A single cable ran up, down, up, and back down to pull both moving stages up. This results in the innermost stage moving first, then the second stage moving.
We ran the cables through Igus chain to keep everything organized. This still created a loop sticking out the back of the robot while the elevator was in motion.
The second, for Logomotion, was cascade:
There were two cables - one from the motor, up over the fixed stage and down to the bottom of the second stage, and one from the fixed stage over the second stage and down to the innermost stage. This results in the motor lifting the second stage, which also lifts the first stage - both stages move at the same time.
With this design, we ran the wires up the fixed stage to a pulley at the top of the second stage, then down to the innermost stage. Since the two stages moved in concert, as the elevator moved up, the distance between the top of the second stage and the innermost stage decreased at the same rate the difference between the the fixed stage and the top of the second stage increased - in other words, the distance from start to finish, through the pulley, stayed the same. The wires remained straight without going overly loose or tight. It completely avoided having a loop out the back of the robot that other robots could get tangled on.
I’ve also seen teams utilize wire loops - they attach one end of a tape measure to the base of the robot, and the other end to their elevator, then loop the wire around the tape measure. As the elevator goes up, the tape measure extends to follow it, and the wire loops get smaller.