Quote:
Originally Posted by Mori1578
Well Done!
Great lifter 
Thanks for sharing this info
What motor did you use? And how did you know all those physics?
I finished High School mechanics, and these calculations are scary! 
Again - Thanks!
-Yaar,Team 1578 |
We ended up using a CIM into a DeWalt transmission into a Modified Andy-Mark transmission with a VEXPro large sprocket reduction (we like to support all the FIRST sponsors). There is a picture of it in CD somewhere.
As far as the physics goes, our team has 3 engineering mentors on the team. We work with the kids to brainstorm the plus/minus of the different concepts and then we (the engineers). Most of the older High Schoolers can understand and assist with figuring out the torque and gear ratios.
Motor power * efficiency * time = change in energy (height change of CG for this one).
Transmission ratios should be figured out
speed based and torque based. These are simple calcs and we do them with the kids. I really like making the kids do the math of those sorts of things.
(5 second hang of 120 degrees from a motor that loaded should spin at 4000 rpm. 120/5 sec = 4 rpm therefore we need a ratio around 1000:1). Some rough numbers: Dewalt is around 40:1, the toughbox gets another 5:1 and the chain drive is roughly 5:1. 5*5*40 = 1000:1 This arm also produces about 250 ft*lbs of torque at its shoulder!
Those level of calculation we work with the kids and often by the end of the year, they can do them themselves. The conference room we borrow has a giant whiteboard wall. sometimes the engineers write, sometimes the kids write. We all do the math though. We feel that this is why FRC does not have a mentor involvement limitation rule like FTC, FLL, OCCRA... It is to engage the students in these designs.
Beam bending fatigue, and the "gription" model (we like to make up words) are more advanced. You need basic "Statics" from an Mech Eng. school or a good understanding of free-body diagrams (FBDs) in order to do the gription model. We like to do these calculations in order to make sure that things should work, and to figure out important parameters. This FBD was made by another engineer, and then he and I discussed the details. This helped us to understand that a longer arm added leverage to the traction patch and thus kept us from sliding down the pole. It also helped us understand the loads applied to the pole to ensure that we did not damage the surface of it (we had a 6" contact patch).
I did the beam bending calcs in order to make sure our arm would be robost enough for about 100 matches. These calculations are taught the second or third year of most Mech. Engr. programs. In 2007 we had a large arm bot that had a casastrophic arm and tower fatigue failure at the very last demo we did that year. I consider this one of my greatest successes. As Collin Chapman would say, if the racecar doesn't fall apart after crossing the finish line, you made it too strong. Since FiM has us playing more matches, we have been leaning a little further towards inifinite life designs (Jr./Sr. year ME course work).
We still go over the harder concepts with the kids, but usually do the analysis off-line from our main meetings. This is pretty advanced stuff, and the struggle can be boring and/or misleading for most of the students. We try desperately to not be Tinkers, but to do engineering with the kids.