Thank you John.
I wish someone had shared stuff like this with me a few years ago.
John, these tips are great for any team, rookie or veteran.
We’ve experimented with Passive assist for the First time that I know of, and it made a WORLD of difference. We’re still searching for that point of near weightlessness, but even now, a few bands of tubing is helping us a lot.
From what we’ve found, and it sounds like you’re coming to the same conclusions, the RS775-18 is a GREAT motor. We were running ours for around 20 minutes, with as a lift or two each minute and a solid ten seconds or more of stall with each lift and they didn’t heat up very much at all.
Wow. Thanks John! That’s just the solution I needed to counterbalance the arm without blowing the 120lb limit or introducing the complexities of a ratcheting system. That’s just too simple.
We planned to use the FP motor with it’s gearbox and a rack-and-pinion for our vertical lift, and knowing how the FP’s do not like to stall (I think they were the original source of “magic smoke” back in the days before CIM’s when they were one of the choices for drive train motors) we decided to use constant tension springs available in a wide range from McMaster. We assembled everything that was going to be lifted, weighed it, divided by 2 and ordered 2 springs in that range. No close fits required - the ID of the spring is over an inch so we just stuck a 3/8" bolt through it, attached it to structure and screwed the other end to the arm. Works great - without the motor even installed you can move the arm to any position and it stays there.
We cheated to learn this by just copying 148’s arm in 2007, works quite well 
So did we. Can we claim collaboration?
Well we did it in 07 during the fix it windows, this years pretty different on our end.
So if collaboration can be applied over long time periods maybe
This is (as you’d expect from John) a great write up. There is one conditition, however, where our CIM’s have become hot enough (outer case over 60 degrees C) that I started to worry about them. That was using them as drive motors in repeat practice matches. One year we were through tech well before most other teams and the field crew wanted robots on the field to test their systems and make sure everything was “broken in”. Our drive team was MORE than happy to oblige. At some point (on the third battery change, if I recall… we’d been going at full “match” power for over half an hour) I mentioned to one of the students that we should check the motor temperature.
The student replied “Ouch!” We grabbed our IR thermometer and I can’t remember exactly what the temperature of the case was, but it was hot enough that even if the CIM wasn’t flinching, I was. It took about half an hour to cool down to the point where I felt good about firing the motors back up again.
Now, the good news was that the CIMs were still fine, so I’m not disagreeing with the quote. But there may be occasional circumstances where even the mighty CIMs shall succumb to heat.
Of course, if you use them in an arm, and counterbalance the arm like John is suggesting (that’s kind of the point of the whole post) then you shouldn’t have a problem.
Jason
P.S. If you want to see some cleverly counterbalanced arms, just check out a VEX tournament.
P.P.S. Oh, the many benefits of being through tech at 9:00am!
You should see how hot a CIM gets after 6 hours of Robowrangler practice… but they keep running.
I want a video of the hot-dog roast taking place over the drive train at the end of the day! 
I wonder if anyone has actually smoked a CIM… maybe they ARE indestructible in an FRC context.
Jason
Our kicker mechanism from 2010 was the linear type using a lead screw and surgical tubing, and while the idea had merit, it was poorly executed. There was no shortage of binding on the system, and the CIM driving the lead screw would get very hot, besides the Jag faulting every 5 seconds, until we got everything tuned just right… (Only to be knocked out of tune 30 minutes later! :ahh: )
Of course, the CIM has shown no sign of permanent damage, and just keeps chugging. Yay CIMs!
We still have the original little banebots motor driving the arm of our 2007 robot. It’s a long arm on that robot, too. We used a gas spring.
This year we’re using a single window motor to raise the arm…it’s also a long arm.
Here’s the lower attaching point for our passive assistance–easy to adjust, just push it up or down the pole, it stays where you put it, because of the magic of friction and leverage.
http://photos.project1726.org/albums/userpics/10010/2011bot10.jpg
Great post JVN! as usual
Hooray for passive assistance! Our 2007 bot was able to lift a tube quickly with the assistance of a gas spring, powered by a single 540 Banebots motor. This year’s arm is more interesting, but fortunately we have math to compensate (and bigger motors!).
We learned passive assistance in VEX, specifically last year when we wanted to design a 254-esque arm to fling the game objects without any pneumatics. We actually gave our basket “negative weight”. Footballs flew.
Since then, passive assistance on every arm we’ve done, including on the VEX robot that made it onto JVN’s blog. It’s that useful.
Yes, 190 has. I do not remember the year, 2004 maybe. They did a mecanum drive and were overheating the CIMS big time. The robot rule that says “thow shalt not modify motor housings” belongs to 190 because they cut a window in the CIM housing and had small muffin fans on the other side/end pulling air through the windings. a really neat solution, actuallly.
Great post John; it should add a bit for knowledge to many varying levels of experience.
For our lift, I’ve done some math. We too will be using some surgical tubing to aid in the lift. We’re using a 5/8" thick 2"/turn threaded rod at 50% of its critical speed – we can increase that if we decrease its load. The hope is that we can use only 1 RS-775 to power it, yet we may still add a second one just to be on the safe side if we have weight to spare.
The surgical tubing serves a second purpose too, however. At the top and the bottom of the rod’s range of movement, we’ve lathed off the threads in order to prevent the nut from getting ripped apart should the programmer’s 3 safety pieces fail. At the bottom the tubing will combine with springs to lift it out of the bottom dead zone, thereby enabling us to continue scoring even sensors/code fail.
(Yea I know, threaded rod is heavy – but we ‘got a guy’ who knows what he’s doing with it, so I’d rather error on the heavy-but-quality side than try our multiple failed winch ideas again…)
Most of our VEX robots this year have passive assistance on their arms. The rack/pinion on a slider is great, the ability to “up gear” to move the arms faster is a must. The latex tubing makes this possible since there is less weight for the motor to move.
Nice post John, is this the famous “Subsystem 0” we keep hearing about on your blog?
Related question:
Does anyone have a stress-strain curve for surgical tubing? Anecdotal evidence suggests that it’s possible to exit the linear-elastic region under FRC conditions. While it’s generally not that big of a deal, given how FIRST teams use surgical tubing, I’m still curious.
Calculator here: http://www.primelineindustries.com/tools.html
Results and explanation here: http://www.chiefdelphi.com/forums/showpost.php?p=1011716&postcount=9