What would be the best way to achieve a mechanically retracted elevator system that doesn’t rely on gravity? I’ve prototyped a continuous elevator design but even with a closed loop tension is released when trying to retract.
There will always (probably) be an infinite number of ways to build a robot in FRC.
The beauty of a design comes from the ability to complete the given task with the most simple solution.
The “best” solution will come from what your team decides are the most important tasks to achieve during this year’s game, nobody on CD will be able to tell you what is best for your robot, nor should you want them to (where’s the fun in the competition if you just do what other teams told you to do?)
Thanks for the input, I’m not looking for the best way to compete, but the best way to actuate a mechanism. As a fairly new team it’s always nice to hear from more veteran teams.
I recommend looking at the McMaster Carr (mcmaster.com) website for ideas. Check the “Power Transmission” section.
Mechanically retracted elevator systems were fairly common in 2015 as well, when it was advantageous to be able to “snap” down a set of totes. However, that year allowed for 6.5 foot tall robots, so there weren’t many mechanically retracted multi-stage elevator systems. This year we have the 55" restriction whereas many elevators are going to go above 80.5" to attach to the rung. With a climber design like this, whatever is mechanically retracting the elevator has to be strong enough to pull up the entire robot.
I’m not exactly sure where we can look to find inspiration for a 2-3 stage elevator system that doubles as a robot lifter. Do others have any idea?
Take a look at 330’s robot from 2004, or even 2001 or 2000 if you can find pictures. There’s a few in CD-Media, but none really show the system well.
Cable rig “cascade”, single down-cable. Lifted the robot up near the start of the match, locked in the “retracted” position, stayed there despite all attempts to get the robot down.
Let’s say say you have a winch and one end of the cable goes through the cascading lift and returns where it ends at the same winch, but wound the opposite way. Now picture three winch divided into 2 sections: a smaller drum responsible for raising the lift and a larger one for pulling it back down.
The trick is in figuring out how big the diameter of each half should be relative to the other.
Hope this helps.
How about this?
Maybe try using some surgical tubing to pull it back down faster than gravity.
Something like this?
To avoid hooke’s law which makes surgical tubing and springs pull with more force the farther you pull up, you could use constant force springs to assist with the pull down.
My team has done 2 elevators. 2008 was a cascade style with no powered return, just gravity. Worked flawlessly. 2011 was a continuous style, with a powered return. Also worked great.
With continuous, the length of wire you pull in as you pull up is the same amount you need to let out for the return. And vice versa. Run both lines on the same diameter spool, driven from the same motor.
With cascade, it’s not quite as simple. You need different diameter drums. When you pull up, you get approx twice as much height as the cable you pull in. So the return line (which just runs direct to the final stage of the elevator) needs to let out twice as much cable as you pull in. So the down drum needs to be twice as big (circumference) as the up drum.
These are only approximations. Your best bet is to put it all in CAD and figure out the length of the line (both up and down) from your winch drum at both extremes. That’ll give you the actual ratio you need.
The return line on a Cascade elevator should be idler on both ends of the 2nd stage, and thus stays 1:1.
Much easier
I would encourage everyone who is thinking about building an elevator to check out the 973 RAMP videos on their (2011?) elevator. Adam does an excellent job of explaining how the elevator works and it makes a great starting point. We’ve adapted almost every part of our elevator off the design of the one in that video.
Thanks Adam for the great resource!
There are a number of ways to skin the proverbial cat of powered down stages on an elevator lift.
The most mechanically straight forward is to have a second motor and pulley attached to your last stage that is programed to spool and unspool at the same rate as your up motor system.
My preferred solution is to do a single pulley that winds up and unwinds string at the same time. This type of solution does require more attention to mechanical detail than a dual drive system. The advantage is that you only have one drive system that needs to be controlled as well as saving the mass of the second drive system. For running the powered down for the 2nd or 3rd stages I would recommend doing an inverted cascade string setup. The most common flaw in these types of systems is sine/cosine error caused by the string or chain not being completely vertical.
We detail the implementation of this type of solution in the REV 1" Lift build guide.
Though this guide is referencing the REV lift kits the principles described are applicable to any lift system your team chooses to use.
-Frank
