I am in the process of designing a spool/cable driven elevator system, and have two questions regarding the design:
Currently, I have a powered up, gravity down continuous cable rigging system. However, I would like to also be able to power my elevator down. How do I change my cable routing to achieve this functionality?
For my counterbalance, I wish to use surgical tubing rather than a constant force spring,since teams in the past have had pretty serious injuries with constant force springs. What/where have teams purchased their surgical tubing to properly counterbalance their elevators.
Look at our 2007,2008, and 2011 robot pictures. There should be quite a few of the 2011 robot out there on our website, which is the most refined version. Fundamentally they are all the same though. This picture shows you what’s happening. The return cable is attached to the bottom of the carriage and then runs to a pulley at the base and then is taken up by the spool that the “up” cable is wound on.
McMaster has all the surgical tubing you’d need, but it’s subpar as your force drops off as soon as you start lifting and the carriage will only be partially supported. Constant force springs are much better. I know 1717 used a device in 2011 that integrated a constant force spring into a box with a cable that extended/retracted. I think they were called “pull boxes”, though as I recall it they were only good for up to 6 lbf or so.
I know 846 used one of these integrated tool balancer constant force spring assemblies in 2011. They’re distributed via SDP-SI, and should have enough oomph to counterbalance an elevator. However, they are a tad expensive.
Does anyone have links to photo galleries of 254 or any other good elevator designs? Specifically, it would be nice to get some pit scouting pictures from Champs that do a good job of getting close ups of the different parts of the robot.
Personally I thought 1114 had the best elevator design in 2011, it was driven up/down through a chain and belt system. http://www.frc-designs.com/CAD2011.html has models of ’11 Bots with elevator style lifts, but not 1114. So I contacted them and asked if they would share a CAD model for an off season student elevator style lift project which they graciously provided. However our server crashed awhile back taking all our work with it.
330 did something similar when they did a lift; usually no pulley was involved to get the cable to the spool, though. The key trick is to make sure that the “down” cable is going out when the “up” cable is going in.
I was fortunate enough to be able to take some detailed pictures of 254’s lift while in St. Louis in 2011. Here are some of the details I think should be noted:
Here is a picture of the rope run Cory mentioned. You can see both the lift and return runs. Up is on the left and lower on the right.
Here you can partially see the drive train for the lift. I’m sure someone can provide the details of this mechanism.
Here, on the left end of the lift spool, you can see the “counter balance” they used. It is a surgical tubing powered rope wound around a spool on the end of the lift spool.
And now the magic used to keep everything in alignment and still allow it to move freely.
Here is the design we’ve used variations of since '96.
It’s a simple chain loop and one box extrusion sliding inside another.
It doesn’t get much simpler.
The top cross member is stationary and mounted to the inner extrusion.
The motor is stationary at the bottom of the robot, and the chain is linked to the lower cross member to drive the whole assembly up.
In 2011, we created a very cool lift system to elevate our grabber from the ground to the height of the scoring pegs. It’s a bit different from 254’s, as it incorporates a constant force spring motor to counterbalance the system (we preferred not to use exposed constant force springs, as if they snap it can be very scary and unsafe). By doing this, we were able to drive the system with a very small motor, but still have a very fast lift system. Video (go to 1:07): https://www.youtube.com/watch?v=UzZJO0pQafY
The three stage system was nearly perfectly counterbalanced with a constant force spring motor assembly from SDP-SI (https://sdp-si.com/eStore/Catalog/Group/580), as mentioned by DampRobot somewhere back in the thread. The line from the constant force spring motor was looped over a pulley on the second stage and connected to the third stage to counteract the weight of the manipulator. Even without the motor attempting to move the elevator, the system would stay put wherever it was, which greatly simplified the programming of the system.
The first stationary stage has a motor mounted to it (an RS-550, as it only needed to move the system, and didn’t have to handle the weight of the manipulator), which turns a chain run that is connected to the second stage. The chain moves and it pulls the second stage with it.
The third stage (grabber) is slaved to the second stage very simply. A line is connected to the top of the third stage, runs over a pulley at the top of the second stage, and is connected to the bottom of the first stage (base frame member of the robot). When the second stage moves up, the third stage moves up with relation to the second stage as a result.
If you want more information, I can provide some. Also, our CAD is online at frc-designs if you want to take a closer look for yourself.
