I have been designing an elevator and I am not sure how much I should counterbalance it. Do I completely counterbalance it or do I just counterbalance it enough for the break on the motor controller to prevent backdrive? Any advice would be helpful.
Passenger elevator codes have strict guidelines. see ASME A17.1
Whatever your application, this is a general explanation of what you are trying to accomplish with the counterweight.
From How Stuff Works
“The ropes that lift the car are also connected to a counterweight (4), which hangs on the other side of the sheave. The counterweight weighs about the same as the car filled to 40-percent capacity. In other words, when the car is 40 percent full (an average amount), the counterweight and the car are perfectly balanced.
The purpose of this balance is to conserve energy. With equal loads on each side of the sheave, it only takes a little bit of force to tip the balance one way or the other. Basically, the motor only has to overcome friction – the weight on the other side does most of the work. To put it another way, the balance maintains a near constant potential energy level in the system as a whole. Using up the potential energy in the elevator car (letting it descend to the ground) builds up the potential energy in the weight (the weight rises to the top of the shaft). The same thing happens in reverse when the elevator goes up. The system is just like a see-saw that has an equally heavy kid on each end.”
Constant force springs would be a good method to counter-balance an elevator. They’re pretty simple to use, you could have one for each stage. I believe 846 and 973 used them in 2011 on their elevators with success, i’m sure many other teams used them as well just none come to mind. Hope this helps.
As far as how much to counter-balance I would not completely counter-balance it, just enough to prevent back-drive. This is just what I would do if building an elevator though, I only have experience designing an elevator, not building or using one for an FRC application.
In 2011, we used a constant force spring from McMaster Carr. Worked great all season. We used a two stage cascade elevator with a single spring. With the drive disconnected, it was pretty much neutrally balanced. Here’s a photo.
Here’s a link to a previous post that I wrote about our experiences with constant force spring motors (in a thread that is ripe with tons of other goodies). Our elevator in 2011 was without a doubt the best part of the robot; it was speedy, going to full extension in a second, and was only run off a tiny little RS550; all made possible by counterbalancing the lift so that it would be able to stay in whatever position it was left in when unpowered.
If you want any more information on our lift and how it works, I’d be more than happy to help you out, just message me. Some of the finesses on that lift are things that I still find really cool, 3 years down the road. Also, do some searches of Chief Delphi; you’ll find that there are a lot of threads on this subject already, and they contain information that you might find extremely useful.
Reading the description and looking at the pictures, I think there may be a step missing from the descriptions.
If the constant force spring (CFS) is stationary on stage 1 (as it appears to be), then I believe the path of the CFS line should be the following:
Bottom of Stage 1 -> Top of Stage 1 -> Bottom of Stage 2 -> Top of Stage 2 -> Top of Stage 3.
Is this correct or am I missing something?
- Sunny G.
You caught an error in my wording; it should have said “The line from the constant force spring motor was looped over a pulley at the top of the first stage and connected to bottom of the second stage to counteract the weight of the manipulator.”
The third stage (manipulator stage) is slaved to the second stage (explained in the post I linked); this is why you don’t need to attach the constant force spring to the third stage directly.
What would be an advantage of slaving the 3rd stage over a continuous cable?
In a continuous cable system you have to pull much more cable to raise the hi-lo to full height. As that cable/rope wraps around the winch the effective diameter of the winch increases. That changes the tension on your rope and can cause issues. The use of flat spectra or very small diameter cable or rope can help that situation. In-line springs can also take up any change in tension.
Slaving the two, however, requires much less rope be wrapped on the winch for a full hoist. We’ve never had to worry about a spring system in a multistage slaved setup. The stretch in the rope has been enough so that we don’t have to worry about a spring loaded tensioner. We just tension it all by hand.