I saw a lot of teams use Vulcan springs that wrap up on their elevators and release as they go up. Can anyone tell me why this is, how it is done and what the benefit is? Thanks for your help.
Those are constant force springs, which many teams use on their elevators to reduce the load the lift motors have to move. This reduces the amount of current drawn and eases the process of motion profiling since the lift motors can maintain the elevator carriage position more easily.
This is often done to “pull up” on the elevator which means there is less downward force (due to gravity) when trying to hold the elevator at a position. This means the motors have to draw less current which can prevent brownouts and motors stalling out.
An additional benefit can be achieved with serpentine elevators when the spring is used between the carriage and the intermediate stage. It will help make sure the carriage moves first. The intermediate stage won’t move until the carriage reaches the top of the intermediate stage.
Is a serpentine elevator the same as a continuous elevator?
Based on the context, I believe you are correct.
Having the springs between the carriage and the intermediate stage is also advantageous in a cascade elevator as this gives the springs a 2:1 mechanical advantage relative to the motor, so you get double the lift force for free.
This isn’t quite the case. The spring actually will have a 1:2 mechanical advantage and you’ll get half the lift force.
As a thought exercise, say your spring is able to lift the load on its own from the bottom to fully extended. Just to make it easy let’s say each stage moves 4 feet.
Then for 4 feet of spring retraction, your final load on the carriage will have moved 8 feet. You don’t magically get double the distance and double the force. You get half the force, and double the distance of motion. If your carriage weighs 20 lbs, it would require a 40 lbs constant force spring to achieve perfect counterbalance.
Drawing free body diagrams of all of the stages would also illustrate this same effect, but is a bit more complex to show through words.
My team used one this year. We were originally considering it for the reasons given by Ben_the_Builder and Shelby_Lamp, but it became a necessity due to an error in our elevator gearbox design.
We did a cascading elevator, and accidentally designed the gearbox based on the speed of one stage, when really the speed was twice that because both stages move at once. So the gearbox as built baaaarely had enough torque to lift the carriage, and was sketchy enough that we were worried about burning out a motor. Instead of redoing the gearbox we decided to follow through on the constant force spring, which made the motors able to comfortably lift the carriage.
Motor torque has two jobs to do when powering a mechanism:
- Overcome the static load (weight), and
- Overcome the dynamic load (inertia).
Constant force springs deal with part of the weight, leaving more of your motor’s torque available to handle the inertia. That makes your mechanism quicker; i.e., increases available acceleration. Quickness = faster scoring cycles = more points.
Another benefit of letting springs handle the static load is less heating.
Of course, you can also address both quickness and heating by using more or larger motors – but that adds weight.
Hmm. This is a fun one to think about.
When I get a thought experiment like this, I always move back to energy equations to double check. Potential energy and length of motion is much more straight forward to think about.
To break it down a step further, removing weight from the equation also simplifies your controls. Without constant force springs, moving the elevator up loads the motor/gearbox differently than moving the elevator down. Gravity works with the motor/gearbox going down and against it moving up. With constant force springs that mostly counterbalance the carriage weight, both up and down can be treated the same. While not a huge deal either way, every little bit helps.
The way we have our springs mounted, the carriage travels four feet relative to the springs for four feet of spring extension. If you draw a FBD of just the carriage (assuming it’s only powered down) there’s only 3 forces, gravity pulling the carriage down, spring force pulling the carriage up, and tension from the rope line.
However, adding a down-pulling rope does complicate things. I might try to draw out this FBD.
Yeah, make sure you keep this in mind when calculating spring force.
On 930 we had 68 lbs of upward spring force to neutralize our elevator.
Neutralizing your elevator has a lot of advantages as the motors are effectively only lifting the game piece. This puts a lot less stress on your elevator motors and allows you to gear your elevator super fast. I think with all of our weight from our manipulators we could lift our elevator from bottom to top in 0.8 sec, maybe even less.
That is really smart! Do you mind explaining to me kind of how you came to that point, maybe what equations you used to figure out the spring tension, stringing, and weight? Thank you!
Time to re-think that one!
The reduction required for each stage is simply the stage number.
What you may be thinking is:
What happens is this:
If you make the simplifying assumption that each stage weighs the same, then the force required to lift N stages (ignoring the load) is N(N+1)/2 times the force required to lift one stage; this should be the design force for your spring. The force required to lift the load (ignoring the elevator itself) is simply N times what it would have been for a single stage elevator; this (after padding for friction and uncertainties and safety) should be the design force for your motor.
Honestly, the way we came to 68lbs of spring force was through trial and error. We started out with 2x 18lbs springs at the base of the of the elevator and when that wasn’t enough, we switched it to 2x 25lbs springs. When that still wasn’t enough we added another 18lb spring on the carriage and that did the trick.
The right way to find out how much spring force we needed would probably have been to use the equations in this thread, but with all the constant changes to our manipulators it was often hard to determine how much everything weighed on the elevator.
In most FRC mechanisms that fight against gravity, it is often advantageous to use elastic potential energy to achieve “neutral buoyancy” – whether it’s an elevator or an arm, the ideal situation is that it takes virtually zero force to lift the mechanism to any given spot, and once you remove the force, it stays there on its own.
This lets you do things faster, with less gear reduction (and thus weight), draw less current and so tax your batteries less, and/or use smaller pneumatic cylinders (saving weight and taxing both your compressor and your battery less), and control things easier as well.
Perfect neutral buoyancy is often hard to achieve, but the closer you can get to it with any given mechanism, the better that mechanism is going to perform.
In addition to constant force springs, torsional springs, surgical tubing, and plain-old extension springs are also popular choices to achieve neutral buoyancy or mechanical assist. Constant force springs are probably the most popular because, well, they output a(n approximately) constant rather than a proportional force.
As usual, Gus is correct. After making a (rather complicated) free body diagram and a quick solidworks sketch, I came to the conclusion that my understanding and numbers were off. It is, in fact, a linear relationship rather than an exponential one. Thank you for pointing this out!
I’ve deleted my previous post to prevent anyone from using my misinformation.