WARNING: LARGE IMAGES
Due to popular demand, I thought I’d explain a little about uniformly tensioned lifts and why they are useful. I’ve explained the uniformly tensioned rubber band system in my robot reveal, but as the pictures are not working and editing does not work, I thought I’d go more in-depth.
As you all know, adding a rubber band tensioning system to a lift is very important to increase the amount of game objects the lift can carry, to reduce strain on the motors, and even to change the lift’s gear ratio to a faster one.
This system is great due to complexity vs. effectiveness. Just by putting a couple of pieces of metal (in this case, standoffs and screws) and rubber bands, the stress on the motors is greatly reduced.
However, some of you may have come across some flaws with this sort of tensioning system. The rubber bands have a greater upward force when the lift is at the bottom, while the upward force starts to diminish while the lift rises. What this means is that as you lift upwards, the assistive force of the rubber bands start to disappear, as the motors begin to have to do more work. The lift then reaches a point where the rubber bands are completely slack and has no assistive force on the lift.
Obviously, the numerical values are not accurate as the rubber bands won’t be producing a 100N of force on the lift, but the concept is the same. The Graph also may not even be linear, but the point is that the force diminishes as the lift angle increases.
In addition to the fact that the rubber bands do not work effectively when the lift is raised, high upwards force near the bottom of the lift is also not ideal. With no game pieces in your robot, the lift may start to rise on its own because of the strong upwards force of the rubber bands. One may combat this problem by taking off rubber bands, further reducing the effectiveness of the rubber bands. Others may put a small constant motor value in the code ( while (lift is down) motor value = -10; ) but this is still not ideal.
My discovery of the uniformly tensioned rubber band system (UTRB for the sake of the phrase’s length) made its way during the 2012 gateway season. While creating a pneumatic lift, I discovered that a UTRB was absolutely necessary due to the fact that pneumatic pistons are quite weak, but also have a constant force along its stroke (save for minimal amount of loss force due to the fact that air gets released every time you activate one). After the robots completion, the robot was able to lift 6 game pieces (3 pounds) to 30” and still be able to bring the lift down with 0 pieces (no weight) all while having a pneumatic lift. To put this in perspective, the pistons could not lift the manipulator up at all without any type of rubber band system. This should demonstrate the effectiveness of the UTRB.
How it works:
This means that the force acting on the lift by the rubber bands is very small.
R (crossproduct) F = Torque
R sin(theta) F = Torque.
When theta approaches 180, sin(theta) approaches 0.