pic: Magnetic Grappling Hook

[Chris Hapstack]

Originally Posted by Boltman

The acceleration applied to get magnet to engage on bar is relatively slow. So to properly position (big one) , then engage, then collapse then winch seems like that would be troublesome to say the least in 20 seconds. Don't see how the magnet helps much.

I took two separate videos to showcase the two actions of the grappling hook (firing and latching), but in reality they would all happen in one fluid motion. We would pull up on the batter, fire the grappling hook, and then immediately start winching in the attached rope. Pulling on the rope is what actually causes the hooks to latch closed, so the time between firing the hook and the robot starting to lift off the ground is controlled by the slack in the rope. Even if we wanted to be conservative and leave a couple feet worth of slack, the robot would start rising a couple seconds after firing the hook. If you’ve seen 118’s or 842’s climbers, we’d be shooting for a similar motion. It shouldn’t take appreciably longer than theirs.

The magnets were an attempt to help the hook align itself if the firing position was slightly off. They seem to have a small effect in testing so far, but it’s not significant enough that we’re stopping here.

Here’s a different view of the grappling hook firing upwards. This test was without the polycarb hooks attached, so it was lighter and thus went higher. Even with the hooks attached, though, the assembly went from release height to bar height in under a second. https://www.youtube.com/watch?v=FsrqKvVxamk

When it comes to the actual acceleration of the grappling hook, we didn’t calculate it, because knowing it is unnecessary. What we DID calculate was the spring size and displacement needed to launch the hook to the height of the bar (done by setting the potential energy of a compressed spring equal to the gravitational potential energy of the hooks at the height of the bar). This allowed us to have “getting the hooks to stop around 6ft in the air” as our goal, and we could size our spring to meet that goal. Because the hooks are thus reaching the top of their projectile path right at the height of the bar, you can assume it takes as much time to reach the bar as it does to fall from the bar to the height of the launcher. Using the equation t=sqrt((2*s)/g) where t is time in seconds, s is height above the ground in feet, and g is the acceleration due to gravity (32 ft/s^2), you get 0.61 seconds for the hooks to travel upwards (if the bar is 6 feet above the robot).