Post-005
Jan. 15th, 2022

Pixar Lamp video part 2: building the rail system.

The units of specific density is waters per water.

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So last time we said we would re-build the linear rail system — while also using paint to match the green screen but we came up with a better idea in the shower this morning — aircraft cable! If we anchor aircraft cable to the ground and the ceiling we could use a ratchet to tension the aircraft cable and that could work as a super thin linear rail. Then we could also suspend the bungees from the ceiling and get the lamp to jump that way, and the whole system would be adjustable — so that’s what we are going to do. Time for a trip to Home Depot (one of my favorite stores). 

Let’s just sketch this out real quick. In the sketch below we see the anchors the aircraft cable hooks into on the ground. At the top the aircraft cable hooks onto a ratchet strap that connects to the anchors on the ceiling that allows us to tension the cables. On the lamp itself we have these cable guides that screw into the lamp but slide along the tensioned aircraft cable. The bungee hooks directly from the ceiling to the top of the lamp.

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Now for a small physics lesson. In 2.74 Fischer and I tried many many things to get this system to jump up and down and we finally came up with the following control strategy. The lamp is a little too heavy to jump on its own so the bungee chord provides assistance. When we hook the lamp to the bungee the bungee moves from the un-stretched length to a loaded length like if we hang a mass from a spring. We will call this the neutral position of the bungee.

If we allow the foot of the robot to touch the floor in this position we can use the motors to move up and down close to the resonant frequency of the bungee/lamp system and therefor pump energy into the system exciting the bungee making the lamp jump up and down. I’ll make a post later detailing how we make the lamp do this, but for now that’s the general concept. The lamp pumps energy into the spring-mass system at the right frequency to make it oscillate up and down - this is appropriately named the energy pumping control strategy. 

Ok so here's what we got from Home Depot in a list:

  • 1/16" x 50ft steel cable

  • 1/16" cable crimps and clamp set 

  • husky 300lbs tie-downs

  • concrete screws w/ a concerete drill-bit

  • a single ratchet tie-down

The Home Depot guy actually gave us a great tip, for the cable crimps we of course want to use those to make a loop in the wire and secure it so we can hook the ends to the tie-downs. He recommended we use a hammer or a vise which is exactly what this online article suggested after googling. Saved us 30 dollars on a swaging tool. 

The above time-lapse is part of the build, it's me installing the concrete anchors into the floor and ceiling of my room. This was actually a lot harder than I thought it was going to be and it took all afternoon. After the process I'd recommend a hammer drill and patience, maybe an ice pack for your arm too. 

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So we attempted this with a hammer and a vise grip. The vise didn't work so well but the hammer did a great job. Now time to secure the crimped end of the cable to our tie downs, and we shall use some carabiners we have lying around on the end that connects to the anchor, we'll loop through the bottom and add the ratchet on the other side. We might cut the ratchet strap so it isn't obnoxiously long.

The only problem is when we cut a ratchet strap the end starts to fray so we need to find a lighter and burn the end and since it's a nylon strap, the plastic will melt together sealing the end up. If you're going to try it at home tho, make sure you know the rope you're burning is plastic based or fire may be a side-effect of your actions. Here's an article on preventing rope ends from fraying. We could also use heat shrink but I have none in my room (need to change that :/). Below are a few pictures of the tensioned "rail" setup. Now it's time to modify the lamp.

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For the lamp, hopefully all we have to do is remove the linear bearings from the back and replace them with the little U-Bolts we got today. We want them to be tight enough where they constrain the lamp but not so tight they don't move up and down on the tensioned cable. 

Our U-bolts are all the wrong size, but for now we're just going to take some zip-ties and use that to secure the lamp to the cable. If that doesn't work we may think of something fancier.

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Ok that's enough adventure for one day. The problems we still have to solve are the bungee chord, the tensioning in the steel cable (if we can go higher), and if we want a better method of securing the lamp to the bungee or if we can make the "rail" we built smoother somehow.

Next time I want to run some tests with dropping the lamp and making it land on the soft floor and seeing how well it absorbs impact and how it compares to the original.

#pixar #concrete_screws #robotics