19 Februari 2013

POV Globe - Part II - Mechanics

This is in continuation to: POV Globe - Part I - Introduction

Initially, I completely underestimated the complexity of the mechanics. "Yeah, let's take a 60cm hoop with LEDs and turn it at 50 RPM..." Then, one night I did the simple kinematic calculations, only to find that this means about 3000G(!). Yes, that's right: 3000 times gravity, or simply put, each gram of mass (on the widest part of the hoop) is going to pull the hoop outwards with a force of 3 kilograms. Now that takes a rather rigid structure to support. I started thinking bicycle wheels and the like, but I reckoned that building a machine that is beautiful enough to justify potential killing of a few people is rather challenging. So I started re-thinking the structure, trying to find a way to make is both light and suitable for the kinds of forces that operate here. I also figured that drag is going to be rather substantial, so I'll need a rather sturdy motor to support that.

Eventually, I came up with this structure:

There isn't really any hoop - just "wings" with lengths and spaces chosen carefully that comprise a frame for mounting boards with white LEDs on one side (16 boards, 16 LEDs each) and a single board with the RGB LEDs on the other. Those wings will be made of a strong and light material, preferably transparent, probably poly-carbonate or similar. This part has not yet been built (waiting for a friend with a laser cutter to rescue me).

Turning a shaft fast enough with the ability to resist friction and vibrations is also not a simple matter. The solution that has been chosen is to mount the shaft on a washing machine bearing and drive it with a washing machine motor. This part has been really fun, since I have never worked with motors of this size or with AC. The controller has also been taken from the same washing machine (it even included a pot for speed control!), so I was only left with some wiring and mounting. I also needed to build a pulley and belt system to achieve the desired transmission ratio and a case to keep everything in place. My friend Shachar contributed his golden hands for this task.
Here are some photos of the build process:


Currently, it is very noisy. I'm still considering my options: possibly I'll fill the case with some acoustic isolating material.

Here's a video showing the turning mechanism working slowly:

And this video shows some fast spinning and variation of the speed:

Next: POV Globe - Part III - Electronics


View the original article here

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