Monday, August 9, 2010

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


  1. Dude, you'd be better off putting the "hoop" on to control the wind resistance (which will not only drag, but introduce substantial warping of the wings). You can also much more easily balance the frame, mount your electronics, and lower the death toll if it breaks up.

    Don't forget to make use of the good aspects of the air speed over the frame. You can use it to fine tune support and balance just off the top of my head.

  2. That's a really valuable feedback. Indeed, I was expecting to still have to tackle air & balance problems.
    Since I'm not a mechanical eng. - could you point me to some information sources on balancing?

  3. Why don't you just use 100W light bulbs on that thing?

  4. Sssshhhhh... You are ruining the surprise :-)