Hacker News Reader

Comment by sowbug | Hacker News Reader

A couple ideas for improvement. If you have extra identical motors and disassemble two, you can fashion a rotating power transmission system from the brushes in two assemblies. You sacrifice two motors for each unit, but it's a perfect fit and very reliable (with a cap and rectifier), and you don't have to worry about batteries anymore.

The rectifier also provides a signal that the assembly has completed a rotation, so you can maintain image stability based on actual position, rather than guessing how long a cycle is.

Transmitting power via induction might work, but I was never able to deliver it efficiently enough, so to make it work I had to turn up the source voltage so high that I worried about fires.

This advice comes from my 2001 Burning Man art project. A very sad early prototype is pictured here: https://github.com/sowbug/tqw/blob/master/photos/side.jpg. The final installation worked great.

View on HN

That's all true, but I liked his solution of using an IR sensor for the sync. It was so cool how he was able to rotate the display by changing the position of his finger. No guessing involved.

That is a pretty good idea! The brushes will last a long time. Luckily now the rise of wireless phone charging means you can get kits like [1] or [2].

It makes me wonder how different LIDAR vendors manage it.

1: https://www.adafruit.com/product/1407 2: https://www.adafruit.com/product/2162

Another idea: why not put a tiny TFT display on the motor? You can get cheap ones at aliexpress.

Would be cool to see how that would look like. Or even a transparent OLED or such.

Standard displays are 60 Hz. You need a much higher framerate because not only do you want 60 frames a second, but you also want some number of frames per angular rotation. For 1° of angular resolution, you would need: 360°*60 Hz = 21,600 Hz display. Liquid crystals can be modulated at KHz speeds, but you're not going to find associated driving circuitry to do that. It's not easy, and there's no demand for it.

A TI DLP DMD can modulate at those high speeds, and there's readily available driving circuitry for it. However, it's a small reflective based display designed for projectors, and you would then need a light source to reflect off of it.

MicroLEDs would let you increase your pixel pitch with fast modulation frequency, but the display area is still small at the moment because of low yields. You also need a custom chip to drive the microleds at the required high framerate.

You seem to know a lot about this. Have you worked on similar things?

I work on VR/AR headsets where there's a drive for higher refresh rate displays to do things like multiple focal planes with the extra frames.

What do you think is on the horizon of cool or interesting within your work?

The big push is currently towards microLED displays because of the high brightness and fast modulation speed. Yield is not yet good, and there's difficulty in growing all colors on the same substrate. Picking and placing different color LEDs onto the same panel is not cost effective. Red microLEDs are also not yet as bright as blue and green.

There was a push towards micro liquid crystal panels with laser illumination to create holographic images with depth. There are several startups still pursuing that, but the image quality isn't very good at the moment.

The latest advancement has been in the optics with moving to pancake lenses to reduce the length of the optical path from the display to the eye. The Meta Quest 3 has a smaller form factor than previous generations because of this.

I saw a cool design where they stacked transparent displays

If anyone is interested in that then search for Sean Hodgins on YouTube, he made one like this.

This is interesting info. I guess you can get higher angular resolution if you are able to turn off the screen fast enough, and turn it on at the correct angle. Of course, you won't be able to light all voxels during the same revolution, but perhaps that is not a problem.

What about keeping the motors intact and using the second as a generator?

I might be misunderstanding the idea, but I don't think it addresses the original problem that wires can't connect the two parts of something that rotates in a single direction.

I think what he means is having the second motor act as a generator driven by the first motor (maybe with some geared arrangement to have the generator shaft rotate at a different rate from the body), so that the resulting power can be used for the rotating electronics.

Sorry if I'm being dense, but the second motor (the generator) must stay stationary relative to the first motor to do the generating. Which means that the generator is on the same "plane" as the power source. The problem all the solutions are trying to solve is how to transmit power from one plane to another rotating plane, because physical wires can't do it without getting twisted up. So if the generator is on the same plane as the power source, then it can't contribute to solving the problem.

It shouldn't have to stay stationary I think? As long as the shaft rotates at a different rate to the body, there should be some power generated. That's why I mentioned the need for gears.

Overall though it's certainly not going to be good enough to justify the complexity compared to either slip rings or wireless transmission.

I think I get it now. Maybe the main rotor rotates at 6,000 rpm, the geared assembly has a 10-1 reduction so it rotates at 600 rpm (giving the LEDs 10 fps), and the 5,400 net rpm difference is used to generate power. Yes, maybe that would work.

If the drive motor was asynchronous you could use the inductive windings on the rotor to power your rotating electronics, but the phase difference may get pretty extreme

He has actual positioning built in with the ir transceiver.

Reply on news.ycombinator.com