The Inception Drive, just invented by SRI International, is an extremely compact, infinitely variable transmission developed for robotics, but which could have all sorts of other applications. You have to watch the video and think about it for a while to see what's going on and even then, you'll have to think a little more to picture it in your mind. When you watch the video, keep your eyes on the shafts in the center to see the output slow and change direction.
The point to remember is that it has no clutch and the input, an electric motor in the case of robotics, is always turning at a constant speed in one direction, but by varying the size of the pulleys, you get not only a variable transmission in one direction, but it can come to a complete stop and then reverse direction of the output without any change in speed or direction of the input! It's called an IVT or infinitely variable transmission instead of a CVT because it has a zero point, it can stop the output entirely within the transmission, no clutch necessary and then smoothly go in the other direction.
There are a few very important things going on here that are very hard to see, because they’re happening inside the drive itself. The first thing is how the two pulleys interact with each other. This happens through the V-belt, of course, but it’s not as though one pulley turns, which turns the belt, which causes the other pulley to turn, as in a conventional transmission. Rather, the outer pulley is fixed and doesn’t turn at all, while the inner pulley, which is nested inside the drive, has a wobble in it, since it’s mounted off-center to the input shaft.
It's still brand new so there are a lot of things to work out, but It's amazing to see what they've done here, the size and weight savings alone will get your attention, but I can't help but keep going back to how it actually works. I love it!