Have any of you lot run into an issue when you want to control the speed of something rotating with a percent slider? All seems fine until you want to slow it down to a stop gradually. You'll notice that the rotation goes nuts, backwards, all kinds of speeds, before finally stopping. I've made an Xpresso rig to do just that!
It's not a recommended method of controlling speed anyway, and until I saw a few other attempts I was still using 0-100% sliders. That would only work if the percentages were remapped to a real value. Even if you've got further than that in your xpresso, you might have noticed that the rigged object jumps every time you click on something - in other words, every time the viewport redraws and the xpresso loop reevaluates. Otherwise, sometimes you'll have set keyframes and noticed that the rotation doesn't exactly repeat every time, making your keys more or less worthless.
This is exactly the same issue as with using an Expression Slider in After Effects as a speed multiplier - the per-frame evaluation just makes the thing almost useless. Whereas Dan Ebberts' website will steer you in the right direction using AE expressions, we have to ram our heads together until something oozes out. I've been doing rigging for Dewalt power tools for a couple years now and thankfully I've never had to show the thing slowing down, and to be honest, never really thought it was an issue. Until the last one, a power strimmer (weed whacker). The weed whacker had to be shown changing speeds, but the bloody thing was going backwards and speeding up before coming to a rest. After some avid forum trawling I found this xpresso tag by dgbatten of c4dcafe which almost worked. It jumped around when the play head was stopped. So I took the pliers to it, and it's so useful that it's worth sharing.
That said, if you're Tim Clapham and you know that I'm just being ignorant of a fundamental element of trig or 3D, please do tell me. All this fanfare over a spinning null must piss the real pros off, but I have to put something on the website, don't I? If anyone is wiser than I, please do get in touch. I'd love to know that adding a Quaternion Tag could have saved me years of bother.
Download Xpresso Motor (Made in r15, but it should work r12-16)