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It seems to me that worm gears and other ready-made screws wouldn't work, because you need a left-hand and a right-hand thread on the two sides...?

A while ago I tried to do something along the same lines with mechanical automation. It drives forward until it hits an obstacle, then reverses and turns for a set distance, then goes forward again...

Wow, that's a very intriguing idea! Did you try it on carpet? Maybe a screw made of lots of these might work?

Thanks all for the kind remarks! @2LegoOrNot2Lego...: Yes, by far the best way to understand an add/sub mechanism is to build one! I would start with the basic 2-differential version, for which there are many designs avaiable, e.g. here. Basically, it lets you control a tank-style vehicle using one motor for forward/reverse and one for turning. Here I used a more complicated variant to handle 4 wheels and 3 motions. I used something somewhat similar for a Killough platform here. @Zblj: needless to say, none of this would be possible without your wheels! I must indeed have got the black 12t bevel gears from Gigamesh - I didn't plan this, but was happy to find I had them! The black 20t double bevels are more common, of course. @rien: yes, below is the joystick (probably this is enough to understand how it's made). And I'm working on instructions for the whole thing... There are several other joystick designs around, e.g.: this thread And here are some more pics.

Here is something I've been wanting to make for a long time, finally made possible by Zblj's amazing design for Mecanum wheels. Like Zblj's and esrtiece's impressive vehicles, it can drive and turn in any direction. What makes this one different is that it is controlled by a system of differentials (similar to an add/subtract mechanism but a little more complicated). This means it can be controlled by just 3 motors: one for forward/reverse, one for sideways, one for turning. With a lot of effort I was able to get the mechanism quite compact - it uses only 4 differentials. I usually settle for pure functionality, but this time I made some attempt at styling too . Enjoy!