Twisted string actuators...

Twisted string actuators...  I had this idea about 15 years ago, after using an old-fashioned twisted wire-and-stick to straighten up a fence gate.  I did some investigation of it, never realizing that anyone else had done so already.  I came to the same conclusion others did: that the problems outweighed the benefits:

We spent a fair bit of time evaluating twisted string actuators to build low-cost robots. Unfortunately, they have some major problems that are tough to overcome for our specific application(s). For one, their lifecycle is measured in the many thousands instead of millions -- despite our best efforts at material engineering. For another, they are highly nonlinear in response (twists vs displacement), which requires very precise calibration for precision movements. Plus, they're inherently linear motors; for rotary motion, you need two opposing motors or spring-return mechanisms. That said.... there are probably a lot of applications that could benefit from this type of actuator!

It's a pity, because these actuators are ridiculously easy to build.  The non-linear part was especially frustrating to me.

Analyzing these things was surprisingly easy – it turns out that it breaks down into a simple trigonometry problem, with the key variable being the diameter of the string or wire.  I had the most success with graphite-lubricated mono-filament fishing line, and silicone grease lubricated mono-filament line came close...