A few years back I learned about an actuator that is surprisingly simple, has a high gear ratio, was known since ancient times, and can be produced with nothing more than a cheap hobby motor and some string. Crazy, right?! Up until recently I was barred from discussing the actuator due to NDAs with an anonymous startup, despite plenty of academic work on the subject. Thankfully those NDAs expired, and I've received permission to discuss some of that company's findings publicly. Read on for details!

Twisted String Actuator Concept

The concept is surprisingly simple: hook up two wires to a motor on one side and a load on the other; twist the wires, which causes their effective length to shrink and linearly pull the load:

The actuator is so simple; it appears in several kids' project books (eg. "Ink Sandwiches, Electric Worms, and 37 Other Experiments for Saturday Science" and "Vacuum Bazookas, Electric Rainbow Jelly"), where they're used to build rudimentary robots. This is Arduino territory right here:

The worm comprises a set of identical pad-motor units that slide on the ground and are linked by arched wires (springs) that push the units apart and by twisted-string actuators that pull the units together.

And the basic concept has been known since ancient times:

Twisted String Actuators in Academia:

The actuator's simplicity masks some interesting physics. Hod Lipson (etal.) wrote one of the better papers, entitled "Mini Twist: A Study of Long-Range Linear Drive by String Twisting" (PDF). This paper looks at theoretical models as well as empirical data regarding displacement, tension, force, etc for a variety of string configurations (number strands, materials, etc):

Meanwhile, Dexmart (various sources: 1, 2, 3, 4, 5) is probably the most-experienced of the academic labs at building and employing the actuators in real robotic systems. For example, this small actuator is able to lift 7 kilograms, despite being powered by a 12.4mm diameter motor (aka, a pager motor).

Dexmart has also used these actuators to build tendon-driven robot hands that are somewhat anthropomorphic.

A few other, relatively-obscure papers, like "Twisted Strings-based Elbow Exoskeleton" out of Korea, also discuss the practical considerations for mounting and affixing the strings to pulleys and cams:

Another fun fact: Jorge Cham (of PhD Comics fame) worked on some twisted string actuator fingers at Stanford in the late 1990's:

Twisted String Actuators: Caveats

So why aren't twisted string actuators taking over the world? The startup I mentioned, which asked to remain anonymous, had this to say (paraphrased):

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!

Indeed, I believe toys are probably the sweet spot for these actuators -- whether eldercare robots like Paro (below left), a toy teddy bear (like the one from the movie AI), or even an inflatable toy Baymax from Disney's Big Hero 6 (this was the idea I alluded to earlier when chatting about inflatable robots).

So the final takeaway: These actuators are pretty interesting, especially for a certain class of applications. Definitely worth adding to your bag of tricks!

A Different type of "Twisted String" Thermal Actuator

In 2014, researchers discovered another type of "twisted string" actuator.... but this one was much different than the one I've discussed here. It's made of nylon string (fishing line or sewing thread), and is heat-activated. In many respects, it's like more-traditional shape memory alloys -- including the same drawbacks (thermal actuation, cycle time, etc). But it is exceedingly interesting!

It was published in Nature: "Artificial Muscles from Fishing Line and Sewing Thread." Actually, there's a RobotsPodcast about this technology too. To quote from some of the press coverage: