Robot Reviews

A closer look at shape memory alloys and how they can impact the future of robot motion mechanisms. Are we getting close to stuffing robots with metal tissue?

We've already featured two robots that make use of shape memory alloys (SMAs): the robot bat and sunflower robot. But just what are shape memory alloys? Sit back as we digress from discussing the robots themselves to focus on one of the technologies that are revolutionizing the designs for their body structures and the way they move.

Essentially, SMAs (a.k.a. smart metals, memory alloys, or muscle wires) are alloys that can return to their original shapes after being deformed once heat is applied; hence the term 'shape memory'. Hit that video file to see what we mean.

SMAs have been around for quite some time now. In fact, their origin can be traced all the way back to the 1930's. Today's shape memory alloys are known by 3 main types: copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium (NiTi). You've probably heard of Nitinol, which stands for Nickel Titanium Naval Ordnance Laboratory. It is a (relatively) popular trade name of the 3rd alloy mentioned, NiTi.

Aside from it's most recent use in robotics, older applications range from the field of medicine, sports, cell phone technology, fashion, and piping, where they are highly favored due to their extreme lightweight, flexible and durable qualities. 

Here are a few specific examples.

* Shape memory coupling, a system for connecting oil line pipes. You can easily find them right beneath your local gasoline pumping stations.

* eyeglass frames 

* dental braces and root canal files 

* golf club inserts

* mechanical watch springs

* retractable antennae in earlier cell phones. 

* wires in underwire bras

If you use SMAs in lieu of hydraulic, pneumatic, or motor-based systems, you can produce much lighter and flexible structures. This is the reason why robotics engineers are training their sights on these materials. 

By applying heat via electric currents, a wire made of SMA can contract or change its length. What's more, from your high-school physics, you know that by changing the length of a wire, you are also changing its electric resistance. Once measured, the wire can serve as a source of sensory input. This dual benefit, coupled with the qualities mentioned earlier (lightweight, flexible, and durable), make a bundle of such wires the perfect candidate for metal replacements of skeletal and muscular systems.

The video below shows a robotic hand whose mechanism is a result of hot and cold water pumped over SMA-made wires.

Note: Researches are also being undertaken on special alloys, known as ferromagnetic shape memory alloys (FSMA) or Magnetic Shape Memory (MSM) alloys, that react to magnetic fields. Transformation in such alloys are much faster since exposure of the material to the magnetic fields can be instantaneous, whereas those dependent on heat, due to the gradual nature of heat transfer, are relatively slow.