< > [Motion](../../motion) > Thermal Actuators
## Thermal Actuators
#### High stress, low efficiency, low bandwidth
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### Shape Memory Actuators
Shape memory actuators come in two primary flavors: shape memory alloys, and shape memory polymers:
#### Shape Memory Alloy
Nitinol is one of the most common shape memory alloys. As its name suggests, it's an alloy of Nickel and Titanium. It exhibits a relatively unusual property in which, once deformed at one temperature, can revert to its previous undeformed shape once heated above its transformation temperature.
> #### Fun physics aside...
> What's behind this is a mechanism called twinning. Essentially, some deformations of the material are possible without breaking atomic bonds because atomic planes are allowed to slip past each-other.
><img src="nitinol_lattice.jpg" width=500>
>The same is true in reverse. By stretching a nitinol wire and looking at it with a thermal camera, we can see that local spots are undergoing this transformation and releasing heat in doing so:
><img src="nitinol_stretch.gif">
>Then, we we quickly move our test fixture back to the original position, it immediately cools down, bowing initially and then reverting to its undeformed state.
><img src="nitinol_release.gif">
#### Okay, back to what you can do with shape memory actuators...
<a href="http://news.mit.edu/2010/actuators-slideshow-0406"><img src="sma_actuator.jpg" width="500"></a>
<a href="http://iopscience.iop.org/article/10.1088/0964-1726/24/9/095014/meta"><img src="sma_coil.jpg" width="500"></a>
#### Shape memory polymers
e.g. "pre-stretched polystyrene":
- https://spectrum.ieee.org/automaton/robotics/robotics-hardware/
self-folding robots:
- https://www.youtube.com/watch?v=X1Wf8xm8TLw
#### Twisted Coil Polymer Actuator
(Also known as "fishing line actuators")
- http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7523604
- http://iopscience.iop.org/article/10.1088/1361-665X/aa84e4/meta
<iframe width="560" height="315" src="https://www.youtube.com/embed/Tba8Nf02OSI" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
#### Phase-Change Actuators
Phase change actuators exploit the difference in volume caused by a phase-change of a material. A very common version of this actuator uses the phase-change behaviour of wax.
Essentially, the actuator simply consists of an enclosed volume of wax, a plunger or piston, and a heat source. When the wax is heated, it liquifies and expands in volume considerably (5% - 20%). This change in volume is then channeled into a useful actuation of the plunger.
These actuators are extremely powerful since their motion is essentially hydraulic. However, this force comes at the cost of speed: because the actuation is dependent on the phase-change of a material, its motion is quite slow. They also require a bias spring to return the piston to its contracted position.
These are often used in cases where reliability is the primary concern. Those applications include things like aerospace controls to control hydraulic systems but are also used in dishwashers and laundry machines to open latches and release detergent.
This is what the wax motor that opens the latch in your dishwasher looks like:
<img src="wax_motor_dishwasher.jpg" width=300>
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