Nitinol as a sensing and actuating material

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Fred Henny - 1269925


Description of Nitinol as a sensing and actuating material


Shape memory alloys such as Nitinol (a NiTi alloy) are alloys that act differently depending on the crystallization structure of the material which can be either in the Martensite state or in the Austenite state. In short this allows the material to act in different ways depending on temperature. This allows for use of the material in special application for which normally, complex sensing and actuation systems would be required. Nitinol allows for the construction of relatively simple structures, performing functions that cannot be achieved with other materials. Its unusual characteristics seeming almost unreal and magical to people unfamiliar with the material, making it a material which can be very inspiring and useful within a wide range of applications.

Nitinol Austenite and martensite.jpg
Source: http://upload.wikimedia.org/wikipedia/en/e/eb/Nitinol_Austenite_and_martensite.jpg


Working Principle


Nitinol, an alloy of Nickel and Titanium, is available in many different compositions, each of them having different characteristics. However, the basic working principle of the memory-effect remains the same in all.
When nitinol is above a certain temperature (depending on its exact composition) it will be in the austenite state, which is often referred to as the parent state. In this state, the material has a cubical crystallization structure. At lower temperatures (also depending on the composition) the material enters the martensite state, which is a more complex ‘monoclinic’ state.



Actually there are four transition temperatures:


• Ms: When fully austenite, the material starts to form martensite crystallization (upon cooling)
• Mf: At this temperature the material will be fully in martensite state
• As: At this point, a fully martensite material will start to form austenite (upon heating)
• Af: At this point the heated material will be fully austenite.

Figure 1 constantstress.jpg

Source: http://jmmedical.com/resources/211/Measuring-Transformation-Temperatures-in-Nitinol-Alloys.html


The special thing is that the states of the material are reversible, which is crucial for explaining and understanding its memory effect. When the material is shaped (at temperatures above Af) it has a certain crystallization structure that defines its shape. Upon cooling and thus state transition, the material can be deformed and shaped in any way preferred, without ´wanting´ to go back to its to its original shape. However, upon heating (from As to Af) the material will regain its original austenite structure and thus regains the shape it had before turning martensite.


Applications


Medical / Orthodontic applications


Currently, Nitinol is most known for its application within the orthodontic and medical fields. For instance, nitinol wire is utilized as a means of applying a certain pressure within an orthodontic brace. The memory capabilities of the metal allow for the application of a constant pressure in a ‘programmable’ shape.
Also, the material is used in stents that can be implanted into arteries in order to strengthen them. The biocompatibility of the material allows it to be surgically implanted without danger to the patient.

Nitinol stent.png
Source: http://www.phiengineering.com/nitinol.htm


Heat to mechanical energy conversion


Heat engines are one of the most promising, yet not yet industrialized possibilities for Nitinol or any other Shape Memory material. Efficient engines were already designed and built in the early 1970’s1. By designing mechanical systems that transfer the energy released by the nitinol upon repetitive cooling and heating cycles into a mechanical energy such as rotational- or linear motion, it is possible to create a heat-engine. This can be used to directly power mechanical systems or to produce electricity. However, the limitations of the material in terms of shaping and production seem to make these kind of systems unreliable and hard to build and maintain.

HeatEngine.jpg

Nitinol heat-engine driven by cold and hot water (Source: http://www.unitednuclear.com/index.php?main_page=product_info&cPath=28_42&products_id=711)


Active vibration control


By employing both the capability of the material to act as a sensor device for changes in temperature or strain and the actuation capabilities it also has, it is possible to construct systems that are able to actively measure and subsequently react to changes in dynamical systems such as flexible cantilever beams2.


Active acoustics enhancement


Research has also been done in the field of (re)active acoustics enhancement by using a similar setup as described above. By measuring sound pressure using nitinol (by using the heat converted into the nitinol sensors) one can subsequently use the nitinol as an actuator to react to the mearured sound, leading to actively controlled sound radiation systems.


HCI


Because Nitinol reacts on temperature change (which can also be induced by electricity) it can be used as both an input device (sensor) and as an actuator, even within the same application.
This is very interesting for domains such as biomedical- and biomechanical engineering but also for Human Computer Interaction.
The material can be applied to provide tactile user feedback but also to sense user input by reacting on temperature/strain differences.


Ubiquitous potential


The ubiquitous potential for materials such as nitinol are very exciting and might lead to exciting new control systems and actuation and sensing equipment. By what has been described above, one can think of the endless possibilities. Sensing of environmental conditions and subsequent activation and active control of other systems using the direct input can lead to innovative and semi-intelligent systems such as automatically closing venetian blinds. While the way of controlling such a system is far different from the usual approach using electronic systems, it allows for the conception of for example bio-inspired design solutions that are as elegant as they are efficient.

Robolobsters.png

Robot driven by muscle-wire that drives the legs. (Source: http://skitterbot.com/blog/category/biomimicry/)



Recent Developments


Currently, shape memory plastics and shape memory textiles are under development. These act similar to the material described in this wiki, but further extend the possibilities for application, such as adaptive clothing depending on temperature changes.
The most prominent and possibly most promising developments for the material can be found in the medical domain, where nitinol is used for the development of delicate instruments for use in lapararoscopic surgery and stents. In this field it allows for instance for miniaturization of equipment that could otherwise only be used in open-surgery, promising less invasive surgery and less risks.

Nitinol basketretr all72dpi 2067 large.jpg

Source: http://www.endoscopy.com/store/pc/viewCategories.asp?idCategory=246


Also, some artists are experimenting with the material, creating beautiful structures showing the potential of the material.
Hylozoic001.jpg

Source: http://www.philipbeesleyarchitect.com/sculptures/0635_HyloSoil_Large/hylozoic.html


References & Further reading


1.http://www.winstonbrill.com/bril001/html/article_index/articles/1-50/article12_body.html
2.http://www.sciencedirect.com/science/article/pii/0022460X9090760W
3.http://spiedigitallibrary.org/proceedings/resource/2/psisdg/2715/1/220_1?isAuthorized=no
4.http://www.youtube.com/watch?v=fmaYNfrZPhI
5.http://www.youtube.com/watch?v=ZoAFc_DeWVQ
6.http://fkeel.blogspot.com/2011/02/more-experiments-with-nitinol-paper.html
7.http://www.sciencedirect.com/science/article/pii/0022460X9290565F
8.http://www.google.nl/patents?hl=nl&lr=&vid=USPAT3516082&id=n9hVAAAAEBAJ&oi=fnd&dq=nitinol+sensor&9.printsec=abstract#v=onepage&q=nitinol%20sensor&f=false
10.http://softcircuits2011.wordpress.com/2011/04/11/working-on-microphone-breath-sensor-and-nitinol-wire/
11.http://www.colorsproject.com/nitinol_articles_science_digest_1982_sanders.pdf
12.http://www.ccmr.cornell.edu/education/modules/documents/Nitinol.pdf



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