Introduction: Caress the Gaze: 3D Printed Structures Using SMA Actuators
This intractable is an attempt to provide a framework for design of a dynamic, shape changing object using SMA actuators. The final intention for me was to design an interactive wearable called "Caress of the Gaze" (for the final result see: www.behnazfarahi.com )
Information provided here aims to demonstrate how you can design dynamic 3D printed objects activated using SMA actuators. The application areas can be wearables, buildings, industrial objects and so on. The printer used in the "Caress of the Gaze" was an Objet Connex500 3D Printer. This technology allows the fabrication of composite materials with varying flexibilities and densities, and can combine materials in several ways with different material properties deposited in a single print run.
However, you can also use MakerBots or any 3D printers to do some interesting dynamic prototypes (some filaments such as PLA have interesting elastic properties which make it perfect for designing SMA actuators).
Step 1: Inspiration: Biomimetics
Why does motion design matter?
Humans have always been fascinated with producing artificial motion since ancient times when they developed mechanisms producing mechanical motions such as catapults in ancient Greece or windmills in medieval Europe. Historically, the 17th century marked a significant increase in the phenomenon of human or animal automata, that is, self-operating machines. None of these designs, however, demonstrated the performance and behavior of biological locomotion caused by muscle functions.
Muscle functions have inspired numerous researchers and scientists to explore the potential of developing smart material soft and compliant actuators such as EAPs (Electro-Active Polymers), SMAs (Shape Memory Alloy) and so on. There are a number of types of smart materials However, the three main categories are color changing materials, light emitting materials and moving materials. Having said this, please note that despite similarities in their performances, in the end none of these actuators has the complexity of behavior of actual muscles. You have probably heard of muscle wire or Shape Memory Alloy. This (seemingly magical) alloy metal changes its shape when heated to an activation temperature and returns to its initial stage. When it cools down, it can be remodelled to any particular form. In other words, it is called Shape Memory Alloy because it has a memory of its initial form. By means of special heat treatment, a piece of SMA can be made to ‘remember’ a shape. For example, a length of wire can be made to remember that it should be a letter "A" at temperatures above 70°C. If you bend and deform this wire at normal room temperature, it stays bent. However, if you place it in a glass of water whose temperature is above 70°C, it immediately goes to the letter "A". In this intractable I would first like to demonstrate how you can work with SMA in order to design various motions and then show how 3D Printing can help to integrate motion into designed objects.
By the way, if you are interested in reading more about bio-inspired muscles I would recommend this book: 'Biomimetics: Biologically Inspired Technologies' edited by Yoseph Bar-Cohen. (Here is the book: https://www.crcpress.com/Biomimetics-Biologically-...
Step 2: Fabrication of Actuating Cellular Auxetic Structures Using SMAs and 3D Printing
One way to design compliant actuators is to use cellular structures which can demonstrate interesting mechanical properties such as auxetic behavior and activating them with SMA actuators.
What are auxetic materials? Auxetic materials show an unusual mechanical behavior due to a negative Poisson's ratio. In other words, when stretched these materials become thicker perpendicular to the applied force. This occurs due to their particular internal structure and the way this deforms when the sample is uni-axially loaded. ( Check out this video:
I also found this paper extremely useful on the design of auxetic structures suggesting a systematic approach not only to the design of 2D Auxetic Structure, but also to 3D ones: http://iopscience.iop.org/article/10.1088/0964-172...
But can these structures be activated? How can materials come alive?
Some interesting research at MIT Assembly Lab is exploring the notion of living matter and 4D Printing which is very exciting. Their research on 4D Printing is a new process for printing customizable smart materials.
However, in my research the actuators are assembled in post printing process due to difficulties in embedding SMAs during the process or changing the resin materials for the printer. Here are some of my attempts:
This video shows one of the MIT projects on smart matter:
Step 3: Cellular Structures and Flexible Joint Systems Using Multi-Material 3D Printing
My first step was to try to understand 2D auxetic design, and its fabrication and behavior. As you can see in this diagram the basic 2D structures have periodic boundary conditions:
Through multi material 3D printing using Objet Connex500 3D Printers different material properties are assigned to these parts of the structures in order to study their mechanical behavior. As mentioned above, multi material 3D printing allows the fabrication of composite materials with varying flexibilities, densities, which can combine materials in several ways with different material properties deposited in a single print run. Therefore, the design started by assigning soft material (Tango black+ in Objet Printer) to joints and stiff material (Vero White) to sections in between. The result was an interesting contraction/ expansion behavior.
Please note: there are different techniques for printing in multi-materials, but one straightforward way is to exports various "stl" files to the printer. (Basically if you have two or more layers of objects, export them separately and assemble them for printing, assigning various materials in 3D printer software, such as Objet Studio). In Objet Studio you then have to select and insert them all and click on "assembled". Other ways of printing in multi material is bit map 3D printing in which you send a series of PNG files showing the gradients of the pixels. Monolith seems to be a good software for a voxel-based modeling engine for multi-material 3D printing. (I haven't tried it myself but here you can find it:http://www.monolith.zone/#introduction)
After printing these prototypes I started to study their mechanical behavior and ways of activating them using SMA actuators. I used variety of SMA actuators in terms of their thickness, actuation temperature, spring pitch, coil size and so on, as you can see in this video:
Next I am going to demonstrate the basics of using SMA actuators.
Step 4: Actuation of Cellular Structures Using SMA Actuators
SMA wires are fun to work with but also very tricky. SMAs are quiet and light, and are also called motorless motion actuators. They can make different motions which are organic, gentle and beautiful to watch. David Benjamin, Marcelo Coelho, Philip Beesley, Jei Qi, Rob Ley are among people who have been using these materials for their projects.
I found SMA springs great to work with tensegrity structures. You can see one of my projects which uses these actuators in a dynamic tensegrity structure here: http://www.behnazfarahi.com/204244/913126/gallery...
First you have to know what Shape Memory Alloy is. Choosing SMA as an actuation material requires an understanding of the limitations and rules of working with this material. As mentioned above, it can be deformed and then “remember” its original shape when triggered by a specific activation temperature. SMAs are often manufactured as wires. Since these are small, strong, durable and easy to trigger, they can be used in many products. And since they move silently and organically, they could be a good tool for making a livelike movement. These small diameter wires contract typically 2% to 10% of their length. The diameter of the wire is one of the really important factors to actuating them. Higher diameter wires have more pulling force than lower diameter wires. Also higher diameter wires have longer off times (it takes longer for the wires to return to their original, uncontracted shape). Higher-diameter wires have lower resistance and need more power. Thus, they are more likely to overheat and lose their original contraction abilities. Wires that are 0.006” or smaller can be kept on constantly without fear of overheating. High-temperature (90C) wires have faster off-times than low-temperature (70C) wires.
Where can you order SMAs? You can order them from different manufacturing companies:
1. http://www.kelloggsresearchlabs.com/ Helpfully they list down all the different variations of their product.
2. http://musclewires.com/This is a typical muscle wire company. Check to see if they have student/researcher discounts on their products.
How much current is required to activate them? The most tricky part of working with SMAs is to control the current just enough so you don't overheat the wires. If you overheat them you can even see the smoke coming out and after a few times the SMA stops responding. To understand how much current is required check the flexion technical data sheet: http://www.kelloggsresearchlabs.com/
As with any electrical circuit, one way is to use Ohm's law:
V (voltage) = I (current) x R (resistance).
So let’s imagine you want to use 0.020" diameter wire and you have a 6 Volt power supply but you don't know how long the wire is supposed to be. First from the data sheet you know that the current required is 4000 mA or 4 A. (which is a lot in this case). From the Ohm's law resistance is 6/4 = 1.5 ohms. We know that the wire itself has 0.16 ohm/inch resistance which means that we can activate: 1.5/0.16= 9.37 inch. You can hook it up to your Arduino using a transistor (for instance TIP 120) or relays.
How can you control the speed? This is something interesting which I want to explore further, but technically changing the power before the actuation point, changes the motion speed. In other words, if you activate your wire with 12V the full contraction speed is faster than 6V. You just have to make sure you don't overheat them by this experiment.
Step 5: Three Types of Actuator Design Using SMAs
(Here I want to thank my artist/ mechanical engineer friend Paolo Salvagione for his help in designing various SMA actuators: http://salvagione.com/)
One of the challenges in designing a dynamic system with SMAs or flexinol is that unlike servo motors which you can control their motions and location, you have less control of their motion. Precise SMA position control is actually possible but not so easy.
One of the main drawbacks with SMA wires is that their cooling and heating curvatures are not equal. You basically have to wait for the material to cool down. Based on the following paper, working with SMA actuators there is no standard model which prescribes temperature, load, and material geometry for a desired performance; the convention therefore is to derive an actuator model’s thermo-mechanical properties experimentally, either partially or fully: (https://micro.seas.harvard.edu/papers/SMS10_Paik.p...)
The following paper explains this problem in detail and offers active cooling and pre-stress systems in order to overcome the response time of SMA: http://www.utdallas.edu/~ytt110030/data/SMA.pdf
Based on this, I would suggest that there are three ways you can design a SMA actuator:
1) Using Weight/ Gravity for SMA
You can use gravity by simply adding some weight to your SMA actuator. This is a great way to lift up materials. Having said that, this is an experimental process which requires test and trial to understand the force required for retracting the SMA wire/ spring back. Or possibly if you can find the data sheet of SMA you are using you can calculate exact force which SMA can lift up.
2) Using a Biased Spring
You can also use a biased spring to push the SMA into its stretched state.
3) Using Material Properties
The material behavior plays an important role
in this method by storing and releasing bending energy during the heating and cooling phases of the SMA wire. Therefore it can help the retracting process. The mechanical properties of the material which is inherently springy applies an initial tension to the SMA wire. Therefore, by heating the wire, it contracts and produces additional curvature in the structure. If the structure also has springiness to push the SMA stretched then this would be a much faster process. This was the main focus of studying cellular systems in this project.
Step 6: Using Material Properties for the Design of a SMA Actuator
As discussed above, the material properties can provide enough force to retract SMA actuators to their stretched (initial) stage. Here is a video showing one of the prototypes which Paolo and I have developed showing relatively equal cooling/ heating times in response of SMA actuators. As you can see, as the SMA is charged with electricity it contracts and bends the PLA members, and when there is no current it slowly goes back to its full length due to the mechanical force of the PLA. The actuator in this video consists of SMA spring wire, PLA member (printed with MakerBot) and clear acrylic for the base.
To develop this design further, as you can see in this image, this approach has been implemented for the actuation of cellular structures by assembling SMA actuators between nodes of stiff/hard material. Therefore:
1) Force never transfers to the soft member directly but is always a transition in force distribution from hard to soft.
2) The material properties of the PLA plus cellular structure provide an interesting organic response through heating/cooling cycles of the SMA. This approach to the design of the motion can provide an organic, silent, life-like motion to your design.
Ok. Let's try and make a small prototype together. You can apply this logic to almost anything you would like to design. Attached you can find 5 different STL files which need to be assembled and assigned with various digital materials. Imagine you have a gradient of materials, from Soft to Hard. Working with Objet 3DPrinter, I select from Still to Hard (TangoBlack+> Shore 60> Shore 85> Grey 40> Grey 25> VeroWhite). The logic is that, wherever you apply a force (SMA actuators) then the material needs to be stiff (VeroWhite in this case) and in between nodes the materials gradually get softer.This way, you would have a softer transition for force distribution not dissimilar to structures in nature. After printing this piece, you need to attach the SMA actuator. For that, I used a MakeBot to print connection members made of PLA which were later assembled into the piece, and then I mounted the SMA (see the pictures). Attached you can find the STL file for PLA files as well. How can the PLA parts be attached to our cellular grid? Using nuts and bolts from McMaster http://www.mcmaster.com/mv1444941043/#catalog/121/... you can attach your members together and you do not have to worry about their connections at all. Also, please note that you cannot solder anything to SMA actuators. Therefore both for connections to the electrical wire as well as for joints, you would need to use crimp tubes (you can find a variety of these products here: http://www.beadalon.com/). I also used tiny eye terminals for the connections between the SMA and the screws.
As you can see in these photos, the next step was to design the form of these modules. I was very inspired by animal and fish scale systems. Therefore, the design process explores how a cellular structure can be expanded into 3D dimensions to create a formal expression. All the forms are generated computationally using Grasshopper in Rhino -which in itself can be a new instructable-. This was a highly iterative, back and forth process between digital design and physical testing using 3D Printer.
Here is the video of the final 3D Printing process:
I hope this project can provide a new approach to motion design for our daily objects which is less based on conventional mechanical systems by introducing a soft and organic motions into matter. And hopefully I have provided a small step toward a design of a programmable matter.... Good luck and happy making!
Special thanks to:
Sebastian Morales Prado
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