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Basic Theory
Basic Theory
The basic theory behind a super-coiled polymer (SCP) actuator is similar to that of the shape-memory alloy (SMA). Heat generated through electric current is used to untwist the muscle-like coil of thread, causing the actuator to contract.
The SCP itself is constructed out of a thread, such as nylon, carbon fiber, or even sewing thread, which is coated with conductive metal, such as silver or copper. The thread itself is made up of polymer fibers (made of a chain of molecules) which are arranged parallel to each other along the length of the thread.
Twisting the thread creates a helix formation (similar to diagrams of DNA) that causes polymer fibers to cross over one another. This tightens the fiber at a microscopic level, allowing them to untwist when heated.
Untwisted thread. Polymer fibers are composed of many connected units, called monomers, which comprise the material of the thread.
After twisting, the polymers are turned into a helix formation, resembling DNA. The diagram does not depict the surface of the thread, which would now be bumped instead of smooth.
We then coil the twisted thread on itself (pictured below) in order to create a macroscopic twist. When heat is applied in this configuration, the polymers will attempt to lengthen. Due to their helical structure, however, this lengthening instead causes the polymers to squeeze together. Coiling turns this squeeze into a full-fledged contraction, in the direction of the actuator.
After coiling, the thread takes on a spring-like shape. The cross-section of this spring shape is pictured above. On a microscopic level, the twisted polymer helices will squeeze together when heat is applied, causing a contraction that is scaled up through coiling.
Design Considerations
Design Considerations
Some key design considerations for SCP actuators include the type of conductive thread used, the amount of electricity you can safely feed into the actuator, and the way you bundle different SCP units together.
Thread Type and Electrical Safety
Thread Type and Electrical Safety
An important note is that SCP actuators rely on heating the polymer fibers in a thread, rather than heating the metal coating the thread. We chose silver-coated nylon as a purchasable option, although many online vendors such as Sparkfun have stopped selling this material in favor of stainless steel thread (which would not work, as it does not contain a polymer fiber). In theory, any plastic-based thread coated with any conductive metal can work, but different metals have different electrical resistance, while different threads have different heat capacity
Related to this is how much power can be safely fed into the actuator. In general, an electric resistor can be thought of similarly to a lightbulb. While a lightbulb takes in electricity and converts part of it into light, a resistor takes in electricity and converts part of the energy into heat. Metals have very small, yet nonzero, resistances, so various metals will generate varying amounts of heat. The equation for electric power comes into mind here:
P = I x V
or:
P = I^2 * R
where: I is electric current,
R is the metal's resistance,
P is power, a rough measure of how much energy (as heat) our metal will give off for a certain current and resistance.
Since the resistance of a metal is fairly constant, it falls onto you to determine how much heat you can safely generate without causing the thread itself to melt. In turn, this determines how much current you can safely put through the actuator. Ohm's Law can be used to convert this current limit into a voltage limit, given that you can estimate the resistance of your actuator (which, in turn, depends on length, metal type, and the cross-sectional area of the thread).
Bundling
Bundling
If you wish to use multiple SCP's in parallel for your design, be careful not to have them in contact with one another. If bundled in contact with one another, the total result is similar to putting a large number of wires together -- greater resistance, less energy efficiency, and overall degraded performance.
However, if spaced apart, using multiple SCP's can achieve the desired effect of greater force output. The below video displays some creative bundled SCP designs from the Advanced Robotics and Controls Lab at UCSD, which you can take inspiration from for your own designs.
References
References
Wu C, Zheng W. A Modeling of Twisted and Coiled Polymer Artificial Muscles Based on Elastic Rod Theory. Actuators. 2020; 9(2):25. https://doi.org/10.3390/act9020025
A. Simeonov et al., "Bundled Super-Coiled Polymer Artificial Muscles: Design, Characterization, and Modeling," in IEEE Robotics and Automation Letters, vol. 3, no. 3, pp. 1671-1678, July 2018, doi: 10.1109/LRA.2018.2801469.
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