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Tipping Points and Stability with a New Type of Soft Actuator

Soft robotics is a subfield of robotics that deals with robots made from soft materials such as rubber or silicone. Soft robots are useful for interactions with humans and carrying out tasks in uncertain environments. However, most soft robotic systems are not completely made out of soft materials. They often need large compressors or pumps to actuate and due to these bulky hard components, most soft robots cannot operate untethered. A new soft actuation method was recently discovered: it combines hyperelastic membranes and dielectric elastomer actuators (DEAs). It does not involve pumps, compressors, or valves. The DEAs activate and inflate the membranes into various stable configurations.

Hyperelastic membranes do not have a linear relationship between membrane volume and internal pressure because of snap through instabilities. For example, balloons (a hyperelastic material) can change volume drastically at certain pressures). Because of this, the relationship between volume and pressure is not linear, but instead, peaked. If the membrane is inflated to just before the peak and then a DEA induces some stress in the membrane, the membrane will be pushed beyond this peak to the next stable state. There is a tipping point before the peak: an unstable equilibrium. An upward stress will bring the membrane to the next equilibrium point. This new soft actuator is a dynamical system whose stable and unstable equilibria can be calculated similar to how we found equilibria in class.

In class, we modeled a market for a good with network effects. The relationship between the price of the good and the fraction of consumers that will buy the good is peaked similar to the model of the soft actuator. The points where a particular price intersected the fraction of the consumers willing to buy the good were the equilibrium points. An intersection before the peak was unstable, and an intersection after the peak was stable. This analysis of a market for a good is also applied to the soft actuator to find its equilibrium points. At a particular pressure, multiple volumes of the membrane can be equilibrium points. When multiple individual membranes are combined into a system, they too can have multiple stable configurations at a particular pressure.



Hines, L., Petersen, K. and Sitti, M. (2016), Inflated Soft Actuators with Reversible Stable Deformations. Adv. Mater., 28: 3690–3696. doi:10.1002/adma.201600107




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