Combustion-driven actuators (CDAs) are a class of soft actuators. They use the ignition of combustible mixtures (i.e. air-methane) to drive actuation. The Functional Material Laboratory at ETH Zurich has developed a fabrication technique for production of silicone monoblock structures (i.e. single continuous parts with no seams). These structures are both soft and able to withstand the high stresses imposed by combustion.
The basic idea of these actuators is similar to that of air pressurized systems. Instead of applying pressure to inflate channels, the expansion is created directly at the needed spot by igniting an air-methane mixture. The combustion process generates high energy gases which force the soft outer walls of the actuator to expand. The reaction energy is so large that the volume of the CDA can expand to several times its original size (for example the volume of the CDA presented in this documentation increases five-fold). The resulting motion is controlled by the usual variables such as geometry, material thickness or material choice (just as seen in PneuNets Bending Actuators).
CDAs can be triggered by a simple control set-up, consisting of a programmable logic controller (PLC), mass flow controllers (MFC) and a spark transformer. Therefore, powering soft actuators with hydrocarbons not only enables direct conversion of fuel into work, but also reduces weight (i.e. energy density of hydrocarbons is higher than for Li-batteries). This has great potential for soft robots operating outdoors.
The CDA design presented in this documentation is only a demonstration of the CDA concept. This design is not used for actuation purposes directly. Nevertheless, the concept of producing monoblock actuators by casting virtual lost-wax molds is essential for ensuring long-term performance of any combustion-driven actuator design. The design of such molds is explained in the first part of this tutorial. Next, the fabrication process to produce the CDA that has been designed is explained. Using few equipment components, a CDA is then set up for control and testing in the next section. Possible applications for CDA are demonstrated in the form of case studies. Lastly, all files, including a detailed Bill of Materials listing all parts and materials needed to build the combustion-driven actuator designed in this documentation can be downloaded here.
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