The combustion-driven actuator documented here consists of one single combustion chamber. This chamber has three channels to plug a gas injection nozzle, a spark gap (to trigger ignition) and an exhaust channel. This CDA is designed for demonstration purposes, so there is a horizontal cylinder that serves only as a handle to hold the soft actuator in position for testing. It can be seen in the picture below running perpendicular to the exhaust channel above the combustion chamber.
Upon a combustion event, the expansion will inflate the combustion chamber. Due to the chosen geometry of the CDA, the thin wall will stretch, while the thicker, horizontal section undergoes almost no movement (shown below). This repulsive motion can be extremely useful in robot design, where fast actuation plays a key point.
Combustions impose an incredible stress on the soft material. The first combustion driven soft robot presented by Shepherd et al. broke down after very few ignitions. One of the main reasons that robot failed was the use of two layers glued together to form the body. This revealed that a monoblock structure made from a single material is essential for combustion-driven actuators. This is why the main focus of this documentation is on the lost-wax mold design to produce these actuators.
The next two sections provide some background on material selecting criteria. Additionally we have a detailed tutorial on making the solid models of the actuator and the corresponding lost-wax-like mold using NX. The resulting part files as well as .stl files for 3D printing the lost-wax-like mold can be downloaded here. The part files include .stp formats for users of other CAD software, but we suggest following along with the tutorial in spirit and attempting to mimic the results in your own software.