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.
The material choice is much more important for combustion-driven actuators than for others. No company actually tests their silicones under simultaneous stress (i.e. thermal and mechanical). Therefore, silicone data sheets only tell half the story. However, we found several silicones that are able to withstand thermal and mechanical stress up to 30,000 combustion events. We'll start with some information about our selection criteria.
We use room temperature vulcanizing (RTV) silicones from Altropol GmbH*. They offer a wide range of two component silicone systems.
*Note: Like all of our guides, this was written by contributors from their own experience. Users in other countries may have to search for alternative suppliers.
The following points largely influenced our material selection:
Condensation systems:
Note: Condensation based systems will shrink with time. This is because the condensation product, a short alcohol, evaporates from the silicone.
Addition systems:
We achieved over 1,000 combustion events using the RTV 23 and over 30,000 for the RTV 1701. The exact handling of the presented silicone types is described in the fabrication section.
Lost-wax casting is a technique normally applied to cast metal. The process begins by producing a duplicate of the final part made from wax. This wax part is then placed in a box, which is filled with cement. After curing, the cement block is heated up and the wax melts and pours out. The remaining cement is now a negative of the final part. This negative mold is then filled with melted metal. After cooling, the cement mold is destroyed and removed, leaving a cast metal part.
Our fabrication process uses the same casting idea but uses other materials than metals. We virtually design a part and then create a virtual mold by inverting the part design (see Computer Aided Actuator Design). This mold is then 3D printed and filled with uncured silicone. After curing, we destroy the mold by dissolving it in solvent.
We designed this actuator using the computer aided design (CAD) software Siemens NX. The following short tutorial shows how to design such a combustion driven actuator. Note that other CAD software (i.e. SolidWorks, Inventor, AutoCAD,…) can be used, but this tutorial is specifically for NX software. The main reason to use NX is its simplicity in creating free forms. This might be helpful especially if you want to design complex geometries. If you do not have access to NX, we encourage you to follow along through the guide to understand the design process in principle and even to attempt to replicate our design in your own software of choice. Downloads of the part files for the actuator design in this tutorial can be found here which include formats compatible with most CAD software.
In the following sections, we show you how to create the simple combustion driven actuator shown below. All necessary sizes are indicated in the figure below. The design is comprised of a cylinder, which partially drives through a hollow sphere. Three channels, aligned at different angles towards the sphere center, form the gas inlet, exhaust and igniter electrode connection.
Again, all part files can be found here.
Now that the actuator has been designed, we need to make the mold that will allow us to cast the part. To do this, we will "invert" the design from the previous steps. This inversion process essentially involves making a part that has the same shape, but with slightly larger dimensions in order to enclose the actuator design and then subtracting the actuator design to create a hollow mold. We then have to add filling and venting holes to allow silicone to enter and air to escape the mold during casting. This section will detail how to create the basic mold for the actuator designed in those previous steps.
Note: The minimum wall thickness of the mold should be 1.5 mm. Otherwise, your 3D printer might not be able to successfully print the mold. Also, the silicone filling becomes quiet difficult in terms of leaking spots.
One of the main problems when creating and filling a mold are dead volumes. These dead volumes can trap air and therefore destroy your design upon filling uncured silicone. Hence, the design not only has to be “inverted” to form the mold but also specific connections (in the form of vented holes) to the outside have to be made. The vented holes enable the escape of air, in order to allow silicone to fill the pockets that would otherwise be filled with this trapped air. Our design needs four of these vents, which are all positioned at the end of a channel inlet.