Step 1: Design 3D Model and Decompose Into Panels
With the model in hand, we use Rhino (available as a Work In Progress download for Mac) to unroll "developable" surfaces which lie flat and can be sewn together into the original model.
Next, we need to add 10 mm seam allowances and arrange the parts onto printable pages. Using Illustrator or Inkscape, we can take the DXF output of Rhino and use 'Path Offset' to add seam allowance. The panels with allowances can be arranged manually to fit on printable pages. This method works well but can be really tedious with many panels.
As an experiment, we read the output of Rhino and programmatically added seam allowances and nested the parts on paper stock.
Step 2: Print Panels and Cut Them Out
Once the panels are printed, we need to cut the shapes out of the fabric. For small parts, you can cut the paper and fabric in one step, but for larger ones it is often easier to cut the paper first. Use weights to ensure the paper pattern doesn't slip around on the fabric while cutting.
Step 3: Sew Panels Together
To stuff and service the bladder, we need access to the internal cavity. To this end, we put in a zip down the back side of the robot. This also provides access for a human driver...
Finally, to access the inflation valve, we reinforced a small hole just below the zip. For historical reasons, the valve is invariably placed in the standardized position.
Step 4: Stuff Small Cavities With Foam
Step 5: Make the Bladder
The polyurethane bladder is oversized and stretchy so the fabric envelope takes all the force and hence dictates the shape of the robot. Meanwhile, the internal bladder stays bunched up inside, only providing the airtight seal. This system is like the tube and tire of your bicycle wheel. Generally, the bladder should be oversized enough to avoid stretching.
Because of this, the shape of the bladder can be greatly simplified. We make a simple 2D approximation of the robot that can be easily constructed by heat bonding. After cutting the panels of polyurethane, we use the impulse sealer to assemble each half before joining them around the edges into a large inflatable bag.
After the shape is done we still must add a nozzle to pump air in and out. Stick-on one-way valves make this easy and can be bought from kite builders' supply shops like this one.
Finally, to keep the bladder appropriately distributed inside the fabric envelope, we need attachment points at each extremity. We use the impulse sealer again, bonding small loops of polyurethane at the ends of the arms, legs, and head.