Otherlab has been doing a lot of inflatable fabrication lately, and we thought it was about time to write an Instructable about it. What better subject to inflatable-ize than the Instructables robot itself? Here's the high level view of the fabrication process.
Step 1: Design 3D Model and Decompose Into Panels
For us, the first step to constructing an inflatable is designing the 3D model using a CAD program like AutoCAD or Solidworks. Using artistic license, we added a third leg to make the robot more stable. Further, inflatable shapes tend towards a sphere when inflated, so some allowance must be made. For example, to prevent the legs from splaying we pointed them inwards.
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
Now we print out the panels with seam allowances. We used a Roland Versacam with 48" bed, but any printer will do if you tile the panels across multiple pages. After printing, the tiled pages can be cut and taped together.
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
Using the patterns as guides, identify which panels are connected by which seams. For most of the robot, we use a plain seam sewn flat. For large diameter sections (like the belly) the stress on the fabric is higher, and care must be taken to maximize seam strength. For smaller appendages, strength is not as critical and simpler, easier seams can be used.
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
Some cavities, like the fingers, eyes, antennae, and buttons, are too small to be conveniently inflated by the internal bladder. For these parts, we just stuff foam inside to give them support, simplifying the bladder construction.
Step 5: Make the Bladder
The fabric envelope is not airtight, so we need an internal bladder to keep the robot inflated. Like a bouncy castle, though, a continuous blower could keep the robot inflated without a bladder. This can also be used to quickly test the inflated shape before making a 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.
Step 6: Stuff and Inflate
Everything is done now, we just need to stuff the bladder and inflate it. Starting with the extremities, tie the loops on the bladder onto the envelope. Push the nozzle through the inflation hole and distribute the bladder evenly around the inside of the envelope. Talcum powder helps the bladder slide into position, and sit better. Start pumping air into the bladder, all the while massaging the robot to eliminate spots where the bladder doesn't properly fill out the fabric envelope. If all goes well, you'll have a robot punching bag!