I’m quite interested in robots with organic movement, the sort of thing that looks creepy due to its unnatural naturalness and with this in mind I’ve started looking into building a robotic/animatronic tentacle arm (Japanese schoolgirls beware!). My first step was to have a look at the sort of thing floating about on youtube already, I’ve compiled some of the best examples into a playlist.
The basic structure seems to be a set of ribs with guide holes for wires, spaced along the length of a flexible tube. Control wires run up through the ribs and are tied off at the end of the arm so that when they are pulled at the base the arm flexes outward toward the side that the cable is on. To get a feel for the workings I decided to knock up my own simple version using cardboard ribs and 3 control wires. I made the template rib shown below and copied it multiple times to fill a sheet of A4 paper (print @ 300dpi for correct scaling).
I printed out my page of ribs and glued it down to some scrap cardboard, then I cut out 8 or so (I would have done more but got bored). Once I had a stack of arm segments I looked around for something suitable to use as the flexible core, the first thing that came to hand was the inner tube of an old biro laying on my desk. I made holes in all the rib segments using a pin and scissors, then started to thread them onto my biro core. To keep the segments separated I cut short (15mm approx.) lengths of clear plastic air tubing, the sort used in fish tanks. These fitted snugly over the core and were stiff enough to add strength to the arm.
Once I’d stacked up as many segments as would fit (7), I cut 3 lengths of fishing line and ran them up through the ribs to the top end of the pen, where I tied a simple slipknot in the end of each piece and tightened them over the biro. To hold the whole lot vertical I simply clamped the biro tube in a small vice.
I was thinking about making some kind of joystick contraption to control the arm to begin with but after a quick sketch and a cup of tea realised that I was over-thinking things considering the rough state of the arm, and I decided instead to use a marionette style controller made from two pencils taped into a cross shape. I did cut a flat into the middle of each pencil to stop them twisting away from square, but even that might have been overkill. I notched the ends too so that the fishing line could be secured easily, again using slip knots, and that was pretty much it.
It works quite well given the simplicity of the setup and has definitely made me want to go on to a slightly more complex version, probably using my CNC machine to cut the next set of ribs from MDF or plywood.