Tentacle Coathook

John Kestner - Techniques for Design and Fabrication, Fall 2008

I've been meaning to make a nautically-themed coatrack. Since I've acquired some new skills, I was thinking about making it interactive. Maybe certain coats are extended to you depending on the day's weather, or the rack refuses to hand you a jacket unless you twist its other arm. For my final project, I decided to prototype a single hook on this coatrack. It would take the form of an octopus tentacle, and use a force-sensitive resistor and servo to curl up and "grab" clothing that was hung on it. (I made two different tentacle shapes; some pictures may show one or the other.)

Skin

  1. I started by carving and sanding the tentacle form out of yellow foam.
  2. Next, I sealed the foam using spackling to prevent the silicone from sticking to the foam, though I left it a little rough to create some opacity in the silicone. I also sprayed mold release on the form.
  3. I mixed up some Dragon Skin silicone, scraped a bit of pastels in for color, and spread it on the form, letting it flow around to even out into a fairly uniform coat.
  4. I rotated the form around until the silicone had set, then let it cure. Dragon Skin silicone only takes about an hour to cure.
  5. The silicone peels right off.

Bones

  1. The project came together when brainstorming suggested that a toy wooden snake was a structure that could provide smooth, organic motion. I threaded a cable through staples pressed into the top side of (half) the snake, and anchored it at the head. The idea was to mount the snake to a plate and use a servo to pull the cable, pulling up the structure until it smoothly curled up.

    I wedged small pieces of shrink tubing and coffee stirrers between segments on the bottom side, to keep the snake from drooping too much when not actuated.
  2. I had a FSR with the sensor on the end of a long strip, the length of which I had made sure the tentacle mold matched. I attached the sensor to the tip of the snake and soldered on leads.

Brains

  1. To measure the FSR and control the servo I programmed an AVR mega88.
  2. Like most servos, the one I used is controlled by square wave pulses. It expects a 50Hz cycle (20ms), and the position of the arm (in a 180-degree range) is controlled by the length of the pulse (between about 1ms and 2ms). So I used the PWM timer on the AVR to generate pulses keyed to the sampled resistance of the FSR.

    I had two issues with this part. First, the resolution of the timer I initially used was 8-bit. This combined with the nearest timer scale I could use meant that there wasn't more than maybe 3 steps from 1ms and 2ms. This was solved by switching to the 16-bit timer, which provided a nice smooth gradient. Second, I didn't realize that my first servo, having been modified for continuous rotation, didn't listen to positioning commands as expected. Once I figured that out, I was good to go on the servo end.
  3. I had more trouble with the FSR once everything was hooked up together. (That's one thing I learned only from actually making this - adding mechatronics can throw all your best-laid circuits into disarray.) I haven't yet figured it out precisely, but somewhere between the linearity of the force resistance and the fluctuations caused by the sensor being fixed right to a curling arm, the tentacle got very jittery (it had tested smoothly when controlled with a pot). I tried dampening the reading in software but haven't got it right yet.
  4. For demonstration purposes, I found a cardboard box that had an opening perfect for wedging the tentacle arm onto and giving free play to the cable and wires. The final coatrack will be mounted onto a wooden plank on a wall, and will be larger - this arm/servo is probably not enough to support more than the weight of a scarf.

I enjoyed playing with form and materials and figuring out the mechanics of this. I learned a lot about the difficulty of combining sensors with moving parts, and debugging a microprocessor without seeing much information. I intend to carry out my original plan once I debug the small-scale tentacle.

I should state for the record that I am not a pervert, nor did I have any perverted intentions with this project - though I'm glad it's brought much joy to the Media Labbers who have seen it in action. I suppose I'm like Mother Theresa in that regard. On the other hand, one good source of information on working with silicone forms that I found happened to be a furry site documenting the construction of a suit for a character who is a lion, who is also a Roman soldier. At least I didn't do that.

A video of the tentacle in action will be up shortly.