Close

Hairspray surprise (update, confirmed!)

A project log for WEFT electrovibration demo board

This board is for prototyping electrotactile interactions - use an electronic signal to dynamically change the texture of a surface.

akaAKA 11/17/2015 at 20:4110 Comments

Olivier Bau, one of the authors of the two Disney Research papers the WEFT project is based on, was kind enough to meet with me recently and answer some of the questions I had after trying to implement the circuits described in the "Revel" paper.

The most surprising revelation was the choice of dielectric - the paper called for a thin non-conductive layer to be applied over the conductor, but no substance in my tests worked very well (I tried Kapton tape, packing tape, and a couple of other substances). Offhand, Olivier mentioned that spraying the copper with hairspray was ideal, and even enhanced the electrovibration sensation. Who knew?!

So once I recover form Hacakday SuperConference jetlag, I'll try this out. The procedure he recommended was to really drench the pad in hairspray and then let it dry vertically, so the excess runs off and leaves a thin, even layer as it dries.

Discussions

Eric Hertz wrote 11/20/2015 at 17:42 point

This reminds me of the bed of my old flat-bed pen-plotter. Not sure if the technology is similar or completely different, but the bed would activate electrically and attract the paper to the surface. It was smooth and slippery when "off" but held in place when "on." As I recall the bed consisted of a long "snake" PCB-trace and two leads. Never could figure out how it worked. 

Cool use of whatever this technology is for user-feedback! And Hairspray! Who'da thunk?

  Are you sure? yes | no

AKA wrote 11/20/2015 at 19:24 point

So I think it's a similar phenomenon at work here - the plotter probably used higher voltage (in the kV) to hold the paper down, much like a balloon clings to your head when statically charged.

The trick with electrovibration is that because the hi-v signal varies over time (sine wave, sawtooth wave, etc) AND your finger is moving, you feel the clinging force (which is really tiny, imperceivable when finger's not moving) as it builds and diminishes (as the sine wave goes from max to min, etc).

The physical effect is similar to the Stick-slip phenomenon https://en.wikipedia.org/wiki/Stick-slip_phenomenon which is the mechanism behind how violins and other string instruments work...

  Are you sure? yes | no

Eric Hertz wrote 11/27/2015 at 12:05 point

Great explanation, thanks. Tangentially(orthogonally?)-relatedly, that "stick-slip phenomenon" seems to be what I'm running-into with my latest linear-actuator project (not yet documented). Thanks for the name/link!

  Are you sure? yes | no

Roobotics wrote 11/20/2015 at 05:49 point

Why not simply leave the pad covered and let the solder mask be applied overtop of it if you're going to have to seal it up again? Just a thought!

  Are you sure? yes | no

AKA wrote 11/20/2015 at 16:28 point

Thanks for the idea!

The research papers actually specify that having a thin, insulating dielectric over the copper pad is desirable. However, before meeting up with Olivier, I had tried quite a few materials with no success.

Packing tape, butcher paper, rolling paper, and kapton tape were all such good insulators (or were too thick) that the electrovibration sensation was completely suppressed.

I think hairspray's chemical makeup makes it a pretty good dielectric, and the ability to make a really thin coating probably helps it not over-insulate.

Regarding the soldermask idea, I think it's worth a shot - because the first board is so big (and thus expensive), I've moved on to fine-tuning the WEFT board (the little one with BLE capability). It might make sense, though, to prototype a pad-only daughterboard that connects to the WEFT board. I'll definitely update this project if/when that happens!

Thanks again,

AKA

  Are you sure? yes | no

AKA wrote 11/20/2015 at 00:44 point

So I got some cheap "Suave" brand hairspray and really drenched the copper pad (everywhere else was masked off with packing tape).

Once the hairspray had dried (an hour or so, clipped and held upright so the spray residue would drip off), I was really amazed how much more effective the device was at delivering consistent signal across different people.

I'll be working on more permanent solutions (does anyone know if hairspray will eventually wear off?), but thought I'd share this surprising result!

  Are you sure? yes | no

zakqwy wrote 11/20/2015 at 05:21 point

I think it's water soluble?

  Are you sure? yes | no

AKA wrote 11/20/2015 at 16:29 point

Oh snap, that would make sense. Maybe I will try some lacquers or varnishes? I am trying to figure out how much of the improvement is due to the layer being really thin and how much is due to the hairspray itself being a good dielectric...

  Are you sure? yes | no

EK wrote 11/18/2015 at 16:56 point

Interesting!

  Are you sure? yes | no

zakqwy wrote 11/18/2015 at 13:47 point

!!! So hairspray isn't just for spud launchers and 3D printers anymore??

  Are you sure? yes | no