January 22, 2020 · project

Pressure Matrices

Whilst my MVP prototype did manage to achieve the bar of detecting if it had been petted. I found the use of capacitivc touch sensors to be a little cumbersome and unpredictable.  For this reason I decided to investigate a different kind of touch sensor. Enter: the Piezoresistive Sensor Matrix.

These sensors are a lot more predictable than capacitive touch sensors. They use a grid of conductive material, that, when touched, measures the resistance across the multiple grid locations. This is most easily achieved with Velostat, an electrically conductive fabric that can sense pressure. The more you squeeze it, the lower it's resistance. Using some documentation and photographs from the excellent Kobakant blog, I constructed 3 different designs for a pressure matrix. Two of the designs use Velostat, and one is just some copper tape woven into strips of felt.

A selection of woven and non-woven pressure matrices. Left: Woven felt. Right: Velostat sandwiched between cotton sheets. Bottom: Woven Velostat
Silhouette of the non-woven pressure matrix

Using some handy visualisation code, again courtesy of Kobakant, we can see the pressure matrices in action! They require almost no force to actuate at all, which is very different to the experience I was having with the capacitive touch sensors. The resistance ranges seem a lot more reasonable as well, since the resistiveness of Velostat goes down when you touch it, not up.

Visualisation of pressure applied to non-woven velostat matrix
It works on the felt matrix, too!

I was surprised when The readings on the woven felt matrix were similar to the velostat matrices. Even without velostat, the program is able to work out the resistance across each point on the grid. Sadly, this is where the fun with pressure matrices starts to fall apart. Throughout this testing, it became clear that you need direct contact with the conductive surface of the matrix. Without it, the pressure sensing doesn't work at all. Sadly, this makes it a non-starter for my project, as I need to mount my sensors under layers of material to get the required visual effect I am looking for.

Going back to our wonderful friends at Kopakant, it does seem possible to build these matrices with conductive thread embroided into another piece of fabric. Sadly I don't have access to an embroidery machine, and my sewing skills are beginner at best. It looks like I'm going to have to go back to using capacitive touch sensors, but I'm hoping I can iterate somewhat on the design from last time.

Satisfying...