Hi-Res, Body-Sized Pressure Sensor Mat

Hackers often find uses for pressure sensitive materials, detecting footfalls during walking or keypresses in a synthesizer being two examples. [Marco Reps] decided he’d make a hi-res, body-sized pressure sensitive mat mainly for computer-guided physiotherapy, though he wouldn’t rule out using it for gaming (twister anyone?). That meant making the equivalent of a body-sized matrix circuit of around 7000 sensors, as well as a circuit board with a multitude of shift registers. The result has a surprisingly good resolution, capable of making clearly distinguishable the heel, arch and front part of a foot.

His choice of pressure sensitive material was Velostat, a polymeric foam available as large sheets. The foam is impregnated with carbon black to make it electrically conductive, but being a foam, its resistance changes when pressure is applied. The first layer of the mat is made up of one centimeter wide strips of copper tape laid out lengthwise and spaced one centimeter apart. That’s followed by the Velostat and then another layer of copper tape oriented horizontally this time. The pressure sensors are the sandwiches formed by where the tapes overlap. In the first video below he shows how he measured and graphed the Velostat’s dynamic range to help decide to use one centimeter squares. He also puts together a smaller prototype, with good results.

Testing the mat
Testing the mat

For the body-sized mat, we count around 50 by 140 overlapping areas for a total of around 7000 one square centimeter sensors. And of course to measure each sensor in that large matrix, as you can imagine, he made up a custom circuit board with shift registers. The board works by applying positive voltage to the columns one-by-one, while each time going through all the rows and reading their voltages. Making the board was in itself was an adventure, taking a chance on a Chinese manufacturer asking only $2. But watch the second video below where he evaluates the result, including trying unsuccessfully to delaminate a board. Sadly he forgot to include places on the board for diodes, one for each column, and fixing that is another adventure he walks us through. Patience was definitely a prerequisite here, not only for making the mat, and fixing the diode problem, but also for connecting up 96-pin ribbon cables. We applaud his efforts, and his results. Check out the second video below for the making of the large mat and the circuit board.

As we said, Velostat has found all sorts of uses in the hacker community. One example is to put it in footwear to light up LEDs whenever the foot hits the pavement. Another we’ve seen is to capture keypresses in a polyphonic synth. What use can you come up with or have used it for?

13 thoughts on “Hi-Res, Body-Sized Pressure Sensor Mat

  1. This was a well done project but nothing surprised me more than seeing him take a door off its hinges for use as a work table. I consider myself resourceful, but I doubt I would have thought of this.

    1. Hi, I’m not using it in quite the same way. I am trying to use patches of velostat as pressure sensors. I’m pressing the patches against a PCB which has strips of exposed copper interleaved. Half of these are connected to one common terminal and the other half connected to a second terminal. I’m using this as a pressure sensitive variable resistor in a resistor divider and sampling the voltage to get a reading of pressure/load. I’ve been struggling to get a decent set of readings because as soon as there’s a bit of pressure the resistance has travelled through most of its range and changes in load that I’m interested in are barely showing up in the voltage readings. I’m about to start a proper look at linearising the output but if anyone has any ideas I’d love to hear them.

      1. Hi Smerrett,

        Actually I have the sema project with you. I will read pressure and change it to load. Actually there is no properly way to change data from pressure to load, If you change the pressure area or applied location, load might be change unlineer. Pressure mapping systems are not good idea to measure load. If you mail me, I can send you information how you can calibrate pressure.

        By the way I need this software of matrix project. I have installed many project but their softwares could not work. balbursa@gmail.com

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