Archive for August, 2010

Pressure mapping with DIY foam load cells

I had an idea a while back to make a pressure sensing pad for testing saddle fit on horses. The intent was to create an array of pressure sensing cells, which could then be used to produce a pressure map that would represent any pinch points on a horse’s back. I found that you can buy this sort of thing, but it’s way too expensive for the average guy. I decided to try and make my own for cheap. I ended up making one from a handful of copper-clad PCBs and 1/4″ shipping foam.

Pressure map pad and electronics

The active area of the pad is approximately 2′ x 2′. I think the foam is polyurethane open-cell foam but I’m not sure. It’s the stuff you use to pack shipping crates. The load cells are made by sandwiching the foam in between 1″ circle cutouts of copper-clad FR4 PC board. I used single sided board and a hole saw with the pilot bit removed (use a drill press and a clamp). The capacitance varies as the foam compresses, and the amount of capacitance is directly related to the thickness and density of the foam, as well as the area of the copper conductors (plates). So you can create any size or thickness load cell you want really. A bigger plate results in more capacitance, as does placing the plates closer together. I estimated the capacitance I would have in my application with this calculator I found at Daycounter engineering services.

I created an array of 64 cells by making 8 rows and 8 columns, each with 8 copper-clad discs. Wherever the row/column discs align a load cell is created. An AVR typically has eight available ADC inputs along with another eight control outputs, so this way you can scan down through the rows and columns to measure each cell. A square wave is sequentially output on the columns, and after some analog proccessing the AVR’s ADC scans each row. The analog voltage present represents the amount of pressure (capacitance) at each site. › Continue reading

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Monday, August 30th, 2010 Electronics 9 Comments