Here's a project you can complete in a half hour or less with materials you probably already have at home or can obtain locally. It's a "load cell" or force sensor whose resistance is inversely proportional to the compressive force applied to it. It uses a "pellet" of black anti-static foam. This kind of foam is loaded with carbon particles. When the foam is squeezed, the carbon particles become compressed together, conducting electricity more readily, providing a variable resistance that can readily be measured.
1" x 2" piece of copper-clad circuit board.
Small piece of black anti-static foam.
1½" x 1½" piece of springy foam, the same thickness as the anti-static foam.
Some insulated wire.
Double-stick tape (e.g. carpet tape).
1/4" hole punch.
Hacksaw to cut circuit board.
Fine-grit sandpaper or Scotch-Brite® pad.
X-Acto® or other razor knife.
1. Polish the copper-clad circuit board with the sandpaper or Scotch-Brite® pad until it's shiny.
2. Cut the circuit board into 1" x 1" squares. Then nip one corner off of each one. (Illustration below.)
3. Solder wire ends to identical corners adjacent to the cut corners of both circuit boards. (Illustration below.)
Materials and Circuit Board Preparation.
4. Punch a hole in the black foam, retaining the little round piece that gets punched out.
5. Cover both sides of the springy foam with double-stick tape, leaving the paper liners on the non-foam sides.
6. Draw a 1" square on one of the liners.
7. Punch a hole in the foam/tape sandwich about 1/3 of the way into the marked square. (Illustration below.)
Holes Punched in Both Foam Pieces
8. Trim the foam to size along the lines you drew.
9. Peel the paper off one side of the foam sandwich and adhere it to one of the circuit board pieces. The off-center hole should be positioned opposite the wire/notched end of the board.
10. Insert the black foam pellet into the hole. Make sure it's pushed in all the way.
11. Remove the other paper liner. (Illustration below.)
12. Adhere the other circuit board to the newly exposed adhesive. Squeeze the two circuit boards together tightly to get a good bond with the tape.
13. With the razor knife, trim away the excess foam exposed by the notched circuit board corners. This completes construction. (See illustration below.)
"Open-faced Sandwich" with Pellet Installed; Finished Load Cell
With an ohmmeter connected to the wires, you can test your load cell with various finger pressures to see how the resistance changes. I used a postage scale in conjunction with an ohmmeter and a drill press to measure the resistance at various measured forces and plotted the results, shown below. (See explanation below for why there are two curves.)
Graph of Load Cell Resistance vs. Force
You can interface your load cell to a BASIC Stamp, using the RCTIME setup to measure its resistance. If your sole interest is measuring the force against a fixed threshold, a resistive divider, with the load cell as one leg, will be adequate. Just pick a resistor value that places the divided voltage at the logic threshold of your input when the applied force is at its desired threshold.
Load cells (the expensive kind) are often used in electronic scales. So a natural question would be, "How suitable is this load cell for weighing stuff." The answer is, "Not at all." The reason is two-fold: 1) Both foams have memory — especially the antistatic foam. This means that it won't bounce back quickly — or even completely — like a spring does and will eventually fatigue altogether. 2) The conductive properties of the anti-static foam will change over time as the foam fatigues and the carbon particles shift. Both these properties will conspire to compromise the repeatability needed for precision weighing.
The memory property mentioned in 1) above is also called hysteresis, the tendency fo a system to stay in the state it's in, even when external conditions change. That's why there are two curves on the above graph. One curve shows the resistance as the applied force is increasing; the other, as the force is decreasing. As you can see, there's a wide disparity between them, rendering any absolute measurement questionable, at best.
Nonetheless, for sensing things like relative finger pressure, bumps, footfalls, robotic grip pressure, etc., a simple sensor like this one may well suffice. In any event, they're simple and cheap to build and fun to play with.
'Took new measurements using a drill press to stabilize the applied force, this time in both directions to demonstrate the foam's memory property.
Post Edited (Phil Pilgrim (PhiPi)) : 11/16/2008 7:36:08 AM GMT