## Introduction: How to Make a DIY Force Plate

Force plates can be very useful in bio-mechanics. As part of a workshop in conjunction with RPI, we taught high school students how to build and use their own force plates from very inexpensive materials. These custom force plates were used to test their own hypotheses about bio-mechanics. It is our hope that this built their understanding of biology, math, physics, and making.

To make this force plate you're going to need the following components:

## Step 1: Prepare Strain Gauge

Take the strain gauge and wrap the wire around some stripped insulated wire and solder them together. In addition to wrapping the wires (as one normally does with soldered contacts) wrap both wires together, and then tape them to the plate to relieve strain. Keep in mind that the contacts on the strain gauge are fragile and not meant to flex.

The strain gauges come in some thin plastic sheet which is excellent for this application. So you can double it over into an ellipsoid. This will be out sensor mount. When the plastic is compressed, it will bend and the strain gauge taped to it will stretch with it generating a signal.

## Step 2: Setup the Circuit

The signal created by the strain gauge is about a change in resistance. The simple way to measure the change is to create a voltage divider. By putting another equal resistance in series with this one, and measuring the voltage between the two, as the resistance changes, so too will the voltage. So start by putting a 350 ohm resistor in parallel. you may quickly find that the signal is very small, we'll need a better setup to measure it.

In particular, the signal changes from about 2.500 to 2.503. If only we could measure the change and get rid of those pesky 2.5... to do this let's setup a parallel circuit and use that to to compare with. Put two 350ohm resistors in series and measure the voltage between the two. Now it's only a change between 0 and 0.003.... well, now the change is obvious, but it's still very small, let's make it bigger.

Run the two signals through voltage followers so that the signal, which is very small, won't be impacted by the amplification. Now let's subtract and amplify the signal. Run the voltage followers into the subtractor, and amplify it by 1000x. To do this connect one voltage follower to the positive via a small resistor, and then to the ground by a resistor 1000x larger, and the other to the negative by a resistor, and that one to the output by 1000x greater resistor.

Snap for arduino is a great way to graph the output of the sensor, and a good place to start if you've never programmed before. Alternately, you can use just the arduino IDE for very high speed / high accuracy output.

## Step 3: Build the Plate

Now that we have a circuit and displacement sensor, let's take the final step and make the force plate. There are a ton of ways to do this, but we want something cheap so you can go to any hardware store and get some pipe insulation which is very springy. Using this as a spring we should be able to use hooks law F=kX, to determine the force on the plate.

Take your insulation and cut it up. Typically it will have a slit down the side, and cut it into similarly sized cylinders. Depending on the strength of the force you want to apply, you may want different areas. For example, if you're learning about boxing and force, you may want a smaller spring area, but if you're making a platform to measure jumping you'd want a larger one. Make sure that your springs are about as tall as your displacement sensor. Alternately, if you only care about forces > than a certain size, you can make it slightly smaller.

When you have your springs, place your sensor between them, and place a box or stiff piece of plastic over the top. If you're going to be punching it, make sure to put a bit of padding on the top just in case.