# Measure impact force on a surface

Hi. I'm new to the forum, so I apologize in advance if I'm breaking any rules.

A friend and I are making an art project that is basically like one of those "Test Your Strength" games at a carnival, where you hit a pneumatic thing with a sledge hammer as hard as you can and send a ball up a tube to ring a bell at the top. The difference is, we're trying to do it with an accelerometer built into the striking device, sending the data wirelessly to a controller which will light up some LEDs based on how hard the player strikes. For the sake of the game, we can't attach anything to the object being struck, only to the striking object (for reference, picture a foam LARPer sword hitting a moving person.)

We have successfully gotten the accelerometer to talk to the controller, although our first iteration wasn't wireless. We're taking the average acceleration across all three axes and using value = sqrt(x^2 + y^2 + z^2) as our reading. So we can successfully find max acceleration, but what we really need to do is find periods of sudden deceleration, and we don't know how to do that.

We don't actually need to measure force itself, just force relative to a hard strike or a soft strike, which we can calibrate to once we know those values. But we're having trouble figuring out how to deal with things like backswing and bounce, and figuring out what measurements to actually take in order to find a period of sudden deceleration, and determining when to take the measurements. Do we start a timer when we detect acceleration above a certain threshold and then wait for the motion to change suddenly? If so, how do distinguish between an actual impact and someone just flicking it back and forth?

Has anyone worked on something like this? Is it even a good idea to use an accelerometer for this, or is there some way of using a force-sensitive-resistor that's actually accurate enough and tough enough to withstand repeated impacts?

Thanks for any help!

Can you log the data you are currently getting, e.g via the serial terminal, then plot it e.g. in Excel

Actually, you may need to log to ram or SD card and then read the data off the SD card.

I suspect what you want to know is the maximum force in the positive direction, but until you can via a plot its hard to know precisely the best approach

what we really need to do is find periods of sudden deceleration

Deceleration is exactly the same as acceleration. The difference, which is semantic, is that one acts to increase velocity, the other to decrease it. The only way you can tell is to look at the directions of the velocity and acceleration vectors (they are completely independent of each other).

Incidentally, it is possible to have constant acceleration but no change in speed, the magnitude of the velocity. This is the case for something moving in a circle at constant velocity.

A 3D accelerometer is ideal for your purpose. You just have to learn to use it properly, which includes monitoring acceleration direction as well as magnitude.