Use the wii for a science fair project?

Greetings,

My son is doing a 7th grade science fair project involving the measurement of force (He wants to measure the force of various kicks in karate using a big rubber "bob" training dummy. We're wondering if it might be possible to use the accelerometer in a wiimote to do it? You all seem to be WAY smarter than I am, so I thought I'd ask if you all have any ideas... Thanks!

Well, I will just think out loud here. Maybe something I say can be helpful to your son...

The accelerometers don't measure the force, they measure the speed of movement. However, in order to use them to measure the speed of a kick, it would have to get attached to the foot, which takes me to the following:

1) Would the wiimote break with the kick?
2) Might the wiimote harm the person who kicks with it attached to the foot?

Still, if that didn't happen, quick kicks would mark greater results than strong kicks, so it wouldn't really be measuring what is purposed to be.

In order to measure force, a pressure plate is required, like the ones contained inside the WiiFit balance board, however, I think that the balance board is too hard to be kicked :)

Some years ago, in the arcades, there were some games where you had to punch a dummy with a glove, and it measured the force of the punch in pounds. I'm trying to remember how it worked, and I recall the dummy was pushed backwards with the force of your punch and it smashed a pressure plate behind it. A similar concept could be achieved with the wiimote if it is placed inside the dummy, but then the dummy couldn't be fixed, it would have to be able to be pushed back when kicked, so it creates a movement backwards based on the force of the kick.

In this scenario the speed with which the dummy moves back would be directly proportional to the force it was kicked with, so the accelerometer could measure this speed and by doing some conversion (maybe a table for this would need to be created) the force it was kicked with could be determined.

Still, I believe the best idea is to use pressure plates instead of accelerometers if what you want to measure is the gross force :)



Edited 2 time(s). Last edit at 10/04/2010 04:05PM by Axel.

Axel,

Thank you! I input is greatly appreciated. Looks like he'll have to re-think it. I wonder if we could mount a Wiimote on the back of the "bob" - it's a soft rubber form on top of a heavy plastic base - it does move to some degree when hit with enough force... maybe use the pressure plate idea - if we could mount it in a stand so it just makes contact with the back of the Bob, when the Bob is kicked, it would push back into the balance board. Not sure how he could measure either one - if you have any thoughts they would be - like your previous input - greatly appreciated!

If you want to see what I'm referring to, I've included a link to a "Bob"

Thanks again!

You should be able to mount the wiimote to the back of the bob; I see every reason why this would work. However, to my knowledge, the software you would need to measure and output the data from the wiimote does not exist. Writing such an applications should be very easy, and if there were any difficulties, most of the necessary code probably already exists in other open source applications anyway. Therefore, if you have any C/C++ experience, you probably will have no trouble doing it yourself. Otherwise, you will need to find someone else to do this for you.

Yeah, I also believe the Wiimote can be fixed on the back of the Bob, however, the dummy Bob will do need to MOVE back when kicked, otherwise the wiimote will not detect any movement speed.

The problem I see is that 7th grade kids might not be able to kick it strong enough to make it move :(

Also, the more the dummy moves, the best the accelerometer will work. Thinking on this, I could suggest using a punching sandbag dummy hanging from the ceiling. That would definitely move when kicked an enough distance to be detected by the Wiimote. Just a thought.

Doesn't the accelerometers measure... acceleration??? If that's true (which it is), you could go with the sandbag idea. That is, attach the wiimote to a sandbag with known mass, then kick/punch it, then obtain its acceleration from the wiimote and then calculate the force with this simpole formula:

F = m * a,

where F is force, m is mass of the sandbag plus the wiimote and a is the acceleration of the wiimote.

heh... the bob dummy will move. :-)
Pretty sure my son has enough technique to move it enough to measure - and he wants his Sensei to do the kicking... it may be more a question of not knocking over the bob and/or breaking the wiimote/board. If you lighten the base, it will move easier than a hanging bag. Like your arcade example, the pressure plate balance board mounted on a stand-type-thingy behind the bob sounds most feasible to me - but it's not my project.

I would have to find out exactly what and how the pressure plate works - what it measures. Once I know what it does, I can see if I can find someone to help with the data collection and output part. I know enough to know I don't do C/C++ ... For me, it would be more fun to build it than code it. But I know folks who have more fun with the code part.

Thank you again for all of your input!

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profetylen
F = m * a

How could I have missed that! :)

Yeah, it is the more general formula of Weight calculation: W = m * g , where the gravity is a constant for acceleration.

I guess the wiimote can then be used with no problem to calculate the force of the kicks, as long as the dummy moves and you know its mass. It does not matter if it falls back, it is the initial movement acceleration the only one which is required.

Regards,



Edited 3 time(s). Last edit at 10/06/2010 05:27PM by Axel.

BTW, there's a much simpler (as it doesn't require any wiimotes (which also makes it less fun, but since it requires more calculation it is also more fun)) way of measuring force if you have a sandbag of known mass. You could kick/punch it and then observe the height difference from its starting position (with eyes, a camera or an attached wiimote). Then you could calculate the energy obtained from that kick/punch through:

E = m * g * h,

where E is the energy, m is the mass of the sandbag, g is the gravity of earth (around 9.81 m/s²), and h is the height difference of the sandbag.

Then use E to calulate the initial speed of the object through:

v₀ = sqrt(2 * E / m),

where v₀ is the initial speed of the sandbag and sqrt means "square root of".

Acceleration a will equal initial speed divided by the time, t the acceleration occurs:

a = v₀ / t

You to estimate a good value for t. A guess (based on a vague memory from physics class 3 or 4 years ago) is that a good estimate is 0.1 seconds.

Then you can obtain force, F with the previous mentioned formula:

F = m * a

Edit: A fatal error in the method was fixed.



Edited 1 time(s). Last edit at 10/07/2010 12:49AM by profetylen.

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profetylen
BTW, there's a much simpler (as it doesn't require any wiimotes (which also makes it less fun, but since it requires more calculation it is also more fun)) way of measuring force if you have a sandbag of known mass.

That is absolutely true, but it is also true that that would never make a science project, since all of that are already proved facts. So instead of doing a science project they would be making a demonstration of applied physics. :)

I think the idea of using the wiimote is pretty neat, and indeed it can be used to obtain the initial acceleration of the object (either it is a sandbag or a dummy bob), since as you mention the initial acceleration is instantaneous.

And making a wii application that registers the initial acceleration through the accelerometers of the wiimote, performs the conversion and displays the resulting force in screen would do for a really attractive science project in the school. Don't you believe? :D

For the record: The pressure plates do measure directly the force applied, so no conversion would be needed. However, now that I think of it, in the arcade machines I mentioned, after punching the dummy, while it made its way back to the pressure plate some energy was lost, hence the force displayed was less than the one from the original punch. At the end that didn't even matter much as it was only a game. ;)



Edited 2 time(s). Last edit at 10/07/2010 12:07AM by Axel.

@Axel: Yeah, you're probably right.

However, while eating my nightly sandwiches before bed, I started thinking and realized that the latter method I described contained a flaw; Acceleration is not instantaneous, only close to instantaneous... and if it was instantaneous, then it would also be infinite (since t, as is now described in my previous post, would equal 0.)

Edit1: Spelling
Edit2: Forgot to add "Edit1: Spelling"



Edited 2 time(s). Last edit at 10/07/2010 12:57AM by profetylen.

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profetylen
Acceleration is not instantaneous, only close to instantaneous... and if it was instantaneous, then it would also be infinite (since t, as is now described in my previous post, would equal 0.)

Yup, that I also correct and I was thinking in that (it appeared to me that the /s you added to the formula should actually have some value), but I am not sure if t tends to 0, since the closer t gets to 0, the greater the acceleration becomes. For t = .0000001s we would have a really huge acceleration value.

Maybe a negative acceleration could be calculated, with ( V_f - V₀ ) / t, where t is the time the dummy takes to reach the highest point. However, I am not sure either, since we might be entering into the angular acceleration field, given the sandbag will describe a curve as it is hanging from the ceiling. I don't remember how to calculate angular acceleration, tho. It's been a while since I studied vectorial calculus :)

Perhaps just obtaining the initial acceleration with the accelerometer of the wiimote is the best idea after all :D



Edited 1 time(s). Last edit at 10/07/2010 01:17AM by Axel.

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Axel
Perhaps just obtaining the initial acceleration with the accelerometer of the wiimote is the best idea after all :D

I agree :D

This is the type of project I want to do for my science fair project. Did the wwiimote work?