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TucoRamirez
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« on: September 10, 2013, 09:23:51 21:23 »

hi, i want to design a 'strong man' game (you know the one with the hammer to hit in a surface then you have a barmeter...
to develop it with a lot of students in our lab course.
as the first thing i'm looking is a cheapo solution ( cheapo force sensor or strain gauge ) to start with.. (because students will destroy tons of sensors (from personal experienc !)  and my responsables don't want to buy tons of sensors to dump it by damage...

I was searching a little bit and i found resistive force sensors (FSR family from interlink electronics)   but as long as i have no personal experience with i would like some advice of the best solution to perform a decent  system.
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Checksum8
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« Reply #1 on: September 10, 2013, 10:42:22 22:42 »

Here's an idea for a cheap solution and good learning experience.

I remember seeing a show on TV where they were studying the biting force of sharks. The scientist sandwiched a square of hard conductive foam between to squares of copper clad pcb. Then measured the resistance between the two copper surfaces. The higher the force the lower the resistance. You could calibrate it by placing known weights on it. Protected it with outer layers of plywood.
« Last Edit: September 10, 2013, 11:18:16 23:18 by Checksum8 » Logged
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« Reply #2 on: September 11, 2013, 07:00:52 19:00 »

another idea could be to use a similar solution as they use to meter flow of gas when welding with a small ball
in a tube measuring height in it. looks like this. where u measure the hight to measure strenght.

http://www.thermalmassflow.com/variable-area-flow-meters.html
http://www.amazon.com/Argon-Flowmeter-Regulator-Welding-Welder/dp/B00CBIZWY8
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TucoRamirez
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« Reply #3 on: September 11, 2013, 09:48:30 21:48 »

looks like an alcohotest ^^

Good stuff anyway but not cheapo ^^ :p   (but i'll take note for doomsday devices)
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solutions
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« Reply #4 on: September 12, 2013, 07:08:13 07:08 »

Two parallel plates with a flat silicone sheet to hold them apart (they have sheets used for cooking/baking that would be perfect). Piezo disk sitting in a well (if necessary) in the lower plate.

The next trick is to figure out how to drop or transform the high voltage it'll generate

Second option is to forget the piezo and simply measure the capacitance between the two plates...fast measurement....

Also, you could conceivably run magneto-constriction backwards
« Last Edit: September 12, 2013, 07:11:31 07:11 by solutions » Logged
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« Reply #5 on: September 12, 2013, 07:51:46 19:51 »

I built a very similar device some years back, it is for a "basketball passing contest" - contestant do passing (throwing the basketball with two hand started with the position near the chest) to a target (about 50x50cm) from a distant, the target has a sensor at the center which measure the force when the ball hit it.

It is very similar with Solutions first idea, two plates of 6mm phenolic / pertinax with casted silicon rubber in between - in the center sits a load cell taken from a weighing scale, fixed to the lower plate.

It works like a charm, many people had fun with it...  Cool

-Ichan
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Parmin
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« Reply #6 on: September 13, 2013, 12:34:09 00:34 »

Hum

E=½mv²  


You got a finite mass, so all you got to do is to find the velocity.
And to do that you can use an accelerometer.

so, either stick an accelerometer on a large block of wood (for the hammer strike area), and measure the displacement,
or stick it in the hammer to measure the velocity.

I rather use it on the hammer, and log all the velocity changes for the swing, and get just the last reading before it reverse itself (due to rebound) or stop.
Just put it in a box and screw it onto the hammer handle near the mass.
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« Reply #7 on: September 13, 2013, 01:45:22 01:45 »

a long time a go.. we made a "ring the bell" fun fare game, for a charity fund raising event.
we used a stylus from a record player to respond to a hammer hit on the peg.
an array of op-amps detected the level to display the power delivered to the peg.
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solutions
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« Reply #8 on: September 13, 2013, 05:40:55 05:40 »

What the heck is a "stylus", ya old fart?   Tongue
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TucoRamirez
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« Reply #9 on: September 13, 2013, 08:49:57 08:49 »

hahahah Stylus = record player cartdrigde =  Needle  +  piezo mumbo jumbo i never opened ... ohhh but i have no time machine to buy that
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« Reply #10 on: September 14, 2013, 05:27:55 05:27 »

Ok back to topic.

The easiest way is to use analog output accelerometer such as ADXL335, and hook this up to appropriately set bargraph driver such as LM 3914..

And, there you go, the higher the acceleration, the more LED turns on.. 


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