DIY force sensor / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

DIY force sensor

previous 1 2

Move Thread

LAN_403

Dr. Slack

Fri Feb 28 2014, 09:06AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

@klugesmith ... or you could always simulate a wooden board with a bit of wood

Thinking about making the instrumentation fit the task - do you want the force on a board that is rigidily clamped at the ends, or one that is being held loosely so that it moves away when struck? Not doing martial arts, I'm not sure that there's an obvious answer, and I can think of situtations where either might be the desired measurement, and where fast/light and slow/heavy would give different ranking results for the two situations.

If it's the first, to a rigidly suported board, then a 'plank' made from two strips of plywood, bolted together spaced with seperators at the ends to create a gap between them, with a capacitive transducer across the gap, could work well. The reference for the sensing spring is the 'inifinite' mass of what supports the board.

If it's the second, then I guess you're measuring acceleration of the target itself, and you could do worse than to simply buy one of these MEMs accelerometer thingys to stick on it. Attached rigidly to the back of the target, the 'low mass' and 'high mass' ends of the sensing spring in it are swapped. The important point is the ratio of the masses is still very large, so that the deviation of the large mass from 'inifinity' is not important.

If it's somewhere in between, say a board held by a person, your acceleration measurement will have a significant dependence on the dynamics of the restraint they can apply, and you won't be measuring the kick in isolation. Think very carefully about how you can deploy a sensing spring, and what its deflection means, in this case.

You might want to make a mechanical 'kicker', with some elastic bungee straps and a bit of 2x2, to apply repeatable impacts to your measurement system.

Dr. Slack

Sat Mar 01 2014, 03:20PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If good accuracy is not too much of a concern, I offer up the following suggestion. However reading back along the posts, it's the same geometry and essentially the same principle as Shrad's suggestion in post #4, but using capacitance rather than resistance.

This is a plan sketch of the force transducer. Yellow is the target plank or board, presumably held up by the person who picks the shortest straw. Green and gold is copper covered PCB (or any conductor on any insulator for that matter, alli foil, cardboard). Grey is a thin sheet of resilient foam, this is the spring. Black is flexible wire or tape. The use is pretty obvious, when the earthed top cover is kicked, the foam gets compressed, and the capacitance from the sense electrode to the ground electrodes increases.

Advantages - very quick and cheap to put together, probably very well damped depending on the foam qualities. Easy to iterate, change dimensions, change materials.

Disadvantages - not linear output with force, and depending on the foam chosen, possibly very bad drift under constant load, so attempting to calibrate it with a weight would probably be doomed to failure.

I suspect bubble-wrap (air in LDPE) would have a much lower drift under load than a foam, though it would still drift with temperature, so it could be calibrated with a weight before and after use. It might require some pre-load, unless you just accept that it's very inaccurate when it's not loaded.

Earthing both the outer and inner large electrodes is important, as you want the sense electrode fully shielded so that the approach of the foot doesn't change the capacitance, only the contact.

How to sense the change in capacitance? Make it part of an oscillator, or part of a Wheatstone bridge, or part of a potential divider. With the latter two, sensing the changing output amplitude on a sound card would be a very simple method to turn delta C into distance.

Sulaiman

Sat Mar 01 2014, 04:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

How about a ballistic pendulum to measure the energy of the kick ?
( I think that energy goes to bone breaking and organ bursting,
momentum goes to knock-down etc.)
hang a (strong) sack full of sand from a rope and use ballistic pendulum equations to derive impact energy.

With a camera and strobe you could work out
the initial acceleration to give a measure of the force applied
OR
an accelerometer in/on the sandbag would give a good indication of force
OR
The sand bag would be a good place to attach your force sensor
simulating a real blow to a moveable deformable body.

P.S. you could use different size sand bags to simulate head, torso etc.
Since force, energy and momentum are involved
I'd expect to see a different force measurement for different sandbag masses
and also different results for dry (easily deformable)
and wet (less easily deformable by impact) sand ....
also
instead of suspending the sandbag on a rope you could
attach the sand bag to a wooden plank (or other 'springy' thing)
and use a strain gauge .......

Newton Brawn

Tue Mar 04 2014, 02:38PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Yes, as Sulleiman advices, a balistic pendulum appears to be a good choice.

Napoleon Bonapart has used the ballistic pendulum to measure the energy of cannon projectills. (1800AD)

THE CANNON IS PART OF THE PENDULUM, and the projectil is released free in the space. If you know the mass of cannon and how much the cannon goes back you can calculate the energy given to the projectil !
In others words you are going to measure the energy stored in the cannon, that is the same that is stored in the projectil
Give us detail of your machine....

Dr. Slack

Tue Mar 04 2014, 04:32PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you know the mass of cannon and how much the cannon goes back you can calculate the energy given to the projectil !
In others words you are going to measure the energy stored in the cannon, that is the same that is stored in the projectil

Nope. The momenta of the cannon ball and cannon are equal and opposite (neglecting the momentum of the ejected gas), but the energies are different. As the cannon gets heavier, its absolute energy decreases. As the mass of the cannon tends to inifinity, the absolute kinetic energy it ends up with tends to zero, but with the same momentum

kimbomba

Tue Mar 04 2014, 05:45PM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

Thanks all for the great advice, the ballistic pendulum seem to be a great solution, i will definately give it a try.

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob