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DIY force sensor

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kimbomba

Thu Feb 27 2014, 02:43AM

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

Hi, I need a force sensor for a thesis project, forces are on the order of 10,000 newtons for a fraction of a second. Precision does not need to be high, 20% error is acceptable. I wonder if there is a way DIY way to make a force sensor. Anyone has experience on this? Thanks in advance.

Patrick

Thu Feb 27 2014, 05:58AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

There are load cells that can easily do this but if you want DIY, then you may be best looking at predictable deformation type system, like a bending lever, with scratch or ink marks , then calculate out the result...

Doesn't piano wire get to 10 mm in diameter?

Sulaiman

Thu Feb 27 2014, 06:11AM

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

for a wider variety of suggestions, a little more info. may help;
is the force from an explosion, or a ballistic mass, electromagnet etc.?
what do you mean by 'a fraction of a second ?
big difference between 500ms and 1 ns etc.

Shrad

Thu Feb 27 2014, 08:00AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

my idea for pressure sensors has always been the following :

antistatic foam with a given resistivity per unit of width will see its resistivity reduced by the amount of compression it takes

add two copper foil electrodes to each faces of a maxim sample pad, vacuum pot it with soft silicon, and you have a resilient pressure sensor

this is yet to be tested ;)

must I specify this design is under intelectual property and I revendicate any share of the business done with my idea ;)

Dr. Slack

Thu Feb 27 2014, 09:16AM

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

The general 2 stage principle I follow is
a) use the force to deflect a spring of some type
b) measure the deflection in some way

Springs can be anything from a long bit of steel wire (very compliant) to a short fat block of the stuff (very stiff). Alternative materials for springs - GRP, glass, BeCu. There are several geometries which will give good things like more or less linear motion, a positive end-stop to avoid over strain, and cutting slots and drilling holes to change the compliance over a large range. Choose the material for availability, cost, creep (no plastics if creep is important), your ability to work the stuff.

Once you have the deflection, then how you sense depends of what form you want the output to be and how fast. A capative transducer, varying the capacitance in an RC oscillator, the frequency counted by a uC, is a very cheap and cheerful way, if you are into uCs. I'm not, but am into Python, so would use the same oscillator front end, but detect the frequency on audio in with PyAudio and scipy. Laser pen pointed at a sliver of mirror on the end of the spring, no electronics involved, is very handy for manual use and absolute calibration. A few others worth mentioning - magnet and Hall sensor, LVDT, and of course a stick-on strain gauge.

You have some tradeoffs in how stiff you make the spring

Very stiff - high bandwidth, good linearity, but need extreme sensitivity in the motion to reading sensor which may then have poor drift

Very compliant - eases the drift and gain requirements on the motion readout, but may be non-linear, and will have a low bandwidth, may need extra damping to avoid being bouncy

Goldilocks - somewhere in the middle, optimised for your particular conflicting requirements

kimbomba

Thu Feb 27 2014, 06:10PM

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

Sulaiman: I plan to measure the force of a tae kwon do kick.

Patrick

Fri Feb 28 2014, 12:44AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i suggest a Greek pancration gastriza kick... you might be able to cripple a man with one good strike.

so now that we know what your measuring and why, duration, total displacment from initial contact, and force will all be needed to evaluate relative degrees of harm to your would be human target.

Electra

Fri Feb 28 2014, 01:03AM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

Perhaps a piezoelectric sensor might work for something like this. They wonâ€™t be able to measure a static load, only dynamic force but lookâ€™s like thatâ€™s what your only interested in. most types seem to be for compression or bending forces, so youâ€™d have to transmit the force from a large pad or whatever was the target, through the sensor to the support fixture.

By the way do a little bit of kickboxing myself,(I know not exactly the same) if someone is strong and trained well there is a lot of force in one kick, been on the receiving end enough to know it does hurt.

BigBad

Fri Feb 28 2014, 02:46AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

You could attach an accelerometer to a mass and then measure how fast it accelerates when struck?

klugesmith

Fri Feb 28 2014, 03:10AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

How about simulating a wooden board with a similar-size piece of tough plastic, with a well-chosen thickness?
Support it as you would a board to be broken with a kick.

Measure the (reversible) bending deflection of the plastic board.
That could be done with strain gauges, proximity sensor, etc.

It can be calibrated by supporting the board between bricks and standing in the middle.

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