Need an impact sensor for project 1

[masamune]

Hi everyone,

I am looking for a device to provide a signal output to a microcontroller (or maybe a PIC16f876/ etc) to get a useful reading to evaluate force exerted on a pad mounted on a stand.

The idea is that a martial arts practicioner can strike this pad (and several others mounted at differing positions) and a simple display will show the relative strength of his/her strike. The stand is ideally fixed with hardly any oscillation and 'give'.
I just want to be able to calibrate the reading based on a benchmark strike or perhaps derived from a table or equation.
I realise that force delivered can vary due to striking 'through' target or just 'at' it.
Evidently one black belt has been said to deliver 2800 Newtons into a board @ 14.0208 ms-2 (does that seem right? seems a little slow....)
The pads will weigh .5kg (roughly)

The main problem is finding what will do the job.
I have looked into an ADXL 150 (single axis +/- 50g) and an HBM B12 acceleration transducer (nominal 200ms-2 up to 1000ms-2 up to 250Hz).


Would these be suitable?
Does anyone have experience in such transducers and any advice?

any help is much appreciated,

Thanks

 

[IRstuff]

http://www.findarticles.com/p/articles/mi_m1511/is_5_21/ai_61692484

You might try something like:

http://www.tekscan.com/

http://www.sensorprod.com/pressure-mapping-system.html

from:

http://www.dogpile.com/info.dogpl/search/web/foot+force+tactile+sensor

TTFN

 

[sreid]

Go to Analog Devices

http://www.analog.com/

and look at their MEMs accelerometers. Small and cheap. For testing you could by their development boards.

 

[masamune]

Interesting,
Thanks for the reply. The ADXL 190/3 seem to be new additions to last years. I picked up an ADXL150 and didn't get around to using it as I didn't like it's max g rating.

I might grab a few ADXL190/3s...

Any tips on interfacing, reducing noise, effects of vibration, powering, A/D conversions etc ?

I haven't much experience on interfacing these sorts of devices to embedded controllers, just a bit of theory and low-end practice (keypads, displays etc)

Cheers

 

[Warpspeed]

Hmmm interesting problem. How about fitting a pressure transducer to one end of a flat coil of rubber tubing beneath a flat leather pad ?

When you strike that, it would have a bit of give, and it would measure the peak impact pressure into a "human body" target. That might be more Representative than total energy or acceleration.

It would be fairly easy to make, low cost, and reasonably indestructible (hehehe). Just measure and display the peak pressure achieved should to do it. And it could easily be calibrated with a static weight.

A microprocessor with a continuously sampling analog to digital converter could very easily display and hold a peak pressure reading, it could auto zero, and all you need is a reset to zero button after each hit, or it could time out automatically.

Interesting project.

 

[masamune]

That's a very good idea.
I was thinking along similar lines, as I don't think acceleration is very representative of how much "power" is actually in the punch i.e. how much resistance is needed to stop it.
I would need a transducer that can handle, with accuracy, low power to very hard punches...

I am not exactly sure of the setup you described though, there has to be a totally sealed space between the attachment to the striking pad and the input of the transducer.
Could you please describe what you meant? I'm not sure how the deflection of the pad will change the pressure in the line...

I envisaged a hard rubber target, which is glued to a piece of sheet metal then attached to some reasonably sturdy contruction plywood, this would then be fixed to the frame.
This would have an air-line input into the transducer, which is mounted in a fixed position?
I'm haven't used these sensors before and they aren't all that cheap, do you recommend any?
I found that Omega had a good range, but most sensors were expensive, especially loadcells.

http://www.omega.com/ppt/pptsc.asp?ref=PX209_PX219&Nav=preb03

 

[IRstuff]

I still think that you need a pressure mat. One problem with any single sensor is thatyou will only measure force or acceleration in the axis of the sensor.

If the force is off-axis, you'll get a lower number. Unless the user is extremely consistent, your measurements will have huge variations.

TTFN

 

[Warpspeed]

I would just use a long length of ordinary soft rubber tubing with one end plugged, and a pressure transducer fitted into the other end. The whole length of tubing could then be coiled into a flat spiral, and attached to something really solid, how about a thick steel plate glued to a concrete slab ?

For a suitable low cost plastic bodied pressure transducer, perhaps something like this :

http://content.honeywell.com/sensing/hss/pressure/pdf/scc.pdf

A similar idea is often used to count passing vehicles by placing a rubber tube across the road. Just coil up the tube. It is probably the simplest solution.

 

[masamune]

IRtuff - I think a pressure mat may be an expensive way to go? as I will need several of them. I am not up to speed on all of the available mats, but aren't they more juston/off type detector pads rather than proportional transducers?

Warpspeed - I know what you mean now. It sounds similar to a bellows-type arrangement, where the air inside a "container" is displaced to feed the sensor.

I need to design this so that the resistance of the rubber and the displacment of air is proportional linear to the amount of force applied i.e. axial force insensitive so that all the force applied displaces the air and doesn't get directed into the framing or needs to overcome the "springiness" of the rubber air vessle.
I thought perhaps strips of vertically running rubber, but to have them "feeding" one sensor or input involves some construction issues.
The concrete slab thing might get a bit heavy to lug around as well.
Maybe I should get a bagpipe or accordion specialist to design me a special displacement device vessle....

 

[IRstuff]

It depends on the type. There are strain gage pressure mats that should be fairly inexpensive.

The problem with single detector and/or balloons is that you cannot tell whether the force was correctly applied.

As you know, technique trumps strength. If the student applies lots of force ineffectively, it's worthless. In the one form of karate I'm even vaguely familiar, Shotokan, the forced had to be directed through the knuckles of the fore and index fingers.

Presumably, if you had a pressure mat, you'd be able to see the distribution of force and whether it's focussed through the two knuckles. Moreover, with the pressure mat, you'd be able to determine the corrective action, based on the force distribution as well as the magnitude.

TTFN

 

[ScottyUK]

You could look at the industrial accelerometers from the likes of Bruel and Kjaer. Unlikely to be cheap, but may give you some additional avenues to explore.

http://www.bksv.com/

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[masamune]

I agree that the target can still be hit incorrectly from a technical point of view, but as long as the total displacement of air can be related to the force generated, then that is ok.
I know some people will not be able to strike properly that the way they strike will have an effect on how much force is translated, but this isn't for the market (yet), so I just want to get it up and runnning.
you seem to be keen for the pressure mat, do you have one product in particular? i seem to only find the safety ones..

ScottyUK - I was thinking of acceleromters but I don't think that will give me a good reading of force considering the time involved during the strike, I may still come back to it though.

 

[IRstuff]

Not really, it's just the pure scientist in me; there's never enough data ;-)

TTFN

 

[Warpspeed]

Not sure what effect it might have, but the coiled rubber tube idea could always be filled with some sort of fluid other than air. That might transmit impact force much more directly, with less actual crushing of the tube needed to do it.

I suppose how hard it feels when you hit it matters a great deal too. It would not want to be either too firm or too mushy, but feel "just right".

It must be forty years since I last bowed in a dojo. How time flies.

 

[ScottyUK]

masamune,

Applying good ol' F=M.a, and mass being constant, this might be a relatively easy calculation. It depends on whether 'M' can be given some freedom of movement to give a measurable 'a'.


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[masamune]

Hi Scotty

The mass of the target is definitely fixed, but the user can vary their "seen" mass by using their whole body or just their arm, wrist, whatever, which is why I was thinking about another sensor. Punching technique varies by quite a lot, so I need to get an total force reading.

It may get expensive to use a pressure transducer, but I think it is the best way to go (for now), I just have to come up with a good idea to translate impact force to the sensor....

 

[IRstuff]

The previous idea floated of a balloon-like target is probably the easiest to implement to mitigate the directionality of the punch. You could probably make it like the round target gloves I've seen.

An additional thought, you should make the sampling rate at least 2 kHz, maybe more, to adequately sample the transient behavior of the punch. My understanding is that you do not want a "follow-through" for the punch. Instead, you want to punch and retract almost instantaneously.

TTFN