Prototype Project - Looking for some help

From your description you don't measure impact, which is a very demanding measurement in terms of amplification equipment and the accuracy required in calibration. You measure force applied, on static known mass, completely different animal. For this an accellerometer with peak recording instrumentation will do the job. Just a notion (from lifetime in test department of aerospace industry). And NO, am happily retired.

Have fun

Stanislaw

Stanislaw,

Thanks for the reply. The machine is stationary though so I don't know if an accelerometer would work for this application. The sensors I'm looking to use would be of a variable resistance type so it would measure impact force unless I'm not understanding what you mean?

Basically I want to identify that there was an impact, and how hard that impact is. The system will allow for setting a target force so that if the user hits the machine and the force is less than the set target, it will either not register the hit, or notify the user that the hit was insufficient by means of some analog signal being a light or sound.

The system needs to record the force of the hits so it can create statistical data and do some other calculations that I would like produced.

I'm throwing around the idea of creating two systems, one that only has generic contact sensors and one that uses the variable resistance force sensors.

- Jace

"Strain gauges?" would do if: the 'impact' shock wave moves along the test sample without degradation and is longer than the strain gauge dimention, the attaching glue transfers this in true form so NO shock absorbing action takes place. Otherwise your readings recorded and reality have nothing in common.

If you insist on 'impact' as your measure stick better find someone experienced in this kind of measurements. And such maybe rare like chicken teeth.

Good luck

Stanislaw

Stanislaw,

I've been doing some research into Strain Guages and I don't know if it would work for my application only because according to the description on Wikipedia, "as the object is deformed, the foil is deformed, causing it's electrical resistance to change", and the object the user would be hitting is a 8" diameter steel vertical cylinder. Unless we're counting micro-deformations, the "object" shouldn't deform. But I understand the principle, it's the same as the piezoresistive force sensors just instead of using a chemical pressure sensitive ink, the strain gauges cause variable resistance based on the shape.

I know this goes against most all electrical engineering ideals, but this system doesn't have to be super accurate. Even rounding to the nearest pound or point of a pound of force would be sufficient. Hence why I said I'm considering not using force sensors at all for one model. I've contacted Tekscan and described what I needed thoroughly with them and they said that their product is 100% suited for my application. The only problem is that I can't afford, personally or as a per-unit cost, to have a custom run of 10,000 sensors made to my size requirements and their standard "off the shelf" sensors are the size of a penny and I need a contact area of roughly 6" tall by 24" wide. They have some leftover sensors from the company that makes those force sensing brake pedals so that trailer brakes can be applied in addition to the truck brakes, but they don't make a habit of selling other people's leftover custom sensor designs, and those are only 3"x5" anyway.

Basically it comes down to cost versus accuracy. I need to keep the per-unit cost of my system down under the $1k range to make it affordable for my prospective clients but I want to give them the best experience for their buck. For what I'm doing, there are other products on the market, but in my opinion, they don't fulfill the needs of their (our) target market.

And just as additional info to take into consideration for the cost projection, I need either 15 6"x24" sensors or 45 6"x8" sensors to complete each unit. Using Tekscan's estimate of about $25 per sensor (for the 3"x5" of which I would need about 90) I would be spending over $2k just on sensors, not including the equipment to process, record and control them or the cost to actually have them built.

So if you, or anyone else, has any recommendations for some way I can inexpensively measure the force of a hit, either some kind of rig to use 2 or 3 small piezoresistive sensors that can accept accurate readings from a 6"x24" curved contact area, I would appreciate any ideas.

Oh, one more thing, I thought of this last night.

What if I had some kind of extremely thin rubber air chamber created that could wrap around the steel tube and put an air pressure sensor on it. So when it's impacted, the force of the air could be measured and it wouldn't require expensive piezoresistive sensors?

Just a thought.

- Jace

Based on this and previous posts you could just as well use a microphone !

Baron,

Not sure if that's a joke or not? If not, what do you mean?

- Jace

No ! It wasn't. Impact implies vibration=noise. Something acoustic ?

Ok, that makes sense but do you think a system could be setup to use the vibration to detect the force though?

The padding would absorb some of the impact and depending on what direction the device is hit from, it might not register the same force if it was from the front or the side?

I love the suggestion and would be interested if anyone could elaborate how something like that would work. If that is a plausible solution, it would remove all wearable parts from the impact surface which would lower maintenance costs considerably considering the machine is literally getting beaten to death.

- Jace

Well since there selling you something, they would, wouldn't they ?

Just what is the target market ?

Flying by the seat of my pants here...... You describe a hollow cylinder! Is it closed at both ends ? What does the cylinder contain ? How is it mounted ?

You describe that it is hit (Impacted.) Your words. In what way ? from what direction ? What with ?

It has meaning for me;^) Baron went the physical way, what physical quantity you want to record, in what accuracy, and suggested a "sensor?" that responds to those requirements. My mistake was taking your description of _impact_ literally, and when you gave your proposed dimentions of areas involved, I comprehended that "impact" is nowhere in neighbourhood. The 'normal' impact wave in any kind of solid is much smaller than those dimentions. So you are measuring force, transitional or static, so use a load cell ( four strain gauges) and get the signal from that. Easy. (On 100.000lb load cell we ?weighed? a 60kg person to 1kg precission, and recorded a hammer hit "impact"; just for fun) But the equipment we used is out of the use for casual customers, our sworn accuracy was 1% or better on static _and_ dynamic readings. Full stop, and the design people sometimes required better answers and got them.

Have fun

Stanislaw

Stanislaw,

I'm not sure I'm 100% on the same wavelength as you. I did mean impact in the literal sense. I am going to physically whack the machine with a blunt object, be it a fist, foot or weapon of some sort. The area that would be struck can be anywhere on the circumference of the steel pipe that is facing the user, hence why I can't use a single small sensor at a specific point. It would be as if you wrapped a parking meter with sensors and then started wacking it with a stick. Inside the parking meter would be the measurement equipment.

Baron,

The target market is for entertainment / sports / fitness applications.

The cylinder is hollow, standing vertically with the circuitry inside and is capped at the top and the bottom is fastened to the floor. It is hit with a large blunt object. The user can choose to hit it with their fist, foot or use a weapon, most likely a staff or some sort of club. Think of a bop the weasel game but instead of the weasel you're aiming for lights and hitting this cylinder instead of weasel heads.

That's about as detailed as I can get without describing the rest of my machine and nullifying the whole point of selling it :P. But basically it's a sports machine that can be used for entertainment and physical training. It has multiple components but this is the only one that I'm stuck on; the rest have already been worked out.

- Jace

OK ! I get the picture. Use a simple microphone capsule, say electret. Take the signal from it and manipulate it to get what you want. Amplitude will tell you how hard the device was thumped and the frequency will tell you where.

Obviously you are going to have to play around a little to get the optimum mounting/location for the mic. I would suggest suspending it on a stalk inside near the bottom but above any circuits.

Baron,

I appreciate the idea but I still don't think it'll work for my application only because, and this is my fault for not including it, there are 5 different sections on the tube that are to be impacted and I think it would be too difficult to try to pinpoint which section was hit with the microphone setup.

Again, thanks for the info and sorry I left that detail out.

- Jace

put four or five microphones in it and the impact point can be determined by the time-lag between the microphone responses.

In theory at least, it should be possible to identify where in a known tube the sound originated by analysing the waveform produced by the microphone. In practice this may not be sufficiently accurate, or may simply be too hard a signal-processing problem to be worthwhile. In any case, the fact that your tube could be hit with nearly anything, rather than a known striker, probably rules out any detection based on sound.

I think the best idea so far is the air-bladder one, or possibly a variation using some kind of liquid or gel in the bladder.

Pete