Measurement of Force (Newtons) generated

yodon

Staff member
Super Moderator
#1
Hi again folks,

I'm back to "when in need, ask the cove."

I put this in the FDA group for lack of a better place and because this is for a medical device. Moderators: feel free to move it to a more appropriate location if there is one.

The best way I can describe what I need is through an analogy. Remember the old suction-cup-tipped dart guns? You would push a dart into the barrel compressing a spring until the trigger latch mechanism was engage. Pulling the trigger would release the latch and the spring would launch the dart at the target.

We are developing a device that would, essentially, not release the dart - only push it out maybe 3 - 6mm. Can anyone offer any advice on how we could measure the force (Newtons) generated?
 
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Tim Folkerts

Super Moderator
#2
How about just a simple weight scale?

Weight is a type of force, so any scale calibrated to measure weight (or mass) can also be used to measure force. Put the "dart" against the scale and "pull the trigger". The reading will tell you the force.


  • make sure the scale is appropriately positioned. some scalse only work when horizontal, for example.
  • if the reading is in kg, multiply by g = 9.8 ( the acceleration due to gravity) to get Newtons. Other units (oz, lb, g) can similarly be converted. (The value 9.8 varies slightly from one location to the next - but typically good to much better than +/- 1%. If greater accuracy is needed, you might have to determine the local value of g.)
  • if the weight of the "dart" is significant, you might want to have the "gun" horizontal and the scale vertical.
Tim F
 
J

jfgunn

#3
Based upon your description, I am picturing a lancet (spelling???) which is used to prick your finger to test your blood sugar. Anyway.....

There are a few questions to be asked in order to determine what you need:

1.) What force range do you expect? 10 N, 20N, 1000N?
2.) How accurately do you need to measure that force?
3.) How quickly do you expect the force to be reached? 1 second, fraction of a second?

Depending on answers to these questions..... To make this measurment, you should most likely use a force measurement test stand. Some manufactuers are Instron, Tinius Olsen, Ametek. Ametek sells Chatillon and Lloyd brand names.

Instron is proably the best known name with a price tag to match.

If you were in Northeast Ohio, I would reccomend the Chatillon line as it is one I sell. They have recently come out with a new line that I belive would work well. www.chatillon.com. I would reccomend getting a demonstration from a local dealer. I find that the newly released TCD225 has all of the powerful control aspects of an Instron at about 1/2 the cost.

Whatever machine you look at, get the distributor to give you an on-site demonstration. Again, if you were in Ohio, we would show it to you and let you use it for a week or two before buying. Hopefully someone in your area would do the same.

The largest pitfall in this potential measurement is How quikcly the force is exerted. All force measurement devices have a "capture rate" which determines how many readings are taken per second (Hz). Some might take 60 readings per second. Some might tke 1000 readings per second. If your force is exerted quickly, the 60 Hz machine would not capture the reading as well as the 1000 Hz machine.
 

Miner

Forum Moderator
Staff member
Admin
#4
You may also want to consider a device that can capture the force measurement as a function of travel distance. Do you want a constant pressure throughout the travel, a peak at the max travel or a decay, etc.?
 

yodon

Staff member
Super Moderator
#5
How about just a simple weight scale?
Weight is a type of force, so any scale calibrated to measure weight (or mass) can also be used to measure force. Put the "dart" against the scale and "pull the trigger". The reading will tell you the force.
Hmm... that just might work. I don't think accuracy is going to be required down to a significant level. We will have to give that a try. Great suggestion.

Based upon your description, I am picturing a lancet (spelling???) which is used to prick your finger to test your blood sugar. Anyway.....

There are a few questions to be asked in order to determine what you need:

1.) What force range do you expect? 10 N, 20N, 1000N?
2.) How accurately do you need to measure that force?
3.) How quickly do you expect the force to be reached? 1 second, fraction of a second?

Depending on answers to these questions..... To make this measurment, you should most likely use a force measurement test stand. Some manufactuers are Instron, Tinius Olsen, Ametek. Ametek sells Chatillon and Lloyd brand names.

Instron is proably the best known name with a price tag to match.

If you were in Northeast Ohio, I would reccomend the Chatillon line as it is one I sell. They have recently come out with a new line that I belive would work well. www.chatillon.com. I would reccomend getting a demonstration from a local dealer. I find that the newly released TCD225 has all of the powerful control aspects of an Instron at about 1/2 the cost.

Whatever machine you look at, get the distributor to give you an on-site demonstration. Again, if you were in Ohio, we would show it to you and let you use it for a week or two before buying. Hopefully someone in your area would do the same.

The largest pitfall in this potential measurement is How quikcly the force is exerted. All force measurement devices have a "capture rate" which determines how many readings are taken per second (Hz). Some might take 60 readings per second. Some might tke 1000 readings per second. If your force is exerted quickly, the 60 Hz machine would not capture the reading as well as the 1000 Hz machine.
Thanks for the vendor suggestions. If it comes to that, these are good to know.

The force will be between 50 and 400N. It will just be a 'pop' so just a fraction of a second.

You may also want to consider a device that can capture the force measurement as a function of travel distance. Do you want a constant pressure throughout the travel, a peak at the max travel or a decay, etc.?
I don't think we will care about the pressure throughout the travel, only the max force delivered.

Thanks all for the great feedback. I knew I could count on the cove.
 

Tim Folkerts

Super Moderator
#6
For my suggestion to use a simple scale, I should add that this would only work in a fairly "static" situation, since most scales aren't designed to read very rapid variations.

For example, you could slowly compress the spring to get the maximum force before it locks into place. This should be (approximately) the maximum force that it presses out again when "fired".

You could also measure the force each mm as you compress the spring, but this would be a bit more work, since you would need to measure both position and force. (On the other hand, most springs are very close to linear - if the force to compress 1 mm is 100 N, then it would be very close to 200N for 2 mm, 300 N for 3 mm, etc)

If you need something as fancy as force at each position or rapid measurements during an actual "firing", then I expect that a device designed for such a purpose - as jfgunn suggested - might be a better solution.

And even for something basic, if you are going to buy something, you might as well buy a "force gauge" instead of a scale. The price can't be much different. If you have the scale already, then by all means try to see of that will work well enough.


Tim F
 
Z

zancky

#7
sorry but I'm confused if the dart leaves the spring and I wolud like to get more informations

let me explain:
You can talk about force to compress the spring but after the trigger has released the "dart" You should investigated the energy of the dart when it leaves the spring (i.e. half of the moving mass times the square of his speed).
therefore You should measure the speed and the mass.
How deep the dart will dig into the target will depent on that energy and how sharp is the dart vs target resistance/reaction.
 

Al Rosen

Staff member
Super Moderator
#8
sorry but I'm confused if the dart leaves the spring and I wolud like to get more informations

let me explain:
You can talk about force to compress the spring but after the trigger has released the "dart" You should investigated the energy of the dart when it leaves the spring (i.e. half of the moving mass times the square of his speed).
therefore You should measure the speed and the mass.
How deep the dart will dig into the target will depent on that energy and how sharp is the dart vs target resistance/reaction.
I'm curious as to how or what you would use to measure the speed of this dart with a displacement of 3-6mm.
 

yodon

Staff member
Super Moderator
#9
I may have introduced a bit of confusion into the discussion.

The "dart" doesn't actually leave the gun. And it's not really a dart in the sense that it sticks into anything. It's more like a mini-jackhammer - only you can't hold the trigger down to get a continuous hammering - either a single 'pop' or ~5 'pops' depending on the mode. There's a solenoid that pushes the 'hammer' (with force) the 3 - 6mm and a spring that pulls it back in.

In the single 'pop' mode, we would want to measure the force of that pop. In the multi pop mode, we would only need to know what the maximum force of the pops was.
 
D

Daniel Walker - 2011

#10
You need a force gauge with a peak hold funtion (holds the highest reading on the display until reset) and a very high sampling rate. I don't want to push any particular manufacturers on here, but I think a link was supplied in a previous post that will get you to a good source. Although, to cover the range you are testing with any accuracy at all, you may have to purchase two seperate units.
 
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