Finger Pressure Sensor?

@wrhastings 

Is that not accurate enough, or is it just too clumsy ?

@will I’ve always thought it was clumsy. I’m not sure if my high tech version will work, but it will be fun to work on.

@wrhastings 

Ah, I see, it's more of an interesting substitute than a need for precision. I have to agree that it sounds like an interesting project 🙂

Good luck.

Posted by: @davee

Hi @wrhastings,

At risk of sounding over pedantic in pointing out the obvious, pressure and force are intrinsically different measurements.

Measuring with a spring balance measures a force in pounds (weight) or kilograms (weight),  or Newtons.

By contrast, pressure is a 'force per unit area' such as 'pounds per square inch', so that the pressure is dependent on not only the force, but also the area to which the force is applied to.

I think you would like  to measure force, but you are proposing a pressure sensitive approach.

To achieve the same force, someone with 'fat' fingers would probably experience a smaller pressure than a person with 'thin' fingers, as the force will be distributed over a greater area with the 'fatter' fingers.

If you are only measuring the same person, pulling the same bow, etc.. then it may be possible to 'calibrate' pressure measurements in a manner that they can be converted into force measurements in a useful way?

Otherwise, maybe 'sandwich' the pressure sensitive materials between stiff layers so that the force is always applied across the same area? This might make the measurements more repeatable, but the extra stiff material might be too intrusive.

Just a thought or two. Best wishes with your project. Dave

Technically you are exactly right.  And personally, I really think this material is going to just permanently deform (as in plastic vs elastic deformation) if not tear.  The pressure (stress) that will be exerted across my three fingers would be 100 lbs / 1.75" / 0.02" (bow string diameter) = 2900 psi.  Being inside a glove may help distribute the load.  Oh! - This is purely hypothetical... I can barely pickup an 80lbs bag of concrete.   I'm pretty sure I couldn't pull 100lbs with three fingers.  🤣  

Next -  Even if it doesn't tear, there is question how this material works.  I suspect its resistive vs pressure properties must happen over a large percentage of the sample to even budge the resistance measurably.  Again, the area under the string is so small that it wouldn't budge the resistance at the ends where the wiring is attached.  I imagine the wires have to be placed such that the current flows through the deformed area and if there is a large area that is undeformed, the current will take the path of least resistance... just ignoring the deformed area.

But - If it does budge the resistance enough to measure (and use), I would contend it would be a function of load.  Intuitively, I would think that if the load is applied over larger area it would move the resistance just a significantly as a higher load... thus... A fat fingers person has a lower pressure, but over has a larger area... QED... the output is the same and more related to the Load and not the pressure.

Next - Then there is the next issue - The glove would have to have the same stuff in all fingers.  I imagine that an archer can cause less pressure to be applied to the middle finger simply by relative lifting of it.  If it was the only finger instrumented, the load reading would go down.  If all three were instrumented the average might be suitable. As the middle finger is lifted it get less load and lower reading, but the other two would have higher load and higher reading... average should be about the same as long as the material is linear in behavior.  This intuitive example also highlights that the reading should be load dependent and not pressure dependent.

But - As pointed out all this is conjecture and most anything having to do with polymers is completely non-linear.  

Net-Net - I think it is a very interesting problem, and I would really appreciate hearing @wrhastings results from testing the material, but a practical archery load cell coming out is seems unlikely.  

VBR,

Inq

P.S. - I may have to get some of this stuff to play with it.  Load cells are expensive... if they could be made of this stuff, I can see lots of projects coming out of it.

@inq All well thought out presumptions, and as the laws of physics will ultimately determine the outcome, it is also possible that using piezo-resistive and the piezo-electric principle is widely used in industrial instrumentation, such as strain gauges,pressure transducers, and a variety of other transducers that can utilize those physical properties. The accuracy of some of those instruments can be > 0.075% of span.. far greater that the ADC used to convert the measurement signal.( a typical Industrial processor's ADC is typically 24 bit.. thus very high resolution with high sample rates. 

The following link is for Piezoelectric force fabrics article I found.. https://www.advancedsciencenews.com/textile-piezoelectric-force-sensors/

An interesting subject to be sure..

regards,

LouisR

Posted by: @inst-tech

@inq All well thought out presumptions, and as the laws of physics will ultimately determine the outcome, it is also possible that using piezo-resistive and the piezo-electric principle is widely used in industrial instrumentation, such as strain gauges,pressure transducers, and a variety of other transducers that can utilize those physical properties. The accuracy of some of those instruments can be > 0.075% of span.. far greater that the ADC used to convert the measurement signal.( a typical Industrial processor's ADC is typically 24 bit.. thus very high resolution with high sample rates. 

The following link is for Piezoelectric force fabrics article I found.. https://www.advancedsciencenews.com/textile-piezoelectric-force-sensors/

An interesting subject to be sure..

regards,

LouisR

https://k9f7a9j9.rocketcdn.me/wp-content/uploads/2013/02/Textile_piezoelectric_force_sensors-e1360330533353.jp g" target="_blank">https://k9f7a9j9.rocketcdn.me/wp-content/uploads/2013/02/Textile_piezoelectric_force_sensors-e1360330533353.jp g"/>

Pretty cool stuff - piezo.

I got a bunch of these one time and wanted to put them in my hiking boots and use harvesting techniques to gather the energy to keep a cell-phone charged while in the back-country for days, week, etc.  Saw the kids shoes that use them to blink LED's.  But the power generation is just way low as compared to cell-phone requirements.  

VBR,

Inq

I want to get some piezoelectric sensors and make some electronic drums. 😀 Dream on!

Maybe the strain gauge used in an electronic scale could be used?
The human finger can probably measure fairly accurately at an unconscious level the force being applied. It all takes place in sensory/motor memory. You just "know" how to apply a force of just the right impulse and direction to land that ball in the basket.

Here is a robot that was given the task to learn to shoot arrows.

https://phys.org/news/2010-09-icub-robot-archery.html

@robotbuilder Very cool!.. robots sure have come along ways since AI was introduced in 1959..

In the industrial realm where i worked for over 40 years, we've been using "Fuzzy logic, and neural networks since the 1990's in some of the advanced  process control logic using supervisory computers to interact with cascade control loops. This works well with complex schemes like Boiler plant master controls, among other complicated control schemes.

regards,

LouisR 

Posted by: @yurkshirelad

I want to get some piezoelectric sensors and make some electronic drums. 😀 Dream on!

Give me the broad side of a barn any day.  I'll hit it.  🤣   

I can't imagine anyone hitting one of these regularly... Even someone as portrayed in "Whiplash".