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Finger Pressure Sensor?

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wrhastings
(@wrhastings)
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I'm considering a project that would require reading a pressure applied to a finger. The pressure point could be anywhere along the face of the finger, from the tip to the base. The max pressure would be 100 lb. The sensor needs to be flexible enough to completely open and close the fingers. I've ordered some 3M Velostat, a pressure sensitive sheet, and will try to build a useable sensor. I wanted to know if you might have another suggestion for a finger pressure sensor and/or material?


   
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Inq
 Inq
(@inq)
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Sounds like an interesting problem you're tackling.  Just curious...

  • Are you primarily wanting to sense any touching of the surface... say to trigger a reflex action
  • Or are you looking for a method to keep from crushing, say... an egg as a hand grasps it to pick it up?

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wrhastings
(@wrhastings)
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@inq It's for measuring the actual pressure on the finger(s). I want to measure the actual draw weight of an archery bow, by measuring the pressure the string puts on the finger(s) at full draw.


   
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Ron
 Ron
(@zander)
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@inq @wrhastings And I am assuming based on your description it is NOT a capacitive touch sensor.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
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My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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Inq
 Inq
(@inq)
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@wrhastings What are you looking to use to do the drawing of string?

 

3 lines of code = InqPortal = Complete IoT, App, Web Server w/ GUI Admin Client, WiFi Manager, Drag & Drop File Manager, OTA, Performance Metrics, Web Socket Comms, Easy App API, All running on ESP8266...
Even usable on ESP-01S - Quickest Start Guide


   
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wrhastings
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You are correct. The Velostat is resistive, resistance changes with pressure. I don't know much about it, yet. It may not be repeatable enough to use. I asked about alternatives, just in case.


   
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wrhastings
(@wrhastings)
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@inq Your fingers. I want to measure the draw weight under actual conditions. The sensor would be inserted into a glove or tab, and the archer will draw the bow, and determine the actual draw weight, "at the fingers." It's an important value used when selecting new arrows.


   
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Inq
 Inq
(@inq)
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Ah... guess I have robot making mind-set. 😉 I'm always thinking of the robot doing the heavy lifting pulling.  So my idea wouldn't be worth anything.

Second thought, then...

I don't really know anything about archery... at least in the realm you must be in.  Wouldn't the pull weight be the same for a bow pulled to the same distance?  And isn't the goal of archers to always precisely pull the same distance for consistency?  I guess what I'm wondering... wouldn't a low tech method of using a fish-scale to make a chart of weight = f(pull distance) suffice?  

I had never heard of the 3M Velostat so I had to look it up.  I couldn't find anything on its resistance versus pressure properties.  It also didn't show what kind of pressure it can tolerate and not permanently deform or tear.  Looking at a pull string across a couple of fingers will be applying about 1500 psi under the string.  Also, seeing it's a thin film, I would bet it will need the pressure applied a large area for it to show readable change in resistance.

Third thought...

Don't Olympic archers use some kind of pull-thing that even has a trigger so they don't actually touch the string with their fingers???  I don't know what its called or even looks like.  I just noted how those archers hands seem to be gripping something.   How about putting a load cell built in to it?  

3 lines of code = InqPortal = Complete IoT, App, Web Server w/ GUI Admin Client, WiFi Manager, Drag & Drop File Manager, OTA, Performance Metrics, Web Socket Comms, Easy App API, All running on ESP8266...
Even usable on ESP-01S - Quickest Start Guide


   
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wrhastings
(@wrhastings)
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Don't want to get into a lengthy discussion of archery, but...

The low tech method you described is how it is usually done, now. It's kind of clunky, though, because you first need to draw the bow to your anchor point, and someone needs the mark the arrow so it can be drawn to precisely that point when the scale is attached. Anyway, what's the fun in that? 😉

Yes, I'm concerned about the Velostat, as well.

Only compound archers use a release to trigger the bow. Compound archery is not part of the Olympics, only recurve bows are used.


   
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Inst-Tech
(@inst-tech)
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@wrhastings , Hi, reading your post it struck me that using Embroidered Resistive Pressure Sensors might also be a possibility.. See the video on this Link:

regards,

LouisR

LouisR


   
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(@davee)
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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


   
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wrhastings
(@wrhastings)
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@inst-tech Thanks. Looks interesting.


   
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wrhastings
(@wrhastings)
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@davee Thanks. These are all things that need to be considered. 


   
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Will
 Will
(@will)
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@wrhastings 

How about just setting the bow on the floor, stand on the handle and use a luggage or other weight spring to pull the bowstring up to the desired draw length ?

Anything seems possible when you don't know what you're talking about.


   
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wrhastings
(@wrhastings)
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@will That’s the way it’s done now, just not on the floor. Check out my previous replies.


   
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