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I would like to electronically measure the width (really the compression/extension) to which an object's constituent blades have been stretched. Or alternatively, I could measure some other parameter which I can then map to the displacement by data calibration.

In particular, I am dealing with a miniature tool (a few cm long) whose outline resembles an alligator-clip (or pliers). So consider something like either of the objects in the below image.

How can I electronically track/measure the below spacing to a resolution of about 0.1 mm over a full-scale extent of ~10 mm?

I cannot think of any transducer or phenomenon that I could map to this movement and get such fine resolution. For example:

  • Perhaps I can measure parallel-plate capacitance -- would that be accurate enough in this scenario?

  • Or maybe I could place a spring between the two jaws and measure the compression -- but how would I measure the compression of the spring?

  • CV isn't a viable option because I wish to make the sensor compact enough that that I can log the data wherever I wish.

measure

EDIT: Updated with illustration added based on @Michael Karas's Answer and @Matt Young's suggestion. enter image description here

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  • \$\begingroup\$ Attach it to a digital outside caliper? \$\endgroup\$
    – AndreKR
    Jun 18, 2013 at 16:33
  • \$\begingroup\$ @AndreKR: Great idea but that would be a little too big perhaps. Ideally, I wish to make the clip itself the sensor and have some sort of transducer attached to either the top end or the bottom end of the clip, and log the data in-situ. Perhaps I'll look to see if there is some variant of that outside caliper which is more compact. \$\endgroup\$
    – OrCa
    Jun 18, 2013 at 16:38

3 Answers 3

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I think you want to look into small strain guages. Strain guages are a resistive element that changes resistance in relation to very small changes in the longitudinal stress introduced into the senesor element. These are generally made of a thing plastic type carrier material with loops of resistive material printed upon it. Generally a sensor is comprised of two resistive elements in series like a voltage divider. These are then wired into a bridge circuit that is monitored via an instrumentation amplifier.

The sensors often are made up with a stick on glue to attach them to substrate. In the case of something like your allegator clip you could use two sensors just attached along the outside length of the allegator clip assembly.

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  • \$\begingroup\$ That was my first thought, but the OP doesn't state what these things will be gripping. If the opening is to be tracked without an object being gripped, there won't be an strain the jaws and a strain gauge will provide no useful information. \$\endgroup\$
    – Matt Young
    Jun 19, 2013 at 0:05
  • \$\begingroup\$ @Michael: This is a great idea. But I'm trying to understand: Suppose I were to attach these resistive elements on the outside faces of the alligator clip. And let's assume the clip grips an object, say a cable of 6mm diameter. There would obviously be a force applied because of the object in between the clip arms, but longitudinally ALONG the surface of each arm, there wouldn't be any strain, would there? Or am I not understanding where the strain gage element is to be placed? \$\endgroup\$
    – OrCa
    Jun 19, 2013 at 3:03
  • \$\begingroup\$ @MattYoung: Yes, the clip would indeed be used to grip rigid objects, and measure their thickness variation over time. The clip-like sensor in my case would be a natural way to allow for the object to expand or contract. \$\endgroup\$
    – OrCa
    Jun 19, 2013 at 3:06
  • \$\begingroup\$ @OrCa You'll want to put the strain gauges on each side of the of alligator clip, outside, and right over the pivot point for maximum strain. Have two will give you temperature compensation. You'll also have to consider the non-linearity of the spring that holds the jaws closed. \$\endgroup\$
    – Matt Young
    Jun 19, 2013 at 3:30
  • \$\begingroup\$ @MattYoung: Something like the above? (See my new illustration added in the Question). But the gage elements moved closer to 'C' and 'D', so as to center exactly over the pivot point ideally, I suppose. \$\endgroup\$
    – OrCa
    Jun 19, 2013 at 3:43
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I'd look at a flex sensor such as this one from Sparkfun. Resistance varies depending on the amount of flex. You might mount it as a 'U' loop inside of your scissors so that as the scissors open or close the sensor flexes into a tighter or looser 'U'.

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  • \$\begingroup\$ Unfortunately, these flex sensors and stretch sensors in my experience provide nowhere near the precision I require, and also there appears to be drift over time. \$\endgroup\$
    – OrCa
    Jun 19, 2013 at 3:10
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A micro potentiometer might work depending on your tool configuration. ... I'm terribly new at this

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