to measure displacement...

Thread Starter

toffee_pie

Joined Oct 31, 2009
235
hi guys,

i need a design that measures distance, nothing too complex and distances will be not too great, 10/15 mm or so

what is the best way to approach this?

i am thinking strain gauge, proximity sensors, op amps, or some software even such a labview which i quite like but have not used it for years.

just want to get some thoughts here, again i suppose the key work here is simplicity as i don't want something over kill.

cheers.
 

Thread Starter

toffee_pie

Joined Oct 31, 2009
235
i am going to do a simple wheatstone bridge and use a op amp to hook up to a scope on the output voltage.

i am not sure what value strain gauge to get or what type, again i need something pretty simple and this works for me.

i am measuring displacement >| --- - |<

accuracy not paramount but will be in the order of mm.

i need 2 strain gauges either side of my source object which will expand under test.

any idea on what value resistance of strain gauge?

thanks
 

KMoffett

Joined Dec 19, 2007
2,918
"Expand 15mm"? 15mm on a strain gauge doesn't sound logical sensor choice.
Maybe you can explain exactly what you are trying to do. What is the overall project.

Ken
 

BillB3857

Joined Feb 28, 2009
2,570
I agree that 15mm is a very wide stretch for a strain gauge. For those not versed in mm, that is a little over 1/2 inch. Again, the question would be..... How will the measuring device be physically connected to whatever it is you are trying to measure? What is moving, what is stationary?
 

BillB3857

Joined Feb 28, 2009
2,570
I work on video cameras that measure the diameter of a stream of glass. Observe the video signal on a scope and measure the width of the pulse defined by the edges of whatever you are measuring.
 

Thread Starter

toffee_pie

Joined Oct 31, 2009
235
its actually a pouch cell, it will expand during discharge etc, i want to measure its displacement to see how much., 10mm would probably be the maximum, i think 5mm might be more realistic.

i got a instrumentation amp i can use for the +/- inputs and measure the output voltage on the amp.

i want to use 2 strain gauges so a half bridge or full bridge circuit, not sure what value resistors on the gauge though.?
 

KMoffett

Joined Dec 19, 2007
2,918
Call me dense, but I still can't visualize how you are going to arrange two strain gauges to measure the pouch expansion?

Ken
 
Last edited:

#12

Joined Nov 30, 2010
18,224
Strain gauges don't move much. You would have to put a spring in the mechanism so the strain gauge is not fractured. Then measure the results as per the usual way, opamps, etc.
 

Thread Starter

toffee_pie

Joined Oct 31, 2009
235
the units involved will be relatively small (from the strain gauge) but i hope i can get eligible results out of the instrumentation amp.

the pouch cell will expand and will cause restiveness changes in the Wheatstone bridge and thus on its output terminals, these will be fed into the amp

well that's the plan anyway.

any other methods are over kill at the moment, ie buying specific hard ware or the like.
 

KMoffett

Joined Dec 19, 2007
2,918
Yes, strain gauges put out a very small signal, and yes that can be amplified. Strain gauges output a voltage proportional to force, not distance. So I still can't see how you are going to mechanically implement them in your apparatus. Maybe you could post a sketch.

Ken

Got to correct myself. The output is proportional to distance, but on the microscopic scale.
 
Last edited:

THE_RB

Joined Feb 11, 2008
5,438
Tiny magnet and linear hall effect sensor. That will work well from 0-15mm.

The non-linearity should be easy enough to compensate in software, depending on accuracy and resolution needed.
 

atferrari

Joined Jan 6, 2004
4,764
As far as I know, LVDTs can translate displacement to voltage with good repeatability.

/edit
Didn't know that somebody had posted about them already!
/edit
 

Thread Starter

toffee_pie

Joined Oct 31, 2009
235
Linear variable transformers are overkill also, have you seen how much they cost..

So, i will have to make do with a strain gauge and back to my initial question of a resistance that will give me maximum amplification factor for a op amp?. , INA337AIDGKT i think is ok to use.

regards
 
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