Strain gauge calibration and force distribution.

Taylor Turner

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Join Date
Sep 2020
Location
Midwest
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78
This is 99% a structural design flaw, but most people here have the capacity to manage this.


First, some R&D background. I have a strain gauge on the throat of a C frame press and the actuator has a platen face that meets a mobile load cell and is sitting on a stationary platen.
I can calibrate the strain gauge to the load cell, when it is centered. But when the load cell is positioned to the front or back, the strain increases or decreases, which is not accurate to the actual force.



Additional inquiry, do load cells, with direct force measurement, share this same calibration dissonance when the force is not in the same as the calibration location?


Without exception, the stain gauge will be moved to the actuator rod to measure force more directly. I have doubts that this will be good enough because of my last thought that a load cell itself will show this flaw.


I have thought to include two strain gauges on the actuator rod, 90 degrees out of phase, and sum a certain expression when a constant torque or pressure is generated on the newly installed tool as a calibration cycle.



Where 1 gauge is x, the other y…


Force offset = ycalibrated+( ycalibrated-ymeasured)+xcalibrated+( xcalibrated-xmeasured)


Does anyone have other thoughts?
 
Strain gauges that are affixed directly to a structure, only measures the deformation of the structure at that very limited place. So yeah, you can only use such as setup if the conditions are always 100% identical from measurement to measurement.

One or more sketches or photos to illustrate what you describe in words would be useful to understand your setup.

What is the straing gauge(s) / load cell(s) measurement used for ?
How precise must the measurement be ?
 
The screen shot of the model shows 3 locations of applied force. The +/- 2 is arbitrary, and only describes the change in strain at the location of the sensor.


The chicken scratch is an actuator cross section with two strain sensors. There are two offset load points in relation to the calibration point.



I can not guarantee the placement of the tool on the platen will have the same force distribution as the calibration distribution. This seems like a flaw in load cells all together.


(Edit) I didn't answer your questions fully. This is to measure the tonnage of a press. The measurement in all the location are surprisingly precise, but not consistent from one tool location to the next.

Location1.png Location2.jpg
 
Last edited:
I still have no idea of how you intend to measure the force by the actuator rod.
And you havent informed us the intended precision.
And "This is to measure the tonnage of a press" doesnt even begin to describe what you intend to measure. The sum of all force being applied ? The local force at certain points on the press surface ? The highest force value over the press surface ?

If this is something that must be precise, then I think it is a very difficult task.
You are not measuring directly inline with the force, but highly offset and within a bending beam. Obviously if the offset varies, so does the deformation at the strain gauge, even if the force does not vary. I can also imagine that the shape of the pressing can impart forces that are not inline with the press.
 
https://www.kistler.com/files/document/002-552e.pdf


This is a really good read for understanding how strain measurement works.


The rod will have a flat area machined for the sensor mounting. As the area undergoes compression, the sensor will spit out magic crystal electrons.


The shape does matter too, that is a good point. But it should be consistent from one part to the next.


As far as precision, obviously I'm trying to minimize as much error as possible. I'm asking for a way to minimize that error with some result better than the current. <= +/- 5%? and lets say we are at +/-10%? Not sure that helps.


https://www3.nd.edu/~jott/Solids/Solids_lab/Lab3/Interface_Load_Cell_1210-Data-Sheet.pdf


This is another style of load cell. I would have to imagine the same offset strain problem occurs here as well.


I was looking for a programming solution or just raw experience advice with load cell/strain gauge measurement inconsistency.
 
The only way to be accurate is to put the load cell on the end of the rod. The problem wit what you are doing is that you are actually measuring bending or torque. Torque depends on force x distance so forces near where the bending occurs will not have as much effect as the same force farther away. Much depends on how the press makes contact with the work piece and where. If it is always in the same place and n the same way you could probably calibrate the load cell and be done with it.
 

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