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I'm going to experiment with a parallel beam load cell like this one: https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/TAL220M4M5Update.pdf. I've read somewhere that for a setup shown on this picture:

enter image description here

the response of the cell is largely independent of the place the force is applied to the top plate, e.g. a mass placed in the spot indicated by red or green arrow, they will produce the same load sensor response. Is that accurate?

In my use case, the force will be applied where the green arrow points. I want to make sure that I'm not exceeding the safe overload for my sensor.

EDIT

The gist of my question is whether the sensor response will be proportional to force times distance from the center of the cell (moment of force) or there is some other formula.

EDIT'

Here is the drawing showing compression and tension directly measured by bending beam load cell:

enter image description here

BTW, I'm not interested in highly accurate measurement, just an approximate detection of an object with a mass above a certain threshold.

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  • $\begingroup$ This is more of a mechanical engineering question, I will move it to engineering.SE $\endgroup$ Jul 12 at 23:13
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    $\begingroup$ Paul, the load cell should come with a datasheet. And the datasheet, in this case, should specify a loading area. The load cell should be mounted in the center of the load area and the weight should never be placed outside the boundary that the datasheet specifies. So long as the load stays within that area, the reading will be relatively accurate and the load cell will not be damaged. We cannot tell you, looking at arrows, whether or not that point is within or exceeds the loading area. If the datasheet doesn't specify a loading area, then you need to find it somehow. $\endgroup$
    – jonk
    Jul 12 at 23:36
  • $\begingroup$ @jonk I linked the datasheet, which doesn't specify the load area. I've seen a number of datasheets for parallel beam cells, and none of them had this datum. $\endgroup$ Jul 13 at 0:50
  • $\begingroup$ @PaulJurczak It's an essential component to the question. Just a second and I'll track down some video on youtube. There must be something on this.... Ah! Try this. I've set it to start right at the point I want to bring to your attention!! Hope that helps you see what I'm trying to suggest. $\endgroup$
    – jonk
    Jul 13 at 0:55
  • $\begingroup$ @jonk Thank you. This is in agreement with an article I read. I would like to understand the mechanics of it, e.g. the formula for strain when the load is outside the loading area and the "magic" of (nearly) constant strain when the load is within the loading area. $\endgroup$ Jul 13 at 1:00
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The datasheet of the TAL220 mentions parallel beam type. This is (probably) the same as what is more commonly know as double bending beam (however the shape can also also allude to shear load cell). In any case all those types are fairly insensitive to the application load.

If I were in your shoes (i.e. I was stuck with a product), and if you are certain that the point of load remains constant (changes less than 10%) all I would do is calibrate the signal. I.e. put different loads on, see the response, and interpolate.

E.g. I would put a weight at the threshold (e.g. 3kg) and see the values at the closest, furthest and most likely position. If you got 3.5,3.8, and 3.6 respectively, and you are ok with a $\pm$0.5 kg error, then I would just set the 3.6 as a threshold.

(Although, I expect that the above procedure would give you acceptable results,) in the event that the desired accuracy is not achieved, I would try to redesign the platform to bring the load application closer.

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Correct. The position of forces will affect the reactions at points A & B as shown.

$\sum M_A = 0$

$R_B = \dfrac{W*a + P_1*b + P_2*c}{a}$ (Compression)

$R_A = W + P_1 + P_2 - R_B$ (Tension)

Note: W = Weight of the top plate (conveniently assumed its centroid falls on point B). The weight of the lower lever arm is ignored in the calculation.

enter image description here

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  • $\begingroup$ Thank you, but forces at points A and B is not what the load gauge is measuring. I'm adding a drawing, which shows tension and compression measured by the sensor. $\endgroup$ Jul 13 at 1:56
  • $\begingroup$ This concept holds true whether you are measuring force, pressure, displacement, or strain. All are linearly proportional to the location of forces. $\endgroup$
    – r13
    Jul 13 at 1:59
  • $\begingroup$ That was my first impression, but it is not accurate. Datasheets of bending beam load cell claim constant strain for the loading force applied within a large area, e.g. 800 x 800 mm, see: assets.minebea-intec.us/fileadmin/fm-fal/intec_media/… $\endgroup$ Jul 13 at 2:08
  • $\begingroup$ Sorry, find nothing but the specification. Don't they have an example problem? $\endgroup$
    – r13
    Jul 13 at 2:39
  • $\begingroup$ Look at these parameters: Max. platform size and Corner load error. 0.0233% error over 800 x 800 mm area. $\endgroup$ Jul 13 at 3:58

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