System calibration of a cantilever with strain gauge atteched

[Nick Dwyer]

I have written a vi to measure the number of chocolate bars (this is a second year engineering design project) using a cantilever with a strain gauge applied. Part of the calibration is to plot readings of applied loads. Is it possible to add to the vi to correct for this? ie change the reading to the applied load. If this is possible how do I do it?

[mikeporter]

If you are generating a curve that essentially shows the conversion between ounces (pounds?) of chocolate (load) and strain, you can use that data as a lookup table. What you need is a 1D array of strains and a 1D array of loads that correspond to the stain values in the other array.

Use the reading you are getting (in strain) to threshold the array of strain readings. The result is a fractional index where the value from the array below the index is less than the threshold value and the next value from the array above the index is greater than the threshold value. To come up with the fractional part of the index I believe the code does a linear fit between the two bounding points, but you should check that.

(By the way, an interesting feature of the thres
hold function is that unless the first value in the array being thresholded is less than the threshold value, it won't work... An error condition to check for...)

Now use this fractional index to interpolate the array of loads and you should have a weight close enough to tell how many candy bars are sitting on the dohickey.

Mike...