5 point calibration Step 7

I have seen 5 point calibrations on different pieces of equipment and I would like to do something like it in Step 7. I have a weight scale that I can zero and I have a multiplier that adjust the scale. I would like to calibrate with 5 different weights. For example now I zero the scale with no wieght and I put on max weight and adjust the mulitiplier till it reads max weight. In the middle of the weight range I am off just a little. So if I could set up different mulitipliers through out the range of the scale I could be more accurate in the middle ranges.

Is there any easy way to do this?

You could use a lookup table with as many points as you feel necessary and then use linear interpolation between the points.

It depends.

You could use a look up table or you could use an equation at fit if to your points. You would end up with an equation that looks like:

y=a+b*x+c*x^2 or
y=(c*x+b)*x+a // This is the way to execute the function on a S7.

The c*x^2 term would account for the slight bow in your data.

The coefficients a,b and C need to be calculated off line using Excel or some other math package. It would be too messy to do this in a PLC.

Search for threads about Excel's LINEST. This will show you how to find the coefficients to a function like y=a+b*x+c*x^2.

So what are your 5 points? I have a book with an example similar to what you want to do I just have to modify it a little.

The scale max is 1000g and I have a tare weight of about 150g. The calibrated weights that I have are 2g, 10g, 50g, 100g, 200g. I am only off a couple of tenths on the low end but I think I could do better with this 5 point calibration.

We need the counts for each measured weight. The computer will find the coefficients so that equation will return the minumum squared error.