How do I define custom derivation rules for calculating error marigin in formula?

Hi

there was a discussion aout this topic not too long ago:

http://communities.ptc.com/message/228086#228086

This might give you some ideas abotu what is possible & definitely what the pit falls can be.

regards

Andy

Rafal Grasman wrote: Formula: U = I * R Error formula: error_in_U / U = error_in_I/I + error_in_R/R

Hmmm!? Lets substitute tolerance for error and let R=(10 +/- 2) ohm, I=(2 +/- 1) A. Smallest result for U would be 8 V, largest result is 36 V. So we don' get a symmetric interval around the nominal 20V. According to the rule you provided for multiplication we would add 2/10 + 1/2 = 7/10. So according to this calculation the relative error for U should be 70%. So the lenght of the interval for U (6 to 34) is the same, but the absolute limits aren't.!!??

It is also not said that you just have to consider the minimum and the maximum value of a variable to get minimum and maximum of a calculation. Let x=(5 +/- 2), so we have a range from 3 to 7 (the examples use an unusal big tolerance to better show the point). The calculation we have to do is y=x*(12-x). We get the minimum for x=2 with y=27, but the maximum value is y=36 for x=6, which is neither end of the x-range.

Using you rule you would get that y has a tolerance of 80%, which would be a range for y from 7 to 63 around the nominal 35, while in real we have to expect the range from 27 to 36 for y.

Calculations with tolerances or errors can be very tricky and I think it would not be practical and worth the effort implementing appropriate routines for the most used operations or even functions like I did with +/- in the thread Andy pointed you to.

Considering all those points led me in trying a brute force approach to find the minumum and maximum value of any calculation using values with tolerances http://communities.ptc.com/message/199660#199660. Any variable is represented by a vector with selectable number of elelemnts and then all those values are combinated and the calculation done to find min and max at the end. A calculation which uses 5 variables and if each variable is represented by 100 values within its range has to do the calculation 100^5 times. So the drawback is clearly apparent - calculation time, possibly in prohibiting dimensions.