96.1 Introduction to wrstcse

wrstcse is a naive go at interval arithmetics is powerful enough to perform worst case calculations that appear in engineering by applying all combinations of tolerances to all parameters.

This approach isn’t guaranteed to find the exact combination of parameters that results in the worst-case. But it avoids the problems that make a true interval arithmetics affected by the halting problem as an equation can have an infinite number of local minima and maxima and it might be impossible to algorithmically determine which one is the global one.

Tolerances are applied to parameters by providing the parameter with a tol[n] that wrstcase will vary between -1 and 1. Using the same n for two parameters will make both parameters tolerate in the same way.

load ("wrstcse") loads this package.

96.2 Functions and Variables for wrstcse

Function: wc_typicalvalues (expression, [num]) ¶

Returns what happens if all tolerances (that are represented by tol [n] that can vary from 0 to 1) happen to be 0.

Example:

(%i1) load("wrstcse")$

(%i2) vals: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 2000.0*(1+tol[2]*.01)
 ];
(%o2) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 2000.0 (0.01 tol  + 1)]
                                                          2

(%i3) divider:U_Out=U_In*R_1/(R_1+R_2);
                                R_1 U_In
(%o3)                   U_Out = ---------
                                R_2 + R_1

(%i4) wc_typicalvalues(vals);
(%o4)             [R_1 = 1000.0, R_2 = 2000.0]

(%i5) wc_typicalvalues(subst(vals,divider));
(%o5)            U_Out = 0.3333333333333333 U_In

Function: wc_inputvalueranges (expression, [num]) ¶

Convenience function: Displays a list which parameter can vary between which values.

Example:

(%i1) load("wrstcse")$

(%i2) vals: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 2000.0*(1+tol[2]*.01)
 ];
(%o2) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 2000.0 (0.01 tol  + 1)]
                                                          2

(%i3) wc_inputvalueranges(vals);
        [ R_1  min = 990.0   typ = 1000.0  max = 1010.0 ]
(%o3)   [                                               ]
        [ R_2  min = 1980.0  typ = 2000.0  max = 2020.0 ]

Function: wc_systematic (expression, [num]) ¶

Systematically introduces num values per parameter into expression and returns a list of the result. If no num is given, num defaults to 3.

See also wc_montecarlo.

Example:

(%i1) load("wrstcse")$

(%i2) vals: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 2000.0*(1+tol[2]*.01)
 ];
(%o2) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 2000.0 (0.01 tol  + 1)]
                                                          2

(%i3) divider: U_Out=U_In*(R_1)/(R_1+R_2);
                                R_1 U_In
(%o3)                   U_Out = ---------
                                R_2 + R_1

(%i4) wc_systematic(subst(vals,rhs(divider)));
(%o4) [0.3333333333333334 U_In, 0.3311036789297659 U_In, 
0.3289036544850498 U_In, 0.3355704697986577 U_In, 
0.3333333333333333 U_In, 0.3311258278145696 U_In, 
0.3377926421404682 U_In, 0.3355481727574751 U_In, 
0.3333333333333333 U_In]

Function: wc_montecarlo (expression, num) ¶

Introduces num random values per parameter into expression and returns a list of the result.

See also wc_systematic.

Example:

(%i1) load("wrstcse")$

(%i2) vals: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 2000.0*(1+tol[2]*.01)
 ];
(%o2) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 2000.0 (0.01 tol  + 1)]
                                                          2

(%i3) divider: U_Out=U_In*(R_1)/(R_1+R_2);
                                R_1 U_In
(%o3)                   U_Out = ---------
                                R_2 + R_1

(%i4) wc_montecarlo(subst(vals,rhs(divider)),10);
(%o4) [0.3365488313167528 U_In, 0.3339089445851889 U_In, 
0.314651402884122 U_In, 0.3447359711624277 U_In, 
0.3294005710066001 U_In, 0.3330897542463686 U_In, 
0.3397591863729343 U_In, 0.3227030530673181 U_In, 
0.3385512773502185 U_In, 0.314764470912582 U_In]

Function: wc_mintypmax (expr, [n]) ¶

Prints the minimum, maximum and typical value of expr. If n is positive, n values for each parameter will be tried systematically. If n is negative, -n random values are used instead. If no n is given, 3 is assumed.

Example:

(%i1) load("wrstcse")$
(%i2) ratprint:false$

(%i3) vals: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 1000.0*(1+tol[2]*.01)
 ];
(%o3) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 1000.0 (0.01 tol  + 1)]
                                                          2

(%i4) assume(U_In>0);
(%o4)                      [U_In > 0]

(%i5) divider:U_Out=U_In*R_1/(R_1+R_2);
                                R_1 U_In
(%o5)                   U_Out = ---------
                                R_2 + R_1

(%i6) lhs(divider)=wc_mintypmax(subst(vals,rhs(divider)));
(%o6) U_Out = [min = 0.495 U_In, typ = 0.5 U_In, 
                                                max = 0.505 U_In]

Function: wc_tolappend (list) ¶

Appends two list of parameters with tolerances renumbering the tolerances of both lists so they don’t coincide.

Example:

(%i1) load("wrstcse")$

(%i2) val_a: [
   R_1= 1000.0*(1+tol[1]*.01),
   R_2= 1000.0*(1+tol[2]*.01)
 ];
(%o2) [R_1 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_2 = 1000.0 (0.01 tol  + 1)]
                                                          2

(%i3) val_b: [
   R_3= 1000.0*(1+tol[1]*.01),
   R_4= 1000.0*(1+tol[2]*.01)
 ];
(%o3) [R_3 = 1000.0 (0.01 tol  + 1), 
                             1
                                    R_4 = 1000.0 (0.01 tol  + 1)]
                                                          2

(%i4) wc_tolappend(val_a,val_b);
(%o4) [R_1 = 1000.0 (0.01 tol  + 1), 
                             2
R_2 = 1000.0 (0.01 tol  + 1), R_3 = 1000.0 (0.01 tol  + 1), 
                      1                             4
R_4 = 1000.0 (0.01 tol  + 1)]
                      3

Function: wc_mintypmax2tol (tolname, minval, typval, maxval) ¶

Generates a parameter that uses the tolerance tolname that tolerates between the given values.

Example:

(%i1) load("wrstcse")$

(%i2) V_F: U_Diode=wc_mintypmax2tol(tol[1],.5,.75,.82);
                                          2
(%o2) U_Diode = (- 0.09000000000000002 tol ) + 0.16 tol  + 0.75
                                          1            1

(%i3) lhs(V_F)=wc_mintypmax(rhs(V_F));
(%o3) U_Diode = [min = 0.5, typ = 0.75, max = 0.8199999999999998]