For the first case we found that - depending on the
chosen membership function - it is possible to omit
some rules. When we were using triangles or trapezoid
membership functions for the sparsely filled input
matrix e(k)/de(k), the controller was ''falling into the
hole of missing rules" from which it could not recover
[7]. When we used a bell shape membership function
instead (as in fig. 2), the system could react to
any input due to the fact that the membership functions
have values greater zero all over the input matrix
e(k)/de(k). Nevertheless, the rules have to be chosen
carefully.