Operating invalid feature-based models

A valid feature-based representation is one where instantiated features in a model agree with the features' expected behaviours, available and defined as a library. Invalid feature-based models happen when manipulations on the model change the interrelationship among features therefore changing the behaviour of an instantiated feature. Freedom of manipulation is an intrinsic advantage of using a CAD system and it is taken for granted. However, even the most basic manipulation, such as "adding" a feature to a model, is capable of disrupting the validity of a representation. Furthermore, invalid models could compromise the usefulness of any following analysis on it. Thus, identifying means to operate on an invalid model to make it valid, through "revalidation operations", is a necessity in Feature-based CAD systems. It allows conventional CAD systems (usually more preoccupied with representing and producing feature-like shapes within a geometrically constrained environment) to interface more easily for example with CAPP systems (usually more preoccupied with planning problems than with the correctness of the representation). The framework of a feature-based validation system, called FRIEND (Feature-based Reasoning system for Intent-driven Engineering Design), and a discussion on representation validity analysis is presented with emphasis on identifying and discussing "revalidation operations”.