4.14 A note on other forms of knowledge representation

    Graphs and semantic networks are a means of representing knowledge-based on the relationships between objects. The nodes correspond to these objects, and links (lines or arcs) between them describe the dependencies. This is only a kind of representation without capabilities of processing. Final implementation of this kind of representation usually uses rule-based systems.

    Examples of Fig. 3.3 and Fig. 3.4, extracted from [Melendez, 1998], are a partial representation of the global process behaviour. In this case, graphs were used to assist the expert system configuration. The necessity of describing time dependencies to differentiate situations in the process behaviour, can be observed. These are the main difficulties when dealing with dynamic systems, because the reasoning must be performed not only about sampled data, but also about the temporal evolution of signals (trends). The graph of Fig. 3.4 shows the necessity of supplying these trends (E¯ , or E­ for example) for interfacing coming data in the on-line application. In those cases, the imprecision of human knowledge is difficult to be interfaced with numeric process variables. In the previous graphs, the arrows are used to describe the human perception of increasing or decreasing tendencies of process variables in a sample interval. The translation of this observances into a rule-based system becomes a difficult interfacing problem. The use of specialised tools to abstract qualitative representation from process variables are needed to interface process input variables with expert knowledge.

    Frames and object oriented representations.

    A frame is a data structure for representing objects, situations, facts or other kind of knowledge by breaking it down into their constituent parts. A simple example of using frames consists in structuring data in tables. The use of frames involves a set of mechanisms (access, new, reset and change of values) to deal with frame attributes or slots. Inheritance is another important property of frames. Graphical representation of frames offers an easy to use interface to users. For instance, frames are present in G2 to represent knowledge related to process variables and objects. Attributes as sampling time, length, connections and other are used to describe data and relationship between them. Copies of these graphical objects conserve all the attributes (inheritance).

    The actual object oriented languages are very extended to support this type of representation with additional features. Languages such as C++ or Smalltalk permit to declare complex data structures, called classes, embedding data (object attributes) and methods (to manage and/or to access to its attributes). Main benefits of Object Oriented Programming (OOP) are resumed in the following properties :