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3.1.4 Graph-based stamping process plan
After the mapping from stamping features to a set of stamping operations, the remaining process planning task is to assign each stamping operation to the relevant die station according to an optimal sequence of stamping operations. Stamping operations are sequenced in a progressive manner by creating stamping operation relations and using them to form a stamping process plan. This formal description of operation relations forms the foundation of automatic strip layout design.
A graph-based approach is used to arrange the stamping operation objects in a stamping process plan. The graph consists of a set of nodes that store information about the stamping operations, and a set of arcs that store information about the operation relations. Stamping operations are related to one another through two kinds of relationship, “cluster” or “precedence” relations. Cluster stamping operations are executed simultaneously and can be staged at the same die station. Stamping operations in precedence must be performed in sequence and so they are staged in adjacent die stations. Cluster relation, and precedence relation are represented by dashed ellipses and directed solid line, respectively, as shown in Fig. 2. Note that stamping operations B and C work simultaneously, and are staged at the same die station, while stamping operation A precedes operation B, and is staged in a die station immediately prior to the one for the operation B.
The strip layout can be generated by a computer automatically using the graph-based stamping process plan, which is suited for computer implementation and leads to efficient formulation and solution procedures.
3.2 Knowledge sources (KSs)
3.2.1 Specialist KSs
The planning objects on the blackboard outlined above are not isolated data structures, but are interrelated to each other by a set of specialist KSs that resemble experts by embodying the problem solving knowledge.
The specialist KSs are independent chunks of knowledge and do not communicate directly with each other. Instead, they participate in the problem solving process by contributing theirpartial solutions to the blackboard, or updating the contents of the blackboard.
The perse specialist KSs related to stamping process planning include, but are not limited to, unfolding knowledge to produce a flat pattern, nesting knowledge to produce a blank layout, mapping knowledge to transform stamping features into stamping operations, and staging knowledge to sequence the stamping operations. Methods such as structured interviews, observation, structuring techniques [26] and knowledge modeling approach [27] can be used for eliciting the perse knowledge. Due to the modularity of the blackboard framework, it is convenient for the experts to expand the KS space in the system by integrating different methods of knowledge representation, such as procedures, rules and objects with the aid of knowledge engineers. Unfolding and nesting knowledge are represented as conventional procedures and will not be elaborated further because this topic has been well-researched and is mature technology [4, 9, 13, 28]. Our work focuses on the mapping knowledge and staging knowledge.
3.2.2 Object-oriented knowledge organization of KSs
Building upon the hierarchical knowledge organization concept by Kamel and Quintana [29] that is proven effective in management of heterogeneous KSs in knowledge system development, we employ the object-oriented approach to organize KSs in a hierarchical architecture. The top knowledge is abstract and general, while the descendent knowledge is specific and is used to solve practical problems. Each KS is modeled as an object that resides in an independent module. The implementation of the KS depends on the attributes encapsulated inside the KS object. In other words, the KS is activated if its precondition is satisfied. Instead of immediately executing the activated KS, a knowledge source activation record (KSAR) is created first and is placed into the agenda pending execution of the KS body. Hence, referring to Fig. 3, the abstract KS objects can be represented as “Abstract_KS” class with the basic attributes including “problem_domain”, “precondition”, and “KS_body”. Problem_domain indicates the domain of KS application. KS_body may accommodate a hard-coded procedure or a production rule. The Abstract_KS class also contains the basic methods such as “reset ( )” and “evaluate_blackboard ( )”. Reset ( ) is used to restart the KS, and evaluate_blackboard ( ) is used to evaluate the state of the blackboard.