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1 Computer Group Engineering Department University of Science and Culture S. H. Davarpanah

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1 1 Computer Group Engineering Department University of Science and Culture S. H. Davarpanah

2 Constructive Problem Solving2 Expert Systems Constructive Problem Solving I cf. Jackson, Chapter 14 Constructive Problem Solving II cf. Jackson, Chapter 15

3 Constructive Problem Solving3 Task Areas of Expert Systems System-Based View of XPS Task Analysis Tasks (Interpretation of System) – Diagnosis – Classification Synthesis Tasks (Construction of System) – Construction – Configuration – Design – Planning

4 Constructive Problem Solving4 Solution constructed by choosing and assembling solution elements. Examples: develop a plan for a robot to bring a cup of coffee from IQ; construct a system, e.g. a computer, by assembling a set of components, like HD, CPU etc. Solution elements are described as components (maybe with parameters); assembly is subject to constraints (robot cannot go to IQ if it cannot take the elevator; certain CPUs need certain power supply); solution might be subject to evaluation function

5 Constructive Problem Solving5 Constructive PS and Task Areas Planning solution elements = actions solutions = sequence of actions constraints = e.g. physical or logical constraints Design solution elements = components solutions = combination of components constraints = e.g. physical or logical constraints Diagnosis of multiple disorders solution elements = disorders solutions = sets of disorders to explain the symptoms ; determine 'best' set according to evaluation

6 Constructive Problem Solving6 Constructive PS Approach Choose combination of solution elements – set – sequence – complex arrangement Combined according to constraints – order (sequence of steps; arrangement of components) – time (sequence of actions in time; limit of time) – spatial arrangement (layout in space, e.g. floor plan) – features and their agreements (e.g. matching voltage for electrical components, matching colors for cloths) –...

7 Constructive Problem Solving7 Constructive PS as Search Search Space: all combinations of solution elements Search Space can be huge! Restrict search by selecting solution elements based on known constraints; selection of new components is restricted through constraints. Constraints can be formulated in rules: IF device requires battery THEN select battery for device IF select battery for device THEN pick battery WITH voltage(battery) = voltage(device) IF device requires battery AND device = watch THEN select micro-battery

8 Constructive Problem Solving8 Constructive PS - R1/XCON R1/XCON developed to design DEC VAX computer systems (early 1980ies) R1 uses a database of computer components a rule base specifying design rules and constraints a working memory (WM) to store interim structures, in particular the partial computer configuration generated so far

9 Constructive Problem Solving9 R1/XCON – Sample Component Jackson, p. 262, Figure 14.1

10 Constructive Problem Solving10 R1/XCON – Types of Rules Rule types related to tasks in PS: 1.Operator Rules create and extend partial configurations 2.Sequencing Rules determine order of processing (contexts, modules) 3.Information-Gathering Rules access database of components; perform various computations

11 Constructive Problem Solving11 R1/XCON – Sample Rule Jackson, p. 262, Figure 14.2

12 Constructive Problem Solving12 R1/XCON – Basic Principles R1 selects component from DB; entered as token (instance) into Working Memory (WM). Rules specify configuration patterns (conditions, constraints) and actions for extending partial configurations (consequences). Rule set divided into "contexts" (modules) according to sub-tasks. Strategy – Finish a sub-task before starting a new one. Implementation – Add contexts to condition part of rules. – Switch to new context in the action part.

13 Constructive Problem Solving13 R1/XCON – Sub-Tasks 1.Check and complete order. 2.Configure CPU. 3.Configure unibus modules, prepare cabinets with modules. 4.Configure paneling; assign panels to unibus modules and devices. 5.Generate floor plan. Device arrangement. 6.Cabling.

14 Constructive Problem Solving14 R1/XCON – Problem Solving Propose-and-Apply (Bachant 1988): 1.Initialize Goal (for current task) 2.Propose Operator (plausible next steps) 3.Prune Operator (according to global criteria) 4.Eliminate Operator (pair-wise comparison) 5.Select one Operator (based on 2-4) 6.Apply Operator (extend configuration) 7.Evaluate Goal (okay? or not?)

15 Constructive Problem Solving15 R1/XCON - Conclusion DB of components constraints and actions in rules about 10.000 rules defined and used integrate various experts' knowledge heavily based on "what-to-do-next" follows always one line of reasoning control through contexts (modules)

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