TR020: SightPlan: A Blackboard Expert System for Constraint-Based Spatial Reasoning About Construction Site Layout

SightPlan is an expert system that lays out temporary facilities on construction sites. It demonstrates how one can closely model the steps taken by a person performing layout design, and how interactive graphics combined with an expert system can augment human decision-making. This report describes site layout practice and reviews the state of the art of layout modeling. The work on layout modeling fits within the larger context of spatial reasoning and generic design, so SightPlan can be related to ongoing research in other domains. SightPlan builds upon the domain-independent BB 1 blackboard architecture and uses the ACCORD language for constructive assembly.

Three SightPlan strategies are applied to two case studies of power plant construction (the Intermountain Power Project and the American 1 project). An early-commitment strategy models the way human experts conduct site layout and produces a layout solution that satisfies all constraints. A temporal strategy tests and validates the early-commitment strategy and extends it by explicitly representing and reasoning about objects over time. This allows the global site layout to be animated as a sequence of layouts over discrete time intervals. A postponed-commitment strategy takes advantage of computer capabilities by delaying commitments and then heuristically sampling possible positions to find several satisfying solutions. While these solutions meet hard constraints, they appear "chaotic"; that is, a person viewing them is likely to introduce additional constraints. SightPlan can introduce such constraints at run-time, or a user of the SightView graphical interface can introduce such constraints interactively and feed that information back to SightPlan for further reasoning.