Methods for incorporating imprecision in engineering design decision-making are briefly reviewed and compared. A tutorial is presented on the Method of Imprecision (MoI), a formal method, based on the mathematics of fuzzy sets, for representing and manipulating imprecision in engineering design. The results of a design cost estimation example, utilizing a new informal cost specification, are presented. The MoI can provide formal information upon which to base decisions during preliminary engineering design and can facilitate set-based concurrent design.

Negotiations are common in engineering design, especially on large projects, and are typically conducted informally. Often, negotiation is used to handle the imprecision or uncertainty that is inherent in the design process. Performance targets, initially specified as hard numerical constraints, are adjusted throughout the design process in negotiations between engineers and managers. Crucial unmeasured or unmeasurable aspects of performances, such as aesthetic concerns, are commonly negotiated. Negotiations settle conflicts between engineering groups over values of shared design variables and distribution of limited design resources. In this thesis, a formal description of negotiation in engineering design is presented. This formal model builds on earlier work at Caltech in the modelling of imprecision in engineering design. Negotiation is modelled mathematically as the aggregation of preferences. A complete characterization of the aggregation problem and of the aggregation operators...

Imprecision is uncertainty that arises because of vague or incomplete information. Preliminary design information is characteristically imprecise: specifications and requirements are subject to change, and the design description is vague and incomplete. Yet many powerful evaluation tools, including finite element models, expect precisely specified data. Thus it is common for engineers to evaluate promising designs one by one. Alternatively, optimization may be used to search for the single "best" design. These approaches focus on individual, precisely specified points in the design space and provide limited information about the full range of acceptable designs. An alternative approach would be to evaluate sets of designs. The method of imprecision uses the mathematics of fuzzy sets in order to represent imprecision as preferences among designs: . Functional requirements model the customer's direct preference on performance variables based on performance considerations: the quantifi...

Numerical concept selection methods are used throughout industry to determine which among several design alternatives should be further developed. The results, however, are rarely believed at face value. Uncertainties (or errors) in subjective choices, modeling assumptions, and measurement errors are fundamental causes of this disbelief. This paper describes a methodology developed to predict overall final error ranges and to estimate a confidence measure in the numerical evaluation results, based upon assigning error ranges to the individual ratings and weightings. Each numerical assignment is given an associated error range, and then treated as a probability error to create a simple means to propagate the errors into an overall error range on the final rating value. Further, a degree of confidence is derived, similar to a statistical t-test, to indicate an induced confidence level in the final decision. Two preliminary concept selections are shown, to illustrate the methodology. Resu...

In this paper a general design and catalog representation is proposed and implemented. The representation is applied to mixed continuous variable and catalog search using genetic algorithms. The representation addresses a number of common catalog search design scenarios. In addition to catalogs of static data (e.g., different sizes of pipes), it is possible to model/search subdesigns that can provide performance data based upon operating conditions. The use of catalog hierarchies allows the search to simultaneously consider catalogs from different vendors and at different levels of detail, therefore achieving the capability of modeling solutions containing general information. The capabilities of the representation are demonstrated through an object-oriented computer implementation that uses a genetic optimization and search algorithm. INTRODUCTION In practice, solving many product design problems involves choosing elements from catalogs and combining them with unique custom componen...

A computer-aided-design (CAD) environment was designed and implemented for the initial design and analysis of large dynamic structures. The design of these types of structures represents a difficult and time consuming task with little support provided by existing CAD packages. The results of this work include: ffl Algorithmic synthesis of complex, dynamic three-dimensional (3-D) structural space frame geometry from initial design specifications. ffl Methodology for dynamic analysis of open kinematic chains which is independent of specific joint trajectories. ffl Dynamic analysis of the complete structure throughout the workspace, including computation of "worst case" loading conditions. ffl Complete algorithmic construction of a Finite Element Analysis (FEA) model for each component of the structure, including conversion of the dynamic loads into a useful form. ffl Discrete optimization of each link of the structure, including catalog lookup of existing beam sizes and heuristics t...

This paper presents a novel formulation of the configuration-design problem that achieves the benefits of the concurrent engineering (CE) design paradigm. In CE, all design concerns (manufacturability, testability, etc.) are applied to an evolving design throughout the design cycle. CE identifies conflicts early on, which avoids costly redesign and can lead to better products. Our formulation is based on a distributed, dynamic, interval constraintsatisfaction problem (DDICSP) model. Persistent catalog agents map onto DDICSP variables and constraint agents map onto DDICSP constraints. These agents use a set of operations and heuristics to navigate through the space of possible designs to rapidly eliminate sets of designs until a solution is found. Experimental results show that an architecture where each catalog agent resides on a separate computer has performance advantages over non-distributed approaches.