Daniel E. Whitney

Simplified generation of all mechanical assembly sequences

Bourjault has presented a method which will generate all valid assembly sequences for the set of parts that constitute an assembly. A modification of Bourjaults method is presented which makes practical the application of this technique of assembly analysis to assemblies with greatly increased part count. The salient difference between the two methods is in the form and number of the questions whose answers yield the relations that allow algorithmic generation of assembly sequences. Bourjaults method requires 2l^{2} questions plus an often-large number of subsequent questions whose existence depends on answers to part of the former question set; all have yes or no answers. (Here l is the number of relations between parts.) The modified method requires 2l questions that are answered in a precedence-logical form; the questions are similar to those asked by an engineer contemplating assembly of a set of parts. Applications and techniques for use are presented, and examples are shown for l as great as 18.

Feedback Control of Two Beam, Two Joint Systems With Distributed Flexibility

The control of the flexible motion in a plane of two pinned beams is addressed with application to remote manipulators. Three types of linear feedback control schemes are considered: joint angle and velocity feedback with (GRC) and without (IJC) cross joint feedback, and feedback of flexible state variables (FFC). Two models of the distributed flexibility are presented along with some results obtained from them. The relative merit of the three control schemes is discussed.

An integrated computer aid for generating and evaluating assembly sequences for mechanical products

An integrated computer aid that is useful for assembly-line design and for concurrent design of mechanical products is described. Recognizing that early consideration of assembly sequence is important for producibility, quality control, flexibility, and market responsiveness, the authors have built an integrated set of user-interactive computer programs that generates all feasible assembly sequences for a product and then aids the user in judging their value based on various criteria. The programs use a disassembly analysis for generating sequences and provide online visual aids during generation and evaluation. During evaluation, matters such as avoiding difficult assembly states or moves, stability, fixturing, orientation, refixturing and reorientation count, and inclusion of favorable states are considered to highlight desirable or undesirable sequences. The designer edits the set of sequences according to these criteria, leading to an informed sequence choice or to needed design refinement. The interactive programs provide a rapid means for sequence selection, encouraging their use during early design. >

Historical perspective and state of the art in robot force control

This presentation combines historical lineage, assessment of the state of the art, and representative videotapes on robot force control. The difference between continuous and logic branching strategies is described. The development of various impedance strategies and hybrid methods is traced and compared. The problem of stability is discussed and remedies are related to higher strategy issues.

Modeling and controlling variation propagation in mechanical assemblies using state transition models

Presents algorithms to propagate and control variation in mechanical assemblies using the state transition model approach. It exploits the modeling environment and uses concepts from control theory to model variation propagation and control during assembly. The assembly process is modeled as a multistage linear dynamic system. Two types of assemblies are addressed: Type-1 where the assembly process puts together parts at their pre-fabricated mating features and Type-2 where the process can incorporate in-process adjustments to redistribute variation. Algorithms are developed to determine and control variation in final assembly propagated through the combined effect of individual part variations and choice of assembly methods. Algorithms to propagate variation in the presence of adjustments are also presented. In-process adjustments in Type-2 assemblies are determined by the type of interface features between parts being assembled which are modeled as control inputs to the dynamic system. An optimal control problem is formulated to design these interfaces. Variation associated with final assembly dimensions, cost of making adjustments, and assembly sequence effects are included in the optimization procedure.

A Prototype of Feature-Based Design for Assembly

This paper describes a prototype software system that implements a form of feature-based design for assembly. It is not an automated design system but instead a decision and design aid for designers interested in concurrent design. Feature-based design captures design intent (assembly topology, product function, manufacturing, or filed use) while creating part and product geometry. Design for assembly as used here extends existing ideas about critiquing part shapes and part count to include assembly process planning, assembly sequence generation, assembly fixturing assessments, and assembly process costs. This work was primarily interested in identifying the information important to DFA tasks, and how that information could be captured using feature-based design. It was not intended to extend the state of the art in feature-based geometry creation, but rather to explore the uses of the information that can be captured. The prototype system has been programmed in LISP on Sun workstations. Its research contributions comprise integration of feature-based design with several existing and new assembly analysis and synthesis algorithms; construction of feature properties to meet the needs of those algorithms; a carefully chosen division of labor between designer and computer; and illustration of feature-based models of products as the information source for assembly analysis and process design. Some of its functions have been implemented approximately or partially but they give the flavor of the benefits to be expected from a fully functional system.

Nippondenso Co. Ltd: A case study of strategic product design

Nippondenso Co. Ltd (NDCL) is Japans foremost manufacturer of automotive components. Over the past twenty-five years it has developed a variety of approaches to automating the assembly of products in order to meet the high-variety, just-in-time production requirements of its customers, notably Toyota. The approach evolved by NDCL is to design the product intelligently and to make massive use of the simplest automation technology possible consistent with the technical challenges of the product and its production strategy. The result is the capability to manufacture products with considerable model mix at high volume, with little or no changeover time between models. This is essentially a technological response to a business environment challenge.In pursuit of this strategy, NDCL has categorized the problems of assembly automation into distinct classes, identified applicable solutions for each class, and successively attacked and solved increasingly difficult problems. This paper describes this strategy, gives examples of its evolution, and indicates how NDCL has managed production technology, notably robots, as part of the overall attack. NDCLs approaches to concurrent engineering (CE) and new product risk management are also described. The paper is based both on seven personal visits to NDCL during the period 1974 to 1991, which included extensive interviews with NDCL engineers and managers and plant tours, and on papers published by NDCL and interviews with their authors.

Using simulated annealing to select least-cost assembly sequences

Applies simulated annealing (SA) to find the probable least cost assembly sequence for a mechanical product from among the thousands of feasible sequences. The sequences are represented by the assembly sequence network introduced by De Fazio and Whitney (1987). Candidate sequences are selected one at a time by the SA algorithm and their cost is estimated by designing a minimum unit cost concept assembly system using a dynamic programming algorithm. Alternate sequences are selected by perturbing the path through the sequence network in a way similar to a genetic algorithm. This method differs from that normally used to select least cost sequences, in which an engineer edits the thousands of sequences using judgment and experience, and then obtains the costs of a few that survive editing. Here, the search for a least cost sequence is done without prior editing. In the examples studied, there appear to be a large number of sequences that are technically quite different but of similar cost.<<ETX>>

A mathematical programming procedure for equipment selection and system evaluation in programmable assembly

A math programming model is formulated for selecting assembly stations and assigning operations to these stations so as to satisfy a production volume requirement at minimum system cost. A branch-and-bound algorithm, coupled with a subgradient optimization procedure, is proposed. The model and algorithm are demonstrated by example on a system design problem for assembling automobile alternators. The model is applicable to many kinds of manufacturing systems.

Design-specific approach to design for assembly (DFA) for complex mechanical assemblies

Uses assembly sequence analysis (ASA) to explore design for assembly (DFA), subassembly partitioning, and assembly sequence choice for two complex assemblies. Complex assemblies have very high parts-counts, a final assembly organized as an assembly of subassemblies, and offer limited redesign options. ASA addresses combinatorial aspects of complex assemblies that conventional DFA ignores: choice and partitioning of subassemblies, and assembly sequence choice. The paper describes criterion-based searches for favorable subassembly partitioning and assembly sequences that use genetic algorithm techniques to spread assembly move difficulty across entire final assembly sequences while satisfying all logical constraints imposed on the assembly sequence by part geometry. The measure of assembly move difficulty, a count of kinematic degrees of freedom secured during each final assembly step, is measured on an absolute scale. We find that ASA can pinpoint candidate DFA-related redesigns and can suggest assembly issues to designers. Logical assembly issues dominate quantitatively-characterized issues when selecting assembly sequence or subassembly partitioning. After logical issues are addressed, the sequence choice criterion defined here often duplicates choices made by experienced analysts. Finally, the sequence choice criterion favors in-line over branched final assembly lines.