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.

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

In recent years, features have been introduced in modeling and planning for manufacturing of parts. Such features combine geometric and functional information. Here it is shown that the feature concept is also useful in assembly modeling and planning. For modeling and planning of both single parts and assemblies, an integrated object-oriented product model is introduced. For specific assembly-related information, assembly features are used. Handling features contain information for handling components, connection features information on connections between components. A prototype modeling environment has been developed. The product model has been successfully verified within several analyses and planning modules, in particular stability analyses, grip planning, motion planning and assembly sequence planning. Altogether, feature-based product models for assembly can considerably help in both assembly modeling and planning, on the one hand by integrating single-part and assembly modeling, and on the other hand by integrating modeling and planning.

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Assembly features in modeling and planning

This paper focuses on the planning and scheduling problems in disassembly systems, in which used or end-of-life products are disassembled into basic parts and subassemblies for both economic and environmental reasons. The disassembly planning includes product representation, disassembly sequencing with disassembly level and end-of-life options, and related product design/redesign issues, while the disassembly scheduling is the problem of determining the order quantity of the used product to fulfil the demand of disassembled parts and subassemblies. This paper presents a review of the state-of-the-art articles and suggests the corresponding further research directions. The research directions suggested in this paper can be summarized as

Disassembly planning and scheduling: Review and further research:

This paper presents a new method to decompose complex sequences of assembly operations into skill primitives. This can be realized by analyzing hyper-arcs of the underlying AND/OR graphs representing automatically generated assembly plans. Features like local depart spaces, symbolic spatial relations, and the necessary tools classify the type of assembly operation (peg in hole, placements, alignments, etc.). Skill primitives are robot movements or commands for grippers and tools. The unified modeling language is used to model the robot tasks and skill primitives. A robot control system uses the skill primitives as input to select the desired control scheme (position, force, or hybrid). In addition to this, we use an algorithm to identify assembly process states considering static friction under uniform gravity to execute skill primitives. This enables a robot to select and modify its motion strategies adequately according to the state of the assembly operation.

Automatic decomposition of planned assembly sequences into skill primitives

In this article, a personal computer disassembly cell is presented. With this cell, a certain degree of automatism is afforded for the non-destructive disassembly process and for the recycling of these kinds of mass-produced electronic products. Each component of the product can be separated. The disassembly cell is composed of several sub-systems, each of which is dedicated to the planning and execution of one type of task. A computer vision system is employed for the recognition and localisation of the product and of each of its components. The disassembly system proposed here also has a modelling system for the products and each of its components, the information necessary for the planning of tasks, generating the disassembly sequence and planning of the disassembly movements. These systems co-operate with each other to achieve a semi-automatic disassembly of the product.

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Automatic PC disassembly for component recovery

The authors propose the stability analysis of subassemblies which has several important benefits in assembly planning. Some of the cost criteria such as reorientation, parallelism, and fixturing are identified. Incorrect assembly sequences can be eliminated by identifying unstable subassemblies. A more efficient planner can be built by reducing the search space by pruning out unstable subassemblies early in the planning steps. A more complete stability analysis of subassemblies is presented. Three necessary stability criteria are considered: subassembly-stability, pose-stability, and mating-stability. Force-based reasoning is introduced in the analysis. Algorithms to solve the stability criteria are described. >

Subassembly stability and reorientation

The design of tolerances should take the functionality, manufacturability, assemblability and cost effectiveness of a product into consideration. This paper presents a method of analyzing and evaluating the assemblability of a product under given tolerance specifications in order to provide a tool to aide for tolerance design. Introduced is the range of feasible displacement of a part of an assembly as a result of the accumulation of clearances between mating parts along the serial as well as parallel chain of assembly, which can play a role of compensating for the pose error of the corresponding part due to the accumulation of tolerances. Assemblability is determined by the probability that all the parallel as well as serial chains of the assembly can be f ormed successfully under given statistical properties of tolerances. This requires to numerically compute the probability that a pair of mating parts forming a parallel chain can align themselves within the error margin defined by their mating clearance, accounting for the possible compensation of the ranges of pose errors associated with the mating parts due to tolerance accumulations by the ranges of feasible displacements of the mating parts due to clearance accumulations. Simulation results are shown.

Statistical representation and computation of tolerance and clearance for assemblability evaluation

We present an approach for evaluating the assemblability of a product based on tolerances and adjustable displacements. An adjustable displacement is a functionally permitted free space between two mating features, which can be used for compensating tolerances. The assemblability of a product is computed in term of a multi chain by incrementally solving the parallel chains. We consider both the functionality and assembly sequence constraint in the evaluation. The result of the computation is a statistical measure of the product assemblability which can be used by the designer to evaluate and to optimize the tolerance allocation. In addition, this measure can be used for evaluating assembly sequences. The algorithms and simulation results are given.

Tolerance analysis for multi-chain assemblies with sequence and functionality constraints

The specification of tolerances is an integral part of product design since tolerances directly affect the assemblability, as well as the functionality, manufacturability, and cost-effectiveness of the product. However, there has been little research on the assemblability evaluation based on tolerance propagation which can play an important role in assembly planning. Assembly planners in the past have assumed nominal dimensions for product components for generating the feasible assembly sequences. These sequences, however, may not be assemblable in practice due to propagations and accumulations of tolerances. In this paper, we present a new approach to assemblability evaluation based on tolerance propagation. For the assemblability evaluation, we introduce an important concept of clearance. Clearance represents the possible free space between two mating features. In the assemblability evaluation, clearances are used to compensate for tolerances.

Assemblability evaluation based on tolerance propagation

Fixtures are devices used in manufacturing to immobilize a part far operations such as assembly, inspection or machining. Modular fixtures are configurations of standard components. The goal of a automatic modular fixture design is to find algorithms to determine a set of fixturing contact points automatically, given a set of modular components and the fixturing requirements. This paper considers the problem of generating valid assembly sequences for a given modular future configuration. This is called assembly planning for modular fixtures. Because the application domain is known, we can devise an efficient assembly planning system taking advantage of the characteristics of the fixture domain. In this paper, we propose a fastener-based approach which considers the functionality of fasteners (to hold the fixture components together) to achieve both the efficiency in assembly planning and the stability of the generated assembly sequences. We also describe an accessibility measure that can be used to select preferred assembly sequences. This work complements automatic fixture design such that complete automation can be achieved from the design of a fixture to its assembly, which can be performed manually or with the aid of robots.

Chunsik Yi

Papers

Subassembly stability and reorientation

Statistical representation and computation of tolerance and clearance for assemblability evaluation

Tolerance analysis for multi-chain assemblies with sequence and functionality constraints

Assemblability evaluation based on tolerance propagation

Assembly planning for modular fixtures