An assembly sequence planner is a system which, given a description of an assembly to be manufactured, identifies subassemblies and generates high-level plans for the construction of the each subassembly, including the final assembly. Previous research has shown that feasible plans can be efficiently produced for large assemblies, but the generation of good plans has met with less success. Not only are there difficulties in defining "good plans," but all published methods for finding good plans have computational complexities that make them unlikely to be practical for large assemblies. This paper shows that representing an assembly as a hierarchy of assembly structures can overcome many of these difficulties. A planner is described which uses the structure hierarchy both as a framework for structure-dependent definitions of a good plan, and as a tool for finding good plans more rapidly by using high-level expert advice, by reusing subplans for repeated substructures, and by not fully optimizing the plan. Analytical and experimental results are presented to demonstrate the effectiveness of the approach.

A wide range of different approaches to assembly sequence planning has been described in the literature, but, to date, primarily assemblies of rigid parts have been considered. Yet flexible parts of all sorts are common in real world assemblies. This paper considers the special characteristics of operations on flexible parts in general and string-like parts (wires, cables, hoses, ropes, etc.) in particular. Different types of operations are discussed, and knowledge representation techniques for assembly states and operations are described, together with basic algorithms for queries on string-like assemblies.

This paper describes a fundamentally new approach to assembly sequence planning which views an assembly as a hierarchy of standard structures and uses a strategy of merging partial plans for structures to derive a plan for the whole assembly. Such a planner has been implemented on a Sun Sparc Station 1. It generates a plan in which there are no subassemblies, if such a plan exists, given as a partial order in terms of the sequence of part insertion and their trajectories. The planning algorithm is shown to be correct and complete and the methodology is shown to lessen considerably the combinatorial problems encountered by current planners working on large real world problems.>

During the assembly of a product, it is vital that the partially completed assembly be stable. If the assembly is unstable, then it must be fixtured to stabilize it before retrieving the next part or subassembly This paper presents a stability test and a new approach to automatically generating the positions of a small set of fixture elements (fixels) that will stabilize an assembly. The stability test and the fixel positioning approach consider both the translational and rotational degrees of freedom of each part. Since all the relevant mechanical constraints are linear functions of the contact force magnitudes and the components of the velocities of the parts, linear programming techniques can be used with great efficiency.>

The authors analyze the most commonly used form of constraint languages for assembly planning: mating constraint languages. These constraints are typically composed of relations asserting that a certain pair of parts must be mated before some other pair of parts may be mated. However there are many significant differences between the forms of these constraints used by different authors. It is proven that some of these forms, such as those based on less-than relations and those based on less-than-or-equal-to relations, are equivalent. A variety of theorems that address which kinds of mating constraint languages are able to represent which kinds of sets of plans are included.>

Current assembly planning systems have used different definitions of an assembly plan and applied different restrictions to the planning process. This report surveys, analyzes, and compares several key aspects of assembly planning systems. The most common restrictions applied to assembly plans-sequentiality, monotonicity, and coherence-are defined and discussed. Three distinct definitions of an assembly plan are described and compared. Several enumerative data structures designed to represent large sets of assembly plans are analyzed and compared.>

The author describes a level of assembly process-planning in which a task-level plan is generated directly from a model of the proposed assembly. This involves not only the generation of a series of operations to build the assembly but also the selection or design of an appropriate workspace with suitable fixtures and tools. The author introduces the assembly planning problem and describes an experimental assembly planning system which is capable of rapidly generating plans for most assemblies. The structure of the proposed assembly planner, XAP/1, is described and the performance of the XAP/1 is discussed.>

Grip determination is essential to any task-level planning process. The complete force/moment equations are presented for grasping employing a rigid two-fingered gripper with a thin elastic layer on the contacting surface. The surface contact is modeled as a linear pressure variation. The quality measure of a grip is taken to be the coefficient of friction needed to keep the held object from slipping between the fingers under applied forces and moments. The lower the coefficient of friction, the better the grip. Incorporation of this evaluation into general grip selection strategy is discussed, and several examples are given.

Grip determination is essential to any task level planning process and to situations in which the pose of the part to be grasped is not known a priori. This paper presents the complete force/moment equations for grasping via a rigid two-fingered gripper. Surface contact is modeled as a linear pressure variation. The quality measure of a grip is taken to be the coefficient of friction needed to keep it from slipping under applied forces and moments, the lower the coefficient of friction, the better the grip. The incorporation of this evaluation into a general grip selection strategy is discussed and several examples given.

The problem of automatically determining gripping positions for objects based upon geometrical knowledge of the object and its environment is addressed. Both the question of what constitutes a good grip and the efficiency of the computational algorithms are considered. Two new criteria for the stability of a grip are proposed: resilience to slippage and twisting in the gripper due to external forces and torques. Resilience to slippage is expressed in terms of friction effects of the surfaces involved, the shape of the contact between the gripper and object, and the distance of the grip point from the center of mass of the object. Two new performance measures are introduced to reflect resilience to twisting. A general grip planning strategy is introduced that is both run-time efficient and robust. The problem is divided into three parts: one depends only upon the geometry of the object to be gripped, the second takes into account a priori geometric constraints, and the third handles constraints unknown until run-time.

The interrogation of spatial relations and properties of three-dimensional objects are often involved in applications programs for CAD/CADM and robotics. These interrogations can be expressed in terms of low-level data retrieval queries called access primitives (APs). A data structure is presented for storing spatial relations of three-dimensional objects such that the amount of storage is linearly proportional to the number of entities, and the response time for the APs is constant on the average. Linear storage for this data structure is the lower bound. The data structure is in the form of a symmetric graph. UsingE, the total number of edges of the object, as the unit, it is shown that the storage requirement of the symmetric data structure is 6E. By defining nine general data structure accessing primitives, it is shown that any local topological query can be answered in constant time, on the average.