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.

A new approach to automatically generate polyhedral convex cones from contact graphs for identification of assembly process states is presented. The state of contact between subassemblies is changing as operation proceeds in manipulative tasks such as part-mating. Measured contact forces and the resulting torques provide information about the contact geometry which is used to guide the assembly operation. In this paper, we present a method to calculate contact graphs for polyhedral subassemblies. The nodes of the contact graph represent contact formations of the subassemblies and the arcs represent neighborhoods of contact formations. This information is used to generate polyhedral convex cones for each contact formation considering friction. These polyhedral convex cones describe static equilibrium for each contact formation and are used to identify the contact state by measuring forces and torques during a part-mating.

The execution of automatically generated assembly plans by robots is one of the key technologies of modern and flexible manufacturing. During the execution of assembly sequences the robot comes into contact with the environment. Since there are positional and geometrical uncertainties from object representation, robot motion, and sensor information, compliance is used typically to prevent excessive contact forces. The contact forces and the resulting torques provide information about the contact geometry which is used to guide the assembly operation. We present a fast algorithm to identify assembly process states considering static friction under uniform gravity. This identification of assembly process states enables a robot to select and modify its motion strategies adequately according to the state. We give a symbolic representation of contact states and analyze the static properties of assemblies at each contact state by using the theory of polyhedral convex cones.

Robotic manipulators in contemporary work-cells are often incapable of solving even simple pick-and-place operations. More specifically, most systems require each object to be supplied in the same and pre-defined way. Introducing regrasping to the production cycle relaxes the constraints imposed on supply mechanisms. Regrasping describes the operation that must be performed whenever an object's pick-up grasp is incompatible with its put-down grasp. The paper presents a novel approach to solve the regrasp problem for robots equipped with parallel-jaw end-effecters. The proposed method first evaluates the object's possible grasps and placements. These are subsequently combined to form grasp-placement-grasp triples. A regrasp sequence leading from the pick-up to the put-down grasp is generated by searching through the resulting space of compatible grasp-placement-grasp triples. The algorithm takes kinematic and geometric constraints of both the manipulator and the objects into account. Computational concerns are addressed by subdividing the calculation into an off-line and a fast online phase.

We discuss assembly stability as a constraint for assembly sequence planning. An assembly sequence plan is a high-level plan for constructing a mechanical product from its component parts. The analysis of (sub)assembly stability avoids mating parts which tend to disassemble under the influence of gravity. Furthermore, the number of reorientations which gives a good idea on the value of an assembly sequence depends on the stability of the parts to be assembled. We present algorithms to calculate the set of potentially stable orientations of an (sub)assembly considering static friction under uniform gravity. This set is used for the evaluation of assembly sequences and the minimization of the number of reorientations during plan execution. Therefore, a new evaluation function based on the set of potentially stable assembly orientations is proposed and integrated into the assembly cost evaluation of a high level assembly planning system.

The execution of generated assembly plans by robots is one of the key technologies of modern and flexible manufacturing. During the execution of assembly sequences the robot comes into contact with environment. Since there are positional and geometrical uncertainties from object representation, robot motion, and sensory information, compliance is used typically to prevent excessive contact forces. These contact forces provide information about the contact geometry which is used to guide the assembly operation. In this paper we present an algorithm to classify and recognize contact states between subassemblies. We analyze the local depart space (LDS) given by the assembly planning system for the calculation, classification, and recognition of the contact states. The LDS describes the direction of motion for the separation of subassemblies involved in the assembly sequence. We calculate valid configurations of the active subassembly in an uncertainty region and use the theory of polyhedral convex cones for the recognition of contact states. Therefore, force-torque sensor information acquired in the assembly process is interpreted using state classifiers which are formulated by polyhedral convex cones.

We discuss the stability of assemblies consisting of arbitrary configurations of rigid bodies considering Coulomb friction under uniform gravity. We give a method to calculate the set of all stable orientations of an assembly considering the frictional properties. The presented algorithm is a new method for automatically determining all stable orientations of assemblies considering friction. The potential stability criterion describes a necessary but not a sufficient condition for the stability of assemblies. To compute the potential stability of an assembly we consider the forces which arise at contact points of objects touching each other. Constraints which must be satisfied for a set of valid contact forces are expressed by linear inequalities; we use linear programming to determine such valid contact forces. Beside this we describe a method based on the concepts of the enumeration of vertices of polyhedra to compute the set of stable orientations considering Coulomb friction even in the case of degeneracy in the linear program, Furthermore, we discuss the space and time complexity of the underlying algorithm.