ANALYSIS OF SUBASSEMBLY STABILITY FOR AUTOMATED ASSEMBLY

For automated assembly, disassembly, fixture, and grasp planning, it is necessary to determine the sequence of steps in which the required goal will be achieved. However, this sequence of steps cannot be arbitrarily chosen. It is necessary to satisfy certain constraints such as geometric and manufacturing resource constraints. In addition, it is also important that the components of the system retain their configuration when subjected to the assembly forces and torques, and frictional and gravitational forces during manufacturing. This paper deals with reasoning quantitatively and qualitatively about the stability of a subassembly. The subassembly stability is analyzed quantitatively by deriving the dynamic differential equations of motion of the subassembly and by perturbing it with the assembly forces and torques. If any component of the subassembly tends to move away from its intended location, the subassembly is considered unstable. As friction plays an important role in maintaining a stable equilibrium of the components, the model incorporates friction. In addition to analyzing stability by the numerical simulation technique, the stability of the system is also determined by studying the effective mass, inertia, stiffness, and damping. The subassembly stability is examined qualitatively by the use of velocity, force, stiffness, damping, inertia, and non-linear forces ellipsoids. Further, to incorporate the stability analysis procedure in the design stage, a stability index has been derived based on the variations in the velocities, forces, inertia, stiffness, and damping in the system. The stability index enables comparison of stabilities of various subassemblies and allows the user to choose the best assembly design. The stability reasoner is currently being incorporated in RALPH (an acronym for " Robot Assembly Language Programming Hierarchy"), a task-oriented system under development at the Automation and Robotics Laboratory.