DYNAMIC MODELING AND CONTROL OF A MULTIROBOT SYSTEM FOR ASSEMBLY OF FLEXIBLE PAYLOADS WITH APPLICATIONS TO AUTOMOTIVE BODY ASSEMBLY

The field of multi-robot control has almost exclusively addressed issu es that are of relevance to the manipulation of payloads that are rigi d. A number of studies have examined the multi-robot manipulation of r igid payloads with one or more lower-pair joints such as hand tools, i .e., pliers. In this work, we examine certain modeling and control asp ects associated with the assembly of flexible payloads with a multi-ro bot system. While this particular problem is of a general nature, our work is motivated by the particular problem presented by the assembly of automotive bodies from sheet metal parts. State of the art assembly of automotive bodies involves the use of a great number of costly har dware fixtures that are used to orient and clamp each piece of bent sh eet metal prior to robot welding. Currently under development, a new a ssembly technology called flexible fixtureless assembly is being desig ned to replace fixtures with robotic technology. Each robot grasps one piece of sheet metal, and correctly positions and orients the part to mate them to permit a third robot to weld them. The assembly process is complicated by the fact that the sheet metal parts are flexible, ca nnot be permanently deformed during mating and must be positioned to w ithin a relatively small position tolerance. This article describes wo rk performed to model the dynamics of a multi-robot system consisting of two robot manipulators bringing sheet metal parts of an automotive body into contact. This dynamic model is used as a tool to facilitate the investigation of control strategies fur the execution of this task . To adequately model the system, the sheet metal parts are first disc retized into finite shell elements. The flexible payload dynamics are derived via the Lagrangian formulation and combined with the robot dyn amics to form one robot-payload system. The system equations are first simplified by making use of some of the properties of the assembly pr ocess. This allows certain of the interaction effects between flexible and rigid body coordinates to be ignored. Contact between the sheet m etal payloads during the mating process is modeled with an exponential barrier function. Application of Guyan reduction leads to a lower ord er dynamic model of the sheet metal payloads and a simplified dynamic model of the two robot system suitable for numerical simulation. The m odel developed is then used to investigate several candidate control m ethods for the mating of two sheet metal parts. Simulation results are presented for proportional and derivative control with gravity compen sation, computed torque control, and master slave hybrid position forc e control. Simulation results reveal that all three control methods ar e able to achieve contact force and position stability. Adequate perfo rmance of the proportional and derivative control demonstrates that st andard industrial controls implemented in commercial robots may be use d to control robots for fixtureless assembly tasks. (C) 1996 John Wile y & Sons, Inc.