An integrated approach to the optimal sequencing of robot operations in a workcell

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Virginia Tech

In this research we develop an integrated approach to optimally sequence robot operation in a workcell. The workcell represents a f1owshop operation with multiple robots transporting jobs among machines. A buffer of infinite capacity is available ahead of each machine. A robot transports jobs from buffers to machines and from machines to buffers. The robots used in the system are 5 jOint cylindrical coordinate robots. All the robots are identical in design and capability. For the type of robot used in this study its closed form inverse kinematic solution is known. The objective is to determine the sequence of robot operations so as to minimize the total time needed to complete all jobs (known as makespan).

The integrated approach consists of determining optimal robot task sequences using a branch and bound procedure. and a graphical simulation procedure to display the robots as they perform transport operation. The branch and bound algorithm, which is an implicit enumeration scheme, is used to derive several near optimal robot task sequences. For the branch and bound algorithm. each node is a transport operation. Lower bound on the makespan is machine based and is computed at each node for further branching. The graphical simulation is used to detect interference among robots which is hard to be incorporated in the branch and bound procedure. Infeasible robot sequences are discarded and other solutions from the branch and bound procedure are displayed using the graphical simulation procedure to determine a near optimal and feasible sequence.

The integrated approach is implemented on a prototype system. A command driven general purpose graphics system MOVIE.BYU is used for the graphical simulation of the robotic workcell and robot motion. The entire system is available in an integrated environment. A powerful programming language Rexx is used to manage various programs and data. Also, intermediate Rexx programs are generated during execution to allow MOVIE.BYU to edit and display animation data on the Tektronix 41XX series of terminals. The entire system is flexible and modular to be able to be used for various different applications.