Journal of Scheduling 7: 375–395, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. ROBOTIC CELLS WITH PARALLEL MACHINES: THROUGHPUT MAXIMIZATION IN CONSTANT TRAVEL-TIME CELLS H. NEIL GEISMAR, MILIND DAWANDE, AND CHELLIAH SRISKANDARAJAH ∗ University of Texas, Dallas ABSTRACT We present a general analysis of the problem of sequencing operations in bufferless robotic cell flow shops with parallel machines. Our focus will be cells that produce identical parts. The objective is to find a cyclic sequence of robot moves that maximizes the steady state throughput. Parallel machines are used in the industry to increase throughput, most typically at bottleneck processes having larger processing times. Efficient use of parallel machines requires that several parts be processed in one cycle of robot movements. We analyze such cycles for constant travel-time robotic cells. The number of cycles that produce several parts is very large, so we focus on a subclass called blocked cycles. In this class, we find a dominating subclass called LCM Cycles. The results and the analysis in this paper offer practitioners (i) guidelines to determine whether parallel machines will be cost-effective for a given implementation, (ii) a simple formula for determining how many copies of each machine are required to meet a particular throughput rate, and (iii) an optimal sequence of robot moves for a cell with parallel machines under a certain common condition on the processing times. KEY WORDS: manufacturing, robotic cell, constant travel-time, parallel machines, LCM cycles 1. INTRODUCTION Robotic cells have become a standard tool in modern manufacturing. The efficient use of such a cell is essential in order to compete in this marketplace. As manufacturers implement larger and more complex robotic cells, more sophisticated models and algorithms are required to optimize these operations. These models and algorithms often focus on cell design, sequencing of robot moves, and optimal scheduling of the products or parts to be produced. In this paper, we treat the problem of sequencing robot moves to achieve efficiency in material handling in an important class of robotic cell flow shops: those in which certain processing stages have multiple machines to perform processing. Such machines are called parallel machines. This paper is a general analysis of robotic cell flow shops with parallel machines. The robotic cells considered in this paper consist of a number of stages served by a single robot. Each part is processed at each stage; each part follows the same order through these stages. A three- stage robotic cell is illustrated in Figure 1. Each stage has one machine; such a cell is called a simple robotic cell. The robot arm rotates and moves linearly to handle the intermachine movements of the parts. As in a classical flowshop, each part being processed passes successively from the input hopper ( I ) to machines M 1 , M 2 , and M 3 , and finally to the output hopper ( O). After loading a part onto a machine, either the robot waits at the machine for it to finish processing the part, or it ∗ Correspondence to: E-mail: chelliah@utdallas.edu