Sequencing of parts and robot moves in a robotic cell

Abstract

Abstract. In this paper, we deal with the problem of sequencing parts and robot moves in a robotic cell where the robot is used to feed machines in the cell. The robotic cell, which produces a set of parts of the same or different types, is a flow-line manufacturing system. Our objective is to maximize the long-run average throughput of the system subject to the constraint that the parts are to be produced in proportion of their demand. The cycle time formulas are developed and analyzed for this purpose for cells producing a single part type using two or three machines. A state space approach is used to address the problem. Both necessary and sufficient conditions are obtained for various cycles to be optimal. Finally, in the case of many part types, the problem of scheduling parts for a specific sequence of robot moves in a two machine cell is formulated as a solvable case of the traveling salesman problem.

Bicriteria robotic cell scheduling

“…From these, we can conclude that for greater processing times S 2 2 is preferable to S 2 1 and for smaller processing times vice versa. Observe that K = 6 + 8δ orP 1 +P 2 = 2δ, is the region of indifference in the case of Sethi et al (1992). However, in the settings of this study, in this region S 2 2 dominates S 2 1 .…”

“…This is another result that differentiates this study from the earlier ones, since the decision tree provided by Sethi et al (1992) compares all of the 1-unit cycles with each other and presents the sufficient conditions for each of them to be optimal with any given cell data, where the only objective is the minimization of the cycle time. In other words, the decision tree spans the whole feasible region.…”

“…More specifically, Sethi et al (1992) proved that the number of feasible 1-unit cycles for an m-machine robotic cell is m!. For a three-machine robotic cell there are a total of six feasible 1-unit cycles.…”

“…The remaining three cycles, S 3 1 , S 3 3 , and S 3 5 can only be optimal for the cycle time values that cannot be attained by S 3 6 . Sethi et al (1992) provided a decision tree on conditions for the robot move cycles to be optimal with any given cell data considering only the cycle time. However, the above theorem shows that earlier results are not valid anymore when the manufacturing cost is considered besides the cycle time.…”

“…Although a solution may have a small cycle time value, it may be dominated because of its poor cost performance. Even the basic results of Sethi et al (1992) regarding the two-machine identical parts robotic cell scheduling problem are not valid when the cost is considered as a performance measure simultaneously with the cycle time. This brings additional insights to the problem and provides flexibility for the decision maker by determining the set of efficient solutions.…”