(624i) Simultaneous Optimization of Hoist Scheduling and Production Line Arrangement

Chuanyu Zhao and Qiang Xu*

Dan F. Smith Department of Chemical Engineering

Lamar University, Beaumont, TX 77710, USA

Abstract

In multi-stage material handling processes, such as electroplating and polymeric coating, productivity maximization is always an objective.  Hoist schedule development or hoist scheduling is the most relevant aspect to improve the production efficiency.  Hoist is a controlled robot following a pre-set movement schedule to move various jobs in a production line for processing based on their recipes.  It is reported that as high as 20% reduction in mean job waiting time and 50% improvement in standard deviation of cycle time can be achieved by hoist scheduling.  In previous works, how to feasibly and timely organize cyclic or dynamic schedules has been broadly studied.  However, the productivity of a multi-stage process not only depends on the hoist schedule, but also substantially relies on the production line arrangement, i.e., production units allocation spatially.  This is because a hoist needs to travel among units for different manufacturing steps with or without a job and the travelling time depends on unit locations, which primarily affects the scheduling time or productivity.  Thus, the design and operational scheduling should be simultaneously considered for the best performance of such multi-stage material production line. 

In this paper, a systematic methodology coupling optimization of cyclic hoist scheduling and production line arrangement for maximum productivity has been developed.  It first generates an MINLP model addressing all the major concerned scheduling issues, such as cyclic scheduling for large amounts of jobs with multiple recipes, various production unit capacities, and customized production portfolios.  Then, the MINLP model is fully linearized as an MILP model to ensure the global optimal of the solution.  The methodology can satisfy any customized demand on product ratios among different job types. 

The efficacy of the proposed methodology is demonstrated by a case study of electroplating system.  Three types of jobs with different recipes are processed by a production line, where the overall production rate needs to be maximized.  The discussion for hoist scheduling with and without consideration of production line arrangement will also be addressed. 

Keywords: Cyclic Hoist scheduling, Production line arrangement, Optimization, MILP