Coordination of capacity adjustment modes in work systems with autonomous WIP regulation
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ORIGINAL PAPER
Coordination of capacity adjustment modes in work systems with autonomous WIP regulation Neil Duffie • John Fenske • Madhu Vadali
Received: 27 August 2012 / Accepted: 14 September 2012 / Published online: 5 October 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract A method is presented in this paper for coordinating multiple modes of capacity adjustment in work systems with autonomous WIP regulation with the goal of maintaining desired fundamental dynamic behavior. To prevent overcorrection of capacity, adjustments involving floaters, temporary workers, overtime, etc. need to be coordinated, and it is shown that control-theoretic analysis can be used to develop algorithms for determining combinations of adjustments that result in WIP regulation that is as fast-acting as possible yet non-oscillatory. Results of discrete event simulations in Arena, driven by industrial data, are used to illustrate the dynamic behavior of WIP regulation in an autonomous work system that incorporates such an algorithm and multiple modes of capacity adjustment. Keywords
Capacity Control Dynamics
1 Introduction With the increasing complexity and uncertainty in demand, as well as the rise of global competition that modern manufacturing industries face, superior control of internal processes is an attribute that companies strive for in order to maintain a competitive ‘‘edge.’’ Work-In-Progress N. Duffie (&) J. Fenske M. Vadali Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI, USA e-mail: [email protected] J. Fenske e-mail: [email protected] M. Vadali e-mail: [email protected]
(WIP) regulation is an important aspect of this, with objectives of high utilization and keeping lead times short. It has been suggested that optimization of these conflicting objectives can be approached using the concept of ideal minimum WIP [1]. If WIP deviates from this ideal, or some multiple of it, then loss of performance occurs in the form of lower utilization or higher lead times. Regulation of WIP, in the presence of turbulence in demand, requires flexible capacity; the agility with which capacity can be adjusted is a crucial factor. Beyond simply hiring or laying off permanent employees, several modes of capacity adjustment may be available to manufacturing industries. Each has its positive and negative aspects as well as specific constraints. The use of ‘‘floaters’’ is common. Floaters are cross-trained workers who are able to perform a variety of tasks within a company. In a manufacturing environment, they may be assigned to a different department each day, or assignment changes may occur even more frequently. These personnel are useful for filling in for absent workers or increasing the capacity of a work system that has accumulated backlog. However, training costs are generally higher for these higher-skilled workers and depend on the quantity and difficulty of the tasks they are expected to perform. In addition, the number of these workers available is often limited because the
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