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Julie Simmons Ivy

Rebalancing the System to Reduce Production Disruptions

3/19/2007 --

Manufacturers can improve system performance by combining reconfiguration operations with preventive maintenance.

ANN ARBOR, Mich.—Reconfigurable manufacturing systems have given companies the ability to respond more quickly to changing market and regulatory conditions. However, firms still must perform preventive maintenance on their equipment to replace or repair machinery that may fail due to aging or wear and tear. Typically, these maintenance and reconfiguration problems have been treated separately.

In a new University of Michigan study forthcoming in IIE Transactions, a team of researchers at the Ross School of Business and College of Engineering suggests that by incorporating reconfiguration operations into preventive maintenance actions, manufacturers can actually improve system performance by reducing total cost.

"Since both preventive maintenance and reconfiguration can be used to protect the production operation against machine degradation and failures, there is potential benefit in combining them for a less costly and more reliable production system," said Julie Simmons Ivy, assistant professor of operations and management science at the Ross School of Business.

However, the solution is not quite that simple, she says, because making changes in one thing affects the other. For example, preventive-maintenance activities interrupt production and change the system reliability on which reconfiguration decisions are based. Conversely, reconfiguration activities change the configuration of the overall system, which in turn affects both throughput and the reliability of equipment. Thus, some sort of joint decision-making approach is needed to simultaneously optimize and integrate these two actions.

In their research paper, Ivy and colleagues Jing Zhou, Dragan Djurdjanovic and Jun Ni, explore the best way to couple the reconfiguration action of operation transfer with preventive maintenance action, and discuss the advantages and disadvantages of doing so. They develop what they call an "integrated reconfiguration and age-based maintenance" (IRABM) policy and apply it to a typical parallel-serial reconfigurable manufacturing system.

"The primary advantage of reconfiguration is its ability to lessen the impact of mechanical breakdowns or repairs by rebalancing the system (resulting in a higher throughput) and to reduce the likelihood of a systemwide failure by slowing down the weakest or least reliable machine," Ivy said. "The disadvantage of reconfiguration is the associated cost of an operation transfer. Achieving the optimal tradeoff between these benefits and cost is the focus of the decision-making process in the IRABM policy."

The researchers use simulation-based methodology to optimize the expected total cost of the IRABM policy and then crunch numbers in a numerical analysis to verify the cost benefits of incorporating reconfiguration into maintenance actions. For their model, they utilize age-based preventive maintenance (ABM), which is carried out (when a machine is still operating) on a schedule based on running time, average time between machine failures or calendar time in order to head off costly breakdowns or failures that could shut down the entire production system.

In looking at a hypothetical two-stage manufacturing system with four machines and four operations, two of which are flexible, Ivy and colleagues find that incorporating reconfiguration into maintenance actions results in an estimated 21 percent decrease in the total cost. It also lessens the frequency of preventive repairs, yielding further cost savings, because the system is able to endure (and compensate for) more unexpected failures by flexibly rerouting production through still-operational machines without disrupting production.

The researchers report that the cost benefits of the IRABM policy are higher when the time and unit cost of reconfiguration are lower. However, when those cost factors rise significantly, reconfiguration ceases to be cost-beneficial and the optimal policy is to perform no reconfiguration at all. Similarly, reconfiguration proves more beneficial when the unit penalty for unmet production is high, because the high penalty motivates a company to increase its throughput rate by rebalancing production through operation transfer. On the other hand, once the production goal becomes too high, the cost benefit of reconfiguration declines rather than continuing to increase.

"We show that the IRABM policy has the advantages of rebalancing the system for a larger throughput and reducing the chance of a systemwide failure," Ivy said. "When the savings in maintenance and production costs induced by reconfiguration outweigh the added reconfiguration cost, the total cost is reduced. These results support our conclusion that the IRABM policy can always achieve superior performance in terms of reduced total cost compared to the ABM policy without reconfiguration."

Written by Claudia Capos

For more information, contact:
Bernie DeGroat
Phone: (734) 936-1015 or 647-1847