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"Suri, Rajan"
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Performance analysis of single stage kanban controlled production systems using parametric decomposition
We present a new approach that permits efficient performance analysis of kanban systems with general demand processes, material arrival processes, and service times. The approach is based on parametric characterization of the traffic processes (arrival and departure) in the network and uses two-moment approximations to estimate performance measures at individual stations. We derive traffic flow constraints that are particular to closed queuing networks with synchronization stations and use these to establish relationships between the parameters characterizing arrival and departure processes at the stations in the network. The resultant set of non-linear equations is solved to estimate network performance measures. Numerical studies show that the approach is not only fast but also reasonably accurate when compared to simulation. These studies also provide insights with respect to the impact of different types of variability on the performance of a kanban system. This work also provides a fundamental building block that can be used in the analysis of multi-stage kanban systems. [PUBLICATION ABSTRACT]
Journal Article
Manufacturers can reshore production without increasing costs
Cutting long transit lead times helps businesses regain competitive edge Over the last 20 years, manufacturers in North America and Europe have seen large numbers of jobs being lost while production has been \"outsourced\" to low-cost countries (LCCs). Over the two decades of our work on QRM, we have shown that these long lead times add a lot of indirect costs, including forecasting, planning (and replanning), managing work-in-process, warehousing of finished goods, freight costs for rushed shipping, time from salespeople and administrative staff to manage the delays, and many more such tasks. The reason is that historically, manufacturing companies were organized to minimize the unit cost of each operation and minimize the cost of each purchased item, but not to minimize the time it takes for products to flow through their system - in fact, quite the opposite; most of these decisions resulted in long lead times. Successful industry-university partnership At the QRM Center, we have not only demonstrated the successful partnership that can exist between university and industry but it has also enabled manufacturers to regain their competitive edge, reshore products, increase market share and profitability, and create significant numbers of manufacturingrelated jobs in their communities.
Trade Publication Article
Convergence Rates of Finite-Difference Sensitivity Estimates for Stochastic Systems
A mean square error analysis of finite-difference sensitivity estimators for stochastic systems is presented and an expression for the optimal size of the increment is derived. The asymptotic behavior of the optimal increments, and the behavior of the corresponding optimal finite-difference (FD) estimators are investigated for finite-horizon experiments. Steady-state estimation is also considered for regenerative systems and in this context a convergence analysis of ratio estimators is presented. The use of variance reduction techniques for these FD estimates, such as common random numbers in simulation experiments, is not considered here. In the case here, direct gradient estimation techniques (such as perturbation analysis and likelihood ratio methods) whenever applicable, are shown to converge asymptotically faster than the optimal FD estimators.
Journal Article
Convergence Properties of Infinitesimal Perturbation Analysis Estimates
Infinitesimal Perturbation Analysis (IPA) is a method for computing a sample path derivative with respect to an input parameter in a discrete event simulation. The IPA algorithm is based on the fact that for certain parameters and any realization of a simulation, the change in parameter can be made small enough so that only the times of events get shifted, but their order does not change. This paper considers the convergence properties of the IPA sample path derivatives. In particular, the question of when an IPA estimate converges to the derivative of a steady state performance measure is studied. Necessary and sufficient conditions for this convergence are derived for a class of regenerative processes. Although these conditions are not guaranteed to be satisfied in general, they are satisfied for the mean stationary response time in the M / G /1 queue. A necessary condition for multiple IPA estimates to simultaneously converge to the derivatives of steady state throughputs in a queueing network is determined. The implications of this necessary condition are that, except in special cases, the original IPA algorithm cannot be used to consistently estimate steady state throughput derivatives in queueing networks with multiple types of customers, state-dependent routing or blocking. Numerical studies on IPA convergence properties are also presented.
Journal Article
POLCA helps jobs dance through your factory
Loops of cards between pairs of cells For our example company, let's say certain types ofjobs need to go from the fabrication cell \"N\" to an assembly cell \"S\" and then to a testing cell \"T.\" In POLCA, ifjobs flow between any two cells, these cells are connected by a POLCA loop. [...]as shown in Figure 2, there will be a loop from N to S and one from S to T. Each loop contains a number of cards called POLCA cards that circulate in the loop. Each cell must use the POLCA decision rules (the three questions), so when the job arrives at cell S, the team must first wait till this job is the next authorized job (question one above). Since the job has arrived at cell S, the answer to question two is \"yes.\" [...]POLCA ensures effective use of capacity while also reducing congestion. Next, POLCA works well for high-mix, low-volume and customized production. Since demand is moving more and more in this direction, it is important to have a system that can deal with this environment.
Trade Publication Article
Analysis of a Fork/Join Synchronization Station with Inputs from Coxian Servers in a Closed Queuing Network
Fork/join stations are commonly used to model the synchronization constraints in queuing models of computer networks, fabrication/assembly systems and material control strategies for manufacturing systems. This paper presents an exact analysis of a fork/join station in a closed queuing network with inputs from servers with two-phase Coxian service distributions, which models a wide range of variability in the input processes. The underlying queue length and departure processes are analyzed to determine performance measures such as throughput, distributions of the queue length and inter-departure times from the fork/join station. The results show that, for certain parameter settings, variability in the arrival processes has a significant impact on system performance. The model is also used to study the sensitivity of performance measures such as throughput, mean queue lengths, and variability of inter-departure times for a wide range of input parameters and network populations. [PUBLICATION ABSTRACT]
Journal Article
A TIMELY METRIC
Industrial engineering -- 100 years ago, it was all about measurement. Today it's all about continuous improvement. Now here is a concept that, like a time warp, joins together both ends of this timeline. In today's world, lead-time is critical. Businesses can earn a competitive advantage by offering products or services with short lead-times. Hence many improvement methods aim to reduce lead-time. MCT, an acronym that comes from the phrase \"manufacturing critical-path time,\" is a time-based metric that defines lead-time so that it properly quantifies systemwide waste. MCT is \"the typical amount of calendar time front when a customer submits an order, through the critical path, until the first end-item of that order is delivered to the customer. MCT also should be a factor in sourcing decisions. A supplier with short MCT should be given some preference over one with a long MCT, even if the latter has lower costs.
Trade Publication Article
How quick response manufacturing takes the wait out
A new strategy called quick response manufacturing (QRM) enables companies to simultaneously motivate their employees and give them competitive advantage. Externally, QRM means rapidly delivering customized products. Internally, QRM focuses on reducing lead times for all tasks in a company - this results in improved quality and lower costs as well. QRM espouses a relentless emphasis on lead-time reduction that impacts every aspect of a firm. The biggest obstacle to lead-time reduction is not technology, but mindset. Beliefs that are holding companies back from being successful in the 21st century are presented.
Journal Article
PERFORMANCE ANALYSIS OF AN ASSEMBLY STATION WITH INPUT FROM MULTIPLE FABRICATION LINES
We develop analytical models for performance evaluation of Fabrication/Assembly (F/A) systems. We consider an F/A system that consists of an assembly station with input from K fabrication lines. Each fabrication line consists of one or more fabrication stations. The system is closed with a fixed number of items circulating between each fabrication line and the assembly station. We present algorithms to estimate the throughput and mean queue lengths of such systems with exponential processing times. We then extend our approach to analyze F/A systems with general processing time distributions. Numerical comparisons with simulations demonstrate the accuracy of our approach.
Journal Article