Institute for Systems Research
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Item Modeling Separations of Plant Layout Problems(2014-08) Herrmann, JeffreyThis paper describes the results of a simulation study that evaluated the performance of different separations of the plant layout problem solved by bounded rational decision-makers. Seven problem instances from the literature were studied. We simulated the solution of a problem by a bounded rational decision-maker as a random search over the solution space. The problem was separated by identifying “subsets” of adjacent locations. The subset assignment problem partitioned the departments into subsets corresponding to these subsets of locations. Then, the subset layout problem assigned the locations in the subset to the departments. We considered separations with 2, 3, and 4 subsets. We also considered separations that first aggregated the departments before assigning them to subsets of locations. The results showed that separating the problem can lead to better solutions than solving the problem all-at-once, but some separations lead to worse solutions. Maximizing the flow inside the subsets generated better solutions than maximizing the adjacency of the departments inside the subsets. When fewer subsets are used, minimizing the cost inside each subset generated better solutions than minimizing the total cost. These results show that the quality of the solutions created by a design process is influenced by the choice of subproblems that make up the design process.Item Scheduling Perfectly Periodic Services Quickly with Aggregation(2013-03) Herrmann, JeffreyThe problem of scheduling periodic services that have different period lengths seeks to find a schedule in which the workload is nearly the same in every time unit. A time unit’s workload is the sum of the workloads of the services scheduled for that time unit. A level workload minimizes the variability in the resources required and simplifies capacity and production planning. This paper considers the problem in which the schedule for each service must be perfectly periodic, and the schedule length is a multiple of the services’ period lengths. The objective is to minimize the maximum workload. The problem is strongly NP-hard, but there exist heuristics that perform well when the number of services is large. Because many services will have the same period length, we developed a new aggregation approach that separates the problem into subproblems for each period length, uses the subproblem solutions to form aggregate services, schedules these, and then creates a solution to the original instance. We also developed an approach that separates the problem into subproblems based on a partition of the period lengths. Computational experiments show that using aggregation generates high-quality solutions and reduces computational effort. The quality of the partition approach depended upon the partition used.Item Aggregating Alphabets to Construct Balanced Words(2009-09) Herrmann, Jeffrey W.Balanced words are useful for scheduling mixed-model, just-in-time assembly lines, planning preventive maintenance, managing inventory, and controlling asynchronous transfer mode (ATM) networks. This paper considers the challenging problem of finding a balanced word (a periodic sequence) for a finite set of letters, when the desired densities of the letters in the alphabet are given. We present an aggregation approach that combines letters with the same density, constructs a word for the aggregated alphabet, and then disaggregates this word into a feasible word for the original alphabet. We consider two different measures for evaluating solutions and use the aggregation approach with different heuristics. Computational experiments show that using aggregation not only finds more balanced words but also reduces computational effort.Item The MDLe Engine -- A Software Tool for Hybrid Motion Control(2000) Hristu, Dimitrios; Krishnaprasad, Perinkulam S.; Andersson, Sean B.; Zhang, F.; Sodre, P.; D'Anna, L.; ISR; CDCSSOne of the important but often overlooked practical challenges in motion control for robotics and other autonomous machines has to do with the implementation of theoretical tools into software that will allow the system to interact effectively with the physical world. More often than not motion control programs are machine-specific and not reusable, even when the underlying algorithm does not require any changes.The work on Motion Description Languages (MDL) has been an effort to formalize a general-purpose robot programming language that allows one to incorporate both switching logic and differential equations. Extended MDL (MDLe) is a device-independent programming language for hybrid motion control, accommodating hybrid controllers, multi-robot interactions and robot-to-robot communications.
The purpose of this paper is to describe the "MDLe engine," a software tool that implements the MDLe language.
We have designed a basic compiler/software foundation for writing MDLe code. We provide a brief description of the MDLe syntax, implementation architecture, and functionality. Sample programs are presented together with the results of their execution on a set of physical and simulated mobile robots.
Item A Hierarchical Structure For Finite Horizon Dynamic Programming Problems(2000) Zhang, Chang; Baras, John S.; Baras, John S.; ISR; CSHCNIn dynamic programming (Markov decision) problems, hierarchicalstructure (aggregation) is usually used to simplify computation. Most research on aggregation ofMarkov decision problems is limited to the infinite horizon case, which has good tracking ability. However, in reallife, finite horizon stochastic shortest path problems are oftenencountered.In this paper, we propose a hierarchical structure to solve finite horizon stochastic shortest pathproblems in parallel. In general, the approach reducesthe time complexity of the original problem to a logarithm level, which hassignificant practical meaning.