Henri Tokola

Lean manufacturing methods in simulation literature: review and association analysis

The lean manufacturing philosophy includes several methods that aim to remove waste from production. This paper studies lean manufacturing methods and how simulation is used to consider them. In order to do this, it reviews papers that study simulation together with lean methods. The papers that are reviewed are categorized according to the lean methods used and result types obtained. Analysis is performed in order to gain knowledge about the volumes of occurrence of different methods and result types. Typical methods in the papers are different types of value stream mapping and work-in-process models. An exploratory analysis is performed to reveal the relationships between the methods and result types. This is done using association analysis. It reveals the methods that are commonly studied together in the literature. The paper also lists research areas that are not considered in the literature. These areas are often related to the analysis of variation.

Worker coordination policies in parallel station systems: performance models for a set of jobs and for continuous arrival of jobs

Varying workloads and uncertain processing times in parallel assembly cause idle times for skilled, high-cost workers. This idleness can be avoided and the utilisation of the workers improved by allowing workers to move between the stations to help each other. Worker movement between assembly stations needs efficient and feasible coordination, and therefore, this paper compares four different worker coordination policies: no helping, floater, pairs and complete helping. The dynamics of the policies are modelled by studying the parallel assembly as a continuous-time Markov process. The system is studied with two different job release cases for non-identical jobs (customised products). In the first case, a given number of jobs have to be completed by the entire system. In the second case, new jobs arrive with a Poisson-distributed rate. The models assume that when one worker helps another, their collaborative inefficiency reduces the productivity. The models are used in numerical experiments to compare the performances of worker coordination policies as average job cycle times. The main conclusions from the results suggest the use of the complete helping policy in minor collaborative inefficiency conditions, especially with a given set of jobs. The pairs policy is a reasonable alternative in major inefficiency conditions with the continuous arrival of jobs.

A comparison of rescheduling policies for online flow shops to minimize tardiness

In practical situations, flow shops usually have some policies on rescheduling previously scheduled jobs. This article compares three of these rescheduling policies: an unrestricted one where previously scheduled jobs can be moved freely, one where jobs can only be moved forward in the schedule, and one where jobs that have already been scheduled cannot be moved at all. The comparison is performed by considering the minimization of tardiness. While unrestricted rescheduling should generally give the best solution, moving jobs only forward can be more practical as in general production, material orders can be delayed but seldom advanced. This article points out that moving jobs only forward is not significantly worse than the unrestricted scheduling. When cases with small numbers of jobs and machines are analysed both policies give similar tardiness. Numerical experiments show that the differences between these two rescheduling policies are rather small in larger problems as well.

Block erection sequencing in shipbuilding with general lifting and joining times

This article studies the scheduling of block erection in shipbuilding. A mathematical model is formulated to minimize the makespan, i.e., the time between the start and finish of the erection schedule, when there are different lifting and block joining times. Structural stability and the no-skipped-blocks property are taken as constraints. The model is used to study the erection in three special cases. First, the case with a short lifting time is analyzed. Second, a solution is constructed for the case in which joining times are short and, third, the case in which the lifting and joining times are the same is analyzed. The article shows how these three cases can be solved efficiently, and, in addition, it gives insight into the erection scheduling such as the effects of the number of blocks in the horizontal and vertical directions on different joining times.

Block erection in the event of delays in shipbuilding: A scenario-based approach

Ships are constructed from individual steel structural units called blocks, which are typically erected into the ship one by one in a sequence. This sequence, the block erection schedule, is the main schedule in shipbuilding, and it is followed in other production phases. However, the problems in the production of the blocks can delay the arrival of the blocks in comparison to what was initially scheduled. To deal with these delays, this article studies five different planning methods: erecting blocks unfinished; the use of inventories; the use of rush jobs; sequence changes; and, if the delays cannot be dealt with, delaying the completion of the ship. In order to do that, a mathematical, scenario-based mixed-integer linear programming model is formed. The model is used with numerical results to show that the block structure, delay conditions, and the tardiness costs have significant effect on the optimal use of planning methods. Results show that inventories should be used when delays are probable, whereas blocks should be erected unfinished if the delays are very uncertain and there are high tardiness penalties. The uses of rush jobs and sequence changes depend on the structure of the ship.

Proceedings : Winter Simulation Conference

This paper studies the scheduling of two cranes in automated storage and retrieval systems that have a single output/input location. The cranes are located on a common rail, which restricts their movement, and which also makes the scheduling interesting as the cranes have to dodge each other while operating. The purpose of the paper is to study the scheduling of the retrieval of cartons from the storage to the output location. In order to do that, the paper introduces different scheduling restrictions and constructs a local search heuristic for scheduling the cranes. The heuristic relies on simulation to calculate the length of a given schedule, i.e., the makespan. In the numerical experiments different scheduling restrictions are compared in three different types of automated storage and retrieval systems. The results show how the length of the schedule changes when the input/output location changes in the storage.

The effect of input/output location in an automated storage/retrieval system with two cranes

This paper studies the scheduling of two cranes in automated storage and retrieval systems that have a single output/input location. The cranes are located on a common rail, which restricts their movement, and which also makes the scheduling interesting as the cranes have to dodge each other while operating. The purpose of the paper is to study the scheduling of the retrieval of cartons from the storage to the output location. In order to do that, the paper introduces different scheduling restrictions and constructs a local search heuristic for scheduling the cranes. The heuristic relies on simulation to calculate the length of a given schedule, i.e., the makespan. In the numerical experiments different scheduling restrictions are compared in three different types of automated storage and retrieval systems. The results show how the length of the schedule changes when the input/output location changes in the storage.

The effect of short-term planning delays in multi-item production-inventory systems

This paper studies short-term production planning in multi-item production-inventory systems. In short-term planning we study the delays caused by frozen schedules and planning intervals. We consider three different systems that are here called the machining subcontractor, the electrical machine company, and the assembly factory. Each of these has its own special characteristics. The subcontractor has several batching delays caused by setups, the electrical machine company ships products periodically, and the assembly factory has long production times and a long shipment time for finished products. We study how these characteristics affect their short-term production planning. Simulation results show how the electrical machine company has the most noticeable advantage from planning that does not have any delays.