A base model for discrete event simulation of field operations in agriculture using simula and demos
Within the research project "Optimization of hay harvesting systems" a mathematical model of the entire hay production and utilization process was developed in order to enable the solution of dimensioning and management problems by means of simulations rather than making full-scale experiments. As a part of this project, a model of field operations and management had to be developed, since no existing model fulfilled the demands on high resolution, combined with simplicity to use. A comparison between linear programming (LP) and simulation showed that only the latter technique could yield the resolution necessary for accurately modelling the behaviour of the man-machine system, as well as of most biological processes that might be simulated along with the process of field operations. Since the man-machine system is a typical discrete event driven one, whereas most biological processes are continuous, the model was designed so as to enable so called combined simulation, i.e. discrete event simulation combined with continuous simulation. The model of field operations and management was given a multi-layer design, and was implemented in the object oriented programming language Simula. The simulation package DEMOS was used as the discrete event modelling environment. The model design was kept as simple as possible. It mainly consists of three types of actors: the fields, the gangs, and the manager. The fields contain exactly one material each. The gangs are different combinations of men and machinery and a process, converting the material of the field being processed into another field. The manager takes decisions about starting and stopping gangs. There are also other actors: one keeping track of the starting time of the day, one keeping track of the hours for dawn and dusk, and one keeping track of the hours for rain starts and stops. The individual items of men and machinery however do not take actions of their own. The package was designed as a base model, useful for simulating field operations in general. Specific applications are developed by means of further specifying properties of the actors included in the model. The discrete event sequencing is completely hidden within the model so that the user do not have to bother about it. In order to maintain a flexible system, no decision model was included. This should be entered in the specific application. The base model was validating by means of developing a test bed program which utilized the essential parts of the base model. The results were investigated in order to examine whether the program executed correctly. The results showed that the synchronizations between all the actors involved in the simulation worked as expected. It was concluded that the resolution of the model is high enough for most problems. An exception would be certain transport systems, which would require simulation of the position on the field or on the road for every vehicle. The efficiency of the program with respect to computing speed showed to be satisfactory, but the memory requirements might limit the maximum problem size. It is however possible to execute realistically large problems on a personal computer under the MS—DOS operating system.
machinery; equipment; labor; planning; scheduling; management; programming; operations; research; simulation; optimization; field operations; field work; systems analysis; systems dynamics
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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