T.F. ter Hoeven
A study of the NOMAD model and its possible application on AGV systems
Literature survey,
Report 2004.TL.6904, Transport Engineering and Logistics.
AGV systems have been around for almost fifty years. Although many
improvements have been made, AGV systems still use fixed driving routes.
When AGVs cross each others driving path, this often causes delays in the
system. It is therefore very interesting to determine whether an AGV
system with more freedom of path choice performs better than the
traditional systems. Such a system will have to be controlled by a model
that finds the optimal paths to destinations whilst avoiding collisions
between AGVs.
Serge Hoogendoorn, a researcher at the faculty of Civil Engineering at the
Technical University of Delft, has developed a microscopic pedestrian
model called NOMAD. This model has been used to aid in the design of
public spaces such as airport terminals and train stations. The model
considers pedestrians to be economists, in a way that the interests of a
pedestrian can be seen as costs or utility, which can be optimized. The
question is whether this model can be adapted for use as a control model
of an AGV system.
NOMAD consists of three levels; the strategic, tactical and operational
levels.
At the strategic level, several activity sets are created. These activity
sets act as inputs for the tactical level, where a simultaneous
optimization decision is made for the activity schedule and time,
destination area and route choice. The optimal driving route is determined
by optimizing the so-called running costs. These consist of the factors
travel time, proximity to obstacles, energy use due to walking speeds,
density of the pedestrian field and stimulating effects of the
environment. Every running cost factor has a weighting factor assigned,
which enables the user to set up the model to plan routes in the way that
is demanded for a certain situation. The output of the tactical level is
an optimal velocity path describing the route to a certain destination. At
the operational level, a similar approach is used to determine the
behaviour of pedestrians while following the path that is determined at
the tactical level. Again, an optimum is sought for the running cost
factors, which consist of straying from the optimal velocity path,
proximity to other pedestrians or obstacles, and energy use due to
acceleration or deceleration. The behaviour of the pedestrians can be
changed by assigning different values to the individual weighting factors
of the running costs.
Application of the NOMAD-model on the AGV systems of container terminals
is an interesting prospect. By changing the cost and weighting factors of
both levels of the model, the behaviour of the pedestrians of
Hoogendoorn's model can be adapted to suit the character of AGVs. The
question remains whether an AGV system with this type of control
outperforms the more traditional type of control.
Simulating an AGV container system will increase our insight in the use of
the NOMAD model; because of the bi-level structure of the model, it is
possible to simulate the tactical and operational levels separately. A
logical first step would be to set up a simple simulation with fixed
driving paths to study how the behavioural level of the model copes with
AGV encounters. With this experience, a more elaborate simulation could be
set up to create a simulation for the tactical level.
Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2005.03.11;
logistics@3mE.tudelft.nl
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