B.R.C. Herijgers
Design issues of AGV systems
Literature survey,
Report 2000.LT.5433, Transport Technology, Logistic Engineering.
Material handling is the transfer of materials in the most economic manner
from one location to another. These movements must be made safely,
accurately, efficiently and without damage to the materials. Option for
material handling equipment is an automated guided vehicle (AGV) system.
Automated Guided Vehicles are now becoming popular in automated material
handling systems.
Advantages of the AGV systems are its diversity of use, flexibility,
ability of automatic interface with other facility systems and improvement
of working conditions. Disadvantages are problems with design issues
during the implementation, the costs involved in applying the system and
the possibility of failure.
The design of an AGV system affects the performance of the system and the
design can be very complicated. A number of interrelated decisions must be
made in order to exploit the full benefits of the system. First the kind
of automated guided vehicle has to be decided taking into account the
characteristics of the vehicle like vehicle speed, load capacity and their
docking equipment. AGV system design concerns also the determination of
the guide path layout. The design of the guide path depends on three
factors: location of pickup and delivery points, guide path layout and
path type. The amounts of vehicles that will operate in the system have to
be accurately chosen. Analytical determination as well as simulation can
be used to find the required fleet size of the system. Beside these
physical design aspects operational issues like vehicle dispatching,
routing and scheduling are important. It is important to know in advance
the kind of different dispatching and routing rules that can be used
before designing an AGV system. These rules are in general based on
traditional algorithms from the operations research area.
The dispatching rules used in AGV systems can be classified in several
ways. The most common used classification is work-centre initiated rules
versus vehicle-initiated rules. Work centre initiated dispatching deals
with the problem of matching a task to a vehicle when multiple vehicles
are idle and waiting for a task assignment. Vehicle initiated dispatching
deals with the situation that a vehicle has the choice of a task when
multiple loads are waiting for pickup simultaneously at different
locations in a facility. The vehicle-initiated rule can be further
classified into source-driven rules and demand-driven rules. The source
driven rule operates on a push concept: an idle vehicle selects a part to
move from an output queue that has the highest priority. The demand driven
rule operates on a pull concept: an idle vehicle selects the part that has
the highest demand from its succeeding work centres. Other classification
is the distinction between dispatching rules based on one attribute and
dispatching rules based on several attributes. A single-attribute
dispatching rule cannot, in general, satisfy all the objectives. Some of
the objectives may be in conflict due to the interdependencies within the
manufacturing process. Multi-attribute dispatching rules take into account
different criteria simultaneously in order to produce better performance.
Most of the dispatching rules wait until a vehicle becomes empty and then
assigns a transportation task to a vehicle. These dispatching rules do not
take in account the status of the already assigned vehicles during the
assignment process of a new task. As an alternative the Bid-based dynamic
dispatching is proposed.
In routing a distinction can be drawn between static routing and dynamic
routing. If routing is static, the path taken by an AGV between two given
nodes is fixed. Dynamic routing is based on the current state of the
system therefore dynamic routing offers a higher degree of flexibility.
Dynamic routing can be classified into three major classes: centralised,
local, distributed.
The last section of this report introduces a dispatching rule integrated
with a routing technique. The dispatching rule that will be used is the
Bid Based Dynamic Dispatching rule (B2D2) in combination
with a semi-dynamic routing strategy. Normally first a transportation task will
be matched with an AGV based on the shortest path. Subsequently the
shortest route has to be scheduled to perform the transportation task in
reality. In the case that will be given these two parts will be
integrated. A transportation task will only be dispatched if a route can
be scheduled that is not in conflict without earlier dispatched vehicles.
The proposed procedure can be a starting point for further investigation.
A simulation study can provide insight if the expected benefits,
performance that can be exploited by use of this technique. The results
can be compared with non-integrated dispatching and routing technique. The
situations in which the proposed algorithm can be useful can be
defined.
Reports on Logistic Engineering (in Dutch)
Modified: 2001.01.05;
logistics@3mE.tudelft.nl
, TU Delft
/ 3mE
/ TT
/ LT.