Graphics Programs Reference
In-Depth Information
8.1
Flexible Manufacturing Systems
A Manufacturing System is a production system where (discrete) parts are
machined and/or assembled. The oldest production system organization is
the production line. It was optimized for mass production and was based on
the synchronous operation of machines/workers performing a repetitive task
onto parts transported by a conveyor. Obviously this system can work only
if the processing to be performed can be decomposed into a set of operations
of identical duration.
A more flexible organization is the flow shop, where parts may by-pass some
of the machines/workers on the conveyor path. This system allows the
production of a family of slightly different products.
In the job shop organization, the production line doesn't exist any more.
For each different kind of product a working schedule is defined, specifying
the sequence (or alternative sequences) of machines the parts have to visit
to complete their processing. The sequence of machines is not restricted by
the physical layout of the machines. This organization is very flexible and
allows the production of very different families of products, hence it easily
adapts to changes in the market demand. The operation of this type of sys-
tem is necessarily asynchronous; as a consequence, the machine utilization
can be high only at the price of a very large in-process inventory, or using
sophisticated machine scheduling algorithms.
FMSs were introduced as a trade-off between the e
ciency of old production
lines and the flexibility of job shop organizations. A FMS consists of a set
of flexible machines, an automatic transport system, storage facilities, and
a scheduling system capable of deciding at runtime which operation has to
be done on each machine. The scheduling system must be able to deal with
machine failures and has the objective of maximizing machine utilizations
and of meeting the production deadlines taking into account the limits of
storage facilities. Usually, parts need to be loaded onto supports called
pallets to be transported within the production system.
Due to the complexity of FMSs, scheduling decisions are distributed, and
global coordination is achieved by communicating the local decisions up-
wards in a hierarchy of control levels. At a local level, scheduling rules im-
plement one among several possible inventory management strategies. The
push and pull inventory management strategies are based on two opposite
ideas. In the push control an operation is started every time raw material is
available and a machine is idle. When several operations may be performed
at a given time, decisions are taken based on priority and required produc-
tion mixes. This approach leads to a production schedule that is driven by
the Material Requirement Planning. Of course the Material Requirement
Planning should be guided by some guess on the future demand; however,
this may end up in large in-process inventories in case of an error in the
demand forecast.
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