NW : the automated inventory system is not working either because the
drill or cutter input buffers are full, or because the assembler output
buffer is empty.
Six events determine state transitions:
LOADED : the inventory system has loaded a piece into the drill or cutter
UNLOADED : the inventory system has unloaded a piece from the assembler
FULL : the drill or cutter input buffer is full.
NOTFULL : the drill or cutter input buffer is not full.
EMPTY : the assembler output buffer is empty.
NOTEMPTY : the assembler output buffer is not empty any more.
The work cell system is a typical example of production line that produces
high volumes of a specific type of product (e.g. bolts). Work pieces follow a
predefined sequence of operations and thus the production line is called a
flow line (see Figure 9.5).
A number of simulation models have been proposed in the literature, such
as continuous or discrete, synchronous or asynchronous, deterministic or
stochastic, etc. The flow line described in this chapter is a typical discrete,
asynchronous and stochastic production line. The manufacturing process is
simulated piece by piece. An infinite source of raw parts feeds the first
machine and an infinite sink collects finished products at the end of the line.
The simulation of a discrete event dynamic system consists of modelling
the permanence of the system in a state as the interval time between the
occurrences of two subsequent discrete events. For example, the execution
of the screw cutting operation corresponds to the permanence of the cutter
in the WORKING state. When a LOADED event is notified the cutter enters the
WORKING state and activates the WORKDONE event, whose lifetime is equal
to the duration of the cutting operation. The cutter leaves the WORKING
state when the WORKDONE event is notified.
When the system is made up of a number of interacting subsystems, event
notification must refer to the same simulation time. Thus, the simulation
Figure 9.5 A flow line