Specification of Reactive Hardware/Software Systems - Electronic ...

12.2 Initial Requirements Description 377
12.2 Initial Requirements Description
12.2.1 Purpose Description
A (future) mailing machine consists of a main production line built from mechanical
stations. See Figure 12.1. A mechanical station consists of a substructure on which
zero or more functional modules are built. Further each station can transport mailing
packets. Transport is performed at very high speeds with currently a maximum of about
18 thousand packets per hour. A main production line can have side lines, which can
be as complex as complete production lines. If a mailing packet passes a certain station,
the corresponding functional modules can perform certain services on this packet. The
assembly of a mailing packet starts at the main feeder modules, which form the heads
of a line. Notice that both the main production line and the side line have a main feeder.
A main feeder inserts (feeds) the first magazines of the packets. The other feeders add
particular other products. The gimmick feeder shown in Figure 12.1, for instance, can
add 3.5” diskettes to the packets that pass by. Another type of feeder can blow open
magazines and insert advertisement cards. At the merger station where the production
lines meet, a packet from the side line can be added to a packet on the main line. At the
packer module the mailing products are wrapped in foil. In the next station the foil is
sealed and cut and a mailing address is printed on the packed products. Finally at the
stacker module packets are combined into bundles.
12.2.2 Requirements
The current generation of mailing machines is based on a centralised control concept.
A production line contains numerous sensors and actuators. They are all controlled
by a PLC-based central control station (see Figure 12.1). All stations are connected
to the central control station by dedicated channels each containing numerous wires.
Production steps such as the assembly, the wiring, the testing and the configuring of
every new production line are performed within the Buhrs factory. After approval a
disassembly follows. Then all production steps are repeated again at the location of the
customer. Further, each customer-specific machine requires manual adaptation and fine
tuning of the PLC-software. The same holds if a production line needs to be equipped
with new or modified functionality. All this leads to recurrent high costs for every
supplied mailing machine.
To solve the above problems, the centralised control concept has to be replaced by a
decentralised concept. The new generation of mailing machines must be realised as
a collection of rather independent and autonomous stations that are ’glued’ together
as plug-and-play components. Stations may solely communicate through generic interfaces.
Each station must have its own intelligence and it must take care of its own product
transport. Further, different stations should be supplied with different plug-and-play
interfaces to the functional modules such as the feeders. A production line should be
self-configuring. Currently each new batch-job 2 requires manual configuration of sev-
2 A batch-job is a service that is provided by the purchasers of Buhrs machines for their customers.