wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

25-6 Industrial Communication Systems
Control recipes are structured similarly to the procedural control model and contain sufficient information
with respect to the attributes of a process cell to be linked with the equipment control. Depending
on the control recipe’s granularity and the available software elements, this interlinking can be done
theoretically on any layer. However, in practice, the interlinking of the control recipe, which resides on
a batch server, with the equipment control, which is executed on controllers, is commonly done on the
phase layer (see Section 25.2.3) by the so-called phase logic interface [BR05]. This interface is responsible
to convert steps of the control recipe into commands for the linked equipment control and to return
data and information from the process to the batch server. Most commercially available batch software
tools perform this connection on the phase layer. This means that the higher more abstract parts of the
control recipe (procedure, unit procedures and operations) are controlled by the batch server while only
the phases, which comprise the most precise information about the procedural activities, are actually
executed on controllers such as Programmable Logic Controllers (PLCs).
25.2.6 Classification of Process Cells
Process cells can be organized not only according to the physical model described above but also
categorized according to either the number of different fabricated products or the physical structure of
their installations (i.e., units) [IEC97].
If we categorize process cells according to the number of different fabricated products, we can distinguish
• Single-product process cells that fabricate always the same type of product in each batch though
variations of the procedures or material and production parameters are possible to compensate
changes of the environment conditions
• Multiproduct process cells that are able to fabricate different types of products by applying different
procedures and parameters
A classification according to the physical structure of process cells leads to the following categories:
• Single-path structures represent a group of units that process one or several batches at the same
time strictly sequentially.
• Multipath structures consist of several parallel single-path structures excluding material
exchanges between the paths. However, material sources as well as storages for the final products
may be shared by all paths.
• Network structures may have determined or variable paths. The actually used path for a batch can
be scheduled either at the beginning or also during its processing. Flexibility is therefore a key
issue as even units may be movable within the process cell. Hence, material sources and storages
may need to be accessible for all units.
Evidently, process cells can be either simple in the case of a single-product process cell with a single-path
structure or very complex in the case of a multiproduct process cell with a network structure. Especially,
the latter rather complex category demands flexible control paradigms.
25.2.7 tasks and Functions of a Batch Management and Operation System
The realization of certain tasks is necessary to perform the successful production of a batch. Seven tasks
are identified, which define the means of controlling the equipment [IEC97]. Figure 25.3a displays these
tasks—the connections represent a relation realized by an information flow between the tasks.
• Recipe management comprises activities to create and store general recipes, site recipes, and
master recipes. The final master recipe is provided to the process management that uses it to create
a control recipe.
• Production planning and scheduling is a high-level activity incorporating as its most important
task the creation of a batch plan, which is transmitted to the process control.
© 2011 by Taylor and Francis Group, LLC