INTELLIGENT CONTROL OF MATERIAL HANDLING SYSTEMS - Environmentally Conscious Materials Handling - page 68

• Agility . Agile manufacturing is a key concept. It is the ability to quickly
adapt and reconfigure the manufacturing environment to unanticipated
changes.
• Scalability . It is important that the organization can grow or shrink at any
level when required.
• Fault tolerance . Environments are not static. Failures will occur once in a
while; therefore the system must be able to detect and recover from such
system failures.
The worldwide trend toward low-batch production with an increased variety
has been ongoing for several decades. The growth and advancements in com-
munication technologies have enabled customers to raise the individual desires
or even take part in the development process. Information technology has also
made it much simpler to get valuable customer feedback, which can evolve and
improve the products — thereby also shorten the product lifecycle.
Previously manufacturing sites were optimized by a linear production model
that was suitable for mass series production, such as transfer lines, but long
switching times make them inherently less suitable for flexible manufacturing,
resulting in low utilization.
Both flexible and distributed systems, as presented in the previous section, can
meet many of the requirements mentioned, but fail on others as well. Holonic
systems focus on creating flexible systems through decentralized and cooperative
components, which will benefit the systems with respect to agility and scalability
of fault tolerance, but the heterogeneous environment and enterprise integration
are given no special attention. Distributed systems might solve the issues of
interoperability, as well as open and dynamic structures, but at the local scope
there is no guarantee for flexibility and efficiency.
Computer integrated manufacturing (CIM) has also been proposed as a solu-
tion to cope with the new challenges. CIM is an approach where the entire
production process is controlled by a computer. It is organized in a hierarchical
architecture from the strategic level of the company to the production level, but
with closed-loop control so that feedback is provided back to the subsystems in
order to optimize the entire process (Asai et al.1994, Rembold and Nnaji 1993).
However the centralized and sequential approach to control both planning and
scheduling was found insufficiently flexible and agile for the dynamic production
environment and the changing production styles (Colombo 2006). Huge invest-
ments were required to install the sensory feedback at the physical machine level
and implement them in the centralized monolithic control system. The complexity
of the system made them rigid and inflexible, and often conditions had changed
when the systems were fully operational. The organization of CIM factories is
commonly hierarchical, so a single point of failure could shut down the entire
system. For these reasons, the original approach of a CIM factory was never suc-
cessful in real life (Scheer 1992), as the new requirement of flexibility, dynamic