Page 372 - Sensors and Control Systems in Manufacturing

Industrial Sensors and Contr ol
automated operation, the mean time between operator interventions 327
must be large when compared with the times between manufacturing
setups.
⎛
⎞
/
MTOI = ⎜∑ n τ + τ + τ + τ +L + τ ⎟ n (7.1)
⎝ i 1 2 3 4 4 ⎠
where τ = setup time
i = initial setup
n = number of setups
The processes in use must rarely fail; the operator will intervene
only when such failures occur. In such a setting, the operator’s func-
tion is to ensure the adequate flow of work in progress and respond
to system failure.
Several types of work cells are designed according to the concept
of total manufacturing integration. The most sophisticated cell design
involves fully automated processing and materials handling. Com-
puters control the feeding of work in progress, the performance of the
manufacturing process, and the removal of the work in progress.
Manufacturing systems of this type provide the opportunity for the
most advanced automated and integrated operations. The manufac-
turing system must be modified to achieve closed-loop operations for
all of these functions.
Most manufacturing systems in use today are not very resourceful.
They do not make use of external sensors that enable them to monitor
their own performance. Rather, they depend on internal conditioning
sensors to feed back (to the control system) information regarding
manipulator positions and actions. To be effective, this type of manu-
facturing system must have a rigid structure and be able to determine
its own position based on internal data (largely independent of the
load applied). This leads to large, heavy, and rigid structures.
The more intelligent manufacturing systems use sensors that
enable them to observe work in progress and a control loop that
allows corrective action to be taken. Thus, such manufacturing sys-
tems do not have to be as rigid because they can adapt.
The evolution toward more intelligent and adaptive manufacturing
systems has been slow, partly because the required technologies have
evolved only in recent years and partly because it is difficult to design
work cells that effectively use the adaptive capabilities. Enterprises are
not sure whether such features are cost-effective and wonder how to
integrate smart manufacturing systems into the overall strategy.
The emphasis must be on the building-block elements necessary for
many types of processing. If the most advanced sensors are combined
with the most advanced manufacturing systems, concepts, and state-of-
the-art controllers and control loops, very sophisticated manufacturing
systems can result. On the other hand, much more rudimentary sen-
sors, effectors, and controllers can produce simple types of actions.