One of the most important data in any line
balancing project is the cycle time. In a paced line, the cycle time
is often meant to be the amount of time it takes before the product
leaves a workstation and moves to the next one in the line. By this
definition, the cycle time is the same for all workstations in the line.
However, this definition is often too restrictive: in many real lines,
it is desirable to have a certain “reserve” of time at the
workstations at the end of the line, so that possible perturbations
(e.g. equipment downtime) can be absorbed easily by those workstations.
OptiLine therefore defines the cycle time as
the work time that should be spent on a workstation. As such, the cycle
time is defined for each of the workstations separately, and can differ
from one workstation to another – in particular, it can decrease
towards the end of the line. OptiLine’s optimizing algorithm is
capable of taking the varying cycle time into account.
Since the cycle time is allowed to vary among
workstations, all the workstations and their respective cycle times
must be fixed before the optimization can take place. During optimization,
the cycle time is taken into account in the following way. Since the
objective of the optimization is to balance the workload among workstations,
the optimizing algorithm aims to assign operations to workstations in
such a way that the ratio of the work time and the cycle time be as
equal as possible across all workstations.
For example, suppose that there are two workstations
in the line, and that the first workstation’s cycle time is the
double of the cycle time of the second workstation. In that setup, the
optimizing algorithm will seek an assignment of operations such that
the work time on the first workstation be as close as possible to the
double of the work time on the second workstation.
In cases where there are workstations with several
operators, the work time on a workstation is taken to be the longest
work time among all the operators assigned to the workstation.
Note that in real-world applications, it is
usually impossible to find an assignment of operations to workstations
that would perfectly match the cycle times. As a consequence, if the
cycle times specified for the workstations are too tight w.r.t. the
amount of time necessary to carry out the operations, the cycle time
can be exceeded on some or all workstations. In that case, it may be
necessary to add a workstations (or operators) and rerun the optimization,
in order to find a feasible solution. Conversely, if the work times
obtained in a solution are way below the available time, it may be possible
to eliminate workstations (or operators) and increase the efficiency
of the line.
