  Project scheduling in manufacturing environments often requires flexibility
in terms of the selection and the exact length of alternative production
activities. Moreover, the simultaneous scheduling of multiple lots is mandatory
in many production planning applications. To meet these requirements, a new
resource-constrained project scheduling problem (RCPSP) is introduced where
both decisions (activity flexibility and time flexibility) are integrated.
Besides the minimization of makespan, two new alternative objectives are
presented: maximization of balanced length of selected activities (time
balance) and maximization of balanced resource utilization (resource balance).
New mixed integer and constraint programming (CP) models are proposed for the
developed integrated flexible project scheduling problem. Benchmark instances
on an already existing flexible RCPSP and the newly developed problem are
solved to optimality. The real-world applicability of the suggested CP models
is shown by additionally solving a large industry case.
