When scheduling operations in manufacturing, there comes a point where it is preferable to avoid making last-minute disruptions to the schedule unless there is a problem such as a machine breaking down or a need to rush a crucial order through. Having too much volatility in the schedule in the short-term is inefficient as it can create confusion and nervousness on the shop floor.
There are two ways to control schedule stability in PlanetTogether: the Frozen Span and the Plant Stable Span.
The Frozen Span represents a window of time found at the beginning of the schedule in which jobs and operations are locked and anchored. The Frozen Span is usually reserved for manual scheduling of operations as it can be excluded from the automatic optimization functions. Jobs can be dragged into the Frozen Span but they should remain locked when an optimization is performed.
This is an optional setting that can be configured in the Settings | System Options | Plant Spans tab. The Frozen Span starts at the beginning of the schedule and is denoted by an orange zone on the Gantt Chart.
To further prevent instability in the schedule, the optimize function can be set to start optimization after the Frozen Span. This means that only jobs that start after the Frozen Span will be able to optimize and reschedule.
One important note is that the Frozen Span is universal to all plants found in PlanetTogether. Setting the Frozen Span in the System Options applies it to all plants, which may be undesirable, depending on how similar the plants are.
When managing multiple plants, the Plant Stable Span can provide a better alternative to set different stable spans within each plant.
Plant Stable Span
The Plant Stable Span is specific to each individual plant and functions much the same way that the Frozen Span does. The main drawback to the Plant Stable Span vs the Frozen Span is that jobs cannot be automatically locked or anchored within the Plant Stable Span. However, Optimization can also be set to start after the Plant Stable Span instead of after the Frozen Span.
The Plant Stable Span can be set per plant by going to Settings | Scenario Data | Resources. From there, double-click on a plant to open the plant options dialog and set the "Stable Span".
In either case, Jobs can still be moved manually via the Move Activity dialog or by Drag-and-Drop.