There is a point in the schedule where the plan is set in motion and it becomes inadvisable to disrupt the schedule that is in place unless there is a problem, such as a machine breaking down or the need to rush a crucial order through. Introducing too much volatility into the schedule in the short term creates “nervousness” on the shop floor, causing inefficiencies and confusion later in the schedule. An apt analogy would be similar to how constantly changing lanes in traffic may actually result in a longer wait than simply remaining in the current lane.
To this end, there are two ways to control stable stability: the Frozen Span and the Plant Stable Span.
Frozen Span
The Frozen span is a time span that is designed to reduce the chance of shop floor “nervousness” by locking and anchoring Jobs that start within that zone. This setting is optional and can be set within the System Options Dialog along with how long the Frozen Span will last. The Frozen Span always starts at the beginning of the schedule and is denoted by a pink zone on the Gantt Chart. Additionally, 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 open to APS Optimizing and rescheduling.
One thing to bear in mind is that the Frozen Span is universal across every plant in APS. Setting the Frozen Span in the System Options applies the same Frozen Span to all the Plants, which may or may not be desirable depending on how similar the Plants are.
If, when managing multiple plants, the Frozen Span should be different per Plant, then the Plant Stable Span presents another alternative.
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.
In either case, Jobs can still be moved manually via the Move Activity dialog or by Drag-and-Drop.