Key Differentiator: Weights & Penalty Functions

OVERVIEW

Rules and constraints that apply to planning and scheduling are of different types and originate from different factors.

  • Manufacturing or supplier plant capacity limits cannot be exceeded, especially in the short term. These are classified as “hard” rules. Process routings, bills of material and changeover times fall in the same class.
  • On the other hand “soft” rules and constraints may be relaxed but a cost. The cost might be in the form of reduced throughput, as for example with line balancing type rules, or higher costs or inventories – or a combination of all three. Soft rules are usually prioritized based on the impact of a relaxation or violation – the more the impact, the higher the priority. However, in Optessa rules and constraints can be classified and prioritized by the user.

OPTESSA FEATURES

Optessa offers comprehensive support for hard and soft rules and constraints and their relative priorities – called “weights”. A “penalty function” specifies how a rule violation is to be applied.

Sometimes, in real world planning and scheduling problems, rule violations become inevitable. As a simple example consider the following rule:

  • Car plant schedule cannot contain back to back sunroof units. This rule can only be satisfied if the order mix has 50% or less of sunroof car orders. If 50% or less of the orders are for sunroof cars, then Optessa MLS will find the best schedule. If more than 50% of the orders are for sunroof cars then possibilities that exist are:
    • Case 1 – occasionally schedule back to back sunroof cars but smooth these violations and do not allow more than 2 sunroof cars in a row. This produces more violations but each of low impact.
    • Case 2 – in rare cases allow more than 2 sunroof cars in a row. This produces fewer violations than Case 1, but each violation of higher impact.
    • Case 3- allow no rule violation to the extent possible. This pushes out all excess sunroof cars to the end of the schedule thus creating a large violation, and potentially “unbuildable”  portion, at the tail end of the schedule.

The “penalty function” associated with the rule effectively selects which case applies to the rule.

Prioritization and selection of penalty functions is accomplished by a simplified interface. The interface enables users to define, modify and remove rules and constraints. Optessa customers have been successfully setting and modifying rule  weights and associated parameters.