Sequence Scheduling and Synchronous Processes

The quality of a factory’s Supply Chain planning and production scheduling is a meaningful indicator of its overall efficiency and long-term sustainability. Many companies have been applying the principles of CIP, Kaizen, 6-Sigma or Lean for decades and such topics as 5S, TQM, workplace layout and TPM are important. However, if the planning is inaccurate making cosmetic changes in micro areas will be of little value.

Sequence Scheduling

The leading cause of expensive wastes can be attributed to non-synchronous processes, which in turn, can be traced back to either a lack of or incorrect sequencing – especially in parallel or merging processes. Often resources issue the “wrong” production orders, with incorrect timing. The consequences of which are stagnation, high inventory levels, wait times and long lead times.

Commonly used scheduling software is usually only capable of managing daily scheduling (without sequencing) and relies on the judgement of the foreman or machine operator to determine the process sequencing of orders. If there is no sequence scheduling in place and each process owner determines the sequencing themself, this can increase the lead time several times over (compared to synchronous production).

Process Synchronization

Just as an orchestra has only one conductor, the same approach is necessary to achieve harmonious and synchronous scheduling. If individual resources or resource groups each have their own “conductor” (tact giver) and are only focused on the efficiency in their own area, this results asynchronous processes and a “push” production. The tact should be determined by the delivery time, i.e. the start time of the final process at the end of the value-added chain, or alternatively, a bottleneck process. All other processes should be adjusted accordingly and leveled, if possible.

Consistent, end-to-end integrated sequence scheduling is a prerequisite for process synchronization. Just as accuracy and precise timing in sequence scheduling are key to reaching the highest level of Just-in-Time production.

The Optimal APS

The most important requirement for any scheduling software is its ability to map a factory’s current reality 100%, and that means taking into account all product properties, processes, process rules/restrictions, planning restrictions, etc. If even a supposedly minor aspect cannot be mapped this will compromise the scheduling’s logical consistency, the scheduling result will not be realistic/feasible and you will have to resort to manual scheduling with Excel. ERP and MES systems were not designed to meet the high requirements needed for such comprehensive mapping.

An efficient detailed scheduling tool oversees all processes, including current and planned future orders from a “bird’s-eye” perspective and is focused on maintaining overall efficiency. It issues production orders to the individual resources with the exact specifications necessary to achieve optimal scheduling.

Below are some of the most important features that an optimal APS (Advanced Planning and Scheduling System) should include:

  • Multi-level finite capacity scheduling: for most processes with main resources (machines) and sub-resources (operators, tools, devices, etc.) the finite capacities should be considered simultaneously.
  • The mapping of complex merging and branching processes
  • Sequence optimization in consideration of minimum changeover times
  • Automatic lot size splitting or merging to help reduce lead times
  • Takes into consideration both current and fluctuating material availability
  • Simulation capabilities for short, medium and long-term scheduling, using various scenarios
  • Visualization: offers a variety of graphic displays for viewing the scheduling results – in order to anticipate emerging problems (e.g. capacity bottlenecks, delivery delays, conflicting order priorities) days / weeks in advance.
  • High calculation speed, enables the near real-time mapping of production
  • Predictive KPI: most companies maintain key figures relating to the past; these can no longer be altered. More important are metrics that can be used to drive the future and are based on a proactive and accurate production schedule. Essential predictive KPI are: delivery reliability, inventory levels / costs (raw material, semi-finished & finished goods), production lead time, stagnations times, changeover ratios, value-added ratios, resource productivity, manufacturing costs, etc. – these should be calculated either periodically or according to resource, product, order or customer.