Real Time Re-sequencing & Rescheduling

NEED FOR IN-PLANT RE-SEQUENCING & RESCHEDULING

In assembly lines and mass manufacturing plants, base sequences or schedules are established for horizons like a week or a month. In practice, very few plants are able to operate with 100% FIFO conformance – i.e. where the actual final off sequence or schedule matches the base sequence or schedule.

The deviations from base sequence or schedule can occur due to several reasons:

  • Quality problems in a particular process require pulling units out of sequence; for example, in the paint shop.
  • Some of the more detailed plant level constraints were not considered when generating the base sequence or schedule; for example, no back to back cars with side air bags.
  • Parts supply shortages or quality problems.
  • Re-sequencing or in process buffers may exist.
  • For example: ASRS or mixing banks in auto assembly plants.

The objective of re-sequencing or rescheduling in such situations is:

  • To maintain the integrity of the base solution so as to minimize changes elsewhere on the line, while adjusting for any problems or required changes.
  • To generate a new optimal solution so that costs and inventories are controlled.

NEED FOR REAL TIME RE-SEQUENCING & RESCHEDULING

In many environments, business practices or manufacturing technology require frequent real-time re-sequencing and rescheduling.

Example 1: Modern car plants maintain a buffer inventory of painted car bodies in an ASRS (Automated Retrieval & Storage System) prior to final assembly. Real time sequencing based on the WIP in the ASRS is required to maintain high throughput in the final assembly line.

Example 2: Mass manufacturing plant operating in a vendor managed inventory (VMI) environment or as a “shopping center”. A reschedule is run upon every new customer order or change to any customer order.

REAL TIME RE-SEQUENCING & RESCHEDULING REQUIREMENTS

  • A real time sequencing or scheduling optimizer generates solutions upon receiving a message from a controlling system.
  • The schedule is based on a “state” represented by data. This state also includes a copy of the most recent solution or the base solution so as to enable a bias to stability in the solutions.

A signal is generated by the system controlling the process, as in the following examples:

  • ASRS or lane bank in an auto assembly plant: MES or line control system.
  • Semiconductor fab: MES or RTD system
  • VMI: stock signals.
  • “Shopping center”: order entry system.

The figure below is a conceptual image of an event driven real time sequencing / scheduling setup.

Conceptual image of an event driven Real Time Resequencing scheduling setup