Rescheduling Manufacturing Systems: a framework of strategies, policies, and methods Vieira, Herrmann and Lin

Manufacturing system A manufacturing system is the collection of operations and processes used to produce a desired product. J.T.Black order release shop floor control material handling

ManufacturingSystems Manufacturing Systems They are complex, dynamic and stochastic systems Controlling production activities through  dispatching rules, kanban: myopic  production schedule: plans that state when certain controllable activities should take place. Gives shop floor personnel an explicit statement of what should be done so that managers can measure their performance.

Production Schedule Help managers and supervisors coordinate activities to increase productivity and reduce operating costs  Better coordination between production levels  Controls the release of jobs to the shop  Identifies resource conflicts  Identifies periods for preventive maintenance  Determines whether delivery promises can be met

Scheduling System has a dynamic, stochastic environment need to generate high quality schedules react quickly to unexpected events revise schedules in a cost effective manner. Rescheduling : the process of updating an existing production schedule in response to disruptions or other changes.

Rescheduling factors Unexpected events cause difficulty in following the schedule precisely, as time passes. If deterioration in performance is significant, they trigger rescheduling to reduce the impact.

Most common rescheduling factors Machine breakdown Urgent job arrival Job cancellation Due date change Shortage of materials Change in job priority Quality problems Over/under estimation of processing times Operator absenteeism

Actions that suggest rescheduling Overtime In-process subcontracting Process change Re-routing Machine substitution Limited manpower Setup times Equipment release

Performance Measures Schedule efficiency: time-based measures Schedule stability : stability,nervousness, robustness starting time deviations between the new schedule and the original schedule. measure of the sequence difference between the two schedules. Wu, Storer and Chang (1993) Cost : earliness, tardiness, WIP minimization

Reschedulingcosts Rescheduling costs Computational costs Setup costs Transportation costs

A rescheduling framework Rescheduling Environments Rescheduling Strategies Rescheduling Methods

Rescheduling Environments Identifies the set of jobs that need to be scheduled Static set of jobs Dynamic set of jobs

Static rescheduling environment Deterministic there is a finite set of jobs no uncertainty about future Stochastic some variables are uncertain (random processing times)

Dynamic rescheduling environment No arrival variability Schedule continuously repeated Single scheduling decision needed Arrival variability In a flow shop, steady arrıval rate Schedule should determine when to switch producing other classes of products. Process flow variability In a job shop

Rescheduling Strategies Two common strategies for controlling production in dynamic rescheduling environments: Dynamic scheduling Predictive-reactive scheduling

Dynamic Rescheduling Dispatching rules, pull mechanisms are used. Decentralized production control methods dispatch jobs using current data Computational effort may be low or high Literature of control theoretic models have studies on the control of dynamic manufacturing systems If there are no setup times, proposed dispatching rules are based on least slack policy. If there are setup times, finish all jobs in the same class, then move on to other. Kumar (1994) There exist idling policies which are good Chase and Ramadge(1992)

Predictive-reactive rescheduling Has two primary steps : generates a production schedule updates the schedule Rescheduling may occur frequently in a dynamic environment May be a single revision of the schedule of a stochastic, static environment

Productive-reactive scheduling Iterative process of three steps : evaluation: evaluates the impact that disruption caused solution: determines the best rescheduling solution revision: updates the schedule or generates a new one Wu and Li (1995) Three-phase rescheduling scheme : planning: constructs an initial schedule control: compares the actual progress of operations to the current schedule rescheduling: constructs a revised schedule Yamamoto and Nof (1985)

Rescheduling Policies Rescheduling policies: periodic event-driven hybrid Rolling time horizon approaches : periodic and hybrid

Periodic policy Rescheduling is done periodically and implemented on a rolling time horizon basis. Preferable if there is no on-line data More schedule stability,less schedule nervousness Following the same schedule although there are significant changes worsens the system performance What is the optimal rescheduling period? Short interval scheduling

Event-driven policy Rescheduling can happen repeatedly in dynamic systems or it can be a single event to revise a schedule in a static system. Rescheduling is triggered when Total number of job arrivals reaches a threshold Vieira(2000) Every time a new job arrives Bierwirth and Mattfeld (1999) Time spent for rescheduling & computations is excessive Requires fast and reliable electronic data collection High nervousness, low stability

Hybrid policy Reschedules the system Periodically When special events take place

Rescheduling Methods As part of predictive-reactive scheduling : generate or repair schedules Schedule Generation Nominal Schedules Robust Schedules As the level of uncertainty increases, frequent rescheduling becomes more effective in improving the robustness of the schedule

Schedule repair right shift rescheduling: postpones each remaining operation by the time required to make the schedule feasible partial rescheduling: reschedules only the affected operations by the disruption. preserves the initial schedule as much as possible. complete regeneration : reschedules the entire set of jobs not processed before rescheduling point including the ones not affected

Whether to repair or reschedule Reschedule from scratch if the disruption is highly significant. Revise the schedule if the change is less significant. Two measures to determine the strategy to be used in repair : Utility: measures the benefit gained by using a particular rescheduling strategy. Stability: Compares the start and completion times of the schedules. Cowling and Johansson (2002)

Impact of rescheduling policies Periodic policies are near optimal when order release is periodic. Rescheduling at the arrival of an urgent job with a tight due date is useful. Church and Uzsoy (1992) Higher rescheduling frequencies yields better system performance but increases the number of setups. Lower frequencies lower the number of setups but increases the cycle time and WIP. Vieira (2000)

Impact of rescheduling policies Never reacting to disturbances or reacting to every disturbance are not appropriate. Moderate level of frequency is suggested. Sabuncuoglu and Karabuk (1999) Only at tight due date conditions increasing the rescheduling frequency increases the system performance significantly. Shafaei and Brunn (1999) Decreasing scheduling frequency is advantageous if setups cost significant amounts. Herrmann and Delalio (2001)

Future studies Rescheduling policy must be considered in manufacturing system design. More research is needed to understand how the interactions between rescheduling policies and other production planning functions affect manufacturing system.

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