Page 224 - Mechatronics for Safety, Security and Dependability in a New Era

Ch43-I044963.fm Page 208 Tuesday, August 1, 2006 3:58 PM
Ch43-I044963.fm
208
208 Page 208 Tuesday, August 1, 2006 3:58 PM
manufacturing processes occur in the manufacturing systems. A reactive scheduling method for delays
of manufacturing processes was proposed in the previous research papers (Tanimizu 2002). This
method used Genetic Algorithm (GA) to generate new feasible production schedules. The previous
paper showed that the initial production schedule is modified and improved through the GA based
reactive scheduling processes.

The objective of the research is to propose a new GA based reactive scheduling method for tardiness
minimization scheduling problems, aiming at improving the disturbed production schedule efficiently
and generating suitable production schedules faster than the previous reactive scheduling method. A
prototype of reactive scheduling system is developed and applied to computational experiments.

CURRENT REACTIVE SCHEDULING METHOD
Reactive scheduling process is activated, only when the initial production schedule cannot satisfy the
constraint on the make-span, due to the unscheduled disruptions. It is necessary to consider the
progress of the manufacturing process in the reactive scheduling process.

Figure 1 shows the whole reactive scheduling process. The reactive scheduling process is activated at
the present time T\, only when the delay of the make-span occurs and the predetermined initial
production schedule does not satisfy the given constraint on the make-span. The reactive scheduling
process takes computation time dt to generate a new feasible schedule. The time dt is the time in which
GA creates a new generation of the population representing the modified production schedules. The
computation time dt is estimated based on the time needed to generate a new population of the feasible
production schedules by applying GA. Therefore, the schedule of the operations starting after (T\ + dt)
can be modified in the reactive scheduling process. If the make-span of the newly generated schedule
is shorter than the make-span of the current schedule, the current schedule is substituted by the newly
generated one. The reactive scheduling process is repeated, until the newly generated schedule satisfies
the constraint on the make-span, or until all the manufacturing operations have already started.

Tf new operations start during the reactive scheduling process, the next reactive scheduling process
inherits only the individuals that are consistent with the schedule of the operations starting between T x
and (T x+ dt). It is because that the schedule of these operations should be fixed in the reactive
scheduling process. The other individuals are deleted, and new individuals are randomly created.
Therefore, the proposed GA based reactive scheduling method can continuously modify and improve
the production schedule, taking into consideration of the progress of the manufacturing processes.
Initial production schedule (Predetermined)
Initial production schedule (Predetermined)
Resources delay Job name
( ) 1 , 1 ( ) 3 , 2 ( ) Operation
2 , 1
4 , 3
O
J O ( ) J O
R 1 2 O 1 , 2 1 J 1 , 1 4 J 1 , 4 3 1 , 3
( )
( )
( )
( )
J 3 O 1 , 1 J 4 O 2 , 1 J 1 O 3 , 2 J 2 O 4 , 3 delay
R 2 2 , 3 2 , 4 2 , 1 2 , 2
( ) ( ) ( ) ( )
J 1 , 2 J 2 , 2 J 3 , 3 J 4 , 3
R 3 3 O 3 , 3 2 O 3 , 2 1 O 3 , 1 4 O 3 , 4
Time
dt
T 0 Time
T 1 C
(Constraint on
Delayed Modified t make-span)
Delayed
Modified
Processing schedule I
Processing
schedule
Time datan ... I i ...
Time data
Manufacturing system
Manufacturing system
Figure 1: Reactive scheduling process