For the lot-streaming problem of a single job in a
flowshop, the objective is to simply determine the
optimal sublot sizes. Potts and Baker (1989) consid-
ered a flowshop with makespan criterion and indi-
cated when it is sufficient in a single-job model to
use the same sublot sizes for all machines. Trietsch
and Baker (1993) reviewed the different forms of
single-job lot streaming existing in the literature
and generalized some important struc tural insights.
To minimize the total flow time, Kropp and Smunt
(1990) presented optimal sublot size policies and
two heuristic methods for a single job in a flowshop.
Bukchin et al. (2002) identi fied the optimal solution
properties and developed a solution procedure to
minimize the total flow time in a two-machine flow-
shop with detached setups and batch availability.
For the lot-streaming problem with n jobs
(j =1,... ,n) in a flowshop with makespan criterion,
we need to simultaneously obtain the best job
sequence and the optimal sublot allocation (sublot
starting and completion times). Potts and Baker
(1989) showed that it is not possible to solve the
n-job problem sim ply by applying lot streaming
individually to the single-job problem. Vickson
and Alfredsson (1992) modified Johnson’s rule to
obtain the optimal solutions for the two-machine
and special three-machine prob lems with unit-size
sublots. Kalir and Sarin (2001) investigated equal-
size sublots and developed a bottleneck minimal
idleness heuristic to generate solutions that were
very close to the optimum. To the best of our
knowledge, the n-job model in a flowshop with total
flow time criterion was not considered in the
literature.
With the advent of just-in-time (JIT), the crite-
rion involving both earliness and tardiness penalties
has received significant attention. Yoon and Ven-
tura (2002a) examined lot-streaming flowshop
scheduling with respect to earliness and tardiness
penalties. For a given job sequence, they presented
linear programming (LP) models to obtain the opti-
mal sublot allocation for cases where the buffers
between successive machines have limited or infinite
capacities and the sublots have equal sizes or are
consistent. For the case with equal-size sublot and
infinite capacity buffer, sixteen pairwise interchange
methods obtained by combining four rules to gener-
ate initial sequences with four neighborhood search
mechanisms are considered. The experimental
results showed that the best performance was
obtained using a non-pairwise interchange
mechanism and the smallest overall slack time rule
to generate the initial sequence. Later, Yoon and
Ventura (2002b) provided a genetic algorithm
(GA) that incorporated the LP and pairwise inter-
change method for the lot-streaming flowshop
scheduling with equal-size sublot and infinite capac-
ity buffer. Their computational results showed that
the proposed GA, called the hybrid genetic algo-
rithm (HGA), works well for this type of problem.
Particle swarm optimization (PSO), proposed by
Kennedy and Eberhart (1995), is a novel metaheu-
ristic. To address various types of optimization
problems, both the continuous and discrete versions
of PSO have been developed (Kennedy and Eber-
hart, 1995, 1997). Since then, PSO has been success-
fully applied to many continuous and discrete
optimization problems (Kennedy and Eberhart,
1995; Van den Bergh and Engelbrecht, 2000; Clerc,
2004; Allahverdi and Al-Anzi, 2006).
The literature on PSO applied to the scheduling
problem is limited. Tasgetiren et al. (2004a,b,
2007) proposed PSO algorithms extended from the
continuous version (Kennedy and Eberhart, 1995).
They used the smallest position value (SPV) rule,
borrowed from the random key representation of
GA, to conv ert the continuous position values into
a discrete job sequence. Their PSO algorithms with
local search were effectively applied to solve single
machine and flowshop scheduling problems. On
the other hand, two discrete PSO (DPSO) algo-
rithms exist in the literature for solving flowshop
scheduling problems. Rameshkumar et al. (2005)
proposed a DPSO algorithm where new operations
based on job transpositions are provided to com-
pute particle velocity and update particle positions.
Liao et al. (2007) also developed a DPSO algorithm
based on the discrete PSO version (Kennedy and
Eberhart, 1997 ). In their DPSO algorithm, the par-
ticle and the velocity are redefined and an efficient
approach is developed to move a particle to the
new job sequence.
In this paper, we address the lot-stre aming flow-
shop scheduling problem with the objective of min-
imizing the total weighted earliness and tardiness.
As in Yoon and Ventura (2002b), we assume
equal-size sublots and infinite capacity buffers. Tri-
etsch and Baker (1993)
reviewed three types of sub-
lots including equal-size, consistent and variable
sublots. They indicated that it may be more attrac-
tive to use equal-size sublots in some applications,
such as an application described by Kalir and Sarin
(2001) for scheduling surface mount technology
(SMT) flow line.
C.-T. Tseng, C.-J. Liao / European Journal of Operational Research 191 (2008) 360–373 361