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WANG AND CHAN: ROBUST REPLENISHMENT AND PRODUCTION CONTROL POLICY 3
an optimal state-dependent base-stock control policy based on
a continuous time Markov decision model.
To deal with the uncertainties in manufacturing systems
(e.g., variable processing time, failure of machines, or fluc-
tuation of demands), robust production scheduling is often
applied. Kouvelis et al.[27] measured the uncertainty of pro-
cessing time by the range of its fluctuation, and modeled the
robust scheduling problem by an integer programming model
to minimize the gap between the scheduling performance in
the worst case and that of the optimal scheduling. For the
uncertainty from machine failure, Yellig and Mackulak [28]
developed a robust scheduling policy to avoid frequent
rescheduling. For uncertainties from both processing time vari-
ability and machine breakdown, Goren and Sabuncuoglu [29]
defined three robustness and two stability measures and pro-
posed a branch and bound algorithm and a beam search
heuristic to solve the problem. Robust optimization is an
often-used technique for solving robust production scheduling
problems, where uncertain parameters in production schedul-
ing are usually captured by variables in a known interval [30],
or random variables with a known probability distribution [31].
This technique has been applied to robust production schedul-
ing problems in process industry [32], [33]. It is also applied
to solve the production planning and scheduling problem for
complex manufacturing and supply chain systems with ran-
dom market demand, uncertain yield or inaccurate production
data [34]–[36].
C. Summary
From the above survey on the related work, we find as
follows.
1) Existing research on inventory inaccuracy pays more
attention to inventory control policy for hedging against
the negative impact caused by this inaccuracy. However,
little attention has been paid to the inventory inaccuracy
at the production stage of a supply chain and robust pro-
duction control policies again this inaccuracy. The robust
replenishment and production control problem of a pro-
duction/inventory system with inventory inaccuracy has
not been sufficiently studied.
2) Existing research on the control of a produc-
tion/inventory system has not taken inventory inaccuracy
into consideration, although many other uncertainties
(e.g., stochastic processing time, random demand, fluc-
tuation of production capacity, or variable lead-time)
have been investigated, and many control policies (e.g.,
the policies based on classical control theory, opti-
mal production and inventory control policies, or robust
production scheduling theory) were developed.
Due to the reasons given from the above two perspec-
tives, the robust replenishment and production control problem
for a production/inventory system with inventory inaccuracy
becomes the main issue of this research.
III. P
ROBLEM DESCRIPTION
In this paper, we try to solve the robust replenishment
and production control problem for a single-stage produc-
tion/inventory system with inventory inaccuracy. This system
Fig. 1. Single-stage production/inventory system.
is composed of a replenishment process, a production process,
and a demand satisfying process. In the replenishment pro-
cess, the system orders raw materials from outside to replenish
its raw materials stock, which is located between the replen-
ishment process and the production process and has infinite
capacity. In the production process, the system consumes raw
materials and produces final goods, which are stored in the
final goods storage with infinite capacity. In the demand satis-
fying process, the final goods are delivered to customers. The
structure of this production inventory system is depicted in
Fig. 1.
The dynamics of this single-stage production/inventory
system can be captured by two differential equations: one is
for the replenishment process, and the other is for the produc-
tion and demand satisfying process. The state variables are
the raw materials inventory level and the final goods inven-
tory level. It is necessary to note that the inventories of raw
materials and final goods are inaccurate. We assume that the
errors between observed inventory level and actual inventory
level for the raw materials and the final goods are normally
distributed with known means and variances. The control vari-
ables are the replenishment rate and the production rate. The
lead-time between placing an order for raw materials and the
arrival of raw materials are taken into consideration. However,
we assume that the demand can be satisfied immediately when
the final goods stock is not empty, namely, there is no time-
delay between production and final goods delivery. We also
assume that the demand rate is constant, the production capac-
ity is finite, and the machines are failure-prone. We assume
that the time to failure and the time to repair of the machine
are exponentially distributed. The objective of this problem is
to minimize the average inventory and production cost, which
includes the holding cost of raw materials, the holding cost of
final goods, and the backlog cost of demand.
The mathematical model of this problem can be constructed
as follows:
Minimize lim
T→∞
1
T
E
T
0
c
1
x
1
(t) + c
+
2
x
+
2
(t) + c
−
2
x
−
2
(t)
dt
(1)
Subject to
dx
1
(t)
dt
=
−u(t), if 0 ≤ t < L
r(t − L) − u(t), if t ≥ L
(2)
dx
2
(t)
dt
= u(t) − d(t) (3)