USE
OF EXCEL WORKSHEETS WITH USERFRIENDLY INTERFACE IN BATCH PROCESS (PSBP) TO MINIMIZE THE MAKESPAN
Rony
Peterson da Rocha
Universidade
Estadual do Paraná (UEP), Brazil
Email:
petersonccbpr@hotmail.com
Fabrício
Wesley da Rocha
Universidade
Estadual de Campinas (UNICAMP), Brazil
Email:phabriciowesley@hotmail.com
Mauro Antônio S. S. Ravagnani
Universidade
Estadual de Maringá (UEM), Brazil
Email:ravage@dep.uem.br
Paulo
Roberto Paraíso
Universidade
Estadual de Maringá (UEM), Brazil
Email:
paulo@dep.uem.br
Cid
Marcos Gonçalves Andrade
Universidade
Estadual de Maringá (UEM), Brazil
Email:
cid@dep.uem.br
Submission: 29/04/2013
Revision: 18/05/2013
Accept: 31/08/2013
ABSTRACT
In the chemical industry, the necessity for scheduling
is becoming more pronounced, especially in batch production mode. Nowadays,
planning industrial activities is a necessity for survival. Intense competition
requires diversified products and delivery in accordance with the requirements
of consumers. These activities require quick decision making and the lowest
possible cost, through an efficient Production Scheduling. So, this work
addresses the Permutation Flow Shop scheduling problem, characterized as Production
Scheduling in Batch Process (PSBP), with the objective of minimizing the total
time to complete the schedule (Make span). A method to approach the problem of
production scheduling is to turn it into Mixed Integer Linear Programming
MILP, and to solve it using commercial mathematical
programming packages. In this study an electronic
spreadsheet with userfriendly interface (ESUFI) was developed in Microsoft
Excel. The ease of manipulation of the ESUFI is quite
evident, as with the use of VBA language a userfriendly interface could be
created between the user and the spread sheet itself. The results
showed that it is possible to use the ESUFI for small problems.
Keywords: Excel; Permutation Flow Shop; Mixed Integer Linear
Programming – MILP
1.
INTRODUCTION
In
the chemical industry, the necessity for scheduling is becoming more
pronounced, especially in batch production mode. According to Raklaitis (1995),
batch production can be described as incomeoriented production, with equipment
network connectivity and resource limitations. Mendez et al. (2006)
classifies this type of operation: Single stage, which is subdivided into
single or parallel resources, and multiple stages, which are subdivided into
multipurpose (Job Shop) and multiproduct (Flow Shop).
The
scheduling problem addressed in this work is of batch type  multiproduct 
Permutation Flow Shop. According to Polon (2010), BatchMultiproduct plants are
in general employed for a set of products whose income structure is the same
and production lines are also referred to as Flow Shop.
According
to MacCarthy and Liu (1993) and Baker (1974), Flow Shop is a type of process
where all tasks have a similar flow pattern, that is, they have the same
processing schedule on all resources and the number of resources in each stage
of production equals one.
The
environment of the permutation Flow Shop production scheduling problem is
described by MacCarthy and Liu (1993), Baker (1974), Taillard (1993), and
Moccellin (1995) as a type of Flow Shop in which the processing order of tasks
is the same in all resources. Severo (2007) states that many chemical process
industries such as oil and paint, pharmaceutical and fine chemistry industries
fit into this category.
The
diversity and complexity of a batch production industry recommend, as stated by
Severo (2007), the application of production planning techniques in order to
meet consumer demands. Thus, scientific development in the area of Production
Scheduling is increasing, especially concerning batch processes.
This
work presents the development of a spreadsheet, created using Visual Basic (VB)
with interface with the user, whose goal is to minimize the task completion
time (Makespan) of a Permutation Flow Shop Scheduling environment.
Therefore,
this paper is divided into six sections. The first section presents the research
problem. The second section will be described in a literature review on the
topic in question. The third section will be described the methodology used in
the work. On Wednesday we will present a description of each of the
methodological steps, as well as the formulation of the research problem. The
fifth section will be held to present and discuss research results. Finally,
the sixth section will be held the final considerations.
2.
STATE
OF THE ART ABOUT SCHEDULING PROBLEM
Since
Johnson’s work edition in 1954, about flow shop scheduling , as described by
Gupta and Stafford JR (2006), there was an important progress in this knowledge
area, derived from it, there are endless other researches published all over
the world, about this subject. Gupta and Stafford JR (2006), describes a review
about flow shop scheduling, based on five (5) decades of publications, so the
author leaves it pretty clear that the results of those publications are the
many different ways released in literature about the heuristic solutions
proceeding and of available optimization to solve a huge variety of problems, flow
shop scheduling. So the Gupta and Stafford JR’s work, presents a summary about
the evolution of these problems with the propose of showing a redirection, of
this area, to the future. Frequently, as said by Gupta and Stafford JR (2006),
problems about the proceeding of a certain number of job in a number of
machines, are characterized by different researchers of the area as a problem
of scheduling, dispatching, sequencing, or combinations thereof. The first
term: “scheduling”, has to do not only with the programming, but also with the
sequencing of the production. In this context, even if decades of studies pass,
the scheduling problem is still discussed a lot on its specific literature,
because it involves the accomplishment of certain objectives (function
objective), related with the numberless restrictions of the different study
environments, which leads to another variety of programming problems; as, among
others, a simple scheduling problem, a programming of multiple machines
problem, or a problem of labor programming.
The
initial studies about the flow shop scheduling were published by Johnson
(1954), Bellmane Gross (1954), Bellman (1956) and Bellman et al (1982). Being
JOHNSON’s work (1954), about the flow shop scheduling problem, directed to an
environment with two machines and its forerunner. Therefore, JOHNSON’s study
emerged from a method (algorithm) to optimize the sequencing of a group of a
certain number of jobs to be processed in a production system with only two
machines.
Gupta
and Stafford JR’s (2006) show in their article that a flow shop is
characterized by an unbroken flow of jobs through a variety of serial machines.
The job flow is unidirectional whereas all jobs obey the same technological
route through the machines (considered in the chemical engineering as a
multiproduct plant). So some chances and theorems related to the problem of
scheduling flow shop are represented on Gupta and Stafford JR’s (2006) article
from the Conway et al. work (1967) and Tanaev et al. work (1994). However Gupta
and Stafford JR say that the cases of scheduling flow shop show restricted
structures when it comes to a practical point of view (real cases), with
mathematical formulations that ignored multiple important factors that
interrupt the job flow on the system of production (SP), causing unexpected
delays, are discussed on Ashour’s (1972) literature, such as: lack of
consistence on the performance and absence of the operators; delays on the
material, tools or equipment supply; variability on the processing time;
changes on the job specifications; pressure coming from the clients to a quick
conclusion of the work, etc.
Gupta
et al (1979), present various assumptions to the flow shops, put together by
the job characteristics, machines and policies of the process functioning
(operational policies). Simplifying hypothesis are also imposed to the flow
shop case, although these can increase the generality of the developed models,
so the possibility of finding the ideal scheme, sometimes is lower, because the
models get too far from the real situations. Thereafter, during the years it
have been added lots of modifications to the original model of Johnson (1954),
by researchers, to attend more and more the needs imposed by the system of
production (SP) of the real life, such as the terms: without intermediate
queues; time of preparation dependent of the sequence; flow shops hybrids, and
so on. It has been showing an evolution of this subject facing the reality, for
example: Gupta and Stafford JR’s (2006), present on their article a general
description of the evolution of this area from 1955 to 1964 (first decade),
from 1965 to 1974 (second decade), from 1975 to 1984 (third decade), from 1985
to 1994 (fourth decade), from 1995 to 2004 (fifth decade) and since 2004
(actual times).
On
the first decade, that came after the publication of the Johnson’s classic work
in 1954 about flow shop scheduling to the production programming in two
machines, this area has increased to an environment of production programming
with more machines with the goal of minimizing the total time of conclusion of
the tasks (makespan).
In
this time the researchers studied the environment of production programming
with mmachines, based on a completely theoretical view, using optimization
techniques (mathematical programming) and the simulation technique and
MonteCarlo, as shown in Wagner’s work (1959), Sisson’s (1959), Manne’s (1960),
Muth and Thompson (1963), Dudek and Teutão (1964), and also published by Gupta
and Stafford JR (2006). In this period of time, although just a few techniques
of solution were developed, still some processing configuration and scenarios
were identified. The difficulty to solve a lot of problems considered possible
to solve, existed because of the lack of computer efficient programs and
because of a huge part of the different variant of the flow shop problem with
two machines are considered NPhard, in other words, problems there are
difficult to solve.
The
“second decade” is known by its wide range of solution techniques and by the
appearance of different functions objective. In this decade the works of
Lomnicki (1965), Ignall e Schrage (1965), Brown and Lomnicki (1966), Mcmahon
and Burton (1967), Smith and Dudek (1967), Mcmahon (1969), Gupta (1969B), Gupta
(1970), Gupta (1971), Szwarc (1971), Gupta (1972) and Szwarc (1973) were cited.
A technique used in that time was the “Branch and bound”. The difficulty found
on the development of algorithms of optimization to programming problems in flow
shop environments, resulted on the development and usage of heuristics to find
good or almost great solutions.
On
the “third decade”, the theory of NPCompletude was born; it presented an
important impact to the evolution of the production programming area in flow
shop. A lot of heuristic approaches were developed in this phase, besides in
this decade has occurred a proliferation of a variety of considerations about
programming problems in flow shop, that violated some of the restrictive
assumptions listed at the beginning of this theory.
The
fourth decade is marked by the appearance of flow shops hybrids where each of
its phase can present various parallel machines. In this period of time it
emerged the studies and development of metaheuristics, such as: Tabu Search,
Genetic algorithms and “simulated annealing”. This decade has also witnessed
the usage of techniques based on artificial intelligence. In this phase are cited
authors like: Byrd and Moore (1983); Zweben and Fox (1994); Brown et al.
(1995); Osman and Kelly (1996); RaywardSmith et al. (1996); Aarts and Lenstra
(1997); Allahverdi et al. (1999) and Cheng et al. (2000).
In
the fifth decade there was the continuity of the proliferation of a variety of
programming problems flow shop, function objective and solution approaches. An important
part of this decade was the analysis and solution of (multicriteria) that have
become really popular. It has also been important in this period of time the
studies about lot sizing and programming of machines of lot processing,
efficiency of diverse metaheuristics to the minimization of the makespan into flow
shops with setup times dependents of the sequence, significant progress on the
development of solution process to robotic and equipped with automatic vehicles
flow shops (AGV). It´s also important to say that because of the progress on
the computer area, there was an increasing interest on the job for mathematical
programming approaches of flow shop problems. There are cited here the Works
of: Potts and Van Wassenhove (1992), Tkindt and Billaut (1993), Potts and
Kovalyov (2000), Ruiz, Maroto and Alcaraz (2005), Geismar et al. (2004), Tseng
et al. (2004).
Nowadays
studies are developed considering the problem of preparation time of the
machines, in other words, differently of the initial study of Johnson (1954),
that considered the time of preparation together with the processing time, recent
studies show that if these times are separated, the configuration leads to
better performance. Although the approach of mathematical programming to the
production programming in environments flow shop, for a certain time, was
rejected, because of its excessive computer luggage to solve the mathematical
model that shows the reality, so heuristics proceedings would have to be used
all the time. But recently huge advances on the solution of problems of
“mathematical programming” and the availability of proceedings with approximate
solution shows that the approach of the mathematical programming can be used to
find real answers and with less computer effort. This, as said by Gupta and
Stafford JR (2006), shows that the techniques of mathematical programming and
other programming proceedings in flow shop (heuristic based on mathematical
programming approaches) indicate a trend of search redirected to the future.
GotheLundgren
and Persson (2002) have studied the problem of scheduling in an oil refinery
with the intention of choosing the best way of operation, in order to attend
the demand and the capacity of storage, reducing the cost of production. The
role model was elaborated in the Mixed Integer Linear Programming. Thereby, it
was showed that the model of optimization can be used as a applicable tool to
support the planning of production and programming at the refinery, and that
the model can bear the shipment planning and strategic decisions about new
products and investments in storage capacity.
Sundaramoorthy and Karimi (2005) reported on the
production scheduling problem in the short term for plants in batches multipurpose.
This study was presented one formulation based on mixed integer linear
programming (MILP), using a continuous representation of time slots synchronous
and a new idea of multiple balances (time, mass, resources). The model uses
without bigM constraints, and is equally effective for both maximize profit
and minimize the makespan.
Burkard
and Hatzl (2006) researched the problem of production scheduling in the
chemical industry. The objective of the study was implement an objective
function to minimize the makespan, using heuristics. In the study proposed an
iterative algorithm construction that alternates between phases of construction
and deconstruction. It was also proposed strategies for diversification and
intensification in order to obtain optimal solutions in good moderate running
times. Computational results show the power of this algorithm.
Shakib
and Logendran (2007) studied the problem of production scheduling model as a
mixed integer linear programming (binary) developed for scheduling tasks in
multitasking environments, for which the number of completed tasks is not a
good measure. Fixed issues for small, medium and large, with the aid of a tabu
search algorithm. The solution obtained from the algorithm is compared with
that of the optimal solution or the upper bound found from using the Lagrangian
relaxation.
Pan,
Qian and Li (2008) describe a study on for shortterm scheduling of
multipurpose batch plants using Mixed Integer Linear Programming.
This work presented the network states and tasks (STN) to eliminate the
inconvenience of precedence based formulations which typically include a large
number of batches. Rules have been proposed in the study heuristics for solving
the problem. The results were effective to find the best solution to problems.
Privitera,
Day, Deshi and Long (2011), presented a paper about the application of the
Mixed Integer Linear Programming on the technological school of renewable
energy, with the goal of reducing the CO2 emission. On that paper the authors
implemented the objective function based on the cost reduction. the
implementation was done by using the solver: "Ip_solver", that is a
freefont Mixed Integer that was incorporated in a Microsoft Excel app.
Grossmann (2012) presented a study about
optimization, that is an important subject when talking about process
industries, because of the competitively on the global market. The author
presented a general view in terms of mathematical programming structure such as
techniques of mathematical programming as the Mixed Integer Linear Programming.
In this paper, it is concluding that it is necessary to put the control with
the planning and the programming all together; however, the attention has been
dedicated mainly to the integration of the planning and programming.
Lin
and Liao (2012) presented a paper about a scheduling problem for a twostage
assembly shop in a machinery factory. The objective is to minimize the weighted
sum of makespan, total completion time and total tardiness. A Mixed Integer Programming
(MIP) model is developed for solving smallsize problems, and three heuristics
are proposed for solving medium and large size problems.
Catalá,
Durand, Blanco and Alberto (2013) Developed a research in a farm about the
Mixed Integer Linear Programming usage, to plan the restructuring of an orchard
of pears and apples. The result shows the fruits replacing policy during 20
years. On the model, the maximization of the business current liquid value was
taken into account, and also it's planting structure during a certain time
under different funding conditions. The model can be used not only by
government agencies to counsel private sectors and develop strategic economic
policies, but also by companies to optimize their profit.
Koné,
Artigues, Lopez and Mongeau (2013) have studied the problem of scheduling
problem that takes into account storage resources which may be produced or
consumed by activities. The role model was elaborated in the Mixed Integer
Linear Programming.
Fumero,
Corsano and Montagna (2013) also conducted a study on Mixed Integer Linear
Programming. In this study was conducted a model for both design and scheduling
of flow shop batch plants considering mixed product campaign and parallel unit
duplication. The results showed that a realistic formulation is attained, where
industrial and commercial aspects are jointly taken into account.
Solimanpur
and Elmi (2013) proposed a model of mixed integer linear programming for the
scheduling problem of cells, as well as an application of tabu search approach
to solve the problem heuristically.
Thus,
this study sought to investigate the problem of production scheduling for batch
process (Flow  Shop) through a mathematical formulation implemented in Excel
and VBA, adopting as a principle the state tasks network (STN) formulation and Mixed Integer Linear
Programming to the problem of reducing the total time of completion of tasks
(MAKESPAN).
3.
METHODOLOGY
For
the creation of a userfriendly interface to solve a production scheduling
model for the permutation Flow Shop system, it was necessary to divide it into
five parts:
3.1 Model Development
The
problem proposed by Polon et al. (2006), of a multiproduct batch plant,
characterized as a permutation Flow Shop environment, was analyzed for the
model development.
3.2 Model Formulation
At
this stage, the objective function was defined, considering the case of
production scheduling as a mixed integer linear programming problem (MILP).
In
this permutation Flow Shop programming environment, one research scenario were modeling,
all of them with the aim of minimizing the total task completion time
(Makespan).
The
model constraints are: a) the storage of intermediate products should not be
available between the processing resources, that is, if a product is processed
at resource j and resource j+1 is not available at the time of completion, the
finished product should be kept at resource j, until resource j+1 is ready; b)
after finishing the processing of a product at the last resource (equipment),
this product is immediately sent to the stock of finished products; c) all
resources are initially empty at time zero and the manufacture of any product
may be delayed an arbitrary amount of time to keep it in the previous resource;
d) the ordering of tasks on each resource is the same.
3.3 Model Resolution in Excel Spreadsheets
After
finishing the modeling, the mathematical equations of all one scenario were
manually transcribed into Excel spreadsheets, and solved by the addin Solver.
3.4 Creation of Interface Between User and Worksheet in Visual Basic Language (VB), and Finally
The
VB language was used to create an interface between the user and the Excel
spreadsheet. This way, all the mathematical equations that had been modeled in
the one scenario, along with the addin Solver, were developed in order to
create a generalized programming model for this environment and with the
restrictions imposed by the proposed model.
3.5 Evaluation of the Consistency of the Interface
The
evaluation of the consistency of the spreadsheet interface with the user was
done by comparing the results that were manually obtained with Excel
spreadsheets with those obtained using the VBA language.
4.
USERFRIENDLY
INTERFACE FOR THE RESOLUTION OF A MODEL OF PRODUCTION SCHEDULING IN A
PERMUTATION FLOW SHOP SYSTEM WITH MAKESPAN REDUCTION
4.1 Model Development
To
develop the research was analyzed a research scenario, referring to the
programming environment of three tasks (t1, t2, t3) and five resources, with
the respective processing times shown in Table 1.
Table
1 shows the respective processing times for each of the tasks on each machine.
Table 1 
Processing time (h)

4.2 Model Formulation
The
three scenarios were modeled. However, as all of them are part of the same
programming environment (permutation Flow Shop) and meet the same constraints,
only the modeling of scenario 1 will be presented. In the mathematical modeling
of this scenario, Equation 1 represents the main goal of the problem to be
solved.
(1)
Variable
C in Equation 1 represents the Makespan, while N and M represent the number of
tasks and the number of resources, respectively. Thus, the objective function
expressed by Equation 1, is about finding, among the various combinations of
tasks and resources, the NM sequence that provides the lowest Makespan. For the
scenario 1, N = 4 and M =
(2)
(3)
Variable
i represents the task to be processed at the resource and k
represents the position of this task in the order of sequencing. Equations 2
and 3 thus represent a discrete optimization problem involving the decision
between two alternatives, that is, if a task is processed at a given resource,
the other tasks cannot be processed concurrently at the same resource.
Therefore, X_{ik} is a binary variable defined as: X_{ik}
= 1 if task i is in position k, and X_{ik} = 0,
otherwise.
Each
task is sequentially processed by resources (1), (2), and (3), respecting the
permutation Flow Shop programming environment.
Equation
4 represents another problem constraint, which also involves Makespan. In this
Equation, C_{k,j} is the time of end of processing, by resource j,
of the task occupying position k in the sequence; N is the number
of tasks; X_{s,k} is the binary variable, and TP_{s,j}
is the time for processing task i at resource j.
Generally,
the resolution of the various alternatives of Equation 4 shows that for a
scheduled task to be processed in resource j,
from the second order of sequencing, it must present a Makespan greater than or
equal to the Makespan of a task placed earlier on the same resource j, plus the processing time of the same
task on resource j.
Another
important constraint is Equation 5, which also has to do with the Makespan of
the tasks in the resources. Equation 5 shows that the Makespan of the task of
order k on resource j is greater than or equal to the
Makespan of the same task on resource j1
plus the sum of the products of the binary variable X_{s,k} and the processing times TP_{s,j}, starting with resource (2 ) and finishing with
resource (3).
(5)
The Makespan of the task placed in the sequence 1 of
the production scheduling, at resource j,
is expressed in Equation 6. This Equation presents the constraint that the
Makespan of such task is greater than or equal to the Makespan of the task
placed in the sequence 1 of the production scheduling, at resource j1, plus the sum of the products of the
binary variable X_{s,}_{1}
and the processing times TP_{s,j}.
In this Equation, resource j must
start with resource 2 and finish with resource 3, according to the model
initially proposed.
6)
Another
important constraint is the Makespan of the task sequenced with order 1 on
resource 1, which is represented by Equation 7. This Equation shows that the
Makespan of this task must be greater than or equal to the sum of the products
of the binary variable (X_{1,1}, X_{2,1}, X_{3,1},
X_{4,1}) and the processing times (TP_{1,1}, TP_{2,1},
TP_{3,1}, TP_{4,1}).
(7)
The
constraint presented in Equation 8 shows that the Makespan sequenced in the
order k on resource j must be greater than or equal to the
Makespan sequenced in the order k1 on resource j +1, starting
from the order of sequence 1 on resource 1.
(8)
Finally,
the last constraint of the problem concerns the question of nonnegativity of
the model, which is represented by the general Equation 9.
(9)
4.3 Model Resolution in Excel Spreadsheets
Equations
(1) through (9) of the three tasks and five resources model were inserted in
the worksheet, in different cells. After that, the addin Solver, available
within the Microsoft Excel electronic spreadsheet, was used to optimize the
scheduling of the tasks on the resources.
In
general, for the development of the manual resolution of the processing orders
scheduling model proposed in this study, all available data was first organized
in an Excel spreadsheet, separating the cells representing the decision
variables and the objective function. For each constraint of the problem, a
formula was created in a separate cell of the spreadsheet, corresponding to the
lefthand side (LHS) and the righthand side (RHS) of the restriction.
LHS
and RHS correspond to the transformation of the set of constraints into a set
of equivalent equations, by introducing variables that will represent the gap between
the left (LHS) and right (RHS) sides of the inequalities.
After
inserting all the equations of the PSBP model, representing the objective
function, decision variables and restrictions, in the Microsoft Excel
electronic spreadsheet shown, the addin Solver, available in the tool bar of
this spreadsheet, was used to find the optimal solution for the problem.
4.4 Creation of the Friendly Interface Between the User and the Excel Spreadsheet in the Language Visual Basic for Applications (VBA)
For
the creation of the userfriendly interface with the Excel spreadsheet (ESUFI)
used to solve the model, in order to reduce the Maskespan following the
constraints set forth in the methodology, it was necessary to first define a
standard spreadsheet. The standard spreadsheet refers to a Microsoft Excel
spreadsheet, composed of all the equations described in the study, organized in
a standard model, which means to put each of the equations in an array of rows
and columns, available to perform the generalization of the model. The
generalization of this model was performed using the VBA programming language.
The
implementation of the VBA code in the spreadsheet made possible the
generalization of the model of three tasks and five resources for the
configuration of (n) tasks and (m) resources. With this implementation, a
friendly interface was created between the user and the Microsoft Excel
spreadsheet. At this friendly interface the user should only report the number
of tasks (n) and resources (m), and the processing time of each task at the
respective resource, and then click on the button "Optimize Scheduling
Order" to obtain the best production sequence that minimizes the Makespan.
Figure 1 exemplifies the model of the Microsoft Excel spreadsheet with userfriendly
interface.
Figure
1  Microsoft Excel spreadsheet model with userfriendly interface
To
build the userfriendly interface created for the ESUFI presented in Figure 1,
it was necessary to carry out some validation tests for each of the model
equations. These tests serve to validate the model, that is, to show the
veracity of each of the equations from the expansion of the model of three
tasks and five resources for different situations of (N) tasks and (M)
resources. The tests also served to find a number of combinations of (N) and
(M) possible to be applied with the help of the Solver tool.
4.5 Evaluation of the Consistency of the Interface
The
assessment of the consistency of the generalized equations in the Excel
spreadsheet with userfriendly interface was done by comparing the results that
were manually obtained with Excel spreadsheets with those obtained with the
userfriendly interface, using the VBA language.
To
validate the automated spreadsheet regarding the accuracy of the equations
generated by the expansion of the model, two scenarios were tested in each one
of the classes N Î {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13} and M Î {2, 3,
4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 60}, with time intervals of [1,99],
totalling 360 problems tested (2 scenarios x 12 classes of tasks x 15 classes
of resources).
The
classes of problems N Î {2, 3,
4, 5, 6, 7} and M Î {2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25}, according
to the classification of Boiko (2008), represent the small sized production
scheduling problems. However, for the same author, classes N Î {8, 9, 10, 11, 12, 13} and M Î {30, 40, 60} correspond to mediumsized production
scheduling problems.
In
assessing the consistency of generalized equations in the Excel worksheet with
userfriendly interface, tests were performed in cases of production scheduling
in small and medium businesses. Besides verifying the veracity of equations (1)
through (9), expanded on the Excel worksheet with userfriendly interface, the
tests sought to find the limits of adjustable cells on the spreadsheet.
Adjustable cells mean the largest number of tasks (N) and resources (M) of the
subclasses that have the possibility of solving the objective function
(minimizing the Makespan) in the Excel worksheet with userfriendly interface,
regardless of the final solution.
The
correlation coefficient (r) given by Equation 10 was used to assess the
relationship between the number of tasks (N) and resources (M) of the limiting
subclasses of the Excel spreadsheet model with userfriendly interface
(MOREIRA, 2010, p 327).
(10)
Where:
r = correlation coefficient; n = number of limiting subclasses; X = value of the number of the task subclass
(n); Y = value of the number of the resource subclass (m).
Table
2 was used to interpret the values of r, in the analysis of the relationship
between the number of tasks (n) and resources (m) of the limiting subclasses of
the PSBP model in the Excel spreadsheet with friendly interface.
Table 2  Interpretation of r values
(r) 
correlation 
0 to 0,2 
very low 
0,2 to 0,4 
low 
0,4 to 0,6 
average 
0,6 to 0,8 
high 
0,8 to 1,0 
towering 
According
to data presented in Table 2, if it is found a value between 0 and 0.2 can be
said that the correlation is very low and amounts to between 0.8 and 1.0, there
is a high correlation.
5.
RESULTS
AND DISCUSSION
It
was verified in the tests of each problem of subclasses {2,2}, {2,3}, {2,4},
{2,5}, {2,6}, {2,7}, {2,8}, {2,9}, {2,10}, {2,15}, {2,20}, {2,25}, {2,30},
{2,40}, and {2,60} that all equations were consistent with the generic model
equations. However, one realized that there was still scope to increase the
number of resources, since the expansion of the equations was not at the limit
allowed in the Excel spreadsheet. Therefore, a new set of problems with N Î {2} and M > 60 was tested, and the limit of {2,98}
was found.
The
same behaviour was observed with the subclasses {3,2}, {3,3}, {3,4}, {3,5},
{3,6}, {3,7}, {3,8}, {3,9}, {3,10}, {3,15}, {3,20}, {3,25}, {3,30}, {3,40}, and
{3,60}. This way, a new set of problems with N Î {3} and M > 60 was tested, and the limit of {3,62}
was found.
All equations of the problems of subclasses {4,2},
{4,3}, {4,4}, {4,5}, {4,6}, {4,7}, {4,8}, {4,9}, {4,10}, {4,15}, {4,20},
{4,25}, {4,30}, {4,40}, and {4,60} were consistent with the generic model.
However, subclass {4,60} could not be verified, since it went beyond the limit
of Solver adjustable cells present in the ESUFI. Thus, after conducting further
tests, the limit of {4, 46} was found.
The
same behaviour was observed for the subclasses {5,2}, {5,3}, {5,4}, {5,5},
{5,6}, {5,7}, {5,8}, {5,9}, {5,10}, {5,15}, {5,20}, {5,25}, {5,30}, {5,40}, and
{5,60}, and it was found that subclasses {5,40} and {5,60} went beyond the
limit. New tests led to the determination of the limit {5, 35}.
Further
tests with the subclasses {6,2}, {6,3}, {6,4}, {6,5}, {6,6}, {6,7}, {6,8},
{6,9}, {6,10}, {6,15}, {6,20}, {6,25}, {6,30}, {6,40}, and {6,60}, pointed to
the limit of {6,27}, after it was found that subclasses {6,30}, {6,40}, and
{6,60} went beyond the limit of Solver adjustable cells present in the ESUFI.
Similarly,
in the problems of subclasses {7,2}, {7,3}, {7,4}, {7,5}, {7,6}, {7,7}, {7,8},
{7,9}, {7,10}, {7,15}, {7,20}, {7,25}, {7,30}, {7,40}, and {7,60}, all
equations were consistent with the generic model, although subclasses {7,25},
{7,30}, {7,40}, and {7,60} exceeded the limit of adjustable cells in the ESUFI.
Thus, after performing new tests, the limit of {7, 21} was determined.
For
the same reason, subclasses {8,20}, {8,25}, {8,30}, {8,40}, and {8,60} could
not be verified, when analysing the subclasses {8,2}, {8,3}, {8,4}, {8,5},
{8,6}, {8,7}, {8,8}, {8,9}, {8,10}, {8,15}, {8,20}, {8,25}, {8,30}, {8,40}, and
{8,60}, but all equations were consistent with the model up to the limit of
{8,17}.
During
the tests with the problems of subclasses {9,2}, {9,3}, {9,4}, {9,5}, {9,6},
{9,7}, {9,8}, {9,9}, {9,10}, {9,15}, {9,20}, {9,25}, {9,30}, {9,40}, and
{9,60}, all equations were consistent with the generic model, but the last six
subclasses could not be verified, as they went beyond the limit of Solver
adjustable cells present in the ESUFI.
New tests were performed and the limit of {9, 13} was found.
Analogously,
the last six subclasses of the set {10,2}, {10,3}, {10,4}, {10,5}, {10,6},
{10,7}, {10,8}, {10,9}, {10,10}, {10,15}, {10,20}, {10,25}, {10,30}, {10,40},
and {10,60} were not verified, and the limit of {10,10} was found after
performing new tests.
Following
the same pattern, the limit observed for the subclasses {11,2}, {11,3}, {11,4},
{11,5}, {11,6}, {11,7}, {11,8}, {11,9}, {11,10}, {11,15}, {11,20}, {11,25},
{11,30}, {11,40}, and {11,60} was {11,7}, while for the subclasses {12,2},
{12,3}, {12,4}, {12,5}, {12,6}, {12,7}, {12,8}, {12,9}, {12,10}, {12,15},
{12,20}, {12,25}, {12,30}, {12,40}, and {12,60} the limit was {12,4}.
As
for the results of the tests with the problems of subclasses {13,2}, {13,3},
{13,4}, {13,5}, {13,6}, {13,7}, {13,8}, {13,9}, {13,10}, {13,15}, {13,20},
{13,25}, {13,30}, {13,40}, and {13,60}, all equations were found to be
consistent with the generic model, but only the subclass {13,2} could be
generated on the spreadsheet, as the others went beyond the limit of Solver
adjustable cells present in the ESUFI. Table 3 summarizes the evaluated classes
and subclasses of problems, and the last subclass of each problem corresponds
to the ESUFI limit for the application of the Solver tool to optimize the
Makespan.
Table
3  Summarizes the evaluated classes and subclasses of problems
CLASS 
SUBCLASS 
2 
{2,2};{2,3};{2,4};{2,5};{2,6};{2,7};{2,8};{2,9};{2,10};{2,15}; {2,20};
{2,25};{2,30};{2,40};{2,60};{2,98} 
3 
{3,2};{3,3};{3,4};{3,5};{3,6};{3,7};{3,8};{3,9};{3,10};{3,15}; {3,20};
{3,25};{3,30};{3,40};{3,62}; 
4 
{4,2};{4,3};{4,4};{4,5};{4,6};{4,7};{4,8};{4,9};{4,10};{4,15}; {4,20};
{4,25};{4,30};{4,40};{4,46}; 
5 
{5,2};{5,3};{5,4};{5,5};{5,6};{5,7};{5,8};{5,9};{5,10};{5,15}; {5,20};
{5,25};{5,30};{5,35}; 
6 
{6,2};{6,3};{6,4};{6,5};{6,6};{6,7};{6,8};{6,9};{6,10};{6,15}; {6,20};{6,25};{6,27};

7 
{7,2};{7,3};{7,4};{7,5};{7,6};{7,7};{7,8};{7,9};{7,10};{7,15}; {7,20};
{7,21}; 
8 
{8,2};{8,3};{8,4};{8,5};{8,6};{8,7};{8,8};{8,9};{8,10};{8,15}; {8,17};

9 
{9,2};{9,3};{9,4};{9,5};{9,6};{9,7};{9,8};{9,9};{9,10};{9,13}; 
10 
{10,2};{10,3};{10,4};{10,5};{10,6};{10,7};{10,8};{10,9};{10,10}; 
11 
{11,2};{11,3};{11,4};{11,5};{11,6};{11,7}; 
12 
{12,2};{12,3};{12,4}; 
13 
{13,2}; 
Therefore,
after performing all the tests in the classes of problems N Î {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13} and M {2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40,
60}, with time intervals of [1, 99], for the validation of the automated
spreadsheet regarding the accuracy of the equations generated by the model
expansion, and establishment of the limits of classes N x M of the PSBP model
in the ESUFI, it was possible to conclude that equations (1) through (9) were
correctly expanded in all 360 analyzed problems, thus ensuring the accuracy of
the results obtained with the ESUFI.
Observing
the limits found for the classes of problems N Î {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13} and M Î {2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 60},
with time intervals of [1,99], there is a clear reduction in the number of
resources (M), as the number of tasks (t) increases. This reduction in the
number of resources, concerning the limitation of adjustable cells in the ESUFI
for the use of the Solver tool to solve the objective function of the PSBP
problem, can be assessed by the application of the correlation coefficient (r),
given by Equation 10. The value of 0.89 shows a very high inverse correlation
between the number of tasks (n) and the number of resources (m), that is, for
the adjustable cells of the ESUFI, as the number of tasks is increased, the
number of resources decreases, resulting in a reduction in the number of
resources to be programmed in the ESUFI.
In
analyzing the performance of the model PSBP ESUFI held for class 3, were tested
percentage of success of each
of the subclasses listed in Table 3.
Table 3  Class 3
CLASS 
SUBCLASSES 
INTERVAL
OF TIME 
3 
{3,2};{3,3};{3,4};{3,5};{3,6};{3,7};{3,8};{3,9};{3,10};{3,15};{3,20};{3,25};{3,30};{3,40};{3,62}; 
[ 1,8 ] 
[1,12] 

[1,24] 

[1,45] 

[1,99] 
In
the time intervals [1, 8] and [1, 12], the subclasses {3, 2}, {3, 3}, {3, 4},
{3, 5}, {3, 6}, {3 , 7}, {3, 8}, {3, 9}, {3, 15}, {3, 20}, {3, 25}, {3, 30}, {3,
40} and {3, 62}, showed that 100% of the scenarios tested in the ESUFI (10
scenarios / subclasses) had the objective function of optimization.
In
the time interval [1, 24] subclasses {3, 2}, {3, 3}, {3, 4}, {3, 5}, {3, 6},
{3, 7}, {3 , 8}, {3, 9}, {3, 15}, {3, 20} {3, 25} {3, 30} {3, 40} and showed
that 100% of the scenarios tested in to ESUFI (10 scenarios / subclasses) had
the objective function of optimization, since the subclass {3, 10} presented
90%.
In
the time interval [1.45] subclasses {3, 2}, {3, 3}, {3, 4}, {3, 5}, {3,
6}, {3, 7}, {3 , 8},
{3, 9}, {3, 15} {3,
20} {3, 25} {3, 30} and {3, 40}, showed that 100% of the
scenarios tested in the ESUFI (10 scenarios /
subclasses) had the objective function
of optimization, since the subclasses {3, 10}, {3, 62} showed 90%.
In
the time interval [1, 99] subclasses {3, 2}, {3, 3}, {3, 4}, {3, 5}, {3, 6},
{3, 7}, {3, 8}, {3, 9}, {3, 15} {3, 20} {3, 25} {3, 30} and {3, 40}, showed
that 100% of the scenarios tested in the ESUFI (10 scenarios / subclasses) had
the objective function of optimization, since the subclasses {3, 10} and {3, 62}
showed 80% and 90%, respectively.
In
general, the results of tests of Class 3, show that 93,33% of the scenarios
tested in the ESUFI showed 100% response (optimization of the objective
function), 5,33% had 90% response and 1,33 % showed 80% of responses.
Thus,
it appears that although the literature review we highlight the work of Mixed
Integer Linear Programming to the problem of programming for Batch Production
Systems (FLOW SHOP) with the use of heuristics, this study was focused to solve
a mathematical model using Excel and VBA, which presented good results
computing, as in all scenarios analyzed were solved and obtained an optimal
solution with a good computational time.
6.
CONCLUSION
With
the completion of this study, focused on the development of the Microsoft Excel
spreadsheet with friendly interface, it was observed that this spreadsheet
could be used to minimize the Makespan in the environment of production
scheduling in batch processes.
During
the research, a primary study was conducted on the production scheduling, where
it became clear the vast amount of information involved in this area. Thus, for
the specific case of using the ESUFI in the sequencing of production orders for
minimization of the Makespan, which is an activity involved in the production
scheduling, the ESUFI played a very important role in the production scheduling,
since spreadsheets are known by most computer users and are available in almost
all Office packages.
The
ease of manipulation of the ESUFI is quite evident, as with the use of VBA
language a userfriendly interface could be created between the user and the
spreadsheet itself. To generate the production scheduling that minimizes the
total time of completion of tasks, the only information needed in the worksheet
is the number of tasks and resources and the processing times of the tasks in
their respective resources. However, the ESUFI had some limitations.
In
carrying out the validation tests of the macros used to automate the use of the
Microsoft Excel using Visual Basic Application (VBA), a limitation of the
spreadsheet was found concerning the amount of adjustable cells, that is, as
the number of tasks to be programmed in the spreadsheet increases, the number
of resources decreases. Thus, production scheduling is limited to an amount of
up to 13 tasks, and therefore this shows that within standard Excel, the present
ESUFI can be used for smallscale problems.
As a
suggestion, it is important to develop studies applied to real cases, as well
to use a professional Excel version to increase the number of adjustable cells,
which would extend the limit of variables in the production scheduling and
therefore permit working with larger problems.
Although
the literature review found many studies have been related to the topic in
question by applying heuristics for solving the problem of production
scheduling, it appears from this study that the application by means of
mathematical formulation is also a viable alternative for small problems size,
since in these cases there was obtained a good computational performance.
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