A multiobjective genetic algorithm for a mixedmodel assembly Uline balancing typeI problem considering humanrelated issues, training, and learning
Abstract
Mixedmodel assembly lines are increasingly accepted in many industrial environments to meet the growing trend of greater product variability, diversification of customer demands, and shorter life cycles. In this research, a new mathematical model is presented considering balancing a mixedmodel Uline and humanrelated issues, simultaneously. The objective function consists of two separate components. The first part of the objective function is related to balance problem. In this part, objective functions are minimizing the cycle time, minimizing the number of workstations, and maximizing the line efficiencies. The second part is related to human issues and consists of hiring cost, firing cost, training cost, and salary. To solve the presented model, two wellknown multiobjective evolutionary algorithms, namely nondominated sorting genetic algorithm and multiobjective particle swarm optimization, have been used. A simple solution representation is provided in this paper to encode the solutions. Finally, the computational results are compared and analyzed.
Keywords
Mixedmodel assembly lines Ushaped assembly lines Learning and training effect Humanrelated issues MultiobjectiveIntroduction
An assembly line is a group of successive workstations, joined by a material handling system. In each workstation, a set of tasks are carried out using a predefined assembly process, in which the time required to carry out each task and a set of priority relations which determines the order of the tasks are defined. The current market is severely competitive and consumercentric with high variety in demands. As a result of high cost to establish and maintain an assembly line, the manufacturers produce one model with various features or several different models on a single assembly line. In situations like this, the mixedmodel assembly line balancing problem arises to smooth the production and decreases the cost. Mixedmodel Assembly Line (MMAL) is a kind of production line, where a set of similar models of a product are assembled to respond to the diversity of customer’s demands. There are two types of assembly line balancing problems. The purpose of typeI problems are minimizing the number of workstations. In this problem, the required production rate, assembly tasks, tasks times, and precedence requirements will be given. In typeII problems, the goal is to minimize the cycle time and maximize the production rate with fixed number of workstations or production employees. This study is mainly focused on the typeI problem, which wants to minimize the number of workstations.
Utype line balancing was first invented by Miltenburg and Wijngaard (1994). The Utype assembly line is an attractive substitute for assembly production systems from the time operators became multiskilled by performing tasks defined on different parts of assembly line (Gökçen et al. 2005). The advantage of the Utype assembly line is the flexibility that it offers to choose an appropriate number of operators to satisfy demand changes (Aigbedo and Monden 1997).
Learning effect is another important factor at assembly lines in the time of the new product lunch, or start of production (Baloff 1971). The length of the learning stage has become an important performance indicator for a firm because of some common topics, such as shortened product life cycles, high innovation rates, and, therefore, more frequent product launches. Learning effect has to be considered in firms, because shorter learning stages enable firms to increase sales and, as a result, achieve more profits with the highest revenues, by the time, the new product reaches the market. Learning effects may occur by a highly repetitive execution of certain tasks. “A worker learns as he works; and the more often, he repeats an operation”. Andress (1954) mentioned, learning effects at assembly lines and overall for repetitive operations. According to aircraft construction, Wright (2012) described learning effects at assembly lines and overall for repetitive operations. He figured out that by making the cumulated output double, average construction costs per unit sunk by about 20 %. This observation was formalized as an inversely proportional relationship between unit costs and cumulated output called learning curve. After that, for assembly lines in different industries, the presence of significant learning effects was confirmed. Basically, in mixedmodel Ushaped assembly lines, workers are capable of operating several tasks. As Park (1991) said, training, the process by which workers become multiskilled, has been recognized as a tool for boosting production flexibility. The minimum introduction of worker crosstraining has the most significant improvement from no crosstraining, and the subsequent increase of the crosstraining has a diminishing return. In this research for the first time, a new model is presented considering both line balancing and worker assignment simultaneously, considering humanrelated issues. Two metaheuristic algorithms [i.e., multiobjective particle swarm optimization (MOPSO) and nondominated sorting genetic algorithm (NSGAII)] are used to solve the proposed biobjective problem, and a simple method is applied to represent solutions.
The rest of the paper is organized as follows: in “Literature review”, the relevant literature is reviewed. In “Problem description”, the biobjective problem, the objective function, and a mathematical model are presented. The methodology is described in “Methodology”, and the illustrative examples are presented in this section. In “Parameters tuning”, comparisons and discussion are brought. The study is finally ended by conclusions and future research in “Conclusion”.
Literature review
The existing competitive and consumercentric market and the observed trend of diversification of customer demands and high fluctuations is an important subject that is worth studying. Firms should improve their performance for dealing with these pressures to meet the customers demand within a short delivery time and with the lowest possible cost. Mixedmodel assembly lines are one of the most relevant production environments that deal with these problem. The assembly line balancing problem encompasses assigning tasks to an ordered sequence of stations, such that precedence relations among tasks should not be violated (Erel and Sarin 1998). A mixedmodel assembly line is assembly line, in which some similar product type with some insignificant difference is assembled. Many attempts have been made to solve the assembly line balancing (ALB) problems using the exact solution methods, heuristics, and metaheuristic approaches. Some comprehensive reviews of such studies have been done (Becker and Scholl 2006; Erel and Sarin 1998). Some researches solved the assembly line balancing problem using a ranked positional weight technique (Helgeson and Birnie 1961). Monden (2011) was concerned with the sequencing of assembly lines, such as considering the stability of parts usage rates. Kim et al. (2009) presented a mathematical formulation and a genetic algorithm for the ALBII problem. Some practitioners presented a formal ALBI problem, and they also developed a branchandbound algorithm to solve the problem (Wu et al. 2008). Erel and Gokcen (1999) proposed a study that was concerned with minimizing the task time for different models considering precedence constraints using shortest route formulation. A binary integer formulation for the mixedmodel assembly line balancing problem is developed by Gökcen and Erel (1998). In another work, Gokcen and Erel (1997) extended a goal programming approach which was previously developed by Thomopoulos (1967), using a combined precedence diagram. Vilarinho and Simaria (2002) develop a twostage heuristic method for balancing mixedmodel assembly lines. The application of genetic algorithms (GA) for assembly line balancing has widely been considered in many studies. A genetic algorithm for typeII problems was presented by Anderson and Ferris (1994), and Leu et al. (1994) presented a GAbased approach to solve typeI problems with multiple objectives. Kim et al. (1996) presented a genetic algorithm for work load smoothing. In another study, a hybrid genetic algorithm approach to the assembly line planning problem was developed (Chen et al. 2002). There are only a few studies which use more than one metaheuristic approach to solve their problem, but in this study, two metaheuristic algorithms (i.e., MOPSO and NSGAII) are used to solve the proposed biobjective problem.
Many practitioners studied the mixedmodel straight line assembly line balancing problem which has been reported in the literature (Erel and Gokcen 1999; McMullen and Frazier 1998; Simaria and Vilarinho 2004, 2009; Thomopoulos 1967; Vilarinho and Simaria 2002). Simaria and Vilarinho (2009) proposed a mathematical programming model to formally describe the MMALB problem presenting an ant colony optimization algorithm. One of the effective factors for realizing the objectives of lean manufacturing is workforce planning. Several options of alternative production planning that can be applied for dealing with changing demand patterns, considering use of variable workforce, overtime, seasonal inventory, and planned backlogs have been developed by Hax and Candea (1984). Several classical LP models combining the production, manpower, and inventoryrelated tradeoffs in each of the options mentioned above have been presented (Bhatnagar et al. 2003). Justintime (JIT) is able to adjust to changes in the external environment of the firm, because of several reasons, including efficient facility layouts and multifunctional workers (Monden 2011; Schonberger 1983). Japanese companies are operating with very low level of inventory and recognizing a high level of productivity using the justintime (JIT) manufacturing system which has the goal of continuously reducing and ultimately removing all forms of wastes (Ōno 1988). The replacement of the traditional straight lines with Ushaped production lines is one of the most important changes resulting from JIT implementation (Chiang and Urban 2006). Reducing the work in process inventory and wasted operator’s movement, labor productivity improvement, material handling improvement, zerodefects campaign’s implementation, and higher flexibility in workforce planning in the face of changing demand patterns (Monden 2011) are the main benefits of the Uline as compared to a straight line.
(In some reference, it is shown that one of the best applicable types of line is Ushape line and they illustrate that the benefits are impressive. The main characteristics in a Ushaped line are (Miltenburg and Wijngaard 1994): the Uline arranges machines around a Ushaped line in an operators work inside the Uline; Ulines are rebalanced periodically when production requirements change; the operators must be multiskilled and versatile to do several different processes; it requires operators to walking, when setup times are negligible; Ulines are operated as mixedmodel lines, where each station is able to produce any product in any cycle; when setup times are larger, multiple Ulines are formed and dedicated to different products. Miltenburg and Wijngaard (1994) have a comprehensive article in the subject of Ushaped production line. In his article, the benefits of Ushape line were mentioned, and by some statistic information, they are proved for all). There are several studies on line balancing problems. Most of them assumed that the time of tasks for repetition tasks is independent from learning of workers. A few researchers have examined the learning effect on assembly line balancing problems (Chakravarty and Shtub 1988; Cohen and Ezey DarEl 1998; Cohen et al. 2006). Learning can play a considerable role in manufacturing environments and there are many empirical studies that have proven learning effects (Cochran 1960; Yelle 1979). Learning occurs on the part of workers directly involved into manufacturing of the product (Andress 1954).
Worker skills promotion possibility
Skill level 1  Skill level 2  Skill level 3  Skill level 4  

Skill level 1  –  *  *  * 
Skill level 2  –  –  *  * 
Skill level 3  –  –  –  * 
Skill level 4  –  –  –  – 
Overview of the related literature and contributions of this study
Study  Human–related issues (hiring, firing, and salary)  Training and learning  ALBI  ALBII  Parallel stations assembly line balancing  Ul type assembly line balancing  Simple assembly line balancing  Twosided assembly line balancing  Mixedmodel assembly line balancing  Method 

This study  *  *  *  *  *  NSGAII MOPSO  
Yuan et al. (2015)  *  *  HBMO and SA  
Kucukkoc and Zhang (2014)  *  *  *  Agentbased ACO enhanced heuristics and model sequencing agent  
Manavizadeh et al. (2013)  *  *  *  *  SA  
Özcan and Toklu (2010)  *  *  GA  
Simaria and Vilarinho (2009)  *  *  *  SA  
Kim et al. (2009)  *  *  GA  
Wu et al. (2008)  *  *  Branchandbound  
Aryanezhad et al. (2009)  *  *  Model numerical examples  
Simaria and Vilarinho (2004)  *  *  *  *  Genetic algorithm 
Problem description
In this study, the focus is on minimizing the number of stations to achieve an optimum balance; therefore, the idle time should be minimized and the efficiency of the line should be enhanced. These goals may be achieved by smoothing the amount of workload and maximizing the equalization of the workload among stations. It was assumed that training, which is done to promote workers to upper levels, is performed between periods and it takes zero time. Workers are classified into four types based on their skill levels. The level of each work station indicates types of workers allowed to work at that station. Each worker has exactly one skill and exactly belongs to one skill level. Workers with skill level 4 can work on task levels 1, 2, 3, and 4. Workers with skill level 3 can work on task levels 1, 2, 3, and so on. In each period, workers can be trained to improve their working abilities to operate other task levels. The initial number of workers with skill level O in the beginning of the planning horizon is known. Levels of tasks are known, and the level of each station is equal to the maximum level of tasks which are assigned to it.
Assumptions

Parallel stations are not allowed.

Operator walking time is ignored.

All parameters in the model are assumed to be deterministic.

There is no uncertainty.

Each task must be assigned to exactly one station.

All predecessors or successors of a task have already been assigned to a station (the precedence constraint.

The total time of the tasks assigned to each station, (i.e., the station time), may not exceed the cycle time (the cycle time constraint).

Salary is merely dependent on worker’s skill level and not depending on machine levels.

All of the machine types which need the same skill levels assumed to be similar in worker assignment.

Cost of hiring and firing are given, and they merely depend on skill levels.

Each task needs just one worker.

Training, which is done to promote workers to upper levels, is performed between periods and it takes zero time.

The productivity of experienced workers is assumed to be equal to 100 %.

The productivity of newly trained workers is assumed to be fewer than that of experienced ones, and it depends on the skill level to which they are trained.

Productivity of newly hired workers is assumed to be fewer than that of experienced ones, and it depends on the skill level for which they are hired.

Cost of training from one skill level to another is given, and it depends on both skill levels.
Objective functions
The value of B _{w} is within the value range of [0,1]. In worst case, when only model attributes to the idle time of each workstation, it equals 1, and when all models attribute equally to the idle time at each workstation it equals zero (Simaria and Vilarinho 2009).
The value of WIT is different from one problem to another due to their dependence on the cycle time and task processing of each specific problem, whereas the function B _{b} and B _{w} are always within the value range of [0,1]. An alternative measurement, which is always within a fix range of values, is the weighted line efficiency (WE) (Simaria and Vilarinho 2009).
Mathematical model
 i, b

Index of task
 R

Maximum number of stations
 r, r′

Index of station
 J

Model (product) {1,…, M}
 s

Index of period
 O

Work skills category {1, 2, 3, 4}
 k,k′

Index for station levels {1, … , MS = 4}
 M

Number of models
 V

Number of operators
 I

Total number of tasks in the combined precedence diagram, (i = 1, 2, 3, … , I)
 MS

Number of station levels
 D

The vector presenting the total demand for each model, D = {D _{1}, D _{2}, … , Dm}
 q′

The overall proportion of the number of units of model j
 P _{ ib }

Showing the precedence relationship between task b and i. Equal 1 if task b is the precedence for task i
 su _{ ib }

Showing the succeeding relationship between task b and i. Equal 1 if task b is a successor for task i
 o _{ ib }

A zero–one variable which determines whether or not constraints 2 or constraint 3 is satisfied
 C

Cycle time
 P

Total time in the planning horizon
 id _{ r }

Idle time of station r
 D _{ js }

Demand of model j in period s
 t _{ ij }

Processing time of task i of model j
 w _{ o }

Number of workers of skill category o
 w _{ s } ^{ o }

Number of workers of skill category o working in period s
 pt _{ s }

Regular time rate for workers during period s
 ot _{ s }

Overtime rate for workers during period s
 h′

Total working hours in a period
 h′

Minimum overtime work for operators
 h _{ o,s }

Cost of hiring of a worker with skill level o in period s
 s _{ o,s }

Salary of each olevel worker in period s
 f _{ o,s }

Firing cost of each olevel worker fired in period s
 Co,o′′,s

Training cost of each olevel worker trained for skill level o′ in period s
 α _{ o }

Productivity of each newly olevel worker hired in period s 0 < α _{o} < 1
 β _{ o,o }′

Training productivity of olevel worker trained for skill level o′ 0 < β _{o,o′} < 1
 a _{ ro }

Equals 1 if workers of skill category o can work at processing stage r and zero
 x _{ ir }

Equals 1 if task i is assigned to station r and equal 0 otherwise
 y′ _{ r }

Equals 1 if workstation r is used for assembly and 0 otherwise
 x′ _{ rs }

Total number of overtime hours done by workers at station r in period s
 x _{ rs } ^{ o }

Equals 1 if worker from skills category o is allocated to station r in period s
 U _{ o,s,k }

Number of olevel workers who are hired and assigned to station level k in period s
 E _{ o,s,k }

Number of existing olevel workers who are assigned to station level k in period s
 UX _{o’,o,s,k,}

Number of o′–level workers who were assigned to task level k in period s − 1 and now are trained to skill level o and assigned to task level k′ in period s
 UG _{ o′,o }

Equals 1 if training from skill level o′ to skill level o is possible and 0 otherwise
Methodology
Proposed model in this paper is multiobjective, so the methods for solving the problem are NSGAII and MOPSO. Rabbani et al. (2016a, b) applied these two algorithms for solving a mixedmodel assembly line problem, and the results obtained by these two algorithms were compared to each other. NSGAII is a popular nondominationbased genetic algorithm for multiobjective optimization. It is a very effective algorithm but has been generally criticized for its computational complexity, lack of elitism, and for choosing the optimal parameter value for sharing parameter (Rabbani et al. 2016a, b). Kusiak and Wei (2012) introduced MOPSO for optimizing continuous nonlinear functions, Particle Swarm Optimization (PSO) defined a new era in Swarm Intelligence (SI). PSO is a populationbased method for optimization. The population of the potential solution is called as swarm and each individual in the swarm is defined as particle. PSO is motivated by social behavior of birds flocking or fish schooling Solutions are represented by particles in the search space. The particles fly in the swarm to search their best solution based on experience of their own and the other particles of the same swarm. PSO started to hold the grip amongst many researchers and became the most popular SI technique soon after getting introduced, but due to its limitation of optimization only of single objective, a new concept MultiObjective PSO (MOPSO) was introduced, by which optimization can be performed for more than one conflicting objectives, simultaneously. Coello et al. (2002) described the advantages of using MOPSO in solving multiobjective optimization problem rather than the single objective version of the algorithm.
Representation of solutions
The chromosome is a string of length I which shows the task numbers, where each element represents a task and the value of each element represents the workstations to which the corresponding task is assigned. The maximum number of stations is equal to total number of tasks. For example, for 16 tasks, 9 workstations will be created.
In this research, individuals in the initial population are all randomly generated. While a heuristic procedure can provide good initial solutions, it can cause the solutions to be biased.
Illustrative example
Test problem generation
Parameters  Value  Parameters  Value 

Demand  U(5, 10)  Hiring cost  U(1500, 2000) 
Processing time  (2, 5)  Firing cost  U(1500, 2000) 
Training cost  U(50, 150)  Salary  U(100, 500) 
Initial number of workers with skill level 1 in the beginning of the planning horizon
Skill level  1  2  3  4 

Initial number of workers  5  1  1  0 
Level of tasks
Task 1  Task 2  Task 3  Task 4  Task 5  

Skill level 1  
Skill level 2  
Skill level 3  *  *  
Skill level 4  *  *  * 
Cost of training from skill level O to skill level O′ in periods
From skill  Period/to skill  Skill level 2  Skill level 3  Skill level 4 

Skill level 1  1  118  64  127 
2  125  69  112  
3  118  83  117  
Skill level 2  1  97  130  
2  95  65  
3  106  66  
Skill level 3  1  124  
2  86  
3  146 
Cost of hiring, firing, and salary of each Olevel worker in each period are generated randomly
Skill level/period  Hiring  Firing  Salary  

1  2  3  1  2  3  1  2  3  
Skill level 1  1823  1604  1667  1823  1604  1667  1800  1600  1500 
Skill level 2  1813  1530  1959  1813  1530  1959  1800  1500  1900 
Skill level 3  1910  1677  1509  1910  1677  1509  1900  1677  1500 
Skill level 4  1878  1628  1651  1878  1628  1651  1800  1700  1600 
Processing time related to five task problems
Model  1  2  3  4 

Task  
1  5  2  5  4 
2  4  3  2  3 
3  4  4  4  3 
4  5  3  3  4 
5  5  3  4  3 
Task assignment
4  3  3  2  1 
The results from NSGAII algorithm are shown below:
Training
Period 1  Period 2  Period 3  

Skill level 1  Skill level 2  Skill level 3  Skill level 4  Skill level 1  Skill level 2  Skill level 3  Skill level 4  Skill level 1  Skill level 2  Skill level 3  Skill level 4  
Skill level 1  0  0  0  2  0  0  0  2  0  0  0  2 
Skill level 2  0  0  0  1  0  0  0  0  0  0  0  0 
Skill level 3  0  0  0  0  0  0  0  1  0  0  0  1 
Skill level 4  0  0  0  0  0  0  0  0  0  0  0  0 
Parameters tuning
Total number of hiring and firings
Hire  Fire  

Skill level  1  2  3  4  1  2  3  4 
Number of workers  0  0  0  6  9  2  1  0 
Tuned parameters for NSGAII and MOPSO algorithms
Algorithm  Parameter  

\(\text{Max}\_\text{iteration}\)  \(N_{p}\)  \(P_{c}\)  \(P_{m}\)  \(N_{r}\)  W  \(c_{1}\)  \(c_{2}\)  
NSGAII  100  100  0.7  0.5  …  …  …  … 
MOPSO  125  50  …  …  75  0.7  1.5  1 
Comparative results
Comparison metrics: It is common to compare the performance of the multiobjective algorithms’ performance by means of some specific comparison metrics; to compare proposed algorithms with each other, three comparison metrics are employed (Rabbani et al. 2016a, b).
1. Number of Pareto solutions (NPS): The quantity of nondominated solutions that every algorithm can discover.
Smallsize problem
Computational results for smallsize problem
Number of tasks  NPS  SM  MID  Diversity  

MOPSO  NSGAII  MOPSO  NSGAII  MOPSO  NSGAII  MOPSO  NSGAII  
5  3  3.66  0.939  0.029  12,104.313  23,473.333  26.697  233.388 
6  2.88  3.8  1.923  1.280  14,280.774  19,886.250  255.612  337.352 
7  6  8.9  1.145  1.4219  8657.736  18,860.888  255.668  480.552 
8  6.3  10.5  1.519  0.612  11,554.265  17,592.728  297.687  442.358 
10  10.4  12  1.103  1.265  27,919.017  23,332.4  502.774  430.670 
Average computational times for smallsize problems (in seconds)
Number of tasks  NSGAII  MOPSO 

5  7.812923  2.629137 
6  4.666816  2.084193 
7  5.151678  2.364705 
8  41.968083  2.780 
10  47.039556  6.261575 
Largesize problem
Computational results for largesize problems
Number of tasks  NPS  SM  MID  Diversity  

MOPSO  NSGAII  MOPSO  NSGAII  MOPSO  NSGAII  MOPSO  NSGAII  
16  11.5  12.5  1.307  1.244  61,031.23  54,649.58  513.695  508.78 
17  10  7.4  1.598  1.013  64,565.99  58,431.00  543.438  257.06 
18  10  5  1.914  1.224  68,312.35  66,500.80  472.476  255.95 
19  11.3  13  1.068  1.024  71,492.21  67,707.84  452.856  498.853 
20  13  13  1.31991  1.23546  75,269.613  73,343.53  673.46195  560.282 
Average computational times for largesize problems (in seconds)
Number of tasks  NSGAII  MOPSO 

16  460.982041  143.715486 
17  2620.811549  2972.869897 
18  1067.034342  1200.195 
19  1518.914096  6041.283304 
20  13,680.837762  16,997.6252 
Conclusion
This research deals with balancing a mixedmodel assembly Uline considering humanrelated issues. The objective function consists of two separate components. The first part of the objective function is related to balance problem. In this part, objective functions are minimizing the cycle time, minimizing the number of workstations, and maximizing the line efficiencies. The second part is related to human issues and consists of hiring cost, firing cost, training cost, and salary, and the labor assignment policy was defined. In this research, workers are classified into four types based on their skill levels. The level of each work station indicates types of workers allowed to work at that station. Two metaheuristic algorithms (NSGAII and MOPSO) are used for solving a biobjective problem presented in this paper. In smallsized problem, MOPSO outperforms NSGAII with respect to computational time, but in largescale problem in all problems except the problem with 16 tasks, the operation of NSGAII is better than MOPSO with regard to computational time. In most problems, including small and largesized problems, the number of Pareto solutions (NPS) generated with NSGAII is more than MOPSO. Spacing metrics obtained by the NSGAII provide nondominated solutions that have a less average value of the spacing metrics. These data reveal that the nondominated set obtained by the NSGAII is more uniformly distributed in comparison with the MOPSO algorithm. In two other comparison metrics, the obtained results do not show any superiority of each algorithm with comparison another one. The algorithms provided approximated Pareto solutions for decision maker to choose from them, but in some real cases, especially in critical industries, where any error has catastrophic results, finding approximated solutions cannot be helpful for decision makers.
Future developments will be devoted to investigate the effects of human resource planning policies on balancing of a mixedmodel assembly Uline in uncertainty conditions, given the fact that human activities are not deterministic. In addition, solving a problem by exact methods, such as goal programming and goal attainment, can have great managerial insights to make decisions more precisely.
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