Formulations , Features of Solution Space , and Algorithms for Line-Pure Seru System Conversion

The line-seru conversion is usually used to improve productivity, especially in volatile business environment. Due to the simplicity, most researches focused on line-pure seru system conversion. We summarize the two existing models (i.e., a biobjective model and a single-objective model) of line-pure system conversion and formulate the three other usually used single-objective models in an integrated framework by combining evaluated performances and constraints. Subsequently, we analyze the solution space features of line-pure seru system conversion by dividing the whole solution space into several subspaces according to the number of serus. We focus on investigating the features between Cmax (and TLH) and subspaces. Thirdly, according to the distinct features betweenCmax (and TLH) and subspaces, we propose four effective algorithms to solve the four single-objectivemodels, respectively. Finally, we evaluate the computational performance of the developed algorithms by comparingwith enumeration based on extensive experiments.


Introduction
The seru production, conceived at Sony, is an innovation of assembly system used widely in the Japanese electronics industry and recognized a new production pattern.Seru production is proposed to overcome the less flexible shortcoming of assembly line, especially confronted with the dynamic and volatile business environments.Seru is an assembly unit including several simple equipment and one (or more) multiskilled operator(s).Seru is the pronunciation of cell in Japanese; therefore, Seru production is also called Japanese style cellular production.In seru, worker(s) must be multiskilled [1][2][3][4] because workers need to operate most or all the processes of production.In general, there are three types of seru: divisional seru, rotating seru, and yatai [5,6].In a divisional seru, tasks are divided into different sections and workers are partially cross-trained.Each section is operated by one or more workers.However, workers in rotating seru or yatai are completely cross-trained and do all tasks.In this research, rotating serus or yatai are considered.
Seru system can be generally divided into two types: (1) pure seru system with only seru(s) and (2) hybrid system with seru(s) and short line.As Stecke et al. [5,6] claimed, with combined strengths from Toyota's lean philosophy and Sony's one-person production organization, seru system is a more productive, efficient, and flexible system than conveyor assembly line.As we know, the productivity of the assembly line is decided by the worker with the worst skill.It is amazing that seru system can greatly decrease the influence of the worker with the worst skill on the productivity by worker reconfiguration.Seru is more flexible than assembly line, because the workers in seru system are multiskilled operators who can operate multiple tasks and process multiple products.Seru's flexibility is different with cellular manufacturing, where workers operate the certain part/product family.Seru system can response to market change quickly, due to movable workstations, light equipment, and multiskilled workers.Moreover, seru system can be continuously improved, because workers in seru can study to process more tasks and products.
Due to the merit of seru system [5][6][7], seru system has successfully been applied by many leading Japanese companies such as Sony, Canon, Panasonic, NEC, Fujitsu, Sharp, and Sanyo.Presently, many companies converted assembly line into seru system (line-seru conversion) to increase the productivity [8][9][10].By adopting line-seru conversion, Canon and Sony reduced 720,000 and 710,000 square meters of floor space, respectively [5,6].Moreover, Canon's costs were reduced significantly, by 55 billion yen in 2003, and by a total of 230 billion yen from 1998 to 2003.As a result, Canon emerged as a leading electronics maker [11,12].Other benefits [13,14] include the reductions of throughput time, setup time, required labor hours, WIP inventories, and finishedproduct inventories.For example, the makespan was reduced by 53% at Sony Kohda and 35,976 required workers, that is equal to 25% of Canon's previous total workforce, have been saved.Seru systems also influence profits, product quality, and workforce motivation in a positive way.
The essence of line-seru conversion lies in how to convert traditional conveyor assembly line into a seru system to obtain the optimal conversion solution.It is a very hot and important decision making problem because the total productivity of manufacturers may be improved dramatically [15,16].Such technical and decision making problems were defined as line-seru (or line-cell) conversion problems [10,16].Kaku et al. [10,16] built a biobjective line-seru conversion model with minimizing makespan (they use the term "total throughput time") and TLH.They claimed that their model considered three types of seru systems, that is, (1) pure seru system, (2) hybrid system with serus and short line, and (3) assembly line.However, the third one, in fact, is not considered as a seru system, because it does not contain any seru.Therefore, there are only two types of seru systems, that is, pure seru system and hybrid system with serus and short line.
Pure seru system is the seru system only containing seru(s) and is very simple and a special case of all other seru assembly systems.The results obtained from pure seru system models not only provide insights into the pure seru system environment but also provide a basis for heuristics that are applicable to more complicated seru system environments, for example, hybrid system with serus and short line.In practice, problems in more complicated seru system environments are often decomposed into subproblems that deal with the pure seru system.Therefore, many literatures focused on the conversion of assembly line to pure seru system (line-pure seru system conversion), such as Yu et al. [17][18][19][20] and Sun et al. [21].A simple case of line-pure seru system conversion is shown in Figure 1, where two serus are constructed, that is, workers 2 and 5 in seru 1 and workers 1, 3, and 4 in seru 2.
To establish the main mathematical models is very important for the research on line-pure seru system conversion.Yu et al. [17] formulated the biobjective model of Min- max and TLH.Subsequently, the researches of Yu et al. [18,20] were based on the biobjective model.Considering the biobjective model has higher computational complexity, Sun et al. [21] formulated the single-objective model of Min- max with TLH constraint.Additionally, there are several other usually used single-objective models in line-pure seru system conversion.We should not formulate one model in one research but integrate these models into a common framework.Therefore, the first objective of the research is to formulate the five models of line-pure seru system conversion in an integrated framework by combining the evaluated performances and constraints.
Moreover, the algorithms for line-pure seru system conversion are also the key.However, the existing algorithms for line-pure seru system conversion were based on enumeration (i.e., GA and local search) or metaheuristic and did not consider the distinct features of solution space of line-pure seru system conversion.For example, enumeration in Yu et al. [17] searched the whole solution space; Yu et al. [18,20] and Sun et al. [21] used GA or local search to seek the optimal solution in the whole solution space.In fact, solution space of line-pure seru system conversion has the distinct features; that is, the solution space can be divided into several subspaces according to the number of serus, and minimum  max and minimum TLH usually exist in the special subspaces.That is to say, considering the features of solution space of line-pure seru system conversion, we maybe do not need to search the whole solution space to seek the optimal solution.Therefore, the second motivation of the research is to investigate the distinct features of solution space of line-pure seru system conversion.Subsequently, the third of objective is to develop the effective algorithms for the different models according to the obtained features of solution space of line-pure seru system conversion.
This paper, originally motivated by line-seru applications of Sony and Canon, has three purposes.First, two existing models (i.e., a biobjective model and a single-objective model) and the three other usually used single-objective models in line-pure system conversion are formulated in an integrated framework by combining evaluated performances with constraints.Subsequently, we analyze the solution space features of line-pure seru system conversion by dividing solution space into several subspaces according to the number of serus.We focus on investigating the features between  max (makespan) and TLH (total labor hours) and subspaces with different number of serus.Thirdly, we propose four effective algorithms to solve the four single-objective model, respectively, according to the distinct features of line-pure seru system.
The remainder of this research is organized in the following ways.By combining the evaluated performances and constraints, Section 2 formulates five models usually used in line-pure seru system conversion in an integrated framework.Section 3 investigates the solution space feature of line-pure seru system conversion based on W divided subspaces according to the number of serus.Subsequently, we focus on analyzing the features between  max (and TLH) and subspaces with different number of serus.In Section 4, according to the distinct features of solution space, especially the features between  max (and TLH) and subspaces with different number of serus, we propose four algorithms to solve the four single-objective models, respectively.Section 5 uses extensive experiments to evaluate the computational performance of the proposed algorithms by comparing with enumeration.Finally, we end the paper with conclusions and with suggestions for future research in Section 6 and give Figure 1: A case of line-pure seru system conversion [20].
the further research emphases, that is, the conversion from assembly line to the hybrid system with serus and short line.

Formulation of Several Main Models of Line-Pure Seru System Conversion
2.1.Assumptions.The following assumptions are considered in line-pure seru system conversion: (1) The types and batches of products to be processed are known in advance.There are  product types that are divided into  product batches.Each batch contains a single product type.(2) In the line-pure seru system conversion, most assembly tasks within a seru are manual, so need for only simple and cheap equipment and the cost of duplicating equipment is ignored [5].(3) A product batch needs to be assembled entirely within a single seru.(4) All product types have the same assembly tasks.If tasks of some product were unique, we assume the task time for these unique tasks was zero.(5) The assembly tasks within each seru are the same as the ones within the assembly line.Therefore, the total number of tasks equals .(6) In the assembly line, each task (or station) is in charge of a single worker.That means that a worker only performs a single assembly task in the assembly line.In contrast, a seru worker needs to perform all assembly tasks and assembles an entire product fromstart-to-finish, and there is no disruption or delay between adjacent tasks.
(ii) Parameters : size of product batch .
: cycle time of product type  in the assembly line.
: setup time of product type  in the assembly line.
: setup time of product type  in a seru.
: upper bound on the number of tasks for worker  in a seru.If the number of tasks assigned to worker  is more than   , worker 's average task time within a seru will be longer than her or his task time within the original assembly line.
: worker 's coefficient of influencing level of doing multiple assembly tasks.
: skill level of worker  for each task of product type .
(iii) Decision Variables ( (iv) Variables   : coefficient of variation of worker 's increased task time after line-seru conversion, that is, from a specialist to a completely cross-trained worker.If the number of worker 's tasks within a seru is over her or his upper bound   , that is,  >   , then the worker will cost more average task time than her or his task time within the original assembly line.  is given in (3).
: assembly task time of product batch  per station in a seru.In a seru, the task time of product type  is calculated by the average task time of workers in the seru.  is represented as (4).
: flow time of product batch  in a seru.  is represented as (5).
: setup time of product batch  in a seru.Setup time is considered when two different types of products are processed consecutively; otherwise, the setup time is zero.For example, in (6), two adjacent assembled products in a seru are expressed as  and However, if the product types of  and   are identical, that is,   =     = 1, then   is 0.
: begin time of product batch  in a seru.
There is no waiting time between two product batches so that   is the aggregation of flow time and setup time of the product batches processed prior to product batch  in the same seru.  is represented as (7): 2.3.Evaluated Performances.The following two performances are usually used to evaluate seru system's productivity.

Makespan.
Makespan ( max ) of the pure seru system is the due time of the last completed product batch and can be expressed as

Total Labor Hour.
The total labor hours (TLH) are the work time of all workers assembling the total product batches: 2.4.Constraints.Each decision variable determines one process.Therefore, the line-pure seru system conversion is a twostage decision process, that is, seru-formation and seru-load, decided by   and   , respectively.According to the two decision processes and the two evaluated performances, we divide the constraints into the following categories.

Seru-Formation Constraints
Equation ( 10) is the seru's worker constraint which ensures that each formatted seru contains least one worker and most  workers.Equation ( 11) is the worker assignment constraint which guarantees that each worker must be only assigned to a seru.Equation ( 12) is the worker number constraint which guarantees that the total number of workers in all formatted serus equals , that is, the total number of workers in the assembly line.

Seru-Load Constraints. Consider
Equation ( 13) is the seru-load constraint; that is, a product batch is only loaded to a seru.Equation ( 14) guarantees a product batch must be assigned to a seru in which at least one worker is assigned.
Equation ( 15) constrains that pure seru system's  max is not worse than the given value.For example, we can set the given  max as 90% of the assembly line's  max .

TLH Constraint. Consider
TLH of pure  system ≤ given TLH.( 16) Equation ( 16) constrains that total labor hours (TLH) of pure seru system are not worse than the given value.

Several Main Models of Line-Pure Seru System Conversion.
By combining the above two evaluated performances of  max and TLH with four constraints of seru-formation, seru-load,  max , and TLH, we formulate several main mathematical models of line-pure seru system conversion.10)-( 14)).Model of Min- max is to minimize makespan (i.e., (8)) of pure seru system.Therefore, objective is expressed as min (8) and constraints include seru-formation constraints and seru-load constraints, that is, (10)-( 14).

Model of Min-TLH with 𝐶 max Constraint (Model of Min-
TLH with  max Constraint: min (9), s.t. ( 10)-( 14), (15)). max constraint sometimes can be considered in model of Min-TLH, and so model of Min-TLH with  max constraint is formulated as above.
2.6.Researches on the Five Models' Formulations.So far, most researches focused on the biobjective model of Min- max and TLH in Section 2.5.5.For example, Yu et al. [17,20] formulated the biobjective model of Min- max and TLH and investigated the mathematical analysis and influence factors.In addition, as the biobjective model has higher computational complexity, Sun et al. [21] formulated the single-objective model of Min- max with TLH constraint in Section 2.5.2.
Although the three other models were not researched until now, they are usually considered in line-pure seru system conversion.In fact, they are similar to model of Min- max with TLH constraint in Section 2.5.2, because they are single-objective models.It is unacceptable to formulate one model in one research.Therefore, the first contribution of the research is to summarize the existing two models and to formulate the three other usually used single-objective models in a framework by combining the above two evaluated performances and four constraints.
In addition, previous researches did not investigate the distinct features of solution space of line-pure seru system conversion.As a result, the existing algorithms for linepure seru system are not developed according to the distinct features.

Features of Solution Space of Line-Pure
Seru System Conversion

Complexity of Solution Space Satisfying Seru-Formation
and Seru-Load Constraints.In line-pure seru system conversion, seru-formation is the first step.It is to determine how many serus to be formed and how to assign workers into the serus [17]and it is decided by decision variable   .
A feasible solution of seru-formation must satisfy seruformation constraint, that is, (10)-( 12).Yu et al. [17] proved that seru-formation is an instance of the unordered set partition and an NP-hard problem.The number of all feasible solutions of seru-formation can be expressed recursively as the following: where (, ) is the count of partitioning  workers in assembly line into  serus and can be expressed as the Stirling numbers of the second kind [22].The numbers of (1) to (10) are 1, 2, 5, 15, 52, 203, 877, 4140, 21147, and 115975, respectively [23][24][25].Obviously, () increases exponentially with .Seru-load is the second step of line-pure seru system conversion and is decided by decision variable   .It determines which product batches are dispatched to the serus formed in seru-formation [26,27].A feasible solution of seru-load must satisfy seru-load constraint, that is, ( 13) and (14).Seru-load is NP-hard.Therefore, most researches used typical dispatching rules, such as FCFS (first-come, firstserved) and SPT (shortest processing time).Even though the typical dispatching rules are used in seru-load, line-pure seru system conversion still is NP-hard, because it contains seruformation which is NP-hard.
Yu et al. [20] proved that line-pure seru system conversion with SPT is an instance of the unordered set partition and the complexity of solution space can be expressed as (17).Linepure seru system conversion with FCFS is an instance of the ordered set partition and the complexity of solution space can be expressed as The numbers of (1) to (10) are 1, 3, 13, 75, 541, 4,683, 47,293, 545,835, 7,087,261, and 102,247,563, respectively.In the research, we use FCFS rule to assign product batches to serus.

Solution Spaces of the Five Models.
In the above five models, model of Min- max , model of Min-TLH, and biobjective model of Min- max and TLH have the identical solution space described in Section 3.1, because all of them only include seru-formation constraints and seru-load constraints.
However, model of Min- max with TLH constraint and model of Min-TLH with  max constraint have the different solution space, because the two models contain the other constraints except seru-formation constraints and seru-load constraints.Therefore, the solution spaces of the two models must be not more than that of Section 3.1.

Existing Algorithms for the Five Models.
Most researches focused on the algorithms for the biobjective model of Min- max and TLH in Section 2.5.5.Yu et al. [20] proposed a based-NSGA-II algorithm to solve the biobjective model; Yu et al. [18] combined local search into NSGA-II to improve the running time and solutions qualification; Yu et al. [17] used enumeration and based-NSGA-II algorithm to solve the biobjective model to analyze the impact factor on  max and TLH improvements by line-pure seru system conversion.Sun et al. [21] developed a variable neighborhood search (VNS) for model of Min- max with TLH constraint in Section 2.5.2.
However, the algorithms for the three other models were not researched until now.
The existing algorithms for line-pure seru system conversion were based on the metaheuristic (i.e., GA and local search) and did not consider the distinct features of solution space.For example, enumeration in [17] searched the whole solution space; Yu et al. [18,20] and Sun et al. [21] used GA or local search to seek the optimal solution in the whole solution space.In fact, solution space of line-pure seru system conversion has the distinct features; that is, the solution space can be divided into several subspaces according to the number of serus, and minimum  max and minimum TLH usually exist in the special subspaces.That is to say, considering the features of solution space of line-pure seru system conversion, we maybe do not need to search the whole solution space to seek the optimal or suboptimal solution.
Therefore, we investigate the distinct features of solution space of line-pure seru system conversion and then propose effective algorithms to solve the four single-objective models based on the distinct features of solution space.

Subspaces with Different Number of Serus
Definition 1. Subspace in solution space of line-pure seru system conversion is the set of solutions with the same number of serus.
For example, of ( 18), the whole solution space can be divided into  subspaces according to the number of serus, that is, the subspaces with 1, 2, . .., and  serus.Therefore, (18) can be expressed as the following: Table 1 summarizes the number of solutions in subspace with  serus for the lines with 6-9 workers.
To show the proportion of solutions in each subspace to the total solutions in the whole solution space, Table 2 normalizes the data in Table 1.
From Table 2, we can see that the number of solutions in subspaces with few or more serus is very small.Therefore, if minimum  max and minimum TLH exist in such subspaces, we can dramatically decrease the searching space.

3.5.
Value of  max and TLH in Subspaces.Yu et al. [17] performed 64 arrays of full factorial experiment to analyze influence factors and proposed some managerial insights.Two insights on  max and TLH are as follows: (1) to minimize  max , the pure seru system with fewer serus should be created and assign the workers with similar skill levels for product types to the same seru; and (2) to minimize TLH, the pure seru system with more serus should be created.
Yu et al. [17] used the solutions with 1 seru and  serus to briefly explain the two insights: (1) they proved that when  ≤   , if the seru system with a seru is formatted, then the  max performance of pure seru system must be better than that of the assembly line.Additionally, they stated that the pure seru system with fewer serus is easy to balance  max among serus than the pure seru system with more serus.Therefore, the pure seru system with fewer serus can produce better  max ; and (2) they proved that when  ≤   and  ≤ , the pure seru system with  serus should be formatted in order to minimize TLH, because this makes full use of each worker's skill.
To further investigate the two insights, in the research, we analyze the features of  max and TLH in each subspace in The detailed data of the used cases with 6-8 workers are described in Section 5.1.

Distinct Features between 𝐶 max (and TLH) and Subspaces.
By observing  max and TLH in Figures 2-4, we can investigate the distinct features between  max (and TLH) and subspaces as follows.
Feature 1. Minimum  max usually exists in the subspaces with few serus.
Explanation.As shown in Figures 2-4, the minimum  max always exists in the subspaces with 2 serus.
Feature 2. Except the subspace with 1 seru, the minimum  max of the subspaces with fewer serus usually is less than that of the subspaces with more serus.
Explanation.As shown in Figure 3, the minimum  max of the subspaces with  (1 <  < 7) serus always is less than that of the subspaces with  + 1 serus.The trend can be found in all of Figures 2 and 3. Table 3 shows the detailed information on the minimum  max of the subspaces with  serus for the cases with 6-8 workers.
Features 1 and 2 mean that it not necessary to search the whole solution space to obtain minimum  max , especially in single-objective models.To fast obtain minimum  max , therefore, we can search the subspaces from fewer serus to more serus.Consequently, we propose effective algorithms to fast find minimum  max .The detailed procedure is described in Section 4.1.Feature 3. Minimum TLH usually exists in the subspaces with more serus.
Explanation.Assume   express the maximum .In Figures 2-4, the minimum TLH always exists in the subspace with   serus.Feature 4. Minimum TLH of the subspaces with more serus usually is less than that of the subspaces with fewer serus.
Explanation.As shown in Figures 2-4, the minimum TLH of the subspaces with J serus always is less than that of the subspaces with  − 1 serus.Table 4 shows the detailed information on the minimum TLH of the subspaces with  serus for the cases with 6-8 workers.
Features 3 and 4 mean that it not necessary to search the whole feasible solution space to obtain the minimum TLH, especially in single-objective models.To fast obtain minimum TLH, therefore, we can search the subspaces from more serus to few serus.Consequently, we propose effective algorithms to fast find the minimum TLH.The detailed procedure is described in Section 4.2.

Algorithms for the Single-Objective Line-Pure Seru System Conversion
As mentioned above, most existing algorithms were for the biobjective line-pure seru system conversion.The algorithm of Sun et al. [21] was for single-objective model.However, these algorithms were not developed based on the features of solution space of line-pure seru system conversion, which decreases their running efficiency.Therefore, we propose algorithms to solve the 4 single-objective models based on the distinct features of solution space.

Algorithms for Models of Min-𝐶 max and Min-𝐶 max with TLH Constraint.
To avoid the obtained solution with minimum  max being locally optimal, the algorithm does not search the subspace with the least seru (i.e., 1 seru) but searches subspaces from fewer serus to more serus to seek    minimum  max .Therefore, the minimum  max may be obtained by searching only a part of solution space.
For an instance of Min- max with  workers, after obtaining the optimal solution in subspace with  seru, the algorithm searches the optimal solution in subspace with +1 serus.If the latter is not better than the former, then the algorithm will stop; otherwise, continue, until the optimal solution with +1 serus is not better than the optimal solution with  serus.The procedure can be described as shown in Algorithm 1.
In Steps (2) and (3-3),  =  + 1 means to search solution space from few serus to more serus.Step (3-3) is to judge if the algorithm stops.If yes, output ; otherwise, continue to search the optimal  max in the next subspace.
Similar to Algorithm 1, algorithm for models of Min- max with TLH constraint can be expressed as shown in Algorithm 2.
Distinguished from Algorithm 1, Algorithm 2 needs to calculate TLH and to judge if TLH constraint is satisfied.They are described in steps (1-1) and (1-2).

Algorithm for Model of Min-TLH and Min-TLH with 𝐶 max
Constraint.To avoid the obtained solution with minimum TLH being locally optimal, the algorithm does not search the subspace with the most seru (i.e.,  seru) but searches subspaces from more serus to few serus to seek the minimum TLH.Therefore, the minimum  max may be obtained by searching only a part of solution space.
For an instance of Min-TLH with  workers, after obtaining the optimal solution in subspace with  seru, the algorithm searches the optimal solution in subspace with −1 serus.If the latter is not better than the former, then the algorithm will stop; otherwise, continue, until the optimal solution with −1 serus is not better than the optimal solution with  serus.The procedure can be described as shown in Algorithm 3.
In Steps (2) and (3-3),  =  − 1 means to search solution space from more serus to few serus.Step (3-3) is to judge if the algorithm stops.If yes, output ; otherwise, continue to search the optimal TLH in the next subspace.Similar to Algorithm 3, algorithm for models of Min-TLH with  max constraint can be expressed as shown in Algorithm 4.
Distinguished from Algorithm 3, Algorithm 4 needs to calculate  max and to judge if  max constraint is satisfied.They are described in steps (1-1) and (1-2).

Combining Features 1 and 2 into the Existing Algorithm.
As mentioned above, Sun et al. [21] developed a variable neighborhood search (VNS) for model of Min- max with TLH constraint.However, their VNS seeks the optimal solution in the whole solution space.Therefore, considering Features 1 and 2, that is, to search minimum  max satisfying TLH constraint in the subspaces with fewer serus by redefining the neighborhood strategies, Sun's algorithm running time will be improved.5-9 show the parameters, data distribution and detailed data of level of skill of workers, coefficient of influencing level of skill to multiple stations for workers, and data of batches used in experiments, respectively.From Table 5, it can be observed that the lot size of each batch is (50, 5) and the ability of workers is also different with stations and (0.2,0.05).Table 6 shows that the mean of skill level (  ) of each worker for processing  product types  8 and 9, respectively.In fact, data in Tables 5-9 is the part of data used in Yu et al. [20].

Computational Experiments
For the instance with  workers, we use the following data set from Tables 5-9: entire Table 5, first  rows of Table 7, first  columns of Table 8, and entire Table 9.

5.2.
Hardware and Software Specifications.The four algorithms were coded in C# and executed on an Intel Core6 2 processors at 2.66 GHz under Windows XP using 3.49 GB of RAM.

Comparative Results with Enumeration.
Considering the similarity between Algorithm 1 (or 3) and Algorithm 2 (or 4), we used the Algorithms 1 and 3 to solve the models of Min- max and Min-TLH, respectively.The performance comparative results to the enumeration are shown in Table 10.
From Table 10, we can see the optimal solutions obtained by Algorithms 1 and 3 are same as the results from enumeration.Moreover, the running time is much less than that of enumeration, because algorithms search the corresponding subspaces according to the distinct features between  max (and TLH) and subspaces instead of searching the whole solution space.In addition, the running time of Algorithm 3 is more than that of Algorithm 1.That is because the subspace with more serus has more feasible solutions than the subspace with few serus, as shown in Table 1.

Conclusions and Future Research
In the research, three main contributions can be summarized as follows.
Firstly, by combining two common evaluated performances of  max and TLH with four constraints of seruformation, seru-load,  max , and TLH, we summarize the two existing models and formulate three other usually used models in line-pure seru system conversion.The two evaluated performances, four classified constraints, and formulated models are summarized in Table 11.
Secondly, we investigate the solution space feature of line-pure seru system conversion by dividing them into  subspaces according to the number of serus ().Subsequently, we focus on analyzing the features between  max (and TLH) and subspaces in detail.The obtained distinct features are shown in Table 12.
Thirdly, according to the distinct features of solution space, especially the features between  max (and TLH) and subspaces, we propose four algorithms to solve four singleobjective models, respectively.Table 13 summarizes each proposed algorithm, based-on-features, and solved model.
The research on the line-seru conversion is relatively lacking.A thorough research problem list can be found in Yin et al. [20], such as partially cross-trained workers (i.e., a worker cannot perform all assembly tasks), different products
Features Content 1 Minimum  max usually exists in the subspaces with few serus.

2
Except for the subspace with 1 seru, the minimum  max of the subspaces with fewer serus usually is less than that of the subspaces with more serus.
3 Minimum TLH usually exists in the subspaces with more serus.

4
Minimum TLH of the subspaces with more serus usually is less than that of the subspaces with fewer serus.
have different assembly tasks, cost of karakuri (a Japanese word meaning duplication of equipment), and human and psychology factors.In addition, line-hybrid seru system conversion (i.e., the hybrid system with serus and short line is the second type of the line-seru conversion defined by Kaku et al. [10]) is more complex and more real than line-pure seru system conversion.Therefore, the further research should be focused on the line-hybrid seru system conversion.

Figure 2 :Figure 3 :
Figure 2:  max and TLH in subspaces for the line with 6 workers.

Figure 4 :
Figure 4:  max and TLH in subspaces for the line with 8 workers.

Table 1 :
Number of solutions in subspace with J serus.

Table 2 :
Proportion of solutions in subspace with J serus.
detail.Figures2-4show  max and TLH of all solutions in each subspace for the cases with 6-8 workers, respectively.The results of Figures2-4are exactly obtained by enumeration.

Table 3 :
Minimum  max in subspace with J serus.

Table 4 :
Minimum TLH in subspace with J serus.

Table 5 :
The parameters used in experiments.Product types Batch size

Table 6 :
Data distribution of worker's level of skill (  ).

Table 7 :
Data of worker's level of skill (  ).

Table 8 :
Coefficient of influencing level of skill to multiple stations for workers (  ).The detailed data of   are given in Table7.The detailed data of   and batches are given in Tables

Table 9 :
Data of batches.

Table 10 :
Performance comparative results to the enumeration.

Table 13 :
Algorithm features and solved model.