Keywords

1 Introduction

In present time, the manufacturing environment has become highly uncertain, and it is continuously changing. It has shorter life cycles of products and technologies, shorter delivery time, increased level of customization at the price of the standard product, increased product variety, quality as well as demand variability, and intense global competition. Academicians, as well as practitioners, agree that uncertainty will continue to grow in the twenty-first century [1,2,3]. Due to the advancement of technologies, complexity is generated and it generates the uncertainty [4]. To deal with uncertainty, flexibility is required. It has become one of the most necessary and useful tools in unpredictable markets [1].

A lot of flexibility in the system increases the complexity and cost. Therefore, the concept of reconfigurability with customized flexibility has emerged. The systems which have reconfigurability are known as reconfigurable manufacturing systems (RMSs). Reconfigurable manufacturing systems have capacity and functions required for a part family; then, these are reconfigured for another part family when a need arises. Reconfigurability is the ability of a manufacturing system which is used to adjust functions and capacity of the system or machine at low cost and time. A reconfigurable manufacturing system has flexibility, but it is customized [5,6,7,8].

Since these two terms, flexibility and reconfigurability, are important properties of a manufacturing system, therefore there is a need to discuss and differentiate these. The intention of this article is to provide a clear understanding of flexibility and reconfigurability so that it should become clear that what is the need and what is the difference between the both. Further, this paper discusses the concepts of FMS and RMS and compares both.

2 Literature Review

A lot of authors have worked on flexibility. Some of the researchers are Vokurka and O’Leary-Kelly [9], Buzacott and Mandelbaum [10], Bernardes and Hanna [11], Jain et al. [1], Pérez et al. [12], Alexopoulos et al. [13], Chou et al. [14], Chang [15], Fernandes et al. [16], Urtasun et al. [3], etc.

Vokurka and O’Leary-Kelly [9] reviewed the empirical research done for flexibility in manufacturing system. Alexopoulos et al. [13] discussed a method to estimate the flexibility. The concept was based on dynamic behavior analogy between manufacturing system and mechanical system. Buzacott and Mandelbaum [10] defined three aspects of the flexibility: prior flexibility, state flexibility, and action flexibility.

Bernardes and Hanna [11] reviewed flexibility, responsiveness, and agility. Chou et al. [14] discussed the effects of range and response dimension in process flexible structure. Chang [15] discussed the degree of environmental uncertainty and flexibility improvement in uncertainty. Fernandes et al. [16] proposed a model to support firms making important investment decisions associated with the acquisition of new equipment aimed at allowing firms to increase their manufacturing flexibility to produce both standard and customized products.

Jain et al. [1] reviewed manufacturing flexibility and various issues especially need, concept, measurement, dimensions, etc. Barad [17] described two modeling perspectives of flexibility: bottom-up perspective and top-down perspective. Urtasun et al. [3] discussed the relation between human resource management practices and manufacturing flexibility. Pérez et al. [12] reviewed manufacturing flexibility.

Many researchers have worked on reconfigurable manufacturing system. The concept of RMS has been proposed by Koren and Ulsoy [8]. A survey report on FMS has been presented by Hytler and Ulsoy during 1997 in Engineering Research Center (ERC) for RMS. The details of the survey have been given in [18]. The report describes that many industries are not adapting FMS because it is too expensive and complex. Elmaraghy [19] compared FMS and RMS. This paper also described the opinions of the experts on FMS and RMS. Lee [20] has given some relocation rules for machines. Galan et al. [21] presented a methodology for the selection of part family. Prasad and Jayswal [5, 22] considered reconfigurability in the manufacturing industry. Puik et al. [23] proposed a method to compare alternatives to implement reconfigurations considering resources and lead time.

Gu et al. [24] defined throughput settling time, production loss, and total underproduction time. System resilience was measured, and measured values were used for designing of RMS. The designing factors used were system configuration, buffer capacity, and level of redundancy. Effect of the factors on system resilience was investigated. Dahane and Benyoucef [25] proposed a mathematical model for machine selection problem for machine reliability constraints. Goyal et al. [26] proposed methods to measure operational capability and reconfigurability of the reconfigurable machine tool (RMT). The developed performance index along with cost was considered for machine assignment. The problem was solved by using NSGA-II and TOPSIS. Hasan et al. [27] used the concept of bowl phenomenon for RMS planning.

In the literature related to flexibility, all the authors have described the need for flexibility and literature related to reconfiguration, and researchers have described the need for reconfigurability. Therefore, from all the literature, it becomes clear that there is a need of both flexibility and reconfigurability in the manufacturing system. Flexibility is required to deal with uncertainty, and lot of flexibility increases the cost and complexity of the system.

3 Flexibility

Flexibility is a wide concept, and its meaning changes in different contexts. Early definitions of flexibility in manufacturing system were based on the adaptability of the system to uncertainties [28, 29]. Many definitions have been given about flexibility. Mascarenhas [30] defined it as “the ability of a manufacturing system to cope with changing circumstances or instability caused by the environment” [30, 31]. Cox [32] defines it as “the quickness and ease with which plants can respond to changes in market conditions.” Nagarur [33] defines it as “the ability of the system to quickly adjust to any change in relevant factors like product, process, loads and machine failure.” However, a more comprehensive definition might be “the ability to change or react with little penalty in time, effort, cost or performance” [34].

More flexibility in a manufacturing system means that it has more ability to change itself with customer’s need and respond to the competitive pressure. Since flexibility is the ability to change, therefore, thinking what can be changed in the system gives the understanding about flexibility. It should be noted that all the resources contribute to flexibility and it costs money. Various types of flexibility can be measured in the manufacturing system, but all of them cannot have the same priority. In a manufacturing system, flexibility is considered at different levels such as production resource, task of production function, performance of the production function, competitive performance of the company [28].

3.1 Types of Flexibility

On the basis of the literature review, at least ten types of flexibilities can be identified [19, 35, 36]. These are:

  1. 1.

    Machine Flexibility: It is related to ease of making the changes in the machines that are required for the production of given set of products. It is related to number of operations performed without changing the machine setup.

  2. 2.

    Material Handling Flexibility: It is related to the number of paths available for a product due to material handling devices.

  3. 3.

    Operation Flexibility: It is related to the number of various process plans which can be used for the manufacturing of the product.

  4. 4.

    Process Flexibility: It is related to a group of part types that can be manufactured without major setup changes.

  5. 5.

    Product Flexibility: It is related to ease of making a new product into products setup.

  6. 6.

    Routing Flexibility: There are two major definitions of routing flexibility. First, it is ratio of number of all routes of all part types to number of part types [19]. Another definition is that it is the ratio of number of operations assigned to a machine to the operations that can be assigned to alternative machines [37].

  7. 7.

    Volume Flexibility: It is related to changes in the production volume.

  8. 8.

    Expansion Flexibility: It is ease of changing the capacity and capability of the system.

  9. 9.

    Control Program Flexibility: It is related to control software, algorithms, and intelligent machines.

  10. 10.

    Production Flexibility: It is related to the number of all products that can be manufactured without any setup change.

4 Reconfigurability

Various researchers have viewed reconfigurability in various ways. NSF Engineering Research Center for RMS has defined it as “the ability to adjust the production capacity and functionality of a manufacturing system to new circumstances by rearranging or changing the system’s components” [38].

Lee [20] defines it as “the ability of a manufacturing system to be reconfigured at a low cost and in a short period of time.” According to Setchi and Lagos [39], “the essence of reconfigurability is to enable manufacturing responsiveness to a change in market conditions - that is, the ability of the production system to respond to disturbances that may be caused by social or technological changes.” Wiendahl et al. [40] define it as “it is the operative ability of a manufacturing or assembly system to switch with minimal effort and delay to a particular family of workpieces or sub-assemblies through the addition or removal of functional elements.” According to Galan et al. [21] “reconfigurability does not necessarily arise solely from the market or customers but can also emanate from within the company for the sake of relevance.” Basically, it is adjustment of the setup of the system at low cost to adjust fluctuation in demand and variety when required.

Table 1 shows the difference between flexibility and reconfigurability. Both flexibility and reconfigurability cost money. Therefore, it becomes a research area that what should be flexibility and reconfigurability of a manufacturing system.

Table 1 Comparison of flexibility and reconfigurability
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4.1 Measurement of Machine Flexibility and Machine Reconfigurability

Machine flexibility is defined as the ratio of the number of operations that can be performed in the machine without setup change to total operations that can be performed in the machine.

Machine flexibility of pth machine in qth configuration, MFpq can be calculated as

$$ {\text{MF}}_{pq} = \frac{{N_{pq} }}{{N_{p} }} $$
(1)

where Npq is the number of operations that can be processed on pth machine in qth configuration; Np is the total number of operations on pth machine for all the configurations.

Machine reconfigurability is quick adaptability of the reconfigurable manufacturing system in response to the dynamic environment. A reconfigurable machine can be changed in many configurations by adding, removing, or adjusting its auxiliary modules. For ease of reconfiguration, reconfiguration effort (RE) should be minimum. It can be calculated as [7]

$$ \begin{aligned} {\text{RE}} & = \alpha \frac{\text{No. of modules added}}{\text{Total modules}} + \beta \frac{\text{No. of modules removed}}{\text{Total modules}} \\ & \quad + \gamma \frac{\text{No. of modules readjusted}}{\text{Total modules}} \\ \end{aligned} $$
(2)

where α, β, γ are weights assigned for modules addition, removal, and adjustment, respectively. Generally,

$$ \alpha > \beta > \gamma \quad {\text{and}}\quad \alpha + \beta + \gamma = 1. $$

Total reconfiguration effort of machine configuration pth machine in qth configuration, TREp,q,

$$ {\text{TRE}}_{p,q} = \mathop \sum \limits_{j = 1,j \ne q}^{{j_{p} }} {\text{RE}}_{j} $$
(3)

where jp is number of configurations of pth machine.

Machine reconfigurability MRp,q can be calculated as

$$ {\text{MR}}_{p,q} = \frac{{\left[ {j_{p} - 1} \right]^{z} }}{{n_{p}^{q} \times {\text{TRE}}_{p,q} }} $$
(4)

where nq is number of machines needed to satisfy the required demand rate; z is the power index.

For example, consider company ABC has M1, M2, M3, M4, and M5. M2 is modular machine, and machine M2 has three configurations \( M_{2}^{1} \), \( M_{2}^{2} \), and \( M_{2}^{3} \). A number of operations that can be processed on machine configuration \( M_{2}^{3} \) are 6. A total number of operations on machine M2 for all the configurations are 12. If configuration of machine M2 is changed from \( M_{2}^{3} \) to \( M_{2}^{1} \), number of modules added = 4, number of modules removed = 3, and number of modules adjusted = 1. If configuration of machine M2 is changed from \( M_{2}^{3} \) to \( M_{2}^{2} \), number of modules added = 1, number of modules removed = 3, and number of modules adjusted = 1. Required number of machine configuration \( M_{2}^{3} \) for the operation = 3, α = 0.5, β = 0.4, γ = 0.1, z = 2. Then, flexibility and reconfigurability of the machine configuration \( M_{2}^{3} \) can be calculated as follows.

Number of operations that can be processed on machine 2 configuration 3, N2,3 = 6; total number of operations on machine 2 for all the configurations, N2 = 12; machine flexibility of configuration \( M_{2}^{3} \) for operation 2,

$$ {\text{MF}}_{2,3} = 6/12 = 0.5 $$

Machine M2 has three configurations, \( M_{2}^{1} \), \( M_{2}^{2} \), and \( M_{2}^{3} \), i.e., jp = 3.

Reconfiguration effort from changing configuration \( M_{2}^{3} \) to \( M_{2}^{1} \),

$$ {\text{RE}}_{1} = 0.5 \times 4/8 + 0.4 \times 3/8 + 0.1 \times 1/8 = 0.4125. $$

Similarly, reconfiguration effort from changing configuration from \( M_{2}^{3} \) to \( M_{2}^{2} \), RE2 = 0.36.

Machine reconfigurability of MF2,3,

$$ {\text{MR}}_{2,3} = \frac{{\left[ {3 - 1} \right]^{2} }}{{3 \times \left( {0.4125 + 0.36} \right)}} = 1.73. $$

4.2 Measurement of System Flexibility and System Reconfigurability

Measurement of system flexibility and system reconfigurability becomes slightly complicated. For flexibility, there are ten flexibilities in the manufacturing system. Each one is measured separately, and combined effect of this flexibility can be calculated by Multi-Attribute Utility Theory (MAUT). According to it, total evaluation is calculated as [41]

$$ y\left( x \right) = \mathop \sum \limits_{i = 1}^{n} w_{i } u\left( {x_{i} } \right) $$
(5)

where y(x) is the total evaluation, w is the weight assigned to each parameter, and u(x) is the value of each parameter.

If MF = machine flexibility, MHF = material handling flexibility, OF = operation flexibility, PF = process flexibility, PDF = product flexibility, RF = routing flexibility, VF = volume flexibility, EF = expansion flexibility, CPF = control program flexibility, PDTF = production flexibility

then system flexibility (SF) can be measured as

$$ \begin{aligned} {\text{SF}} & = w_{1} {\text{MF}} + w_{2} {\text{MHF}} + w_{3} {\text{OF}} + w_{4} {\text{PF}} + w_{5} {\text{PDF}} + w_{6} {\text{RF}} \\ & \quad + w_{7} {\text{VF}} + w_{8} {\text{EF}} + w_{9} {\text{CPF}} + w_{10} {\text{PDTF}} \\ \end{aligned} $$
(6)

Weights are assigned as per requirement, and all the flexibilities are needed to normalize.

For reconfigurability, key characteristics of RMS are considered. These are modularity, convertibility, scalability, diagnosability, customization, and integrability [42]. In brief, these can be defined as

  1. 1.

    Modularity: It is related to small identity module which can be added/removed in the system.

  2. 2.

    Convertibility: It is ease to convert the system from one setup to other. It includes convertibility of configuration, machine, and material handling system. Convertibility of system (CV) is measured as [43]

    $$ {\text{CV}} = \theta_{1} C_{c} + \theta_{2} C_{m} + \theta_{3} C_{h} $$
    (7)

    where Cc, Cm, Ch are the configuration convertibility, machine convertibility, and material handling convertibility. θ1, θ2, θ3 are the weights assigned.

    $$ C_{c} = \frac{RX}{I} $$
    (8)

    Where R is no of routing connections, X is minimum number of replicated machines at a stage, and I is minimum increment of conversion.

  3. 3.

    Scalability: It is related to minimal capacity increment which is needed to add in the system to adjust its capacity. It is defined as [44];

    $$ {\text{scalability}} = 100 - {\text{smallest incremental capacity in percentage}} $$
    (9)
  4. 4.

    Diagnosability: It is related to error detection ability.

  5. 5.

    Customization: System is designed for a part family. Therefore, it is related to part family formation and customized flexibility.

  6. 6.

    Integrability: It is related to ease to which any module can be added to the system.

If MD = modularity, CV = convertibility, SC = scalability, DT = diagnosability, CS = customization, IN = integrability, then reconfigurability of system (RS) can be calculated by using MAUT as

$$ {\text{RS}} = w_{1}^{\prime} {\text{MD}} + w_{2}^{\prime} {\text{CV}} + w_{3}^{\prime} {\text{SC}} + w_{4}^{\prime} {\text{DT}} + w_{5}^{\prime} {\text{CS}} + w_{6}^{\prime} {\text{IN}} $$
(10)

Weights are assigned as per requirement, and parameters are needed to normalize.

The author did research work for consideration of reconfigurability in an industry. Reconfiguration effort of the system was considered by removing or adding the modules of the machine shown in Table 2. Details of the measurement are given in [5].

Table 2 Machines with machine configurations
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5 Flexible Manufacturing System

A flexible manufacturing system, as its name means, has a very high flexibility. As defined by Groover [45] is “A flexible manufacturing system (FMS) is a highly automated GT machine cell, consisting of a group of processing stations (usually computer numerical control [CNC] machine tools), interconnected by an automated material handling and storage system, and controlled by an integrated computer system.” According to Brown et al. [35], “A flexible Manufacturing System is an integrated, computer controlled complex of automated material handling devices and numerically controlled (NC) machine tools that can simultaneously process medium-sized volumes of a variety of part types.” According to Tetzlaff [46], “A flexible manufacturing system can be defined as a computer controlled production system capable of processing a variety of part types.” According to Mehrabi et al. [18] “Flexible manufacturing system is a programmable machining center configuration which incorporates software to handle changes in work order, production schedules, part programs, and tooling for several part families.”

From above definitions, some points are clear; FMS has a high level of automation. There is computerized control of machines, loading, unloading, transfer, etc. It can produce a variety of parts. Generally, it has CNC machines. Some systems which use flexible transfer line have been said as flexible manufacturing system [35]. But nowadays FMS cannot be imagined without CNC machines. Brown et al. [35] have classified types of flexible manufacturing system as: flexible machining cell, flexible machining system, flexible transfer lines, flexible transfer multi-line.

When the concept of FMS was introduced, it attracted the attention of many researchers. Many industries have started to use FMS. But a survey on FMS was conducted, and its conclusion was [18]; two-third of the responded said that FMS is not living up to its full potential, over half reported that they purchased FMS of excess capacity and features; the problems identified with FMS were training, reliability, maintenance, software, cost, and reconfigurability.

Generalization of the feature of FMS increased its cost and very high-level automation stared problem in maintenance. Because of these limitations, a new type of manufacturing system has been introduced named as reconfigurable manufacturing system. However, a flexible manufacturing system is used in many industries in India and worldwide. Even many academic institutions have a flexible manufacturing system for research purpose.

6 Reconfigurable Manufacturing System

Reconfigurable manufacturing system is a new type of manufacturing system which can change its capacity and functionality very easily and quickly whenever required. Reconfigurable manufacturing system (RMS) has capacity and functionality exactly what is required. RMS is adjustable to the fluctuating demands, and it can be easily upgraded with new process technology [5,6,7,8]. RMS has six key characteristics which are modularity, integrability, scalability, convertibility, customization, and diagnosability. These have been described in the previous section. The key characteristics, customization, scalability, and convertibility, are essential RMS characteristics, while the other three (modularity, integrability, and diagnosability) reduce the system configuration time and its ramp-up time [5, 47, 48]. RMS combines features of dedicated and flexible systems.

Reconfigurable manufacturing system has been evolved from dedicated manufacturing system. With the concept of using the modular machine, the concept of reconfiguration arises. But it is not limited to modular machines. Some researchers have given the concept of reconfiguration by material handling systems [49], reconfiguration by relocation [20], reconfiguration process plan [50], etc.

Koren and Shpitalni [51] have given the concept of practical reconfigurable manufacturing system using cell gantry and spine gantry. It is like a special type of layout of flexible manufacturing system. Later, reconfigurable machines were added [42]. Reconfigurability has been reviewed in mining industry [52], mold and die making industry [53], Arvin Meritor industry [54], powertrain industry [42], Continental Automotive [5, 22], etc.

Table 3 compares the features of DML, FMS, and RMS. Figure 1a, b shows the difference between dedicated system, FMS, and RMS. The functions of FMS are very high, but it also increases the cost. The capacity of FMS is lowest, and the reason for it is that in FMS there are CNC machines which use the single-point cutting tool.

Table 3 Comparisons of system features of dedicated system, RMS, and FMS
Full size table
Fig. 1
figure 1

Comparisons of DMS, FMS, and RMS [8]

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Some points can be given based on the comparison: FMS has generalized flexibility, while RMS has limited flexibility. FMS has evolved by combining CNC machines with transfer lines, while RMS evolved by introducing modular machine in dedicated transfer lines. CNC machines in FMS are single-point cutting tool which reduces the production capacity. In RMS, multi-point cutting tool machines can be used. In FMS, very high level of automation is required, while in RMS, it is required as per need. Mostly FMS has been used in machining, while the concept of RMS has been used in machining, mining, mold and die making, etc.

7 Conclusions

Manufacturing flexibility and reconfigurability are widely recognized as the critical components to achieving a competitive advantage in the marketplace. These are the most sought-after properties for manufacturing enterprises. This paper synthesizes the vast literature review on manufacturing flexibility and reconfigurability. This paper discusses the measurement of flexibility and reconfigurability, Sects. 4.1, 4.2, and compares the both, Table 1. This paper also discusses the concept of flexible manufacturing system and reconfigurable manufacturing system and differentiates between them, Sects. 5 and 6. It has been found that both flexibility and reconfigurability have importance, Sects. 3 and 4. But rather than having a lot of flexibility it is better to have some flexibility and some reconfigurability. Both flexibility and reconfigurability cost money, Fig. 1. Therefore, it becomes a research area that what should be flexibility and reconfigurability in a manufacturing system.