Yicha Zhang
email: yicha.zhang@utbm.fr.

Chen Zheng

Xiansheng Qin

Benoît Eynard

Jing Bai

Jing Li

SME-oriented flexible design approach for robotic manufacturing systems

Keywords: Small and medium-sized enterprises, manufacturing system flexibility, robotic manufacturing system, configuration design

Introduction

Over the past few years, manufacturing SMEs have played a significant role in the global economy, constituting 99% of all enterprises and offering more than 60% of all employment [START_REF]Accelerating energy efficiency in small and mediumsized enterprises[END_REF][START_REF] Mittal | A critical review of smart manufacturing & Industry 4.0 maturity models: Implications for small and medium-sized enterprises (SMEs)[END_REF]. In the US, manufacturing SMEs are considered as the backbone of the U.S. economy. According to the survey carried out by the U.S. Department of Commerce in 2017, 96.4% of manufacturing exporters are SMEs and they contribute 20.3% of the sector's $798 billion in exports, while 93.5% of manufacturing importers are small-and medium-sized; they account for 14.5% of the sector's $826 billion in imports [START_REF]A profile of U.S. Importing and Exporting Companies[END_REF]. In the EU, SMEs make up 99.8% of all enterprises in the non-financial business sector, in which manufacturing sector is one of the most important among the five key sectors [START_REF] Skolud | Integration of manufacturing functions for SME. Holonic-based approach[END_REF][START_REF] Muller | Annual report on European SMEs[END_REF]. In China, over 90% of Chinese enterprises can be categorised as SMEs, which contribute to more than half of China's tax income and more than 80% of employment [START_REF] Hu | The change of production systems through consultancy involved projects: a multiple case study in Chinese SMEs[END_REF]. In recent years, more than 29 million manufacturing SMEs have emerged in China, which contributed to 60% of China's industrial output, and the number of Chinese industrial SMEs will steadily with an average annual growth rate of 8% [START_REF] Luo | Ranking Chinese SMEs green manufacturing drivers using a novel hybrid multi-criterion decision-making model[END_REF].

For the manufacturing SMEs, various products of very similar type are always required, but sometimes their quantity to be produced is quite small [START_REF] Tseng | A collaborative control system for mass customization manufacturing[END_REF]. As a result, most of the manufacturing SMEs still adopt handwork manufacturing process due to the diversity of products and their small batch sizes. Moreover, continuously growing pressures from competitive market require manufacturing enterprises to be more productive but with less risk and cost. Robotic flexible manufacturing systems are considered as an effective solution in solving the aforementioned problems. There are several similar definitions of flexible manufacturing systems proposed by different researchers from various perspectives. The term of the flexible manufacturing system is proposed by Stecke for the first time as "an integrated, computer controlled complex of automated material handling devices and numerically controlled machine tools that can simultaneously process medium-sized volumes of a variety of part types" [START_REF] Stecke | Formulation and solution of nonlinear integer production planning problems for flexible manufacturing systems[END_REF]. Chang et al. propose the definition of the flexible manufacturing system as "an integrated system of equipment and processes via computer or network of computers that is capable of producing a variety of products economically" [START_REF] Chang | Computer-aided manufacturing[END_REF]. Borenstein et al. define flexible manufacturing systems as "automated, integrated systems of equipment and information flow, arranged for the economic production of small batches of complex components" [START_REF] Borenstein | A systemic and integrated approach to flexible manufacturing systems design[END_REF]. According to the above definitions, robotic manufacturing systems are flexible because they produce a variety of products very quickly and economically. However, Koren and Shpitalni state that flexible manufacturing systems should not only facilitate the changes in products manufactured, but should mean more than that [START_REF] Koren | Design of reconfigurable manufacturing systems[END_REF]. For example, Browne et al. believe that a flexible manufacturing system should have the capability of building a system, and expanding it as needed, easily and modularly [START_REF] Browne | Classification of flexible manufacturing systems[END_REF]. Holzner et al. mention that attention should be also paid to the personal deployment flexibility, which means that the robotic manufacturing system should be easily operated by a variable number of employees or workers with different skills [START_REF] Holzner | Systematic Design of SME Manufacturing and Assembly Systems Based on Axiomatic Design[END_REF]. Wadhwa define the flexibility in the context of manufacturing SMEs as the ability to response efficiently to the changing demands of customers [START_REF] Wadhwa | Flexibility in manufacturing automation: A living lab case study of Norwegian metalcasting SMEs[END_REF].

As previously mentioned, one of the hallmarks of robotic flexible manufacturing systems is to improve the reliability, productivity, versatility and availability of manufacturing enterprises. After the implementation in large enterprises to achieve better product quality, lower production cost and higher customer satisfaction rates, robotic flexible manufacturing systems have attracted more attention from manufacturing SMEs. Not only does the robotic flexible manufacturing system permit significant manufacturing time and cost saving for SMEs, it also frees up time for increasing the product diversity and the ability to adapt to small production batches. However, it is difficult for the manufacturing SMEs to adopt robotic flexible manufacturing systems due to the lack of concentrated market power and powerful original equipment manufacturers (OEMs) and their limited technical and financial resources [START_REF] Mittal | A critical review of smart manufacturing & Industry 4.0 maturity models: Implications for small and medium-sized enterprises (SMEs)[END_REF]. To further understand their difficulties, the authors research team not only conducted a number of semi-structured interviews within the CEOs, managers and manufacturing consultants of five Chinese manufacturing SMEs, whose scopes of manufacturing activities include composite manufacturing, shipbuilding, shoemaking, mechanical component manufacturing and lens manufacturing, respectively, but also organised in-depth discussions with the engineers from three industrial automation companies, which offer industrial robotic solutions to manufacturing companies. The duration of each interview was 0.5-1h. During this period, 25 participants from SMEs and 15 participants from industrial automation companies were asked a series of questions according to their own understandings of the challenges to find out the factors which hinder the implementation of robotic flexible manufacturing systems in SMEs, these being: Fig. 1 presents the results of the interviews with the participants from SMEs and industrial automation companies. From the perspective of customers from SMEs, two of the top three factors which hinder the implementation of robotic flexible manufacturing systems are related to the system costs. Moreover, many interviewees are afraid that their required functionalies may not be realised completely by designers. From the point of view of participants from industrial automation companies, they believe that the high development cost and long development lead-time make it difficult to implement robotic flexible manufacturing systems in SMEs. Additionaly, they also insist that the difficulty in capturing customer requirements severely affect the successful implementation of robotic flexible manufacturing systems in SMEs.

Fig. 1 Results of interviews with participants from SMEs and industrial automation companies

Analysing the results of the interviews, the authors propose the challenges regarding the implementation of robotic flexible manufacturing systems in SMEs as follows:

• Conflict between professional designers and non-professional manufacturing SMEs:

designers define the architecture of flexible manufacturing systems according to the requirements of SMEs because there is always a lack of ability for SMEs to develop their own manufacturing systems. However, different perspectives from designers and SMEs may lead to totally different comprehension of manufacturing systems. On the one hand, designers may miss some industrial problems which SMEs need to solve by the manufacturing system; on the other hand, SMEs may propose some special requirements which can hardly be realised by designers due to the limited basic engineering knowledge in SMEs. As a result, sometimes the manufacturing system cannot fully meet requirements of SMEs [START_REF] Li | A design pattern for industrial robot:user-customized configuration engineering[END_REF]. The conflict of professional designers and non-professional SMEs indicates that it is necessary to shift from the "professional designers-leading" design approach to the design process in which manufacturing SMEs can participate.

• Conflict between development cost of flexible manufacturing systems and financial burdens of SMEs: although flexible manufacturing systems provide designers with a lowcost manufacturing process; however, the high cost of development of such flexible manufacturing systems becomes one of the most significant barriers toward the implementation of flexible manufacturing systems in SMEs due to their scarce financial resources [START_REF] Spena | Requirements for the design of flexible and changeable manufacturing and assembly systems: a SME-survey[END_REF]. Most of current SMEs still adopt traditional manufacturing mode, e.g., handwork manufacturing process due to their heavy financial burdens [START_REF] Lin | Design of a global decision support system for a manufacturing SME: towards participating in collaborative manufacturing[END_REF]. However, traditional manufacturing mode not only leads to long manufacturing lead-time, but also results in environmental pollution and resource abuse problems [START_REF] Luo | Ranking Chinese SMEs green manufacturing drivers using a novel hybrid multi-criterion decision-making model[END_REF]. Therefore, a low-cost development approach for flexible manufacturing systems are necessary for designers. On the one hand, it can encourage SMEs to choose flexible manufacturing systems rather than traditional manufacturing process, so that the environmental pollution can be greatly reduced. On the other hand, both sales and profits of flexible manufacturing systems can be increased for the side of designers.

Considering the aforementioned two challenges, the authors propose two types of flexibilities which should be attained for the implementation of robotic flexible manufacturing systems in SMEs.

• Flexibility in customisation: in the design process of manufacturing systems, the iterations between designers and manufacturing SMEs often lead to increasing development cost and lead-time because the needs of manufacturing SMEs are not satisfied completely. Flexibility in customisation allows SMEs to customise the required manufacturing system by themselves. In other words, the final manufacturing system is not only completed by the designers, but by the SMEs according to their own needs as well [START_REF] Li | A design pattern for industrial robot:user-customized configuration engineering[END_REF].

• Flexibility in design: flexibility in design is the capability of the design for new robotic manufacturing systems based on existing systems as needed, easily and economically. Similar terms, such as changeable manufacturing systems, design for variety, variety management, and product family architecture have been proposed by researchers in engineering and business. Thanks to flexibility in design, designers can develop new manufacturing systems easily by making appropriate changes from the existing ones, and the development cost can be therefore greatly reduced [START_REF] Browne | Classification of flexible manufacturing systems[END_REF].

The paper intends to propose a SME-oriented design approach for robotic flexible manufacturing systems by focusing on the aforementioned two types of flexibilities. Section 2 reviews the current design methods for flexible manufacturing systems and evaluates each method by the challenges stated previously. Section 3 introduces the proposed design approach which can be considered as an effective solution to the challenges relating to the implementation of flexible manufacturing systems in SMEs. The application of the proposed design approach is demonstrated by industrial case studies in Section 4. Section 5 provides a detailed discussion of the proposed design approach. Finally, the authors draw the conclusion in Section 6.

Related works

According to the principle of systems engineering, solving a complex engineering problem often requires a technique to decompose the problem into smaller, more manageable subproblems that are easier to solve, and then to join them together [START_REF] Sage | Handbook of systems engineering and management[END_REF]. The design of robotic flexible manufacturing systems is an example of such complex problems; decomposing the robotic flexible manufacturing systems into smaller and manageable elements is thus a common technique to help designers to obtain the design solutions, which results in the agent-based design paradigm and configuration design paradigm. The former demands designers to break a flexible manufacturing system down into manageable agents, while the latter decomposes the manufacturing system into reconfigurable modules. Considering different dimensions of manufacturing flexibilities, current design approaches for robotic flexible manufacturing systems based on the two design paradigms will be reviewed hereafter.

Agent-based design paradigm

Agent is composed of states; social, environmental and personal knowledge; a perception function; and behavioural rules [START_REF] Adam | Role-based manufacturing control in a holonic multi-agent system[END_REF]. The agent-based design paradigm is therefore a distributed problem-solving paradigm that decomposes a complex manufacturing problem into a large number of small manageable agents which are autonomous having intelligence to take their own decision [START_REF] Jana | Dynamic schedule execution in an agent based holonic manufacturing system[END_REF]. Similar to the concept of "agent", the word "holon" is proposed to describe the hybrid nature of sub-wholes/parts in real-life systems by Koestler at the end of 1980s [START_REF] Koestler | The ghost in the machine[END_REF].

Holon is defined as an autonomous and co-operative building block of a manufacturing system for transforming, transporting, storing and/or validating information and physical objects. In fact, a holon can be regarded as a special kind of large-grain reactive agent [START_REF] Wang | Combined strength of holons, agents and function blocks in cyberphysical systems[END_REF].

Based on the agent-based design paradigm, different agents (or holons) are able to exchange information efficiently, and communicate and cooperate effectively [START_REF] Brussel | A conceptual framework for holonic manufacturing: Identification of manufacturing holons[END_REF]. Therefore, it is widely used in solving scheduling and planning problems to achieve the flexibility in automated line during manufacturing process [START_REF] Chou | A bio-inspired mobile agent-based integrated system for flexible autonomic job shop scheduling[END_REF]. Currently, different agent-based design approaches are proposed to aid manufacturing systems in achieving the flexibility in automated line. The agentbased design approaches can be generally classified into the following three types: (1) Modelling manufacturing stages by agents: in this type of design method, each manufacturing stage is considered as an autonomous agent. The action that each manufacturing stage or task takes is based on the local state and the messages received from other agents [START_REF] Sikora | Coordination mechanisms for multi-agent manufacturing systems: applications to integrated manufacturing scheduling[END_REF][START_REF] Shih | Distributed Artificial Intelligence in manufacturing systems control[END_REF][START_REF] Sahin | A multi-agent based approach to dynamic scheduling with flexible processing capabilities[END_REF]. [START_REF] Mittal | A critical review of smart manufacturing & Industry 4.0 maturity models: Implications for small and medium-sized enterprises (SMEs)[END_REF] Modelling stakeholders by agents: based on this proposition, stakeholders in charge of cost estimation, assimilability evaluation, process planning, manufacturability evaluation, etc. are modelled as different agents. The coordination and conflict detection among stakeholders during the design process can be realised by the communication among different agents [START_REF] Wernz | Decision strategies and design of agent interactions in hierarchical manufacturing systems[END_REF][START_REF] Mishra | Cloud-based multi-agent architecture for effective planning and scheduling of distributed manufacturing[END_REF][START_REF] Sun | A distributed multi-agent environment for product design and manufacturing planning[END_REF]. (3) Modelling control parts by agents: the control part is regarded as an agent, which manages information and data flows in flexible manufacturing systems so that the adaptability to change the manufacturing environment can be achieved [START_REF] Chirn | A holonic component-based approach to reconfigurable manufacturing control architecture[END_REF][START_REF] Park | An autonomous manufacturing system based on swarm of cognitive agents[END_REF][START_REF] Choi | Multi-agent hybrid shop floor control system[END_REF].

Previous review indicates that the problems related to flexibility in automated line (e.g., manufacturing scheduling, production planning, etc.) during manufacturing process can be well solved by the design approaches based on the agent-based paradigm; however, none of them can fully support the aforementioned two types of flexibility (i.e., flexibilities in customisation and design) which play a significant role in the implementation of flexible manufacturing systems in SMEs. A non-exhaustive list of design approaches for flexible manufacturing systems based on the configuration design paradigm are presented hereafter.

Configuration design paradigm

Configuration design was firstly proposed by Mittal and Frayman in 1989, which is defined as "taking a set of predefined components, add the search for an assembly of components that satisfies a set of requirements and obeys a set of constraint" [START_REF] Mittal | Towards a generic model of configuration tasks[END_REF]. When applying the configuration design paradigm on the design of manufacturing systems, the term of component always represents the physical artefacts, i.e., reconfigurable modules. Based on the configuration design paradigm, designers should select and integrate reconfigurable modules to satisfy customer requirements and accomplish certain manufacturing tasks. National Research Council insists that the configuration design paradigm is a priority technology to address the issue of flexibilities related to manufacturing systems [START_REF]Visionary manufacturing challenges for[END_REF].

The idea of decomposing a complex manufacturing system into reconfigurable modules to achieve rapid change in architecture has received attention from both academia and industry for long time. In 1995, the concept of Reconfigurable Manufacturing Systems (RMS) is proposed in a proposal from University of Michigan [START_REF] Koren | Design of reconfigurable manufacturing systems[END_REF]. According to the definition proposed by Koren et al., RMS can be designed at the outset for rapid change in structure, hardware and software components, in order to quickly adjust production capacity and functionality within a part family in response to sudden changes in market or regulatory requirements [START_REF] Koren | Reconfigurable manufacturing systems[END_REF][START_REF] Bortolini | Reconfigurable manufacturing systems: Literature review and research trend[END_REF]. However, compared to the design of traditional manufacturing systems, the design of RMS represents a significant challenges, such as how much variety to cover in the RMS, how to determine the level of reconfigurability [START_REF] Andersen | Towards a generic design method for reconfigurable manufacturing systems: Analysis and synthesis of current design methods and evaluation of supportive tools[END_REF]. Considering the challenges relating to the design of RMS, different approaches for the design of RMS have been proposed. After pointing out the core characteristics and design principles of RMS, Koren and Shpitalni discuss how RMS are configured and propose a means for calculating the number of possible RMS configurations based on the number of machines in the system [START_REF] Koren | Design of reconfigurable manufacturing systems[END_REF]. Galan et al. believe that RMS allow the production of different products together to help customers to achieve product flexibility. They propose a methodology for calculating the similarity values between products in order to form the best set of product families [START_REF] Galan | A systematic approach for product families formation in Reconfigurable Manufacturing Systems[END_REF]. Lameche et al. discuss the possibility to adopt modular design principles for the design of RMS. In their proposition, Design Structure Matrix (DSM) is adopted to help designers to organise subsystems and components in RMS [START_REF] Lameche | Modularity in the design of reconfigurable manufacturing systems[END_REF]. Al-Zaher et al. proposes a design methodology by using the design principles of reconfigurable manufacturing systems for automotive framing systems. This modular design methodology can both eliminate the coupling design and enable increases of variants within a product family during production [START_REF] Al-Zaher | RMS design methodology for automotive framing systems BIW[END_REF]. Compared to the multi-agent manufacturing systems focusing on the flexibility in automated line, RMS are considered as an effective support to aid customers in achieving product flexibility [START_REF] Maganha | Understanding reconfigurability of manufacturing systems: an empirical analysis[END_REF]; however, current design approaches for RMS seldom take the context of manufacturing SMEs into consideration. Therefore, the flexibilities in design and customisation cannot be fully supported by current design approaches for RMS.

Besides RMS, the configuration design paradigm has been widely adopted by current design approaches for manufacturing systems. The configuration design for manufacturing systems requires designers to assemble a set of predefined modules to satisfy customer requirements and obey a set of constraints [START_REF] Wielinga | Configuration-design problem solving[END_REF]. Based on this principle, various configuration methods are proposed to aid designers in selecting the "best" option from different alternative assembly results. Current configuration methods can be generally divided into the following three classes: KBE (Knowledge Based Engineering) -based approaches, ranking methods, and mathematical optimisation methods.

Knowledge based engineering is initially proposed to capture the knowledge and the historical trace of experiences in a particular discipline of interest, and it can help designers to make better-informed decisions during the design process. Wielinga and Schreiber confirm the possibility and necessity of knowledge in configuration-design problem solving [START_REF] Wielinga | Configuration-design problem solving[END_REF]. Mellichamp et al. propose a knowledge base of the expert system for manufacturing system design. Knowledge and experiences of previous genuine designers related to production output (i.e., production output of manufactured parts per unit of time by the manufacturing system) and capital investment (i.e., the cost per part manufactured by the manufacturing system) are stored in the expert system to help designers to choose the best components of manufacturing systems among potential alternatives [START_REF] Mellichamp | FMS Designer: an expert system for flexible manufacturing system design[END_REF]. Alsafi and Vyatkin use ontological knowledge to help designers to produce the new configurations of manufacturing systems reacting on changes in the requirements or the manufacturing environment [START_REF] Alsafi | Ontology-based reconfiguration agent for intelligent mechatronic systems in flexible manufacturing[END_REF]. For the KBE-based approaches, it is the designers who are in charge of capturing of the design knowledge from previous design cases or experiences and defining the architecture of the required manufacturing system, and SMEs are not involved in such design process. Therefore, current KBE-based approaches do not allows SMEs to customise the required manufacturing system by themselves.

Ranking methods need designers to score each of the attributes in a quantitative or qualitative way and provide related weights for them according to customer requirements. Different models or methods, such as the multi-attribute utility analysis (MAMU) [START_REF] Butler | A multiple attribute utility theory approach to ranking and selection[END_REF][START_REF] Malak | Multi-attribute utility analysis in set-based conceptual design[END_REF], the fuzzy methods [START_REF] Zhu | Applying fuzzy multiple attributes decision making for product configuration[END_REF][START_REF] Deciu | Configurable product design using multiple fuzzy models[END_REF], the analytic hierarchy process (AHP) [START_REF] Galan | A systematic approach for product families formation in Reconfigurable Manufacturing Systems[END_REF][START_REF] Gupta | Configuration selection of reconfigurable manufacturing system based on performance[END_REF], are applied to evaluate all of the scored attributes, giving a comprehensive index value for each alternative as for ranking. This type of methods can order alternatives from the best to the worst with the analysis result of customer requirements [START_REF] Zheng | Knowledge-based engineering for multidisciplinary systems: Integrated design based on interface model[END_REF]. Designers complete the main processes of ranking methods, such as selecting the evaluation attributes, assigning the attributes' weights and scoring the alternatives after carefully considering customer requirements. However, how to capture design requirements, especially the informal requirements from customers and usage scenarios related to SMEs is still a critical issues for designers, because the capture of informal design requirements is not merely a technological challenge, but a complex social process [START_REF] Macaulay | Requirements capture as a cooperative activity[END_REF]. The raking methods require designers to capture and specify customer requirements, and customers are not really involved during the ranking methods; therefore, the different perspectives from customers may lead to totally different comprehension of customers' real requirements.

As for mathematical optimisation methods, objective functions are constructed to represent different factors or attributes along with their interrelationships. Designers aimed to find a set of ideal optimal solutions among the infinite solution space in theory when selecting the best configuration. The objective function of optimisation always considers the profit maximisation, customer satisfaction, development lead-time, etc. [START_REF] Afrouzy | A fuzzy stochastic multi-objective optimization model to configure a supply chain considering new product development[END_REF]. Similarly to the ranking methods, the main difficulty within the mathematical optimisation methods is that the suitable objective functions are selected by designers according to their understanding of customer requirements, which may lead to the misunderstanding of customers' real intention due to different perspectives between designers and SMEs [START_REF] Zhang | An integrated decision-making model for multi-attributes decision-making (MADM) problems in additive manufacturing process planning[END_REF].

In order to implement the configuration design paradigm for the design of robotic manufacturing systems, a common representation for reconfigurable modules is needed, because one manufacturing system should be configured from modules by designers of different disciplines. In order to provide designers with a common representation, the first task is to clarify which kind of information should be represented for the modelling of modules and interfaces. Du et al. [START_REF] Du | Joint optimization of product family configuration and scaling design by Stackelberg game[END_REF] propose that each module can be described by some attributes; and each attribute assumes certain values. Jiao and Tseng [START_REF] Jiao | A methodology of developing product family architecture for mass customization[END_REF] use design parameters to represent module functional and structural aspects. Tsai and Wang [START_REF] Tsai | The development of modular-based design in considering technology complexity[END_REF] do not propose the internal structure of module; on the contrary, they believe that exploring the parameter flow between modules is a feasible approach to represent the function of modules. Four types of flows, i.e., geometry constraints, mechanical strength, energy flow and signal flows, are proposed to represent the relationship between different modules. Pahng et al. [START_REF] Pahng | Distribution modeling and evaluation of product design problems[END_REF] integrate data and mathematical models to represent modules. They point out that modules interact with each other if the interfaces are compatible, but details of interface model and interface compatibility have not been presented. Li et al. [START_REF] Li | A design pattern for industrial robot: User-customized configuration engineering[END_REF] adopt function realisation and internal structure to describe the module, while the interface which connects different modules is represented by interface size, interface location and communication standard. Above studies focus on the information which should be expressed by modules and interfaces; however, a unified form in which the information is structured should be also proposed to facilitate the understanding of designers from different disciplines.

Object-oriented technology was initially proposed to offer a revolution in software engineering discipline that is resolving the problems inherent in developing and managing organisational information processing capabilities [START_REF] Gamma | Design patterns: elements of reusable object-oriented software[END_REF]. The concept of modularity is the basis of the objectoriented modelling approach in software engineering discipline [START_REF] Bonfe | Design and verification of mechatronic object-oriented models for industrial control systems[END_REF]; therefore, by integrating with the configuration design paradigm, several researchers intend to use the object-oriented modelling approach to model the modules. The module models based on the object-oriented technology provide a unified form in which the information is structured to facilitate the understanding of designers from different disciplines. Various object-oriented models, such as STandard for the Exchange of Product model (STEP) [63], Core Product Model [START_REF] Fenves | CPM : a core model for product data[END_REF], Product-Process-Organisation (PPO) model [START_REF] Noël | The PPO design model with respect to digital enterprise technologies among product life cycle[END_REF], have been developed by experts worldwide. However, to achieve the flexibility in customisation, customers should be also involved in the design process, and the customers' view about the information representation of reconfigurable modules is not considered by current studies on module model.

Table 1 shows the assessment result of current design approaches regarding the flexibilities in design and customisation. The design approaches based on the agent-based design manufacturing paradigm [START_REF] Sikora | Coordination mechanisms for multi-agent manufacturing systems: applications to integrated manufacturing scheduling[END_REF][START_REF] Shih | Distributed Artificial Intelligence in manufacturing systems control[END_REF][START_REF] Wernz | Decision strategies and design of agent interactions in hierarchical manufacturing systems[END_REF][START_REF] Mishra | Cloud-based multi-agent architecture for effective planning and scheduling of distributed manufacturing[END_REF][START_REF] Chirn | A holonic component-based approach to reconfigurable manufacturing control architecture[END_REF][START_REF] Park | An autonomous manufacturing system based on swarm of cognitive agents[END_REF] mainly focus on the flexibility in automated line, while the design approaches for RMS focus on the flexibility in product. In other words, none of them can fully support the flexibilities in customisation and design simultaneously, which play a significant role in the implementation of flexible manufacturing systems in SMEs. Some design approaches based on the configuration design paradigm can partially support the flexibilities in design and customisation for the design of manufacturing systems, but the context of manufacturing SMEs has not been taken into consideration. Therefore, the authors propose a SME-oriented design approach based on the configuration design paradigm for the robotic flexible manufacturing systems. Modelling manufacturing stages by multiagent [START_REF] Sikora | Coordination mechanisms for multi-agent manufacturing systems: applications to integrated manufacturing scheduling[END_REF][START_REF] Shih | Distributed Artificial Intelligence in manufacturing systems control[END_REF][START_REF] Sahin | A multi-agent based approach to dynamic scheduling with flexible processing capabilities[END_REF] No (only focus on the flexibility in automated line)

No (only focus on the flexibility in automated line)

Modelling stakeholders by multi-agent [START_REF] Wernz | Decision strategies and design of agent interactions in hierarchical manufacturing systems[END_REF][START_REF] Mishra | Cloud-based multi-agent architecture for effective planning and scheduling of distributed manufacturing[END_REF][START_REF] Sun | A distributed multi-agent environment for product design and manufacturing planning[END_REF] Modelling control parts by multi-agents [START_REF] Chirn | A holonic component-based approach to reconfigurable manufacturing control architecture[END_REF][START_REF] Park | An autonomous manufacturing system based on swarm of cognitive agents[END_REF][START_REF] Choi | Multi-agent hybrid shop floor control system[END_REF]

Configuration design paradigm

Design approaches for RMS [START_REF] Koren | Design of reconfigurable manufacturing systems[END_REF][START_REF] Koren | Reconfigurable manufacturing systems[END_REF][START_REF] Bortolini | Reconfigurable manufacturing systems: Literature review and research trend[END_REF][START_REF] Galan | A systematic approach for product families formation in Reconfigurable Manufacturing Systems[END_REF][START_REF] Lameche | Modularity in the design of reconfigurable manufacturing systems[END_REF][START_REF] Al-Zaher | RMS design methodology for automotive framing systems BIW[END_REF][START_REF] Maganha | Understanding reconfigurability of manufacturing systems: an empirical analysis[END_REF] No (only focus on the flexibility in product)

No (only focus on the flexibility in product) Design approaches based on KBE [START_REF] Wielinga | Configuration-design problem solving[END_REF][START_REF] Mellichamp | FMS Designer: an expert system for flexible manufacturing system design[END_REF][START_REF] Alsafi | Ontology-based reconfiguration agent for intelligent mechatronic systems in flexible manufacturing[END_REF] Partial (do not consider the context of manufacturing SMEs) Partial (do not consider the context of manufacturing SMEs)

Ranking methods [START_REF] Galan | A systematic approach for product families formation in Reconfigurable Manufacturing Systems[END_REF][START_REF] Butler | A multiple attribute utility theory approach to ranking and selection[END_REF][START_REF] Malak | Multi-attribute utility analysis in set-based conceptual design[END_REF][START_REF] Zhu | Applying fuzzy multiple attributes decision making for product configuration[END_REF][START_REF] Deciu | Configurable product design using multiple fuzzy models[END_REF][START_REF] Gupta | Configuration selection of reconfigurable manufacturing system based on performance[END_REF] Mathematical optimisation methods [START_REF] Afrouzy | A fuzzy stochastic multi-objective optimization model to configure a supply chain considering new product development[END_REF] Module representation approaches [56-60,63-65]

SME-oriented design approach for robotic flexible manufacturing systems

Normally, designers should capture customer requirements to define the architecture of complex systems. However, it is a challenging task to capture the customers' informal design requirements. If customer requirements are not fully understood by designers, the performance of the product will not entirely meet customers' expectations. In this case, designers should modify their design, and sometimes this process will be repeated a few times. On the other hand, due to the limitation of basic engineering knowledge, SMEs may propose some requirements, the development cost of which may not be afforded by SMEs, or even sometimes their requirements may hardly be realised by existing technologies. In order to achieve such flexibilities in design and customisation, the authors propose a design approach which allows the SMEs to choose their own architecture from different choices provided by designers after the designers carefully consider customer requirements and financial resources. In this section, the template model of robotic flexible manufacturing systems will be firstly introduced, which is considered as the key part of the design approach. Then, based on the template model, the SME-oriented design approach for robotic flexible manufacturing systems will be presented.

Template model of robotic flexible manufacturing systems

Based on the template model, designers can define the architectures of different robotic flexible manufacturing systems by carefully considering their design experiences, existing technology accumulation and customer requirements, which provides the SMEs with various design solutions. The proposed template model of robotic flexible manufacturing systems is illustrated in Fig. 2.

Representation of configurable modules in template model

The configurable module is considered as the encapsulation of functional realisation, and can be integrated with other modules through the interfaces. The modelling principles of the configurable module will be illustrated by using Fig. 2 as follows:

• Class Module: four attributes, i.e., name, workingPrinciple, cost and necessity, are proposed in the class Module. The attribute name is used to store the name of the module and to distinguish it from the others. The attribute workingPrinciple briefly describes the working principle of the module. The attribute cost stores the development cost estimated by designers. The attribute necessity adopts a Boolean data type to express whether the module is necessary to fulfil the basic function or to provide an additional function for the robotic manufacturing system. For example, for a robotic welding system, the welding module is considered as a necessary module because the robotic welding system cannot achieve its basic function without the welding module; however, the vision module can provide an additional function to the robotic welding system, which generates the collision-free trajectory by capturing the workpiece's welding seams with cameras instead of analysing workpiece's CAD model. Therefore, the vision module is unnecessary for the realisation of the basic welding functionality for the robotic welding system. SMEs can choose such unnecessary module by considering its development cost and their financial resources.

• Class Common module and class Specific module: the modules of robotic manufacturing systems can be generally divided into two types, i.e., common modules and specific modules. Common modules are the modules required in different robotic manufacturing systems, such as the control module, power supply module, movement modules, etc., while the specific modules are used to achieve specific manufacturing tasks, such as the welding module of the robotic welding system, the cutting modules of the automated cutting system. In Fig. 2, the generalisation relationship is adopted to represent the relationship between the class Module and the classes Common module/Specific module.

• Class Component: Fig. 2 clarifies the relationship between the terms of module and component because both of them play an important role for the configuration design, but they are often confused by designers. For example, the switch, the power transmission line, the transformer, etc., are the basic components of a robotic manufacturing systems, but none of them can be called as "modules". However, the power supply sub-system composed by the aforementioned components can be considered as a module, because according to the definition of Gu et al., modules are the cluster of components, in which the interactions of components are maximised [START_REF] Gu | Product modularization for life cycle engineering[END_REF]. The relationship between classes module and component can be depicted by the aggregation relationship in UML, which is represented as a hollow diamond shape in Fig. 2.

• Class Component Document: designers usually have several alternative combinations of components provided by different component suppliers to realise the function of one module. The class Component Document is proposed to store the information related to the components provided by different component suppliers, such as the logistics, supplier location, price, delivery time, etc.

• Considering the information stored in the classes module and component, designers can select their preferred modules or components from the list of possible choices. The selection process is essentially a multi-attribute decision-making problem. The operation configuration() contained in the class architecture is therefore defined as a smart search algorithm based on the multi-attributes decision-making model to help designers to select the most suitable combination of modules or components. The proposed multi-attributes decision-making model is composed of two sub-models, a deviation model and a similarity model. Hence, it has two metrics for evaluation as compared with other mono-metric models using "distance-based" evaluation. Another advantage of this model is that it is more suitable to deal with vector-represented solutions in the discretised solutions space.

For more details of the multi-attributes decision-making model, the interested readers are referred to [START_REF] Zhang | An integrated decision-making model for multi-attributes decision-making (MADM) problems in additive manufacturing process planning[END_REF].

Representation of interfaces in template model

The interface is defined in the paper as the logical or physical relationship through which two modules (or two components) interact with each other. Fig. 2 also presents the modelling principles of the interface:

• Class Interface: Type and Configuration are proposed as two main attributes of the class Interface. The attribute type represents the transfer information occur through one interface. One enumeration type is created to detail the attribute type, in which four different types of transfers, i.e., geometric, energy, control and data, are presented. The attribute configuration describes which two modules (or two components) are linked by the interface. The operation compatibility() contained by the class Interface is used to test the interface compatibility in order to guarantee the different modules (or components) to integrate correctly. For the details of the compatibility rules and its implementation in computer-aided design platform, interested readers are referred to [START_REF] Zheng | Multidisciplinary interface model for design of mechatronic systems[END_REF].

• Class Port: the port refers to the primary location through which one module (or component) interacts with other modules (or components), and every interface links two modules (or components) through the ports. Therefore, the relationship between the class port and the class component or module is the whole/par relationship and can be represented as the composition relationship, which is graphically rendered as a filled diamond shape in UML. The attribute direction of the class Port signifies the direction of the information transferred through the port, and an enumeration type is used to represent the direction of the information (i.e., in, out and in/out).

• Class Port Document: the class Port Document is used to store the documents describing the port, such as the parameters or the brief description. The attributes value, maxValue, minValue and unit are used to express the value or the interval of the value and its unit of the port's parameter, such as the input voltage, output power, etc. However, sometimes the value of the port's parameters may not be accurately determined during the early design phase, and designers can just roughly describe the information related to the port. In this case, the attribute portDescription is proposed to store such brief description.

•

Design approach for robotic flexible manufacturing systems

Based on the proposed template model, the technology process of the SME-oriented design approach for robotic flexible manufacturing systems is illustrated in Fig. 4. The details of each step will be presented as follows: (2) Proposition of customer requirements: SMEs propose their requirements in this step. However, the manner in which designers capture customer informal design requirements is not merely a mathematical or technological challenge, but a complex social process [START_REF] Macaulay | Requirements capture as a cooperative activity[END_REF]. Therefore, the iterations still exist between customers and designers in order to understand customer expectations.

(3) Proposition of system template: after analysing customer requirements, designers can put forward several design solutions based on previous successful design experiences. Candidate templates of the required robotic manufacturing system can be therefore obtained by configuring existing modules and checking the compatibility of the interfaces between modules. Sometimes, some specific modules may be required by customers to achieve special manufacturing tasks, but they may not be contained in the module base. In this case, designers can update the module base by adding new instances of the specific modules required by customers.

(4) System template selection: in this step, SMEs can select the configuration of modules by their own. Due to the lack of basic engineering knowledge, the SMEs can check the basic working principle and the cost of each module, and the technologies to achieve the functionality of each module is encapsulated in a black box. They can choose the suitable configuration according to their design requirements and financial resources. The flexibility in design can be therefore achieved because even though the customers change their requirements or put forward new requirements, designers can always propose a new configuration of modules with a few changes.

(5) Proposition of component list: after SMEs select their ideal system template, designers should further decompose the modules of the selected template into components. However, the required functionality of one component can be achieved by one or more solutions. Therefore, the designers should propose a component list which contains different component alternatives, from which designers select the most suitable combination.

(6) Release of component information: the components suppliers can be industrial robot companies, automatic device manufacturers, or even designers themselves. Components suppliers are required to release the information related to components, such as logistics, supplier location, price, delivery time, and other lifecycle issue in the open design platform by instantiating the proposed template model.

(7) Selection of optimal combination of components: various candidates of one component may be provided by different components suppliers; therefore, designers should evaluate the performance of combinations of different possible alternative components and select the optimal one based on multiple criteria.

In this section, the authors present the details relating to the SME-oriented design approach for robotic flexible manufacturing systems. The template model is considered as the key part of the design approach, which provides different views for both designers and customers. Based on the template model, the process of the design approach is described, bridging the gap between the customer requirements and the designers' design solution.

The flexibilities in design and customisation for the implementation of robotic flexible manufacturing systems in SMEs can therefore be achieved by adopting the proposed design approach. On the one hand, various templates of the robotic flexible manufacturing system are proposed by designers according to their existing technology accumulation; therefore, the previous successful design experiences can greatly reduce the development time, cost and risk. On the other hand, the proposed design approach provides a means by which customers can choose and decide the architecture of their required manufacturing system according to their design requirements and financial resources, so that the possible misunderstanding of customer requirements by designers can be avoided at the beginning of the design process. In summary, the design approach based on the template model is proposed to support the design process of robotic manufacturing systems in a collaborative way, in which costumers and designers define the architecture of the system together. The applicability of the proposed design approach for robotic flexible manufacturing system is demonstrated by industrial design cases in the following section.

Case study

The case study selected for demonstrating the application of the proposed design approach is the industrial projects carried out in the authors' research team for two SMEs. The authors' research team in the School of Mechanical Engineering utilises its advanced engineering experiences to provide reliable industrial automation solutions with an eye to the robotic flexible manufacturing to satisfy industrial manufacturing requirements of different customers.

According to the proposed design approach, the design team firstly classifies their existing modules into common and specific modules (Fig. 5 The authors' design team is required by two SMEs of different industrial manufacturing domains to develop two robotic manufacturing systems. The SME A is a superhard material company in Xi'an (an inland city of China), and it needs an automated ceramic matrix composite materials (CMC) cutting system to cut workpieces made of CMC materials into various forms, according to different industrial uses. The SME B is a shipbuilding company in Shenzhen (a coastal city of China), which requires the authors' design team to develop a robotic manufacturing system for robotising welding tasks.

The authors' design team captures the design requirements of the automated CMC materials cutting system and robotic welding system, and then propose several candidate templates of the required manufacturing system by selecting existing modules from the module base and checking the compatibility of the interfaces between modules (Fig. 6). Considering the template selection result, designers further decompose the modules of the selected template into components. Table 4 illustrates the design alternatives for each component of the robotic welding system.

Discussion

Considering the challenges of the implementation of robotic manufacturing systems in SMEs, the authors present an SME-oriented flexible design approach in the paper. The proposed method provides the following advantages:

• Firstly, as to the solution to the conflict between development cost of flexible manufacturing systems and financial burdens of SMEs, the proposed design approach provides designers with a template to define the basic architecture of manufacturing systems. Moreover, the template offers designers the possibility to rapidly switch the development of one manufacturing system to another (i.e., configurable product family), so that the development cost can be greatly decreased.

• Secondly, the authors propose the design approach in which SMEs can configure the manufacturing system through selecting modules among various alternatives by their own. In this design approach, the template selection and system configuration are not only completed by the professional designers, but also by the SMEs according to their own needs, thus solving the conflict between professional designers and non-professional SMEs.

However, although the authors' research team have applied the proposed design approach successfully in industrial design cases, several perspectives for future research based on this approach are still recommended.

• Continuously growing pressures of increasingly complex market demand companies to develop new strategy to deal with various needs of customers with acceptable quality and affordable cost by considering the socio-economic context of the targeted market. In this new challenged context, frugal innovation theory is put forward by different authors.

Frugal innovation is defined as an innovation strategy to create significantly more value by minimising the use of resources such as energy, capital, and time [START_REF] Radjou | Frugal innovation: how to do more with less[END_REF]. Therefore, frugal innovation provides the SMEs with a compact physical architecture and more degree of freedom of movement for agile production, which can deal with mass customisation production despite financial, technological, material or other resource constraints. In fact, the design approach in the paper is proposed with respect to one of the basic principles of frugal innovation theory, that is, selecting the suitable alternative components respecting the requirements from regional markets. The authors' research team has already begun the research on the configuration design for defining product architecture and production network to reach frugality goals [START_REF] Belkadi | Co-definition of product structure and production network for frugal innovation perspectives: towards a modular-based approach[END_REF]. However, in order to achieve frugal innovation for robotic manufacturing systems, there are still a lot of further studies to be carried out in the future.

• Although SMEs play an important role in the global economy, however, they are also considered as a significant source of pollution. A report completed by the Commission of the European Communities points out that SMEs are responsible for 64% of the overall environmental impact in the European Union, including greenhouse gas emissions [START_REF] Calogirou | SMEs and the environment in the European Union[END_REF]. In China, more than 80% of SMEs have environmental pollution problems and account for 60% of the total pollution in the country [START_REF] Luo | Ranking Chinese SMEs green manufacturing drivers using a novel hybrid multi-criterion decision-making model[END_REF]. Therefore, "dematerialisation" becomes a new development trend for SMEs. In other words, how to aid SMEs in creating nonphysical service that provide users with the same level of satisfaction as the physical product with an inherently lower environmental burden has attracted increasing attention from both academia and industry. The proposed design approach in the paper can help designers to design robotic manufacturing systems for SMEs to produce physical products. However, it cannot fully support the service design for SMEs. Therefore, large amount of efforts is still required to propose a novel approach to help SMEs to develop both physical product and non-physical service.

• Successful development projects require accurate cost defining. Cost estimations predict the resources and their costs required during the development process of robotic manufacturing systems, which helps designers to satisfy customer requirements within the approved budget. However, defining the development cost is a difficult task. On the one hand, the development process can be affected by various risks, so it is significant to build allowances into cost estimations. Risk identification and allocation of contingency reserves are the most common approaches. On the other hand, even considering all risks when estimating the development cost, the estimation may still contain some degree of uncertainty. Therefore, the estimates should be overestimated rather than underestimated, and designers should establish tolerance levels for cost deviation. Although various cost estimating software and different development cost estimating methods have been developed to help designers to define the development cost, however, providing an accurate cost estimates is still a challenging issue, which not only demands estimating techniques, but also requires designer experiences. Interested readers are referred to [START_REF] Nicholas | Project management for business, engineering, and technology[END_REF] for more information about the techniques relating to the cost estimation.

• Future work should also focus on the implementation of the proposed design approach in an expert home-made PLM system. Even though the authors' research team has developed a design platform based on 3DEXPERIENCE of Dassault Systèmes to help designers partially achieve the automated design for mechatronic systems [START_REF] Zheng | Knowledge-based engineering for multidisciplinary systems: Integrated design based on interface model[END_REF]. However, large amount of efforts is still required by the development of an expert system which can realise the automated conceptual design process for robotic manufacturing systems as well as more complex interdisciplinary system design problems in a life cycle view.

• Conclusion

Considering the challenges of the implementation of robotic manufacturing systems in SMEs, the authors present an SME-oriented flexible design approach in the paper. The main contributions of the work lie on the following two levels. From the designers' perspective, the proposed design approach provides designers with a template to aid them in defining the basic architecture of robotic manufacturing systems, which offers designers the possibility to rapidly switch the development of one manufacturing system to another, so that the flexibility in design can be therefore achieved because even though the customers change their requirements or put forward new requirements, designers can always propose a new configuration of modules with a few changes. Therefore, the development lead-time and cost can be greatly decreased. From the customers' perspective, the authors propose the design approach in which SMEs can configure the manufacturing system through selecting modules among various alternatives by themselves according to their design requirements and financial resources, thus solving the conflict between professional designers and non-professional SMEs and reaching the goal of flexibility in customisation.

( 1 )

 1 High development cost (2) Long development lead-time (3) Large space requirement (4) Complex operating process (5) Difficulty in capturing customer requirements (6) System unstability (7) High maintenance cost (8) Laws or regulations which constrain the development of robotic manufacturing systems in SMEs A five-level scoring method is used to indicate the grades of each factors which hinders the implementation of robotic flexible manufacturing systems in SMEs.

Fig. 2

 2 Fig. 2 Representation of template model in UML class diagram

Fig. 3

 3 Fig. 3 Implementation of proposed template model in an open collaborative design platform

Fig. 4 ( 1 )

 41 Fig. 4 Technology process of SME-oriented design approach for robotic flexible manufacturing systems

  (a)), represents them in structured forms by instantiating the proposed template model (Fig.5(b)), and stores them in the module base (Fig.5(c)).

Fig. 5

 5 Fig. 5 Classification, representation and storage of existing modules

Fig. 6 (

 6 Fig.6(a) shows the proposed architecture of the two robotic manufacturing systems. The main modules are represented as blue boxes, while the interfaces among modules are represented in purple. The details relating to the proposed architecture of the two robotic manufacturing systems is presented as follows: supported by the position detection module (CM7 ) which can detect the welding seams (or the cutting shapes), the off-line program sub-system (CM 2 ) generates the collision-free trajectory. Executing the off-line program, the control module (CM 4 ) enables the movement module (CM 1 ) to move the end effector module (SM 1 ) to the starting point of the welding seams (or the cutting shapes). Fig.6(b) illustrates solutions selected from the module base by designers for the proposed common and specific modules.

Fig. 6

 6 Fig. 6 Proposition of system templates for two manufacturing systems

FollowingFig. 7

 7 Fig. 7 Component information representation and final component selection -example of robotic welding system

Table 1

 1 Table 1 illustrates the relationship between the scores and grades. Relationship between scores and difficulty levels

	Importance level	Score
	Very low	1
	Low	3
	Neutral	5
	High	7
	Very high

Table 2 .

 2 Assessment of design approaches regarding flexibilities in design and customisation

	Design approach	Flexibilities in design	Flexibility in customisation
	Agent-based
	design
	paradigm

Table 3

 3 Table 3 presents the customers' template selection results of two manufacturing systems. Template selection results of two manufacturing systems

	Module	Robotic welding system	Automated CMC cutting system
	CM 1	CM 2 1 : Two robots & gantry	CM 1 1 : Single robot & gantry
	CM 2	CM 2 2 : Off-line program	CM 2 2 : Off-line program
	CM 3	CM 1 3 : Power supply module	CM 1 3 : Power supply module
	CM 4	CM 1 4 : Control module	CM 1 4 : Control module
	CM 5	CM 1 5 : HMI module	CM 1 5 : HMI module
	CM 6	CM 3 6 : Hybrid method	CM 3 6 : Sensor-based method
	CM 7	CM 2 7 : Vision-based detection method	CM 1 7 : CAD-based detection method
	CM 8	CM 1 8 : Safety fence	CM 1 8 : Safety fence
	SM 1	SM 1 1 : Welding module	SM 2 1 : Cutting module

Table 4

 4 Components and their alternatives -example of robotic welding system

	Selected module of
	robotic welding	Components of each module	List of alternative components
	system
		M 1 : Robot	M 1 1 : Yaskawa AR1730 M 2 1 : KUKA KR16-2C
		M 2 : Servo-motor	2 : Bosch Rexroth MSK061C-0300-NN-S1-M 1 UG1-NNNN
	1 CM 2	M 3 : Linear motion rail	M 2 2 : Delta ECM-E2M-D11315RS0 M 1 3 : THK HSR-35B
		M 4 : Linear bearing	M 1 4 : THK LR50130
		M 5 : Gearbox	M 1 5 : SHXTM ZF80
		M 6 : Other mechanical parts of gantry	M 1 6 : Local machining company
	CM 2 2	M 7 : Off-line program	M 1 7 : Proposed by authors' research team
	3 CM 1	M 8 : PLC power supply unit M 9 : Cables	M 1 8 : SANTAK Rack3KS M 1 9 : Local cable manufacturer
	4 CM 1	M 10 : Robot controller M 11 : Gantry controller (programmable logic	M 1 10 : KUKA KR C4 M 2 10 : Yaskawa YRC1000 M 1 11 : BECKHOFF C6930
		controller -PLC)	M 2 11 : SIMATIC S7-400
		M 12 : Graphic touchpanel with integrated modular	M 1 12 : SIMATIC HMI KTP400
	CM 1 5	PLC M 13 : Robot control panel	M 2 12 : BECKHOFF CP6942-0001 M 1 13 : KUKA SmartPAD holder M 1 14 : Yaskawa Smart Pendant
		M 14 : Collision detection algorithm	M 1 14 : Proposed by authors' research team
	CM 3 6	M 15 : Collision avoidance sensor	M 1 15 :TBi KS-2 Safety-off mechanism M 2 15 : ABIROB W cpl
	7 CM 2	M 16 : Camera M 17 : Image reconstruction algorithm	M 1 16 : Baumer XC100 M 2 16 : Cognex CIC 2000 M 1 17 : Proposed by authors' research team
	CM 1 8	M 18 : Safety fence	M 1 18 : Local machining company

Acknowledgements

This project is supported by National Natural Science Foundation of China (Grant No. 51805437 and 51805438).