Reconfigurable machine tools

2.5. Reconfigurable machine tools

In the view of machine equipments, traditional machine tools such as DMTs and Flexible Machine Tools (FMTs) cannot meet the requirements to compensate for frequent demand fluctuations. DMTs are designed to produce a single product at a high rate of production, which is achieved by utilizing all tools simultaneously. The only way to enhance the ca- pacity is to purchase additional equipment. While this covers for peaks in demand, these tools may stay idle or remain underutilized at normal demand levels. FMTs can produce a variety of products according to respective demands, but are very expensive and typically their full functionality is not exploited which results in unnecessary costs due to software development and installation. As they are not designed for structural changes, thy are not suitable to react to persistent market fluctuations. Hence, considering a manufacturer’s point of view, a sustainable machine tool is required which allows for increasing flexibility, improving the productivity, reducing energy consumption, shortening delivery times and enhancing responsiveness in the presence of demand fluctuations [83].

Aiming to fulfill these wishes, RMTs were developed as a new class of machine tools, which combine the high throughput of DMTs and flexibility of FMTs [84], cf. Table 2.1. These machines are designed modularly for customized operation requirements and may be cost-effectively, quickly and permanently reconfigured in an open-architecture, which renders them ideally suited for a machinery based capacity adjustment in the presence of short or mid-medium fluctuation in demand [56, 84], cf., Figure. 1.1. In contrast to rebuild- ing, reconfiguration as the key factor to handle the exceptions and performance deteriora- tions in manufacturing systems [85] does not focus on the preparation of the machine to complete an order, but to adapt the structure, technology and functionality including the functional capacity of the manufacturing site.

Table 2.1.:RMT combines features of dedicated and CNC machines [1]

Dedicated RMT/RMS CNC/FMS System structure Fixed Adjustable Adjustable Machine structure Fixed Adjustable Fixed

System focus Part Part family Machine

Productivity High Medium Low

Scalability No Yes Yes

Flexibility No Customized General

Cost Low Medium High

2.5. Reconfigurable machine tools 17

The illustration of a workflow of a manufacturing process with RMTs is given in Fig- ure 2.4. In the first place, the customers launch order requests to industry and place the orders. Second, technicians convert these orders into design requirements and send them to designers who are responsible for establishing the manufacturing system to satisfy the requirement. Third, operators adjust the manufacturing system and handle those orders on the shop floor. Last, when the parts are completed, they will be delivered to the customers. In this thesis, we consider the operational layer which is the case of the third step. Recent development of RMTs was investigated in [86], indicating that the contributions mainly in- clude the three fields: architecture design, configuration design with optimization as well as system integration and control. Within this thesis, we cannot consider all references avail- able concerning study of RMTs. Instead, we choose some key contributions which were published in the high level journals. Also, we pay attention to the development of RMTs, especially for the recent years.

Figure 2.4.:Workflow of a manufacturing process with RMTs [5]

2.5.1. Architecture design

In order to fulfill a variety of operations to meet productivity and functionality, the designed RMTs have to take the mechanical requirements including kinematic viability and structure stiffness into account. The modular design of RMTs on hardware level was conducted in [87, 88, 89, 90, 91, 92]. The control and mechanical requirements of RMT was presented in [84, 93, 94, 95]. Several types of RMTs have been developed for the design of manufacturing processes, such as module 3-axis RMT, arch type RMT, which allow the implementation of multiple operations [96, 97].

2.5. Reconfigurable machine tools 18

2.5.2. Configuration design with optimisation

In [98], the authors optimized the production process plan associated with the configura- tions of RMTs via a genetic algorithm to improve production capacity and reduce produc- tion cost. In [99], the authors investigated the optimal design of dedicated and reconfig- urable manufacturing systems for adapting to a variable market demand with the mini- mization of investment costs. Such kind of combinatorial optimization problem is NP-hard, which can be solved by meta-heuristics strategies to achieve suboptimal solutions, e.g., genetic algorithm, particle swarm optimization, ant colony optimization, etc. In [97], the authors developed a hybrid generic design AND-OR tree and AND-OR graph for the opti- mal design of a reconfigurable process and of configurations of RMTs. The solutions were later identified by a multi-objective hybrid optimization. To balance the trade-off between economy and responsiveness, [100] applied the non-dominated sorting genetic algorithm to solve a multi-objective optimization problem for the optimal selection of machine con- figuration. The performance index indicated cost, operational capability and machine re- configurability. Then these obtained solutions were further analyzed and ranked by the “technique for order preference by similarity to ideal simulation” approach.

In [69], relying on production capability of RMTs, the authors studied a single prod- uct line to satisfy demand changes. The production capacity was increased or decreased through adding or removing auxiliary modules for performing different operations, and also purchasing new RMTs to increase capacity while minimizing reconfiguration cost and capital investment cost. To exploit the best configuration, a mixed integer linear program- ming problem was formulated. Two cases concerning cost management were presented to demonstrate the efficiency of the proposed method. In [5], based on graph theory method, a tree-based decision approach for the determination of a configuration design of RMTs was proposed. The satisfactory configuration was conducted to meet operational requirements while minimizing cost and maximizing reconfigurability. The most satisfactory configura- tion from the feasible design space could be derived based on the performance indicators. In [101], the authors built a multi-objective optimization model incorporating configurabil- ity, cost and process accuracy for RMTs design. A modified fuzzy-Chebyshev programming approach was proposed which allows to dynamically adjust objective weights in the search space. The derived solution was obtained by particle swarm optimization.

2.5.3. System integration of control

In [102], the authors formulated reconfigurability of RMTs in a discrete event system re- garding its corresponding configurations. A polynomial-time algorithm was proposed for the construction of a reconfiguration supervisor. The configuration states were monitored by means of supervisor control and allowed controller modifications during time run. In [103], an intelligent Fuzzy PID controller was designed to provide a viable solution in an