9. Discussion
9.4 MRMs in Reconfigurable Manufacturing Systems
MRMs were designed for the manufacturing of general products as in the case of CNC machines;
however the modularity of the system permits its flexibility to be customized as in the case of DMTs and RMTs. A comparative analysis of MRMs is provided in Table 9.1, this comparison is between the properties of MRMs, DMTs, CNCs and RMTs. MRMs display a clear advantage over other types of machines in terms of the modifiable mechanical architecture, the scalable nature of the control system and the interchangeability of axes and cutting heads. The property of customizable and expandable flexibility in MRMs has resulted in the prediction of a relatively moderate cost in comparison to CNCs.
9.4 MRMs in Reconfigurable Manufacturing Systems 9.4.1 Reconfigurable Functionality in MRMs
The MRM library of modules consisted of twelve units of hardware that were ultimately used to create nine machines that displayed distinctly different combinations of cutting processes and kinematic abilities. In addition to the manufactured hardware, commercial off the shelf enhancements such as steady rests and tail stocks could be integrated with MRM platforms to provide additional levels of functionality. The full range of modules is documented in Appendix B. All MRM configurations created with the library of modules are documented in Appendix D.
MRMs address the necessity for reconfigurable functionality in RMSs by providing a variation in processing operations and DOF through the structural reconfiguration of a machining platform. A change in processing operations enables different part features to be produced while a change in DOF enables different tool paths. Reconfigurability of this nature holds three important implications:
(i) The flexibility of individual machines may be expanded, thus enabling RMSs to produce different part families with a minimum investment in additional hardware.
(ii) MRMs need only possessed the exact level of functionality required to complete an operation, any excess modules may be removed and distributed to other machines in the system.
(iii) Unused machines may be decomposed into modules and reassembled into other machines that are needed in the system.
9.4.2 Initial Capital Investment in Hardware
MRMs are machines that are able to display expandable machine flexibility. The implication is that manufacturers may begin the operation of a system with the minimal level of functionality required at the outset. As the product portfolio of the system evolves, the machines in the system may be enhanced with additional modules. From an economic perspective this means that manufacturing systems may be initialized at a minimum cost and the flexibility of the system may be gradually increased at a later stage as the system begins to pay back the initial capital investment. Moreover, the increase in flexibility is derived from the upgrading of existing machines as opposed to the purchasing of new machines, therefore promising cost savings in hardware investments.
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9.4.3 Scalable System Capacity
Figure 9.1: Example Demand Characteristics: Part A (above), Part B (below) [74]
In manufacturing systems producing multiple product types, a redistributing of system resources between operations will often be required as the demand characteristics of the individual products vary over a period of time. Changes in demand characteristics are most profound when new products are launched into markets. Consider the example of Figure 9.1 which illustrates two parts belonging to two different products. Part A corresponds to Product A, which is being phased out of production. Part B corresponds to Product B, which is replacing the old product.
Fourth Month Tenth Month
Capacity: Stream A Capacity: Stream B Capacity: Stream A Capacity: Stream B
Figure 9.2: Reconfiguration of Production Stream Capacity by the Reconfiguration and Redistribution of MRMs
During the phase of introducing the new product the demand for the older product will decline while the demand for the new product will increase as illustrated. Part A requires a 3-axis drilling machine for the machining of holes on its various flat surfaces while Part B requires a 4-axis drilling machine to cater for the machining of additional holes on inclined surfaces. In the case presented a manufacturer would reallocate a portion of the system resources previously used in the production of A to the production of B based on the demand characteristic.
Demand/ Capacity Vs Time
0 5000 10000 15000 20000
1 2 3 4 5 6 7 8 9 10 11 12 Months
Units Per Month Demand - Product A
Demand - Product B Capacity Requirement - Part A
Capacity Requirement - Part B
MRM MRM MRM
MRM MRM MRM
MRM MRM
MRM MRM
MRM MRM MRM
MRM MRM MRM
MRM MRM
MRM MRM MRM MRM
MRM
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This is achieved by reconfiguring the 3-axis drilling machines into 4-axis drilling machines by the addition of modules to the platforms. The 4-axis machines are then allocated to the production stream of product B. Figure 9.2 illustrates how the process of reconfiguration may used to vary the capacities of both production streams by the redistribution of system resources.
9.4.4 High Product Variety and Product Customization
MRMs are able to display expandable mechanical flexibility. An expansion in machine flexibility further implies an expansion in the process flexibility of the manufacturing system. Process flexibility is concerned with the set of part types that can be produced by current process configurations. MRMs will therefore enable systems to cope with a higher part variety over a period of time; this is presently a significant challenge in modern manufacturing.
MRMs possess the potential to aid in product customization, which was also identified as a significant challenge in modern manufacturing. Specific product configurations may require unique machine configurations for their production. In this instance an MRM may be suitably reconfigured to match a customized derivative of a product platform. This eliminates the need to purchase an unpopular machine for the customized product. This is a particularly unattractive scenario as the machine may only be used for a limited run of products. With MRMs the manufacturer need only invest in a specialized module to impart the required functionality to the system. The machine may then be reconfigured for other operations once a customized product feature is no longer required.
The modularity of MRMs may in future, grant manufacturers the platform to develop customized modules for their machines (as opposed to purchasing them). This will further enhance product customization and enable manufacturers to optimize MRMs to meet their specific requirements. A customized MRM is expected to be quicker to assemble than the building of customized machines. User customization will require the drafting and publication of open standards for the interfacing and control of MRM modules.
9.4.5 Expansion Flexibility and System Life Span
The expandable machine flexibility, process flexibility and the ability of MRMs to aid in capacity scaling imply high expansion flexibility for RMSs. DMSs are limited in their expansion flexibility due to the rigid nature of the systems, while FMSs would usually require the purchasing of new types of machines to alter the functionality available in the system. The lifespan of RMSs are therefore expected to be significantly longer than DMSs, and the cost of extending its lifespan much lower than FMSs, if MRM technology is refined and implemented.
9.4.6 MRMs and the Five Essential Characteristics of RMSs
The five essential characteristics of RMSs are modularity, convertibility, customization, integrability and diagnosability. These characteristics are to be enabled in RMSs by the technologies implemented in these systems. In MRMs these characteristics were imparted to the system at multiple levels. The Mechatronic engineering approach was applied to the design of MRMs and necessary features in the mechanical, electronic and software systems were identified as early as the conceptual design phase. The individual subsystems either display each of the five characteristics directly, or support other subsystems in displaying these characteristics.
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Table 9.2 summarizes how features in the various subsystems of an MRM support the five essential characteristics of RMSs.
Table 9.2: MRMs and the Five Essential Characteristics of RMSs
Mechanical System Electronic System Software System Modularity Modularized axes and
cutting heads. Modularized control hardware and networked axes.
The concentration of generic software functions on the host PC and the location of module specific control functions on distributed control drives promoted mechanical and electronic modularity.
Convertibility Reconfiguration of modular assemblies to convert machines to produce new products.
Modular control hardware supported the conversion of the mechanical platform.
A fully comprehensive G-Code command set inbuilt to support a large variety of mechanical configurations.
Customization The functionality of the mechanical system could be customized by allowing only the necessary modules to be present on a platform.
The electronic control system was modular and scalable, ensuring that only those modules necessary for the control of the current MRM
configuration are present in the control system.
The selection of active axis combinations by drop down menus customized the active G-Code command set.
Commands that were inconsistent with the selected combination were rejected by the text interpreter.
Integrability Mechanical modules possessed a series of standard mechanical interfaces for integration with each other.
Standardized 8 and 11 wire connections were used to provide a consistent power and control interface between mechanical and control modules.
Standardized 8 and 11wire connections for interfacing of control modules with the mechanical platform.
The use of standardized communication protocols such as I2C supported the integration of control modules into networks.
USB communication provided a standard means of interfacing distributed control dives with the host PC.
The software system supported the USB communication protocol.
The ability to configure USB port addresses for spindle and servo control further enhanced the integrability of hardware with the host PC.
For modular axes the G-Code word addresses corresponded on a 1:1 basis with their I2C addresses, supporting the easy addressability of servo control modules.
Diagnosability All automated mechanical modules contained sensors.
Limit switches for collision detection.
Accelerometers for the measurement of vibrations.
The MRM GUI contained a warning box, status box, progress bar and LCD for the display of diagnostic information to the user.
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