Many firms are being confronted with increasing market pressures induced by competitiveness, reduced prices, better qualities, minimal response times and a rise in product diversity. In the last few decades, the increasing development of flexible manufacturing systems and cells has emerged to simultaneously fulfil the requirements of efficiency, quality and flexibility. Manufacturing systems are large, as well as complex systems that are made up of a variety of numerically controlled machine tools, Coordinate Measuring Machines (CMM), machining centres, storage-loading-unloading-clamping-unclamping areas and pallet transport systems. Manufacturing cells are reduced-scale manufacturing systems controlled as automation islands dedicated to the manufacturing of small sets of product classes and acting as a computer controlled stand-alone entities of an overall manufacturing system. Flexible manufacturing systems and cells are considered as the shop floor start-up of a Computer Integrated Manufacturing architecture involving contributions and effort from all the departments of a company.
As one of the major goals in manufacturing systems, quality concerns the whole life-cycle of both product and process, thus covering all quality management activities, including quality planning, control and monitoring with appropriate feedback actions.
The quality objective stands on horizontal and vertical integration flows in a Computer Integrated Manufacturing structure. For the horizontal viewpoint, in-process quality assurance and process certification methods require the significant use of in-process quality sensors and deterministic metrology methods supported by a reactive architecture. The QIA (Quality in automation) project has made big efforts on this subject. Vertical integration of quality assurance operates from the manufacturing system automation stage to the Computer Aided Design (CAD) stage by means of an information system with quality management features.
Quality control has to be integrated into the whole manufacturing process, as close to the production operations as possible to induce feedback actions on the operational and/or informational stages of the system.
Optimal management of manufacturing operations requires the setting up of feedback control loops within the system architecture. To achieve this, the system must be equipped with loop-controlled functions. The difficulty here is to survey manufacturing control loop solutions using existing and heterogeneous equipment, to specify necessary modifications for their integration, and to propose adaptable and reconfigurable solutions for various types of equipment.
Thus, a solution based on a centralised production control organisation, as well as coordinating NC machines without initiative would be implemented easily but would prove much too sensitive to perturbations: the smallest equipment failure can lead the manufacturing system towards a degraded and sometimes incoherent functioning mode. For these reasons, a distributed computer control solution that attempts to grant more intelligence and also more autonomy to the machines is preferred. Many studies have demonstrated the profitability of such an approach.
To increase the autonomy of manufacturing system, the production management of the shop floor and the cell must be coupled as closely as possible. The manufacturing orders can be created differently, either from:
Information received from a next cell (Kanban),
Information determined by the shop floor (MRP), or
(hi) information decided by the cell (OPT).
These differences alter the data flow consequences. Once the cell has received the manufacturing orders, it can sequence them, but only in a very short-term way. The integration of defects and manufacturing exceptions into the planning of computing time is quite impossible. It does not take into account the due dates which are computed by the MRP (material requirement planning) as priorities, and thus the FIFO criterion is used. As for the Kanban and the OPT, the estimated processing time of the manufacturing operations may serve to compute the due dates.
The cell may also use the Kanban or the OPT criteria, or any other criterion, in maintaining both its autonomy and the decisional framework defined by the entire manufacturing orders. The cell may decide to perform more quality control tasks especially if it is not overloaded or if it is not a bottleneck. The cell also fulfils the reporting activity, which concerns both the executed tasks and the obtained qualities. Nevertheless, the reports must be suited to the production management method because this method does not consider the same indicators.
The manufacturing abilities and control must be able to manage varying manufactured articles (for example, using a group technology configuration), or different products conceived to match the customer’s requests. The equipment flexibility in the manufacturing system is managed by the product system design and the control flexibility by the chosen control architecture. The former is related to the integrated management of the quality, and the latter needs modular software and hardware. In the long term, control system reconfiguration depends on the facility to substitute the software in the computers, the numerical controllers, and the programmable logic controllers. In the manufacturing process, the new incoming product should be able to:
Explain the manufacturing specification using the design stage information (the CAD/CAM product data exchange standards enable the consistency of data format from the design stage to the manufacturing and inspection stage); perform the report of the adapted manufacturing, at least to satisfy Statistical Process Control (SPC) methodology.
The manufacturing system adjusts itself on the control variation following the quantity and the due date in JIT (just in time) and OPT ways. The manufacturing capabilities must change with the control estimates. That is not within the cell’s power, but it affects the manufacturing management method applied to the factory or the shop floor. On the other hand, the control criterion relative to this order can progress along with the methods and the manufacturing systems. Thus the manufacturing control system must be adaptive: its programming is parameterized by the criterion.