تأثیر کنترل کیفیت بر روی عملکرد سیستم تولید

Quality and production logistics have been traditionally considered as separate fields, both by researchers and industrialists. However, during the design phase of production systems, the decisions taken as an answer to productivity requirements have an impact on product quality; similarly the decisions taken to meet quality requirements affect the productivity performance of the system. The paper proposes an approach to evaluate the overall performance of the system considering both quality and production logistics. The results obtained by the application of the method provide new insight in the relations among the two areas and pave the way to the joint design of production logistics and quality control systems.

With the globalisation of the markets and the growth of competitiveness in the manufacturing sector, quality of products has become a key factor of success. Moreover, the turbolence of demand leads to the continuous need of modifying production targets. In order to rapidly react to these changes, manufacturing systems must be endowed with a right degree of changeability [1] and reconfigurability [2]. In this environment, both quality and logistic performance measures get a fundamental importance in the configuration / reconfiguration phase of production systems. Even if these considerations are widely shared both in the scientific and the industrial research community, quality and logistics have almost always been investigated as two separate research areas. Logistic performance of production systems have been analysed in the literature by using several tools, mostly developed to predict the main productivity performance measures of manufacturing systems. Simulation [3], analytical methods [4] and logistic operative curves [5] are some of the most commonly used tools. These techniques have different characteristics and fields of application as discussed in [6]. The diffusion of these techniques contributed to create a solid knowledge in the manufacturing system design field. Considerable attention has also been dedicated by researchers to quality issues. Statistical Process Control, SPC, techniques [7], have been developed to monitor the behaviour of production systems, in terms of quality performance. Several techniques have also been proposed to solve design problems such as the location of inspection devices in production systems,the optimisation of control charts parameters and the design of inspection plans. Recent works investigated the impact of the process and the production system design on the quality of the produced parts. Among them, the stream of variation theory [8] has been developed with the objective of determining how the variability of products propagates through the stages of a production system and how the causes for variability can be attributed to different production stages. In these works the impact of different production system configurations is considered, even if logistic performance measures are not directly taken into account.Recently, some researchers [9] [10] and industrial practitioners have started to recognize the importance of integrating the two fields of quality and production logistics. In particular in [11] cases from General Motors Corporation are reported showing how different system design decisions impact on production quality. The objective of this paper is to analyse the intersection among quality and productivity and to propose an innovative method for evaluating the overall performance of production systems, considering both Statistical Process Control and system logistics. Indeed, since the quality control system can decide to stop the machines to restore the “in control” conditions, it does affect the production rate of the system. On the other hand, the level of Work In Progress in a system has an impact on the responsiveness of the quality control system. The application of the method provides some results and insight in this new research area and paves the way to the joint design of production systems considering both quality and logistics requirements.

In the paper, an approximate analytical method has been proposed for evaluating the performance of production systems monitored by SPC. The method is general in the sense that it can be applied to different system architectures and different types of control charts. Provided results show that the quality control system configuration impacts on the system productivity performance measures. On the other hand, the impact of the production system architecture on the quality system control performance is also relevant. Therefore quality and logistics requirements must be jointly considered while designing a production system. Future developments include the introduction of a cost function synthesizing both the effects of quality and logistics performance and the generalization of the method to consider the following issues: • unreliable buffers and buffers delay due to non zero travelling times of parts in the conveyors; • off-line time consuming inspections, performed by stations shared among different sub-systems; • multiple modes of out of controls of machines and dependency of features between different production stages; • rework loops of repairable products; • more general production stopping rules, based on the number of non-conformities produced in sequence or other run-rules; • parts produced in batches.