Multi-objective optimization of product variety and manufacturing complexity in mixed-model assembly systems

Product variety has increased dramatically as manufacturers compete for market shares. While higher variety of products may satisfy a broader range of customers, it also introduces complexity in manufacturing. A multi-objective optimization approach is proposed to balance product variety and manufacturing complexity when designing a product family and the mixed-model assembly system. Relative complexity is introduced to measure the complexity and to find the best set of product variants to be offered while balancing market share and complexity. Numerical examples are provided to demonstrate the approach.     

Evaluation of assembly sequences using generalized flexible assembly systems scheduling problem

The evaluation and selection of assembly sequences are performed in the planning stage of assembly processes. This paper deals with the effect of selecting particular assembly sequences to the performance of flexible assembly systems(FAS). The performance of FAS is evaluated using the generalized FAS scheduling problem(GFASSP). Compared to the conventional FASSP, GFASSP tries to select the most efficient assembly sequences for each product as well as minimize cycle time to complete the assemblies in FAS environment while considering the transfer time of subassemblies.     

Algorithms for determining capacities of individual buffers in assembly/disassembly systems

We deal with a buffer allocation problem of assembly/disassembly (AD) systems with a finite buffer capacity. The problem is to determine capacities of individual buffers for a given total available capacity of buffers with the objective of maximizing throughput rate. We first present an algorithm (BA-G) in which a conventional gradient search is used for finding an initial solution and a two-optimization procedure is used for improving the initial solution. Since this algorithm may require an excessive computation time, especially when a procedure for performance evaluation employed in the algorithm takes a long computation time, a new algorithm (BA-P) is developed in which a sophisticated procedure is used for finding an initial solution. In this procedure, capacities of individual buffers are determined based on the efficiency of their upstream and downstream machines. Results of computational experiments show that the latter algorithm gives the same solutions as those obtained by the former algorithm in much shorter time.     

Development of a tablet test for the precision assembly efficiency assessment

This study explored if an individual's aiming ability can be used to predict his or her overall performance in a precision assembly task. A tablet aiming test was developed. Ten participants in the Pilot group was asked to do both the tablet aiming test and the simulated assembly task to determine the number of trials that were required for participants to get familiar with each operation. Thirty participants in the Test group performed the both tasks at certain times based on the number of trials determined in the pilot test. Pearson correlation analysis shows that, among the seven performance measures extracted from the tablet aiming test, the Number of Miss (r = 0.39, p = 0.03), the Hit Rate (r = −0.38, p = 0.04), and the Miss Rate (r = 0.39, p = 0.03) each had a significant correlation with the time required to complete the simulated assembly task (p < 0.05). The findings provide insights into the potential development for predicting the efficiency of a precision assembly task.     

Utilizing variant differentiation to mitigate manufacturing complexity in mixed-model assembly systems

A measure of product variety induced complexity has been proposed for mixed-model assembly systems with serial, parallel and hybrid configurations. The complexity model was built based on the assumption of identical parallel stations, i.e., same product variants are produced at all parallel stations in the same volume and with the same mix ratio. In this paper, the existing complexity model is extended to general mixed-model assembly systems with non-identical parallel stations in the presence of product variety. Then it is discussed that how to reduce the system complexity using the variant differentiation, based on which a mathematical formulation is developed to minimize the complexity of a mixed-model assembly system. The formulated problem is a non-linear programming problem and then solved by genetic algorithm. Last the developed complexity mitigation model is applied to the configuration selection of assembly systems, i.e., to identify the system configuration with the minimum complexity.     

Lead time control in multi-class multi-stage assembly systems with finite capacity

In this paper, we model multi-class multi-stage assembly systems with finite capacity as queueing networks. It is assumed that different classes (types) of products are produced by the production system and products’ orders for different classes are received according to independent Poisson processes. Each service station of the queueing network specifies a manufacturing or assembly operation, in that processing times for different types of products are independent and exponentially distributed random variables with service rates, which are controllable, and the queueing discipline is First Come First Served (FCFS). Different types of products may be different in their routing sequences of manufacturing and assembly operations. For modeling multi-class multi-stage assembly systems, we first consider every class separately and convert the queueing network of each class into an appropriate stochastic network. Then, by using the concept of continuous-time Markov processes, a system of differential equations is created to obtain the distribution function of manufacturing lead time for any type of product, which is actually the time between receiving the order and the delivery of finished product. Furthermore, we develop a multi-objective model with three conflicting objectives to optimally control the service rates, and use goal attainment method to solve a discrete-time approximation of the original multi-objective continuous-time problem.     

Integrated Set Parts Supply system in a mixed-model assembly line

The mixed-model assembly line (MMAL) is a type of assembly line in which a variety of product models are assembled on the same line. The use of highly variant parts on the assembly line need to be considered carefully to enable satisfactory material flow control and allow for smooth production. To increase the quality of parts supply and parts assembly in MMAL, Toyota has introduced an innovation system known as Set Parts Supply (SPS). In this paper, we investigate the parts supply issues in SPS implementation using a case study in the automotive industry. The linkage of parts supply strategies with Manufacturing Execution System (MES) is introduced to improve the SPS implementation which are (i) synchronized parts supply, (ii) e-kanban system and (iii) Synchronized Supply Sheet. From the research findings, the integration with MES has contributed to the Just In Time in parts supply at the supermarket area and assembly line.     

An evaluation of buffer management strategies for relational database systems

In this paper we present a new algorithm, DBMIN, for managing the buffer pool of a relational database management system. DBMIN is based on a new model of relational query behavior, thequery locality set model (QLSM). Like the hot set model, the QLSM has an advantage over the stochastic models due to its ability to predict future reference behavior. However, the QLSM avoids the potential problems of the hot set model by separating the modeling of reference behavior from any particular buffer management algorithm. After introducing the QLSM and describing the DBMIN algorithm, we present a performance evaluation methodology for evaluating buffer management algorithms in a multiuser environment. This methodology employed a hybrid model that combines features of both trace-driven and distribution-driven simulation models. Using this model, the performance of the DBMIN algorithm in a multiuser environment is compared with that of the hot set algorithm and four more traditional buffer replacement algorithms.This research was partially supported by the Department of Energy under Contract No. DE-AC02-81ER10920 and the National Science Foundation under grant MCS82-01870.     

嵌入式系统缓冲区溢出攻击防范技术研究

针对嵌入式系统在缓冲区溢出攻击下的脆弱性问题,对开源嵌入式操作系统μC/OS-Ⅱ的内存管理机制进行分析,提出了一种基于块表的内存保护方案。该方案将属于同一任务的内存块归纳到一个域内,并建立块表进行管理,实现了任务地址间的隔离;通过对内存块的访问进行越界检查和访问控制,有效地防范了针对嵌入式系统的缓冲区溢出攻击。最后,对该方案进行了有效性分析并在Nios Ⅱ平台上进行了实验测试,结果表明所提方法可行。     

基于ZigBee的生产线监控系统的设计与实现

根据生产线的实际需要,设计并实现了用于生产线的无线信息传输网络节点,研究了系统结构,完成了系统的软硬件设计,详细分析了ZigBee组网通信、应答机制建立等关键技术。测试结果表明,系统传输距离远、可靠性高,能够满足实际应用需要。     

嵌入式系统安全中缓冲区溢出防止技术的研究与实现

提出将嵌入式系统下缓冲区溢出分为堆栈溢出攻击和指针攻击两类,并基于该分类提出软硬件结合的防范缓冲区溢出方法,针对两类缓冲区溢出分别采用基于硬件的堆栈保护技术和指针保护技术来防范。与[2]中的软硬件防范技术(HSDefender)相比,本文的方法进一步实现了数据指针的保护。     

Bounds on buffer overflow probabilities in communication systems

A model to analyze the buffer behaviour in a multiplexor is derived, based on the analogy between the buffer occupancy in a discrete time model of multiplexing and the waiting time of a GI/G/1 queueing system.The bounding techniques developed earlier by Kingman and Ross are extended to the discrete time model. Simple and useful bounds are obtained for the buffer overflow probabilities under general assumptions concerning incoming message traffic characteristics. Numerical examples are presented and compared with other methods.     

Near optimal buffer allocation in remanufacturing systems with N-policy

We introduce a near optimal buffer allocation plan (NOBAP) specifically developed for a cellular remanufacturing system with finite buffers where the servers follow N-policy. The term N-policy is used for the situation where the server leaves primary work to tend to an external workload assigned to him (such as processing additional tasks or performing preventive maintenance of equipments) every time the server becomes idle and does not return back to his primary work until the queue size in front of the primary work reaches a threshold value of N (?1). The remanufacturing system considered here consists of three modules, viz., the disassembly module for returned products, the testing module and the remanufacturing module. In order to analyze the system we propose an algorithm that uses an open queueing network, decomposition principle and expansion methodology. The buffer allocation algorithm distributes a given number of available buffer slots among the remanufacturing system stations to optimize the system’s performance. The algorithm has been rigorously tested using a variety of experimental conditions. From the results, it is clear that the algorithm’s performance is robust, consistent and produces excellent results.     

Simultaneously solving the transfer line balancing and buffer allocation problems with a multi-objective approach

Two of the most researched problems on transfer line, transfer line balancing problem (TLBP) and buffer allocation problem (BAP), are usually solved separately, although they are closely interrelated. When machine tools have different reliability, the traditional balancing approaches lead to a deviation of the production rate from the actual throughput, which is used as the objective of the following optimization on BAP. This may not only reduce the solution space of BAP, but also bring about a biased overall result.In this paper, the simultaneous solution of these two problems is presented, which includes transfer line balancing problem, BAP, and selection of line configuration, machine tools and fixtures. Production rate computed through simulation software and total cost considering machine tools and buffer capacities are used as two objective functions. The problem is solved applying a multi-objective optimization approach. Two well-known evolutionary algorithms are considered: Non-dominated Sorting Genetic Algorithm-II (NSGA-II) and Multi-Objective Particle Swarm Optimization (MOPSO). A real case study related to automotive sector is used to demonstrate the validity of the proposed approach.     

Performance evaluation of supply chain networks with assembly structure under system disruptions

Global supply chain networks (SCNs) with assembly structure are common in industry. System disruptions, due to various causes, can significantly influence the performance of the whole system. Differing from previous studies on supply disruptions, this paper aims to develop a discrete time model, which could describe the characteristics of network-wide system disruptions and provide rapid performance evaluation of SCNs with assembly structure. Considering system disruptions at any stage of the whole system, we develop new iterative methods to obtain the key performance measures of SCNs with assembly structure in both lost sales and backorder scenarios. The proposed methods are verified through a series of numerical experiments. Our results suggest that component suppliers with higher reliability in the downstream stages will deliver better system performance in both lost sales and backorder scenarios.     

Allocation of maintenance resources in mixed model assembly systems

Mixed-model assembly systems (MMASs) have been well recognized for their ability to handle product variety for mass customization. With multiple products to produce, the prioritization of maintenance work-order becomes more crucial and challenging. This paper develops a quantitative method to systematically determine maintenance work-order priorities in an MMAS using on-line production information. The effect of product mix ratio on maintenance decisions is studied in detail. Based on the proposed approach, one can search for an optimal maintenance plan that will lead to improved productivity within the optimization horizon.     

An analysis of buffer sharing and prefetching techniques for multimedia systems

In this paper, we study the problem of how to maximize the throughput of a continuous-media system, given fixed amounts of buffer space and disk bandwidth both pre-determined at design time. Our approach is to maximize the utilizations of disk and buffers. We propose doing so in two ways. First, we analyze a scheme that allows multiple streams to share buffers. Our analysis and preliminary simulation results indicate that buffer sharing could lead to as much as 50% reduction in total buffer requirement. Second, we develop three prefetching strategies: SP, IP1 and IP2. As demonstrated by SP, straightforward prefetching is not effective at all. In contrast, IP1 and IP2, which prefetch more intelligently than does SP, could be valuable in maximizing the effective use of buffers and disk. Our preliminary simulation results show that IP1 and IP2 could lead to a 40% improvement in throughput.     

Genetic algorithm for supply planning in two-level assembly systems with random lead times

This paper examines supply planning for two-level assembly systems under lead time uncertainties. It is supposed that the demand for the finished product and its due date are known. The assembly process at each level begins when all necessary components are in inventory. If the demand for the finished product is not delivered at the due date, a tardiness cost is incurred. In the same manner, a holding cost at each level appears if some components needed to assemble the same semi-finished product arrive before beginning the assembly at this level. It is assumed also that the lead time at each level is a random discrete variable. The expected cost is composed of the tardiness cost for finished product and the holding costs of components at levels 1 and 2. The objective is to find the release dates for the components at level 2 in order to minimize the total expected cost. For this new problem, a genetic algorithm is suggested. The proposed algorithm is evaluated with a variety of supply chain settings in order to verify its robustness across different supply chain scenarios. Moreover, the effect of a local search on the performance of the Genetic Algorithm in terms of solution quality, convergence and computation time is also investigated.