Minimum Cycle Time Research Articles

Parametric algorithms for 2-cyclic robot scheduling with interval processing times

Consider an m-machine production line for processing identical parts served by a mobile robot. The problem is to find the minimum cycle time for 2-cyclic schedules, that is, schedules in which exactly two parts enter and two parts leave the production line during each cycle. This work treats a special case of the 2-cyclic robot scheduling problem when the robot route is given and operation durations are chosen from prescribed intervals. A strongly polynomial algorithm of time complexity O(m 8log?m) is proposed.

A graph-based analysis of the cyclic scheduling problem with time constraints: schedulability and periodicity of the earliest schedule

We consider in this paper a set of generic tasks constrained by a set of uniform precedence constraints corresponding to a natural generalization of the basic cyclic scheduling problem. The two parameters of any uniform constraint (namely the value and the height) between two tasks may be negative, which allows one to tackle a larger class of practical applications. We firstly study the structure and the existence of a periodic schedule. A necessary and sufficient condition for the existence of a schedule is then deduced. As there are no resource constraints, tasks following the earliest schedule have minimum average cycle times. The permanent state of the earliest schedule is characterized and we point out that the minimum average cycle times of tasks are equal for the earliest schedule and an optimal periodic schedule. An algorithm to check the existence of a schedule and to compute these minimum average cycle times using linear programming is lastly presented.

Multidisciplinary optimization of injection molding systems

The design of injection molding systems for plastic parts relies heavily on experience and intuition. Recently, mold makers have been compelled to shorten lead times, reduce costs and improve process performance due to global competition. This paper presents a framework, based on a Multidisciplinary Design Optimization (MDO) methodology, which tackles the design of an injection mold by integrating the structural, feeding, ejection and heat-exchange sub-systems to achieve significant improvements. To validate it single objective optimization is presented leading to a 42% reduction in cycle time. We also perform multiple objective optimization simultaneously minimizing cycle time, wasted material and pressure drop. Sensitivity analysis shows a large impact of the sprue diameter (>1.5 normalized sensitivity) highlighting the importance of the feeding subsystem on overall quality. The results show substantial improvements resulting in reduced rework and time savings for the entire mold design process.

Minimum “safe cycle time”: Selecting the frozen layer thickness

AbstractPredicting the minimum safe cycle time in injection molding is complicated by the cyclic nature of the process. The mold thermal state changes with continuous molding until a quasi steady state is achieved. In the first part of these series, we presented and experimentally tested a simple approach to predict part and mold surface temperature during continuous molding. In this work, we evaluate the minimum frozen layer thickness required before the part can be demolded and how the growth of this frozen layer is affected as the mold thermal state changes with continuous molding cycles. Experiments are used to relate the minimum frozen layer thickness to dimensional stability. Based on these experiments, statistical tools are used to select an ejection criterion to obtain a minimum safe cooling time. We show that the ejection criteria should be adjusted depending on the precision required from the molded parts. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

A Petri net-based heuristic for simple assembly line balancing problem of type 2

In this paper, a Petri net-based heuristic is presented to solve simple assembly line balancing problem type-II (SALBP-2). Petri net is a mathematical and graphical tool to model and analyze for discrete event systems. SALBP-2 minimizes cycle time for given number of workstations. The presented heuristic determines available tasks and assign task to current workstation by using reachability analysis, one of the main properties of Petri nets, and token movement. Solution of SALBP-2 is implemented by iteratively solving the problem for several trial cycle time. If the cycle time is infeasible for given number of workstations, the heuristic increases the cycle time by a value until finding a feasible solution. To improve the solution, a binary search procedure is implemented between the first feasible solution and the last infeasible solution. Three versions of the heuristic are developed by integrating with forward, backward, and bidirectional procedures. All of them are coded in MATLAB, and their efficiencies are tested on benchmark datasets with 302 instances. Also a comparison study is made with a direct procedure and five heuristics based on differential evolution algorithm. Test and comparison results show that the proposed heuristic obtains good results for SALBP-2. Its performance is superior especially in large assembly lines.

Design and validation of heuristic algorithms for simulation-based scheduling of a semiconductor Backend facility

A simulation-based scheduling system is discussed which was developed for a semiconductor Backend facility. Apart from usual dispatching rules it uses heuristic search strategies for the optimisation of the operating sequences. In practice hereby multiple objectives have to be considered, e.g. concurrent minimisation of mean cycle time, maximisation of throughput and due date compliance. Because the simulation model is very complex and simulation time itself is not negligible, we emphasise to increase the convergence of heuristic optimisation methods, consequentially reducing the number of necessary iterations. Several realised strategies are presented.

Design of conformal cooling channels for an automotive part

The cooling system plays a crucial role in determining the productivity and quality of an injection molding process. With the current growth of Solid Freeform Fabrication (SFF) techniques, mold designers have been striving for not only improvement in cooling system performance but also for a method to do so automatically. In this paper, a method is proposed for developing a conformal cooling system that facilitates uniform cooling over the entire mold surface with minimum cycle time. Based on the temperature distribution after the filling stage, the mold surface is split into zones which will be cooled by optimized subconformal channels obtained from the optimization process. The optimization process in which the objective function is stated as minimization of the cooling time with boundaries ensuring a realistic design will optimize the cooling system layout in terms of cooling channel size and location. Finally, all subcooling channels are combined to generate the entire conformal cooling system for the injection mold.

Minimizing cycle time in the manufacturing system based on the flow of various jobs

The paper deals with cyclic production system providing a mixture of various products, each of which is manufactured by unique sequence of operations on machines called job. The aim is to find the cyclic schedule of minimal cycle length under “no store” policy. The problem belongs to scheduling area, is called the job shop scheduling problem with no-store constraints, is strongly NP-hard, and has been considered very rarely in the literature. In this paper there are shown: mathematical model of the problem, some of its theoretical properties, two-level solution method with special algorithm based on paths in a graph on the lower level and TSAB-relative procedure on the upper level.

A Parametric Algorithm for 2-Cyclic Robotic Scheduling with Interval Processing Times

Consider an m -machine production line for processing identical parts served by a mobile robot. The problem is to find the minimum cycle time for 2-cyclic schedules, in which exactly two parts enter and two parts leave the production line during each production cycle. This paper treats a case of 2-cyclic robot scheduling in which the robot route is given and the operation durations are to be chosen from prescribed intervals. A strongly polynomial algorithm of complexity O ( m 4 ) is proposed.

Optimal robot placement using response surface method

This paper is concerned with a new approach for optimal placement of a prescribed task in the workspace of a robotic manipulator. The approach is resulted by applying response surface method on concept of path translation and path rotation. A robotic optimization tool based on this approach is developed as an add-in to RobotStudio. The methodology has been carefully verified by optimizing the position of several kinds of industrial robots and paths in four showcases to attain minimum cycle time. The results indicate that an increase in productivity up to 37%, compared to the admissible location with the highest cycle time, is achieved by optimally positioning the task in the robot workspace.

Evaluation of performance measures for representing operational objectives of a mixed model assembly line balancing problem

Performance evaluation of a mixed model assembly line is complicated in nature. For some configurations of the mixed model line, operational objectives can directly be evaluated; however, in others some performance measure is required for evaluating the operational objective. This paper reports a comprehensive comparative study of performance measures as to their relevance in representing the operational objectives of a mixed model assembly line balancing problem of type II. Two performance measures are proposed in this work. A genetic algorithm has been developed for comparing the proposed measures with eight others in an effort to investigate their ability to equalize workload along with minimizing cycle time. Through statistical analysis, one of the proposed performance measures has been established to perform better than the others. Application of the performance measures to balance an assembly line taken as part of an industrial case study also corroborates the results obtained by statistical analysis.

An efficient approach for type II robotic assembly line balancing problems

In the past decades, robots have been extensively applied in assembly systems as called robotic assembly lines. When changes in the production process of a product take place, the line needs to be reconfigured in order to improve its productivity. This study presents a type II robotic assembly line balancing (rALB-II) problem, in which the assembly tasks have to be assigned to workstations, and each workstation needs to select one of the available robots to process the assigned tasks with the objective of minimum cycle time. An innovative genetic algorithm (GA) hybridized with local search is proposed for the problem. The genetic algorithm uses a partial representation technique, where only part of the decision information about a candidate solution is expressed in the chromosome and the rest is computed via a heuristic method. Based on different neighborhood structures, five local search procedures are developed to enhance the search ability of GA. The coordination between these procedures is well considered in order to escape from local optima and to reduce computation time. The performance of the hybrid genetic algorithm (hGA) is tested on 32 rALB-II problems and the obtained results are compared with those by other methods.

A simplified approach to predict part temperature and minimum “safe” cycle time

AbstractThe largest component of the injection molding cycle is the cooling time. Thus it is highly desirable, to be able to predict what its minimum acceptable value can be. This is becoming even more important with increased competition from regions where the labor cost is low. In an injection molding operation, the mold thermal state changes from its initial value until a quasi steady state is reached. The minimum required cooling time increases with continuous molding until a steady state value is achieved. Improving the mold thermal design will decrease the cooling time thus reducing total cycle time. The overall goal of this work is to develop a simple reliable method to predict a minimum safe cycle time for steady production. In this article, we discuss software capable of simulating the thermal state of the part and mold for multiple injection molding cycles while balancing simulation time with results accuracy. Three case studies are presented, one done in our labs and two on‐site at an automotive manufacturing facility. The three case studies are used to evaluate the ability of the software to predict part surface temperature for continuous molding. We also discuss how this software can be used as the basis to establish the minimum safe cycle time. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Cycle time variance minimization in dynamic scheduling of single machine systems

In this paper, we study the minimization of variance of cycle times in a dynamic single machine system where jobs arrive continuously over time. Numerous production systems give rise to single machine models, and a multiple-machine environment where the performance of a bottleneck machine determines the performance of the entire system reduces to a single machine problem [16]. Minimizing cycle time variance helps in safe predictions of the completion of job production and thus in providing the same quality of service to the customers. This allows an improved ability to meet the due dates reliably, and thus the greater coordination with further downstream operations on the jobs, as highly preferred in semiconductor manufacturing. Scheduling the bottleneck process to minimize the time of presence of the jobs in process minimizes the deterioration of cycle time-related performance measures. Low values of cycle time-related measures are also preferred for low risk of wafer contamination associated with it during processing. New scheduling rules are developed to minimize both the cycle time variance and the maximum cycle time for single machine system, wherein the machine/process is always utilized to the maximum extent and in the extreme case is heavily loaded with jobs for processing. The performance of the proposed rules is compared with the rules available in the literature and the results are presented for the objectives of minimizing cycle time variance and maximum cycle time at higher levels of machine utilization.

The use of the simplex method and its derivatives to the on-line optimization of the parameters of an injection moulding process

The sequential simplex method and its derivatives, the modified and the super modified simplex methods, are used to optimize on-line the levels of the parameters of an injection moulding process (producing bottle caps) in order to minimize the process cycle time and to reduce the rejects. Nine process parameters are selected to conduct the optimization: screw rotation speed, back pressure, injection rate, injection pressure, cooling time, holding pressure, holding time, nozzle temperature and opening stroke. Optimum conditions are achieved using 17 experiments only. The obtained results clearly show that the process cycle time is very sensitive to the effect of two parameters: cooling time and holding pressure. Low levels of these two parameters are needed in order to minimize cycle time. In addition, the evaluation of rejects in the course of the simplex evolution shows that this additional response is also sensitive to the same parameters. In order to minimize rejects, these two parameters have to be set to low levels. Finally, this study allows us to conclude that the simplex method may be easily used and reused on-line to drive the process continuously towards its optimal conditions without any disruption of production or additional operating costs.

A polynomial algorithm for 2-cyclic robotic scheduling: A non-Euclidean case

In this paper we consider the problem of no-wait cyclic scheduling of identical parts in an m -machine production line in which a robot is responsible for moving each part from a machine to another. The aim is to find the minimum cycle time for the so-called 2-cyclic schedules, in which exactly two parts enter and two parts leave the production line during each cycle. The earlier known polynomial-time algorithms for this problem are applicable only under the additional assumption that the robot travel times satisfy the triangle inequalities. We lift this assumption on robot travel times and present a polynomial-time algorithm with the same time complexity as in the metric case, O ( m 5 log m ) .

A universal DC characterisation system for hard and soft magnetic materials

A fully automatic system has been designed for the accurate measurement of the DC magnetic properties of soft and hard ferromagnetic materials utilising discrete calibrated instruments in order to provide a traceable calibration route separate from the transfer of standard magnetic test samples. Custom written software is used to operate the system in one of three modes, constant d H/d t, variable d H/d t and a second quadrant demagnetisation curve mode. The first two of these modes are utilised for soft magnetic materials with the second mode varying d H/d t in order to keep d B/d t relatively constant. Both modes use cycle times of between 60 and 300 s and may utilise a variety of test configurations including a bar permeameter, electromagnet, ring samples or Epstein frame. The minimum cycle time and the most appropriate mode is dependent on the particular sample and the effect of this on materials with a large d B/d H is significant. Measurements on soft materials include major BH loop, minor BH loops, first-order reversal curves, remanence, coercivity, normal magnetisation curve, peak permeability and loop area. The third mode is used with an electromagnet to measure the demagnetisation curve of hard magnetic materials up to a maximum demagnetisation field of 1.6 MA/m. The measurement algorithm modulates d H/d t depending on d B/d t and incorporates holdback in order to accommodate rare earth materials which exhibit high viscosity.

The reliable design of one-piece flow production system using fuzzy ant colony optimization

In this work, one-piece flow production system is designed with the purpose of ensuring just-in-time production. Three approaches are applied to achieve the goal: adopting straightforward schedule policies, relaxing the Takt time and decreasing the risk of machine failures and operator mistakes. Consequently, a multi-objective design model is proposed, whose aim is to minimize cycle time, changeover count, cell load variation and the number of cells and maximize the extent to which items are completed in a cell. The fuzzy ant colony optimization (FACO) is also presented to solve the formulated problem. In FACO, the fuzzy logic controller (FLC) is used to adapt the evaporated and deposited value of pheromone trail based on the ant's fitness and pheromone trail age. Furthermore, domain knowledge of facility layout, generated based on the travel chart method, is also adaptively injected to improve the performance of FACO. The proposed method is evaluated with the real-world data and experimental results demonstrate that our method outperforms many other methods in efficiency, solution quality and facilitation measures.

A New Instrument for Real-Time Ethernet Performance Measurement

Ethernet is now becoming a popular physical layer for industrial communications due to its reliability, performance, and widespread use. Its actual cutting edge is a minimum cycle time on the order of 100 mus and a jitter of 1 mus. Usually, high-cost laboratory devices are used to log network traffic, but they are still not suitable for an industrial environment. In this paper, a field-programmable-gate-array-based instrument is proposed to log 100-Mb/s Ethernet frames, assigning them a high-resolution time stamp. Full-duplex full-rate network traffic is duplicated and sent through two auxiliary Ethernet ports to a personal-computer-based monitor station for frame analysis. A prototype has been realized to verify feasibility and performance. Results show that the proposed instrument introduces a maximum delay of about 500 ns. The measurement capabilities are comparable with commercial instruments: The time-stamp assignment has a jitter of 150 ns measured on a test frame set.

Process planning for circuit card assembly on a series of dual head placement machines

This paper proposes an approach for prescribing the inter-related decisions that prescribe a process plan for a series of dual head placement machines. The goal is to provide a means of rapidly prescribing a process plan that seeks to minimize cycle time (equivalently, maximize throughput rate) by balancing workloads assigned to heads when assembling a given type of circuit card. Our approach decomposes process planning decisions into four related problems. This paper explores list-processing heuristics for the first of these problems and adapts an optimizing method for the fourth. It also integrates these two methods with others to optimize the second and third of these problems, reporting computational tests that evaluate the overall approach to all four problems and assessing the degree to which the decomposition approach is able to balance workloads assigned and the run time required to do so. Resulting cycle times are also analyzed. These test results demonstrate that the overall approach provides a means of prescribing effective process plans within a run time acceptable to process planners.