Load-Oriented Order Release (LOOR) revisited: bringing it back to the state of the art

In the workload control literature, the Load-Oriented Order Release (LOOR) approach has been neglected since its robustness was questioned at the end of the 1990s. This paper revisits LOOR and evaluates whether its performance can be improved in two ways. First, an intermediate pull release mechanism is added to avoid starvation between periodic release events. This mechanism was recently shown to be effective at improving the performance of a state-of-the-art release method known as LUMS COR. Second, an integer linear programming model is used to manage the trade-off between the timing and load balancing functions of order release. The two refinements are assessed using simulations of different shop configurations, which allow us to evaluate robustness. Results demonstrate that the refinements contribute to improving the performance of LOOR such that it can even outperform LUMS COR. Perhaps counter-intuitively, putting more emphasis on load balancing than on the urgency of individual orders is shown to lead to a lower percentage of tardy orders. Overall, the improvements mean that concerns about LOOR’s robustness are no longer valid – it now appears suitable for a wide range of shops found in practice.     

Workload Process,Waiting Times,and Sojourn Times in a Discrete Time MMAP[K]/SM[K]/1/FCFS Queue

Abstract

In this paper, we study the total workload process and waiting times in a queueing system with multiple types of customers and a first-come-first-served service discipline. An M/G/1 type Markov chain, which is closely related to the total workload in the queueing system, is constructed. A method is developed for computing the steady state distribution of that Markov chain. Using that steady state distribution, the distributions of total workload, batch waiting times, and waiting times of individual types of customers are obtained. Compared to the GI/M/1 and QBD approaches for waiting times and sojourn times in discrete time queues, the dimension of the matrix blocks involved in the M/G/1 approach can be significantly smaller.     

Judicious order acceptance and order release in make-to-order manufacturing systems

This paper explores a two-stage input control system for fixed capacity make-to-order manufacturing systems (with heavy job tardiness penalties), that selectively accepts incoming orders and holds the accepted ones in a pre-shop queue prior to releasing them to the shop floor. Single-stage input control systems that only allow orders to be delayed in a pre-shop queue (i.e. they do not allow some orders to be rejected) have been previously investigated and found to negatively impact overall due-date performance. The hypothesis motivating this research is that judiciously rejecting a subset of incoming orders can prevent the order release queue from being overloaded when a surge of demand occurs. The input control system is evaluated via experiments using a discrete-event simulation model of a fixed capacity manufacturing system. The experiments reported here suggest that holding orders in the pre-shop queue does not improve due date performance, and that judiciously rejecting orders on its own is a viable alternative mechanism of input control that can deliver improved performance.     

Healthy lower backs in the construction industry in Sweden

This corss-sectional study exmaines the relationship between healthy lower and psychosocial and physical factors in a random sample of 1773 male construction workers. Infoamtion on lower back status, lifestyle habits, stress, psychosomatic and psychological symptoms, psychosocizal demands and resoruces and physical workload were collected by means of a postal questionnaire. Measures of psycholocial and physical factors were based on factor analysis of the data. The criterion variable 'healthy lower back' (HLB)—no lifetime history of low back pain—proved to be valid compared with an interview and a physical examination. A total of 216 workers (12%) reported HLB. The prevalance rate decreased significantly with increasing age. The prevalence rate of HLB was 6% among workers reporting high stress levels. It was postively influenced when there was a balance between demands and resoruces. A low level of physical workload also increased the prevalence rate of HLB. When age, lifestyle and physical factors were kept constant in a multivariate analysis high scores on the discretions index and low scores on eh psychosomatic, psychological and stress indices cosntributed significantly to an increase prevalence rate of HLB.     

Perceived exertion under conditions of sustained work and sleep loss

Abstract

The rated perceived exertion (RPE) for treadmill walking carrying a 22 kg pack was measured in two studies with heart rate, workload and psychological measures. Study 1 used an initial treadmill speed and grade producing 40% of maximal oxygen consumption. Exercise subjects walked 1/2-h, then did other tasks for 20-h for 16 sessions on two 20 h long days. Group I had a 4 h nap, while Group 2 had 4 h of rest between workdays. Subjects maintained the 40% workload throughout the two days with a 3 h nap between days, one group starting exercise at midnight and one at noon.

A steady increase in RPE over the sessions each day was found in each study. There were no differences in RPE between the Nap and Rest groups in Study 1 or between the midnight and noon groups in Study 2. RPE was higher on Day 2 in each study. RPE correlated significantly with heart rate, distance walked or elevation at most time periods on the first workday in Study 1, but not Study 2. RPE was positively correlated with fatigue, symptoms, sleepiness and negative mood, and negatively correlated with vigor at most of the time periods day 1 in Study 2; with similar significant correlations at only one time period on day 1, Study 1. RPE was lower the first session Day 2 as compared to the last session Day 1 in both studies. This pre-post break recovery did not differ between groups in either study.     

Scheduling algorithms for computer-aided line balancing in printed circuit board assembly

Generalized flexible flow line (GFFL) is a scheduling environment comprising several machine banks which the products visit in the same order but can skip some machine banks. The type of machines in a bank can differ but they are suitable for performing the same manufacturing tasks. To change one product to another demands a set-up operation of the machine. This paper describes several scheduling algorithms for the GFFL problem. The overall structure of these algorithms is similar, consisting of machine allocation and sequencing phases. The algorithms have been integrated into an interactive production scheduling system for electronics assembly. Sample cases are used to illustrate the operation of the system in practice.     

Subjective experiences of stress workload, and bodily discomfort as a function of age and type of computer work

An increasingly older workforce and a growing emphasis on jobs involving interaction with computers have resulted in a need to more carefully examine the relationship between ageing and computer-based work. In this study, a sample of 394 subjects ranging in age from 20-75 years performed a computer task across a 3-day period. Three different types of computer-based jobs (data entry, information retrieval, and accounts balancing) performed at three large US companies were simulated. Age differences in the subjective experience of stress, workload, and bodily discomfort were evaluated. The results indicated that age effects for these measures varied according to task. The older subjects perceived greater workload for the more mentally challenging problem-solving oriented accounts balancing task (Which involved a graphical user interface) than the younger participants, even with increased exposure to the task. However, the older subjects generally experienced less stress than the younger subjects on an information retrieval task that involved a more socially interactive telephone component. A positive relationship between the frustration component of workload and the measure of stress was also found, suggesting an important link between the constructs of stress and workload. Overall, the outcomes of this study provide important insights into design interventions intended to accommodate older as well as younger persons in the workforce.     

Semi-interchangeable machines: implications for workload control

Workload control (WLC) is developed as a production planning and control concept for make-to-order job shops. Effective WLC requires a profound decision regarding order release to allow balancing the workload across capacity groups. WLC design has to reflect the company characteristics, especially machine characteristics. In practice, machines that perform the same type of operations are generally not completely identical but semi-interchangeable. The importance of the load balancing function requires careful consideration of semi-interchangeability within WLC. This paper develops different control alternatives to deal with semi-interchangeability and tests them in a simulation study. The results of the study indicate that the most intuitive control option—grouping semi-interchangeable machines into a single capacity group and making the routeing decision at dispatching—does not give the best performance, despite the advantages of pooling synergy. For a low degree of interchangeability it is more attractive to place semi-interchangeable machines in separate capacity groups and to make a routeing decision at order release. This enables more detailed load balancing resulting in shorter throughput times. More generally, it is shown to be advantageous to postpone the final routeing decision until dispatching, though considering separate capacity groups for a preliminary routeing decision at the time of release.     

Job characteristics as predictors of ill-health and sickness absenteeism in different occupational types--a multigroup structural equation modelling approach

The objective of the present study was to test for multigroup invariance in measurement models and structural models between job characteristics, psychosocial intervening variables, health outcomes and sickness absenteeism. Four types of occupation were represented in the study: blue-collar workers ( n = 241), white-collar workers ( n = 209), elderly-care workers ( n = 338) and child-care workers ( n = 336). A first-order, six-factor multigroup confirmatory factor analysis model (i.e. measurement model) composed of two perceived job characteristics ( job autonomy and skill discretion), appraised workload, job satisfaction, stress-related ill-health and sickness absenteeism provided a good model fit. Invariance tests showed that the six-factor model fits well for all occupations. A partially recursive mediated multigroup structural model showed both similarities and differences across occupations as regards the relationships between independent latent variables ( job autonomy, skill discretion), intervening latent variables (appraised workload, job satisfaction) and dependent latent variables (stressrelated ill-health, sickness absenteeism). By comparing a generic model with occupation-specific models across occupations, this study showed that occupation-specific models were more plausible. The results indicate that it is important to examine different occupational contexts in detail to better understand how certain psychosocial factors at work influence strain in different occupations. Since job characteristics can potentially be amended, the findings have important implications for the differentiation of prevention and intervention in different occupations.     

Deconstructing bottleneck shiftiness: the impact of bottleneck position on order release control in pure flow shops

Bottleneck shiftiness is an important managerial problem that negatively affects shop floor manageability. It has therefore received much research attention. Yet research has focused on how protective capacity can be used to influence bottleneck shiftiness rather than on assessing its operational impact. The latter is complex to evaluate since changing the degree of bottleneck shiftiness influences utilization, which makes the results of different experimental settings non-comparable. To overcome this problem, we take a different approach. Bottleneck shiftiness is decomposed by investigating its underlying phenomenon: the impact of the bottleneck position. Using simulation, we demonstrate that tighter control can be exercised, and better performance achieved, the further upstream the bottleneck is positioned. It is consequently important to be aware of the direction of the bottleneck shift. If the bottleneck shifts upstream, performance is likely to improve rather than deteriorate as is implicitly assumed in the literature.