van Hpg Henny Ooijen

Workload based order release and productivity : a missing link

In this paper we propose an extension of the job shop models that are used in the theoretical studies, that can explain the differences between the results of theoretical studies and empirical studies into the effects of workload control on performance. The extension assumes that the workload level affects the effective processing times in a shop and is based on the results of empirical research on the relationship between workload and shop performance. Using computer simulation we investigate the effects of workload based work order release on the shop performance under the new job shop model. The simulation results confirm the effect of workload control on shop performance found in empirical studies, and show that, under certain conditions, without workload control the shop performance can become unstable

Economic due-date setting in job-shops based on routing and workload dependent flow time distribution functions

The trade-off between the length of the lead times quoted to the customers and the delivery reliability has been investigated by many authors. However, only a few studies do this in an economic setting. In this study, the setting of cost optimal due dates taking into account lead-time related and tardiness related costs is investigated. More specifically, in setting the internal due dates, which are used for determining the priorities on the shop floor, and in determining the expected order flow time probability density functions which are used for setting the external due dates, the work load is taken into account. From this study, it follows that this approach leads to (much) lower costs as compared to the situation with workload independent order flow time p.d.f.s.

The effects of a simple arrival rate control policy on throughput and work-in-process in production systems with workload dependent processing rates

In this paper we investigate the control of throughput and work-in-process (WIP) in job shops where operations processing times in the work centers depend on the workload in the shop. We assume that production efficiency is high if the workload in the shop is such that the workers experience a stimulating work pressure. For higher and for lower workload we assume that the production efficiency is lower because either the workers experience a too low work pressure, or a too high work pressure. Such production systems are unstable if the arrival rate is exogenous and larger than the order completion rate under high workload. Therefore a high throughput can only be obtained if the order arrival rate can be controlled in response to the workload in the system. In this paper we investigate the effect of a simple workload dependent arrival rate control policy on the throughput and WIP of a simple model of a job shop. We use numerical analysis of a queueing model to investigate the performance of the shop under various combinations of parameter values in the arrival control policy.

Integrating material coordination and capacity load smoothing in multi-product multi-phase production systems

In all papers on capacity load smoothing today, the production situation is always studied in isolation, i.e. not considering it as part of a manufacturing chain. If we do not study it in isolation the materials aspect will play an important role. As well the availability of components for the next production phase as the availability of materials might influence the structure of the load-based work-order release rule used. In this research we have investigated this by not only retarding the release of work-orders but also advancing work orders, which in general is possible due to a surplus of material. We called this proactive load-based work-order release. Furthermore, we distinghuished two types of components, critical and non-critical components, leading to what we called selective load-based work-order release. The effects on the delivery performance and the inventory holding costs have been investigated by a simulation study. It turns out that proactive release is better than conventional load-based work-order release but still worse than immediate release. Selective load-based work-order release might lead to a positive effect on the inventory holding costs, however this depends on the ratio of the inventory holding costs for the critical and non-critical components.

Customer order lead times for production based on lead time and tardiness costs

In practice decisions regarding customer order lead time and customer order processing are taken in different parts of the organization. Definite lead times are quoted by the sales department and job flow decisions are taken by the production department. This split up of decision responsibilities is in accordance with the differences in scope and control areas in complex organizations like job-shops or engineer-to-order firms. In this paper we present a simple economic model of sales and production in a job shop. Production aims at realizing the jobs by using its capacity to perform the operations of the jobs. Sales, using a model of the performance of production, aimes at maximizing expected profit by quoting lead times to customer orders, given that penalties are put on quoting long lead times and on tardy deliveries. Analysis of the model shows that for any combination of penalties on lead times and tardiness, there exist job flow times distributions for which it is optimal for sales to quote unrealistically short customer order lead times. Specifically if the job flow time has a high mean value and/or a high variance sales tends to quote a lead time with zero lead time penalty. In practice we often observe that the job flow times are long and have a high variance. The research presented in this paper shows why this may stimulate sales to neglect flow time information and to quote unrealistically short lead times. Our model shows that under a wide range of economic conditions a job flow time distribution with a small mean and a small variance seems to be a necessary condition for sales to quote realistic customer order lead times.

Delivery performance improvement by controlled work-order release and work-center load balancing

The concept of load-based work-order release has been studied by many researchers. The general finding is that controlled work-order release in combination with a FCFS release rule (earliest release date) leads to a poorer delivery performance as compared to immediate release. The main reason for this is that this type of release policy leads to unnecessary resource idle times. In this research we investigate a more sophisticated form of controlled work-order release. As reference point, we use an immediate release situation with Poisson order arrivals times and a general order lead time equal to the average work-order flow time under immediate release. In particular, we investigate release rules which give priority to orders that require resources which currently are underloaded and delay orders which require resources that are overloaded. The research shows that this type of release policy can lead to a better delivery performance than immediate release.

Flow rate flexibility in complex production departments

Consider a production situation where it is desirable that the production orders of different product types have different flow rates, independent of their production characteristics such as processing times. Flow rates are mainly determined by the work centre waiting times, thus the product types should have different production order waiting times at the work centres. In this paper we derive a method which makes this possible in a controlled way: given certain values for some parameters the different waiting times (and thus the flow rates) are predictable, By means of simulation this method has been tested for a production situation where only two different flow rates are required per work centre. This simulation study shows that, using a simple balance equation and operation due-date sequencing, it is possible to create different predictable flow rates. It turns out that until a required waiting time reduction for one of the (categories of) products of about 60% has been achieved a one-to-one relation ...

Controlling different flow rates in job-shop like production departments

Abstract We consider production departments where it is desirable that the production orders of different product types have different flow rates, independent of their production characteristics like processing times. Since flow rates are mainly determined by the work centre waiting times the product types should have different production order waiting times at the work centres. In this paper we derive a method which makes this possible in a controlled way: given certain values for some parameters the different waiting times (and thus the flow rates) are predictable. By means of simulation this method has been tested for two production situations where, per work centre, only two different flow rates are required. In the first situation both (categories of) products have the same average routing length, whereas in the second situation the average routing length for both (categories of) products differ. This simulation study shows that, using a simple balance equation and operation due date sequencing, it is possible to create different predictable flow rates. It turns out that up to a required waiting time reduction for one of the two (categories of) products of about 60% a one-to-one relation approximately exists between the scheduled (required) waiting time reduction and the what will be called, normalized waiting time reduction.