Hermann Lödding

Decentralized WIP-oriented manufacturing control (DEWIP)

DEWIP is a manufacturing control system for job shop environments aiming at achieving short and reliable lead times by establishing WIP control loops between the manufacturing work centres. The paper describes the mode of function, the setting of parameters and simulation results of the new manufacturing control system. The setting of parameters is done with the aid of the funnel model and the theory of logistic operating curves, both developed at the Institute of Production Systems at the University of Hanover. The simulation is conducted using industrial data and makes it possible to assess DEWIP with regard to lead times, WIP level, performance and schedule reliability. DEWIP is compared both with an uncontrolled process and with the manufacturing control systems Load oriented order release (LOOR), Conwip and Polca. The results suggest that DEWIP and the models employed for the setting of parameters are suitable for job shop production and therefore offer a valuable alternative to prevailing centralized manufacturing control systems.

Modelling lateness and schedule reliability: how companies can produce on time

Schedule reliability is a very important target in operations management: if a company cannot produce and deliver on time, it has to make up for it by incurring a penalty on delays and using high cost express delivery. If that option is not available, customer dissatisfaction is inevitable. When the customers production system operates with low inventories and just-in-time deliveries, low schedule reliability of the suppliers will eventually ruin that customers schedule reliability as well. In spite of this high importance, industry analyses show that the schedule reliability of many factories is quite low: a part of the orders is completed late and endangers the delivery reliability of the company; other orders are completed before the scheduled date and increase finished goods inventories. This article shows how lateness can be modelled and it describes the application of the models using an industry example. Appropriate measures to enhance schedule reliability result from the analysis.

Modelling schedule reliability

The significance of schedule reliability as a particularly important logistic objective is indisputable. In comparison to the objectives WIP, utilisation and throughput time, the modelling of the schedule reliability is, however, not very advanced until now. This paper transfers the process capability indices used in quality management to schedule reliability and demonstrates two methods for depicting the schedule reliability as a function of its influencing parameters.

INCAP – applying short-term flexibility to control inventories

Inventory based capacity control (INCAP) is a very simple method that allows inventory levels to be effectively controlled by using short-term capacity flexibility in make-to-stock settings. Moreover, INCAP can be used for finished goods inventories as well as for semi-finished goods inventories. The basic idea is to define upper and lower inventory limits and to adjust capacities if the inventory level reaches either limit. Should the inventory fall below the lower limit, capacity is increased to prevent stock-outs. Similarly, in order to avoid excess inventories the capacity is decreased should the inventory rise above the upper limit. INCAP is thus able to control inventory levels within a defined inventory range. In order to do so, it applies short term measures like over-time or extra shifts to increase capacities and cancels work hours or shifts to decrease capacities. Simulation experiments based on data from the automotive industry show that INCAP is able to improve the performance of a manufacturing system in that it significantly reduces the inventory levels necessary for guaranteeing a satisfactory service level. Overall, INCAP is found to be a straightforward but powerful method, able to cope both with uncertainties in production output as well as with varying demand. However, some restrictions do apply: INCAP depends not only on a minimum level of short-term capacity flexibility, but also in the standard set-up presented here it is limited to make-to-stock environments with similar products.

State-Oriented Productivity Analysis in One-of-a-Kind-Production

Traditional productivity analysis has emerged in mass production and cannot be adopted one-to-one on One-of-a-kind production (OKP). Due to the non-repetitive character of the processes in OKP, productivity improvements do not reproduce like in mass production. In addition, preparatory activities such as orientation, material handling and positioning usually consume a lot more time than the actual value-adding activities in OKP. Therefore, OKP requires analysis methods that deliver: 1) a generic working cycle to enable repetitive productivity improvements; 2) activities of personnel in production processes, which include the preparatory activities. We introduce a state-oriented approach for productivity measurement in OKP. With a case study we show how to capture, visualize and evaluate state data of an OKP.

A manufacturing control model

Be it teaching or discussing logistic target attainment with practitioners, there is often a confusing gap between the manufacturing control methods applied on the one hand and the logistic objectives on the other hand. Which method is appropriate to increase on-time delivery, which to decrease the WIP? When configuring manufacturing control the discussion often quickly turns to choosing one method over the other and too often the results are disappointing. In order to provide a framework for a more systematic approach, a model has been developed that links the tasks of manufacturing planning and control with the logistic objectives.

The surprising effectiveness of earliest operation due-date sequencing

Abstract In make-to order production, schedule reliability is very important but still not sufficiently accomplished in industrial practice by the vast majority of companies. It has been known for long that processing the orders at a workstation in the order of their operation due-dates can compensate for lateness in the arrival at the workstation within certain boundaries. The paper analyses the effectiveness of earliest operation due date (EODD) sequencing by comparing it to an optimistic theoretical boundary. The surprising result is that EODD can nearly fully exploit the theoretical potential. It should therefore be used in practice whenever schedule reliability is important, with only few exceptions. Its effectiveness though is increasing with the workstation’s WIP level and thus is in conflict with the objective to reduce WIP levels and throughput times. A simple forecasting model allows to assess the extent to which lateness can be compensated by EODD sequencing and which schedule compliance can be achieved.

Bewertung des Aufwands von Lean-Methoden

Kurzfassung Lean Production-Methoden sind in der Praxis weit verbreitet. Unternehmen bewerten das Aufwand-Nutzen-Verhältnis der Methoden jedoch teilweise kritisch. Angesichts dieser Beurteilung stellt sich die Frage, wie sich der Aufwand von Lean-Methoden bewerten und reduzieren lässt. Dieser Beitrag beschreibt, wie Unternehmen ihren Aufwand für die Anwendung von Lean-Methoden strukturiert beurteilen und vermindern können.

An approach of capability representation for improving capacity planning

With the growing interest in dealing with the volatile demand fluctuations, there is a renewed interest to enhance responsiveness in capacity planning and, in particular, to address the dynamic changes in product mix. However, with traditional MRP-II and ERP systems, capacity planning assumes that all machines in the work system are either the same or completely different. Similarities among products and resources (such as production equipment) are not a part of the consideration. As a result, inherent flexibility of the system is lost in the planning process. In this paper, we propose a new representation for the capability by characterizing critical capability drivers and their range. With a more refined representation, overlapping and non-overlapping capability regions can be identified, and their inter-relationships with respect to capacity can then be applied for improving the match between the capacity and production orders. Thus, better planning and capacity utilization can be achieved. An example is presented to demonstrate the proposed approach and to compare it with traditional MRP-II planning.

Modeling Lateness for Workstations with Setup Cycles

Sequence-dependent setup times force companies to bundle similar products to avoid setup efforts. While this increases the output rate the schedule reliability tends to decrease due to the sequence deviations enfaced by this sequencing policy. Our paper presents a model to predict the impact of different strategies for setup-optimized sequencing and their actuating variables on the sequence deviation. Through this it enables a positioning in the trade-off between a high output rate and a high schedule reliability.