Modelling input–output workload control for dynamic capacity planning in production planning systems

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Abstract

Workload control has been described as one of the new production planning and control concepts available for practical operations. The main principle has been defined by as to control the lengths of the queues in front of work stations on the shop floor. If these queues are to be kept short, then waiting times and hence overall manufacturing lead times will be controlled. There are four levels at which this control of queues can be attempted; priority dispatching level, job release level, job acceptance and job entry level. The first of these is a relatively weak mechanism for the control of queues if used alone. A stronger instrument, controlled job release, entails maintaining a `pool’ of unreleased jobs in the production planner's office, which are only released onto the shop floor if doing so would not cause the planned queues to exceed some predetermined norms. The main aim of workload control, for those who advocate its use as a job release method, has been defined as to control the lengths of the queues in front of work stations on the shop floor. However, the true objective is to process the jobs so as to meet the promised delivery dates with the machine and workforce capacities and capabilities available. The job release stage can itself only be fully effective if the queue of jobs in the pool is also controlled. Otherwise, jobs may remain in the pool for too long so missing their promised delivery dates. Thus a comprehensive workload control system must include the customer enquiry stage, (the job entry stage), to control the input of work to the pool as well and plan the capacity to provide in future periods so the shop floor queues are also controlled. A methodology and systems to do this at both the job release and the customer enquiry stage have been presented in previous papers. The purpose of this paper is to provide a theory for workload control in a mathematical form to assist in providing procedures for implementing input and output control. It enables dynamic capacity planning to be carried out at the customer enquiry and order entry stages for versatile manufacturing make-to-order companies. The theory shows that attention should be concentrated on controlling the differences between the cumulative inputs and outputs over time, and not the period individual inputs and outputs. Although aimed at make-to-order companies, the theory and procedures give a general capacity planning method for other production planning methods; for example determining the master production schedule in MRP systems.

Section snippets

The input–output transformation model for production systems and workload control

The input–output model is a very useful and commonly accepted way of looking at production and operations. Production is regarded as a transformation process that takes inputs and transforms them to outputs that are of a higher value than the inputs. This can be viewed at the macro system level, the whole of the organisation or the whole of the production function, or at the micro level of the individual transformation operations or activities. These latter are the work centres at which the

The basis of workload control for a single work centre

A work centre, for production planning purposes, is defined by its capacity. This is an output rate, the amount of transformation work that the work centre can perform per time period. That capacity is a rate is a crucial aspect to grasping the production situation and the problems of production planing and control. A common usage of the term capacity is the amount of some substance that a container can hold, for example the water in a bath tub. In the production situation, this water is the

Input–output control for the shop floor network of work centres at the order release stage

However, in the VMC sector of manufacturing and service customer orders turn into jobs that require processing on many work centres. The amount of transformation effort required on each work centre, the routing through the work centres and the sequence of work centres needed will differ from job to job. Jobs will arrive at and join the queue at any work centre not just from new orders but after having had transformation work carried out at other work centres. This means that there will be

The hierarchy of lead times and associated workloads.

The total delivery lead time, DLT, from the customer's point of view is the time between the receipt of the customer enquiry and its delivery to the customer. There are two other stages where jobs might be in a company that add further lead time onto the MLT just defined in addition to the time in the job pool or on the shop floor. Working backwards from the job pool, it may be a confirmed job, a bid in response to a customer enquiry that was accepted by the customer, which is awaiting the

The need for input–output control at the customer enquiry and order entry stage

There are workloads, or backlogs of work, for every work centre on the shop floor which are the direct amount of work in the queue of jobs in front of the work centre plus the indirect work of all jobs at upstream work centres and in earlier stages of the overall process. If manufacturing lead times are to be controlled then the total work load has also to be controlled, not only overall but its occurrence over time. There are four levels at which this control of work can be attempted:

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The theory of input–output control at the order acceptance stage

All the work in the company in any of the four stages for each work centre gives the total workload of work the company has to process. The time required to process all of this work, plus the processing times for the new enquiry thus becomes the delivery time that will be required for a new enquiry made at the current time, if it is processed normally without any priority and without any change to the capacities currently planned for future periods. The planned workload can now be projected

The bidding and order acceptance planning problem

The problem may be to check if a new enquiry can be manufactured within given possible delivery lead times. It is then merely a question of increasing the outputs, Wn,t, and the inputs, In,t, for the relevant work centres in the relevant time periods and checking whether there is a solution satisfying the constraints. It is necessary to ensure that the variables Zn,t, the actual amounts of work to carry out at work centre n in time t, satisfy the constraints (1)–(7) above with all being

Dealing with a new enquiry and planning capacity decisions

The enquiry may come with a fixed delivery date and merely ask for a price. In this case the first step is to check whether this can be achieved. Taking the specified delivery date, backwards scheduling through the work centres needed gives the latest release date (LRD) the job has to leave the pool. Its ERD will be this LRD minus the pool delay. The time from now to the ERD has to be checked to ensure it is sufficient to cover the customer confirmation time plus the delivery of the necessary

Setting the buffer transfer times

Deriving a good estimate of the average queuing time at work centres is necessary for the planned lead times to be a good predictor of the actuals. An initial value can be derived by management setting the maximum MLT it will offer for the largest acceptable order. Expressing this in working days, deducting the actual set-up and processing times at all work centres and dividing the remainder by the average number of work centres per job will give an initial `buffer norm'. This then needs to be

Conclusions

Production planning and order acceptance are difficult management problems in produce-to-order companies because the arrival of orders into the company is a stochastic process. The arrival of enquiries cannot be predicted in advance. Whether an enquiry turns into an order depends upon the bid the company makes in response to a customer enquiry and how it compares with bids from competitors. Furthermore, each enquiry (order) tends to be different, requiring different amounts of processing work

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