Dynamic Scheduling for Flexible Manufactures - Introduction

Introduction

All possible quality criteria of the first SCOR (Supply Chain Operation Reference) model level relate to one of the four categories: customer service, economical indices, demand satisfaction flexibility, product development.  The criteria of the last category are not usually considered in production planning, and demand satisfaction flexibility is ensured by strategy selection – “make-to-stock” or “make-to-order”. Therefore production planning quality mainly depends on the customer service level and production cost. The high customer service level (efficiency) may only be achieved through timely order completion. However, prompt order completion contradicts a high level of utilization and increases expenses. This trend is known as the dilemma of operation planning [Nyhuis and  Wiendal, 2008]. Below we will examine some properties of industrial schedules, which are essentially important in the modern supply chain models.

Dynamic Scheduling

Dynamic of shop scheduling is caused by the total set of events realized in the industrial shop during a plan period. Let us enumerate such event’s kinds.

• Receipt of a task with the job list for the directive plan horizon. The task includes estimated arrivals for pieces and materials. Jobs at the task usually are connected with customer orders, which are realized at the enterprise or may be concluded in the future.    
• Actual arrival of pieces and materials, which are provided by the task, into the shop.
• Arrival of pieces and materials to the machine for processing.
•  Operation completion at the machine.
• Processing order arrival, which is immediately supplied with pieces and materials.
• Sudden events connected with breakage, staff absence, material spending and so on.

Among these events, only the task arrival and its data are strictly deterministic. Other events are either completely casual or having traits of casualness. Usually one out of two existing principles for shop scheduling is applied. At the first chance, the methods based on the job priority are used. At the second chance, there are different algorithms for scheduling with heuristic methods of decision search.

The plan task allows shop managers to elaborate a production schedule taking into account fulfillment of current jobs and plan dates for pieces and materials arrivals for new jobs. The notion “plan period”  G is equal to some of working days. After the expiration of G, production schedule has to be calculated regularly. Such recalculation is obligatory to take account of accumulated changes in production situation, even when schedule is realized completely. In this paper, the plan period is recommended to be equal to a working week - usually 5 working days.

Existence of schedule does not determine its complete and suitable fulfillment, but it is extremely useful for industrial process preparing and organization.  The schedule is calculated for a period, which is limited with plan horizon H in calendar hours. Plan horizon usually is equal or exceed plan period G. At the latter instance, the plan has be inevitably corrected. Correction also may be carried out at the every moment of plan execution owing to arising unforeseen circumstances. 

Dynamic scheduling must be associated with the working calendar of the shop. It is necessary to kick-off the counting during scheduling from zero at the first plan calendar day. The commence moment of scheduling fulfillment here is equal to the moment of the first working day in format 18/09/2016 8:00 am. The shop plan staff   usually works at the first (day) shift and elaborates or corrects the schedule for the next working day.

The scheduling calculation is always carried out with some accuracy. This paper recommends that owing to inaccuracy of initial data the time parameters have to be rounded off to 0.1 hour, or 6 minutes. So, value of 6 minutes is the time quantum during scheduling.

Quantity of working shifts is a main parameter of dynamic scheduling. Here it is supposed that the first shift begins at commencement of a working day, which is usually equal to 8 a.m.  Duration of the working shift may be changed depending on industrial process. The most common duration is equal 8 hours, but it is not mandatory. When shop is loaded with work, two shifts both equal to 8 hours can used. If load is too big, or technology forbids interruptions, twenty-four-hour work may be applied. It may be the version of 3 shifts by 8 hours, or 2 shifts by 12 hours, or any other variant.  

It is necessary to note that at twenty-four-hour work only the first shift applies to the current calendar day entirely. When a shop works with two shifts by 8 hours, the second shift finishes at the end of a calendar day too.  However, if a shop works in 3 shifts by 8 hours, the last shift is working at the beginning of the next calendar day.

Sometimes it is necessary to take into account the possibility of duration changes in certain shifts. For instance, in pre-holiday or pre-rest day, the last shift (or even the single) shift may be shortened or cancelled. At the work day, which follows the rest day, commencement of the first shift usually coincides with the fixed commencement moment of the enterprise.

During a shift, the work interruptions may be introduced for food, sports or otherwise. As duration of such breaks is alike to original data accuracy, these breaks may be overlooked.

Flexible manufacturing

Flexible manufacturing facilitates to management of diverse changes at production surroundings.  This manufacturing has some special properties:

• objects of production must have accurate identification system;
• manufacturing should be easy controlled by the planning and dispatching system;
• quick readjustment of manufacturing should be implemented.

There are 7 kinds of flexibility: operation flexibility of the single machine; flexibility of manufacturing for various modifications of the produces; possibility for production output alternation; possibility for conversion to a new produce; flexibility of technological operation sequence; flexibility of the output value; service flexibility for the ready produces.

Flexibility of a certain machine using depends on duration of changeover, on complication of elaboration and input of computer program for operations, and, to considerable degree, on staff qualification. Peculiarities of machine design have great importance, which allowed various operations for produces of diverse sizes and volumes.  

Necessity of manufacturing for various produce modifications is caused by the sparse of needs for various customers. In this case, production flexibility depends on the resources of production designers and managers, and on operational flexibility described above.

Alternation of production output of various kinds is essentially determined by production batch values. The lots and their sequence are the main task of production planning, and this paper is dedicated to such tasks.  

Possibility for conversation to a new produce is generally and primarily estimated by economical point of view. Here, it is necessary to into account the means of production design, the equipment availability, assimilation difficulties and financial resources.

Flexibility of technological operation sequence exists when there is sufficient quantity of equipment, which is able to carry out the necessary technological operations. Besides, such equipment has to be provided with corresponding tools, materials and so on. When computer numerical control is used it has to be supplied with needed computer programs.

Change of the output value may be implemented when the conditions for sale are altered. There are some factors for such changes:  equipment availability; sufficient staff number; financial resources of the enterprise. The problem of the output value may be not only the extension but diminution of volume.  In such a case, the special arrangements are needed for staff and for equipment.   

After production, the enterprise is often obliged to service the produced items. Flexibility here depends on cost of the special stations and working places for service, on personnel training, transportation, special apparatus and so on.

As a rule, it is impossible for an enterprise to achieve all kinds of flexibility simultaneously, so there is the problem of choice for the main aspects. When great flexibility is achieved, speed of production reaction on external influence essentially grows. In such a case, we have the agile manufacturing. 

Group methods in scheduling

For the most types of manufacturing there are some problems listed below:

1. timely order fulfillment;
2. reduction of throughput time;
3. reduction of changeover time;
4. diminution of transportation cost;
5. sufficient equipment load;
6. even staff load;
7. increase of material using coefficient.

Let us to learn these questions in detail.

It is often assumed that the main task for a shop is to not frustrate the given due dates of manufactured objects for following treatment and assembling. In reality though, the enterprise management is usually aimed at tardiness revealing, and after this, to applying certain corrective actions over such contractors. In the same time, manufacturing before the due date is never controlled. However, in such a case the manufactured objects are kept in a warehouse for a long time, and this fact furthers immobilization of working capital and cluttering of a shop. Therefore we have to strive not only for attainment of due dates, but to make pieces, parts and assemblies at the moments near due dates.  This method is known as Just-in-Time, which proved really useful in large-scale production. 

Reduction of the throughput time depends on the decrease in the pre-processing time and transport time during manufacturing. Issues listed as c) and d) are caused by aspiration to diminish labor costs on processing and transportation. When these costs increase, the manufacturing duration, the throughput time and the total cost increase too. Said labor costs are spent on the each technological or transfer batch, so decreasing of batch number costs to go down.

In many cases, especially when the new expensive equipment is applied, enterprise managers strive to increase the machine load to pay the expenses.  Of course, any equipment may not be loaded by 100%, but there are such attempts sometimes. It is reasonable to follow the policy to load the equipment sufficiently (70-80 %), allowing for time intervals for the working task corrections and for the systematic maintenance.

Regards for staff during manufacturing has great influence over shop work. It is necessary to take into account the natural wishes of workers for the high and stable wages, even engagement over a week and a working day and to avoid the tiredness caused by prolonged monotonous work.  Unfortunately, an enterprise working at the market is subjected to demand variations for its production, and this factor inevitably leads to changes of staff quantity.    

The result of any production is not only ready produces, but waste too. Of course, waste amount has to be as low as it is possible. For instance, the rational cut cards may be used. However, if the production plans change often and abruptly, the cards elaborated beforehand quickly become outdated.

As a result, we must note that majority of the issues identified, namely b), c), d), e), g), are economical in nature, i.e. production costs. Indeed, increase of labor amount in b), c), d) immediately entails to  wages increase; insufficient equipment load in e) entails to depreciation costs increase; taste in g) entails to  material costs increase.

At the same time, need for timely order completion in a) cannot be reduced to the costs only. Any attempts for so-called “penalty for delay” in scheduling are subjective and, as a rule, do not correspond to the current reality.  The issue in f) is of social and psychological kind and cannot be reduced nor to economic or any other aspects.

Moving to the methods to solve said issues we must note that timely order completion, which provides the high efficiency rate, may be attained usually at the expense of batch diminution. However, this way contradicts to request of low production costs.  Aspiration for improvement of these two indices simultaneously is “dilemma of planning” mentioned above.  

As improvement of the cost criteria indices always makes the efficiency criteria go down, we, as a rule, cannot receive the single solution by optimization methods. Therefore, computing can only achieve the set of non-dominated (Pareto) solutions, and the user has to make the final decision.  For this aim it is convenient to use the decision support systems, which are able to simulate scheduling at various input parameters and to find most suitable version for the current situation.    

The main method for described tasks solution is the group planning. It includes calculation of rational production batches and elaboration of plans and schedules. Size of lots and their sequence are essentially depends on order volumes and due dates. This connection may be briefly formulated as:  manufacturing is made by batches, but shipping is made by orders.

The group planning is the natural development of the group industrial technology. The latter one is the conception of production and organisation which uses likeness of construction and technologic parameters of various objects. The grouping is made when similar objects are found in the set. The similar objects are joined into the families, and within the family objects are classified by various traits. The lowest level, the family division always ends with an object type: this is a set of objects in a family, which has very near technological parameters in current manufacturing.

Group technology allows considerable possibility in decreasing of changeover time and cost. However, it is not always realized. The degree of group technology using is defined by perfection of the current calendar planning.

Attempts for group technology were made during the last 80 years, and they resulted in successful production when manufacturing was stable. However, in the last decades there are quick changes within all fields of human activity, and in production activity too. Therefore, grouping of working tasks now may only be effective if it is made on-line or just moments before the manufacturing starts.

Now, at each planning on any level there is a new set of working tasks. Therefore, technology   grouping demands to calculate a corresponding plan quickly, and it is now possible owing to computers accession.   The most simple plan grouping may be designed for types of objects, for example, on the construction designations. In particular, when the production plan is elaborated for some part batches with designations distinguished by versions these batches may be associated with the common type and possibly grouped. 

Group planning may be carried out on the inter-shop level and on the intra-shop level. This paper shall cover scheduling inside the shop only. For this aim dynamic schedules are elaborated with two criteria simultaneously: the first criterion is the timely order completion, the second criterion is the costs of processing and changeovers.

The remainder of this paper is organized as follows. Section 2 provides order parameters the paper is based on.  Section 3 determines the scheduling tree.  Section 4 describes criteria for cutting of non-perspective branches. Section 5 formulates the standard scheduling algorithm. Section 6 is dedicated to brief description of the designed computer programs. In Section 7 some program examples are described. Section 8 contains some concluding remarks.