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PRODUCTION PLANNING

Production planning is the function of establishing an overall level of output, called the production plan.


The process also includes any other activities needed to satisfy current planned levels of sales, while meeting the firm's general objectives regarding profit, productivity, lead times, and customer satisfaction, as expressed in the overall business plan. The managerial objective of production planning is to develop an integrated game plan where the operations portion is the production plan. This production plan, then, should link the firm's strategic goals to operations (the production function) as well as coordinating operations with sales objectives, resource availability, and financial budgets.
The production-planning process requires the comparison of sales requirements and production capabilities and the inclusion of budgets, pro forma financial statements, and supporting plans for materials and workforce requirements, as well as the production plan itself. A primary purpose of the production plan is to establish production rates that will achieve management's objective of satisfying customer demand. Demand satisfaction could be accomplished through the maintaining, raising, or lowering of inventories or backlogs, while keeping the workforce relatively stable. If the firm has implemented a just-in-time philosophy, the firm would utilize a chase strategy, which would mean satisfying customer demand while keeping inventories at a minimum level.
The term production planning is really too limiting since the intent is not to purely produce a plan for the operations function. Because the plan affects many firm functions, it is normally prepared with information from marketing and coordinated with the functions of manufacturing, engineering, finance, materials, and so on. Another term, sales and operations planning, has recently come into use, more accurately representing the concern with coordinating several critical activities within the firm.
Production planning establishes the basic objectives for work in each of the major functions. It should be based on the best tradeoffs for the firm as a whole, weighing sales and marketing objectives, manufacturing's cost, scheduling and inventory objectives, and the firm's financial objectives. All these must be integrated with the strategic view of where the company wants to go.
The production-planning process typically begins with an updated sales forecast covering the next 6 to 18 months. Any desired increase or decrease in inventory or backlog levels can be added or subtracted, resulting in the production plan. However, the production plan is not a forecast of demand. It is planned production, stated on an aggregate basis. An effective production-planning process will typically utilize explicit time fences for when the aggregate plan can be changed (increased or decreased). Also, there may be constraints on the degree of change (amount of increase or decrease).
The production plan also provides direct communication and consistent dialogue between the operations function and upper management, as well as between operations and the firm's other functions. As such, the production plan must necessarily be stated in terms that are meaningful to all within the firm, not just the operations executive. Some firms state the production plan as the dollar value of total input (monthly, quarterly, etc.). Other firms may break the total output down by individual factories or major product lines. Still other firms state the plan in terms of total units for each product line. The key here is that the plan be stated in some homogeneous unit, commonly understood by all, that is also consistent with that used in other plans.
PRODUCTION SCHEDULING
The production schedule is derived from the production plan; it is a plan that authorized the operations function to produce a certain quantity of an item within a specified time frame. In a large firm, the production schedule is drawn in the production planning department, whereas, within a small firm, a production schedule could originate with a lone production scheduler or even a line supervisor.
Production scheduling has three primary goals or objectives. The first involves due dates and avoiding late completion of jobs. The second goal involves throughput times; the firm wants to minimize the time a job spends in the system, from the opening of a shop order until it is closed or completed. The third goal concerns the utilization of work centers. Firms usually want to fully utilize costly equipment and personnel.
Often, there is conflict among the three objectives. Excess capacity makes for better due-date performance and reduces throughput time but wreaks havoc on utilization. Releasing extra jobs to the shop can increase the utilization rate and perhaps improve due-date performance but tends to increase throughput time.
Quite a few sequencing rules (for determining the sequence in which production orders are to be run in the production schedule) have appeared in research and in practice. Some well-known ones adapted from Vollmann, Berry, Whybark and Jacobs (2005) are presented in Operations Scheduling.
THE PRODUCTION PLANNING AND
PRODUCTION SCHEDULING INTERFACE
There are fundamental differences in production planning and production scheduling. Planning models often utilize aggregate data, cover multiple stages in a medium-range time frame, in an effort to minimize total costs. Scheduling models use detailed information, usually for a single stage or facility over a short term horizon, in an effort to complete jobs in a timely manner. Despite these differences, planning and scheduling often have to be incorporated into a single framework, share information, and interact extensively with one another. They also may interact with other models such as forecasting models or facility location models.
It should be noted that a major shift in direction has occurred in recent research on scheduling methods. Much of what was discussed was developed for job shops. As a result of innovations such as computer-integrated manufacturing (CIM) and just-in-time (JIT), new processes being established in today's firms are designed to capture the benefits of repetitive manufacturing and continuous flow manufacturing. Therefore, much of the new scheduling research concerns new concepts and techniques for repetitive manufacturing-type operations. In addition, many of today's firms cannot plan and schedule only within the walls of their own factory as most are an entity with an overall supply chain. Supply chain management requires the coordination and integration of operations in all stages of the chain. If successive stages in a supply belong to the same firm, then these successive stages can be incorporated into a single planning and scheduling model. If not, constant interaction and information sharing are required to optimize the overall supply chain.
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Production Plan

Preparing Your Production Plan
What is a production plan?


A production plan is that portion of your intermediate-range business plan that your manufacturing / operations department is responsible for developing. The plan states in general terms the total amount of output that the manufacturing department is responsible to produce for each period in the planning horizon.
The output is usually expressed in terms of pesos or other units of measurement (e.g. tons, liters, kgs.) or units of the aggregate product (this refers to the weighted average of all the products in your company). The production plan is the authorization of your manufacturing department to produce the items at a rate consistent with your company's overall corporate plan.
This production plan needs to be translated into a master production schedule so as to schedule the items for completion promptly, according to promised delivery dates; to avoid the overloading or under loading of the production facility; and so that production capacity is efficiently utilized and low production costs result.
Why is it important to have a carefully developed production plan?
Production planning is one of the planning functions that a firm needs to perform to meet the needs of its customers. It is a medium-range planning activity that follows long-range planning in P/OM such as process planning and strategic capacity planning. Firms need to have an aggregate planning or production planning strategy to ensure that there is sufficient capacity to meet the demand forecast and to determine the best plan to meet this demand.
A carefully developed production plan will allow your company to meet the following objectives:
• Minimize costs / maximize profits
• Maximize customer service
• Minimize inventory investment
• Minimize changes in production rates
• Minimize changes in work-force levels
• Maximize the utilization of plant and equipment
How is a production plan prepared?
Activity 1 Determination of Requirements
The 1 st activity in Production Planning is the determination of the requirements for the planning horizon. Demand forecasting plays an important role in the conduct of these three tasks. Managers thus need to be aware of the various factors that would affect the accuracy of the demand and sales forecast.
Activity 1 involves the conduct of the following tasks:
ACTIVITY 1
Tasks
Description
1 Draw up the sales forecast for each product or service over the appropriate planning period
2 Combine the individual product / service demands into one aggregate demand
3 Transform the aggregate demand for each time period into staff, process, and other elements of productive capacity
There are company factors that could influence the level of demand for the firm's products. These internal factors include the company's marketing effort; the product design itself; the strategies to improve customer service; and the quality and price of the product.
There are also external factors or marketplace factors that significantly affect demand such as the level of competition or possible reaction by competitors to a firm's business strategy; the perception of consumers about the products and the consumer behavior as affected by their socio-demographic profile. Lastly, there are random factors that could affect the accuracy of demand forecasts such as the overall condition of the economy and the occurrence of business cycle.
Activity 2 How to Meet the Requirements
The next major activity involves the identification of the alternatives that the firm may employ to meet production forecasts as well as the constraints and costs involved. Specifically, this activity involves the following tasks:
ACTIVITY 2
Tasks
Description
1 Develop alternative resource schemes to meet the cumulative capacity requirements
2 Identify the most appropriate plan that meets aggregate demand at the lowest operating cost
Once the most appropriate plan has been selected, then the firm evaluates the plan and later on finalizes it for implementation. For more efficient and effective planning process, the formation of a production planning team composed of managers from manufacturing, marketing, purchasing and finance, is recommended.
What are the inputs to the production planning process?
To be able to perform the aggregate planning process, the following information should be available to this production planning team. These data include the following:
• Materials / purchasing Information
• Operations / manufacturing Information
• Engineering / process Designs
• Sales, marketing and distribution Information
• Financial and accounting information
• Human resources information
How do you address the demand fluctuations?
There are three basic production planning strategies that the company can choose from to address demand fluctuations. These are the (1) Chase Demand strategy, (2) Level Production strategy, and the (3) Mixed Strategy.
Strategy Description
Demand Chase Strategy Matches the production rate to the order or demand rate through the hiring and firing of employees as the order rate varies
Level Production Strategy Maintains a stable workforce working at a constant production rate with the shortages and surpluses being absorbed by any of the following: • Changing the inventory levels • Allow order backlogs (commit to the customer that you will deliver the product (s) at a much later date) • Employ marketing strategies (e.g. promotional activities)
Mixed Strategy The strategies here could include combination of any of the following: • Having a stable workforce but employ variable work hours (e.g., increase no. of shifts, flexible work schedules or overtime) • Subcontracting / outsourcing
• Changing inventory levels
Source: Dilworth, James B. Production and Operations Management: Manufacturing and Services . Fifth Edition. McGraw-Hill, Inc. 1993
What are the important considerations in selecting the production planning strategy?
Demand Chase Strategy
Specific Methods Costs Remarks
Hire additional workers as demand increases Employment costs for advertising, travel, interviewing, training, and others
Shift premium costs if additional shift is added Skilled workers may not be available when needed
Layoff workers as demand decreases Cost of severance pay & increases in unemployment insurance costs The company must have adequate capital investment in equipment for the peak work force level
Level Production Strategy
Specific Methods Costs Remarks
Produce in earlier period and hold until product is needed Cost of holding inventory Service operations cannot hold service inventory
Offer to deliver the product or service later, when capacity is available Delay in receipt of revenue, at minimum; company may lose customers Manufacturing companies with perishable products often use this method
Exert special marketing efforts to shift the demand to slack period Advertising costs, discounts, other promotional programs Exemplifies the inter-relationship
among functions within an organization
Mixed Strategy
Specific Methods Costs Remarks
Work additional work hours without changing the workforce size Overtime premium pay The time available for maintenance work without interrupting production is reduced
Staff for high production levels so that overtime is not necessary Excess personnel wages during period of slack demand Work force may be used for deferred maintenance during periods of low demand
Subcontract work to outside firms Continuing company overhead; subcontractor's overhead and profits The capacity of other firms can be utilized, but there is less control of schedules and quality levels
Revise make-or-buy decisions to purchase items when capacity is fully loaded Waste of company skills, tooling and equipment unutilized in slack periods These methods require capital investments sufficient for the peak production rate, that will be underutilized in slack periods


How can you monitor effectiveness of your production plans?
The important considerations in monitoring the effectiveness of your production plan are shown below:
Systems and Procedures
Consideration Present? Remarks
(if any)
Yes No
• Is there a current documentation of production planning and control systems and procedures? Has this been communicated to all concerned?
• Does production planning and control have a formal monitoring system to maintain and update master scheduling records?
• Is there a system of coordination between sales forecasts to be prepared in sufficient detail so that these maybe readily translated to specific production plans?
Production Planning
Consideration Present? Remarks
(if any)
Yes No
• Does production planning and control prepare a master production schedule with all the production assignments and time allocation?
• Do the production schedules permit adequate planning of purchases and inventory levels?
• Are there signs of significant lost time or low rate of worker productivity? Are the numbers of such orders appear to be significant?
Production Control
Consideration Present? Remarks
(if any)
Yes No
• Can the status of any order or work in progress be readily determined?
• Do actual production levels deviate significantly in comparison with planned schedules?
• Do actual shipments of orders almost always occur according to schedule?
• Are essential production control records and reports maintained to cover current and future production loads?
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Production plan

The sales program (based on the sales forecast, once the marketing strategy is defined) is the starting point for the production plan.


Once you have information as to the quantities that are to be sold, you will have to foresee the quantities that will be produced and the level of stocks of raw materials and finished products. Broadly, the production plan describes how the company is going to produce its products or services and all critical decisions required by the process. Here you also predict production costs as well as the costs by each sold unit.
Basically, the production plan may be translated into the following chart:


The production plan also gives relevant information for the reader of the business plan since it identifies the production costs (which, when compared with the sales forecasts, allows you to assess profitability) and allows you to know which are the areas of possible cost reduction.
To list all production costs is essential in this plan because it will allow you to define the stocks policies (the relevant resources which must be ensured, even if it is necessary to "buy" in advance), the installed capacity (what should be produced at internal level and what should be contracted) and the price of each product (of course, according to the price policy, defined in the marketing mix).
The production plan should also identify the capacity threshold, that is, as from what quantities it is required to increase the capacity, that is, as from what point it is necessary to make new investments.
Finally, you should make a reflection on the possible constraints to production and to prepare contingency plans in order to face them (for instance, if a supplier does not deliver the raw-material in the agreed deadline, who can I contact in order not to delay the delivery to a client).
Once the production plan is finished, you should be able to fill in the following table, per product:
Sales Costs Product A Product B Product C
Cost of raw materials
Man's hours x hourly average wage
Other direct costs
Unit cost price

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Production plan

What makes a good production plan?


No manufacturing or service industry can be productive without a sound production plan. Effective planning is fundamental in any business; it's a complex process that covers a wide variety of activities that ensure that materials, equipment and human resources are available to complete the work. Production planning is like a roadmap to reach your destination. It helps you know where you are going and how long it will take you to get there.
Advantages of an effective production plan and scheduling:
reduces labour by eliminating wasted time and improving process flow
reduces inventory costs by reducing the need for safety stocks and excessive work-in-process inventories
optimizes equipment usage and maximizes capacity
utilizes human resources to their full potential
improves on-time deliveries of products and services
Key factors of a production plan
Effective planning hinges on a sound understanding of key activities that entrepreneurs and business managers should apply to the planning process. Here are some examples:

Forecast market expectations: To plan effectively you will need to estimate potential sales with some reliability. Most businesses don't have firm sales or service figures. However, they can forecast sales based on historical information, market trends and/or established orders.

Inventory control: Reliable inventory levels feeding the pipeline has to be established and a sound inventory system should be in place.

Availability of equipment and human resources: Also known as open time, this is the period of time allowed between processes so that all orders flow within your production line or service. Production planning helps you manage open time, ensuring it is well-utilized, while being careful not to create delays. Planning should maximize your operational capacity but not exceed it. It's also wise not to plan for full capacity and leave room for the unexpected priorities and changes that may arise.

Standardized steps and time: Typically, the most efficient means to determine your production steps is to map processes in the order that they happen and then incorporate the average time it took to complete the work. Remember that all steps don't happen in sequence and that many may occur at the same time.

After completing a process map, you will understand how long it will take to complete the entire process. Where work is repeated or similar, it is best to standardize the work and time involved. Document similar activities for future use and use them as a base-line to establish future routings and times. This will speed up your planning process significantly.

During the process map stage, you may identify waste. You can use operational efficiency/lean manufacturing principles to eliminate waste, shorten the process and improve deliveries and costs. BDC Consulting can assist businesses in process mapping and other operational efficiency principles and tools.

Risk factors: Evaluate these by collecting historical information on similar work experiences, detailing the actual time, materials and failures encountered. Where risks are significant, you should conduct a failure mode effect analysis method (FMEA) and ensure that controls are put in place to eliminate or minimize them. This method allows you to study and determine ways to diminish potential problems within your business operations. This type of analysis is more common in manufacturing and assembly businesses.
How to plan work
All other activities are initiated from the production plan and each area is dependant on the interaction of the activities. Typically, a plan addresses materials, equipment, human resources, training, capacity and the routing or methods to complete the work in a standard time. In order to do a good sales forecast, you should base it on a history of firm orders.
The production plan initially needs to address specific key elements well in advance of production in order to ensure an uninterrupted flow of work as it unfolds.
Material ordering. Materials and services that require a long lead time or are at an extended shipping distance, also known as blanket orders, should be ordered in advance of production requirements. Suppliers should send you materials periodically to ensure an uninterrupted pipeline.
Equipment procurement. Procuring specialized tools and equipment to initiate the production process may require a longer lead time. Keep in mind that the equipment may have to be custom made or simply difficult to set up. This type of equipment may also require special training.
Bottlenecks. These are constraints or restrictions in the process flow and should be assessed in advance so you can plan around them or eliminate them before you begin production. When you assess possible bottlenecks, be aware that they may shift to another area of the process. Dealing with bottlenecks is a continual challenge for any business.
Human resources acquisitions and training. Key or specialized positions may demand extensive training on specialized equipment, technical processes or regulatory requirements. These employees should be interviewed thoroughly about their skills. When hiring them, allow sufficient time for training and be sure that they are competent in their work before the job begins. This will ensure that your process or service flows smoothly.
The production plan provides a foundation to schedule the actual work and plan the details of day-to-day activities. As sales orders come in, you will need to address them individually based on their priority. The importance of the sales order will determine the work flow and when it should be scheduled. After this, you should evaluate whether or not you are ready for production or to offer the service. You will need to determine:
If the inventory is available at the point where work is to start? If not, then the work needs to be rescheduled when supplies become available. There is no point in scheduling work that you will not be able to complete.
Are your resources available? Do you have the necessary staff to complete the task? Are the machines being used?
Does the standard time fit within the open time allowed? If not, then the work should be rescheduled.
You should be careful to minimize risk factors; allowing too many what-ifs can delay delivery and be counter productive.
Communicate the plan
After you have determined that you have met the criteria to start production, you will need to communicate the plan to the employees who will implement it. You can plan the production on spreadsheets, databases or software which usually speeds the process up. However, a visual representation is preferred as a means to communicate operation schedules to floor employees. Some businesses post work orders on boards or use computer monitors to display the floor schedule. The schedule also needs to be available to employees ahead of time and kept up to date.
Consider change
One of the many challenges of production planning and scheduling is following up with changes to orders. Changes happen every day; you may lack materials; delivery time is moved up or work parameters have to be adapted. You will need to adjust your plan in line with these changes and advise the plant. Dealing with change is not always easy and may take as much effort as creating the original production plan. You will need to follow up with the various departments involved in order to rectify any problems. As well, computer software can be helpful in tracking changes, inventory, employees and equipment.
Get outside help
BDC Consulting has experienced consultants who can help businesses with production planning, scheduling and lean manufacturing principles. They provide simple and effective solutions for a full range of business needs.
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Production schedule making method

A method for making a production schedule of an instant process which produces a plurality types of products and supplies the products to a plurality of following processes by trucks.


A truck delivery schedule including a number of truck deliveries and times is taken into account when the production schedule of the instant process is made. On the production schedule, a production order of the products is determined, and a stocking schedule also is made on the production order schedule
1. A method for making a production schedule for an instant process that produces a plurality of types of products and supplies the products to a plurality of second processes by trucks, the method comprising the steps of:
recording a second process schedule which includes numbers of products needed by the second process with respect to respective product types and respective days;
recording a truck delivery schedule from the instant process to the second process and information included in a card called a KANBAN which is carried by each truck between the instant process and the second processes and which includes information about delivery types;
determining a product shipment schedule of the instant process based on the second process schedule, the truck delivery schedule, and the KANBAN information;
recording a working condition and a production condition of the instant process;
determining a production schedule of the instant process based on the product shipment schedule, the working condition of the instant process, and the production condition of the instant process; and determining a production order of the products to be produced at the instant process based on the production schedule of the instant process.
2. A method according to claim 1, wherein each of the second processes comprises a process including a normal working condition in which no work is scheduled on a weekend or holiday, and the KANBAN comprises an ordinary KANBAN in which there is a time lag between delivery of the KANBAN and delivery of the products corresponding to the KANBAN.
3. A method according to claim 1, wherein at least one of the second processes comprises a process including an extra working condition in which work is scheduled on a weekend or holiday, and the KANBAN comprises an ordinary KANBAN in which there is a time lag between delivery of the KANBAN and delivery of the products corresponding to the KANBAN.
4. A method according to claim 1, wherein at least one of the second processes comprises a process including an extra working condition in which work is scheduled on a weekend or holiday, and the KANBAN comprises an special KANBAN in which there is no time lag between delivery of the KANBAN and delivery of the products corresponding to the KANBAN.
5. A method according to any one of claims 3 and 4, wherein the following process schedule comprises a schedule in which the numbers of products needed by the second process are modified so as to include also a number of products needed by the following process on the extra working day.
6. A method according to any one of claims 1, 2, 3, and 4, and further comprising a step of making a stocking schedule of the instant process based on the determined production order of the products to be produced at the instant process.
7. A method according to any one of claims 1, 2, 3, and 4, wherein the truck delivery schedule comprises a schedule specifying times when the trucks reach the second processes.
8. A method according to any one of claims 1, 2, 3, and 4, wherein the KANBAN comprises a KANBAN specifying a number of truck deliveries per day and a time lag between delivery of the KANBAN and delivery of the products corresponding to the KANBAN.
9. A method according to any one of claims 1, 2, 3, and 4, wherein the step of determining a product shipment schedule of the instant process includes steps of:
dividing the numbers of the plurality of types of products needed by the second processes specified in the following process schedule by the number of truck deliveries specified in the KANBAN and distributing the divided numbers over the respective truck deliveries; and
advancing the distributed numbers by the time lag specified in the KANBAN.
10. A method according to any one of claims 1, 2, 3, and 4, wherein the step of determining a production schedule of the instant process includes a step of distributing the numbers of products specified in the product delivery schedule of the instant process over actual working periods of time as evenly as possible.
11. A method according to any one of claims 1, 2, 3, and 4, wherein the step of determining a production order of products to be produced at the instant process includes steps of:
calculating desired production magnitudes with respect to respective types of products and respective actual working times; and determining a product type having a maximum desired production magnitude at an actual working time as the product to be produced at the time.
12. A method according to claim 6, wherein the step of making a stocking schedule of the instant process comprises step of: determining an inspection order of products based on the production order of products; determining cumulative numbers of inspected products with respect to respective times and respective types of products; determining cumulative numbers of delivered products with respect to respective times and respective types of products based on the product shipment schedule; subtracting the cumulative numbers of delivered products from the cumulative numbers of inspected products to obtain numbers of products stocked at the instant process with respect to respective types of products and respective times; and
adjusting initial stock numbers of products at the instant process so that the numbers of stocked products at respective times are not negative.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for making a production schedule for an instant process in a case where the instant process produces a plurality of types of products and supplies the products to a plurality of following processes by trucks, taking a truck delivery schedule also into account.
2. Description of the Prior Art
Japanese Patent Publication NO. SHO 63-265791 discloses a method in which a plurality of types of products (for example, engines) produced at an instant process (for example, an engine assembly line) are delivered by trucks to following processes (for example, car assembly lines) by a production schedule that determines a production number of products and a production order of different types of products, and that controls inventory or stocking by merely advancing the numbers of products needed at the following processes by periods of time necessary to deliver the products from the instant process to the following processes. When a working condition of the instant process is different from a working condition of the following process, the time to be advanced is measured on the basis of an actual working time.
However, in this prior production scheduling method, only the transportation times are taken into consideration; the numbers and times of truck deliveries are not considered. Since the flow of products will change corresponding to a change in the numbers and times of the truck deliveries, production at the instant process and the following processes may suffer a lack of supply of products from the instant process to the following process or an excess stock at the instant process or at the following process, if the truck delivery schedules are not taken into account.
SUMMARY OF THE INVENTION
An object of the invention is to provide a production schedule making method of an instant process which can prevent both a lack of supply of products from an instant process to following processes and an excess stock at the instant process and the following processes.
The above-described object is attained by a method according to the present invention for making a production schedule for an instant process that produces a plurality of types of products and supplies the products to a plurality of following processes by trucks. The method includes steps of entering a following process schedule which includes numbers of products needed by the following processes with respect to respective product types and respective days; entering a truck delivery schedule from the instant process to the following process and information about delivery of product types (this information being included in a card, called a KANBAN, that is carried by each truck between the instant process and the following process); determining a product shipment schedule of the instant process based on the following process schedule, the truck delivery schedule, and the KANBAN information about delivery of product types; entering a working condition and a production condition of the instant process; determining a production schedule of the instant process based on the product shipment schedule, the working condition, and the production condition of the instant process; and determining a production order of the products to be produced at the instant process based on the production schedule of the instant process.
In the above-described method, since the production schedule of the instant process is determined by taking into account not only the following process schedule but also a truck delivery schedule, an optimum production order will be obtained at the instant process to prevent both a lack of supply from the instant process to the following processes and excess inventory or stock at the instant process and the following processes.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features, and advantages of the present invention will become more apparent and will be more readily appreciated from the following detailed description of the preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a system flow chart illustrating a production scheduling method in accordance with the present invention, applicable to any embodiment of the present invention;
FIG. 2 is a table of numbers of products needed per day by a following process, in accordance with a following process schedule in a first embodiment of the present invention;
FIG. 3 is a table of daily arrival times at the following process in accordance with a truck delivery schedule in the first embodiment of the present invention;
FIG. 4 is a table of a product shipment schedule of an instant process corresponding to each truck arrival on each day at a following process in accordance with the first embodiment of the present invention;
FIG. 5 is a table illustrating a working condition and a production condition of the instant process in accordance with the first embodiment of the present invention;
FIG. 6 is a table illustrating a production schedule of the instant process in accordance with the first embodiment of the present invention;
FIG. 7 is a table illustrating a detailed production schedule of the instant process in accordance with the first embodiment of the present invention;
FIG. 8 is a table illustrating a production order schedule of the instant process in accordance with the first embodiment of the present invention;
FIG. 9 is a table illustrating a stocking schedule of the instant process in accordance with the first embodiment of the present invention;
FIG. 10 is a table illustrating a following process schedule in a case where the following process has a normal work schedule, in accordance with a second embodiment of the present invention;
FIG. 11 is a table illustrating a schedule similar to that of FIG. 10, but for a following process having a non-normal (extra working) schedule, in accordance with the second embodiment of the present invention;
FIG. 12 is a table of numbers of products needed by the following process for a modified following process schedule in accordance with the second embodiment of the present invention;
FIG. 13 is a table of arrival times at the following process for a truck delivery schedule in accordance with the second embodiment of the present invention;
FIG. 14 is a table illustrating a product shipment schedule of the instant process in accordance with the second embodiment of the present invention;
FIG. 15 is a table illustrating a working condition and a production condition of the instant process in accordance with the second embodiment of the present invention;
FIG. 16 is a table illustrating a production schedule of the instant process in accordance with the second embodiment of the present invention;
FIG. 17 is a table illustrating a detailed production schedule for production of type a products in accordance with the second embodiment of the present invention;
FIG. 18 is a table illustrating a detailed production schedule for production of type b products in accordance with the second embodiment of the present invention;
FIG. 19 is a table illustrating a production order schedule of the instant process in accordance with the second embodiment of the present invention;
FIG. 20 is a table illustrating a stocking schedule of the instant process in accordance with the second embodiment of the present invention;
FIG. 21 is a table illustrating a following process schedule in a case where the following process has a normal work schedule in accordance with a third embodiment of the present invention;
FIG. 22 is a table illustrating a schedule for a following process that has an extra working schedule in accordance with the third embodiment of the present invention;
FIG. 23 is a table of numbers of products needed by the following process for a modified following process schedule in accordance with the third embodiment of the present invention;
FIG. 24 is a table of arrival times at the following process for a truck delivery schedule in accordance with the third embodiment of the present invention;
FIG. 25 is a table illustrating a product shipment schedule of the instant process with respect to production of type a products in accordance with the third embodiment of the present invention;
FIG. 26 is a table illustrating a product delivery schedule of the instant process with respect to production of type b products in accordance with the third embodiment of the present invention;
FIG. 27 is a table illustrating a working condition and a production condition of the instant process in accordance with the third embodiment of the present invention;
FIG. 28 is a table illustrating a production schedule of the instant process in accordance with the third embodiment of the present invention;
FIG. 29 is a table illustrating a detailed production schedule of the instant process in accordance with the third embodiment of the present invention;
FIG. 30 is a table illustrating a production order of the instant process in accordance with the third embodiment of the present invention; and
FIG. 31 is a table illustrating a stocking schedule of the instant process in accordance with the third embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Three embodiments will be explained. A first embodiment corresponds to a case where a following process has a normal work schedule only, and is illustrated in FIGS. 1-9. A second embodiment corresponds to a case where a following process has an extra working schedule and product shipment is instructed by a normal KANBAN, and is illustrated in FIGS. 10-20. A third embodiment corresponds to a case where a following process has an extra working schedule and product shipment is instructed by a special KANBAN, and is illustrated in FIGS. 21-31. FIG. 1 is applicable to any embodiment of the invention.
The first embodiment will be explained first. As illustrated in FIG. 1, a following process schedule is entered at step 11. As shown in FIG. 2, the following process schedule includes information about numbers of product types a, b, c, . . . needed by each of the following processes A, B, C, . . . (for example, car assembly lines) on respective days. For example, FIG. 2 shows that following process A needs four products of type a and twelve products of type b on each of the 29th day, the 30th day, and the 31st days of a month, needs no product on each of the 1st day and the 2nd day of the following month because they fall on a weekend, and needs twelve products of type a and four products of type b on each of the 3rd day and the 4th day. FIG. 2 also shows a switching by an operator on the 3rd day when the production requirements of the following processes are changed from those of the previous week.
A truck delivery schedule is then entered at step 12 of FIG. 1, and information is entered from a card (called a KANBAN) at step 13 of FIG. 1. A KANBAN is here defined as an instruction card which is carried between the instant process and the following processes by trucks and which includes an instruction or information about production and delivery of products. This entered information is stored in a memory of a computer. FIG. 3 illustrates a typical example of such a truck delivery schedule. The truck delivery schedule of FIG. 3 shows that trucks arrive at the following process A at ten, fifteen, twenty-three, and four o'clocks per 24-hour day (which, in the example of this application, starts at 8.00 am rather than the traditional hour of midnight) in a case of a normal working condition in which there is no working at the following process on weekends and holidays. FIG. 3 also includes a KANBAN information "1-4-2". This KANBAN information "1-4-2" means that there are "four" truck deliveries with respect to "one" day and that detailed information on the products carried by a given truck is delivered to the following process by a "2nd" preceding delivery truck, which carries the corresponding KANBAN. Therefore, there is a time lag corresponding to two truck deliveries between delivery of a KANBAN and delivery of the products listed on the KANBAN, in the delivery of type "1-4-2".
At step 14 of FIG. 1, a product shipment schedule of the instant process is made based on the following process schedule, the truck delivery schedule, and the KANBAN information. One of the features of the invention is that the product shipment schedule of the instant process is made by taking the truck delivery schedule also into account.
FIG. 4 illustrates how to make the product shipment schedule of the instant process. More particularly, the number of products of each type needed by each following process (in this case, process A) in a day is divided by the number of truck deliveries on the day, and the divided numbers are inserted in "process-product" lines A-a and A-b of FIG. 4. For example, four products of type a are needed on Friday the 31st and twelve products of type a are needed on Monday the 3rd, respectively, by the following process A, and these numbers are divided by the number of truck deliveries per day (4) to obtain the numbers 1 and 3. These numbers 1 and 3, which are inserted in line A-a, mean that the products represented by the numbers should reach the following process A by the respective truck delivery times of Friday the 31st and Monday the 3rd.
If the product delivery schedule at the instant process of FIG. 4 is determined so that the products should be ready for dispatch or shipment from the instant process at times preceding the respective truck arrival times at the following process by the time lag between delivery of the KANBAN and delivery of the products that is specified in the KANBAN ("2" of "1-4-2"), the numbers 1 and 3 inserted in line A-a are advanced by the time lag of "two" truck deliveries to determine the product shipment schedule of the instant process. For example, the one product of type a which reaches the following process at fifteen o'clock on Friday the 31st should be listed on the KANBAN of the truck that makes the delivery at four o'clock on Thursday the 30th and should be ready for shipment from the instant process by that time. Similarly, the three products of type a which reach the following process at ten o'clock on Monday the 3rd should be listed on the KANBAN of the truck that arrives at the following process at twenty-three o'clock on Friday the 31st, and the three type a products should be ready for dispatch by that time. The product delivery schedule of the instant process of FIG. 4 shows this time advance by arrows. This advance also means that, as shown in FIG. 4, the switching of the cycle table for completion of products ready for shipment from the instant process will occur at 15:00 o'clock on Friday the 31st, to correspond to the switching of the cycle table for arrival of products at the following process A that occurs at 8:00 o'clock on Monday the 3rd (as shown in FIG. 2).
A working condition of the instant process and a production condition of the instant process are entered at step 15 and step 17 of FIG. 1, respectively. Typical examples of these conditions are illustrated in FIG. 5. For example, in a normal or ordinary working condition having no work on a weekend or holiday, there are two work shifts, that is, a first shift from eight to seventeen o'clock and a second shift from twenty-one to six o'clock, and two rest periods, that is, a rest period for the first shift from twelve to thirteen o'clock and a rest period for the second shift from one to two o'clock. Also, in the example of FIG. 5, one hour is provided as a lead time, which is defined as a time by which completion of production should precede dispatching of products, and one hour is provided as a tact-time, which is defined as a time needed to produce one product.
A production schedule of the instant process with respect to product types a and b is made at the next step 18 of FIG. 1. This production schedule of the instant process is made based on the product shipment schedule of FIG. 4, the working condition of the instant process entered at step 15 of FIG. 1, and the production condition of the instant process entered at step 17 of FIG. 1, these two conditions being shown in FIG. 5.
FIGS. 6 and 7 illustrate how to make the production schedule of the instant process. In FIG. 6, the second, fourth, sixth, and eighth columns under "31(FRI)" represent the four truck delivery times at following process A, and columns one, three, five, and seven represent corresponding times advanced by the lead time of FIG. 5 (one hour). The numbers inserted in "process-product" lines A-a and A-b are the same as the shifted numbers in the product shipment schedule of FIG. 4. These numbers are then further advanced, as shown by the arrows to the lines below the "process-product" lines, by the lead time (for example, one hour) specified in FIG. 5. Production of the products represented by these numbers should be completed by the times of the columns to which the numbers are advanced. For example, on Friday the 31st, production of one product of type a should be completed by nine o'clock, production of one product of type a should be completed by fourteen o'clock, production of three products of type a should be completed by twenty-two o'clock, and production of three products of type a should be completed by three o'clock, so that they will be ready for dispatch, respectively, at ten, fifteen, twenty-three, and four o'clock. The cycle table switching time is also shifted by one hour (from 15:00 in FIG. 4 to 14:00 in FIG. 6).
FIG. 7 illustrates a more detailed production schedule of the instant process wherein the numbers of products of FIG. 6 are distributed over actual working periods of time as evenly as possible. FIG. 7 has hourly columns grouped into working, rest, and shift change periods according to the working condition schedule of FIG. 5. In particularly, according to FIG. 5, the actual working periods for each working day include periods from eight to twelve o'clock, from thirteen to seventeen o'clock, from twenty-one to one o'clock, and from two to six o'clock. For example, corresponding to the product completion numbers in FIG. 6 (1 in the 9:00 o'clock column, 1 in the 14:00 o'clock column, 3 in the 22:00 o'clock column, 3 in the 3:00 o'clock column of the 31st, and 3 in the 9:00 o'clock columns of the 3rd day of the next month), in FIG. 7, a number 1 is inserted in the working period prior to the group that contains nine o'clock, a number 1 is inserted in the working number 3 is inserted in the working period prior to the group that contains twenty-two o'clock, a number 3 is inserted in the working period prior to the group that contains nine o'clock of the next working day.
Then, these numbers inserted in the "process-product" lines A-a and A-b of FIG. 7 are then distributed as evenly as possible over respective actual working periods of time as shown in the next line below each of the "process-product" lines. Therefore, for example, the one product of type a which should be completed by nine o'clock on Friday the 31st is produced from four to five o'clock on Thursday the 30th, and the one product of type a which should be completed by fourteen o'clock on the 31st is produced from ten to eleven o'clock on the same day. Of the three products of type a which should be completed by twenty-two o'clock on the 31st, the first one is produced from fifteen to sixteen o'clock, the second from sixteen to seventeen o'clock, and the third from twenty-one to twenty-two o'clock on that day, and of the three products of type a which should be completed by nine o'clock of the 3rd day of the next month (because of the intervening weekend) the first is produced from four to five o'clock on the 31st, the second from five to six o'clock on the 31st, and the third from eight to nine o'clock on the 3rd. The number of type a products which should be produced from eight o'clock on the 31st to eight o'clock of the next working day (skipping the nonworking weekend days of Saturday and Sunday) is one from eight o'clock on the 31st to a time of switching of the cycle table (fourteen o'clock on Friday the 31st) and eight from the switching time to eight o'clock of the next working day (Monday the 3rd). In this way, the production schedule of FIG. 7 is made.
A production order schedule of the instant process is made at step 19 of FIG. 1 based on the production schedule obtained at step 18 of FIG. 1 and a production condition entered from a file of production condition. For example, as illustrated in FIG. 7, the number of type a products to be produced at the instant process for delivery to the following process A should be one in the period from eight to fourteen o'clock on Friday the 31st, and eight in the period from fourteen o'clock on the 31st to eight o'clock on the next working day (Monday the 3rd). Similarly, the number of type b products to be produced for delivery to the following process A should be four in the period from eight to fourteen o'clock on Friday the 31st, and three in the period from fourteen o'clock on the 31st to eight o'clock on the next working day. Therefore, one product of type a and four products of type b should be produced at the instant process from eight to fourteen o'clock on Friday the 31st, and eight products of type a and three products of type b are produced from fourteen o'clock of the 31st to eight o'clock on the next working day at the instant process.
FIG. 8 illustrates how to determine a production order for products of type a and for products of type b, to cause the instant process and the following processes to operate smoothly. More particularly, in FIG. 8, desired production magnitudes Ka and Kb for products of types a and b at eight o'clock on Friday the 31st are selected to be equal to the numbers of products to be produced from eight to fourteen o'clock (cycle switching time) on the 31st, as shown in FIG. 7, so that Ka is selected to be 1 and Kb is selected to be 4. Then, a desired production magnitude Ka' at nine o'clock with respect to product type a is determined by equation, Ka'=Ka+Ka-(Ka+Kb)×P, and a desired production magnitude Kb' at nine o'clock with respect to product type b is determined by equation, Kb'=Kb+Kb-(Ka+Kb)×Q. In this connection, P is 1 when there is a product of type a to be produced in the period from eight to nine o'clock on Friday the 31st, and P is 0 when there is no product of type a to be produced from eight to nine o'clock on the 31st. Similarly, Q is 1 when there is a product of type b to be produced in the period from eight to nine o'clock on the 31st, and Q is 0 when there is no product of type b to be produced in this period.
For example, a desired production magnitude of type a products at nine o'clock is 2 (=1+1-0), and a desired production magnitude of type b products at nine o'clock is 3 (=4+4-(1+4)). Similarly, a desired production magnitude of type a products at ten o'clock is 3 (=(1+1-0)+1-0), and a desired production magnitude of type b products is 2 (=(4+4-5)+4-5). A desired production magnitude of type a products at eleven o'clock is -1 (=(1+1-0+1-0)+1-5), and a desired production magnitude of type b products is 6 (=(4+4-5+4-5)+4-0). Skipping the rest time (twelve to thirteen o'clock), a desired production magnitude of type a products at thirteen o'clock is 0 (=(1+1-0+1-0+1-5)+1-0), and a desired production magnitude of type b products is 5 (=(4+4-5+4-5+4-0)+4-5). Desired production magnitudes of types a and b at each working hour from fourteen o'clock on Friday the 31st to eight o'clock on the next working day (Monday the 3rd) can be obtained in the same way as described above by selecting the desired production magnitudes of types a and b products at fourteen o'clock on the 31st to be 8 and 3, respectively, as shown in FIG. 7.
The desired production magnitudes of types and b products are then compared with each other at each working hour (eight o'clock, nine o'clock, . . . ), and a product type having a larger desired production magnitude is selected to be the type of product to be produced at the time. In this way, a production order from eight to thirteen o'clock on Friday the 31st is determined to be b, b, a, b, b, and a production order from fourteen o'clock on the 31st to eight o'clock on the 3rd (the next working day) is determined to be a, b, a, a, a, b, a, a, a, b, a. This production order determining method stabilizes the production line.
At the next step 20 of FIG. 1, a stocking schedule is made based on the production order determined at step 19 and the product shipment schedule determined at step 14. At step 21, a stock supervision is executed based on the cumulative stock condition stored in a memory 22.
FIG. 9 illustrates how to make the stocking schedule. In a production order line of the table of FIG. 9, the production order determined by FIG. 8 is inserted. Since all the products are inspected, the same order as that of the production order but delayed by a lead time (for example, one hour) is inserted in an inspection end order line of FIG. 9. An inspection period of time is included in the lead time.
Then, cumulative numbers of the inspected type a products are inserted in "inspected product, a" line. More particularly, since one product of type a is inspected at eleven o'clock and the next product of type a is inspected at fifteen o'clock, the cumulative number is 0 up to eleven o'clock, 1 from eleven to fifteen o'clock, and 2 from fifteen o'clock. According to the product delivery schedule of FIG. 4, however, the number f type a products required to be ready for shipment from the instant process is one by ten o'clock, one by fifteen o'clock, three by twenty-three o'clock, and three by four o'clock, so a cumulative number of type a products dispatched is one from ten to fifteen o'clock, two from fifteen to twenty-three o'clock, five from twenty-three to four o'clock, and eight from four to eight o'clock, as shown in "delivery of product a" line of FIG. 9.
A stock number or inventory of type a products is calculated by subtracting the cumulative numbers of shipped products from the cumulative numbers of inspected products. More particularly, in a case where the stock number is initially zero, the stock number is 0 from eight to nine o'clock, 0 from nine to ten o'clock, -1 from ten to eleven o'clock, 0 from eleven to twelve o'clock, and so on. However, since a minus stock number means lack in stock of products, an alarm will occur in such a case so that an operator notices the lack. An initial stock number is determined so that no lack in stock of products will occur. For example, an initial stock number of 1 is selected for the type a products. The stock numbers of type b products are determined in the same way as for type a products.
Next, the second embodiment of the present invention will be explained with reference to FIGS. 10-20. The second embodiment corresponds to a case where extra work is scheduled on a holiday or weekend and the product delivery on the extra working day is controlled by a KANBAN for a normal working schedule. In this connection, the explanation made about FIG. 1 also applies to the second embodiment.
FIG. 10 illustrates a following process schedule which the second embodiment would have if the following process had no extra working. The schedule of FIG. 10 includes numbers of products needed by the following process with respect to respective product types a, b, . . . and respective days.
FIG. 11 illustrates an example where extra working is scheduled on Saturday the 1st at the following process A. More particularly, FIG. 11 illustrates that eight products are needed by the following process on the extra working day, that the number of products needed by the following process on the 3rd and 4th days is reduced by eight, and that the eight products to be delivered to the following process on the extra working day are produced on the 30th and 31st days at an instant process.
Because of the extra working, the following process schedule of FIG. 10 is modified to a schedule of FIG. 12. More particularly, it is planned that six products of type a and two products of type b are delivered to the following process on the 1st day, and eight are reduced from the numbers inserted in the 3rd day column and the 4th day column of FIG. 10.
Products are transported by trucks from the instant process to the following process in the numbers indicated in FIG. 12, in accordance with a truck delivery schedule and a KANBAN instruction which are shown in FIG. 13. In the example of FIG. 13, there are only two truck deliveries at ten o'clock and fifteen o'clock on the 1st day and there is a time lag of two truck deliveries between delivery of each KANBAN and delivery of the respective products, as instructed by the instruction "1-4-2" of the KANBAN.
As illustrated in FIG. 14, a product delivery schedule from the instant process is made based on the modified following process schedule of FIG. 12 and the truck delivery schedule of FIG. 13. More particularly, the numbers of products to be supplied to the following process are divided by the number of truck deliveries and are evenly distributed over the respective truck deliveries. Then, the distributed numbers are advanced by the time lag specified in the KANBAN, whereby the product delivery schedule from the instant process is made.
FIG. 15 illustrates a working condition and production condition of the instant process. As shown in FIG. 16, a production schedule of the instant process is made based on the product delivery schedule of the instant process of FIG. 14 and the working condition and production condition of the instant process of FIG. 15. This production schedule is made by advancing the product delivery schedule by a lead time (for example, one hour as shown in FIG. 15). In this instance, the products which should be supplied to the following process on the extra working day (Saturday the 1st) are planned to be produced on Thursday the 30th and Friday the 31st.
FIG. 17 illustrates a detailed production schedule of the instant process with respect to type a products. In FIG. 17, the numbers to be produced are distributed over actual working periods of time as evenly as possible. The numbers inserted in the first and second lines from the top of FIG. 17 correspond to the numbers of products to be produced which will be delivered to the following process on normal working days, and the numbers inserted in the third and fourth lines from top of FIG. 17 correspond to the numbers of products to be produced which will be delivered on the extra working day. In this instance, the number "8" in the third line corresponds to the number of products to be supplied on the extra working day, and the numbers "1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0" in the fourth line correspond to the numbers of products to be produced on Friday the 31st. The remaining four products will be produced on Thursday the 30th, but they are not shown in FIG. 17. The numbers in the fifth line from the top of FIG. 17 are summations of the numbers of the second line and the numbers of the fourth line. From this table, it will be understood that two products of type a are produced from eight to fourteen o'clock on the 31st, and eleven products of type a are produced from fourteen o'clock on the 31st to eight o'clock on the 1st.
In the same way as for FIG. 17, a production schedule of the instant process with respect to type b products is made, as illustrated in FIG. 18.
Then, desired production magnitudes Ka and Kb of products of types a and b are calculated as illustrated in FIG. 19, based on the production schedules of FIGS. 17 and 18, in the same way as for the first embodiment, which was explained with reference to FIG. 8, and then the production order schedule of the instant process is made by selecting the largest desired production magnitude at each production time, to stabilize the production lines.
FIG. 20 illustrates a stocking schedule of product types a and b that is made based on the production order schedule of FIG. 19. This stocking schedule is made in the same way as that of the first embodiment, which was explained with reference to FIG. 9.
Next, the third embodiment of the present invention will be explained with reference to FIGS. 21-31. The third embodiment corresponds to a case where extra working is scheduled on a weekend or holiday and the product delivery on the extra working day is controlled by a special or extra KANBAN. In this connection, the explanation made about FIG. 1 also applies to the third embodiment.
FIG. 21 illustrates a following process schedule which the following process would have if the following process had no working on a weekend. The schedule of FIG. 21 includes the numbers of products needed by the following process with respect to product types and days.
FIG. 22 illustrates one example where extra working is scheduled on Saturday the 1st at the following process A. More particularly, FIG. 22 illustrates that twelve products are shifted from the normal working days of Monday the 3rd and Tuesday the 4th to the extra working day on the 1st. Therefore, twelve products will be delivered to the following process on Saturday the 1st, and twelve products are reduced from the products handled on Monday the 3rd and Tuesday the 4th. FIG. 22 also illustrates that the twelve products to be supplied to the following process on the 1st are produced on the 30th and 31st at the instant process.
Because of the extra working, the following process schedule of FIG. 21 is modified to a schedule shown in FIG. 23. More particularly, it is planned that nine products of type a and three products of type b are supplied to the following process on Saturday the 1st, and nine products of type a and three products of type b are reduced from the products to be supplied on Monday the 3rd and Tuesday the 4th in FIG. 21.
Products are supplied from the instant process to the following processes in accordance with a truck delivery schedule and a KANBAN instruction which are shown in FIG. 24. As illustrated in FIG. 24, there are only two truck deliveries at ten o'clock and at fifteen o'clock on the extra working day of Saturday the 1st, and delivery of the products to be supplied on the extra working day is controlled by a special KANBAN in which there is no time lag between delivery of the KANBAN and delivery of the corresponding products. The special KANBAN has an instruction of "1-2-0" which means that there are two truck deliveries on one extra working day, and there is no time lag between delivery of the KANBAN and delivery of the corresponding products.
As illustrated in FIG. 25, a product delivery schedule of the instant process is made based on the modified following process schedule of FIG. 23, the truck delivery schedule of FIG. 24, and the KANBAN instruction of FIG. 24. More particularly, with respect to the products to be supplied to the following processes on the normal working days, the number of the products is divided by the number of truck deliveries, and the divided numbers are distributed over the respective truck deliveries as evenly as possible. Then, the numbers of the products distributed over the respective truck deliveries are advanced by the time lag between delivery of the KANBAN and delivery of products specified in the normal KANBAN. On the other hand, with respect to the products to be supplied to the following processes on the extra working day, the numbers of the products to be supplied to the following process are distributed over both the extra working day itself and the preceding normal working day or days as evenly as possible. The distributed numbers are not advanced because there is no time lag between delivery of the KANBAN and delivery of products according to the special KANBAN. FIG. 26 illustrates a product delivery schedule of the instant process with respect to products of type b, which is made in the same way as that of products a.
FIG. 27 illustrates a working condition and production condition of the instant process including a lead time and a tact-time.
As shown in FIG. 28, a production schedule for the instant process is made based on the product delivery schedules of FIGS. 25 and 26 and the working condition and production condition of the instant process of FIG. 27. This production schedule is made by advancing the product delivery schedule by a lead time (one hour, see FIG. 27). In this instance, the products (nine products of type a and three products of type b) which should be supplied to the following process on the extra working day (Saturday the 1st) are scheduled to be produced over both Saturday the 1st and Friday the 31st. FIG. 28 also shows that three products of type a are produced and one product of type b is produced on the 1st.
FIG. 29 illustrates a detailed production schedule of the instant process with respect to product types a and b. In FIG. 29, the numbers of products to be produced shown in FIG. 16 are distributed over corresponding actual working periods of time as evenly as possible.
Desired production magnitudes Ka and Kb of products a and b are then calculated as illustrated in FIG. 30, based on the production schedule of FIG. 29, in the same way as for the first embodiment which was explained with reference to FIG. 8, and a product type that has the largest desired production magnitude is selected at respective production times so that the production order of product types a and b is determined.
FIG. 31 illustrates a stocking schedule of the instant process that is made based on the production order schedule of FIG. 30. This stocking schedule is made in the same way as that of the first embodiment which was explained with reference to FIG. 9.
In accordance with any of the first, second and third embodiments, since a production schedule of the instant process is made taking a truck delivery schedule into consideration, both a lack in supply of products and an extra stocking of products are effectively prevented. As a result, the production lines are stabilized.
Although a few embodiments of the invention have been described in detail above, it will be appreciated by those skilled in the art that various modifications and alterations can be made to the particular embodiments shown without materially departing from the novel teachings and advantages of the present invention. Accordingly, it is to be understood that all such modifications and alterations are included within the spirit and scope of the present invention as defined by the following claims.
0 nhận xét

Production schedule

The master production schedule (also commonly referred to as the MPS) is effectively the plan that the company has developed for production, staffing, inventory, etc.
It has as input a variety of data, e.g. forecast demand, production costs, inventory costs, etc and as output a production plan detailing amounts to be produced, staffing levels, etc for each of a number of time periods.


This production plan:
• operates at an aggregate level (that is it does not usually go into great detail about parts to be used, etc - hence the name aggregate planning); and
• is cost driven, that is it attempts to meet the specified requirements at minimum cost.
The idea of a master production schedule can best be illustrated by means of an example.
Example
In our example we have just a single product being produced.
Production takes place each period (week) either in the normal (regular) production shift or in overtime associated with that shift. There is only one shift (i.e. not operating a two/three shift system - such as with "round-the-clock" working).
Completed items can also be "bought-in" from a subcontractor (at a cost).
We are allowed to hire/fire workers (again at a cost). Backorders are also allowed (recall here that backorders are customer orders that cannot be satisfied in the required period, but the customer allows the order to remain open to be fulfilled in a later period). Lost sales are not allowed.
The diagram below illustrates the situation and the types of factor with which we are dealing graphically.


The data for the example we consider is as below, where we have shown the initial data entry screen from the package.

In the above screen we have chosen the "General LP Model". This is the most general of the options allowed by the package. LP stands for linear programming and is a generalised way of modelling decision problems. To ease data entry we have not crossed the "Part Time Allowed" box - if we had then we would have had the option of dealing with part time employees.
We have also not crossed the "Lost Sales Allowed" box - if we had then we would have allowed lost sales. In general a company may allow lost sales because the company finds that customers simply do not backorder - i.e. a lost sale is automatic if the product is not immediately available; or the company is prepared to allow lost sales as it may be better to allow orders to be lost than to allow such orders to become backorders (thereby incurring backorder costs).
The remaining boxes have been crossed and so we can deal with:
• overtime
• hiring/firing
• subcontracting
• backorders
In our example above we have just 4 periods (weeks) - this is our time horizon (planning period). We are dealing with employees working hours in each week. Two employee hours are required to produce one unit of each product and the initial number of employees is 10. At the start of the planning period there is no initial inventory (nor are there any backorders).
The data for our example entered into the package in the light of the choices made at the initial screen is as below:

The meaning of each of these lines of data is given below:
Forecast Demand - this is the forecast demand for the product in each of our 4 periods (weeks).
Initial Number of Employee - this is the initial number of employees in each week, here just the 10 employees we have currently.
Regular Time Capacity in Hour per Employee - this is how many regular hours each employee works per week, here 35 hours
Regular Time Cost per Hour - this is the cost per hour of regular time worked, here £15
Undertime Cost per Hour - this is the cost per hour of not using a worker to their full regular capacity, here zero
Overtime Capacity in Hour per Employee - this is the maximum number of hours each employee can work in overtime per week, here 10 hours
Overtime Cost per Hour - this is the cost per hour of overtime, here £25
Hiring Cost per Employee - this is the cost of hiring one employee, here £500
Dismissal Cost per Employee - this is the cost of dismissing (firing) one employee, here £2000
Maximum/Minimum Number of Employee Allowed - here we can set limits on the maximum and minimum number of employees, here M signifies there is no limit on the maximum number and the minimum number is 8. In general there may be an upper limit on the number of employees due to physical capacity constraints.
Initial Inventory (+) or Backorder (-) - the initial inventory available or backorders outstanding, here zero
Maximum/Minimum Ending Inventory - here we can set limits on the maximum and minimum number of product units in stock at the end of each week, here M signifies there is no limit on the maximum number and the minimum number is zero. In general there may be an upper limit because we have a limited space in which to store stock. The minimum number corresponds to safety stock that may be kept in case of unforeseen demand.
Unit Inventory Holding Cost - this is the cost of holding one unit in stock at the end of each period, here £3
Maximum Subcontracting Allowed - this is the maximum number of product units we are allowed to buy in from the external subcontractor, here there is no limit on the amount that may be bought in. In general there may be a limit on the total amount the subcontractor can supply to us each period.
Unit Subcontracting Cost - this is the cost of each unit bought from the external subcontractor, here £60
Maximum Backorder Allowed - this is the maximum number of backorders allowed at the end of each period, here there is no limit on the number of backorders that can be held at the end of each period.
Unit Backorder Cost - this is the cost of each backorder outstanding at the end of each period, here the M signifies that each backorder is very expensive. The effect of M here will be to ensure that (if at all possible) backorders will be avoided.
Other Unit Production Cost - this is the cost of producing one unit of the product that is not already accounted for by employee costs - here zero
Capacity Requirement in Hour per Unit - this is the number of employee hours that are required to produce one unit of the product, here 2 hours
In order to ease understanding of the problem most of the above data items take the same value in each and every period (week). However it would be perfectly possible for them to have different values in each week.
Consider for a moment this example as we have defined it so far. We have a single product, are planning over 4 time periods, have regular time and overtime, can buy from an external subcontractor, and are allowed to hire and fire employees. Some of the decisions we must make are shown below:
Period 1 2 3 4
Amount to produce using regular time ? ? ? ?
Amount to produce using overtime ? ? ? ?
Amount to purchase from subcontractor ? ? ? ?
Number of backorders ? ? ? ?
Number to hire ? ? ? ?
Number to fire ? ? ? ?
You can see from this matrix that there are already 24 decisions which we have to make. For such problems decision models (such as the decision model used within the package) are much better at decision-making than people. Moreover such models can guarantee to make decisions at minimum cost, something people cannot do.
Note here that even the (cheap) package used here is extremely flexible in terms of the situations it can consider.
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Solution
The solution to the problem is shown below:


It can be seen that we immediately hire more employees, and that these are employed throughout the planning period of 4 weeks. Note that the number hired is 10.57 - i.e. it includes a fraction of an employee. This often happens in aggregate planning and can usually be ignored (simply round to the nearest appropriate whole number). Reflect that we are producing a plan for production over a 4 week period. It is unlikely that our demand forecasts will be completely accurate and hence this rounding need not concern us unduly.
With 10+10.57 = 20.57 employees working 35 hours a week we have a regular time capacity of 20.57x35 = 720 employee hours (approximately) and at 2 hours per unit produced this corresponds to a regular time production of 360 units - precisely as above, i.e. over the 4 week planning period we are planning to work all of our employees to their full regular time capacity. As can be seen above we are planning no overtime or subcontracting.
Note the build into inventory that occurs in various periods. As inventory costs us money let us be clear about why the above (the minimum cost solution) involves build into inventory. It is to meet future demand. Demand for the product increases over the 4 week planning period (from 250 to 450 units) and the package has determined that the most cost-effective way to ensure that this demand is met is to build into inventory in earlier periods. Note that an alternative strategy to meet this increased demand would be to buy from the subcontractor, were this cheaper the package would have adopted that strategy.
The costs associated with the package solution can be seen below:

The production and employment strategy given above is the minimum cost strategy since the package uses linear programming to calculate a schedule for production and staffing that meets the forecast demand and also satisfies the other constraints that we place upon the problem at minimum cost. It would be impossible to find the minimum cost solution manually - consider the solution shown with an increase in employment and with varying amounts built into inventory - could that ever be produced by a person in the time (fraction of a second) it takes the package to produce it?
Level and chase strategies
We may be interested in a solution that consists of a fixed number of workers (a level strategy). This can be seen below where we have fixed the maximum and minimum number of employees to 10 (the current number) in each and every period.

The solution for this level strategy is shown below



It can be seen that with this solution we use both overtime and subcontracting but do not build into inventory. The total cost of 63,400 is much more than the previous (non-level) strategy which had a total cost of 49475.71
If we change the hire/fire costs to zero and reset the limits on the maximum/minimum number of employees then we will produce a chase strategy (ramp workforce up/down as required). This solution can be seen below.




Backordering
In our original situation considered above, with costs for hiring and firing, we were prohibiting backorders by making them very expensive. Suppose now that backorders cost us £1 per period (week). The effect of this on the solution is shown below.



It can be seen that we produce nothing - the cheapest solution is simply to allow backorders to build up over the planning period. This seems silly and for this reason it is usual to insist that there are no backorders outstanding at the end of the planning period. This is an assumption that is, by convention, applied and is a reasonable assumption when planning over a relatively long time period. Moreover unless this is assumed it can happen that the best thing for the company to do over the planning period is simply to allow backorders to build up (as above).
To ensure that there are no backorders outstanding at the end of the planning period we enter a zero for "Maximum Backorders Allowed" in the last period (week 4), as below.

The solution is


which is effectively the same as the initial solution we considered. However to illustrate that backorders can play a role suppose that we:
• have a level strategy with exactly 10 employees in each and every period (week); and
• restrict the subcontractor capacity in each and every period
then the input is as below:

and the output is:



where backorders do occur since production capacity (both regular time and overtime) together with subcontractor capacity is insufficient to met demand in some periods.
Rolling horizon
In practice we would probably deal with the situation described above on a "rolling horizon" basis in that we would get an initial production plan based on current data and then, after one time period (say), we would update our problem/data and resolve to get a revised production plan. In other words even though we plan for a specific time horizon (here 4 weeks) we would only ever implement the plan for the first week, so that we are always adjusting our 4 week plan to take account of future conditions as our view of the future changes. We illustrate this below.
Period 1 2 3 4 5 6 7 P = plan
P P P P D = do (follow) the plan in a period
D P P P P
D P P P P
D P P P P

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