WO1993005476A1

WO1993005476A1 - Method of constant rate of production scheduling

Info

Publication number: WO1993005476A1
Application number: PCT/US1992/007504
Authority: WO
Inventors: Henry Engelman
Original assignee: Timephaser Corporation
Priority date: 1991-09-03
Filing date: 1992-09-03
Publication date: 1993-03-18
Also published as: CA2116982A1; EP0603326A1; EP0603326A4

Abstract

A method which applies the critical path method to repetitive tasks having constant rates of production allows a user to plan a schedule and to track progress of repetitive tasks for large and complex jobs, with minimal data entry, and which is updatable to reflect actual job progress. Data for a template unit is transformed into a critical path method (CPM) network for an entire job by determining repetitive tasks (2) with their respective durations and precedence relationships for each additional unit in the job.. Precedence relationships between same tasks but different units are also considered (1100-1145). These latter precedence relationships take into account the movement of crews through units and/or the movement of units through work stations. Special constraints may be incorporated into the network in order to provide continuous work for the crews (1201-1213). The CPM network is used to calculate earliest and latest possible starting and finishing dates for each task performed on each unit (6). The planning schedule is produced in a concise, easily-interpreted format which is an improvement over traditional Gantt bar charts (7).

Description

METHOD OF CONSTANT RATE OF PRODUCTION SCHEDULING

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method of managing repetitive tasks, and more specifically to a process which applies the critical path method to repetitive tasks having constant rates of production.

2. Description of Related Art

Complex projects consist of a plurality of interrelated tasks. Optimizing production schedules for large projects requires the synchronization of these tasks. Many of these tasks are performed on a plurality of individual units, and may be termed

"repetitive" tasks. Examples of repetitive tasks are found in a wide variety of settings, including factory assembly lines and construction sites. Staking out individual lots for a tract-home subdivision, mounting picture tubes to television set cabinets, and bolting transmissions to engines illustrate various types of repetitive tasks within larger projects.

One method of planning production schedules consisting of a plurality of repetitive tasks is known as the line of balance method, also referred to as the constant rate of production method. A two-dimensional graph is manually produced, where the axes are the number of completed units and time. FIGURE 1 is a graphical representation of continuous rates of production for a plurality of interrelated tasks.

The X-axis is labelled in units of time such as days or hours. The Y-axis is labelled to designate the individual units on which tasks are to be performed. The work performed by a crew completing a task is shown on the graph as a line having a slope proportional to the output for that task. In a factory environment, the graph may be used to indicate the outputs of work stations along an assembly line. Lines having steep slopes indicate tasks for which the rate of completion is relatively rapid, whereas lines having more horizontal slopes indicate tasks for which the rate of completion is slow. Many of these tasks have precedence relationships, meaning that a given task cannot be performed until one or more previous tasks have been completed.

If a first task has a precedence relationship with a second task, the lines representing the tasks on the constant rate of production graph must not intersect or cross. This constraint is used to optimize schedules for multi-unit projects. However, in order to synchronize all tasks, a large number of graphs must be analyzed and manipulated, rendering this method tedious, complicated, and impractical for large, complex projects.

Another example of a line of balance calculation is shown in FIGURE 2. The line of balance specifies the types of components which must be available at specific points in time to achieve a constant rate of production. A shortcoming of the line of balance method is that it is very difficult to update the graphical displays to reflect progress that deviates from the original schedule, or unexpected delays. In many cases, virtually the entire series of graphs has to be redrawn to provide an accurate, updated schedule. In practice, however, line-of-balance graphs are seldom updated, due to the difficulty involved.

It is possible to automate the calculation of line-of-balance graphs. However, currently available automated representations merely present graphical or tabular displays comparing the projected total units completed at given points in time with the actual number of units completed. Thus, these displays are not used to calculate production schedules or to identify which unit is scheduled at which time.

FIGURE 3 is a graphical representation of another prior-art method for schedule planning, termed the critical path method (CPM). CPM analyzes a complex project by dividing the project into individual tasks with serial and parallel time-precedence relationships. Serial relationships refer to interdependent tasks, where a first task must be completed prior to commencing a second task. Parallel relationships involve tasks which may be implemented simultaneously. The interrelationships between the tasks may be modeled as a network. Each task is represented by a point or node on the network. A start date is specified for the first task, and a duration is assigned to each task.

By way of background, it is also important to understand two special aspects of CPM. First, there are a number of types of relationships between two logically-connected tasks. The standard relationship is called Finish-Start, and means that task A must finish before task B can start. Another type of relationship is called Start-Start, and means that task B can start after task A has started. A time lag can also be applied to these relationships. For example, if task B is preceded by task A with a precedence relationship of type Start-Start with a time lag of 2 days, task B can start 2 days after task A has started.

The second special aspect of CPM is that there are a number of special types of constraints, or "plugs", that can be applied to a work item above and beyond the logical relationships. One of these constraints is As-Late-As-Possible (ALAP). If an ALAP plug is assigned to the early start date of task A, then the early start date will be delayed as much as possible, i.e., until any further delay would cause negative float (delay of a required due date further in the network).

The CPM scheduling process accepts data representing the coded network interactively and/or in batch mode. A forward pass and backward pass are executed through the network to establish dates and priorities for all tasks in the network. During the forward pass, the earliest possible start and finish dates for each task are determined in a step-wise fashion. The process commences at the project starting date and the durations of individual tasks are added to the starting date, until an expected finish date is determined. The earliest start date of a task is equal to the latest starting date of the preceding tasks plus the duration of the preceding task. The finish date of each task is its start date plus its duration, minus one working day. In this manner, the finish date of the last task determines the completion date of the project.

The backward pass through the network performs the aforementioned operations in reverse order to establish the latest possible completion dates. The latest possible completion date may be defined as the last date for which a task may be accom¬ plished without missing the target project completion date. The difference between a task's earliest possible completion date and latest possible completion date is termed "slack" or "float '.

The path of work items through the network which has the longest cumulative duration is known as the critical path and usually will be equal to the project duration. Items along this path will have zero float, indicating that delay of any one of them will delay project completion. Kerns not on the critical path have positive total float. CPM reports include tabular reports (FIGURE 4) and Gantt or bar charts (FIGURE 5).

The critical path method (CPM) is a comparatively recent engineering development particularly adapted for use in the construction industry for the planning and scheduling of construction activities. For a detailed analysis of CPM, reference is made to the book entitled CPM IN CONSTRUCTION MANAGEMENT, Second Edition, by James J. O'Brien (McGraw-Hill Book Company, 1971).

Although the critical path method provides a useful tool for schedule planning, the amount and complexity of data entry required to use CPM for repetitive jobs, and the volume of printed reports required to interpret the schedules, renders CPM practical for only a small number of repetitive tasks on a small number of jobs. For example, a typical construction tract of 100 homes requiring 150 tasks per home would involve data entry of 15,000 tasks and determination of at least 30,000 precedence relation¬ ships. Using the most compact reporting format available, such a report would generate several hundred pages of data.

One method of representing CPM tasks by two dimensional geometric objects is shown in U.S. Patent 5,016,170, issued on May 14, 1991 5/1991 to Pollalis. Pollalis essentially teaches a tool for manual optimization of resources (men per crew) across dissimilar tasks. Although a mechanism is described for replicating templates, it is applicable only to simple reproduction and uses a matrix solution and graphical representation which is suitable only for small problems. (The 100 home tract example given above, with 150 tasks per home, would require about 200 bytes per task per home, requiring a matrix of 3 million bytes. This is about 50 times the maximum capacity of most compilers). Further, Pollalis teaches an iterative approach that requires user-interaction to successively improve a heuristic solution. A direct solution would be preferable.

Another prior-art method of production scheduling has been employed in environ¬ ments such as factory assembly lines. MRP II (Manufacturing Requirements and Planning) for assembly line scheduling is based on heuristic solutions of different capacity resource planning algorithms addressing idle machine time and uncompleted assemblies. This approach, plus the inherent changeability of data and size of the problems considered here, render MRP II unsuitable for scheduling a large number of repetitive tasks.

What is needed is a practical method for scheduling tasks which is adaptable to relatively complex projects having numerous repetitive tasks. The scheduling method should be flexible to allow for frequent updates in accordance with actual job progres- sion. The method should also provide ease of updating. The graphical output should provide essential scheduling information in a concise, easily-interpreted format. Furthermore, required data providing and data entry operations should be kept to a minimum and be capable of being performed with minimal training and expertise.

The present invention provides such a method.

SUMMARY OF THE INVENTION

The invention provides an improved process for scheduling production tasks. The process combines the constant rate of production method with the critical path method, thereby providing an enhanced process for creating a production schedule. The process allows a user to plan a schedule and to track progress of repetitive tasks for large and complex jobs. The process requires minimal data entry, and may be updated to reflect actual job progress. A production schedule is produced in a concise, easily-interpreted format.

The process commences when a user enters task data into a data processing device. The user need not enter data for all the units of a given job; task data pertaining to one unit is sufficient. This data preferably includes the description, duration, and preceding work items for each task. Each task is assigned a unique code number.

Next, the user specifies the tasks which are to be repeated on additional units. For these repetitive tasks, the user may also enter the number of units to be completed per day, the number of crews or work stations per task, and the minimum number of units required for continuous work. The total number of units to be produced is also specified.

From the entered information, the data processing device determines a sequence, for reporting purposes, of repetitive tasks for a single "template" unit. The sequence is determined by starting at the first task to be performed and making a forward pass through subsequent tasks. The earliest possible starting dates for the tasks are determined, based upon the duration of previous tasks and the starting date of the first task.

The data for the template unit is transformed into a critical path method (CPM) network for an entire job by determining the repetitive tasks with their respective durations and precedence relationships for each additional unit in the job. Precedence relationships between same tasks but different units must also be considered. These latter precedence relationships take into account the movement of crews through units and/or the movement of units through work stations. Special constraints may be incorporated into the network in order to provide continuous work for the crews.

The data processing device utilizes the CPM network to calculate earliest and latest possible starting and finishing dates for each task performed on each unit. The latest possible starting date is the last day for which a task may be started which permits the project to be finished on time. The calculation of early and late dates is imple- mented using CPM techniques which are well-known. In particular, eariiest possible start and finish dates are calculated by conducting a first forward pass through the data. Next, the latest possible start and finish dates are calculated by conducting a first backward pass through the data.

To calculate schedule dates for continuous work, a second forward and backward pass is conducted, replacing early dates with late dates. In this manner, a planning schedule consisting of start and finish dates for each task and each unit is created. The schedule is displayed in a form which is an improvement over traditional Gantt bar charts. A time line is displayed on the X-axis, and configured so that each column can accept multi-digit unit numbers. A list of work items is displayed on the Y-axis, and includes only those tasks which are repetitive in nature. Conventional horizontal bars within the schedule showing the start and finish dates for each task have been replaced by the unit number being worked on. The unit number is aligned to indicate the corresponding task at the X-axis, and the corresponding date at the Y-axis.

The CPM network may be modified throughout the course of a job, as desired.

Completed tasks are entered into the data processing device and are then removed from the schedule of remaining work. In this manner, an updated schedule is calculated which optimizes production based upon actual task progress and planned performance rates. The updated schedule reflects the most efficient manner in which the remaining unfinished tasks may be completed.

The present invention plans the schedule and tracks the progress of repetitive work for relatively complex jobs using a method that requires minimal data entry. The schedule is presented, and progress data are accepted, in an intuitive and compact form.

The method of the invention converts graphical representations from a line of balance format into a tabular format that is intuitive and compact and able to be produced by automated non-graphical means (i.e., character mode as opposed to all-points addressable, or bit-mapped, mode). The method creates and utilizes an inference machine that accepts as input the CPM network for one unit and determines the various ways to create the CPM network for multiple units with the special line of balance constraints. The CPM calculation method is employed with modifications to meet special line of balance constraints. Progress reporting data is accepted in tabular or graphical format and converted into CPM data.

The details of the preferred embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a prior-art manual graphical representation of the constant rate of production method.

FIGURE 2 is a prior-art graphical representation of the line of balance method.

FIGURE 3 is a prior-art graphical representation of the critical path method.

FIGURE 4 is a prior-art schedule report setting forth a planned schedule for implementing a plurality of interrelated tasks.

FIGURE 5 is a prior-art Gantt bar chart setting forth a planned schedule for implementing a plurality of interrelated tasks.

FIGURES 6 and 7 are flowcharts depicting the schedule planning method of the present invention.

FIGURE 8 is a chart illustrating the information which is input to the schedule planning method of the preferred embodiment of the present invention.

FIGURE 9 is a chart illustrating the information which is output by the schedule planning method of the preferred embodiment of the present invention.

FIGURE 10 is a flowchart illustrating the process of creating relationships between different tasks to be performed on the same unit.

FIGURE 11 is a flowchart illustrating the process of creating relationships between the same tasks to be performed on different units. FIGURE 12 is a flowchart depicting the process of creating special constraints for the purpose of scheduling continuous work.

FIGURE 13 is a diagrammatic representation of exemplary relationships between a plurality of interrelated tasks and a plurality of units.

FIGURE 14 is a schedule printout which illustrates the results of the implementation of special constraints to provide continuous work.

Like reference numbers and designations in the drawings refer to like elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Throughout this description, the preferred embodiments and examples shown should be considered as exemplars, rather than limitations on the present invention.

Overview of the Inventive Process FIGURE 6 illustrates the scheduling method of the present invention, which combines the line-of-balance method with CPM. In describing FIGURE 6 and subsequent drawings, a CPM network for a single "template" unit is defined as comprising a plurality of "tasks" (e.g., "stake lot", "drill foundation"). The term "work item" (Wl) is defined to be a task performed on a particular unit on a job.

The invention generates a CPM network from the template unit CPM network in which each Work Item (Wl) for a job is represented as a node WI_/7. In this notation, / equals the task code and / equals the unit number. Thus, WI_/7 represents a Work Item comprising task/ performed on unit / (a third index, for job number, could be used, but is generally omitted for purposes of clarity). Work Items having / = 0 indicate tasks for the template unit. An example of the Wl codes thus formed is shown in

FIGURE 5. The Wl code "1-002-106" refers to Task #106 (Drill Foundation) to be performed on Unit #002 for Job #1 (Bush Hill development).

Referring now to FIGURE 6, in step 1 , a user enters the task data for a single "template" unit (i.e., any one of the actual units for the job) into a computer. This data includes the code, description, duration, and/or preceding tasks for each task (for all

Wl_0y. where j = 1 ,....,n). FIGURE 8 shows a chart illustrating an example of the information which is input to the schedule planning method of the preferred embodiment of the present invention. The task data for the single template will be used as a template for creating the CPM network for multiple units. ln step 2, the user specifies which tasks are to be repeated in additional units. For these repetitive tasks, the user enters: a. the number of units required to be produced per day, if > 1 ; b. the number of crews or work stations per task, if > 1 ; c. the minimum number of units required for continuous work, if > 1 (the minimum number of units required for continuous work may be defined as the "bucket size").

This data is shown in FIGURE 8 as being entered into corresponding rows with the basic task data for the template unit.

In step 3, a default sequence for listing the repetitive tasks in reports (i.e., down the

Y-axis, as shown in FIGURE 9) is determined by the computer from the data entered in steps 1 and 2. A suitable default sequence is obtained by performing a CPM forward pass on the template, and using a sorted listing of the early start date for all tasks to define the default sequence. A user may override the default listing sequence if desired.

In step 4, the user specifies the number of units m to be produced. In step 5, a CPM network database is created from the input data. The CPM network database consists of: a. a CPM network for the single template unit (including any non-repetitive tasks); b. the repetitive tasks with their durations for each additional unit (i.e., all repetitive Work Items Wl,y); c. precedence relationships between different tasks performed on the same unit for all units (see FIGURES 10 and 13); d. precedence relationships between the same tasks performed in different units that take into consideration the movement of crews through units (or units through work-stations) (see FIGURES 11 and 13); e. special constraints to implement bucket size (see FIGURES 12 and 14); f. extra intermediate sequencing to add new rows in reports where needed for multiple crews and multiple units per day (see FIGURES 9 and 11). Greater detail as to the preferred method for generating such information is set forth below in conjunction with the description of FIGURES 10-14.

Next, in step 6, the CPM method of calculation is applied in known fashion to the CPM network thus created in order to compute the early and late start and finish dates for each Work Item Wl,y. Automated computation of CPM networks is well known (see, e.g., O'Brien, "Scheduling Handbook", McGraw-Hill, p.47, with references on p. 72 and p. 143). The extension to the standard CPM procedure used in the present invention to accommodate the special ALAP constraints involves a second forward and backward CPM pass in which early start and finish dates of the constrained Work Item are replaced with late start and finish dates. In effect, this second pass delays the affected work items as much as possible. For instance, refer to FIGURE 14, where Task 134 ("Pour Piers") has been delayed on units 3 and 4.

In step 7, the computer creates a printed production schedule, a copy of which is the

"turn-around" document to be used by a field superintendent. The preferred format of the turn-around report is shown in FIGURE 9, and is an improvement to prior art Gantt (bar) charts (see FIGURE 5). The preferred report format differs from a Gantt chart in that: a. the time-line (X-axis) is spread out so that each column can accept at least a multi-digit unit number; b. the lengthy list of work items Wl_;y (Y-axis) has been replaced by the shorter list of repetitive tasks (i.e., Wl₀); c. the horizontal bars that showed the start and finish dates of a work item have been replaced by a unit number on which the task shown on the Y-axis is to be performed for the date shown on the X-axis; d. the report format displays the progress required to achieve a constant rate of production (or "line of balance"). Next, at step 8, progress on the job is reported. Preferably, a field superintendent marks the completed tasks for each unit on the turn-around document. After marking on the report the completed work at a given cut-off date, the report is returned to be used for purposes of additional data entry to prepare an updated turn-around report.

Step 9 involves the creation of an on-line (i.e., viewed interactively on a computer terminal) computer display that is essentially a visual duplicate of the turn-around document. While the unit numbers referred to in step 7c above are depicted as simply a row of numbers printed on paper, for interactive data entry at a computer terminal, it is preferable to use discrete fields on discrete records for a data entry clerk to modify.

In step 10, the data entry clerk enters into the computer the completed task and unit information indicated by the entries made by the field superintendent on the turn¬ around document. This may be done, for example, by overwriting with a zero each displayed unit number for which a task has been completed.

In step 11 , the computer updates the original database from which the CPM network was computed by marking as completed those Work Items Wl_;y in the database corre¬ sponding to the entries made by the data entry clerk. Steps 7 through 10 associate tasks with units, for compactness of presentation of the production schedule (FIGURE 9). The data captured in step 10 has to be translated back to Work Items Wl,y. This is done by deducing which Work Item WL corresponds to each field that the data entry clerk has marked as completed. Each such Work Item Wl,y then gets its remaining duration set to zero. When the CPM network is recalculated, work items with remaining duration equal to zero are considered completed and the updated schedule is thus calculated. For example, referring to FIGURE 9, if unit #1 on June 4 is set to "0", then it is simple to deduce that task #100 (Stake Lot) has been completed for unit #1, meaning that the network node for Work Item Wl_{τ 100} will have its duration set to ^H0".

In step 12, steps 6 through 12 are repeated using the modified CPM network database every update, until the end of the project.

Flowcharts Explaining Generation of CPM Network

In further explanation of the preferred embodiment of the present invention, the details of the sub-steps described above with respect to step 5 are set forth below.

FIGURE 10 is a flowchart illustrating the process of creating precedence relationships between different tasks to be performed on the same unit. The process is repeatedly performed in a series of nested loops. The innermost loop repeats the process for every template relationship. Relationships between current work items and preceding work items are established. A mid-level loop repeats the process for every unit/ from 1 to m. The outermost loop repeats the process for every template work item/ from 1 to n.

The program commences at block 1000, where the next preceding work item related to the current work item is read from the input shown in FIGURE 9. A test is performed at block 1002 to see whether the current or preceding work items are repetitive. If not, the program loops back to block 1000. If the current and/or preceding work items are repetitive, the program proceeds to block 1012, where a test is performed to determine whether or not the current work item is repetitive but the preceding item is not repetitive. If these conditions are met, a data record is created linking the current item to the non-repetitive preceding item in a precedence relationship, at block 1018. The current work item-preceding work item precedence relationship data record is added to the CPM network database at block 1022. The program then loops back to block 1000, where the next work item is read. If the conditions at block 1012 are not met, program control transfers to block 1014, where a test is performed to determine whether or not the current work item is not repetitive but the preceding time is repetitive. If these conditions are met, the preceding work time is linked to the current non-repetitive item at block 1020, and the current work item-preceding work item precedence relationship data record is added to the CPM database at block 1022. The program then loops back to block 1000 where the next work item is read. The negative branch from block 1014 leads to block 1022.

FIGURE 11 is a flowchart illustrating the process of creating precedence relationships between the same tasks to be performed on different units. The program is executed repeatedly within a series of nested loops, as previously described with reference to

FIGURE 10. The program commences at block 1100, where a work item data record

(from the input shown in FIGURE 9) is read, and then proceeds to block 1101 , where the program tests to see whether the current work item is repetitive. If not, the program goes on to the next work item at block 1100. If so, the program continues to block 1103, where a test is performed to determine whether or not the number of crews for the work item is greater than 1. The affirmative branch from block 1103 leads to block 1105, where the relationship is identified as a start-start relationship.

The unit counter is incremented, and at block 1107 the unit counter is divided by the number of crews. The result is an integer AA and a remainder BB as indicated at block 1109. The remainder BB is placed into a unit table (table 7) at block 1111.

Table T is used to indicate that extra intermediate sequences of tasks are needed for reporting purposes, resulting in the addition of new rows in reports to indicate multiple crews and multiple units per day. An example of such added rows is shown in FIGURE 9, where tasks 100 and 140 have multiple row entries.

In block 1113, the duration of the work item is divided by the number of crews. The result is an integer CC and a remainder DD, as shown in block 1115. Block 1117 tests to see whether or not the remainder DD is greater than zero. If so, 1 is added to CC at block 1119, and the program progresses to block 1121. If the remainder DD is not greater than zero, the program progresses directly to block 1121. In block 1121, the precedence relationship time lag is set to CC. The current work item- preceding work item precedence relationship data record (which includes the time lag) is added to the CPM database at block 1123, and the program loops back to block 1100.

The negative branch from block 1103 leads to block 1131 , where the program checks to see whether or not the number of units per day is greater than 1. If not, the program jumps ahead to block 1123, where the current work item-preceding work item precedence relationship data record is added to the CPM database. If the number of units per day is greater than 1 , the unit counter is incremented at block 1135, and the counter is divided by the number of units per day at block 1137. The result of the division is an integer FF and a remainder GG, as set forth in block 1139. At block 1141, the remainder GG is placed info table T. A test is performed at block

1143 to ascertain whether or not the remainder GG is greater than zero. If so, the precedence relationship is identified as a start-start relationship at block 1145, and the program then loops to block 1123. If the remainder GG is not greater than zero, the program progresses to block 1123, where the current work item-preceding work item precedence relationship data record is added to the CPM database.

FIGURE 12 is a flowchart illustrating the procedure for creating as-late-as-possible (ALAP) constraints, for the purpose of providing continuous work if feasible. The procedure is performed repeatedly, within a series of nested loops. The innermost loop performs the procedure for every unit / = 1 to m. The mid-level loop performs the program for every template relationship between a current work item and a preceding work item. The outermost loop performs the program for every template work item from / = 1 to n. The program commences at block 1201 , where a counter is initialized. Next, at block 1203, a test is performed to ascertain whether or not the current and preceding tasks are both repetitive. If not, the program loops back to block 1201. Otherwise, the program continues to block 1205, where the program checks to see whether or not (1) the bucket size is greater than 1 , and (2) the duration of the current work item is less than the duration of the preceding work item. Bucket size refers to the minimum number of units required to provide continuous work for a work crew. The negative branch from block 1205 transfers program control back to block 1201. The affirmative branch leads to block 1207, where the counter is divided by the bucket size. The result of this operation is an integer HH and a remainder KK, as shown at block 129.

The program tests to see if KK is equal to zero at block 1211. If not, the program loops back to block 1201 ; otherwise, the program continues to block 1213, where an as-late-as-possible (ALAP) constraint for the current work item is added to the CPM network database. The program then returns to block 1201.

FIGURE 13 is a block diagram illustrating different groups of precedence relationships.

Each block 1301, 1303, 1305, 1307, 1309, 1311, 1313, 1315, 1317, 1319 depicts a work item WL. Precedence relationships between different tasks to be performed on the same unit are denoted by arrows pointing in a horizontal direction (e.g., between blocks 1301 and 1303, blocks 1309 and 1311, blocks 1315 and 1317, etc.). Precedence relationships between the same tasks but different units are denoted by arrows pointing downward and to the right (e.g., between blocks 1303 and 1309, blocks 1305 and 1311, blocks 1311 and 1317, etc.).

FIGURE 14 is a diagram which illustrates the technique of bucketing to provide continuous work for a work crew. In the example shown, bucketing occurs for the task designated by task code 134, "pour piers". The work crew is scheduled to perform this task on three consecutive days (June 13, 19, and 20) for three respective units numbered 3, 4, and 5. Due to the precedence relationships between the tasks, providing continuous work for the pier-pouring crew with respect to units 1 and 2 in the present example would delay the job. The bucket for units 3, 4, and 5 is implemented by starting work on unit 3 as late as possible without delaying the job. Although it may be possible to perform task 134, "pour piers", on unit 3 earlier than June 18, the crew would not be working continuously progressing to units 4 and 5. Therefore, these three units are "bucketed", so that work is scheduled so that the task is performed on all three units continuously.

After the CPM network database is generated as described above, conventional CPM data processing is applied to the database (taking into account all defined precedence relationship types, time lags, and ALAP constraints) to generate a CPM production schedule. From such information, plus the default reporting sequence determined in step 3 and Table T, a report is generated in the format shown in FIGURE 9. That is, each task in the CPM network database is listed at least once along the Y-axis in the sequence determined in step 3. Additional rows are added for a task where indicated by corresponding entries for that task in Table 7. The work item schedule dates determined when the CPM calculation was applied to the CPM network database provide the X-axis coordinates for each unit for each task.

Summary

Thus, the present invention provides an improved process for scheduling production tasks which combines the constant rate of production method with the critical path method. The method allows a user to plan a schedule and to track progress of repetitive tasks for large and complex jobs. The process requires minimal data entry, and may be readily updated to reflect actual job progress. A production schedule is produced in a concise, easily-interpreted format.

Furthermore, the invention provides a one-step direct solution, rather than the prior art iterative approach that required user-interaction to successively improve a heuristic solution. A number of embodiments of the present invention have been described. Neverthe¬ less, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.

Claims

CLAIMS:

1. A method of determining a constant rate of production schedule for perform¬ ing a plurality of repetitive tasks on a plurality of individual units, the method including the steps of: a. determining a task schedule setting forth sequences of tasks to be performed on an individual unit; b. determining repetitive tasks from the determined task schedule to be performed on at least one other individual unit; c. determining the precedence relationships of the determined repetitive tasks for the plurality of individual units, based upon at least the determined task schedule for an individual unit; d. applying the critical path method to the determined precedence relationships to determine a substantially constant rate of production schedule for the repetitive tasks for the plurality of individual units.

2. A method of planning a production schedule for performing a plurality of tasks on a plurality of individual units, the method including the following steps: a. Calculating a constant rate of production relationship for each of the plurality of tasks by determining the relationship between the number of completed units and time; b. calculating the parallel and serial precedence relationships for each of the plurality of tasks by (i) determining a task schedule setting forth sequences of tasks to be performed on an individual unit; and (ii) determining tasks which may be performed simultaneously on an individual unit; c. Producing a production schedule from the constant rate of production relationships and the parallel and serial precedence relationships, the production schedule applying a critical path method to the plurality of tasks.

3. A method of planning a production schedule as set forth in Claim 1 , further including the step of determining the start and finish dates for each of the plurality of tasks to be performed on the plurality of units.

4. A method of planning a production schedule as set forth in Claim 3 further including the step of determining start and finish dates for each of the plurality of units.

5. A method of planning a production schedule as set forth in Claim 3 further including the steps of: a. determining an early start date and an early finish date for each task, the early dates representing the earliest possible dates for performing the task; b. applying the task schedule to the early start dates and early finish dates to develop a production schedule for at least one unit repre¬ senting an early unit completion date.

6. A method of planning a production schedule as set forth in Claim 3 further inducing the steps of: a. determining a fate start date and a late finish date for each task, the late dates representing the latest possible dates for performing the task; b. applying the task schedule to the late start dates and late finish dates to develop a production schedule for at least one unit representing a late unit completion date.

7. A method of determining a constant rate of production schedule as set forth in Claim 1 , wherein the repetitive tasks are performed by a plurality of crews.

8. A method of determining a constant rate of production schedule as set forth in Claim 1 , wherein the repetitive tasks are performed on a multiplicity of units over time.

9. A method of determining a constant rate of production schedule as set forth in Claim 1 , further including the determination of logical precedence relation¬ ships for the case where the rate of production is less than the minimum time unit of calculation.

10. A method of determining a constant rate of production schedule as set forth in Claim 1, further including the determination of logical precedence relation¬ ships for the case where a plurality of work crews is applied to the task.

11. A method of determining a constant rate of production schedule as set forth in Claim 1 , further including the determination of logical precedence relation¬ ships for the case where a plurality of resources is applied to the task, the resources including at least one of labor, manufacturing equipment, and material.

12. A method of determining a constant rate of production schedule as set forth in Claim 1 , further including the determination of logical precedence relation¬ ships for the case where a plurality of work stations is applied to the task.

13. A method of determining a constant rate of production schedule as set forth in Claim 1 , further including the determination of special restraints for creating a schedule where continuity of work must be provided where possible.

14. A method of determining a constant rate of production schedule as set forth in Claim 1 , wherein the schedule is presented in graphical format and includes unit numbers, dates, and task descriptions.