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Description  |
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CROSS-REFERENCE TO RELATED APPLICATIONS
The subject matter of this invention is related to application Ser. No.
07/115,073 filed Oct. 28, 1987, by K. M. Ferriter and R. B. Mathis for
"Automated Interface to Project Management Tool", which is assigned to a
common assignee herewith. The disclosure of the Ferriter et al.
application is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a computer based production
release system and, more particularly, to a plant floor production release
system which automatically interfaces a production release tool to a plant
floor monitoring system to provide an integrated approach to a
manufacturing software design. The system according to the invention is a
conceptual tool that uses a top down release process which involves
creating an order list of production orders to be released to the
manufacturing floor for which all components have been allocated and are
available for release. The invention provides an easy to use user friendly
interface for the production planner on a split screen format to assist
the production planner in releasing orders based on defined management
criteria where the system automatically sets up select priorities for
release based on the management criteria.
2. Description of the Prior Art
The process of designing, developing and manufacturing a set of new
products, or making major changes to existing products, presents many
challanges to product managers and manufacturing managers to bring a
product to market for the least cost, within schedule while maintaining
product quality. In today's highly competitive industries, product
managers and manufacturing managers require information to address many
problems that arise because of the complexity of new products and the
complexity of world-wide production and the changing nature of
competition. The requirement that products be manufactured in as short a
period as possible while maintaining a low level of inventory on the shop
floor to meet customer needs presents conflicting criteria to be analyzed
in order to make timely decisions.
Many authors have published papers and books in the field of production
management. For example, Joseph Orlicky wrote Material Requirements
Planning, published by McGraw-Hill, which has become the industry standard
reference for almost all job shop planning requirements. This concept of
planning and releasing is well accepted and, even today, many vendors are
selling software based on the concept. Nevertheless, this concept does not
lend itself to releasing shop orders in the most efficient manner.
D. T. Phillips and G. L. Hogg published a paper entitled "A
State-of-the-Art Survey of Dispatching Rules for Manufacturing Job Shop
Operation", International Journal of Production Research, vol. 20, no. 1,
(1982), pp. 27 to 45, which provides varying dispatching rules that can be
used in a planning process. Though the areas of planning and scheduling
have been discussed in detail, not a single product has been developed
which provides an easy to use and user friendly method of automatically
assigning priorities to orders prior to releasing them to the
manufacturing floor based on management criteria of increased throughput,
minimized work-in-process inventory and reduced cycle time while
maintaining customer committments. What is needed is an expert system
which is simple to use and user friendly that transforms the management
criteria, i.e., increased throughput, reduced work-in-process inventory,
and minimized cycle time, and provides the planner a mechanism that
automatically assignes priorities to shop orders to be released prior to
the actual release of orders to the manufacturing floor.
Expert systems are a branch of computer science, generally referred to as
artificial intelligence, which exhibit characteristics normally associated
with human behavior including learning, reasoning, solving problems, and
so forth. More specifically, an expert system or "knowledge-based" system
uses certain rules and a database to provide a user interactive
environment in the form of a "consultation dialog", just as the user would
interact with a human expert.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an easy to user
system which implements a top down functional approach to an automated
production release system.
It is another object of the invention to provide a system that prompts the
production planner to alternate decision capabilities by a graphic display
of expected results for varying alternatives.
The automated interface to project management tool described in application
Ser. No. 07/115,073 employs a conceptual design tool to generate the
manufacturing details by item in the product structure. When the user
invokes the project management tool interface, the system prompts the user
to select items of the product structure which are critical. The system
then orders the selected items according to lead times from manufacturing
detail gathered by the conceptual design tool. The ordered data is then
formatted in a file of the project management tool. The formatted file is
then imported into the project management tool. In addition, data modified
in the project management tool can later be formatted for export to the
conceptual design tool to allow the design process to continue with
updated project data.
The subject invention builds on the foundation of the automated interface
to project management tool, although the invention can be implemented and
practiced independently. According to the present invention, the
production planner, after receiving a list of shop orders that are ready
to be released, based only on component availability from a Material
Requirement Planning (MRP) system or similar project management tool,
needs to specify only the priority of the three basic management criteria;
namely, increased throughput, reduced work-in-process inventory, and
reduced cycle time. The system automatically performs the remaining
analysis and provides the planner with recommended priorities of orders to
be released to the floor. The system is also flexible enough so that if
the planner chooses a priority value for a given shop order, it will
re-calculate the priority values of all other orders based on the selected
criteria. The system will give a list of the results and the reasons for
the recommendations.
The process begins with the traditional planning process where the
production planner receives demand data of the manufactured parts from the
customer set. All data is loaded into the planning system. A typical MRP
system will explode the requirements of the end item into its
sub-components and establish the order sequence based on earliest due
dates. This invention takes over from then on. The system requirements for
the invention include a simulation system for the manufacturing floor,
although the specific simulation system and the language in which the
simulation system is written is not critical to the practice of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following detailed
description of a preferred embodiment of the invention with reference to
the drawings, in which:
FIG. 1. is a system block diagram of the planning process showing the
functional requirements for implementing the automated production release
system according to the invention;
FIG. 2 is a block diagram showing a logical functional layout of a
production release system which describes the input form the MRP planning
process giving a list of orders with quantity and due dates;
FIG. 3 is a block diagram illustrating the relative position of the user
with respect to the production release system and a relational database;
FIG. 4 is a flow chart illustrating the logical analysis flow for arriving
at the best combination of order release and dispatching rules under a
given management criteria of throughput, work-in-process inventory and
cycle time;
FIG. 5 is a system funcational flow diagram of a conventional simulation
program which may be used in the practice of the invention;
FIGS. 6A and 6B, taken together, are the simulation logic flow chart of the
simulation program illustrated in FIG. 5;
FIG. 7 is a sample graph of one set of test results and the associated
selection results; and
FIG. 8 is a sample display screen layout.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 1, there is
shown in functional block diagram form the normal set of processes that
take place in a typical manufacturing environment from the time of
planning through production and shipment. The key parts of this system are
the master schedule planning 10, where items are planned at the completed
item level, the material requirement planning system 20, where
requirements of completed items are exploded into sub-components and
planned, and the production release system 30, where shop orders planned
at the MRP level 20 are set for releasing to the manufacturing shop floor.
The remaining two systems are the manufacturing floor control system 40
and the shipping systems 50.
FIG. 2 illustrates the production release system 30 in more detail. Here,
the list of orders and their due dates as received from the planning
system 20 are input to the production release system at function block 31
and reviewed for further analysis in function block 32. The production
release system displays the management criteria priority as a default set
or as input during a previous use and requests the user to input the
current priorities. The user responds and provides this input at block 33.
The system then invokes the common function simulation program of the shop
manufacturing floor in block 34, calculates the various combinations of
order release and dispatching rules as set out in List 1 below for the
orders under evaluation and generates a recommended revised sequence and
priorities in block 35 based on the results of the analysis. The system
automatically sets up management reports in block 36 which can be printed
upon request.
LIST 1
Order Release Rules
(i) Critical Ratio is defined as a ratio which equals (due date --current
date)/lead time remaining. If this ratio is less than 1.0, the job is
critical and consequently will be a candidate for release.
(ii) I/O Balancing is a simple but effective method involving the following
steps:
1. Establish unit of production to be measured.
2. Find appropriate level of in-process inventory.
3. Install means to measure output.
4. Release according to measured output.
(iii) Downstream Pull is a finite loading method that accounts for capacity
and location. Steps for implementation are as follows:
1. Estimate capacities and flow times.
2. Establish means to measure in-process inventory by process.
3. Release according to calculated required and measured actual in-process
inventory.
(iv) Bottleneck Scheduling is a reduced version of downstream pull and it
involves the following steps:
1. Identify the bottleneck process.
2. Apply downstream pull concept over the identified process.
3. Insure that the process is saturated.
Dispatching Rules
Dispatching rules are used to establish the order in which orders are
processed at a particular workstation.
(i) Shortest Imminent Operation Time (SI) dispatches those jobs that have a
short processing time as opposed to long jobs. In order to have a limit on
the maximum allowable time that a job can be waiting, this rule is often
modified such that jobs with short processing times are released provided
no other job in the queue is made to wait more than a given specified
amount of time. The revised rule is often described as SI/T (truncated)
rule.
(ii) Earliest Due Date dispatches jobs that have the earliest due dates.
(iii) First-In-First-Out (FIFO) releases jobs based on the order they
arrive for processing.
(iv) Critical Ratio dispatches jobs based on ratio as described above. This
ratio is also used as a dispatching rule.
FIG. 3 illustrates the key parts of the system from the user point of view.
The key parts are the database 60 and the query system 70. The database
can be any of several products currently available, but for purposes of
this preferred embodiment, IBM's DataBase 2 (DB2) is used. DB2 is a
relational database management system, ,but it will be understood by those
skilled in the art that other databases, including hierarchical databases,
could be used. General information on IBM's DB2 can be had with reference
to publication GC26-4073-2 published by IBM Corp. The query system can be
an expert system, but for purposes of the preferred embodiment, IBM's
Restructured Extended Executor (REXX) language is used. A description of
the REXX language is provided in "Virtual Machine/Systems Product, System
Product Interpreter User's Guide", Release 4, publication SC24-5238-2
published by IBM Corp.
The user 80 can query the current status, completion date and the priority
sequence of any job in question using the query system 70. The query
system 70 interfaces with the production release system 30 which accesses
data in database 60 and provides a preferred sequence of orders 90.
The data base as defined has the capability to capture the decision
variables tested and the results obtained for each test. The user can
access the results using the query facility at a later date, if needed.
This enhances the analysis capability of future test data. This also
provides an additional enhancement to the system.
The flow chart of FIG. 4 illustrates in detail the functional logic of the
release analysis system. The process begins in function block 100 where
the user input preferred management priority criteria is received by the
system. Then the simulation of the manufacturing floor is invoked in
function block 102. The simulation in turn has an additional input derived
from the list of order relationships and dispatching rules, i.e., List 1,
as indicated in block 104. The result of the simulation in block 106 is
the expected throughput, work-in-process inventory and cycle time. A test
is made in decision block 108 to determine if the result of the simulation
meets management criteria. If it does, a further test is made in decision
block 110 to determine if all combinations of the rules have been tested.
If either of the tests in decision blocks 108 or 110 is negative, control
goes to function block 112 which receives as its input List 1 from block
104. In function block 112 a new sequence of order relationships and
combination of dispatching rules is generated to control the simulation of
the manufacturing floor in function block 102. When both tests in decision
blocks 108 and 110 are positive, the best result of the various
combination of rules is selected in function block 114 and reports of the
tested combination of rules and criteria is output in function block 116.
It is appropriate at this stage to discuss the simulation of the
manufacturing floor as described in block 102. Those skilled in the art of
simulation will understand that a simulation of a manufacturing system can
be accomplished in a number of ways. It can be custom programmed in any
language or it can be a customized enhancement to a commercially available
computer program. One such program is "GEMS II", a Generalized
Manufacturing Simulator published by Loadestone-II, Inc. of Bryan, Tex.
This software package has the natural orientation to modeling
manufacturing environments. GEMS II is a network based technique such that
its model is largely represented in a graphical format which resembles
both manufacturing process flow diagrams and a PERT (Program Evaluation
and Review Technique) chart. The model consists of boxes or nodes which,
in general, represent various manufacturing and decision activities of the
system, and arcs which show the procedural relationships among the
activities. The logic of GEMS II recognizes queues (in-process
inventories, production backlogs, etc.) and assembly processes. Further,
it recognizes competition among activities for limited resources, such as
tools, fixtures, space and manpower.
The functional flow diagram of the GEMS II simulator program is shown in
FIG. 5. The simulator consists of a main program 120 and a plurality of
subroutines. Its design incorporates the interaction of five components;
data input via the input module 122, logic control via the logic control
module 124, statistical data collection via the statistical collection
module 126, simulation via the simulation module 128, and output via the
output module 130. The main program 122 initiates the program, initializes
variables, and transfers control of the program to the several operational
modules.
The logic control module 124 is the executive routine which organizes the
execution sequence of the other four modules. The data input module 122
reads the input data and constructs the simulation data base. The
simulation module 128 provides the support for systems including
simulation, generation of random numbers and deviates for each simulation
run. The statistical data collection module 126 collects and maintains
relevant statistics. The output module 130 writes the report of the
simulation.
FIGS. 6A and 6B, taken together, are a simplified flow chart of the basic
logic of the GEMS II simulation. The process begins in function block 132
where variables are initialized for simulation. All the GEMS II source
boxes are then put in the waiting list in function block 134. The
transactions in the waiting list are scanned in function block 136 to
select an activity to be scheduled. The activity selected is the one which
is feasible to schedule and has the highest priority level. A test is made
in decision block 138 to determine if the selected activity exists, and if
it does, it is transferred from the waiting list to the in-process list in
function block 140. When a transaction is enetered into the waiting list,
it is associated with a particular list location. Associated with each
location is a box number, a pointer to the next transaction in the list,
and a series of attributes. When the transaction is transferred from the
waiting list to the in-process list, all that actually occurs is that the
pointers in both lists are updated. The procedure is repeated until no
feasible transaction are found in the waiting list. At this time, the test
in decision block, at which time control goes to connector A in FIG. 6B.
With reference now to FIG. 6B, the activity whose completion time is the
smallest is selected from the in-process list. The activity (transaction)
is then deleted from the in-process list and TNOW is set to the completion
time of the activity, as indicated in function block 142. The number
releases (NR) of the follower boxes of the activity are updated in
function block 144. If NR reaches zero, the follower box activity is
entered into the waiting list. A single simulation run is completed when
the specified number of sink box completions have occurred or the
requested simulation completion time has been reached, as determined by
the tests in decision blocks 146 and 148. An exception to this occurs when
segmentable boxes are used in the system model. Segmentable boxes are
those for which an activity can be stopped temporarily for a specified
period of time, in which case the in-process list is scanned to determine
if segmentable activities are in process. If some segmentable activities
are in process, the waiting list is scanned a second time to determine if
any of the activities in the waiting list have a higher priority level
than the segmentable boxes' priority levels. If not, the process proceeds
as before; otherwise, all segmentable boxes with priority levels less than
the waiting activities priority level are preempted and transferred to the
waiting list from the in-process list.
The list of orders to be tested for simulation is treated as an event; that
is, one order will be treated as an event with the average processing
times, i.e., time required to move from station A to station B, etc.,
extracted from the parameter data base which contains each process related
data for the item under study. In FIG. 4, function block 112 causes the
re-sequencing of order release and dispatching rules. After a simulation
is conducted with using one order release rule and a dispatch rule, a
simulation is next conducted using the next rule which has not been
tested. In other words, block 112 acts as a rule changing block which
becomes the next test case for simulation analysis. The logical flow
describes the procedure to test all possible combinations of order release
and dispatching rules before the test is completed and results analyzed.
FIG. 7 is a graph showing a sample result. As can be seen in this figure,
the downstream pull rule of order release and dispatching provides the
least amount of work-in-process inventory and cycle time. However, the
output value is less than that of the value with the order release rule as
MRP and critical ratio as the dispatching rules. If the preferred
management criteria priority is set to work-in-process inventory reduction
as highest priority followed by cycle time reduction and throughput, the
selected order release and dispatching rules would be downstream pull. If
output required the highest priority, the selected rule would be MRP
followed by critical ratio as order release and dispatching rules,
respectively. Based on the system selected rules, the system automatically
sets priorities for the shop orders in question for release.
FIG. 8 illustrates the split screen format adopted for orders planned and
recommended priority for release. The split screen display of the
manufacturing manager's management criteria alongside the effects due to
the priority input is a valuable tool for the planner/user. This is very
practical for the user as he or she can immediately see the effect of a
priority sequence. As mentioned, the system will also be able to define
priority sequence of orders among the list after a specific order or sets
of orders are pre-defined. The user can make modifications to management
priority and see what it does to the priority sequence of the remaining
orders. This split screen concept is very user friendly and provides
improved decision making capabilities to the user. This analysis thus
provides an element of artificial intelligence to the system.
To further illustrate the artificial intelligence provided by the system,
reference is again made to FIG. 4. The user as defined earlier is prompted
through the split screen format to list the management priority for
throughput, work-in-process reduction and cycle time for the manufacturing
process. As is common in any manufacturing process, the manager would
always prefer to maximize the throughput, minimize the work-in-process
inventory and reduce the manufacturing cycle time. Unfortunately, in the
real world, these criteria are not complementary requiring the manager to
accept compromises in his decision making. In general, if it is desired to
maximize the output, all machines in the manufacturing process need to be
functioning all the time, which means that there should be enough material
ready for the process. However, this tends to increase the work-in-process
inventory. On the other hand, if a small order is waiting behind a large
order, even if all the machines are working to their full capacity, if not
properly scheduled certain orders may be waiting for more than a desired
time thereby increasing the manufacturing cycle time.
In block 100, the input requirement from management regarding the priority
settings is specified. This is followed in block 102 by the simulation
function of the manufacturing shop floor. This capability to be able to
simulate any environment is assumed as a requirement of the system
according to the invention. Using the list of orders to be released and
the list of order release and dispatch rules (List 1), the system
calculates varying different possible results. After a thorough search of
the results, the order sequence that provides the desired throughput,
work-in-process inventory and cycle time for a selected set of order
release and dispatching rules is selected. This capability for selecting
the desired order release sequence for a given management objective is the
artificial intelligence that the system according to the invention
provides.
The invention has been described using one commercially available
simulation program. Other simulation programs, such as IBM Corp.'s GPSS
generalized simulation program, could be used. What the invention does is
to provide the manufacturing manager with a powerful planning tool by
using the simulation program to test various management priorities and
display the results in a split screen format which greatly enhances the
manager's decision capabilities. Thus, while the invention has been
described in terms of a single preferred embodiment, those skilled in the
art will recognize that the application can be practiced with modification
within the spirit and scope of the appended claims.
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