ABSTRACTLow volume production associated with space systems
manufacturing is inherently expensive, time-consuming, andrisk-laden. At the root of this problem is an inability toadequately predict, monitor, and control the productdevelopment and sustainment process. This paper describes theASSIST system, an intelligent knowledge management systemdesigned to address inefficient information managementprocesses and improve space system affordability. ASSIST isdesigned for collaborative engineering, manufacturing, andtesting within a company, between companies, and betweenlocal and remote individuals. ASSIST uses web-basedstandards (including HTTP and XML) as the common messageapproach connecting its components. Automated processplanning and production scheduling is a key component ofASSIST. This paper describes an integrated process planningand production scheduling problem and discusses the solutionapproach.
Keywords: Internet based design and manufacturing.
1. INTRODUCTIONLow volume production associated with space systems
manufacturing is inherently expensive, time consuming and riskladen. Typical characteristics of low volume manufacturing,such as lack of standard design and common parts, frequentchanges to design, costly test environments, and loosely
coupled product teams, including suppliers, are obstacles toachieving space systems affordability. At the root of thisproblem is an inability to adequately predict, monitor, andcontrol the product development and sustainment process. Thistranslates to an inability to have effective Integrated Productand Process Development which, as identified by theDepartment of Defense’s Lean Aerospace Initiative (LAI), isessential for manufacturing affordability.
A team led by Lockheed Martin Space Systems Co.Missiles and Space Operations (LMSSC-MSO) is providing thesolution: the Affordable Space Systems Intelligent SynthesisTechnology (ASSIST) for Manufacturing program. ASSIST isan intelligent knowledge management system that dramaticallyreduces cost and cycle time in low volume space systemmanufacturing. This team also includes NexPrise, Inc., OrbitalNetwork Engineering, Inc., Nabh Information Systems Inc., andthe University of Maryland. The ASSIST Program is a 43-month program sponsored by the Air Force ResearchLaboratory, Materials and Manufacturing Division, as aTechnology Investment Agreement under the ManufacturingTechnology for Affordable Space Systems (MASS) initiative.
The goals of the ASSIST program are a reduction of 50%in subsystem design man hours, 15% in procurement cost, 50%in launch site support hours, and 50% in particular pre-launchtesting regimens. The ASSIST System will be demonstrated ina series of validation pilots involving LMSS-Missiles & Space
Operations (MSO) satellite propulsion subsystems. Thesereductions will be made possible by the five components ofASSIST: electronic collaboration, electronic procurement,virtual product modeling & prototyping, automated processplanning and production scheduling, and automated testoperations management. All components will be available tothe user through a single ASSIST graphical user interface(GUI).
The principal objective of ASSIST is process andenterprise affordability improvements within the spacemanufacturing enterprise that will reduce cost and cycle time,while demonstrating improvements in quality for current orplanned Air Force spacecraft subsystems. ASSIST addressessuch problems as the lack of interoperability between currentinformation management tools used in manufacturing, whichyield poor access to data and inadequate team integration.Inadequate team integration leads to isolated, redundant, costlyand time consuming efforts such as design changes after toolinghas been produced, redundant design efforts for similarelements, and manual testing, with little traceability torequirements or design changes.
Automated process planning and production scheduling is akey component of ASSIST. ASSIST includes a processplanning and production scheduling application (called IPPS)that is flexible and general enough to handle complexmanufacturing systems such as those used to manufacture spacesystems. Space system manufacturing consists of manycomplex, labor-intensive tasks. Assembly, integration, andtesting require skilled personnel, expensive equipment, andspecialized facilities. Although the production quantity is low,the production complexity is high. Dramatically reducing themanufacturing cost and time requires utilizing the availableresources effectively. In such complex manufacturing systems,a large number of equipment and human resources must worktogether harmoniously to meet production goals efficiently.Given a set of tasks to complete within a given time horizon,production planning and scheduling activities must assignindividual resources, or combinations of resources, to specifictasks and decide when the tasks should begin. Resourcesinclude trained personnel, machines, infrastructure, andcomponents from inventory. In some cases, the particularcombination of resources applied to the task affects the amountof time that the task requires. This type of resource allocationand scheduling problem is encountered in many manufacturingsystems where personnel have a variety of skills and machinescan be assigned to several different types of tasks.
In this setting process planning denotes using informationabout a specific product design and resource availability todetermine which tasks should be done and how they should bedone. Production scheduling refers to assigning specificresources to each task and determining a task start time. Thus,some portion of process planning must be done dynamically,since changes in resource availability (which can occurunexpectedly) may require finding a different resourcecombination to perform the task.
Figure 1. The applications that ASSIST integrates.
The remainder of this paper proceeds as follows. Section 2describes the intelligent synthesis technology that is thearchitecture of ASSIST. Section 3 discusses the processplanning and production schedule component (IPPS). Section 4concludes the paper.
2. INTELLIGENT SYNTHESIS TECHNOLOGYThe ASSIST approach leverages collaborative and
information management technologies that have beensuccessfully demonstrated in stand-alone environments, andintegrates them in an innovative and extendible infrastructure(see Fig. 1). The resulting environment, which provides aseamless electronic communication management system,overcomes enterprise-wide manufacturing cost and timeconsumption due to limited availability and inefficient use ofinformation. ASSIST facilitates practices which improve teamdynamics through all phases of the space systems acquisitioncycle. This is accomplished by managing technical, cost andschedule information so that all project participants aresupported in,and guided to,more efficient practices.
The technical approach being used to develop the ASSISTsystem consists of:♦ defining the objective of ASSIST♦ defining requirements for the system based on the business
case for the LMSS-MSO standard Propulsion Subsystem(PSS) (which is the first testbed for the technology)
♦ gathering user-defined requirements♦ analyzing current “as-is” processes♦ identifying and evaluationg commercial off-the-shelf
(COTS) products and other leveraged technologies that willsatisfy the requirements
♦ adhering to software industry standards♦ defining the ASSIST architecture♦ developing prototype implementations of the architecture♦ testing prototype modules♦ providing modules to users♦ refining the ASSIST system
♦ validating the technologies through validation pilotsDuring the requirements gathering phase, the "as-is"
design, procurement, manufacturing/assembly, testing andlaunch operations processes and associated product and datamodels of the PSS were examined. Interactions andcoordination with suppliers were also included in the processanalysis, since ASSIST will impact processes from all phases ofthe satellite PSS development, i.e., from design, toprocurement, to manufacturing/assembly, to testing and launchoperations.
In addition to user requirements, issues related toscalability, portability, and utilization of standards are addressedin the approach. The ASSIST development team is alsofollowing the standard LMSSC/MSO software developmentprocess. As part of that process, they have adopted a spiralsoftware development approach.
The development approach is based on a spiraldevelopment model, where the spiral process cycles throughdesign, development, delivery, and evaluation phases severaltimes. Spiral development supports rapid prototyping inenvironments such as ASSIST, where developers are taskedwith evolving an architecture as the technology changes.LMSSC-MSO is building an integrated system of disparateparts, which include legacy, commercial, and internal systems.
A series of ASSIST validation pilots will be the formalassessments of the impact of the ASSIST technology on the PSSprocesses and will provide feedback on the system, as appliedto the production environment by the actual end-users. Thisfeedback will also be used in the refinement of the ASSISTsystem as the program progresses through development spirals,and as input for the market and business plans for thecommercialization feasibility of the technology.
There are three views that are used to develop and explainthe ASSIST architecture: the operational view, the system view,and the technical view. This breakdown into three views isborrowed from the US Defense Department’s C4ISRrequirements for describing architectures. The operational viewis an operational picture of the use, and provides background onwhy a particular architecture decision was made. The systemview identifies what components are in the architecture, andtheir interrelationships. The technical view relateshow thearchitecture will be built, identifying the technical standards andmethods used in the architecture.
2.1 OPERATIONAL VIEWThe operational view is an operational picture of the
intended use for an architecture and provides background onwhya particular architecture decision was made. Operationally,the ASSIST system is intended to be used for collaborativeengineering and manufacturing/testing within a company,between companies, and between local and remote individuals.In Fig. 2, the internal users have been identified, as well astraveling users, and external users (suppliers). These users mustbe able to obtain and update information while external to theircompany system and must be able to communicate that
information with external companies and individuals. Tooperate on this information, the users require a variety oflegacy, commercial, and internal tools. Thus the primaryrequirement for ASSIST is to provide an integrated view anduse of the different ASSIST information and tools (whethercommercial, legacy, or internal).
Table 1 lists the operational architectural requirements thathave been distilled for application to the ASSIST architecture.These include availability requirements, integrationrequirements, and security requirements.2.2 SYSTEM VIEW
The system view identifieswhat functional components arein the architecture, and their interrelationships. ASSIST has acomponent-based, client-server architecture (see Fig. 3), andthere are two categories of components: infrastructure modulesand application modules. Infrastructure modules are used as aframework for the application modules, connecting people,tools, and information into a process. Application modules arefocused on supporting a particular task, such as automatedtesting of satellite propulsion or maintaining an engineeringnotebook. IPPS is one such module. The other modulesinclude the workflow agent (WORK), the notification agent
Table 1. The ASSIST operational architectural requirements.Availability RequirementsProvide a Web front end to the ASSIST systemNeed to interact with supplier and other systems across firewallsNeed for the ASSIST system/user interfaces to interact withmultiple serversNeed to interact with legacy systemsBusiness logic independence from “plumbing” details and userinterfaceScalability for large amounts of users, data, and projectsHigh availability and easy fail recoveryIntegration RequirementsEliminate or reduce vendor-lock in for COTS toolsNeed to interact with common COTS tools (Excel, IDEAS,Microsoft Project, etc.)Need to interact with various databases (Oracle, Sybase,Informix, etc.)Need to support transactions that span across various systems(i.e., provide workflow capabilities)Need for tools that help set up process / data flowLeverage investments in legacy codeSecurity RequirementsAbility to locate components and execute them securely (i.e., aDirectory Service)Authentication of users and servers (e.g. PKI, CA, two-wayauthentication)Provide secure communications (e.g. HTTPS, IIOP-SSL)Collaboration services (Workflow, Notification)
(ALERT), the virtual product server (VPS), security (LOCK),the test operations manager (TOM), the document vault (DV),the Internet collaboration notebook (ICN), and the smartprocurement manager (SPM).
ASSIST’s client-server approach has several systemimplications. The first is that an emphasis on a web-basedinterface to system components without internal heavyapplications that require long downloads or unusual plugins. Atthe same time, the system must support real engineering andmanufacturing operations such as propulsion testing as well assupporting client applications such as LabView, and MicrosoftExcel and Project.
The system view figure (Fig. 3) shows a cluster of clienttools talking with the server-side modules using the ASSISTframework. Note that there is a need to cross the firewallsbetween companies and a need to support client interfaces toserver-based tools. Both of these imply that the ASSIST systemshould adhere to industry standards and use commercial off-the-shelf (COTS) software wherever possible.
2.2 TECHNICAL VIEWThe technical view relateshow the architecture will be
built, identifying the technical standards and approaches used inthe architecture, as well as the reasons for adopting the
standards. This section describes the capabilities desired, thenrelates qualities for industry standards, and finally discusses atiered architecture that enables these capabilities and qualities.
The ASSIST system is to incorporate previously developedcollaboration and special COTS technologies. Some of thesetechnologies exist as a part of their own framework (e.g. theASSIST DV module is the incorporation of the ipTeam iVaultCOTS product which operates within the ipTeam framework).In addition, there are several legacy software and hardwaretools that users would access during its operation. Users,however, do not want a fragmented view of the ASSIST systemnor should they need to interact with multiple frameworks anduser interfaces. Thus a “component-based approach” thatsatisfies both these potentially conflicting requirements is beingused. This approach has four key features, which the followingparagraphs describe.
Encapsulation of individual products as “Components.”Components are objects that communicate via a standardizedremote communication protocol. The ASSIST modules are atype of component. Given the mix of computational platformsidentified in the operational view, ASSIST uses web-basedstandards (HTTP – hypertext transport protocol) and XML(eXtensible Markup Language) as the common messageapproach connecting our components. Once the applicationmodules have been encapsulated as components, the approachthen consists of building customized graphical user interfacesthat can access multiple ASSIST components and yet present anintegrated, user-friendly view of the underlying informationbase.
Separation of Business Logic and “ilities.” ASSISTprovides a clean separation between the actual functionality and“ility” issues such as scalability and security. This will allowdeployment to the same business object in different executioncontext without any re-implementation of the logic.
Single-Sign-On. We plan a centralized user authenticationmechanism that is used by all ASSIST infrastructure modules.This will eliminate the need to log on separately for each of theASSIST components.
Dual User Interface. ASSIST users fall into twocategories. The first and smaller category of users includesprocess owners who configure the information flow within theASSIST environment. Typically these users need a “heavy-weight” user interface and would typically interact with thesystem from one desktop. Thus ASSIST will use Javaapplications (as opposed to applets or HTML interfaces) thatprovide such user interfaces. The other category of everydayASSIST users include managers, engineers, administrators thatneed lightweight user interface and access from wherever theyare located. ASSIST will include a set of web-based userinterfaces for this class of users.
3. PROCESS PLANNING AND SCHEDULINGAs described in Section 1, planning and coordinating
production in complex manufacturing systems such as thoseused to manufacture space systems becomes a resource
allocation and scheduling problem. Modeling such a problemcan require a large number of discrete variables (for taskassignments), continuous variables (for task start times), andnonlinear constraints. Many special cases of the problem forminteresting machine scheduling problems, and many workershave studied these and proposed solution techniques. For anoverview of scheduling results and research, see Pinedo [1].This research has studied parallel machine problems, flow shopproblems, and project scheduling problems. The specialstructure of these problems reduces the problem formulationand the solution space.
As manufacturing systems become more complex,however, the limitations of these special cases cause problems.Thus, it is necessary to consider a general formulation thatincludes resource combinationsand the characteristic that atask's duration depends upon the particular combinationperforming the task. The generalized problem is more flexibleand can address a broader range of manufacturing systems.IPPS formulates a very general model and employs an approachthat can harmoniously incorporate the discrete and continuousvariables that are necessary.
3.1 IPPS OVERVIEWThis section gives an overview of IPPS by describing the
typical actions that a user would complete to plan andcoordinate production. IPPS has seven modules, through whichthe user can perform various IPPS functions (see Fig. 4). Eachmodule is designed to organize a set of functions. Through thefunctions of IPPS, the user can perform the following types ofactions:• View and edit information needed for scheduling• Construct a scheduling problem• Solve the scheduling problem• View and edit a schedule• Publish a schedule
To begin, the user updates information about the orders,materials, and resources in the factory. When the user adds anew order, IPPS locates critical design information about thespecific system that has been ordered. This specific system willbe a customized version of a certain model. A process templatedescribes the generic set of tasks needed to produce one unit ofthis model and rules used to update the process template tocreate a process plan for the specific system ordered. Theprocess plan specifies the set of required tasks, the taskprecedence constraints, all feasible resource combinations foreach task, and the task duration for each feasible resourcecombination. A resource combination is one possible methodfor performing the task. It identifies the specific types ofemployees and equipment needed for that method and howmany that method requires. Different resource combinations forthe same task specify different types of employees andequipment. Note that the factory may have many employees or
Figure 4. The IPPS user interface.
tools of the same type. Thus, IPPS has to determine whichmethod (resource combination) should be used to perform atask (a process planning activity) and which individualemployees and tools will perform the task (a schedulingactivity). Thus IPPS integrates process planning andproduction scheduling.
IPPS first uses the critical design information and theprocess template to create the process plan. This process planis still incomplete since some tasks may have multiple feasibleresource combinations. The schedule optimization engine willcomplete the process plan based on resource availability at thesame time it assigns specific resources (employees andequipment) to the task.
After the needed information has been updated, the usermay release one or more new orders that need to be scheduled.If no additional orders are released, the new schedule willinclude the active and planned tasks in the current publishedschedule. If orders are selected for release, the user will beasked to confirm the release, and the tasks associated with theseorders will be included in the new schedule.
Then, the user may add preferences about specific tasks byspecifying any of the following information about the task:earliest start date, earliest start time, earliest completion date,earliest completion time, latest start date, latest start time, latestcompletion date, and latest completion time. The user mayforce one or more specific resources to perform the task orprohibit such an assignment.
When ready, the user asks IPPS to optimize the schedule.Then, IPPS reads all of the necessary information, generates aninstance of the scheduling problem, and calls the optimizationalgorithm to find a good schedule. (Section 3.2 gives theproblem formulation, and the solution approach is described inSection 3.4.) When the optimization algorithm is finished, theuser can view and edit the new schedule and may publish thenew schedule. The problem size will vary based on the numberof system in process and the number of tasks required for each
system. For the satellite propulsion subsystem factory, a typicalscheduling problem will have fewer than 10 systems andapproximately 35 tasks per system.
3.2 PROBLEM FORMULATION AND RELATED WORKThis section describes problem PSP, the integrated process
planning and production scheduling problem that IPPSgenerates. There exist a finite setT of tasks,T = {1, …, n}, anda finite setR of resources,R = {1, …, m}. The problem is toassign one or more resources to each task and to determine eachtask's start time. Simple task precedence relationships mayexist. We can represent these as a directed, acyclic graphG = (T, E), where the edge (j, k) exists inE if and only if taskjmust be completed before taskk can begin.
Tasks are non-preemptive; once some resources beginperforming a task, that task must continue uninterrupted until itis completed. A resource can perform at most one task at atime. All tasks and resources are available at time 0.
There are two sets of decision variables: the task-resourceassignments and the task start times. LetSj be the start time of
task j in T. Let Aij = 1 if resourcei in R is assigned to taskj in
T. Aij = 0 otherwise. Note that, in a feasible schedule, one or
more resources are assigned to each task. The duration of atask j in T is a function of the resources assigned to it. Eachtask has a duration functionDj(A1j, …, Amj) that is the task's
duration for a given set of resource assignments.Dj(A1j, …,
Amj) = ∞ if the resource combination is infeasible for taskj.
The objective of PSP is to minimize the total flowtime.Given a feasible schedule (a set of assignments and a set of starttimes), letFj be the finish time of taskj:
Fj = Sj + Dj(A1j, …, Amj)
Let Xij (t) = 1 if resourcei in R is performing taskj at timet ≥ 0
and 0 otherwise.Xij (t) = 1 if and only ifSj ≤ t ≤ Fj andAij = 1.
The objective function is the total flowtime:F1 + … + Fn.
A feasible schedule must satisfy the following constraints:Dj(A1j, …, Amj) < ∞ for all j in T.
Fj ≤ Sk for all edges (j, k) in E.
Xi1(t) + … + Xin(t) ≤ 1 for all i in R andt ≥ 0.
Aij ∈ {0, 1} for all i in R andj in T.
Sj ≥ 0 for all j in T.
Research into project management has consideredcontrolling processing times by allocating resources [2, 3].Other workers have studied resource-constrained projectscheduling problems with controllable processing times [4-10].Machine scheduling problems with controllable processingtimes have also received much attention [11-18]. Daniels andMazzola [19], Danielset al. [20], and Olafsson and Shi [21]have studied parallel machine and flow shop problems wherethere exists a set of renewable resources. Assigning one or
more of these (identical) resources to a job affects theprocessing time. The problem is to simultaneously allocatethese resources and sequence the jobs.
PSP, with multiple unrelated resources and a more generaltask precedence structure, includes these problems as specialcases.
Chauvet, Levner, and Proth [22] study a general schedulingproblem where there exist alternative jobs that must be selected.Applications include environments where these exist alternativeprocess plans [23]. PSP could be transformed into such aformulation, where each feasible resource combination becomesan alternative job.
3.3 EXAMPLEThe following example illustrates some of the
characteristics of PSP. There are four tasks and three resources.Two precedence relationships exist: Task 1 must precede Task2. Task 1 must precede Task 3.
The following resource assignments are feasible: Task 1requires either Resource 1 or Resource 2. In either case, thetask duration is 5 hours. Task 2 requires either Resource 1 orthe combination of Resource 1 and Resource 3. Its duration is 4hours if Resource 1 performs the task and 2 hours if Resource 3helps. Task 3 requires either the combination of Resource 1 andResource 3 or the combination of Resource 2 and Resource 3.Its duration is 4 hours if Resource 1 and Resource 3 perform thetask together and 2 hours if Resource 2 and Resource 3 performthe task together. Task 4 requires Resource 3 alone. Itsduration is 7 hours.
Table 2 summarizes the task duration times for all possibleresource combinations. Table 3 lists one combination of starttimes and resource assignments that form a feasible schedule.In this schedule, Resource 1 performs Task 1 and then Task 2.Resource 3 performs Task 4 and then, with Resource 2,performs Task 3. SinceF1 = 5, F2 = 9, F3 = 9, andF4 = 7, the
total flowtime is 30.
Table 2. All possible resource combinations.Taskj Assignment
Table 3. A feasible set of resource assignments and start times.Taskj Assignment
(A1j, …, Amj)Start timeSj
1 (1,0,0) 02 (1,0,0) 53 (0,1,1) 74 (0,0,1) 0
3.4 SOLUTION APPROACHIPPS uses a three-step approach to solving the integrated
process planning and production scheduling problem describedabove as PSP. First, based on information from the currentschedule, the set of additional orders, material availability, andresource availability, IPPS generates a set of tasks that need tobe completed. These include active tasks that have alreadybeen started and planned tasks that have not yet started.
The user is able to review these tasks and define additionalpreferences. These may force specific resources to perform thetask, prohibit specific resources from performing the task, orconstrain the task’s start time or end time. This allows the PSPto capture external issues that are beyond the ordinary. PSP isformulated as described in Section 3.2.
Second, IPPS uses cybernetic optimization by simulatedannealing (COSA) to find a superior solution. COSA is aparallel variant of the simulated annealing (SA) algorithm[24, 25]. The COSA framework utilizes feedback controlmechanisms that enhance the convergence behavior of SA. SAis a metaheuristic and has the advantage of being broadlyapplicable. In addition, it is relatively easy to model problemsfor solutions by SA (and COSA). All that is required is adefinition of an objective function and some neighborhooddefined for each solution. Constraints can be easilyincorporated into most problems by using penalty functions thataugment the objective function. SA however, suffers fromgenerally slow convergence. The COSA approach attempts tomitigate this slow convergence by using what is referred to asprobabilistic feedback controland parallel processing. Thisgives COSA improved performance relative to SA and makes ita candidate for solving PSP.
By determining values for the resource assignment and starttime variables, COSA selects the best process plan for each joband schedules each required task. To guide its search, COSAmeasures the total task flowtime of a schedule and addspenalties when constraints are violated. Thus, COSA attemptsto find feasible schedules that minimize the time that jobs are inthe shop (which minimizes in-process inventory). When COSAcompletes, IPPS uses a heuristic to repair any remaininginfeasibilities.
Third, IPPS allows the user to modify the constructedschedule. The user can change resource assignments and taskstart times. After reviewing and updating the schedule the usercan make it the official schedule by publishing it.
4. SUMMARY AND CONCLUSIONSThe ASSIST system is an intelligent knowledge
management system designed to address inefficient informationmanagement processes associated with low-volume production.The goals of the program are a reduction in subsystem designeffort, procurement cost, launch site support hours, andparticular pre-launch testing. These objectives are met by theintegration of successfully demonstrated individual technologiesinto a seamless electronic communication system, the ASSISTSystem. The system will be validated in a series of pilots andthe results migrated to the industry through dissemination andthe exploration of the commercialization. The ASSIST team isconfident that these goals can be met, as well as achievingdefense manufacturing affordability.
ASSIST integrates process planning and productionscheduling by linking manufacturing operations with designdecision support. The IPPS module performs process planningand production scheduling. The user can view and editinformation needed for scheduling, construct a schedulingproblem, solve the scheduling problem, view and edit aschedule, and publish a schedule.
The development of IPPS has led to a new combinatorialoptimization technique for a very general class of difficultresource allocation and scheduling problems. The newtechnique, based on COSA, has great promise and will be testedextensively against other algorithms.
ACKNOWLEDGMENTSThe development of IPPS was sponsored by Lockheed
Martin Space Systems as part of the Affordable Space SystemsIntelligent Synthesis Technology (ASSIST) for Manufacturingprogram. The University of Maryland performed this workunder subcontract SY01H8901R. The University of Marylandteam is grateful to all of the assistance provided by theLockheed Martin ASSIST team.
The ASSIST Program is a 43-month program sponsored bythe Air Force Research Laboratory, Materials andManufacturing Division, as a Technology InvestmentAgreement (F33615-99-3-5902) under the ManufacturingTechnology for Affordable Space Systems (MASS) initiative.
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