Systems Engineering Interfaces: A Model Based …

1 Systems Engineering Interfaces: A Model BasedApproachElyse Fosse, Christopher L. DelpJet propulsion laboratory , California Institute of Technology4800 Oak Grove DrivePasadena, CA The Engineering of interfaces is a critical function ofthe discipline of Systems Engineering . Included in interfaceengineering are instances of interaction. Interfaces provide thespecifications of the relevant properties of a system or com-ponent that can be connected to other Systems or componentswhile instances of interaction are identified in order to specifythe actual integration to other Systems or components. CurrentSystems Engineering practices rely on a variety of documentsand diagrams to describe interface specifications and instancesof interaction. The SysML[1] specification provides a precisemodel Based representation for interfaces and interface paper will describe interface engineeringas implemented by the Operations Revitalization Task usingSysML, starting with a generic case and culminating with afocus on a Flight system to Ground Interaction.

2 The reusabilityof the interface Engineering approach presented as well as itsextensibility to more complex interfaces and interactions will beshown. Model -derived tables will support the case studies shownand are examples of Model - Based documentation OFCONTENTS1 INTRODUCTION..12 INTERFACEPATTERNS ANDPOINTS OFVIEW..13 MODELINGINTERFACES WITHSYSML ..24 OPERATIONSREVITALIZATIONINTERFACEENGINEE RING..65 SUMMARY..8 ACKNOWLEDGMENTS..8 REFERENCES..8 BIOGRAPHY..81. INTRODUCTIONThe topic of Interfaces is at the heart of the multi-disciplinarynature of Systems Engineering . This area covers what isnecessary in order to connect the individual pieces of theSystem together into a working system . To accomplish theconnection the relevant properties and behavior of each partof the system must be specified. It is also necessary to specifythe particular connections between each part and the natureof those connections in terms of limitations, protocols, andvarious operational conditions or describing and specifying of interfaces in a general wayis a challenging Systems Engineeringpractices rely on a variety of documents and diagrams todescribe interface specifications for different Systems as well978-1-4577-0557-1/12/$ 2012 2012 California Institute of Technology.

3 Government sponsorship ac-knowledged1 IEEEAC Paper #2562, Version 2, Updated 5/1 existing industry standards for specific kinds of [1] provides a precise Model Based representation forspecifying the interfaces of parts and integration betweenparts through those interfaces. The language also allows fordomain-specific semantics to be applied as an extension to thebasic modeling capability of interface Engineering work performed by the Opera-tions Revitalization (Ops Rev) Task[2], sponsored by theMultimission Ground Systems and Service Office of NASA,utilized a Model Based approach to represent the interfacesand interactions required for a Mission Operations system (MOS). Mission Operations Systems Engineering concernswith interfaces begin with the Flight-Ground interface . Theinterface focuses on the interaction between the Flight Sys-tem and the Ground system .

4 Within the Ground system theMOS must interface with ground stations, networks, and ahost of organizations. Within the MOS, Mission Services,Operations Roles, and Software must interface with paper will describe the key Viewpoints used to definethe patterns for modeling MOS interfaces. These Viewpointswill then be complemented by examples from the MOS [2]. Beginning with the Flight-Ground interface ,examples from these models will be used to illustrate specifi-cation of Interfaces, Interactions between Interfaces, and thelimitations and operational contexts of those INTERFACEPATTERNS ANDPOINTS OFVIEWOps Revitalization is building a Model of a Mission Opera-tions system (MOS). The interfaces for Mission OperationsSystems cover a broad range of Systems , software, hardware,and human interactions. In order to Model this broad rangeof interfaces and interactions, it is useful to describe thesystem from different points of view [3].

5 Ops Revitalizationhas developed the Mission Service Architecture Framework(MSAF) [4]. The MSAF defines the atomic interface en-gineering components (Table 1) as well as a set of patternsfor modeling this broad set of integrated Systems and otherpatterns in the MOS. These basic Views can be used todescribe the different interfaces related to the 2 identifies the Viewpoints that address important con-cerns with respect to the topic of interfaces. Part InterfaceViewpoints of the system describe what interfaces the interface Layered Viewpoints describe theinterfaces in a recursive manner and are a special case ofthe Part interface Viewpoint. In Systems where software ispart of the system , it is common to have logical layeringof software interfaces on top of hardware. interface Speci-fication Viewpoints describe the individual specifications of1 Table Engineering DefinitionsTermDefinitionInterfaceThe system boundary that is presented by a system for interaction with other SpecificationDescribes the nature of the boundary presented by a system or component in terms of propertiesand instance of an operational entity ( system , organization, or services) interface .

6 The interactioncan connect to other operational entities according to its Interaction SpecificationDescribes how an operational entity ( system , organization, or service) can effect anotheroperational entity when a connection InterfaceIdentify Interfaces for a given are the interfaces for a given Part?Layered Part InterfaceSpecify layering of interfaces such asapplication, protocol and data is the structure of the different in-formation aspects on the interface ? interface SpecificationSpecify a given interface in terms offunctions and properties of that is the detailed set of functions andproperties of a given interface ? interface ConnectionSpecify the integration of 2 or moreParts through their respective Interfacesin terms of the specific conditions andfunction occurrences that define the inte-gration according to the interface Parts are connected to each other?

7 interface Object FlowSpecify how objects (materials, informa-tion) flow across a given integration ofinterfaces for a set of are the flows between parts of thesystem? interface Function OccurrenceSpecification for behavioral interactionacross do functions occur between Parts ofthe system ?Performance and Limitations on In-terfacesSpecify constraints on interfaces suchas policies, agreements and are the expectations and limits ofthe given integration?each interface . The Part interface , Layered Part interface ,and interface Specification Viewpoints describe the part ofthe system in terms of interfaces without explaining how theywill be integrated. Providing these views of the system allowsthe Systems Engineer to evaluate and analyze parts Based ontheir interfaces separately from how they will be remaining set of Viewpoints in Table 2 refer to how Inter-faces may be instantiated into Interactions in order to describecontext-specific integrations and behavior.

8 interface Connec-tion Viewpoints identify what parts will be integrated witheach other and how they will behave when they are Object Flow Viewpoints describe how objects areallowed to flow across the interfaces of the integrated parts inthe context with which the parts are integrated in. The objectscould be physical objects, like shipping a spacecraft to CapeCanaveral, or Radio Waves in deep space. The objects couldalso be logical like data moving through cables. InterfaceFunction Occurrence Viewpoints show behavior interactionbetween parts. From this vantage point the system can bedescribed in terms of the occurrence of functions and eventsamong the interfaces of components. interface ConstraintsViewpoints describe how the interface is constrained. Thiscould be in the form of expected performance or operationallimitations.

9 Together, these Viewpoints describe integrationof parts through their MODELINGINTERFACES WITHSYSMLThe SysML specification provides a precise Model -basedrepresentation for interfaces and interactions. interface spec-ifications are described using SysML flow ports, operations,and signals which correlate to the system or componentinterface properties, as shown in Table Specifications To SysMLInterface PropertySysML PropertyFunctionsOperationsSignalsSignal sInformation I/OFlow PortsMaterials I/OFlow PortsA simple system will be used to describe the Ops Rev Tasks sSysML interface Engineering implementation. The followingfigures and tables are views into the SysML Model describingthe system shown in Figure 1, which is comprised of twosystems, A and B. The previously identified viewpoints willbe elaborated presently using system A s interface and itsinteraction with system B2 Figure 1.

10 system IdentificationInterface Specification ViewpointEvery system or component that is intended to be composedinto a greater system has some functionality that the com-prising system needs. The functionality is a property of thesystem or component, independent from whom the systemor component is intended to interact with. The InterfaceSpecification Viewpoint focuses on identifying the function-ality properties as operations owned by a SysML block. Thefunctionality identified is the generic set of functionality thatis available to any system or component when an 2. Functionality SpecificationFigure 2 illustrates that an interface element named SystemA interface Functionality Specification is used to describethe functionality of system A. The functions that are availablefor use by other Systems or components during an interactionare shown on the interface element as operations.