Semantic Definition Format (SDF) modeling for Digital Twin

A digital twin is defined as a digital representation of an object of interest and may require different capabilities, for example, synchronization and real-time support, according to the specific domain of application. . Digital twin help organizations improve important functional objectives, including real-time control, off-line analytics, and predictive maintenance, by modeling and simulating objects in the real world. Therefore, it is important for a digital twin to represent as much real-world information about the object as possible when digitally representing the object. Nowadays, digital twin technologies are applied in various domains including manufacturing, energy, medical, farm, transportation, etc. And a common format is needed to represent the objects in the domains as digital twins. SDF can be used for modeling objects as digital twins. This document specifies the modeling and guidance on how to use SDF to represent objects as digital twins.

Terminology This specification uses the terminology specified in in particular "Class Name Keyword", "Object", and "Affordance". The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

This section describes SDF structure with the new Class Name Keyword, sdfNonAffordance, to represent a thing or an object as a digital twin. The architecture of a digital twin based on the SDF model is illustrated in , , following the guidelines of . The Physical Layer comprises affordance and non-affordance objects. From the real-world objects, only those deemed relevant are selected for representation as digital twins. The Digital Twin Layer is structured into three sublayers: the Device Communication Sublayer, the Digital Twin Sublayer, and the Application Sublayer. The Device Communication Sublayer is responsible for monitoring and collecting data from both affordance and non-affordance objects. This sublayer provides the necessary data to synchronize the physical objects with their digital twin counterparts. The Digital Twin Sublayer ensures synchronization between the affordance and non-affordance objects and their respective digital twins using the data provided by the Device Communication Sublayer. The Application Sublayer presents the synchronized values of the digital twins to users, facilitating informed decision-making.

Basic Architecture of digital twin

The document is based on the underlying structure defined in , which which standardizes the semantic definition format (SDF) for representing IoT affordance. This specification provides a strong basis for representing individual devices and their features (sdfProperty, sdfAction, sdfEvent, etc.), but additional mechanisms are needed to address the unique requirements of digital twin modeling. Digital twin systems defined in often have to describe virtual representations of various physical assets, including metadata, identity, contextual relationships, historical data, as well as device interfaces.

A new SDF keyword sdfNonAffordance described in is introduced to represent non-functional or metadata elements that describe a device or component without implying direct interaction: Identifier (e.g., UUID, URN) Location (e.g. site, zone, GPS tag) Owner (e.g., representative, ,anufacturer) These field can appear in both sdfObject and sdfThing contexts and follow the same structural pattern as sdfData and is designed for scalability.

To support hierarchical representations (e.g., a boat composed of heater, GPS, and battery subsystems), this document encourages use of sdfThing to aggregate related sdfObject components, along with metadata. The mapping of digital twin attributes to sdf qualities are shown in . Digital twin modeling within sdfThing

Attribute	Recommended Mapping	Description
Identifier	sdfNonAffordance	Globally unique digital twin ID (e.g., URN)
Characteristic	sdfProperty or sdfData	General description or domain properties
Schedule	sdfEvent or sdfData	Time-based actions, availability, or maintenance
Status	sdfAction or sdfProperty	Actual or calculated operating conditions
Location	sdfNonAffordance	Physical or logical location information
Report	sdfData	Measurement summaries, analytics, or logs
owner	sdfNonAffordance	Organization or entity responsible for the digital twin
Relationship	[TBD]	Inter-object/inter-twin relationships

The example of boat007 illustrates how a Digital Twin representation can be constructed for a heater component (heater1) installed on a specific vessel (boat007). The Digital Twin is modeled using the sdfThing structure, which references the heater object defined in the sdfObject section.

An example of SDF mapping for digital twin { "info": { "title": "An example of the heater #1 in the boat #007", "version": "2025-01-27", "copyright": "Copyright 2025. All rights reserved.", }, "namespace": { "heater1": "https://example.com/boat007/heater1" }, "defaultNamespace": "heater1", "sdfThing": { "boat007": { "label": "Digital Twin of Boat #007", "description" : "A ship equipped with heating and navigation systems", "sdfRequired": { "heater1": "#/sdfObject/heater" }, "sdfNonAffordance": { "identifier": { "type": "string", "const": "urn:boat:007:heater:1" }, location:{ "wgs84": { "latitude": "35.2988233791372", "longitude": "129.25478376484913", "altitude": "0.0" }, "postal": { "city": "Ulsan", "post-code": "44110", "country": "South Korea" }, "w3w": { "what3words":"toggle.mopped.garages" } }, "owner": { "type": "string", "default": "ExamTech Ltd." } }, "sdfObject": { "heater1": { "label": "Cabin Heater", "description": "Temperature control system for cabin heating", "sdfProperty": { "characteristic": { "description": "Technical summary of the heater", "type": "string", "default": "12V electric heater, 800W, automatic cutoff" }, "status": { "description": "Current operational status", "type": "string", "enum": ["on", "off", "error"], "default": "off" } }, "sdfEvent": { "maintenanceSchedule": { "description": "Next scheduled maintenance date", "type": "string", "format": "date-time" } } "sdfData" : { report: { "On February 24, 2025, the boat #007's heater #1 was on from 9 a.m. to 6 p.m." } } } } } } }

Overview A digital twin is a partial representation of sdfThing or sdfObject that contains attributes such as sdfProperty, sdfAction and sdfEvent. By representing sdfThing as a digital twin, crucial events that require appropriate action can be quickly detected and controlled. The requirements defined in are applied to represent sdfThings and sdfObjects as digital twins.

Data acquisition Data related to sdfThing and sdfObject, such as sdfProperty, sdfEvent, and sdfAction, should be collected from IP and non-IP devices.

Data analysis The collected data needs to be analyzed to understand the state of sdfThing and sdfObject.

Identification The sdfThings and sdfObjects should contain data that uniquely identifies them as digital twins.

Accuracy The sdfThings and sdfObjects should be represented as digital twins with appropriate levels of detail and accuracy, depending on the application.

Synchronization The sdfThings and sdfObjects should be synchronized with their digital twins in real-time as appropriate for the application. Newly added or removed sdfThings and sdfObjects should be recognized and reflected in the digital twin.

Security Considerations Only authorized users should have the authority to manage sdfThings and sdfObjects.

IANA Considerations This document has no IANA actions.