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 SDF language uses six Class Name Keywords, and sdfNonAffordance is added as a new Class Name Keyword for digital twin. The information attributes for digital twin are defined in . Some of them, for example characteristics and status, can be described using existing Class Name Keywords. And the others, for example location, can be described using sdfNonAffordance. The mapping information attributes of digital twin to qualities of sdfThing are shown in . It is required to define a new quality in sdfThing in order to express the properties of digital twins such as location. Mapping Information Attributes of Digital Twin to Qualities of sdfThing

Information	Quality	Description
Idetifier		A value used to uniquely identify a sdfThing / sdfObject
Characteristic	sdfProperty	A typical or noticeable feature of a sdfThing / sdfObject
Schedule	sdfEvent	Time information bound to a sdfThing / sdfObject
Status	sdfAction	A condition of a sdfThing / sdfObject
Location		Geographical or relative location information of a sdfThing / sdfObject
Report	sdfData	Description of activities done by or onto a sdfThing / sdfObject
Relationship		Connection information between two or more sdfThings / sdfObjects

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/heater1" }, "defaultNamespace": "heater1", "sdfThing": { "boat": { "description" : "A boat including heater1", "sdfProperty": { "status": { "description": "The status of the boat #007 ; false for stop and true for move." "sdfRef": "#/sdfProproperty/status" } }, "sdfObject": { "heater1": { identifier: { "UUID": { "a2e06d16-df2c-4618-aacd-490985a3f763" } } 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" } } "sdfData" : { report: { "On February 24, 2025, the boat #007's heater #1 was on from 9 a.m. to 6 p.m." } } } } } } }

Requirements for digital twin

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.

Procedures for digital twin A procedure for representing sdfThing, as a digital twin in a domain is as follows: defining a purpose for expressing the observable object, as known as a physical asset or an object of interest, as a digital twin in the domain organizing data based on the roles of the observable object in the domain configuring the observable object into the digital twin based on the data for the purposes interworking with a digital twin of each of other domains in which the observable object performs a different role synchronizing the observable object and 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.