WhitePaper: Digital Twins- Classification and Examples

Diverse technical applications utilize digital twins. However, the term “digital twin” encompasses a wide spectrum of meanings, often lacking clarity regarding its specific attributes, and what parts of the real system it represents. This WhitePaper proposes a classification of digital twins based on two criteria: the type of communication between the digital twin and reality, and the simulation speed at which the digital twin mirrors reality. This classification is supported by numerous practical examples illustrating the classification.

Author: Prof. Dr. habil. Michael Schreiner

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During the Apollo 13 mission to the Moon in 1970, technical issues with the service module necessitated an in-flight abort. The revised plan was for the capsule with the service module and lunar module attached to orbit the Moon and then return to Earth. Initially, determining how to operate the engines for this trajectory was a challenge. What happened next was perhaps the first documented use of a digital twin: Houston’s Mission Control assessed the spacecraft’s current position, velocity, and the altered configuration with the lunar module attached. Using this data, they calculated a revised course back to Earth and devised precise thruster usage timings to correct the trajectory. These simulations, conducted on a NASA computer, provided crucial guidance to the crew inside the space capsule. Ultimately, this innovative procedure proved successful, enabling the safe return of the crew to Earth.

So, what happened? The behavior of a spacecraft within the gravitational fields of both the sun and the moon were calculated with a computational model, and communication took place from reality to the program (regarding the position and speed of the spacecraft). Upon computation, directives were transmitted back to the spacecraft. Moreover, the system was able to respond to unexpected requirements, such as the new course or mass of the shuttle with the lunar module docked. This is how we envision a digital twin today!

Of course, the term “digital twin” was not in use in 1970. It appeared later in a NASA publication in a different context and has been widely used in various publications in recent years. However, its usage varies significantly. In the context of Industry 4.0, for example, the term is used as a digital replication of a real process operating as software in tandem with the actual system and engaging in data exchange with it. In the Industrial Metaverse environment, on the other hand, the focus is on interaction with people, and a real system is not assumed.

The German “Gesellschaft für Informatik” gives the following description in its dictionary of Computer Science, available on its website: [1] “Digital twins are digital representations of things in the real world. They describe both physical objects and intangible entities like services, providing comprehensive information and functionalities through a standardized interface. For the digital twin it is irrelevant whether the counterpart already exists in the real world or will exist in the future”. As you can see, the term “digital twin” is used very broadly. It is therefore useful to classify it to better understand what is meant by a digital twin within various contexts.

The objective of this white paper is to categorize various digital twin variations and explain them with practical examples. While their application for services is verlooked, the emphasis is directed towards digital twins associated with real objects.Initially, we will categorize digital twins based on their communication with the actual system. We distinguish between digital twins that

• operate independently without any communication with the real system,

• solely receive data from a real system without transmitting any information back,

• engage in bi-directional communication with a real system.

The working group on Digital Twins within Industry 2025 [2], for instance, identifies bi-directional communication as an essential characteristic of a digital twin. However, our interpretation here diverges from this definition, focusing instead on practical usage. Alongside the categorization based on communication capabilities, we also distinguish whether the digital twin operates synchronously with the real world (i.e., in real time) or not.


[1] gi.de/informatiklexikon/digitaler-zwilling/

[2] www.industrie2025.ch/wissen-industrie-40/arbeitsgruppen/digitaler-zwilling

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