In this chapter

Chapter 3 of 13

  1. 3.1Delimitation of terms
  2. 3.2Normative framework
  3. 3.3Classification in ISO/OSI
  4. 3.4Gap in the training context
All chapters
  1. 01Initial situation
  2. 02Objective and approach
  3. 03State of the art
  4. 04Requirements
  5. 05Solution approaches and concept decision
  6. 06System architecture
  7. 07The test rig
  8. 08The heated-tank model
  9. 09The simulation cycle
  10. 10Five test scenarios
  11. 11Result and limits
  12. 12Outlook
  13. 13Preliminary projects

Chapter 03 Fundamentals

State of the art

Digital model, shadow and twin; normative framework and ISO/OSI classification

By Chapter 3 of 13 2 min read Updated: 30 September 2026

Summary

Digital twins are distinguished from models and shadows by the degree of automation of the data flow; this thesis sits at the twin. The normative framework is DIN EN ISO 23247, VDI 2206 and IEC 61131.

Delimitation of terms

“Digital twin” is used for very different things. The common distinction is based solely on how automatically the data flows (Fig. 16), not on the accuracy of the model.

Digital modelThe real plant and the model exist side by side; data is transferred manually in both directions.
Digital shadowThe plant feeds the model automatically, but not automatically back.
Digital twinData flows automatically in both directions. This is where this thesis sits; the real controller takes the place of the real plant as the physical coupling partner.
The distinction is made solely by the degree of automation of the data flow — this thesis sits at the twin.
Fig. 16 Digital model, digital shadow, digital twin: distinguished by the degree of automation of the data flowSource: Own illustration

Normative framework

Three sets of rules delimit the field:

  • DIN EN ISO 23247 describes the digital twin in manufacturing.
  • VDI 2206 provides the development methodology for mechatronic systems (Chapter 2).
  • IEC 61131 stands for the programmable logic controller itself.

Added to this is real-time capability: a fixed cycle is something different from a guaranteed cycle. This distinction becomes important in Chapter 9.

Classification in ISO/OSI

The coupling can be located in the ISO/OSI reference model (Open Systems Interconnection) (Fig. 12): browser, REST-HTTP and WebSocket at layer 7, the JSON format at layer 6, below that TCP (4) and IP (3), the UART byte framing (2) and at the very bottom USB and signal levels (1) of the serial connection to the signal bridge. There is no separate session layer; the physics model sits above layer 7.

Where browser, REST, WebSocket, JSON and the serial connection sit in the reference model.
Fig. 12 Classification of the system parts in the ISO/OSI reference modelSource: Own illustration

Gap in the training context

Digital twins for laboratory systems exist, but they are mostly tied to a specific plant or require licensed software. For training operations with existing LOGO! controllers, no suitable solution was available in the assessment of the thesis; hence the in-house development (Chapter 4).