In this chapter

Chapter 4 of 13

  1. 4.1Initial state
  2. 4.2Stakeholders
  3. 4.3Functional requirements
  4. 4.4Non-functional requirements
  5. 4.5System boundary
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 04 Design

Requirements

Stakeholders, functional and non-functional requirements, system boundary

By Chapter 4 of 13 2 min read Updated: 30 September 2026

Summary

From the needs of the stakeholders follow six functional requirements (FA-01 to FA-06) and three non-functional requirements (NFA-01 to NFA-03). The system boundary encompasses only the simulation system.

Initial state

The as-is state consisted solely of the exam wall with simulation boards without internal states (Chapter 1). There was no software at the beginning; apart from purchased finished modules for signal conversion, everything was created within the scope of the thesis.

Stakeholders

ApprenticesWant to test their PLC program on a plant that reacts and to understand plant behaviour.
TrainersNeed a robust, didactically usable environment that can be started quickly.
Exam preparationDemands compatibility with the existing controllers and task definitions.

Functional requirements

Six functional requirements support the system:

  • FA-01: read in signals from the controller
  • FA-02: calculate the model in a fixed cycle
  • FA-03: report threshold switches in the exact step
  • FA-04: display the plant state
  • FA-05: be able to add further plants as model plugins
  • FA-06: make errors recognisable

Non-functional requirements

  • NFA-01: operation without external services
  • NFA-02: cycle time of at most 100 ms
  • NFA-03: a model that can be tested without hardware and user interface
Why this belongs together

NFA-03 is the reason why all five test scenarios could run without a connected controller: testing is done directly against the model function compute_step (Chapter 10).

System boundary

The system comprises the simulation system that replicates the plant and displays its state (Fig. 18). Controller, control panel, monitor and operator adjoin it. Only binary signals are exchanged (Fig. 17): the outputs of the controller (Q1–Q12) are inputs of the model, whose reports are applied to the inputs of the controller (I1–I16).

What belongs to the system, what adjoins it and what is exchanged in between.
Fig. 18 System boundary and interfaces: what belongs to the system and what adjoins itSource: Own illustration
Depending on the viewing direction, the same line is called output or input: actuating signals Y1–Y4 and E1 forward, sensors B1–B4 back.
Fig. 17 Signal direction: depending on the viewing direction, the same line is called output or inputSource: Own illustration

Concepts for meeting these requirements: Chapter 5.