> For the complete documentation index, see [llms.txt](https://infosys.selmotech.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://infosys.selmotech.com/selmo-solution/selmo-solution-en/selmo-documentation/3-the-selmo-method-ptf.md).

# 3 The Selmo method (PTF)

\
It is **consciously placed before the model core** and answers a central question:

> **How must one think about a machine,**\
> **before it is modeled?**

The Selmo method prevents that:

* modeling is done too early
* technical details dominate the process
* implicit assumptions enter the model
* complexity grows uncontrollably

***

### Why a method is necessary before the model

A formal model is only as good as the thoughts\
that led to its creation.

Without upstream methodology the following arise:

* overloaded sequences
* technically driven states
* safety logic as a repair
* hard-to-explain models

Therefore Selmo separates **Thinking and Modeling** strictly.

> **Not everything that is technically possible,**\
> **is also functionally sensible.**

***

### PTF – Process · Technology · Function

The Selmo method is based on three clearly separated levels:

* **process**\
  What should happen functionally – independent of technology?
* **Technology**\
  Which physical and technical principles are used?
* **Function**\
  Which verifiable requirements necessarily result from this?

These three levels together form the **PTF structure**.

PTF is:

* not a diagram
* not a tool
* not a substitute for engineering

But:

> **A framework of thought that prepares modeling**\
> **and limits complexity.**

***

### Role of PTF in the overall structure

The classification is clear:

PTF (Thinking & Analysis)\
↓\
Selmo model (formal & deterministic)\
↓\
PLC / HMI / Diagnostics (implementation)

PTF decides:

* *what* is modeled
* *why* it is modeled

The Selmo model decides:

* *how* this behavior is described formally

***

### Distinction from the model core

This chapter describes **no**:

* Sequences
* zones
* States
* Bit-Control
* Operating modes

These elements therefore deliberately belong in the following chapters.

PTF:

* creates clarity
* reduces modeling errors
* prevents implicit logic

> **PTF is the prerequisite for good models –**\
> **not their replacement.**

***

### Structure of this chapter

Chapter 3 is divided into two parts:

#### 3.1 PTF – Process · Technology · Function

→ the method in detail\
→ thinking model and typical errors

#### 3.2 Digital Twin as behavior model

→ distinction from marketing terms\
→ classification of the Selmo model relative to reality

***

### Transition to the model core

After this chapter it is clear:

* which questions must be answered before modeling
* which assumptions **not** belong in the model
* why Selmo models remain lean and explainable

The following chapters describe the **formal core of Selmo**:

* structure
* Logic
* Behavior
* Operation
* Safety

> **Think first.**\
> **Then model.**\
> **Then automate.**


---

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