> 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/11-reference/examples/example-ptf-for-start-button-and-cylinder.md).

# Example: PTF for start button and cylinder

This example shows how the **Selmo method (PTF)** is applied before a sequence is modeled or programmed.

The example is deliberately simple:

* a **Start button**
* a **Cylinder with two end positions**

The goal is not the modeling, but the **structured thinking before the model**.

***

### P – Process

#### Domain process

From a domain perspective the following should happen:

1. The operator presses the start button
2. The cylinder extends
3. The cylinder reaches its front end position
4. The operation is completed

The process describes:

* **what** what happens
* **in which order**
* **without technical details**

It describes **not**:

* how the cylinder is actuated
* which sensors are used
* how long the movement takes

> **Process = domain sequence, independent of technology.**

***

#### Process boundaries

The process includes:

* Start request by the operator
* Movement of the cylinder
* Reaching the target position

Not part of the process:

* Power supply
* Emergency stop
* mechanical design
* maintenance

***

### T – Technology

#### Technology used

The following technology is used for the process:

* a pneumatic cylinder
* a solenoid valve for actuation
* two end position sensors:
  * retracted
  * extended
* an electrical start button

***

#### Technological characteristics and boundary conditions

The following facts arise from the technology:

* the cylinder can **not simultaneously** be retracted and extended
* the cylinder requires energy (compressed air)
* the movement is **not instantaneous**
* the end position sensors provide binary signals
* the start button is a momentary signal

These characteristics are **not negotiable**, but physically given.

> **Technology describes what is physically possible.**

***

#### Technological risks

* Valve defective → cylinder does not move
* Sensor defective → end position is reported incorrectly
* No compressed air → movement does not occur

These risks must be monitored later, but are **not yet modeled here**.

***

### F – Function

#### Derived functions

From process **and** technology the following functions can be formulated:

1. **Detect start request**\
   Movement may only occur after a deliberate start request.
2. **extend cylinder**\
   After a valid start request the cylinder should extend.
3. **Monitor end position**\
   The movement is only considered complete when the front end position is reached.
4. **Detect impossible states**\
   The two end position sensors must not be active at the same time.

***

#### Functions as verifiable statements

Important: Functions are **no states** and **not an implementation**.

Examples of correctly formulated functions:

* "The cylinder may only extend if a start request is present."
* "The cylinder is considered extended when the front end position is active."
* "Both end position sensors must not be active at the same time."

These statements are:

* verifiably
* unambiguously
* technologically justified

***

### Result of the PTF

After completion of the PTF it is clear:

* **which sequence** is to be modeled
* **which technology** is involved
* **which functions are strictly necessary**

This creates the prerequisite for:

* meaningful states
* clear sequences
* correct zones
* clean monitoring

> **Modeling only begins now.**

***

### Transition to the Selmo model

Only on the basis of this PTF are defined:

* States (e.g. "Waiting for start", "Cylinder moving", "Cylinder extended")
* Zones (start button, cylinder)
* Monitoring (end position, pair-check)
* Sequence and behavior

> **PTF decides *****what***** is modeled.**\
> **Selmo decides *****how***** it is described formally.**


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