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.