Selmo structure and user concept – static documentation part

1. Purpose and Scope

This section describes the operation and control concept of the system based on the Selmo method (Sequence Logic Modelling). The contents apply to machines, plants or modular systems whose control was created in Selmo Studio and implemented in the PLC.

The purpose of this description is to make the operating principle, mode of operation and safety-relevant characteristics of the control logic understandable to the operator, maintenance personnel and inspector.

---

2. Basic Principle of the Selmo Method

The control of the machine is based on a deterministic state model (Selmo SEQ). This means:

• The machine sequence is divided into states (switch positions) structured.

• The machine can be at any time only in one state located.

• The transition between states occurs only when all defined conditions are met.

This ensures that the behavior is unambiguous, reproducible and safely traceable at any time. The Selmo method replaces classical program code logic with a formal sequence model.

---

3. Structure of the Control Architecture

The control is structured according to the Selmo system hierarchy constructed:

1. Plant (entire system) The Plant describes the entire machine or system as a logical unit.

2. Hardware zones (HWZ) Each HWZ is an independently controllable area of the plant (e.g., station, module).

3. Sequences (SEQ) A SEQ forms the logical sequence of a functional unit (e.g., clamping, machining, unclamping).

4. Zones (Zone Types) Zones are the switching elements within the SEQ. They represent sensors, actuators or logical functions.

Each level is formally described and coordinated with the others. Changes in one area affect all other levels transparently and traceably.

---

4. The Sequence Engine

The sequence engine controls the logical order of the processes. It operates according to a fixed rule:

IF (AR = 1 AND ¬I AND ΣS = 0) THEN Next State

Meaning of the variables:

• AR (Auto Release) – Automatic release active

• ¬I (No Interlock) – no safety fault active

• ΣS (Sum of Sequence Checks) – all active actions completed

Only when all three conditions are met does the sequence engine automatically switch to the next state.

This ensures:

• The sequence runs only in released automatic mode.

• Safety violations lead to immediate stop.

• Actions are only continued when their feedback has occurred.

---

5. The Zones and Their Function

Zones connect the sequence engine with the real machine. Each zone stands for a technical signal, a physical function or a logical behavior. The zones are standardized into four types:

Type	Meaning	Examples
IN	Input signal	Push button, sensor, switch
OUT	Output signal	Lamp, valve, contactor
IN_OUT	combined action + feedback	Cylinder, axis, calculation
MEM	internal logical memory zone	Parameter, software flag

---

6. Operants – the Behavior of the Zones

Each zone reacts in every state with a so-called operand. The operant determines how the zone behaves:

Operant	Meaning	behavior
S (Sequence Check)	Action active, waiting for feedback	Zone sets output, waits for feedback
I (Interlock)	Safety monitoring	Stops automatic mode on deviation
M (Monitoring)	Diagnostic monitoring	Reports error, sequence continues
0 (Don’t Care)	Inactive	Zone unaffected

These operants form the basis of the bit control in the Selmo system. In operation they are recognizable by HMI colors:

• Green = active action (S)

• Red = Interlock (I)

• Yellow = Monitoring (M)

• Gray = inactive (0)

---

7. Safety in the System

The Selmo structure contains several standardized safety mechanisms:

1. Interlock function Every zone with Operant I monitors a defined safety condition. If it is violated, the automatic release falls away immediately and the sequence is stopped.

2. PairCheck Monitors contradictory feedbacks, e.g. for cylinders with end positions front/back. If both feedbacks are active at the same time, an interlock is automatically triggered.

3. MXIC (Manual Cross Interlock) Prevents unsafe movements in manual operation. Every button is only active when all associated safety conditions are met.

4. Monitoring (M) Provides diagnostics and hints without interrupting the sequence. Supports preventive maintenance.

These mechanisms are part of the standard – they do not have to be programmed additionally.

---

8. Operation via the HMI

The Selmo control displays all relevant information automatically in the HMI (Human Machine Interface):

• Active state (e.g., “Cyl. 1 Extend”, “Waiting for Start”)

• Zone status with color coding

• Zone texts as plain text messages

• Error display with cause and affected zone

• Diagnostic window for monitoring messages

The HMI is thus at the same time the operating interface, diagnostic tool and documentation of the current system state.

---

9. Behavior in Case of Malfunctions

If a zone reports a deviation:

• The sequence engine halts the sequence.

• The affected zone is shown in red.

• The message appears in the diagnostic window.

• The operator can identify the error and fix it in a targeted manner.

• After acknowledgment (reset or reactivate AR) the process is continued.

This ensures that no uncontrolled movement occurs and that every error can be clearly localized.

---

10. Documentation and Traceability

The entire logic is self-documenting. Every SEQ, every zone and every state are defined in Selmo Studio. The HMI texts and diagnostics are generated automatically from these definitions.

Thus the documentation is:

• always up to date,

• be complete,

• verifiable,

• and independent of the programmer.

An inspector or auditor can at any time trace how the behavior is defined.

---

11. Summary for the Operating Manual

The Selmo control differs from classical PLC programs in that it represents behavior formally. For the operator this means:

• Every step in the sequence is visible and explained.

• Errors are localized and described.

• Safety functions are automatically active.

• The control behaves reproducibly in every situation.

For the maintenance technician this means:

• Diagnostics are available directly on the HMI.

• Changes can be traced without risk.

• The behavior remains stable even during updates.

For the manufacturer this means:

• The control architecture corresponds to the state of the art.

• Documentation and CE evidence can be derived directly from the model.

• The entire sequence is logically closed and verifiable.

---

12. Conclusion

The Selmo method defines a consistent operation and control concept. It enables the operator:

• a transparent understanding of the machine functions,

• safe interaction via standardized zones,

• and clear diagnostic possibilities.

Thus the control is not only functional, but explainable – and meets the requirements for modern, standards-compliant and safe machine controls.