Standards-compliant system description - SDEA

📄 Standards-compliant system description: Selmo automaton (extended deterministic state machine)

1. Basic model

The control system is based on an extended deterministic finite automaton (DFA) according to the principle of a Moore machine. This means:

• The control is state-based.

• All outputs are defined exclusively by the active state.

• State transitions occur deterministic, i.e. uniquely and fully traceable.

Each state describes a logically self-contained step in the machine's behavior.

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2. Functional components of the automaton

The automaton is defined by the 8-tuple:

S=(Z,Σ,Γ,δ,λ,z0,B,M)

Symbol	Description
Z	Finite set of modeled states
Σ	Input set from sensors, buttons, feedback signals
Γ	Output set to actuators (e.g. valves, motors)
δ	State transition function taking inputs and memory into account
λ	State output function according to Moore
z0	start state
B	Bit control matrix with signal behavior per state and zone
M	Set of mem-zones (internal state memory)

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3. Bit control matrix (system layer)

The bit control matrix defines the behavior of each zone in the respective state:

• 0: No requirement ("Don't care")

• S: Sequence check → expected action or reaction

• I: Interlock → safety-relevant monitoring requirement with stop

• M: Monitoring → safety-relevant monitoring requirement with documentation

A transition to the subsequent state is only permitted if:

• all S-zones are fulfilled

• no I-zone reports a deviation

• no errors exist in higher-level Constantly Monitoring Zones (CMZ)

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4. Zone structure

Each zone is uniquely typed:

• Input zone: Monitoring input signals

• Output zone: Actuation of outputs = actuators without feedback

• In-out zone: Combination of control and feedback

• Mem zone: Internal state memory for controlling dependent sequences

The zones are connected to the states via a systematic matrix (bit control). This creates a complete behavioral model of the machine.

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5. Memory function (mem-zones)

Mem-zones serve as internal memory function:

• They are explicitly set or reset in a state with S .

• They can be used in transition conditions and logic paths.

• Their status is deterministically included in the sequence model.

• Used for synchronous sequence control to ensure the sequence occurred or to prevent incorrect state

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6. Error detection and state monitoring

The system detects deviating signal states automatically:

• I-zones monitor safety-relevant signals → lead to automatic stop with fault diagnosis in case of deviation

• S-zones display operator guidance in the HMI → state waits actively until all conditions are met

• Error messages are generated automatically, localized and displayed in the HMI

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7. Advanced safety functions (standard Selmo)

In addition to the base logic, the following standard functions are integrated in the model:

• CMZ (Constantly Monitoring Zone): permanent signal monitoring, independent of state

• MXIC (Manual Cross Interlock): safe release conditions for manual movements in manual operation

• Parameter layer: modeled, parameterizable values (e.g. times, counts, thresholds)

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8. Normative classification

The deterministic, formalized structure of the Selmo automaton supports a traceable and risk-minimized machine control in accordance with:

• EN ISO 13849-1 (Safety-related parts of control systems)

  • Function monitoring (e.g. interlock, CMZ)

  • Fault detection and defined reactions

• IEC 61508 / IEC 62061 (Functional safety)

  • State modeling for safe control sequences

  • Determinism, testability and verifiability

• IEC 61131-3 (Programming standard)

  • Separation of control (state) and monitoring (bit control)

• Machinery Regulation (EU) / CE conformity:

  • Traceable, testable control behavior

  • Automatically producible documentation through formal modeling

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9. Advantage of formalization

The complete formal definition enables:

• Automatic code generation

• Automatic validation and simulation

• Complete technical documentation

• Reduction of human programming errors

• Increased traceability in terms of product liability