2.2 The switching mechanism – the sequence engine

Task and significance

The sequence logic – in Selmo Sequence Engine called – is the central control logic. It is a deterministic automaton, which always knows exactly which state the machine is in and when it may continue operating.

You can imagine the sequence logic as a functional block that has three inputs and one output:

• Inputs:

  • AR (Auto Release) – indicates whether the automation is enabled.

  • ¬I (No Interlock) – shows that no safety fault is active.

  • ΣS (Sum of Sequence Checks) – is 0 when all active actions have provided their feedback.

• Output:

  • State_ID – the current switch position, i.e. the active state.

As long as any of these conditions is not met, the sequence logic remains stopped. When all are met, it switches to the next state.

The rule of the sequence logic

Its mode of operation can be reduced to a single formula:

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

This rule is universal. It replaces complex program logic with a formal condition that applies equally to every machine.

Suggested representation: An IEC box with labeling: SEL_SEQ_ENGINE On the left three inputs (AR, ¬I, ΣS), on the right one output (State_ID). Below it the phrase: "Only one active state – deterministic cycle".

Structure in the Logic Layer

In the Logic Layer the states are modeled graphically. Each state is a block in the flow diagram, connected by arrows that represent the transition to the next state.

A sequence logic can contain the following logical elements:

1. State – a normal process state with defined actions.

2. Decision – a branch, e.g. "If part present → Path A, otherwise Path B."

3. Timer-State – a state that proceeds automatically after a time.

4. Repeater – repeats one or more states.

5. Jump – jumps deliberately to another state, absolute or conditional.

These elements are sufficient to fully map any machine process. They are chosen so they remain interpretable in any control system.