Permissive Logic
Key Points
- Checks prerequisites before enabling action
- Common in start permissives and safe sequences
- Supports safe operation in industrial systems
- Used in process control, machinery control, and safety-related systems
- Often complements interlocks and shutdown functions
Definition
Permissive Logic is control logic that confirms conditions are satisfied before allowing an action or sequence to proceed.
Concept
Permissive Logic is an industrial term used for logic that grants permission to operate only when required conditions are true. It exists to prevent operation under unsafe or invalid conditions. It is used in process control, machinery control, and safety-related systems. Permissive logic often complements interlocks and shutdown functions.
Explainer
Permissive Logic is control logic that confirms conditions are satisfied before allowing an action or sequence to proceed. It works by evaluating required readiness conditions and only enabling the action when all mandatory prerequisites are true. It is used in process control, machinery control, and safety-related systems.
Constraints include logic correctness, sensor trustworthiness, bypass management, and the need to align permissives with the actual operating state. Failure modes include false permissives, false blocks, missing prerequisites, and unsafe operation if the logic does not reflect real conditions.
Tradeoffs involve safer operation versus more logic complexity, clear readiness checks versus more configuration, and controlled enabling versus possible operational friction.
Permissive Logic matters because many industrial actions should only begin when safe conditions are confirmed. Cross-industry relevance is strong in manufacturing, process plants, and industrial control.