Learn Automation with MINATA #67: Batch Control and ISA-88 — Separate Recipe, Equipment and Procedure
Batch Control and ISA-88: Separate Recipe, Equipment and Procedure
ISA-88 gives a team one language for batch production: what the product needs, what the equipment is capable of, and how the control procedure carries it out. Separating those three layers is what makes a system extendable and reviewable.
Do not start from tag names
Start from the physical model, not from I/O addresses. ISA-88 divides equipment into levels: a process cell contains units, a unit contains equipment modules, and an equipment module contains control modules. That structure reflects how the process actually works, and it is the frame the control logic hangs on.
Separate the recipe from the equipment
The heart of the standard is the separation of two layers:
- The recipe describes what the product needs: targets, quantities, sequence and limits. It should contain no I/O addresses and no equipment logic.
- Equipment capability describes what the machine can do: the phases the equipment offers — charge, agitate, heat, discharge.
Because the two are separate, changing a product rarely means changing the PLC, and one recipe can run on different equipment as long as it offers the same capability.
The procedural model
Alongside the physical model sits the procedural model: a procedure contains unit procedures, a unit procedure contains operations, and an operation contains phases. The phase is where the real equipment logic runs — open the valve, run the pump to a volume. The recipe orchestrates those phases into the sequence the product requires.
A worked engineering situation
A mixing cell has a reactor unit offering the phases Charge, Agitate, Heat and Discharge. The recipe for product A calls those phases with its own quantities and times; the recipe for product B uses the same equipment with different parameters. When a new product is added, the team writes a new recipe rather than editing phase logic — and reviewing that recipe does not require reading the whole PLC program again.
The physical and procedural models, and the boundary between recipe and equipment, follow the production process and the traceability requirements.
Common mistakes
- Starting from I/O tags instead of the physical model.
- Putting I/O addresses and equipment logic inside a recipe.
- Mixing "what the product needs" with "how the machine does it".
- Rewriting logic every time a product is added.
- Designing the procedure without considering batch traceability.
Batch control checklist
- [ ] A physical model exists: cell, unit, equipment module, control module.
- [ ] The recipe describes the product and holds no I/O or equipment logic.
- [ ] Equipment capability describes the phases the machine offers.
- [ ] The procedure orchestrates phases into the product sequence.
- [ ] Adding a product is mostly writing a recipe, with little PLC editing.
Separating the layers is what lets a batch system grow without being rewritten. #68 turns to the PID loop and the basics of continuous control.
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Previous — #66: Recipes and machine parameters: https://minatavn.com/en/blog/automation-66-recipes-machine-parameters
Next — #68: PID basics: https://minatavn.com/en/blog/automation-68-pid-basics
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