Hierarchical Decomposition

The terms device level, unit level, and cell level describe hierarchical layers in industrial automation and control systems, aligning with standards like ISA-88 (batch control) and ISA-95 (enterprise integration). While useful for modular design, their definitions vary across industries, and their practical implementation depends on system complexity and organizational needs. Below is a refined breakdown with clarifications and caveats.

Hierarchical Levels

1. Device Level

• Scope: Individual actuators, sensors, or instruments (e.g., motor, temperature sensor, valve).
• Role:
  • Handles low-level I/O (e.g., reading sensor data, executing motor commands).
  • Rarely operates in isolation; typically part of a larger control loop.
• Example: A proximity sensor triggering a conveyor belt’s start/stop.
• OOIP Relevance:
  • Encapsulated as function blocks (e.g., Motor_FB with Start(), Stop(), Status methods).
  • Caveat: Over-encapsulating simple devices (e.g., basic sensors) can add unnecessary complexity.

2. Unit Level

• Scope: A functional group of devices performing a specific task (e.g., reactor, mixer).
• Role:
  • Coordinates devices to achieve a process step (e.g., mixing ingredients, heating a tank).
  • Manages interlocks, alarms, and sequencing (e.g., “start pump only if valve is open”).
• Example: A bottling unit with conveyor, filler, capper, and sensors.
• OOIP Relevance:
  • Represented as a composite function block (e.g., Reactor_FB containing Pump_FB, Valve_FB, TempSensor_FB).
  • Caveat: Overlapping unit boundaries can lead to communication bottlenecks (e.g., two units competing for shared resources).

3. Cell Level

• Scope: A production area coordinating multiple units (e.g., packaging cell, blending cell).
• Role:
  • Oversees cross-unit workflows (e.g., material transfer between units).
  • Implements production recipes, schedules, and error recovery.
• Example: A paint shop cell with pretreatment, coating, and curing units.
• OOIP Relevance:
  • Acts as a supervisory controller (e.g., PackagingCell_FB managing Filler_FB, Labeler_FB, Palletizer_FB).
  • Caveat: Over-centralized logic at the cell level can reduce system flexibility.

Key Clarifications

1. ISA-88 vs. ISA-95:
   • ISA-88 (Batch Control): Defines Process Cell → Unit → Equipment Module → Control Module.
   • ISA-95 (Enterprise-Control Integration): Uses Enterprise → Site → Area → Work Cell → Line/Unit.
   • Conflict: The term “cell” differs between standards. Clarify contextually.
2. Missing Layer:
   • Equipment Module (ISA-88): A mid-tier group between device and unit (e.g., a pump station with motor, valve, and sensor). Often omitted in oversimplified hierarchies.
3. Industry Variability:
   • Discrete Manufacturing: “Cell” ≈ assembly line; “unit” ≈ workstation.
   • Process Industries: “Cell” ≈ reactor area; “unit” ≈ individual reactor.

How They Fit Together

Benefits & Limitations

Advantages

• Modularity: Isolate failures (e.g., a faulty motor doesn’t halt the entire cell).
• Scalability: Add/remove units without redesigning the entire system.
• Alignment with OOIP: Natural fit for encapsulation and reuse (e.g., standardize Pump_FB across units).

Challenges

• Overhead: Managing inter-level communication (e.g., cell-to-unit messaging) can complicate systems.
• Standardization Gaps: Not all PLCs/SCADA systems natively support hierarchical programming.
• Skill Requirements: Engineers must understand both control logic and physical process flows.

Practical Applications

1. Automotive Manufacturing:
   • Device: Welding robot actuator.
   • Unit: Welding station (robots, clamps, sensors).
   • Cell: Body shop (multiple welding/painting units).
2. Pharmaceuticals:
   • Device: Temperature sensor in a bioreactor.
   • Unit: Bioreactor system (sensors, agitators, heaters).
   • Cell: Fermentation cell (multiple bioreactor units).

Conclusion

This hierarchical model is a guideline, not a rigid rule. To implement effectively:

• Tailor terminology to your industry (e.g., align with ISA-88 for batch processes).
• Use OOIP judiciously: Encapsulate only where it adds value (e.g., complex units vs. simple devices).
• Validate communication flows: Ensure seamless data exchange between levels (e.g., OPC UA for cell-to-unit links).

By balancing theoretical hierarchy with operational pragmatism, engineers can build systems that are manageable, modular, and adaptable to evolving production needs.