* Question

What are the fundamental principles for dividing software functional modules in numerical control (NC) systems?

* Answer

Software design for numerical control (NC/CNC) systems typically adopts a modular architecture to ensure reliability, real-time performance, and ease of maintenance. When dividing functional modules, engineers follow several core principles to achieve a clean, scalable, and high-performance system.

1. Single Responsibility Principle (SRP)

Each module should be responsible for one clearly defined function only.
Examples:

• Interpolation module
• Servo control module
• Human–machine interface (HMI) module

This reduces complexity and makes testing, debugging, and maintenance more efficient.

2. High Cohesion

Functions that are closely related should be grouped into the same module.
High cohesion ensures that:

• Module logic is focused
• Internal interactions are efficient
• Code readability and reliability improve

For example, all motion-planning algorithms belong in one cohesive planning module.

3. Low Coupling

Modules should minimize dependencies on each other.
Benefits include:

• Lower impact of changes
• Easier module replacement or upgrading
• Higher system flexibility

NC systems often separate:

• Motion control
• I/O management
• Error handling
• Communication interfaces

to reduce interdependencies.

4. Clear Interfaces and Standardized Communication

Modules interact through well-defined, standardized interfaces:

• API calls
• Task/event messaging
• Data buffers
• Shared memory with strict access rules

This prevents hidden dependencies and allows easier integration and expansion.

5. Real-Time Requirements

NC software includes hard real-time components (e.g., servo loops, interpolation) and non-real-time components (e.g., UI).
Module division must reflect timing constraints:

• Real-time tasks separated into dedicated modules
• Non-real-time tasks isolated to avoid interference
• Priority scheduling clearly partitioned

This ensures deterministic system behavior.

6. Functional Independence and Replaceability

Modules should be designed so that they can be independently modified, upgraded, or replaced without affecting the rest of the system.

Example: Replacing a communication protocol module (RS232 → Ethernet) should not impact the motion control module.

7. Scalability and Extensibility

Module division must consider:

• Multi-axis expansion
• New machining processes
• Future hardware upgrades
• Optional advanced functions (e.g., 5-axis interpolation, adaptive feed control)

Good modularity ensures the system can evolve without major redesign.

8. Error Isolation and Fault Tolerance

Modules should isolate faults so that an error in one module does not cause system-wide failures.
Examples:

• Independent error-handling module
• Separate diagnostic and logging module

This improves safety and reliability in industrial environments.

Summary

When dividing functional modules in NC software design, engineers follow principles including single responsibility, high cohesion, low coupling, clear interfaces, real-time partitioning, independence, scalability, and robust error isolation. These principles together ensure that numerical control systems remain reliable, maintainable, and adaptable to future technological requirements.