PLC migration design method programming steps - Database & Sql Blog Articles

1. Analyze how the original system works

Begin by understanding the operation and mechanical behavior of the controlled equipment. Study the relay circuit diagram to fully grasp the working principle of the control system. This step is crucial for identifying how each component interacts and contributes to the overall function.

2. PLC I/O Allocation

Determine which devices are input and output in the system. Assign I/O points to the PLC and draw the external wiring diagram. This helps ensure that all signals are properly connected and mapped within the PLC's architecture.

3. Establish correspondence between other components

Identify how components like intermediate relays and time relays in the relay circuit map to auxiliary relays and timers in the PLC. This mapping is essential for translating the old system into a new one effectively.

The above two steps (2 and 3) establish a clear relationship between all components in the relay circuit and the internal programming elements of the PLC. This is a critical part of the design process. Some key considerations during this phase include:

1) The actuator in the relay circuit should be matched with the PLC's output relay, such as contactors, solenoid valves, electromagnets, or indicator lights.

2) The main electrical devices in the relay circuit, like buttons or Switches, should correspond to the input relays of the PLC. The thermal relay’s contacts can either be used as an input to the PLC or placed in the external circuit, depending on whether the PLC has enough input points. Be careful when dealing with normally open and normally closed contacts inside and outside the PLC.

3) Intermediate relays in the relay circuit should be mapped to auxiliary relays in the PLC.

4) Time relays in the relay circuit should be matched with timers or counters in the PLC. However, note that time relays have two types—power-on delay and power-off delay—while PLC timers typically only support power-on delay functionality.

4. Design ladder program

Based on the above mappings, convert the relay circuit diagram into a "quasi ladder diagram." Then refine this into a structured and efficient ladder logic diagram according to standard programming rules. For complex systems, it's often helpful to break the process into smaller parts, convert them individually, and then integrate the results.

5. Carefully proofread and debug

Once the ladder diagram is created, it must be thoroughly reviewed and tested. Ensure that its control functions match the original system exactly. This step helps catch any errors or inconsistencies before the system is deployed.