The design of a PLC program typically follows an intuitive approach, which relies heavily on the personal experience and judgment of the circuit designer. This method is more subjective and direct, often requiring a trial-and-error process that can be time-consuming. As a result, the final program may vary significantly from one programmer to another, making it difficult for maintenance personnel to understand the logic unless they are familiar with the original designer’s thought process. This lack of clarity reduces the readability of the program, even though programming is often considered a straightforward task. However, many workshop manuals rarely delve into these practical challenges.
To illustrate this concept, let's take the example of a "three-phase induction motor fault alarm control" circuit. The process of converting a traditional electrician’s diagram into a ladder diagram provides a clear insight into how PLC programs are designed. This conversion involves several steps, including identifying input/output components, redrawing the diagram to fit the PLC format, and adjusting the layout accordingly.
1. **Traditional Electrician Diagram**
A standard three-phase induction motor fault alarm control circuit is shown in Figure 1. This diagram represents the physical connections and control logic using conventional electrical symbols.
2. **Action Description**
- When power is normal, only the green light (GL) is on, and the motor remains off.
- Pressing the start button (PB1) activates the contactor (MC), causing the motor to run and the red light (RL) to turn on while the green light turns off.
- Pressing the stop button (PB2) de-energizes the contactor, stopping the motor and turning off the red light while the green light turns back on.
- If a fault occurs (e.g., overload), the thermal relay (TH-RY) triggers, stopping the motor, activating the buzzer (BZ), and turning off the red light while the green light turns on.
- Pressing the reset button (PB3) stops the alarm, turns on the white light (WL), and resets the system.
- After the fault is cleared, the thermal relay must be manually reset to restart the motor.
3. **I/O Coding**
In PLC systems, hardware wiring is replaced by software-based logic. The first step in converting a traditional electrician diagram is I/O coding, where each input and output device is assigned a specific address. For instance, PB1 might correspond to X0, and the contactor MC might be Y1. The wiring method—whether normally open (NO) or normally closed (NC)—is also determined during this phase.
4. **PLC External Wiring Diagram**
Once the I/O addresses are assigned, the external wiring is configured accordingly. The PLC model used here is the Fengqi V-B series, which uses NPN wiring. The wiring diagram is shown in Figure 2, illustrating how inputs and outputs are connected.
5. **PLC Ladder Diagram**
Converting the traditional diagram into a ladder diagram involves rearranging the components to match the structure of a PLC program. This includes repositioning contacts and coils to align with the PLC’s logic flow. The modified diagram is shown in Figure 3, followed by the I/O-coded version in Figure 4.
6. **Adjustments and Corrections**
To make the diagram compatible with standard PLC programming software, the diagram is rotated and flipped. Adjustments are made to ensure that the output coils and contacts follow the correct syntax. These corrections are illustrated in Figure 5.
7. **Instructions**
The final step is to convert the ladder diagram into a set of instructions that the PLC can execute. These instructions are shown in Figure 6.
8. **Conclusion**
The primary goal of PLC development is to replace relay-based control systems with software-based logic. This allows for greater flexibility, as changes to the control sequence can be made simply by modifying the program. While PLCs are based on traditional relay logic, the resulting programs can be difficult to interpret due to their subjective nature. Therefore, proper documentation and clear design practices are essential to improve program readability and maintainability.
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