The design of a PLC program typically relies on an intuitive approach, which means it is based on the personal experience and understanding of the circuit designer. This method tends to be more subjective and direct, often requiring a trial-and-error process. The program must go through several iterations before it meets the required functionality or operational requirements. As a result, the programs created can vary significantly from one programmer to another, making it difficult for maintenance personnel to understand the logic behind the code—essentially, the program lacks readability. Although programming is often described as a "slogan" in some contexts, many workshop manuals rarely cover this aspect in detail. Let’s take a practical example: the "three-phase induction motor fault alarm control" circuit. We’ll walk through the process of converting a traditional electrician’s diagram into a ladder diagram, discussing the steps involved in the program design. You might find that some of the loop conversions or programming techniques could be simplified or bypassed depending on your experience.
1. **Traditional Electrician Diagram**
A standard three-phase induction motor fault alarm control circuit is shown in Figure 1. This diagram represents the physical wiring used in conventional electrical systems.
2. **Action Description**
- When the power is normal, only the green light (GL) is on, and the motor remains off.
- Pressing the start button (PB1) activates the electromagnetic contactor (MC), causing the motor to run, turning on the red indicator light (RL) and turning off the green light (GL).
- Pressing the stop button (PB2) de-energizes the contactor (MC), stopping the motor, turning off the RL, and turning on the GL again.
- During operation, if there is an overload or fault, the thermal relay (TH-RY) activates, stopping the motor, triggering the buzzer (BZ), turning off the RL, and turning on the GL.
- Pressing the reset button (PB3) stops the alarm, turns on the white light (WL), keeps the GL on, and turns off the RL.
- After the fault is cleared, pressing the thermal relay’s return lever allows the motor to restart, with the WL turning off, GL turning on, and RL turning off.
3. **I/O Coding**
In a PLC system, software wiring replaces hardware connections. While the main circuit isn’t replaced by the PLC, the control circuit is. The first step in converting a traditional electrician diagram into a ladder diagram is I/O coding, where each input/output component is assigned a specific address. This includes determining the external input/output terminal numbers and their wiring configuration, such as normally open (NO) or normally closed (NC) contacts.
4. **PLC External Wiring Diagram**
Once the I/O mapping is complete, the external wiring diagram is established. For example, the Fengqi VB series PLC uses NPN wiring, connecting the 24V terminal to the S/S terminal. The external wiring diagram is shown in Figure 2.
5. **PLC Ladder Diagram**
To convert the traditional diagram into a ladder diagram, the following steps are taken:
- Redraw the electrician diagram so that contacts appear before output coils, as required by the PLC ladder logic.
- Replace the original components with their corresponding I/O coded addresses.
- Rotate the diagram 90 degrees to the left and flip it vertically to match the ladder format. Adjust any elements that don't conform to standard PLC programming conventions.
- The final ladder diagram, as shown in Figure 5, reflects these changes.
6. **Instructions**
The ladder diagram can then be converted into a set of instructions, as illustrated in the figure below.
7. **PLC Conversion and Ladder Diagram**
After completing the hardware wiring, the PLC's external inputs and outputs are programmed using software. This allows the system to function as a fully digital control solution, replacing the need for complex physical wiring.
8. **Conclusion**
The primary goal of PLC development is to replace relay-based sequential control with software programs. This makes it easy to modify control sequences simply by changing the software, without the need for physical rewiring. PLCs are built upon traditional relay control loops, symbolizing contacts and coils in a ladder diagram or command language. However, the resulting control logic is still largely influenced by the designer’s experience, making it subjective and less readable for others. Despite this, PLCs have become the standard in industrial automation due to their flexibility and ease of modification.
Mid Temp Self Regulating Heating Cable
Mid Temp Self Regulating Heating Cable,Fire Pipe Electric Heating Tape,Explosion-Proof Electric Heating Tape,Self-Adjusting Heating Tape
JIANGSU PENGSHEN HIGH TEMPERATURE WIRE CABLE CO., LTD. , https://www.pengshencable.com