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Project Analysis

Project analysis is a crucial step in the development of any control system. It involves a detailed examination of the production process, working environment, hardware requirements, and control specifications. This stage serves as the foundation for the entire system design. A poor project analysis can lead to incorrect hardware selection, delays, and even system failures.

I. Understanding the Project

Before any design work begins, engineering and technical personnel must thoroughly analyze the project. This includes understanding the control process, identifying the types of control required for each process, and anticipating potential issues that may arise during implementation.

(1) Analyzing the Control Process

To better understand the control flow, it's recommended to draw a control flowchart. This visual representation helps identify the sequence of operations and the conditions that trigger transitions between steps. By clearly marking each step and its dependencies, engineers can ensure a more accurate design.

(2) Identifying Control Types

PLCs are typically used for four main control types: sequential control, process control, motion (or position) control, and network communication. After analyzing the control flow, engineers should classify the control types based on the complexity of the project. This classification helps in selecting the right PLC and estimating potential challenges early on.

During this phase, it's also important to estimate key parameters needed for PLC selection. For example, in sequential control, the number of I/O points is critical. If an encoder is involved, the output pulse frequency must be calculated and converted into the high-speed counting frequency of the PLC. In process control, factors like analog input precision and response time to feedback signals become essential. Similarly, in motion control, the PLC’s ability to handle high-speed pulses and the presence of appropriate network support are key considerations.

II. Anticipating Potential Problems

Estimating possible problems is one of the most challenging aspects of project analysis. Engineers must have a clear understanding of the site environment, the difficulty of the project, and the potential risks that could arise.

(1) Understanding the Working Environment

The working environment plays a major role in system design. For example, in textile machinery, where humidity and vibration are high, the PLC system must include anti-vibration measures. In a building materials plant with high temperatures and dust, the electrical cabinet needs additional protection against dust and static electricity. Additionally, human factors such as operator skill levels must be considered. If operators are not highly trained, the user interface should be simplified to improve usability.

(2) Evaluating Project Challenges

Identifying the core challenges of the project is essential. For instance, in air jet loom equipment, the key challenge is controlling the solenoid valve quickly and efficiently. The PLC must respond rapidly to ensure smooth operation. Recognizing these difficulties allows engineers to choose a suitable PLC that meets the project's demands.

(3) Pre-Project Risk Assessment

Before the project starts, engineers should anticipate potential hazards. This includes safety measures for malfunctions during testing, such as overpressure or high-temperature risks in process control. Early risk assessment helps improve safety awareness and ensures that all necessary precautions are taken from the beginning.

III. Selecting the Right PLC Hardware

Based on the project analysis and the level of difficulty, the right PLC must be selected. The following principles guide this decision:

1. Special and General Principle

PLC selection should follow the principle of "special first, then general." Different control types impose different constraints. For example, in sequential control, CPU memory and I/O expansion capabilities are key. In process control, the number and accuracy of analog inputs matter. In motion control, high-speed pulse handling is critical. For large-scale projects, network compatibility becomes a primary concern. Engineers should prioritize the most critical requirements to ensure effective and efficient system design.

2. Bottom-Up Selection

This principle emphasizes cost-effectiveness. Engineers should start with lower-end PLC models and gradually move up if the requirements aren't met. Choosing from the top down can lead to unnecessary costs and underutilized features.

3. Input and Output Unit Selection

Digital input and output units are essential for connecting field devices. Inputs receive signals from sensors, while outputs drive external loads.

(1) Digital Input Units

Most PLCs use transistor-based inputs. The choice depends on the number of input points estimated earlier. However, the type of sensor (NPN or PNP) must match the PLC's wiring configuration. Mixing NPN and PNP sensors on the same PLC is not recommended.

(2) Switching Output Units

There are two main types: relay and transistor outputs. Relay outputs are good for heavy loads but have limited life due to mechanical wear. Transistor outputs are faster and longer-lasting but have weaker load capacity.

4. Built-In vs. Expansion Modules

Modern PLCs often come with built-in functions like analog and communication modules. Using these reduces costs, saves space, and simplifies programming.

5. Redundancy Considerations

Redundancy is important for future upgrades and unexpected changes. A typical redundancy range is 20% for small projects and 5–10% for larger ones. Engineers should plan for extra I/O points, analog functions, and communication capabilities.

IV. Programming Best Practices

(1) Segmenting Based on Flowcharts

Breaking the program into segments based on the control flow improves clarity and makes debugging easier. This approach also facilitates teamwork and enhances overall efficiency.

(2) Creating I/O and Memory Tables

Assigning addresses to I/O points and intermediate variables in the PLC memory helps avoid confusion during programming. Clear comments make the code more maintainable.

(3) Simplifying the Code

Using advanced instructions and optimizing the code can reduce memory usage and improve performance. Familiarity with the PLC instruction set is key to writing efficient programs.

(4) Adding Clear Comments

Comments in the code help with debugging and future maintenance. They explain the purpose of each section, making the program more readable and easier to modify.

V. Debugging the PLC Program

Debugging is divided into two stages: simulation and online testing.

1. Simulation Testing

Simulation involves testing the program without connecting actual output devices. Many manufacturers provide simulation software that allows engineers to test the logic before deploying the hardware. This helps identify errors early and reduces the risk of damage.

2. Online Testing

Once the PLC is installed in the control cabinet and connected to the field devices, online testing is performed. This step verifies that the system works as intended under real-world conditions. Any issues found during this phase are addressed immediately to ensure the system operates reliably.

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