Project analysis is a critical step that involves a thorough evaluation of the project's production process, working environment, hardware requirements, and control specifications. This foundational work ensures that all system design decisions are well-informed. Inadequate project analysis can lead to incorrect hardware selection and cause significant delays in project implementation.
Engineering and technical personnel must first conduct a detailed analysis of the project, focusing on the control processes and the nature of each control stage. They should anticipate potential issues that may arise throughout the project lifecycle.
(1) Analyzing the control process: It is recommended to create a control flowchart during this phase. This visual tool helps clearly define the steps involved and the conditions that trigger transitions between them.
(2) Identifying the type of control and estimating PLC parameters: Most PLCs are suitable for four main control types—sequential control, process control, motion control, and network communication. After identifying the control requirements, engineers classify the control types based on the flowchart and determine the complexity of the project. Accurate early-stage analysis of each step’s control type greatly improves the precision of PLC selection and problem estimation.
Alongside analyzing the control type, engineers must also estimate key parameters required for PLC selection. For example, in sequential control, the number of I/O points is crucial. If an encoder is used, the output pulse frequency from the encoder must be calculated and converted into the high-speed counting frequency of the PLC. In process control, factors like analog input values and precision, the response speed of the PLC to servo feedback signals, and the number of high-speed pulses in motion control must be considered. Additionally, it is important to verify whether the selected PLC supports the necessary network communication protocols.
2. Estimating Potential Problems
Estimating possible issues is one of the more challenging aspects of engineering analysis. It requires not only a deep understanding of the site’s working environment and project challenges but also the ability to predict potential emergencies and hazards.
(1) Understanding the equipment’s working environment: Engineers need a comprehensive grasp of the production setting. For instance, if the equipment operates in a high-humidity or high-vibration environment, such as textile machinery, earthquake-resistant measures should be incorporated into the PLC system design. Similarly, in a building materials processing plant with high ambient temperatures and dust, protective measures against dust and static electricity must be implemented in the electrical control cabinet.
Understanding the working environment also involves considering human factors. If operators have limited technical skills, a more user-friendly interface should be designed to simplify operation.
(2) Anticipating project difficulties: Recognizing the core challenges of the project helps guide the selection of the right PLC. For example, in air jet loom equipment, the main challenge is controlling the solenoid valve quickly and accurately. The PLC must respond rapidly to ensure proper weft insertion. Identifying these difficulties early allows engineers to select the most appropriate PLC model for the task.
(3) Pre-project hazard assessment: Engineers should anticipate potential dangers during the early design phase. This includes ensuring safety measures during debugging in sequence or motion control, and checking for hazardous conditions like high pressure, temperature, or toxic substances during process control testing. Early hazard identification enhances the safety awareness of the engineering team.
Second, PLC Hardware Selection
PLC selection is based on the previous project analysis and the level of difficulty. Key principles include:
1. Special-to-General Principle
Most projects have specific constraints that influence PLC selection. For sequential control, CPU program capacity and I/O expansion capability are key. In process control, the number and accuracy of analog inputs matter. For motion control, the PLC must handle high-speed pulse signals. In large-scale projects, network compatibility becomes essential. Engineers should prioritize special requirements first, leading to more efficient and effective selections.
2. Bottom-Up Principle
This principle aims to maximize cost-effectiveness. Engineers should start with lower-end PLC models and compare their performance parameters. Only when the requirements are not met should they consider higher-end options. Choosing from top to bottom often results in unnecessary feature usage, which increases costs.
3. Selecting Switch Input/Output Units
The digital input points of the PLC receive signals from field sensors, while the output points drive external loads.
(1) Digital Input Terminal Selection: Most PLCs use transistor inputs. Engineers should choose based on the estimated number of input points. However, they must also consider NPN and PNP configurations, as mismatched sensors can cause signal conflicts.
(2) Switch Output Terminal Selection: There are two main types—relay and transistor outputs. Relay outputs offer strong load capacity and isolation but have limited lifespan. Transistor outputs provide faster switching and longer life but have lower load capacity. It is advisable to use transistors for internal control and relays for external loads to combine both advantages.
4. Built-in First, Expand Later Principle
Modern PLCs come with built-in functions like analog and communication capabilities. Using these features reduces costs, saves space, and simplifies programming.
5. Redundancy Consideration
Due to site changes and future upgrades, redundancy is essential. A 20% I/O point redundancy is common for small projects, while larger ones require 5–10%. Redundancy for analog, communication, and bus functions depends on the project's needs.
Third, PLC Programming Points
(1) Allocating Blocks by Control Flowchart: Divide the program into segments based on the control flowchart for better structure and easier debugging. Complex projects can be split among multiple programmers to improve efficiency.
(2) Preparing I/O and Memory Tables: Assign addresses to I/O points and intermediate variables, making the program easier to manage and debug.
(3) Simplifying the Program: Use advanced instructions to reduce workload, save memory, and enhance functionality.
(4) Clear Comments: Add detailed comments in the code to aid debugging and future maintenance.
Fourth, PLC Program Debugging Methods
Debugging can be divided into simulation and online stages.
1. Simulation Debugging: Test the program without connecting output devices using LED indicators or simulation software. This helps identify logical errors before actual deployment.
2. Online Debugging: Install the PLC in the control cabinet, connect all components, and run the program. This phase checks the real-world performance and exposes any hardware or software issues.
During the entire debugging process, engineers should test various scenarios, adjust settings as needed, and ensure the system meets all functional requirements. After successful debugging, a trial period is conducted to confirm system reliability.
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