Project analysis is a critical step that involves a thorough examination of the production process, working environment, hardware requirements, and control specifications. This foundational work ensures that the system design is accurate and efficient. If this phase is not properly executed, it can lead to incorrect hardware selection and project delays.
Engineering and technical personnel must first analyze the project by understanding the control process and identifying the type of control required for each stage. They should also anticipate potential issues that might arise throughout the project lifecycle.
(1) Analyzing the control process: When examining the control flow, it is recommended to create a detailed control flowchart. This visual tool helps clearly define each step and the conditions that trigger the next action.
(2) Identifying control types and estimating PLC parameters: Most PLCs are suitable for four main control types: sequential control, process control, motion (or position) control, and network communication. After analyzing the control requirements, engineers classify the control types based on the flowchart. They then combine these types according to the project’s complexity. A clear understanding of each step’s control type at an early stage greatly improves the accuracy of PLC selection and problem estimation.
While determining the control type, engineers must also estimate key parameters needed for PLC selection. For example, in sequence control, the number of I/O points is crucial. If an encoder is used, its pulse frequency must be calculated and converted into the PLC’s high-speed counting frequency. In process control, factors like analog input values, precision, and the response speed of the PLC to servo signals are important. For motion control, the number of high-speed pulses the PLC can output and whether it supports specific network protocols are essential considerations.
2. Estimating Potential Problems
Estimating possible issues is one of the more challenging aspects of engineering analysis. It requires engineers to have a deep understanding of the site’s working environment and the project’s control challenges, as well as to predict potential emergencies or hazards.
(1) Understanding the equipment’s working environment: Engineers need a comprehensive grasp of the production environment. For instance, if the environment is humid and prone to vibration, such as in textile machinery, earthquake-proof measures may be necessary. In a building materials processing plant with high temperatures, dust, and static electricity, additional protection for the electrical control cabinet becomes essential.
The consideration of the working environment extends beyond physical conditions. As PLC applications become more advanced, human factors must also be considered. If operators have low skill levels, a simpler user interface may be required to ensure smooth operation.
(2) Assessing project difficulties: Identifying the core challenges of the project is crucial. For example, in air jet loom equipment, the key challenge is controlling the solenoid valve efficiently. The PLC must respond quickly to ensure proper weft insertion. Once the difficulty level is determined, engineers can select the appropriate PLC that matches the project's needs.
(3) Pre-project hazard assessment: Early identification of potential hazards enhances the safety awareness of engineers. This includes anticipating malfunctions during debugging, checking for high pressure, temperature, or hazardous substances in process control, and implementing necessary protective measures.
Second, PLC Hardware Selection
PLC selection is based on the previous project analysis and the project's difficulty level. The following principles guide this decision:
1. Special-to-General Principle
According to engineering experience, most projects have certain constraints that influence PLC selection. Therefore, the principle of selecting based on special and general requirements is commonly applied.
Special requirements refer to unique control needs. For example, in sequential control, CPU memory capacity and I/O expansion capability are key factors. In process control, the number and precision of analog inputs are the starting point. In simpler motion control, the PLC must handle high-speed pulse signals from encoders. In large-scale projects, network compatibility becomes a primary concern.
Engineers should prioritize control requirements based on the project's core needs, which leads to a more effective and less complex solution.
2. Bottom-Up Principle
The bottom-up approach aims to maximize cost-effectiveness. Most PLC manufacturers offer multiple series. Engineers should start with the lowest model and compare performance parameters. If the requirements are not met, they move to higher models until a suitable option is found. Choosing from top to bottom can result in unnecessary features and higher costs.
3. Selecting the Switch Input/Output Unit
The digital input points of the PLC receive signals from field sensors, while the output points drive external loads based on internal control signals.
(1) Input terminal selection: Most modern PLCs use transistor inputs. Engineers only need to consider the number of input points. However, it's important to note that there are two input modes—NPN and PNP—depending on the wiring configuration. Sensors must match the input type to avoid conflicts.
Most PLCs operate on 24V DC. If connecting different voltage sensors, a relay must be used for isolation to ensure the signal remains at 24V.
(2) Output terminal selection: PLC output terminals are typically relay or transistor types.
- Relay output: Offers strong load capacity and isolation but has a limited lifespan due to mechanical contacts.
- Transistor output: Provides faster response times and longer life but has lower load capacity.
4. Built-in First, Expand Later Principle
With advancements in PLC technology, many models now include built-in functions like analog and communication capabilities. Using these built-in features reduces costs, saves space, and simplifies programming.
5. Redundancy Consideration
Due to initial estimations, on-site changes, and future upgrades, redundancy is essential. Typically, small projects require 20% redundancy, while larger ones may need 5% to 10%. Redundancy in other areas, such as analog and communication functions, should be handled flexibly.
Third, PLC Programming Points
(1) Allocating blocks based on the control flowchart: Breaking the program into segments based on the flowchart improves clarity and makes debugging easier. Complex projects can be divided among multiple programmers to enhance efficiency.
(2) Creating I/O and memory tables: Assigning addresses to each I/O point and intermediate variables in memory helps avoid confusion and improves program readability.
(3) Simplifying the program: Familiarity with the PLC instruction set allows programmers to use advanced instructions, reducing workload and memory usage.
(4) Clear comments: Adding detailed comments in the code helps with debugging and future maintenance.
Fourth, PLC Program Debugging Methods
PLC debugging typically involves two steps: simulation and online testing.
1. Simulation Debugging
This involves testing the program using LED indicators on the I/O unit without connecting actual output devices. Many manufacturers provide simulation software, such as Omron’s CX-Simulator, to test programs virtually. Engineers can force input bits to ON or OFF or modify data components to check if the system functions correctly.
If hardware is used, small switches and buttons can simulate real-world inputs. LEDs on the output unit help verify if the output meets design requirements.
For sequence control programs, the focus is on ensuring that the program transitions between steps correctly, activating and deactivating the appropriate steps and loads.
All possible scenarios should be tested, and any issues should be addressed promptly. During debugging, some timer or counter settings can be temporarily reduced to speed up the process.
2. Online Debugging
Online debugging involves installing the PLC in the control cabinet, connecting input components and output loads, and running the program for full system testing. While simulating the program, the control cabinet can be designed and wired simultaneously.
After installation, the wiring should be tested. Input signals can be simulated via terminal blocks or panel buttons, and output signals can be monitored to ensure they function correctly.
For systems with analog inputs, standard signals can be applied, and adjustments made through potentiometers or program parameters to achieve the desired conversion.
Once the system is installed on-site, all components are connected, and the PLC is placed in operation mode to run the control program and verify system performance.
During debugging, both hardware and software issues are identified and resolved. After successful commissioning, a trial period is conducted to ensure the system’s reliability.
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