1. What is Programmable Logic Controllers (PLCs)?
Programmable Logic Controllers (PLCs) serve as microprocessor-based control devices positioned at remote sites within industrial plants or process systems. Their primary function is to monitor field-based process parameters and adjust outputs based on input signals received. PLCs are compatible with systems employing On-Off (discrete or digital) input devices as well as analog input devices. Similarly, they can interface with discrete or analog output devices.
In simpler terms, PLCs act as intermediaries between input and output devices in industrial processes. They continuously evaluate input data from various sensors and execute stored logic programs to carry out control functions.
The term “Logic” in Programmable Logic Controllers refers to the fact that all programs within a PLC are written using logic-based programming languages, such as ladder diagrams.
In industrial settings, PLCs have replaced traditional hard-wired relay logic systems due to their advantages, including reliability, simplicity, cost-effectiveness, ease of programming, and multifunctionality.
2. What are the applications of PLC?
PLCs find extensive applications across various industrial solutions, ranging from small machinery to large-scale manufacturing facilities. They are even employed in critical process plants that require redundant systems for enhanced reliability.
3. Explain what are different components in PLC?
PLCs consist of several key components, including:
1. Input Interface
2. Memory Section
3. Central Processing Unit (CPU)
4. Programmable Language
5. Programming Tool
6. Output Interface
4. What are the two basic sections of PLC?
The two fundamental sections of a PLC are the CPU (Central Processing Unit) and the Input/Output interface system.
5. What are the three major components of CPU?
The three major components of a CPU (Central Processing Unit) are actually:
1. Control Unit: Manages and coordinates the operations of all the hardware components in the CPU.
2. Arithmetic Logic Unit (ALU): Performs arithmetic and logical operations, such as addition, subtraction, and comparison.
3. Registers: Small, high-speed storage locations within the CPU used for temporarily holding data and instructions during processing.
The memory system and system power supply are important aspects of a computer system but are not typically considered as components of the CPU itself.
Explain what is the programmable language used in PLC?
The programmable language used in PLCs (Programmable Logic Controllers) is typically called “Ladder Logic” or “Ladder Diagrams.” Ladder Logic is a graphical programming language that was originally developed to mimic the traditional relay logic control schemes used in electrical control systems.
In Ladder Logic, you create programs by arranging various graphical elements, such as contacts (representing input conditions), coils (representing output actions), timers, and counters, in a ladder-like structure. This language allows you to design control logic by creating connections and transitions between these elements, similar to how electrical circuits are connected using relays and switches.
While Ladder Logic is the most common programming language used in PLCs, some PLCs also support other programming languages like Structured Text (similar to a high-level programming language), Function Block Diagrams (FBD), and Sequential Function Charts (SFC). These alternative languages offer more flexibility and can be advantageous for certain complex control applications.
7. Explain what does Central Processing Unit (CPU) of PLC consists?
The Central Processing Unit (CPU) of a PLC (Programmable Logic Controller) is indeed the core component responsible for executing control tasks. Here’s a breakdown of what the CPU of a PLC typically consists of:
1. **Microprocessor:** The microprocessor is the heart of the CPU. It carries out arithmetic and logical operations as instructed by the PLC program. It interprets the program, processes input data, executes control algorithms, and produces output signals based on the program’s logic.
2. **Memory:** The memory within the CPU is where information is stored and retrieved during program execution. It’s divided into different types:
– **Program Memory (RAM):** Stores the PLC program and any data that needs to be processed. This memory is volatile and is cleared when the PLC loses power.
– **Data Memory (ROM or EEPROM):** Stores data that needs to be retained even when the PLC is powered off, such as configuration settings, system parameters, and non-volatile data.
3. **Power Supply:** The power supply unit within the CPU is responsible for converting the incoming AC voltage to various DC operating voltages required by the PLC components. It ensures a stable power source for the CPU and other parts of the PLC system.
These components work together to enable the CPU to process input signals, execute control logic, and generate output signals to control machinery and processes according to the programmed instructions. The CPU is essentially the “brain” of the PLC, coordinating all the operations of the controller.
8. Explain what is SCAN in PLC ?
In a PLC (Programmable Logic Controller), “SCAN” refers to the cyclic process by which the controller operates and evaluates the ladder logic program. Here’s a breakdown of how SCAN works:
1. **Sequential Operation:** The PLC follows a sequential operation where it processes the ladder diagram from top to bottom, one rung at a time.
2. **Updating Inputs:** During the SCAN, the PLC reads the status of its input devices (sensors, switches, etc.). It determines whether these inputs have changed since the last scan cycle. Input changes trigger further actions in the program.
3. **Updating Outputs:** Based on the logic defined in the ladder diagram and the current state of input devices, the PLC updates the status of its output devices (relays, motors, lights, etc.). If the logic conditions in a rung are met, the corresponding outputs are energized or de-energized accordingly.
4. **Left-to-Right Processing:** Within each rung, the processing occurs from left to right. As the PLC evaluates each element of the rung, it keeps track of intermediate results and decides the final state of the output devices.
5. **Continuous Process:** The SCAN process is continuous and occurs repeatedly at a predefined interval, typically in milliseconds. The frequency of scanning depends on the PLC’s scan time setting and the complexity of the control program.
SCAN is fundamental to PLC operation as it ensures that the control logic is executed continuously and that the outputs are updated in response to changing input conditions. This cyclic scanning process allows PLCs to maintain real-time control over industrial processes and machinery.
9. What are the PLCs Advantages or Benefits?
PLCs (Programmable Logic Controllers) offer several advantages and benefits in industrial automation and control systems. Here are some of the key advantages:
1. **Higher Reliability:** Once a PLC program is written and tested, it can be easily duplicated and downloaded into other PLCs. This reduces the chances of wiring errors and increases the reliability of the control system.
2. **Simplified Wiring:** PLCs require less and simpler wiring compared to conventional hard-wired circuits. This reduction in wiring complexity not only saves time but also reduces the potential for wiring errors, improving system reliability.
3. **Flexibility:** PLCs offer more flexibility in control system design. It’s easier to create, modify, or expand control logic with software programming. Program modules can be changed whenever needed, allowing for quick adjustments to accommodate process changes or improvements.
4. **Remote Programming and Monitoring:** Many PLCs support remote programming and monitoring, which is especially useful for troubleshooting and maintenance. Operators can modify programs in the field, and security features like passwords and hardware interlocks can enhance system safety.
5. **Cost Savings:** PLCs were initially designed to replace complex and costly relay control systems. For larger control circuits involving more than a few control relays, PLCs are often more cost-effective to install and maintain. They can significantly reduce hardware costs.
6. **Modularity:** PLC systems can be modular, allowing for easy expansion as a process or facility grows. Additional input and output modules can be added as needed without major rewiring.
7. **Real-Time Control:** PLCs provide real-time control and response to changing conditions, making them ideal for applications requiring precise timing and synchronization.
8. **Diagnostic and Troubleshooting Tools:** PLCs offer built-in diagnostic tools and logging capabilities, making it easier to identify and address issues in the control system.
9. **Integration:** PLCs can easily integrate with other automation components, such as Human-Machine Interfaces (HMIs), sensors, and actuators, allowing for comprehensive control solutions.
10. **Safety:** PLCs can be programmed to incorporate safety features and emergency shutdown procedures, enhancing overall system safety.
These advantages make PLCs a popular choice for a wide range of industrial control and automation applications, improving efficiency, reliability, and flexibility in manufacturing and other industries.
Indeed, low-cost PLCs are a great option for beginners and offer several advantages, as mentioned in your description:
1. **Communication Capability:** PLCs excel in communication capabilities. They can easily connect with other controllers and computers in a system, enabling functions like supervisory control, data acquisition, monitoring of devices and process parameters, and program uploads and downloads. This makes them highly versatile in modern automation setups.
2. **Faster Response:** PLCs are designed for real-time applications and operate at high speeds. They provide faster response times compared to traditional relay logic circuits. When an event occurs in the field, a PLC can quickly execute the corresponding output operation, ensuring precise control.
3. **Easy to Troubleshoot:** PLCs come with built-in diagnostics and override functions, simplifying the troubleshooting process. These features help users identify and trace software and hardware errors efficiently, reducing downtime and maintenance costs.
These advantages make low-cost PLCs a valuable tool for beginners and professionals alike, enabling them to implement efficient and reliable control systems in various industrial and automation applications.
10. Explain Advantages of PLCs than Hard wired Relay?
The advantages of PLCs (Programmable Logic Controllers) over hard-wired relay control systems are numerous. Here’s a more detailed explanation of these advantages:
1. **Reliability:** PLCs are known for their high reliability. Once a PLC program is written and tested, it tends to be very stable and less prone to errors compared to complex and error-prone hard-wired relay circuits. This reliability is crucial in industrial and automation settings where downtime can be costly.
2. **Ease of Programming:** PLCs are easily programmable. Unlike hard-wired relay systems that require physical rewiring to change the control logic, PLCs allow for quick and easy modification of control logic through software programming. This flexibility makes it much simpler to adapt to changing requirements.
3. **Compact and Inexpensive:** PLCs are compact and cost-effective. They replace the need for numerous relays, timers, and other control components, leading to reduced hardware costs and a more efficient use of space in control panels.
4. **Communication Capabilities:** PLCs can be equipped with communication capabilities, enabling them to communicate with local or remote computers, HMI (Human-Machine Interface) devices, and other PLCs. This facilitates data exchange, remote monitoring, and centralized control, making them suitable for modern automation systems.
5. **Robust Environment:** PLCs are designed to operate reliably in harsh industrial environments. They are built to withstand temperature variations, electrical noise, and vibrations, requiring less maintenance compared to traditional relay systems.
6. **Real-Time Control:** PLCs provide real-time control and response to changing conditions, ensuring precise timing and synchronization in industrial processes.
7. **Built-in Diagnostics:** Many PLCs come with built-in diagnostic features, making it easier to identify and troubleshoot software and hardware issues. This speeds up maintenance and reduces downtime.
8. **Modularity:** PLC systems can be modular, allowing for easy expansion by adding additional input/output modules as needed. This scalability simplifies system upgrades and adaptations.
Overall, PLCs offer a more efficient, flexible, and cost-effective solution for industrial control and automation compared to hard-wired relay systems, making them the preferred choice in many applications.
11. How to program PLCs ?
Programming PLCs (Programmable Logic Controllers) typically involves the following steps:
1. **Choose the Right PLC:** Select a PLC model that suits your application and requirements. Different manufacturers offer various PLC models with varying capabilities.
2. **Install the Programming Software:** Install the programming software provided by the PLC manufacturer. As you mentioned, Siemens, Allen Bradley, and Modicon, among others, each have their own software platforms for programming.
3. **Familiarize Yourself with the Software:** Learn how to use the programming software. Familiarize yourself with the user interface, tools, and features it offers. Most PLC programming software provides a graphical environment for creating and editing PLC programs.
4. **Select the Programming Language:** PLCs support various programming languages, including Ladder Logic (LD), Statement List (STL), Functional Block Diagram (FBD), Sequential Function Chart (SFC), Instruction List (IL), and more. Choose the language that best suits your application and your familiarity with it.
5. **Create the Program:** Write the control logic for your application using the chosen programming language. This involves creating rungs, adding instructions, defining input and output devices, and specifying the desired behavior of the PLC in response to different conditions.
6. **Test and Debug:** Simulate or download your program to the PLC for testing. Debug the program to ensure it operates as intended. This may involve troubleshooting logical errors, ensuring correct input/output assignments, and verifying the timing and sequencing of operations.
7. **Documentation:** Properly document your PLC program, including comments and labels, to make it easier to understand and maintain in the future.
8. **Download to the PLC:** Once your program is thoroughly tested and debugged, download it to the PLC’s memory. Ensure that the PLC is properly connected to your computer for this step.
9. **Online Monitoring:** Many PLC programming software tools allow you to monitor the PLC in real-time while it’s running. This helps in diagnosing issues and fine-tuning the program as needed.
10. **Backup and Version Control:** It’s essential to regularly back up your PLC program and maintain version control to prevent data loss and ensure that you can restore the program in case of unexpected events.
11. **Commission the System:** After downloading the program, commission the system by connecting all the physical devices, sensors, actuators, and other components. Ensure that the PLC controls the process as expected and make any necessary adjustments.
12. **Provide Training:** If others will be working with the PLC, provide training on its operation, maintenance, and programming.
Remember that the specific steps and features may vary depending on the PLC manufacturer and software used. Always refer to the manufacturer’s documentation and guidelines for detailed instructions on programming and operating their PLCs.
12. What is the meaning of scan time in PLC?
Scan time in PLC refers to the time it takes for the PLC to complete one cycle of reading inputs, processing logic, and updating outputs, typically measured in milliseconds.
13. Why 4-20 ma and not 0-20 ma?
The use of a 4-20 mA signal instead of a 0-20 mA signal is preferred in industrial applications because it allows for the detection of a cable break. In a 4-20 mA signal:
– 4 mA typically represents the lowest expected value or “live zero,” indicating that the sensor or transmitter is functioning correctly and sending data.
– 0 mA represents a fault condition or cable break. This is important for safety and reliability because it signals to the PLC or monitoring system that something is wrong with the signal transmission.
In contrast, with a 0-20 mA signal, a cable break would still be represented as 0 mA, making it difficult for the PLC to distinguish between a fault condition and normal operation. Using a 4-20 mA signal allows for a clear distinction between proper operation and a fault condition, improving the reliability and safety of industrial systems.
14. What Is SIL?
SIL stands for Safety Integrity Level. It is a measure of the performance of safety systems used in industrial processes. SIL is categorized into four levels, with each level representing a different level of risk reduction and safety system reliability. Generally, higher SIL levels indicate a lower probability of failure for safety systems, but achieving higher SIL levels often involves increased cost and complexity in system design and implementation. SIL is an important consideration in industries where safety is critical, such as chemical, nuclear, and oil and gas industries.
15. Name different types of timers used in PLC.
In PLC programming, different types of timers are used to control various timing functions. Some common types of timers used in PLCs include:
1. **On Delay Timer (TON):** This timer starts timing when an input condition turns ON and only allows the output to turn ON after a preset time has elapsed.
2. **Off Delay Timer (TOF):** This timer starts timing when an input condition turns OFF and only allows the output to turn OFF after a preset time has elapsed.
3. **Retentive or Accumulative Timer (RTO):** This type of timer accumulates the total time an input condition is ON, even if the input condition turns OFF during timing. It retains its accumulated value across PLC scans.
4. **Pulse Timer (TP):** A pulse timer generates an output pulse of a specific duration when an input condition turns ON. It’s used to create time-based pulses for various control applications.
These timers provide precise control over time-related functions in PLC programs and are essential for coordinating processes and sequences in industrial automation and control systems. Many other timer types and variations can be derived from these fundamental timer types to meet specific application requirements.
Difference between PLC & DCS.
The primary differences between PLCs (Programmable Logic Controllers) and DCSs (Distributed Control Systems) are as follows:
PLC (Programmable Logic Controller):
1. **Application Focus:** PLCs are designed for discrete control and are typically used for specific machine or discrete process control within an industrial environment.
2. **Control Method:** PLCs operate digitally and use programmable memory to execute logic, sequencing, and control functions. They excel at handling tasks like logic, timing, and arithmetic operations.
3. **Centralization:** PLCs are often used in a more centralized manner, where a single PLC or a few PLCs control specific machines or processes within a facility.
4. **Versatility:** PLCs are versatile and can be easily programmed and reprogrammed to adapt to various control requirements.
DCS (Distributed Control System):
1. **Application Focus:** DCSs are designed for process control and are commonly used in industries such as chemical processing, oil refining, and power generation, where complex and continuous processes require centralized monitoring and control.
2. **Control Method:** DCSs are distributed systems with multiple controllers distributed throughout the plant or system. They are designed to control and manage large-scale, continuous processes.
3. **Centralization:** DCSs are typically more centralized in their architecture, with one or more controllers overseeing various components and subsystems within a process plant.
4. **Integration:** DCSs provide seamless integration of various control and monitoring functions, making them suitable for managing core processes within a facility.
5. **Scalability:** DCSs are highly scalable and can be expanded to accommodate the control and monitoring needs of complex industrial processes.
In summary, while both PLCs and DCSs are used in industrial automation, PLCs are more suitable for discrete control tasks and are often used as subsystems within DCSs. DCSs, on the other hand, are specialized for managing complex, continuous processes and offer centralized control and monitoring capabilities. Industries typically use a combination of both technologies to address their control and automation needs effectively.
Configuration vs. Programming:
Indeed, there is a distinction between configuration and programming in the context of DCS (Distributed Control Systems) and PLCs (Programmable Logic Controllers):
**DCS Configuration:**
– DCS systems are designed with a focus on configuration.
– Configuration in DCS involves setting up the control system using pre-defined control objects or function blocks. These objects represent physical devices or process control elements.
– DCS configuration typically involves connecting these control objects to corresponding graphical faceplates, which simplify the setup process and promote standardization.
– The configuration approach in DCS is more user-friendly and aims to create a visual representation of the control system, making it easier for operators to monitor and control processes.
**PLC Programming:**
– PLCs are primarily designed for programming.
– PLC programming involves writing software code that defines the control logic, sequences, and behavior of the system.
– PLCs use a programming language (e.g., ladder logic, structured text) to create the control program.
– Programmers specify process setpoints, logic conditions, and response actions using the chosen programming language.
– PLC programming requires a good understanding of control logic and the specific requirements of the application.
In summary, while DCS systems focus on configuring control elements using predefined objects and graphical representations, PLCs are primarily used for programming control logic using specific programming languages. Both approaches have their advantages and are chosen based on the complexity and requirements of the industrial control system.
Skid & Packaged Systems:
Skid-mounted and packaged systems are pre-built units tailored for specific plant functions. They save time and cost but pose integration challenges with the main plant’s control system. Standardization and remote monitoring are key to seamless operation.