Programmable Logic Controller (PLC), which uses a type of programmable memory for its internal memory program, performs user-oriented instructions such as logic operations, sequence control, timing, counting, and arithmetic operations, and passes Digital or analog input/output controls various types of mechanical or production processes.
For decades, programmable logic controllers (PLCs) have been a part of the organic components of factory automation and industrial process control. From simple lighting functions to environmental systems to chemical processing, PLC control is inseparable.
These systems have many features, providing a variety of analog and digital input/output interfaces, signal processing, data conversion, and a variety of different communication protocols. All components and functions of the PLC are centered on the controller, while the controller is programmed for a specific task.
The basic PLC components must be flexible and configurable to meet the needs of different plants and applications. Input stimuli (whether analog or digital) come from machine devices, sensors, or process events, expressed as voltage or current. The PLC must accurately provide resolution and convert the excitation signal to the CPU, which in turn determines a set of instructions to the output system that controls the actuators installed in the factory or another industrial environment.
Modern PLCs originated in the 1960s, and in the following decades, the general functions and signal channels changed a little. However, process control in the 21st century presents even more daunting new requirements for PLCs: higher performance, smaller size, and greater functional flexibility. Protection must be built in to prevent potentially damaging electrostatic discharge (ESD), electromagnetic interference, and radio frequency interference (RFI/EMI), as well as large value transients that are common in harsh industrial environments.
Reliable design
PLCs need to operate without problems in industrial environments for years, and this environment is much more damaging to microelectronic components that provide superior flexibility and precision for PLCs. Maxim understands this better than any mixed-signal IC manufacturer because we have been ahead of the competition with superior product reliability and innovative solutions to ensure high-performance electronics are protected from harsh environments. Includes high ESD, high transient voltage swing and EMI/RFI. Designers have generally recognized Maxim's products because they address the challenges of analog and mixed-signal design and will continue to address these issues year after year.
Higher integration
With 4 to hundreds of input/output (I/O) channels, the PLC supports a wide range of applications, so size and power are just as important as system accuracy and reliability. Maxim insists on integrating the right features in the IC and staying ahead of the industry, reducing the overall system space and power requirements for a more compact design. Maxim offers hundreds of low-power, high-precision ICs in a minimum size, enabling system designers to build sophisticated products that meet the most demanding space and power requirements.
Factory Automation - New Inventions
The assembly line is a fairly new invention in human history, and many countries have emerged similar innovations during the same period. Here are a few examples from the United States.
SamuelColt (American* manufacturer) demonstrated a common part in the mid-19th century. Early*s required separate parts for each gun and then assembled separately. In order to achieve automatic assembly, Mr. Colt tried to place all the parts of the 10 guns in different boxes, then randomly grabbed the parts from the box and assembled them into a single gun. In the early 20th century, HenryFord further expanded its mass production technology. He set up a fixed assembly plant where cars were delivered to different workshops. Employees only need to know a little about assembly and only do this in future work. In 1954, George Devol applied for US patent 2,988,237, which marked the birth of the first industrial robot named Unimate. GeneralMotors in the late 1960s? Use the PLC to assemble the car's automatic transmission. DickMorley—the father of PLC, is GM? The first PLC (Modicon) was developed, and his US patent 3,761,893 is the basis of many PLCs today.
Basic PLC operation
How simple can process control be? Let us take a common household heater as an example.
The heater components are sealed in a container for system communication. This concept can be extended to remotely controlled home thermostat heaters with communication distances of a few meters, usually with voltage control.
Now, we are expanding on this small, simple process control system. What controls and configurations does a plant need?
The impedance, EMI and RFI of the long-distance transmission line make the implementation of the voltage control scheme very difficult. In this case, the current loop is a simple and effective solution. According to Kirchhoff's law, the current at any point in the current loop is equal to the current at all other points in the loop, thereby offsetting the effects of the impedance of the transmission line. Due to the low loop impedance and bandwidth (several hundred ohms, and "100 Hz"), the stray pickup of EMI and RFI is minimized. PLC systems are very useful for proper control, such as factory production systems.
PLC current loop transmission
The application of the current control loop began in the early 20th century teletypewriters, the earliest use of the 0–60 mA loop, which was later changed to the 0–20 mA loop, and the PLC system pioneered the 4–20 mA loop.
The 4–20mA current loop has many advantages. Careful calculations are required in traditional discrete device designs, and the circuit takes up a lot of space compared to the current integrated 4–20 mAIC. Maxim has introduced several 20mA devices, including the MAX15500 and MAX5661, which simplify the design of 4–20mA PLC systems.
Any measured current value represents a certain amount of information. In practical applications, the 4–20 mA current loop operates from 0 mA to 24 mA. The 0mA to 4mA and 20mA to 24mA current ranges are used for diagnostics and system calibration. Since currents below 4 mA and above 20 mA are used for diagnostics, it can be considered that a reading between 0 mA and 4 mA indicates that the transmission line is disconnected in the system. Similarly, a reading between 20 mA and 24 mA can indicate a potential short circuit fault in the system.
The enhanced version of 4–20 mA communication is called the High Speed ​​Addressable Remote Sensor (HART® system), which is backward compatible with 4–20 mA meters. In a HART system, a microprocessor-based intelligent integrated field device enables two-way communication. According to the HART protocol, additional digital information can be carried on the same 4–20 mA analog current signal pair for process control.
The functions of the PLC can be divided into several functional groups. Many PLC vendors integrate these functions into separate modules, each with its own features that vary from application to application. Many modules have multiple functions and can be interfaced with a variety of sensors. However, in most cases special modules or expansion modules are designed for specific applications, such as resistance temperature detectors (RTDs), sensors or thermocouple sensors. Typically, all modules have the same core functions: analog input, analog output, distributed control (such as fieldbus), interface, digital input and output (I/O), CPU, and power. We will explain these core functions one by one, and the sensor and sensor interfaces will be introduced separately in other chapters.
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