Programmable logic controller (Programmable Logic Controller, PLC), which uses a type of programmable memory for its internal storage program, executes logic-oriented operations, sequence control, timing, counting and arithmetic operations and other user-oriented instructions, and through Digital or analog input / output controls various types of machinery or production processes.
For decades, programmable logic controllers (PLCs) have been an integral part of factory automation and industrial process control. From simple lighting functions to environmental systems to chemical processing and other applications, PLC control is indispensable.
These systems have many functions, providing various analog and digital input / output interfaces, signal processing, data conversion, and various different communication protocols. All components and functions of PLC are centered on the controller, and 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 signals) come from mechanical devices, sensors, or process events and appear as voltage or current. The PLC must accurately provide the CPU with the analysis and conversion of the excitation signal. The CPU then determines a set of instructions to the output system, and the latter controls the actuators installed in the factory or another industrial environment.
Modern PLC originated in the 1960s. In the following decades, the general functions and signal channels have changed a little. However, the process control of the 21st century puts forward more arduous new requirements for PLC: higher performance, smaller volume and greater functional flexibility. Protection functions must be built in to prevent potentially damaging electrostatic discharge (ESD), electromagnetic interference and radio frequency interference (RFI / EMI), and large-scale transient pulses common in harsh industrial environments.
Reliable design
PLCs need to work in an industrial environment for years of trouble-free operation, and this environment has caused great damage to microelectronic components that provide PLC with excellent flexibility and precision. Maxim understands this situation better than any mixed-signal IC manufacturer, because we were ahead of competitors in the same industry with excellent product reliability and innovative solutions from the beginning to ensure that high-performance electronic devices are not damaged by harsh environments , Including high ESD, high transient voltage swing, and EMI / RFI. Designers have generally recognized Maxim's products because they solve the problems of analog and mixed-signal design and will continue to solve these problems year after year.
Higher integration
PLC has 4 to hundreds of input / output (I / O) channels and supports various applications of different specifications. Therefore, size and power are also as important as system accuracy and reliability. Maxim insists on integrating the correct functions in the IC and always maintains the leading position in the same industry, thereby reducing the overall system space and power requirements and obtaining a more compact design. Maxim can provide hundreds of low-power, high-precision ICs in the smallest size, allowing system designers to build precision products that fully meet the demanding space and power requirements.
Factory automation-new invention
Assembly production lines are fairly new inventions in human history, and many countries have emerged with similar innovative solutions during the same period. We cite a few examples from the United States here.
SamuelColt (American * manufacturer) showed a universal part in the mid-19th century. In the early days, it was necessary to manufacture the parts of each gun independently and then assemble them separately. In order to achieve automatic assembly, Mr. Colt tried to place all the parts of the 10 guns in different boxes, and then randomly grabbed these parts from the box and assembled a gun. In the early 20th century, HenryFord further expanded mass production technology. He set up a fixed assembly plant, and cars were passed to different workshops on the production line. Employees only need to know very little assembly knowledge, and only do this kind of work in the future. In 1954, George Devol applied for US Patent 2,988,237, this patent marked the birth of the first industrial robot, the robot named Unimate. In the late 1960s, General Motors? Use PLC to assemble the automatic transmission of the car. DickMorley, the father of PLC, is GM? Developed the first PLC (Modicon). His US patent 3,761,893 is the basis of many PLCs today.
Basic PLC operation
How simple can process control be? Let's take a common household heater as an example.
The heater components are sealed in a container to facilitate system communication. This concept can be extended to remotely controlled household constant temperature heaters, with a communication distance of about a few meters, usually using voltage control.
Now, we expand on the basis of this small and simple process control system, what control and configuration is needed in a factory?
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, which can offset the influence of the transmission line impedance. Due to the low loop impedance and bandwidth (several hundred ohms, and "100Hz"), the spurious 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 with the teletypewriter in the early 20th century. The earliest use of a 0–60mA loop was later changed to a 0–20mA loop. The PLC system was the first to adopt a 4–20mA loop.
The 4–20mA current loop has many advantages. Careful calculations are required in traditional discrete device design, and the circuit occupies a lot of space compared to the current integrated 4–20mA IC. Maxim has introduced several 20mA devices, including the MAX15500 and MAX5661, which can effectively simplify the 4–20mA PLC system design.
Any measured current value represents certain information. In practical applications, the 4–20mA current loop operates in the current range of 0mA to 24mA. However, current ranges from 0mA to 4mA and 20mA to 24mA are used for diagnosis and system calibration. Since currents below 4mA and above 20mA are used for diagnosis, it can be considered that readings between 0mA and 4mA indicate that the transmission line in the system is disconnected. Similarly, readings between 20mA and 24mA can indicate a potential short circuit fault in the system.
The enhanced version of 4–20mA communication is called a high-speed addressable remote sensor (HART? System), which is backward compatible with 4–20mA instruments. In the HART system, the use of microprocessor-based intelligent integrated field devices can achieve two-way communication. According to the HART protocol, additional digital information can be carried on the same 4–20mA analog current signal wire pair for process control.
The functions of PLC can be divided into several functional groups. Many PLC manufacturers integrate these functions into independent modules, and the functions of each module vary with specific applications. Many modules have multiple functions and can be connected to multiple sensor interfaces. However, in most cases, special modules or expansion modules are designed for special applications, such as resistance temperature detectors (RTD), sensors or thermocouple sensors. Generally, 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 supply. We will explain these core functions one by one, and sensors and sensor interfaces will be introduced separately in other chapters.
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