A Programmable Logic Controller ( PLC ) uses a type of programmable memory to store programs internally, execute user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations, and control various types of machinery or production processes through digital or analog inputs/outputs.
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 and chemical processing, PLC control is indispensable in a wide range of applications.
These systems offer a wide range of functions, including various analog and digital input/output interfaces, signal processing, data conversion, and different communication protocols. All components and functions of the PLC are centered around the controller, which is programmed for a specific task.
Basic PLC components must be flexible and configurable enough to meet the needs of different factories and applications. Input stimuli (whether analog or digital signals) come from machine devices, sensors, or process events, and are expressed as voltage or current. The PLC must accurately provide the CPU with parsed and converted stimulus signals, which in turn determines a set of instructions to be sent to the output system, which controls the actuators installed in the factory or another industrial environment.
Modern PLCs originated in the 1960s, and in the decades that followed, their general functions and signal channels underwent minor changes. However, 21st-century process control has placed far greater demands on PLCs: higher performance, smaller size, and greater functional flexibility. Built-in protection functions are essential to prevent potentially damaging electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI/EMI), as well as large-amplitude transient pulses common in harsh industrial environments.
Reliable design
PLCs need to operate reliably for years in industrial environments, environments that can significantly damage the microelectronic components that provide the PLC's exceptional flexibility and precision. Maxim understands this better than any other mixed-signal IC vendor, having led the industry from its inception with superior product reliability and innovative solutions, ensuring high-performance electronics are protected from harsh environments, including high ESD, high transient voltage swings, and EMI/RFI. Designers have widely recognized Maxim's products for solving the challenges of analog and mixed-signal design, and for their continued commitment to addressing these challenges year after year.
Higher integration
PLCs have 4 to hundreds of input/output (I/O) channels, supporting a wide range of applications. Therefore, size and power are just as important as system accuracy and reliability. Maxim maintains its industry-leading position by integrating the right functions into its ICs, thereby reducing overall system space and power requirements and achieving more compact designs. Maxim offers hundreds of low-power, high-precision ICs in minimal form factors, enabling system designers to build precision products that fully meet demanding space and power requirements.
Factory Automation - A New Invention
Assembly lines are a relatively new invention in human history, and many countries have seen similar innovative solutions emerge during the same period. We have listed a few examples from the United States here.
Samuel Colt (an American gun manufacturer) demonstrated a universal component in the mid-19th century. Early gun manufacturers required each gun's components to be manufactured independently and then assembled separately. To automate assembly, Mr. Colt experimented by placing all the components of 10 guns in different boxes, then randomly picking the components from the boxes and assembling them into a single gun. In the early 20th century, Henry Ford further expanded mass production technology. He established fixed assembly plants where cars were transferred to different workshops along the production line. Employees only needed minimal assembly knowledge and performed only this type of work in their subsequent jobs. In 1954, George Devol applied for US Patent 2,988,237, marking the birth of the first industrial robot, named Unimate. In the late 1960s, General Motors used PLCs to assemble automatic transmissions for automobiles. Dick Morley—the father of the PLC—developed the first PLC (Modicon) for GM, and his US Patent 3,761,893 is the basis for 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 within a container to facilitate system communication. This concept can be extended to remotely controlled household thermostats with communication distances of several meters, typically using voltage control.
Now, let's expand upon this small, simple process control system. What controls and configurations does a factory need?
Impedance, EMI, and RFI of long-distance transmission lines make voltage control schemes very difficult to implement. In such cases, a current loop is a simple and effective solution. According to Kirchhoff's laws, the current at any point in the current loop is equal to the current at all other points in the loop, thus offsetting the effects of transmission line impedance. Due to the low loop impedance and bandwidth (several hundred ohms and <100Hz), spurious EMI and RFI pickup is minimized. PLC systems are very useful for appropriate control, such as in factory production systems.
PLC current loop transmission
The application of current control loops began in early 20th-century teletype machines, initially using a 0–60mA loop, later changed to a 0–20mA loop, and PLC systems were the first to adopt a 4–20mA loop.
4–20mA current loops offer numerous advantages. Traditional discrete device designs require careful calculations and consume significant circuit space compared to current integrated 4–20mA ICs. Maxim offers several 20mA devices, including the MAX15500 and MAX5661, that effectively simplify 4–20mA APLC system design.
Any measured current value represents some information. In practical applications, a 4–20mA current loop operates within the 0mA to 24mA current range. However, the 0mA to 4mA and 20mA to 24mA current ranges are used for diagnostics and system calibration. Since currents below 4mA and above 20mA are used for diagnostics, readings between 0mA and 4mA can be considered to indicate a transmission line break in the system. Similarly, readings between 20mA and 24mA can indicate a potential short-circuit fault in the system.
An enhanced version of 4–20mA communication is called the High-Speed Addressable Remote Sensor (HART® system), which is backward compatible with 4–20mA instruments. In the HART system, intelligent integrated field devices based on microprocessors enable bidirectional communication. According to the HART protocol, additional digital information can be carried on the same 4–20mA analog current signal pair for process control.
A PLC's functionality can be divided into several functional groups. Many PLC manufacturers integrate these functions into independent modules, each with different capabilities depending on the specific application. Many modules offer multiple functions and can connect to various sensor interfaces. However, in most cases, dedicated or expansion modules are designed for specific applications, such as resistance temperature detectors (RTDs), sensors, or thermocouple sensors. Typically, all modules share the same core functions: analog inputs, analog outputs, distributed control (e.g., fieldbus), interfaces, digital inputs and outputs (I/O), a CPU, and a power supply. We will explain these core functions one by one; sensors and sensor interfaces will be introduced in other chapters.
