A Programmable Logic Controller (PLC) uses a type of programmable memory to store programs internally and execute user-oriented instructions such as logical operations, sequential control, timing, counting, and arithmetic operations, while controlling various types of machinery or production processes through digital or analog inputs and 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, a wide range of applications rely on PLC control.
These systems offer a wide range of capabilities, including various analog and digital input/output interfaces, signal processing, data conversion, and diverse communication protocols. All PLC components and functions are centered around the controller, which is programmed for a specific task.
Basic PLC components must be sufficiently flexible and configurable to meet the needs of different plants and applications. Input signals-whether analog or digital-originate from machinery, sensors, or process events and manifest as voltage or current. The PLC must accurately parse and convert these input signals for the CPU, which then determines a set of instructions to be sent to the output system-which in turn controls the actuators installed in a factory or other industrial environment.
Modern PLCs originated in the 1960s, and over the subsequent decades, their general-purpose functions and signal channels have undergone only minor changes. However, 21st-century process control has placed new and more demanding requirements on PLCs: higher performance, smaller size, and greater functional flexibility. Built-in protection features are essential to guard against potentially destructive electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI), as well as high-amplitude transient pulses commonly found in harsh industrial environments.
Reliable Design
PLCs must operate flawlessly for years in industrial environments, which can be highly damaging to the microelectronic components that provide them with exceptional flexibility and precision. Maxim understands this situation better than any other mixed-signal IC manufacturer, as we have led the industry since our inception with exceptional product reliability and innovative solutions that protect high-performance electronic devices from harsh environments, including high ESD, high transient voltage swings, and EMI/RFI. Designers have widely recognized Maxim's products because they solve the challenges of analog and mixed-signal design and will continue to do so year after year.
Higher Integration
PLCs feature anywhere from 4 to hundreds of input/output (I/O) channels to support a wide range of applications; therefore, size and power are just as critical as system accuracy and reliability. Maxim maintains its industry-leading position by integrating the right features into its ICs, thereby reducing overall system space and power requirements and enabling more compact designs. Maxim offers hundreds of low-power, high-precision ICs in the smallest possible form factors, enabling system designers to build precision products that fully meet demanding space and power constraints.
Factory Automation-A New Invention
Assembly lines are a relatively recent invention in human history, with similar innovations emerging simultaneously in many countries. Here, we list several examples from the United States.
Samuel Colt (an American *manufacturer*) demonstrated a universal component in the mid-19th century. Early *required* the separate manufacture of each component for every gun, followed by individual assembly. To achieve automated assembly, Mr. Colt attempted to place all the components for 10 guns in separate boxes, then randomly retrieve these components from the boxes and assemble 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 transported along an assembly line through different workshops. Employees needed only a minimal amount of assembly knowledge and performed only those specific tasks in their subsequent work. In 1954, George Devol filed U.S. Patent No. 2,988,237, which marked the birth of the first industrial robot, named Unimate. In the late 1960s, General Motors used PLCs to assemble automatic transmissions for cars. Dick Morley-the "father of the PLC"-developed the first PLC (Modicon) for GM; his U.S. Patent No. 3,761,893 forms the basis for many modern PLCs.
Basic PLC Operation
How simple can process control be? Let's illustrate this using 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 thermostatic heaters, with a communication range of about a few meters, typically using voltage control.
Now, building on this small, simple process control system, what kinds of control and configuration does a factory require?
The impedance of long transmission lines, as well as EMI and RFI, make it very difficult to implement voltage-controlled solutions. In such cases, a current-loop system is a simple and effective solution. According to Kirchhoff's laws, the current at any point in a current loop is equal to the current at all other points in the loop, thereby offsetting the effects of transmission line impedance. Because the loop impedance and bandwidth are low (a few hundred ohms and <100 Hz), EMI and RFI interference are minimized. PLC systems are very useful for appropriate control applications, such as factory production systems.
Current-Loop Transmission in PLCs
The application of current-control loops began with electromechanical typewriters in the early 20th century. Initially, 0–60 mA loops were used, later replaced by 0–20 mA loops, and PLC systems were the first to adopt 4–20 mA loops.
The 4–20 mA current loop offers many advantages. Traditional discrete component designs require careful calculations, and the circuits take up a significant amount of space compared to today's integrated 4–20 mA ICs. Maxim has introduced several 20 mA devices, including the MAX15500 and MAX5661, which effectively simplify the design of 4–20 mA PLC systems.
Any measured current value represents specific information. In practical applications, the 4–20 mA current loop operates within a current range of 0 mA to 24 mA. However, the 0 mA to 4 mA and 20 mA to 24 mA current ranges are used for diagnostics and system calibration. Since currents below 4 mA and above 20 mA are used for diagnostics, a reading between 0 mA and 4 mA can be interpreted as indicating a broken transmission line in the system. Similarly, a reading between 20 mA and 24 mA may indicate a potential short-circuit fault in the system.
An enhanced version of 4–20 mA communication is known as the High-Speed Addressable Remote Transmitter (HART®) system, which is backward compatible with 4–20 mA instruments. In the HART system, microprocessor-based, intelligent, integrated field devices enable bidirectional communication. According to the HART protocol, additional digital information for process control can be carried over the same 4–20 mA analog current signal pair.
PLC functions can be divided into several functional groups. Many PLC manufacturers integrate these functions into separate modules, with the capabilities of each module varying depending on the specific application. Many modules offer multiple functions and can interface with a variety of sensors. However, in most cases, dedicated or expansion modules are designed for specific applications, such as resistance temperature detectors (RTDs), sensors, or thermocouples. 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 covered separately in other chapters.