The rapid development of the microprocessor technology led to the creation of control systems for technological processes on the basis of industrial controllers. But this does not mean that relays have outlived itself, they simply have a separate niche to be used. Programmable logic controller (PLC) is a microprocessor device designed to collect, convert, process, and store information, as well as to generate control commands, having a finite number of inputs and outputs, sensors, keys, actuators connected to them, to the control object. They designed for operation in real time modes.
The principle of operation of the PLC is somewhat different from the “conventional” microprocessor devices. The software of universal controllers consists of two parts. The first part is the system software. Making an analogy with a computer, we can say that this is an operating system, that controls the operation and the interconnection of the constituent parts, having an internal diagnostics. The system software of the PLC is located in the permanent memory of the central processor and is always ready for operation. On power-up, the PLC is ready to take control of the system within a few milliseconds. The PLCs operate cyclically, using the method of the periodic interrogation of input data.
The duty cycle of the PLC includes 4 phases:
- Interrogation of inputs;
- Running the user program;
- Setting the output values;
- Some auxiliary operations (diagnostics, data preparation for the debugger, visualization, etc.).
The implementation of phase 1 is provided by the system software. After that, the control is transferred to the application program, the program that is written into the memory. According to this program, the controller does what you want, and upon its completion, the control is again transferred to the system level. Due to this, the simplicity of constructing the application program is ensured, its creator does not need to know how hardware resources are managed. It is necessary to know from which input the signal comes and how to react to it. The PLC, depending on the history of events, is able to react differently to current events. Possibilities of reprogramming, time management, developed computational abilities, including digital signal processing, raise the PLC to a higher level, unlike simple combinational automatic system.
One digital input of the PLC is able to receive one binary electrical signal, described by two states, on or off. All discrete inputs (of the general version) of controllers are usually designed to receive standard signals with a level of 24 VDC. The typical value of the current of one discrete input (with an input voltage of 24 V) is about 10 mA. The analog electrical signal reflects the voltage or current level corresponding to a certain physical quantity at each moment of time. This can be temperature, pressure, weight, position, speed, frequency, etc. Since the PLC is a digital computer, analog input signals are necessarily subjected to analog-to-digital conversion (ADC). As a result, a discrete variable of a certain bit depth is formed. As a rule, 8-12-bit converters are used in PLCs, which in most cases, based on modern requirements for accuracy of process control, is sufficient. In addition, ADCs of higher bit depth do not justify themselves, primarily because of the high level of industrial noise typical for the operating conditions of controllers.
Almost all analog input modules are multi-channel. The input switch connects the ADC input to the required input of the module. The standard discrete and analog inputs of the PLC can meet most of the needs of industrial automation systems. The necessity of using specialized inputs arises in cases when the direct processing of a signal is programmatically difficult, for example, it takes a lot of time. Most often, PLCs are equipped with specialized countable inputs for measuring the duration, fixing the fronts and counting the pulses. For example, when measuring the position and speed of rotation of the shaft, devices that form a certain number of pulses in one revolution-rotary encoder, are very common. The pulse repetition rate can reach several megahertz. Even if the PLC processor has sufficient speed, direct counting of pulses in the user program will be very wasteful in time. Here it is desirable to have a specialized hardware input block capable of performing primary processing and generating the quantities required for the application task. The second common type of specialized inputs are inputs capable of very quickly launching the specified user tasks with the interruption of the execution of the main program, interrupt inputs. The digital output also has two states, on and off. They are needed for control: solenoid valves, coils, starters, light signaling devices, etc. In general, the scope of their application is enormous and covers almost all industrial automation.
Programmable logic controller, programmable logic controller, a programmable controller is the electronic component of an industrial controller, a specialized (computerized) device used for the automation of technological processes. As the main operating mode PLC is its long-term autonomous use, often in adverse environmental conditions, without serious maintenance and with little or no human intervention. Sometimes PLCs build numerical control systems for machine tools. PLCs are devices designed to work in real-time systems.
PLCs have a number of features that distinguish them from other electronic devices used in industry:
– In contrast to a microcontroller (single-chip computer), a microchip designed to control electronic devices, the scope of the PLC is usually automated processes of industrial production in the context of a manufacturing enterprise;
– Unlike the decision-oriented computers and operator control, PLCs are oriented to work with machines through advanced input of sensor signals and output of signals to actuators;
– To differ from embedded systems, PLCs are manufactured as stand-alone products, separate from equipment managed with its help.
In control systems of technological objects, logical commands, as a rule, prevail over arithmetic operations on floating-point numbers, which makes it possible to obtain powerful systems operating in the real-time mode with comparative simplicity of a microcontroller (bus width of 8 or 16 bits). In modern PLCs, numerical operations in programming languages are implemented on a par with logical ones. All PLC programming languages have easy access to manipulating bits in machine words, unlike most high-level programming languages of modern computers.