Some people mistakenly think that the term "controller" comes from the word "control" and implies checking or accounting for something. In fact, this concept came to us from the English language and in translation means "management". Based on this, it becomes clear that programmable logic controllers (PLCs) are commonly used to control automated electrical equipment.
Content
- Main role
- Mechanism of action
- Varieties of logic devices
- Processor module
Main role
For the first time, these computing devices were used in the automotive industry in the 60s of the XX century to automate machine assembly lines. The hardware method of programming, on the one hand, made the production process cheaper, and on the other hand, it required a complex reconfiguration of one technological line to another. To simplify the setup process, controllers were released based on a relay connection diagram.
Later, it became possible to program devices using a machine-oriented language. However, such a setup required major changes to the control system, so the development of programming languages began to simplify the use of controllers. Today, the main ones are the classical algorithmic languages and the IEC 61131-3 languages.
Modern programmable microcontrollers are widely used. They are used by:
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in signaling and emergency protection; - in machine tools with numerical control;
- in building life support systems;
- when archiving and collecting information;
- in medical technology;
- to control robotic equipment and spacecraft;
- when testing various products, etc.
Local software controllers play the role of not only stand-alone equipment to control the operation of various process plants. They are also an important part of large scale enterprise systems.
Mechanism of action
There are different models of PLC, but the principle of operation of any type of controller is the same.
Inside the gate is an LED that provides an electrically isolated high signal. The occurrence of such a signal occurs when an alternating current of 120 volts is established between the common and input terminals.
The operation of the programmable controller is determined by a special program compiled and viewed using a computer connected to the PLC programming port. The logic gate program differs from the relay circuit in the absence of real switch contacts or relay coils required to create connections between input and output within the controller. All contacts are imaginary.
The programmable controller is associated with a pushbutton switch. This the switch can be in two positions:
- Off. No input signal. In accordance with the current program, the output will be de-energized. This is evidenced by the extinguished indicator.
- Included. The signal will be sent to the input, all contacts in the program will be activated. From the closed contact, the signal is transmitted to the coil. When voltage is applied to it, the output will be illuminated using the connected indicator.
All signals visible on the computer monitor are not real electrical components, but virtual ones. Their presence in the software is only necessary to remind you of what is happening in the circuit.
In the process of functioning of the software logic controller, the computer plays a secondary role. It is only needed to write a basic program of actions and monitor ongoing processes. The controller performs all incoming commands independently without the participation of a PC.
The logic element program can be changed at any time. When adjusting individual commands no need for a complete reconfiguration of all connected components. Logic algorithms can be created using your own front panel, portable programmer, monitor, mouse and keyboard.
The main advantage of programmable logic control, as opposed to control by means of equipment, is the ability to use signals an unlimited number of times (depending on needs).
Varieties of logic devices
The wide range of tasks that can be solved with programmable devices influences the selection of the right controller.
Wherein the main characteristics of the device should be taken into account:
- temperature regime;
- reliability;
- availability of certificates for use;
- manufacturer.
Several factors are taken into account when classifying a PLC. One of these is the location of the input and output modules. In accordance with this feature, controllers are:
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Monoblock. They have a single design. The I / O device is permanent and cannot be replaced. - Modular. Inputs and outputs are made in the form of separate enclosures, which are located in a single basket. The user can independently choose a combination of plug-in modules, depending on what task they have to solve.
- Distributed. In them, remote I / O modules are connected to the controller module via a network based on the RS-485 interface. This connection method allows the elements to be located at a distance of up to 1 km from the main module.
Certain types of logical devices can be combined. In this way, programmable equipment with an increased number of channels is created.
The PLC can be mounted on a wall, panel, cabinet door, or DIN rail. In special equipment, single-board devices without package can be used.
Different areas use different programmable controllers. They are divided into general purpose, dedicated, communication and PID controllers depending on their application. They are often used to control positioning and movement, as well as control robots.
Processor module
An important functional role in the PLC is assigned to the main processor module. It includes a central and processing unit, memory devices, a watchdog and a clock displaying real time. The main parameters of such a module are considered clock frequency, bit width, type of supporting ports, floating point operations, operating temperature and power consumption.
Devices responsible for processing large amounts of digital data are equipped with an additional mathematical processor module, performing actions with floating point, or a signal processor that performs mathematical operations in one cycle and allows you to speed up convolution. For processors of this type, memory capacity is an important characteristic.
A counter that reads the pulses of the clock generator and periodically restarts with the help of a running processor can be used as a watchdog sensor.
The built-in quartz-type clock is battery powered and continues to display the time even if the logic controller is turned off. They are required to control the operation of street lighting devices, security systems and other equipment, the work of which is tied to astronomical time.
With the help of the main processor module, information is collected from the input modules and sent to the output module, the exchange of information when programming the device, the protocol stack of the industrial network.
Also, the processor is responsible for bootstrapping the operating system and performing basic tasks, execution of the loading module of the program installed by the user and control over exchange acts memory.
