Rugged and shielded field computational hardware for severe conditions (such as dust, humidity, heat, cold). A programmable logic controller (PLC) or programmable automaton is digital equipment that is used for the automation of electromechanical processes, such as the control of machinery on factory assembly lines, amusement rides, lighting devices. PLCs are used in many industries. Unlike computers and / or general purpose computers, the PLC has the following characteristics:
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Discrete or digital signals behave like binary switches, producing only an On or Off signal (1 or 0, True or False, respectively). Buttons, limit switches, and photoelectric sensors are examples of devices that provide a discrete signal. Discrete signals are sent using voltage or current, in a specific range. For example, a PLC can use 24 V DC I / O, with values higher than 22 V DC representing ON and values lower than 2VDC representing Off, and the intermediate values are undefined.Analog signals are like volume controls, with a value range between zero and full scale. These are usually interpreted as integer values (counts) by the PLC, with various ranges of precision depending on the device and the number of bits available to store the data. PLCs often use signed 16-bit processors, integer values are limited between -32,768 and 32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0-10 V or 4-20 mA is converted to an integer value from 0 to 32767.
A control program is generally executed repeatedly while the control system is active. The state of the physical inputs is copied into a processor-accessible memory area, sometimes called an "I / O image table." The program runs from its first to its last code statement. It takes some time for the PLC processor to evaluate all the code instructions and update the I / O image table with the status of the outputs. This scan time or "Time Scan" can be a few milliseconds for a small program or in a fast processor, but in older PLCs with large programs it could take much longer (for example, up to 100 ms) to execute. of the full program or St. time. If the analysis time was too long, the response of the PLC to the process conditions would be too slow to be useful.
As the PLC evolved, methods were developed to change the ladder execution sequence, and the implementation subroutines. This simplifies programming and keeps the scan time low.
Specific purpose I / O modules, such as timer or counter modules, can be used when the processor cycle time is too long to reliably collect I / O. For example, the pulses of an encoder. The relatively slow PLC could interpret the counted values to control a machine, but the accumulation of pulses is done by a specific module that was not affected by the speed of the program execution.
Small PLCs will have a fixed number of input / output connections. Expansions will generally be available if the base model does not have enough I/O.Modular PLCs have a chassis (also known like rack) in which modules with different functions are placed. The processor and I / O modules customized for the particular application. Multiple racks can be managed by a single processor, and it can have thousands of inputs and outputs. A special high-speed serial link is used so that racks (racks / chassis) can be distributed away from the processor, reducing cabling costs for large installations.
Sometimes it is necessary for the PLCs to have to interact with people for the purpose of reporting alarm, monitoring every day, therefore a Human Machine Interface (HMI) is employed for this purpose. HMI are also known as man-machine interfaces (MMI Man Machine Interface) and graphical user interface (GUI). A simple system can use the buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. The most complex systems use control and data acquisition software installed on a PC, which is connected to the PLC through a communication network.
Usually 9-pin RS-232, but optionally EIA-485 or Ethernet are also very frequent. Modbus, BACnet is usually included as one of the communication protocols. Other options include different field buses, such as DeviceNet or Profibus.
Most modern PLCs can communicate over a network to another system, such as a computer running a SCADA (supervisory control and data acquisition) system or web browser.
Peer-to-peer (P2P) communications are equally used. These communication paths are often used for operator panels, such as keyboards or PC-type workstations.
PLC programs are typically written in a special application (development environment) on a personal computer (PC), then loaded via a direct connection cable or networked with the PLC. The program is stored in the PLC, either in battery backup, RAM memory or some other non-volatile Flash memory. Often a single PLC can be programmed to replace thousands of relays.
Under the IEC 61131-3 standard, PLCs can be programmed with standards-based programming languages.
A graphical programming annotation called a sequential function diagram is available on some automation systems. Initially most PLCs use Ladder Diagram (LD) as a programming tool, a model that emulates electromechanical control panel devices (for example, contact and relay coils), which PLCs replace. This model is still common today.
IEC 61131-3 currently defines five programming languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text (ST, similar to Pascal programming language), instruction list (IL, similar to assembly language) and sequential function chart (SFC). These techniques emphasize the logical organization of operations.
While the fundamental concepts of PLC programming are common to all manufacturers, differences in I / O addressing, memory organization, and instruction sets mean that PLC programs are not perfectly interchangeable between different manufacturers. Even within the same product line from a single manufacturer, different models may not be directly compatible.