What is PLC?

PLC is an abbreviation for Programmable Logic Controller.

Originally called a PC (programmable controller), with the advent and spread of PCs (personal computers), it has come to be called a PLC to avoid mixing (in some products and software). , Even now, PLC = PC).
PLC was developed as an alternative to relay circuits and is installed in mechanical devices such as elevators and amusement park attractions as well as controlling automatic machines in factories.
The PLC mechanism controls ON / OFF of output devices according to a predetermined program according to the ON / OFF of input devices such as switches and sensors (turning a motor, turning on a lamp, etc.).
PLC has a program for control and is often made in ladder format in Japan.
Schneider Electric is the first manufacturer in the world to commercialize a PLC
PLC was first commercialized by Modicon (now Schneider Electric) in 1969 at the request of General Motors (USA). The world’s first programmable display first appeared on Pro-face in 1989, 20 years ago. In short, Schneider Electric is the first in the world to commercialize two FA devices. Since then, PLCs and HMIs have grown significantly into important control equipment as the core of FA systems.
PLC type:
PLCs are roughly classified into “package type” and “building block type” in terms of structure. Each consists of the following five parts, and if anyone is missing, it will not function as a PLC.
Input section
MPU calculation unit (CPU)
Memory section
Output section
Power supply part
Package type (compact type):
This type has an input unit, CPU calculation unit/memory unit, power supply unit, and output unit integrated. Generally cheaper than the building block type.


Build type:
The input unit, CPU calculation unit/memory unit, output unit, and power supply unit are independent units and are used in combination. Since each unit is selected by the user himself, the PLC will be according to the specifications, but the selection requires some knowledge. This type is often used in complex programs and equipment that is expected to be large/expanded.
In addition, building block types can be divided into two types depending on whether or not there is a base.
Those that mount each unit on the base
Those that connect each unit directly (high degree of freedom in adding units (not affected by the number of slots in the base), space-saving)
What type of PLC do your customers use? Think about why.
PLC configuration:
You can deepen your understanding of the PLC configuration by reading it together with the Schneider M221 catalogue/spec sheet. There are five basic PLC configurations: “input”, “MPU calculation”, “memory”, “output”, and “power supply”.
Input section:
Connect switches and various sensors to capture digital/analogue signals. The number of input devices to be connected is called the I / O score together with the output device (Input: Input / Output: Output) I / O is built in the package type PLC, and the PLC is based on the I / O score. The model is different. For a building block type PLC, select an I / O unit according to the number of I / O points.
MPU calculation unit (CPU):
The main processing device, which is the brain / central part of the PLC. It reads the signal from the input section, calculates it, and outputs it to a lamp, etc. via the output section. Performance is judged by processing speed.
Memory section:
There are “user program memory” and “data memory” in the memory section. The capacity is always stated in the catalogue and manual. The memory capacity used is determined by the number of I / O points and the complexity of control. Programming with some margin (about 30%) is convenient for changes and extensions.
Output section:
The result calculated by the MPU is output to an external device via the output section. There are three types of output for the M221: transistor output (sink/source) and relay output.
Power supply part
The internal power supply for operating the MPU etc. is supplied from the external commercial power supply. Generally, there are AC100 ~ 200V and DC12 ~ 24V.
Recent PLCs have become more sophisticated, and are equipped with modules such as HMI and positioning, and interfaces for various network communications.
PLC features (comparison with relay control)
Relay control panels are still used in the field. Now let’s look at the advantages of comparing relay control and PLC.
What is a relay?
An electromagnetic relay, which is a component that receives an electric signal from the outside and turns on / off an electric circuit. It consists of an electromagnet that receives an electric signal and changes it into a mechanical movement and a switch that opens and closes the electric machine. Before the appearance of PLC, control was performed by a relay control panel using these relays, timers, counters, etc.
On the relay control panel
Design and wiring work is required for each control target. In terms of maintenance, the probability of poor contact is relatively high due to the life of the contacts. In addition, there are many inspection points, and inspection work must be performed frequently.
PLC features compared to relays
Economical: The cost performance of the PLC hardware itself is very high, and when combined with other features, the cost performance is outstandingly good.
Flexibility: Easy to reuse the program. Equipment can be added on a unit or module basis.
Reliability: PLC has high reliability because its constituent elements are semiconductors / ICs.
Ease of maintenance: It has a monitoring function, an abnormality display, and a self-diagnosis function that are easy to operate even for field workers, and has excellent maintainability.
Advantages of introducing PLC
Recently, PLCs have become more sophisticated and networked, and the merits of introducing PLCs in connection with higher-level systems are also increasing.
To use PLC correctly
Microprocessors and LSIs are often used in recent PLCs, and if only those parts are taken, the reliability is significantly improved compared to 10 years ago, but the other parts that make mechanical contact (connectors). , Etc.) and soldered parts, etc., there are still parts that should be considered for life. PLC manufacturers may also work on it, and users also have points to consider when using it.

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