PLC PROGRAMMING
WHAT IS PLC?
BASIC INFORMATION ABOUT PLC
Programmable logic controller, abbreviated as PLC. PLC is a microcomputer system that processes the information received from the sensors according to the program given to it and transfers the results to the business personnel. It was developed in order to eliminate the negative aspects of the relay control system. Over time, its properties have been improved and made available in industrial control areas for purposes such as sequential control, motion control (linear and rotary motion control), process control (temperature, pressure, humidity, speed), data management (data collection about the machine or process, monitoring and reporting). [one]. PLC (Programmable Logic Controller, English: Programmable Logic Controller) is a special computer used in the control of operators such as control of production lines or machines in factories. It has a microprocessor inside and can be used in programming complex systems. The main difference from PCs is that the signal inputs and outputs work and are processed. [2nd].
Unlike general-use computers, PLCs are arranged to have many inputs and outputs and are designed more resistant to electrical noises, temperature differences, mechanical shocks and vibrations. Programs suitable for the operation of the system to be controlled are loaded into the PLCs. PLC programs scan the input information in the order of milliseconds, and work accordingly to respond in close to real-time output information. [2nd]. Automatic control systems provide various advantages to businesses in terms of speed, security, usage flexibility, product quality and number of personnel. Today, the most effective system providing these advantages is PLC or PC based control systems. An electronic system operating numerically in PLC control is provided in industrial environment conditions. This electronic system controls many types of machines or processes through digital or analog input / output modules [3].
Although the cost of microprocessors is lower, the reason why PLCs are preferred is; it minimizes the time spent on electronic design. At the same time, they are ready-made solutions that can work without being affected by the harsh conditions of industrial environments (magnetic field, large temperature differences, dust, etc.) [2].
There are PLCs with enough Input / Output number to load all automation jobs of a factory. Additional input-output modules, communication modules such as RS232, RS485, modem, ethernet can be added to the modular PLCs developed today. With such features, the existing structure can be improved. In addition, many models are ready for process control ON-OFF (ON-OFF), PID, Fuzzy (Fuzzy) etc. types of controllers are available as standard [2].
HISTORY OF THE PLC
The first programmable controller was developed in 1968 by a firm named Bedford Associates, which was a consultancy in the field of engineering, in order to find a satisfactory answer to the needs stated in the table below for General Motors [1].
It should be possible and easy to change the program content and do this in the factory.
The sustainability of the program should be made possible and should be such that the installed system will run when plugged in.
It should have a much higher reliability than the relay in the factory.
Compared with relay control, its dimensions should be smaller.
The control unit must be able to send data to the host system.
In terms of money, it should not be expensive compared to the relay system.
It should be available at AC 115 Volt as input voltage.
It should also be used at AC 115 Volt, voltage and current values above 2 amperes in the operation of components such as solenoid valves and motor drives.
The main system should be able to expand if the operating system changes [1].
It should have at least 4 Kilobytes of memory that allows for expandability, which can be programmed.
Over time, PLC features were developed and in 1978, a data circuit that allows PLCs to transfer data to each other was designed. In 1980, 84 Micro, a small, one-piece, low-cost, 64 input / output and more powerful PLC was launched [1].
PLCs, which were initially used only for simple operations, have been developed by various companies and have been equipped with features that can solve much more complex problems and have been used safely in industrial control fields [1].
The differences with other programmable computers are gradually decreasing. In a way, a PLC is like a computer without a monitor and keyboard. Another difference is that the data to be processed comes from the real environment and the results are sent to the real environment as analog or digital (digital) [2].
THE STRUCTURE OF THE PLC
As can be seen in the figure below (Figure 1.1), PLC is an industrial microcomputer that contains processor (CPU), input-output interfaces and memory (memory). In PLC control, various input signals and control contents are sent from the programming console to the PLC. As a result, output signals are directed to output elements [1].
In the signal exchange between input, output devices and PLC, a repeater, which is called the input interface and output interface, is required [1].
Figure 1.1: PLC's internal structure [1]
Figure 1.2 shows the connection between PLC and peripherals. Button, switch, selector switch, digital switch, limit switch, photoelectric switch, proximity switch and so on. can be used. Signals from these elements are sent to the PLC's processor via the input interface.
The processor (CPU) controls the output signals according to the content of the program stored in the memory and the input signals [1].
Equipment to be operated is named as output element. Electromagnetic valve, lamp, small power motor etc. If the electrical values are suitable, the receivers can be directly connected to the PLC, as well as transistors, relays and so on. can also be controlled via other control elements
Figure 1.2: Connection between PLC and I / O devices [1]
Reference: The programming console is a programming tool used to write a program using the commands in the command list and save this program by sending it to the PLC [1].
The program stored in memory is read by the processor (CPU). After reading, the content of the program is checked according to its feasibility. Once the program is applicable, it is run by the CPU and proceeds to the next step. The entire program in the memory is processed line by line [1].
1.1.3 COMPARISON OF RELAY AND PLC
They are microprocessor-based devices used instead of control elements such as auxiliary relays, time relays and counters in PLC automation circuits. In these devices, timing, counting, sequencing and all kinds of combinational and sequential logic operations are performed by software [3].
Therefore, it is possible to solve complex automation problems quickly and safely [3].
They are easier and more reliable.
They take up less space and malfunction.
They adapt to a new application more quickly.
They are not easily affected by bad environmental conditions.
They require less cable connections.
It is possible to use ready functions.
The states of the inputs and outputs can be monitored [3].
However, relay and digital (hardware programmed) controls work in real time. In other words, the change in the input information is reflected on the output immediately (in a very short time). This is called parallel signal processing. In PLC (memory programmed control) orders are evaluated depending on time. In other words, a change in input is not reflected on the output immediately. This form of signal processing is called serial signal processing. It is a disadvantage for PLC. This feature does not make much sense when mechanical systems are controlled. These devices are available in various sizes. They are generally distinguished and preferred from each other depending on the elements they have. Control devices are formed by combining CPU (Central Prosessing Unit) input, output, communication module etc. of many modules. Of course, in such a modular system, there is a complete harmony among all building groups if the device is enlarged [3].
These modules, which were initially independent from each other, are interconnected with a "BUS" system. The CPU creates a closed unit over this "BUS" system and organizes the transport of all data and orders [3].
The advantages of PLC usage over relay control in various aspects are stated below [1].
⦁ Economy
Using 10 or more electromagnetic contactors or switches in fabrication is more expensive than using a PLC [1].
⦁ Labor gain in design
Environmental planning of equipment can be simplified. It is easier to design and change the control circuit when PLC is used. If you want to change the working method for the same elements, it will be sufficient to change the program in the PLC [1].
⦁ Production time reduction
There is flexibility in changing the technical characteristics of the equipment and simplifying the assembly of parts. By simplifying the wiring, production time is reduced and thus the overall cost is also reduced [1].
⦁ Small size and standardization
PLC is much smaller in size than relay control. Mass production is possible thanks to the repeated use of the program. (The program is stored in EPROM.) [1] ⦁ Improving reliability
Confusion in relays and timers can be reduced. (Dust on the contacts and vibration of the relay cause confusion in the ON and OFF times of the contacts.) [1] ⦁ Improving ease of maintenance
PLC's lifespan is longer than relay's. By using the fault diagnosis function of the PLC, the error in the system can be found easily and the necessary maintenance can be done easily [1].
PLC OUTPUT TYPES
There are various types of outputs depending on the contact. When the PLC is to be installed somewhere, the characteristic of each output must be taken into account. It is difficult to decide by looking at the shape of the PLC. The output characteristics of each PLC are explained below [1].
⦁ PLC with Relay Output
In a PLC with relay output, the output is in the form of a contact and this is a very common output type. This PLC can be used in both direct and alternating current, since the output circuit consists of only the contact. High-value current such as 2 amperes can be applied to the circuit. (Up to 2 amps of current can be drawn from each contact.) Since it has no poles, it is possible to use it in various areas. For example DC motor, big powerful electromagnetic valve. The disadvantage of these PLCs can be said to be the low response speed and the wear of the relay contacts over time due to the mechanical action [1].
⦁ Triac Output PLC
It is used as triac or thyristor output element in this type of PLC. This type of PLCs are non-contact type PLCs. In Triac output PLC, alternating voltage between 85 - 242 Volts should be applied to the output. In terms of response time, this PLC is much faster than a PLC with relay output, but slower than a PLC with transistor output. The current we can get from the output of this type of PLC is 0.3 amps. Although triac output PLC does not find much place in practice, it can be used for renovation in factories that use alternating current and have PLC with relay output in the control panel [1].
Transistor Output PLC
Photo Coupler is used in PLCs with transistor output. Output current is about 0.5 amperes. Response time in these PLCs is as short as 0.2 ms.
Some of the PLCs with transistor output have a pulse output for position control without a special unit.
Since transistors are used, these PLCs have poles at their output and special attention should be paid to these poles during wiring. Recently, many small electric motors and valves are used in automated factories. These PLCs are used to control such devices. At the same time, high-value currents are not required for the control of the robot or CNC control unit. PLCs with transistor output are preferred because of their fast operation in such areas. Since these PLCs do not have mechanical contact, there is no abrasion and noise, so the output unit has a much longer life and works silently [1].
PLC FUTURE
The memory and processing capacities of PLCs, which have a lot of use in the industry, may be insufficient in the face of complex processes, so with the developing technology, it is possible to switch to more powerful industrial PCs. Nowadays there are PLCs manufactured as Industrial PC. These are both mounted on the touch panel and can be programmed by computer or with the control units on it [2].