We are living in the midst of the digital age, and this—among many other things—means that almost all commercial buildings and industrial facilities have, to a greater or lesser extent, some degree of automation in their mechanical and electrical systems. This is a trend that we know for certain will only continue to grow in the coming years, and these systems will become increasingly complex.
And one of the key components of automated systems is the programmable logic controllers (PLCs) (Programmable Logic Controller). These are essentially industrial computers responsible for processing data from an industrial machine or a group of machines. These computers are designed to control the operation of these machines in real time and in a programmable manner (through a set of rules).
History of Programmable Automata
Long before the advent of microchips and computers, large production lines were controlled by enormous and complex sets of relays, which not only took up a lot of space but were also quite expensive, difficult to replace, and hindered the detection of problems or errors.
The first people to draw attention to the problems with relays used in machinery automation were General Motors; those who allied themselves with Digital Equipment Corporation in the the 1960s, to create a system that would overcome the analog complexity of relays. The result was the PDP-14, a prototype automaton that could only operate using “Yes or No” control.
This system quickly spread to all industries and was also rapidly improved. For the the 1970s, with the invention of microcontrollers, controllers were now able to perform arithmetic operations and communicate with computers. And finally, in the the 1980s The microchip arrived, which would turn them into what we know today: controllers capable of handling complex programming languages, with high response speeds and very small dimensions.
Parts of a Programmable Logic Controller
- Input modules: Also known as field sensors, these are the PLC’s connection to the outside world via physical connections. They can be: digital inputs such as simple switches, bimetallic strips, presence sensors, etc.; or analog inputs such as manual control knobs, a pressure sensor, a voltage meter, etc.
- Processing unit: This is the “brain” of the device; it houses the software that applies the programmed rules to the input signals. It generally consists of: a microprocessor, a memory integrated circuit, and other integrated circuits.
- Output modules: These are the modules that provide the signal to the device being controlled by the PLC, which can be anything from a simple indicator light to a motor starter or a variable-frequency drive.
- Accessory modules: These are various devices that can add functions or features to programmable logic controllers (PLCs), such as a battery, a user interface display, a clock, a calendar, etc.
How does a programmable logic controller work?
Now that we understand the components of a programmable logic controller, we can address the question of its operating cycle. The basic function of these devices is to execute a “output” or command—which has been preprogrammed—in response to a signal generated by any input module, and in accordance with a set of rules.
A process that can be described as follows: stages:
- Input scanning: Receives the signals transmitted by the devices that make up the input module.
- Program scan: The automaton scans the program stored in its CPU to verify what needs to be done; in other words, it verifies the questions it needs to ask based on the received signals.
- Program Logic Execution: In this phase, the questions stored in the CPU are answered, and based on this, the output sequence is determined.
- Output Operation: In this phase, the devices that make up the output module receive and execute the command issued by the programmable logic controller.
- Maintenance: This is the final phase, which involves periodic diagnostics of communications with devices, potential updates, and the generation of reports on system performance or collected data.
It's called scan time the time it takes for the controller to complete all the stages that make up a cycle. This time can vary depending on the PLC’s performance—specifically, its sensitivity, resistance, and system processing time—as well as the type of inputs, since, for example, analog inputs are more complex to process than digital ones.
Is the end of programmable logic controllers approaching?
This question is beginning to arise as the technology becomes more widespread “Internet of Things”, which is demonstrating just how easy it is today to equip almost any object with a microchip and an internet connection. That would mean the physical disappearance of the programmable controller, since all that would be needed is an interface within a computer connected to a network Wi-Fi, fully connected to input and output devices.
In a future of smart sensors and actuators, the machine learning, and from the cloud computing, PLCs face an uncertain future. However, it is true that in the short term, it is unlikely they will disappear, given their simplicity and moderate cost, since it will take a great deal of time and money to adapt all devices to these new technologies.
