Programable Logic Controller Basics Explained – automation engineering

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[Applause][Music] Almost every commercial building and industrial facility relies on the automation of their mechanical and electrical systems. This trend is only set to increase, especially as larger, smarter, and more complex systems and buildings are constantly under construction.

So how do we control these systems, and what devices are used to achieve that? That’s what we’ll be covering in this video, which is sponsored by Telecontrols. Telecontrols has been a leading manufacturer in the automation industry since 1963. Their technology is compatible with every PLC, HMI, and controller on the market, which reduces PLC programming time and saves valuable storage by handling smaller automation tasks directly. Click the link in the video description below to learn how Telecontrols’ products can enhance your PLC applications. You can contact them at [email protected] or via LinkedIn.

PLC stands for Programmable Logic Controller. There are many variations, but they typically look something like this. A PLC is essentially a small computer that can carry out pre-programmed outputs based on inputs and a set of specific rules. They are used in commercial and industrial applications to control systems with minimal and sometimes even zero manual intervention. The operation can be a simple on/off control based on the status of the input or a more sophisticated response based on calculations, sequences, and logic.

Before PLCs, control was carried out via banks of relays, where each relay controlled dedicated inputs and outputs based on physical wiring. Relays would control other relays to form logic controllers. For example, with a simple AND gate, only when two inputs are energized does the relay output energize. These inputs could be sensors or outputs from other relays. To change the operation, the physical wiring had to be altered, making these old banks of relays vast in size and very complex.

This is an example of an elevator relay bank, and this is the relay bank from an old electrical substation. As you can imagine, these are not easy to change, and finding faults can be difficult and time-consuming. With the invention of solid-state electronics and microchips, the command logic part of the banks of relays could be replaced with software logic, and thus PLCs quickly took over.

PLCs vary widely in their applications, but they all monitor their inputs, make decisions based on a stored set of rules, and then output commands to automate a process. We often find relays used in combination with PLCs; the relays can directly handle automation tasks and communicate with the PLC, reducing the amount of programming required on the PLC and freeing up storage space.

PLCs are widely used. For example, when you check a bag in at the airport, the bag is given a barcode and enters the conveyor belt. A PLC scans the barcode and, based on a set of rules, decides if the bag is diverted to either the domestic or international route. The next PLC scans the barcode and decides which city the bag needs to be diverted to, and the next PLC decides which gate it needs to be diverted to. If all goes to plan, the bag will arrive at the correct gate.

First, we have the input modules of field sensors. These are the physical connections between the outside world and the PLC. These can be digital inputs, such as simple on/off switches, bi-metallic temperature strips, presence or motion sensors, or even a float switch. These digital inputs can only provide information on whether something is either on or off. For that, we would need an analog input, for example, a simple control knob that ranges from zero to one hundred percent. This will go through a voltage transformer to give zero volts at zero percent and ten volts at one hundred percent. The PLC can scale the input to match the sensitivity required for very accurate output control.

The amount of current, usually measured in milliamps, tells the PLC whether something is performing between on and off. These inputs could be, for example, a thermocouple or a resistance temperature detector, a pressure sensor, or perhaps a strain gauge. These voltages or currents are converted into a digital equivalent number that can be understood by the CPU.

Input modules perform four main tasks: they sense when a signal is received, convert the signal voltage into the correct signal for the CPU, isolate the PLC from fluctuations in the input voltage or current signal, and send the corrected signal to the CPU.

The CPU, or Central Processing Unit, is the brain of the operation. It holds the program or software that decides what outputs are required by applying rules to the input signals. The CPU typically consists of a microprocessor that does the work based on the input value and the logic in the program, a memory chip to store the program, and this will also store the output history, any faults, alarms, etc.

Then we also have other integrated circuits, which can include things such as Modbus and LAN connections that allow us to remotely communicate with, reprogram, or even monitor the device.

Next, there are the output modules or field output devices, which provide the signal to the device we are controlling. For example, a simple indicator light, a solenoid valve, a motor starter, or a variable frequency drive. There are some other parts, such as a battery to keep the PLC alive in the event of a power failure, a small screen for a user interface to allow some configuration, a time clock and calendar to operate a device at the correct time, and a power supply to provide the low voltage used by the CPU as well as the input and output modules.

The basic operation of a PLC is to perform a pre-programmed output depending on the input signal by following a set of rules. The PLC completes the following stages in its basic operation: first, there is the input scan, which detects the state of the inputs; then the program scan to see what needs to be done; it executes the program logic to implement what the rules state; and finally, it updates the outputs to operate output devices based on the program requirements.

The scan time, which is the time it takes to complete all the stages, depends on the sensitivity, resilience, and system processing time. Analog inputs tend to take longer to process compared to more simple digital on/off inputs.

For example, a water tank might have a very fast scan time of 2 milliseconds to prevent overfilling, while a room temperature control can be much slower, perhaps 100 milliseconds.

Let’s see an example of a simple response: we have a bi-metallic strip temperature sensor, a PLC, and a boiler. The bi-metallic strip bends as it becomes hot and cold, allowing us to detect if the room is at the desired temperature and control the boiler. When the room is at the correct temperature, the circuit is complete, and the PLC receives a signal, so the boiler is off. When the room temperature drops, the circuit is no longer complete, and the PLC detects this change on the input. It reacts by sending an output signal to turn the boiler on.

This is a simple example, and we could also use a simple relay to achieve this. However, a PLC is better because it has a time function, allowing it to check the time before switching on the boiler. For instance, if the building might be empty at night and on weekends, we don’t want the boiler to turn on then. The PLC checks the time and date to see if it’s allowed to turn on and decides accordingly.

We can then add extra functions and inputs. For example, a motion sensor on the input can tell the PLC if the room is too cold. The PLC will check the time to ensure it is allowed to turn the boiler on and can also check if the room is occupied.

In a more sophisticated example, we have a thermistor, a PLC, and an actuator valve. The thermistor can provide a temperature scale rather than a simple on/off input like the bi-metallic strip. The actuator valve can open anywhere between zero and one hundred percent to control how much hot water is provided to heat the room.

For this, we would use a PID control loop, which stands for Proportional, Integral, and Derivative control. Essentially, this will control the valve position to ensure it only opens enough to suit the difference between the room’s desired temperature and the actual temperature.

For example, if the room temperature drops slightly, we don’t want the heating valve to instantly open 100% because the room will heat too quickly and overshoot the desired temperature. Instead, we want the valve to gradually open in proportion to the demand.

Let’s see a more complex example. In many commercial buildings, the heating or cooling system will use a control strategy known as an optimizer. This learns over time how quickly the building heats up and cools down. It then starts the heating or cooling system at the optimal time before the building is occupied.

For example, if staff are due to arrive at 9 AM, the heating system knows it needs to turn on at 7 AM to ensure that the rooms are at the correct temperature. If this system has a PLC with optimizer software installed, it controls an actuator valve for the heating system.

This system also has two pumps set up in a duty and standby configuration, so only one pump runs at a time. The PLC will decide which pump to turn on based on whichever has the lowest number of previous run hours. The PLC will monitor a flow sensor to detect if the pump turns on when told to do so. If the pump fails to turn on, the PLC receives an alarm, cuts the power, and tells the other pump to start.

However, before the heating system and pump start, the PLC will check with the clock to see if the heating should turn on today and at what time the building will be occupied. If the clock says yes, and the scheduled occupancy time is 9 AM, the PLC checks the current temperature of the room and calculates the difference between this and the desired temperature. It then checks the outdoor temperature to calculate how long it will take to heat the building, as on a very cold day there will be greater heat loss, requiring more time.

From this, the PLC calculates what time it needs to turn the heating system on so that the building is at the desired temperature, ready for 9 AM.

There are many advantages of PLCs, but some of the main ones are as follows: the control software is stored locally, so in the event of a building energy management system failure, the PLC can continue working; the connections between the PLC inputs and outputs are made by software rather than physical wires; PLC installations are smaller than hardwired relay banks but can still use relays when needed; PLCs are much easier to reprogram; fault finding is easier and faster; you can load the same program onto multiple PLC units to save time; and you can also expand the inputs and outputs with more cards.

That’s it for this video! To continue learning about controls and electrical engineering, check out one of the videos on screen now, and I’ll catch you in the next lesson. Don’t forget to follow us on Facebook, Twitter, Instagram, LinkedIn, and visit theengineeringmindset.com.

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