Programmable Controller
The Central Processing Unit (CPU) contains the memory and communication system that instructs the PLC how to perform. Therefore, the CPU is the "brain" of the Programmable Logic Controller. It is also where data processing and diagnostics take place. The memory unit provides a permanent storage location to store programs without losing information when the machine is turned off. You can select the size of the memory according to the tasks assigned to the PLC. The communication system allows the CPU to communicate with other devices such as I/O devices, programming devices, and other PLC systems.
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You also need a communication port, a chassis or rack, and a programming device, such as an external computer. Depending on the type of PLC, many of these components are combined into one device. For example, a Compact Module might have a CPU with a certain number of I/Os and a specific communication port. we will discuss each component and the options available for each.
Programmable Controller Inputs and Outputs
Input/Output (I/O) modules pass the necessary information to the CPU and communicate the desired task in a continuous loop. Input and output devices can be in digital or analog form: digital devices are finite values expressed as 1 or 0 values, analog devices are infinite values and measure ranges of current or voltage. Inputs or providers are switches, sensors and smart devices in analog or digital form. Outputs can be motor starters, lights, valves, and smart devices. Many types of I/O modules are available to get the right configuration to run your machine.
Additional Note: Power supplies are either integrated with the base module or sold as separate components. The two most common voltage ranges available for PLCs include 12-24VDC and 110/220VAC. make sure the voltage range you select is sufficient to power the CPU and I/O modules.
PLC Programming
PLCs need to be programmed before they can run and perform a given task. These programs are usually written using a programming device or software. The program is then downloaded into the memory or RAM of the Programmable Logic Controller. Based on the program written, the CPU can read the information provided by the inputs and instruct the PLC to perform the desired outputs. There are many programming languages available to configure your PLC system.
According to IEC 61131, there are five standard programming languages available:
- Ladder Diagram or Ladder Logic (LD)
- Function Block Diagram (FBD)
- Structured Text (ST)
- Sequential Function Chart (SFC)
- Instruction List (IL)
Ladder Logic
Ladder logic is the most popular PLC programming language. It is a highly used and recognized programming language in many manufacturing industries. Due to its visual layout, it is easy to read and program. Ladder Logic is a program written in symbols that convert open and close relays, timers, shift registers, counters, and mathematical operations. The written symbols transfer the user's program to the PLC system.
PLC Operation
If you want to better understand a PLC system, it is beneficial to understand how a PLC controller works once you have written a program and selected the appropriate I/O modules. A programmable logic controller will process given information (inputs), execute instructions from a program, and deliver results (outputs) based on the information provided and the logic written. Once the user has determined the inputs and outputs, the PLC will operate in a repeating cycle.
The four basic steps of PLC operation are:
- Input Scanning - detects the status of input devices connected to the PLC
- Program Scan - executes a user-created program
- Output Scan - operates all output devices connected to the PLC
- Housekeeping - communicates with other devices and runs diagnostics
Types of PLCs
Unitary PLC
A unitary PLC, also known as a compact or integrated PLC, is the simplest type of PLC controller and is suitable for simple processes. A single PLC will have a certain number of I/O points and an integrated CPU with connection ports.Typically, you can connect these controllers directly to the relevant device or application.
Modular PLCs
Modular PLCs, also known as rack-mountable, offer you greater flexibility. You still need a basic module that includes your CPU, inputs, and power supplies. However, you can customize your PLC by adding additional I/O modules, where you can basically go crazy and add as many modules as your PLC will allow. You can even plug in an analog-to-digital signal converter.
How do I choose the type of PLC I need?
Now that you understand the basic components of a PLC and the types available, you may be wondering how to narrow down your choices. The first thing to consider is whether you are implementing a new system or adding to an existing one. If it is a new system, you have a wide range of options. However, if there is already a system in place, you will need to comply with the requirements of that system.
Here are 6 questions to ask yourself when purchasing a Programmable Logic Controller:
1. are you implementing this PLC into a new or existing system?
2. What type of environment will the PLC reside in?
3. What type and how many I/O points does your system require?
4. how complex is the process you wish to control?
5. how will your PLC communicate with other networks, systems, or field devices?
6. what programming language will you use to control the PLC's processes?
PLCs Move Forward
PLCs are everywhere, in places we might never think of, such as traffic signals. PLC controllers have been around for over 50 years and will continue to be around, but with the rise of "Big Data", robotics integration and cloud computing PLCs will have to adapt to these new changes as manufacturing continues to become faster and more efficient. faster and more efficient.
In the future, PLCs will have to become more robust to withstand extreme climate change, operate remotely without the need for human presence, and communicate with sensors through the Industrial Internet of Things (IIoT) to collect and analyze the data choices needed to make better products. Since all industries require cybersecurity, PLCs must be equipped with higher levels of security.
Hopefully, you have a new appreciation for PLCs. It definitely beats trying to manage hundreds of electromechanical relays. Convinced of the value PLCs can bring to your processes? If you still have questions about PLCs, feel free to contact us.