In this article, we’re going to talk about why a Programmable Logic Controller (PLC) is preferable to a Personal Computer (PC) in automation and industrial control systems.
Up until the last few years, the PLC was the hands-down favorite, but the PC is beginning to appear as the chosen Controller. We’ll tell you more about that later.
Let’s start with the basics…
PLCs have been around since the ’70s. Original PLCs were single microprocessor devices. They were designed to replace decision-making, hard-wired relays, and timers with a user-created software program performing the required decisions.
All of the field devices monitored and controlled such as switches, sensors, and motors were wired to add-on modules connected to the microprocessor module.
It didn’t take long for the PLC to evolve and include modules capable of analog control and PID-controlled processes.
The PC has also been around for a long time. The term Personal Computer was coined in the ’80s as the device was intended to be a single-user desktop computer.
Then along came the laptop computer, the tablet, and now the smartphone each capable of performing PC functions.
The PC was not originally intended for use in automation and control.
Okay… so what are some of the defining differences between a PLC and a PC?
A PLC has a proprietary microprocessor and a unique Operating System.
A PC has an Operating System such as Microsoft Windows, Mac OS, or Chrome OS. Technically speaking, one might argue that a computer with a MAC OS or a Chrome OS is not a PC.
A PLC is programmed using proprietary programming software such as Rockwell Studio 5000, or Siemens TIA Portal. This proprietary software is used to develop programs using one of the IEC 61131-3 standard languages for PLCs.
Interestingly, the proprietary software resides on a PC. The resulting control program is written on the PC and then downloaded to the PLC for execution.
PC programming is usually done using structured text, and higher-level programming languages like C++.
A more common use of a PC is acting as the host to a variety of commercially available software such as Microsoft Office for word processing or accounting.
A major difference between a PLC and a PC is the way each executes a program.
A PLC usually follows a scan-based program execution whereas PC software programs are event-driven.
Alright…
What are the advantages of a PLC over a PC?
A PLC was made for pain. A PLC is constructed for reliable operation in very harsh industrial environments. It can withstand heat, cold, dust, and other nasty environmental conditions.
A PC was made to sit on a desk or your lap in an environmentally friendly environment.
A PLC was designed with ease of programming and troubleshooting in mind. A PLC is inherently resistant to viruses and cyber attacks as the Operating system is proprietary.
A PC is very susceptible to viruses and cyber-attacks.
A PLC is designed to easily accept expansion. Input/Output Modules can be added to work with more field devices.
Speaking of expansion modules, PLCs can easily accept modules facilitating industrial communication protocols such as Profibus, CAN bus, Modbus, Ethernet/IP, and others.
A PC is capable of expansion, but not easy to accomplish.
PLC cabinets can be found in almost every industry from sawmills to oil refineries.
PLCs are also used in mobile heavy equipment such as a Hydraulic Mining Shovel or a FRAC Truck.
Leading PLC manufacturers include Allen-Bradley, Omron, Siemens, Phoenix Contact, and Schneider. There are many more and the list seems to grow every year.
At the start of this article, we stated that up until the last few years, the PLC was the hands-down favorite, but the PC is beginning to appear as the chosen Controller.
Let’s explore that statement a bit further. There’s a new kid on the block called an Industrial PC.
It has an Operating System that provides it with the capability to run programs and applications that PLCs can’t. So, this new Industrial PC can do more than just control as with the PLC.
What separates the traditional PC from the Industrial PC is its design.
Industrial-grade PCs are rated for harsh environments and extreme temperatures and are built to stand up to shock and vibration.
A big advantage of the Industrial PC over the PLC is its programming. An Industrial PC uses the Windows operating system which is recognizable by almost everyone.
Traditional PLC programmers are experts in IEC 61131-3 languages such as Ladder Diagram or Function Block.
A new generation of IT professionals can write control programs for the Industrial PC using higher-level languages like C++.
Ok,… let’s review what we’ve covered in this article…
– PLCs were designed to replace decision-making, hard-wired relays, and timers with a user-created software program performing the required logic.
– A PLC has a proprietary microprocessor and a unique Operating System.
– PLCs are programmed using proprietary programming software.
– A PLC usually follows a scan-based program execution.
– A PLC is constructed for reliable operation in very harsh industrial environments.
– PLCs can be found in almost every industry from sawmills to oil refineries.
– Industrial-grade PCs are rated for harsh environments.
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