Control Options: PLC vs Industrial PC
The use of Industrial PC’s has been on the rise in a number of applications, with accelerated innovation blurring the line between the two technologies. Deciding what type of controller to use is no longer a clear choice.
We've summarised an article that first appeared in Control Engineering: “PC versus PLC: Comparing Control Options” to help you identify the differences.
When choosing between a PLC and PC, there are a number of factors that can differentiate the two technologies, such as operation, robustness, serviceability, hardware integration, security, safety programming and cost.
For many years, the PLC led the way for advances in machine automation control and was the controller of choice. The PLC was developed as a flexible and reliable alternative to switch boxes and relay panels, dedicated to specific tasks in the factory. It was required to be robust and consistently perform in challenging environments that contained high levels of electromagnetic interference (EMI), contamination and vibration. Overtime, the PLC evolved to include capabilities of motion control, advanced PID process control, safety, and PC features, such as a web server and networking utilities.
The PC has successfully worked its way into industrial markets, with processor speeds increasing and costs decreasing. The PC traditionally served a higher level role, used primarily for complex calculations, monitoring, measuring and factory networking. The PC eventually evolved to include PLC functionality and became more robust, allowing it to operate in harsh environments.
Consider how the system should run, and how instructions and tasks will be processed. A standard PLC has an embedded real-time operating system (RTOS), with a dedicated processor that ensures a high degree of reliability. Synchronised motion and/or advanced PID control requires a high level of real-time reliability, while non-critical supervisory controller operations, such as monitoring error messages or sending non-critical controller commands or queries, would not. Also, since a PLC only handles automation and/or a process, it does not need to run other utilities, such as anti-virus programs or system updates.
A standard PLC has no moving parts, so it can withstand harsh environments for millions of cycles. However, industrial PC’s are now just as durable as a PLC, although a PC requires additional options to equal the functionality of a standard PLC.
The ease and cost of repair or replacement over the life of the controller is another consideration. PLC external devices can be replaced while the system is in operation, and the compact design enables the whole unit to be replaced with ease. This reduces machine downtime. If a PC has a more modular design, such as a rack or panel-mount system, replacement time is closer to that of a PLC. Trying to find parts for a PC (even after a year or two) can be more challenging than for a PLC, however long-term availability of spare parts is a minimal issue with long-standing, reputable industrial suppliers.
Both a PC and PLC have the ability to control other devices using industrial networks. Although a PC and PLC can offer an array of communication options, a PLC has many of these options built-in, whereas PC’s require additional cards and drivers to function. While a PLC can interface with devices and perform complex operations, it still needs a PC to handle memory-demanding tasks and connect with other devices. A PC is equipped with more flexible interfaces, giving users access to more devices, such as an advanced imaging system, where the images are stored, analysed and compared. A PC is ideal for this task, whereas a PLC would have limited storage and processing capabilities.
There are two aspects: preventing unauthorised access (such as virus attacks and malware) and limiting user access. A PLC is less exposed to unauthorised access through viruses. Although a PC is far more susceptible to virus attacks, strict security measures can be implemented to significantly reduce potential threats, and standard software is available to detect and remove viruses. PC’s and PLC’s offer different levels of user access to keep the contents as secure as desired.
PLC’s have a long history in machine automation, designated communication channels to slave devices to tightly monitor operation, and optional integrated circuits for redundancies. Integrated safety has only recently become available on some PC-based platforms.
Device functionality is only as good as the program running on it. Therefore, the programming environment and language are crucial to optimal machine performance. The main difference between the technologies is how code is executed. A PLC mixes scan-based and event-driven program execution, whereas PC software is typically event-driven. The scan-based execution of a PLC program can take longer because the system needs to complete higher priority actions first. Each execution style requires different programming philosophy, and users often commit to one over the other.
Cost considerations go beyond the initial investment. Many applications can be solved by either a PC or PLC solution, but for some applications, the overall cost can differ widely. Cost comparisons should take into account performance requirements, future expandability and flexibility, development and programming effort. Long-term there can be little cost difference between a PC and a PLC solution.
To read the original article, use this link.