PC-based controls: Busting myths, revealing benefits
PC-based control systems provide many benefits and simplified solutions for industrial applications, including performance, decreased costs, and increased system lifecycle benefits. See five PC-based control myths and nine industrial PC benefits.
Industrial PC-based control systems increase connectivity capabilities for industrial applications while providing powerful, flexible, and cost-effective control. Understand the myths and benefits to help with control system specification.
PC-based control system misconceptions:
Many misconceptions are often attributed to PC-based control systems. Dispelling misconceptions can help with the implementation of and the benefits from PC-based control systems. Test your knowledge of PC-based control with the following five myths of industrial PCs:
Industrial PCs (IPCs) are not real-time because they rely on the clock speed of the PC.
FALSE: Most PC-based control platforms today use a real-time kernel running under the Microsoft Windows user mode. Windows starts the real-time kernel during start-up, and, once initialized, the kernel does not rely on Windows to function, although there is communication between the kernel and Windows.
Industrial PC-based systems are not as fast as a programmable logic controller (PLC).
FALSE: Typically, PC-based control systems leverage the newest and fastest processors; this ensures that the systems feature industry-leading performance while maintaining the lowest possible cost in all areas, including total cost of ownership, hardware, software, and networking. For instance, a system is capable of 50 microsecond (µs) PLC scan rates synchronous with a 50 µs network scan, while also capable of performing control in between the scan down to 1µs, all with accuracy in the nanosecond range. In addition, this can be accomplished with almost zero jitter, which is essential to the accuracy and determinism of the PLC and network cyclic updates. Highly accurate and deterministic cyclic updates enable increased accuracy for coordinated motion control and data samples at precise time intervals. PLCs typically offer scan rates in milliseconds, as opposed to the microsecond level.
Industrial PCs are not deterministic.
FALSE: A PC-based control system runs PLC scans in software with high determinism. When a task (such as PLC code execution) is scheduled to run, the system latency (also known as jitter) is quite low, coming in at less than 5 µs. Using the distributed clock technology available with the Ethernet for Control Automation Technology (EtherCAT) fieldbus, the accuracy of synchronizing fieldbus data runs in the nanosecond range.
Many PLCs on the market today, including some from the largest automation suppliers, do not run deterministic scans at all. Instead, these devices conduct back-to-back code execution cycles and, depending on the logic and execution sequence of the code, each scan's execution time is different for varied PLC program execution times.
One scan might finish in 800 µs, and the next scan could take several milliseconds. There is no way to repeatedly capture an input that might only stay on for 1 µs without special I/O hardware. In addition, with varying program execution times, it becomes extremely difficult to synchronize motion control in a traditional PLC architecture. This is why many vendors implement an additional central processing unit (CPU), either on the same hardware or in a rack-based add-on module, to independently control the motion-control loops.
The added CPU and associated hardware increase costs, as well as increase required hardware, points of failure, and space. Larger, more expensive electrical cabinets are often required, and the machine footprint increases accordingly. PC-based solutions feature a high level of determinism that enables these systems to combine synchronized motion control with the PLC and other tasks, all on one centralized processor.
Industrial PCs don't offer the same level of security as PLCs.
FALSE: According to reports, several of the largest, global PLC manufacturers have experienced hacks into PLCs through the software's "back door." PLC manufacturers bear responsibility for protecting this aspect of customer systems. With a PC-based control system, common and accepted approaches can protect the system and protect many third-party software packages that provide key components for comprehensive security solutions.
Hardware PLC manufacturers continually attempt to add further communication openness that is available with industrial PC-based control systems. Companies seeking to add those protocols and make the hardware PLC more open, while at the same time indicating that the openness of PC-based control is a security risk, are effectively taking both sides of the argument.
PC-based control manufacturers' participation in leading organizations, such as the OPC Foundation, helps facilitate the development of new standard protocols for encrypted, secure data communication between the machine controller and the enterprise, as well as with cloud-based systems. The sum of these efforts has been the creation of the OPC standard, OPC Unified Architecture (UA), as well as standardization of PLC function blocks for OPC UA communications.
Companies are moving more towards distributed control over centralized control, and there is more risk in centralized control if the controller fails.
FALSE: Due to hardware PLCs' performance limitations, there is heightened interest in distributed control as a way to meet the growing demands of new technology implementations for machines on the plant floor. In addition, smarter sensors create a need for protocols that can deal with the parameters available in the sensor, as these receive updates via on-board web servers. Ideally, all parameters for drives, sensors, and other devices across the system can be stored in one central location, so when connecting remotely these can be viewed and updated easily.
Decentralized control also has its place in automation solutions; however, this is not an argument for or against PC-based control. Manufacturers offer both centralized and decentralized control solutions. While the applications are few and far between, there are modular machines that can benefit from a distributed control strategy. If an application would benefit from distributed controls, manufacturers can employ EtherCAT automation protocol (EAP) for communication among many controllers. EAP is an open protocol managed by the EtherCAT Technology Group (ETG), www.ethercat.org providing fast communication using routable transmission control protocol (TCP) communication frames. The data proceeds directly from the real-time kernel and implements a "publish" and "subscribe" model to exchange data between controllers. This is also referred to as "master to master" communications.
To say that risk increases in the event of a failure when human-machine interface (HMI), PLC, motion, and other technologies are combined on a central controller is a misguided notion when the majority of machines cannot operate if any one of these critical functions is eliminated. Perpetuating this misconception simply adds unnecessary costs, complexity, and reduces the lifecycle of the system due to excessive hardware.
Learn more about the nine benefits of PC-based control systems.
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