C hoosing the right hardware and software platform for industrial and control applications is a challenge that every design engineer has to face. Industrial applications need to incorporate integrated hardware and software; offer a short time to market; and be highly reliable, powerful and flexible, low cost, and easy to maintain and upgrade. For nearly three decades, the PLC has been the automation solution of choice for industrial control engineers. Familiar and reliable, the PLC has evolved to incorporate analogue I/O, network communication, and new programming standards. A strong demand for simple low-cost PLCs has spurred the growth of the low-cost PLC market, but has also created a discontinuity in controller technology. Most PLCs are designed to provide solutions for straightforward control applications. However, the PLC has not traditionally delivered viable solutions for applications that require higher loop rates, advanced control algorithms, more analogue capabilities, and better integration with an enterprise network.
On the other hand, the PC provides software capabilities for advanced tasks, offers a graphical, rich development and user environment, and uses commercial off-the-shelf (COTS) components. The PC also delivers unparalleled flexibility, highly productive software, and advanced low-cost hardware. However, PCs are not ideal for control applications. Although they are still used when incorporating advanced functionality such as analogue control & simulation, database connectivity, web-based functionality, and communication with third-party devices, the PCs fail in one key industrial aspect: standard PCs are not designed for rugged environment. This lack of rugged design presents three challenges for PCs in industrial settings – stability, reliability, and a complex and unfamiliar programming environment.
The industrial PC sector has expanded and improved in recent years, but the PC is still a challenging platform for industrial automation applications. Although some engineers use these latest systems with more rugged hardware and special operating systems, they face the task of piecing together a system that has the reliability of a PLC and the added functionality the PC provides.
These hybrid systems, while offering a solution to the design problem, add multiple-vendor hardware and software integration issues as well as the difficulties that come from the inevitable maintenance and system upgrades.
With no clear PC or PLC solution, engineers who have implemented such hybrid systems have worked closely with control vendors to help develop a new class of industrial controllers. The resulting new controllers, designed to address the needs of architecturally complex applications, combine the best PLC features with the best PC features. Industry analysts at ARC named these devices programmable automation controllers (PAC). In its ‘Programmable logic controllers worldwide outlook’ study, ARC identified five main PAC characteristics. These criteria characterise the functionality of the controller by defining the software capabilities, which include:Multi-domain functionality - at least two of logic, motion, PID control, drives, and process on a single platformSingle multidiscipline development platform incorporating common tagging and a single database for access to all parameters and functionsSoftware tools that allow the design by process flow across several machines or process units, together with IEC61131-3, user guidance, and data managementOpen, modular architectures that mirror industry applications from machine layouts in factories to unit operations in process plantsEmploy de facto standards for network interfaces, languages, etc, such as TCPIP, OPC & XML, and SQL queries. Communication with enterprise networks is critical for modern control systems
The software and hardware
Because software is the defining difference between PACs, the first step for PAC vendors is to provide reliability and determinism, which are often not available in a general-purpose operating system such as Windows. This is accomplished through real-time operating systems (RTOS) such as Phar Lap from Ardence (formerly Venturcom) or VxWorks from Wind River. An RTOS provides the capability to control all aspects of the control system, from the I/O read and write rates to the priority of individual threads spawned on the controller. These vendors then add abstractions and I/O read/write structures to make it simpler for engineers to build reliable control applications. The result is flexible software suited for custom control, data logging, and communication.
Although PACs represent the latest in programmable controllers, the future for PACs hinges on the incorporation of embedded technologies that offer the ability for the design engineer to use software to define hardware. This is accomplished by adding field-programmable gate arrays (FPGAs) to PAC hardware. Adding FPGAs to the PAC picture places intelligence in COTS devices, providing flexibility, power, and the ability for proprietary hardware to be completely reconfigured to meet specific application demands. In many cases, this completely eliminates the need to design custom hardware or to break out the soldering iron each time a change to the system functionality is necessary.
The PLC is still a very effective solution for general-purpose control and I/O. The PC, which can fill many gaps the PLC is not able to cover, is a viable solution that addresses environmental challenges and stability issues. However the PAC, due to its flexibility, power, and reliable hardware and software, is quickly becoming the ultimate solution for complex industrial control applications. Add an FPGA to the picture, and the argument for using PACs in even the simpler industrial control applications becomes a strong one as well.
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