Cover Story: Balancing PLCs, PACs, IPCs
Controller-based applications face a divergence when selecting a controller: Simplicity and ruggedness, or openness and functionality? In math, that’s called indeterminacy, but in automation, the engineer should know what controller feature set is best for the application, no matter what you call the logic device.
In 2001, when ARC Advisory Group Analyst Craig Resnick proposed the new term for the new class of controller (PAC, programmable automation controllers), the acronym was coined on the basis of market observations. Functionality of programmable controllers was extended by the main global automation vendors.
Vendors use the PAC acronym to describe a class of products conforming in design and market positioning with similar hardware–software architecture philosophy. Because there are a wide variety of feature sets, it has become difficult to definitively classify each controller as either a PLC or a PAC. Even a small PLC can be programmed with the use of the automatic code generation tool from Matlab/Simulink. The same controller, given 8 GB to 32 GB of data memory, thanks to the compact flash (CF) interface, can be a data acquisition and analyzing tool for predictive maintenance of the machine. The same controller, thanks to virtual interface technology, can be changed into the remote gateway for the whole process.
Programmable automation controllers (PACs) are known for the following main features, differentiating them in functionality from a typical
- Multidomain functionality—logic, motion, continuous control systems can be designed with the same hardware platform.
- Single multidiscipline development platform—data tags are stored in a common database.
- One tool for every programming task—control logic, motion control, HMI design for more than one machine in a process.
- Open, modular architecture lets the engineer use only the equipment he need.
- Use of many communication standards (from asynchronical to deterministic ones) and many programming languages (consistent with IEC 61131-3 as well as the higher level programming languages)—the engineer can design a multivendor system simply and efficiently.
But as PLC technology has emerged, some companies look at PLC and PAC differences and choose to use the PAC acronym for their products, even without offering all the features mentioned above.
Many things have changed for programmable controllers since 2002:
- Openness of communication standards is typical with PLC functionality.
- More tools are IEC 61131-3 compliant, extending the normative list of programming languages (IL, LAD, ST, SFC, FBD) with ANSI C or even C++ and C#.
- Small controllers are equipped with a large amount of data memory (8 GB and more).
- Ethernet TCP/IP has become the most popular programming interface for PLCs.
- Software architecture of PLCs is based on the real-time deterministic multitasking operating systems.
Given the blending of features, it is difficult to define which controllers are PACs and which are PLCs. The simplest definition says that a PAC combines PC elasticity and functionality with the ruggedness of a PLC.
PAC controllers combine PC and PLC hardware/ software architecture, offering flexibility and ruggedness in one efficient system.
Users in recent years have built applications within two operating systems architectures. Examples include Beckhoff Automation (Twin- CAT PLC under Microsoft Windows XP Professional) and Bernecker & Rainer (AR010 system under Microsoft Windows XP Professional), among others.
National Instruments PXI series controllers are available in two-boot operating systems versions— real-time and embedded, with Microsoft Windows XP Professional and with Microsoft Windows 7. This design can be booted only in one system mode, different from what the figure shows.
As the functionality of PLCs has been extended and newer control technologies have been introduced, the range of controller definitions needs to be reevaluated since 2001.
An increasing number of engineering tools are modernized to comply with the Microsoft Windows 7 32-bit operating system. Fewer 64-bit computers are used, but it hasn’t been a problem. The first updates of the programming tools (for development of control applications) for the Microsoft Windows 7 system were released in mid-2010.
Development tools for programmable controllers will continue to receive updates in 2011. Other companies also are working on the 64-bit versions of their tools, with most upgrades expected to be complete by year end.
Automatic code generation
The number of programming tools consistent with IEC 61131 programming languages continues to increase. Users can program applications with more than just a ladder diagram computation model, typically with three or more languages, including structured text.
New programming software offers new opportunities, such as automatic code generation for programmable controllers. This functionality allows users familiar with the MathWorks Matlab/ Simulink environment to rapidly design and implement control algorithms within the control tasks. Research and development of control strategies is one of the main areas of opportunity for new products. Shortening the “time to market” of new product development is the reason for employing the mechatronic approach, also known as the model-based design.
Tools for automatic code generation are listed, with a list of controllers for which they are designed.
Future of control systems
How will control systems evolve? Automation vendors are incorporating safety technology in their systems. More are integrating motion control functionality in available programming tools. Rapid prototyping is the most innovative method of evolution, and applications also will accommodate that functionality. In 2020, control architectures will be so open that there will be no problem with interchanging control solutions and hardware modules from different vendors, maybe even on the level of processors and programming tools. This may speed the natural evolution of today’s continuing expansion of object-oriented programming tools. Tools will decrease in importance as the team of programmers work more efficiently to solve problems, which is the promise of modern control systems.
– Krzysztof Pietrusewicz, PhD, is assistant professor at the Control Engineering and Robotics chair, Faculty of Electrical Engineering at the West Pomeranian University of Technology, Szczecin, Poland, and contributor to Control Engineering Poland. Łukasz Urbański, MSc, is a PhD student at the Faculty of Electrical Engineering, West Pomeranian University of Technology
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