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 the last decade, a passionate debate has raged over which platform provides the best solution for industrial control and automation. The debate has mainly centered around two solutions: programmable logic controllers (PLC) and PC-based control. Each platform has its own strengths and limitations, and in some simple cases the answer is clear.

However, when applications become complex and demand such features as high-speed control, motion, special analog I/O, machine vision, and more, the line between the various platforms for industrial control becomes blurry and challenging to navigate.

The PLC
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 analog I/O, network communication, and new programming standards. Experts from Automation Research Corporation (ARC) and Venture Development Corporation (VDC) estimate:

80% of industrial applications can be solved with digital I/O and a few analog I/O points, and programming requiring simple logic

77% of PLCs are used in small applications (fewer than 128 I/O channels)

72% of PLC I/O is digital

Because engineers meet so many industrial application challenges with traditional PLC solutions, there is a strong demand for simple low-cost PLCs.

This demand has spurred the growth of the low-cost PLC market, but it 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 analog capabilities, and better integration with an enterprise network.

The PC
To address these PLC hardware limitations, many engineers have evaluated using PCs for industrial control. 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 analog control and 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 environments.

This lack of rugged design presents three challenges for PCs in industrial settings. The first and most limiting of these is stability. In the industrial setting, where hourly downtime is often measured in thousands or even millions of dollars lost per hour, the PC's general-purpose operating system has experienced system crashes and unplanned rebooting that can make it a risky choice for control.

The second issue is closely related to the first: reliability. With the majority of computer hardware sources serving the consumer market, many standard PC components cannot withstand the often harsh conditions in an industrial setting.

Heat and dust wreak havoc on rotating magnetic hard drives and power supplies, and humidity and dirty power can lead to shortened life spans and reliability issues with other core PC components.

Finally, the PC has presented automation engineers with a complex and unfamiliar programming environment. Plant operators need the ability to override a system for maintenance or troubleshooting.

With PLCs, they can manually force coils to desired states and quickly patch the affected code to override a system. But with PC systems, operators are required to learn new, more advanced tools.

The PAC
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 newer 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. Understanding PACs
Industry analysts at ARC named these devices programmable automation controllers, or PACs. In its "Programmable Logic Controllers Worldwide Outlook" study, ARC identified five main PAC characteristics.

These criteria characterize the functionality of the controller by defining the software capabilities, which include:

1. "Multidomain functionality, at least two of logic, motion, PID control, drives, and process on a single platform." Logic, motion, process, and PID are simply a function of the software (except for some variations in I/O to address specific protocols such as SERCOS). For instance, motion control is a software control loop that reads digital inputs from a quadrature encoder, performs analog control loops, and outputs an analog signal to control a drive.

2. "Single multidiscipline development platform incorporating common tagging and a single database for access to all parameters and functions." Because PACs are designed for more advanced applications such as multidomain designs, they require more advanced software. To make system design efficient, the software must be a single integrated software package instead of disparate software tools that are not engineered to seamlessly work together.

3. "Software tools that allow the design by process flow across several machines or process units, together with IEC61131-3, user guidance, and data management." High-level graphical development tools also simplify system design and make it easy to translate an engineer's concept of the process into code that actually controls the machine.

4. "Open, modular architectures that mirror industry applications from machine layouts in factories to unit operations in process plants." Because all industrial applications require significant customization, the hardware must offer modularity so the engineer can pick and choose the appropriate components. The software must enable the engineer to add and remove modules to design the required system.

5. "Employ 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. Although PACs include an Ethernet port, the software for communication is the key to trouble-free integration with the rest of the plant.

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 (RTOSs) such as Phar Lap from Ardence (formerly Venturcom) or VxWorks from Wind River.

These RTOSs provide 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.

Choosing an industrial control application hardware platform is not always an easy task, but there are some great solutions available today. 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.

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.

The second article in this two-part series will talk specifically about the process of making the decision between custom hardware and off-the-shelf technology.

Many times designers turn to custom solutions because there is a perception of cost savings or limited off-the-shelf hardware availability to solve the application. We will learn how using a COTS platform with an FPGA provides the flexibility capable of solving many tasks traditionally reserved for custom hardware development.

See Part 2

About the Author:
Arves Stolpe is the CompactRIO product marketing engineer at Arves Stolpe, National Instruments . He joined NI in June 2005 as an applications engineer and holds a Bachelor of Science in electrical and computer engineering from Utah State University.