Real-Time, When You Really Need It

In most industrial applications, real-time control seems to break down into a large population of users who have relatively little hard real-time requirements, and a somewhat smaller group with significant real-time requirements.

Microsoft is doing a brisk business in providing Windows NT/CE/XP operating systems to meet the needs of the relatively low-hanging fruit in the control and automation orchard, and for ample reason. The development environment is well-known by software engineers, connectivity to higher corporate systems is a relative no-brainer, and the operating systems require little, if any, in-house development and maintenance costs.

But for that last segment of the control community--the roughly 10% of players who have hard real-time operating requirements--the choice of operating system becomes much more murky.

Although the end user almost certainly will want connectivity to higher systems and easy operating controls, the machine OEM first and foremost is concerned that the equipment meets its real-time mission responsibilities in a wholly reliable and repeatable fashion. Microsoft products also compete in this arena, but the field is much more open than in higher-level systems.

Asyst Technologies (http://www.asyst.com) has a long history of working with Windows products for motion-intensive wafer handling systems that service the semiconductor industry (Figure 1). Karl Shieh, Asyst senior director of software technology, studied the real-time performance capabilities of both Windows CE and VxWorks, and opted to switch to VxWorks from Wind River Systems.

"VxWorks looks a little better in terms of performance," he says. "We do a lot of servo control and need to know that those motors are moving very smoothly with no delays, and that they have all the information they need in order to move with reliability and repeatability. With VxWorks, for the same code base, we get a 20-25% performance increase."

Shieh quickly adds that the performance boost is due to VxWorks being a smaller operating system--it's not as feature-rich as its Windows counterpart. "That's a double-edged sword for us," he notes. "We realize it doesn't have the same level of protection, inherently. But we trade that off for performance."

Sheih says that Asyst software developers have found the Wind River Tornado development environment to be very similar to Microsoft's Developer's Studio, and therefore the migration to VxWorks was relatively painless for the engineers.

Machine builders that make equipment for use in industries such as the manufacture of semiconductors know their customers, with expensive semiconductor fab lines, consider robotics and other machinery to be something of a necessary evil.

Fab line managers want motion control operations to be completely transparent to the overall operation by being entirely reliable and clean, never causing damage to in-process or finished wafers. Those engineers often chose a control system not for its operating software, but for its overall performance capability.

"The operating system itself was not a key factor in the controller decision for our application," notes the automation engineering manager for a semiconductor tool manufacturer. "The primary factors in the decision were the performance and functionality offered by the controller for an application of this type."

As a result, development ease is not always a pivotal issue. Some engineers believe the up-front investment required to learn a proprietary programming language will be recovered in the end product. Operator ease, however, is almost always a key concern, and therefore the design usually includes a Windows application as a sub-system graphical user interface for set-up, configuration, and troubleshooting, as well as data-management purposes.

Indeed, many industrial OEMs include Windows at some level of their real-time systems, even if they opt to perform real-time control on another OS. To achieve the real-time control on a Windows OS, some machine builders find third-party extensions can provide adequate performance for their control needs. Thus, the end user could maintain a Windows-centric platform corporate-wide.

"Most intelligent equipment today is part of a larger plant information system that the equipment connects with," explains Myron Zimmerman, chief technology officer with real-time extension developer VenturCom (http://www.venturcom.com). "It's easier for the control manufacturers if they tell their customers a Windows operating system is being used on their equipment. That makes everybody feel it's going to be easier to integrate that equipment with other Windows-based plant information equipment."

Some OEMs may opt for an RTOS even if their real-time requirements are less stringent than the sub-millisecond range simply because the system provides a level of assurance to their users. Officials at Gem City Engineering Co. (http://www.gemcity.com), a manufacturer of automation equipment used in the semiconductor, microelectronics, and biotechnology industries (Figure 2), have found that, in such cases, a non-Windows RTOS can be more economical than the Windows systems they install on most of their equipment.

"There are times when I use a real-time system, not because it's a real-time system, but because I can get a single-board computer for a very low price when I have multiple builds of the job," says David Burton, Gem City engineering manager, controls and software. "I don't want to have to put a whole PC or even Windows CE on it. So I go with something that has the capability of being a real-time system, but not necessarily use it at its highest speed."