The world of machine control integrates motion control, logic control, operator interface, supervisory control, data acquisition, and enterprise connectivity.

Over time, most PC-based machine controls have incorporated motion control by adding PC-bus boards including a dedicated processor and operating system. While these PC-bus boards are readily available and capable for computer numerical control (CNC) and general motion control (GMC), they're still a form of proprietary control. As soft motion-or motion control software that executes entirely on the PC's CPU-becomes more available, automation pundits regard proprietary PC-bus boards as interim solutions.

Digital drive and networking technologies, along with PC-based control platforms, allow the motion control system to be distributed for scalability and high-performance.

Open architecture machine control is all about choices. Many machine builders and users want to select off-the-shelf hardware and software from multiple vendors for their projects.

The reason, says the OMAC (Open Modular Architecture Controls) Users Group, is that unbundling hardware and software results in a control system that is economical, maintainable, open, modular, and scalable. Despite other initiatives, Linux, for example, today's automation market has overwhelmingly defined an open control system as based on the PC-with Intel x86 and compatible systems-and Microsoft's Windows operating system platform. Open architecture control and PC-based control have become synonymous in the marketplace.

With the emergence of soft motion, all parts of machine control become software-based. This enhances the opportunity to use one PC for overall control and to more tightly integrate operator interface, logic control, and motion control functions. Results are improved performance, reduced programming time, enhanced program maintenance, and greater data availability for the enterprise. By using the PC as the hardware platform and software-based control, the entire machine control keeps pace with advancements of the PC industry, where speed reportedly doubles every 18 months.

Simplified motor position feedback-combined with SERCOS' fiber-optic connection between
controller and multiple drives-dramatically lowers installation costs, reduces
commissioning time, and makes the system very modular.

As with most paradigm shifts, detractors have been vocal, and the ongoing migration has been slow and painful. However, many successful applications later, most would agree that PC-based controls have progressed well into the early-adopter phase.

For example, according to ARC Advisory Group (Dedham, Mass.), PC-based controls are now primed to move out of that early-adopter phase to play a major role in the automation market. ARC reports that PC-based controls are currently about 5% of the worldwide controls market, growing about 50% per year, while the overall controls market is only growing about 4% per year. Key drivers for PC-based controls are:

• Dramatic increase in ability and the hunger of enterprise systems to access real-time production data;
• Real-time Windows operating systems (NT, embedded NT, CE) as a standard for the factory floor; and
• Availability of software-based control technology on the Windows operating system platform-first soft logic and now soft motion.

According to Sal Spada, ARC senior analyst, 'System integrators, machine builders, and machine users favor open architecture control systems in an effort to increase their flexibility to select components from multiple suppliers and reduce machinery costs.' ARC's new study entitled 'PC-based Open Motion Control Software' reports a $32 million worldwide market in 1999, growing to nearly $254 million by 2004. About 20 major suppliers of open motion control products are covered in the report.

The longstanding interface between controllers and traditional servo drives has been the analog torque or speed command. This made perfect sense for decades, but the analog interface has some limitations including: multi-wire connections (at least 12 wires per drive for command, position feedback, enable, and faults); susceptibility to electrical noise; temperature drift; limited command resolution (usually 10-16 bits); lack of detailed diagnostic information; and time lags from data conversions (D/A and A/D). Today, the internal workings of most controllers and drives are digital.

While it's possible to implement PC-based soft motion with an interface to analog drives, this would not take full advantage of today's low-cost, high-performance digital technology. A better way to implement soft motion is shown in the Optimum Interface diagram.

Digital technology allows the servo drive to contain all the control loops and motor-specific control algorithms. An ac servo system is shown in the interface diagram, but the architecture is not limited to any particular servo technology. (For more details on design and applications, download the free Handbook of AC Servo Systems at www.MotionOnline.com.)

The position command interface minimizes the controller's computational and networking burden while keeping the servo-specific technology out of the controller.

Eliminating the analog interface and providing a standard interface for soft motion is the job of SERCOS (SErial Real-time COmmunication System). This digital drive interface was started in the mid-1980s and recognized as an international standard in 1995 (IEC 61491). While other initiatives exist, SERCOS is the only motion bus standard supported worldwide by more than 120 controller and drive suppliers.

The SERCOS interface is made with a pair of fiber-optic cables so the connection is digital (drift-free with 32-bit values for command and feedback), fast (up to 4 M bit/sec; 16 Mbit/sec available soon), and noise free due to the optical medium.

Fiber-optic connection replaces hundreds of wires in a multi-axis control system. Low-cost plastic fiber supports up to 130-ft node-to-node distances with a maximum ring distance of 32,800 ft. (Glass fiber can increase these distances by a factor of 10.) A notable system feature is that the position feedback cable from the motor only runs to the drive (see SERCOS Topology diagram).

SERCOS-based systems are used in high-performance applications with up to 32 drives and one controller. (The SERCOS specification supports up to 254 drives and I/O stations.) Its communication protocol provides for synchronous transfer of all drive parameters, which permits automatic set-up, deterministic control, and sophisticated diagnostics. Bandwidth requirements of the SERCOS network are relatively low because the controller-to-drive interface is simply the position command-typically updated every 1 or 2 milliseconds.

However, to increase motion system accuracy, digital drives internally micro-interpolate the position command at a much higher frequency. SERCOS also helps increase performance since it allows all loops to close within the digital drive. This minimizes time lags and maintains all drive parameters at maximum resolution. Customers have discovered that their machine operation is significantly faster and more accurate with SERCOS versus traditional analog technology.

As mentioned, Windows NT is the accepted standard for factory-floor operating systems. By itself, Windows NT is only capable of soft real-time (SRT) control; and, on average, it will service events in a timely manner. Some machine control tasks such as operator interface and soft logic control may operate fine using SRT control.

However, another key technology-hard real-time (HRT) extension for Windows NT-is needed to successfully implement soft motion. HRT controls must, without fail, service events within a predetermined time period. While not the only option, VenturCom's (Cambridge, Mass.) RTX (real-time extension) enables Windows NT to function as a general-purpose and a high-performance HRT operating system-simultaneously on the same computer.

Automation Intelligence has been in forefront of developments with its AML soft motion controller (see AML progress at AI sidebar). Other products are also starting to appear for CNC and general motion control markets. Soft motion products like AML are used in general machine control applications in packaging, food processing, printing, converting, textile, material handling, and metal forming, among others.

Now that soft motion technology is available for Windows NT, we will begin to see integrated software packages that include soft motion, soft logic, enterprise connectivity, and even the operator interface. Such unified packages can be very effective, saving time through one programming environment and one database. Lower volume, less complex applications will be a good fit for integrated software products.

On the other hand, combining best-of-breed products on single or multiple PCs may accomplish optimum machine control in terms of performance or price. This approach will be attractive in higher volume and more complex applications.

The best-of-breed machine controller takes full advantage of PC hardware-which is continually getting faster and less costly-and software-based control products that use the Windows NT operating system. (See Best-of-Breed control diagram.) Software-based control products can be enhanced more rapidly than hardware-based products; can be combined with other standard software; and are more easily upgraded to meet future needs.

Industry will continue to see an increased need for automation as a way to:

• Improve processes and productivity;
• Enhance quality control; and
• Reduce costs.

Information technology will continue its rapid growth and enterprise's demand for production data will only increase. Open-architecture machine control with PCs, Windows operating systems, standard networks such as Ethernet and SERCOS, and soft motion will be the only practical way for manufacturers to keep pace with these demands.

Continued growth of PC-based controls will benefit from several developments. Windows operating systems that work in hard real-time without extensions-and are scalable for open or embedded applications-will be a major driver. There is concern about compatibility between software applications when extensions are used and control software suppliers need to leverage single investments across open and embedded platforms.

Broader support for the SERCOS interface from both controller and drive suppliers is another impetus for soft motion. Few would argue the advantages of SERCOS, but suppliers have been slow to replace the complete range of traditional products with SERCOS counterparts. Until this happens, older analog products will continue to be used just due to lower cost and/or smaller package size.

Lastly, enhanced connectivity standards will make it much easier for software applications from different vendors to share data on the same PC and/or on a network. Best-of-breed control systems require accepted and open standards.

It's too late to be a detractor, but it's not too late to enjoy the many benefits of PC-based machine control through soft motion technology.