ations system (SERCOS) -- that first caught on in Europe and in the last two years has begun to establish itself in North America.
SERCOS's raison d'être is to establish communications between controllers and the digital-motor drives that perform complex multiaxis movements. SERCOS defines a standard way for integrators to plug as many as 254 programmable drives into a 4-Mbps fiber-optic ring that becomes the medium for controls and drives to pass data serially to each other. Unlike analog interfaces, SERCOS allows for two-way communication between controls and programmable drives. Basically, it's a Plug and Play bus for manufacturing systems.
"SERCOS
helps us speed integration," says
Ed Chalupa
, president of Advanced Integration Technology (AIT), the Plano, Texas, systems integrator that built the aerospace robotic units. Chalupa adds that his company's manufacturing systems have to be constructed at his customer's site -- without benefit of debugging in an AIT quality-assurance lab. SERCOS eliminates bugs that were common in the wiring between the controller and the programmable motors, he says.
SERCOS alone wouldn't open architectures for motion-control systems. Openness results from teaming the SERCOS standard with PCs and standard development tools. PCs are more economical than programmable logic controllers (PLCs), the likely alternative. Standard development tools let integrators draw on the same programming expertise from project to project. Together, these standard resources help alleviate the frustrations of resellers and integrators about proprietary systems, says Charles F. Carter Jr., vice president of technology f
or the Association for Manufacturing Technology.
However, SERCOS is still evolving, and some integration issues remain unresolved. Add to that the competitive pressures of the marketplace, which induce component vendors to interpret the standard in different ways to give their products a performance edge. The result is that SERCOS is what John Odenthal, vice president of systems integrator Motion Design, calls "an integrator's nightmare -- a nonstandard standard." Yet Odenthal says the benefits of SERCOS outweigh its annoyances, if you know how to work around the rough spots. Here's how to avoid integration problems.
Why SERCOS Now?
Although SERCOS may be unfamiliar to many resellers, it's not new. Its development began in the 1980s, spurred by German manufacturers that wanted a nonproprietary interface for machine-tool systems. Why is SERCOS suddenly important? Two reasons: big endorsements and digital programmable drives.
The biggest endorsement came in 1995, when GM told supplie
rs to its Powertrain and European manufacturing divisions that all future equipment must support SERCOS.
Another recent catalyst for a better and faster interface, beyond interoperability, is the motion-control industry's move from analog to digital components, particularly digital programmable drives.
Digital drives are faster and more precise than analog drives. It's not unusual for them to go from 0 to 3000 rpm in 20 milliseconds or come to a stop within 1/8000 of a revolution, says Jeff Pinegar, spokesperson for Automation Intelligence, a Duluth, Georgia, systems integrator that was one of the first U.S. companies to work with the standard in 1992.
Also, analog drives are electrically noisy. Because the motors are almost constantly starting and stopping, they become essentially a series of antennas for electrical interference. Electrical noise is not a problem with SERCOS's fiber optics.
Obviously, SERCOS isn't the only interface choice for motion-control systems. Many vendors have
proprietary solutions. There are also other alternatives, from Fieldbus to ProfiBus, CANbus, InterBus-S, and DeviceNet, that are often best suited to low-level components, such as motor starters, rather than for more sophisticated multiaxis motion-control components.
Fieldbus, for example, establishes a communications link for I/O data to travel to a number of field devices. Some integrators say that they use DeviceNet and SERCOS side by side, the former for I/O control, the latter for motion control. However, Pinegar says his company often uses SERCOS for both I/O and motion control, eliminating the need for a separate $1000-$2000 I/O card.
Most integrators give SERCOS high marks for lessening wiring woes (
see the figure
). A typical analog-motor drive might have 16 wires coming out of it that lead to a motion controller. A large automated packaging system might have 15 drives, which means an integrator for such a system would have to complete 240 electrical connections. SER
COS reduces this to one fiber-optic ring.
Easy connections mean a faster setup time. "In the past, we would put a card in the PC doing motion control and have two or three 100-pin ribbon cables fan out to 100 terminal blocks," Pinegar says. "With SERCOS, the network goes out from one transmitter to each drive in one clean fiber-optic ring. In terms of systems integration, this simplified wiring is a tremendous advantage." For Chalupa, simplified wiring is important because the systems AIT builds for the aircraft industry are so large. "We do the final installation, wiring, and programming in the field. SERCOS reduced bugs from wiring errors to zero," Chalupa says.
Who's Behind It?
Thanks to GM's endorsement and the rise of digital drives, the number of products that support the standard has finally made it a universal interface. Who's supporting SERCOS?
According to SERCOS N.A., a trade association, the protocol's capability to ease systems-integration tasks especially benefits mach
ine builders. "Networking drives and controls through the SERCOS interface is a straightforward matter of systems integration," says Tom Lauterback, the association's managing director. "With the ability to manipulate every variable in a drive system, SERCOS-compliant digital drives have brought drive setup and optimization to a new level. Today, such tasks are being performed in graphical environments such as Windows," he adds.
Lauterback estimates that more than 27 companies offer SERCOS products and systems-integration services. This includes leading suppliers of motion-control cards and equipment, such as Indramat, Motion Engineering, and Pacific Scientific.
But Wait: There's Less!
SERCOS has advantages, but resellers aren't oblivious to its shortcomings, which range from slow speed for some applications to poor documentation that leads to inconsistency in how the standard is implemented.
Over the past 11 years, SERCOS's creators have tried to address speed concerns. The origina
l specification, which called for a 2-Mbps data transfer rate, has been boosted to 4 Mbps. New SERCOSASICs are capable of 10-Mbps rates. But until 10 Mbps is widely available in programmable drives and SERCOS cards, some integrators will hunger for more speed. "We'd like SERCOS to be faster," Pinegar says. "The more drives you add, the slower the network becomes."
Part of the speed problem comes from the fact that, although the standard calls for a maximum of 254 programmable-drive connections in each fiber-optic ring, the actual drive limit may be less depending on the communications cycle time, how much data is being passed, and the speed requirements of the application. Consequently, applications that require both high speeds and a large volume of data may need multiple SERCOS rings.
Officials at SERCOS N.A. advise integrators to be sure that they factor in the time required to exchange all command data across the SERCOS ring. Integrators must take cycle times and data volume into account, not
just the bus-transmission rate, to decide if the system they are building is fast enough for a given application. Exact timing in the software interrupt routine is more important than speed, they say, because even the fastest software routine isn't useful if timing isn't consistent.
SERCOS was designed with synchronous operation in mind so that transmission-signal jitter is minimized. It was also designed so that actual values are detected and commanded values acted on by all drives simultaneously. Because drives work synchronously, "beat" frequencies or harmonics appearing in the motion-control system are eliminated. Feedback acquisition and commanded value executions are performed in every drive at the same instant, making an effective snapshot for control purposes, according to SERCOS N.A.
The second drawback -- lack of interoperability among SERCOS-compliant hardware -- stems from two reasons: an attempt by the standard's designers to write flexibility into the protocol and documentation that
generally receives poor grades from integrators who have tried to comprehend it. Many integrators say the best information comes from programmable-drive vendors rather than from official SERCOS publications.
Pinegar says interoperability among large programmable-drive vendors is common; problems usually crop up among smaller vendors or those new to the market.
Community Needed
The real problem, say resellers, is that no central repository exists to help integrators exchange solutions. "We need more support for the user community," Chalupa says. Anomalies are "an annoyance" rather than an insurmountable technical problem, adds Odenthal. He would like to see SERCOS documentation available via the Internet, so changes to the standard could be updated regularly and there could be a forum for resellers.
Chalupa also confesses to mild irritation over the fact that the type of fiber specified for SERCOS is a nonstandard diameter. Also, the fiber must have a screw-on connector rather than t
he more conventional bayonet clasp. "All the drive manufacturers sell the fiber, but we like to buy it from a local source in case we break fiber in the field," he explains.
In the end, however, motion-control integrators say SERCOS is becoming a regular part of their work. Digital technology means motion-control systems can be more precise and easier to build, and SERCOS represents progress away from the frustrating days of proprietary prisons. If dynamic, real-time forums spring up to let integrators share design experiences, SERCOS may become an open standard in the truest sense.
Where to Find
ANSI
Phone: 212-642-4900
Internet:
http://www.ansi.com
Plug and Play for Manufacturers
