SERCOS picks up the pace

DN Staff

August 21, 2000

8 Min Read
SERCOS picks up the pace

SERCOS groups shake the world with the release of the next-generation SERCON816 ASIC, a new product called SoftSERCANS, more support from members such as Rockwell Automation and Beckhoff Automation LLC, and an initiative to establish new competency centers. Representatives from North American and European SERCOS (SErial Real-time Communication System; Interest Groups recently met at Design News headquarters to discuss the news. Founded in 1994, SERCOS's 70 member companies worldwide facilitate and promote the use of digital SERCOS Interface Standard (IEC 61491).

Faster data transfer . Many motion-control engineers feel fast data transfer is key to accurate synchronization. The new SERCON816 ASIC operates at 2/4/8/16 Mbit/sec speeds, transmits data 400% faster than the present 2/4 Mbit/sec chip and costs 35% less. Initially available to SERCOS Interest Group members who helped finance its production in July, the new chip is scheduled to go into full production in October, coinciding with the phasing out of the SERCON810B.

"The new chip's higher 8- and 16-Mbaud transmission rate improves SERCOS functionality," says Mahito Ando, product manager for drive communication at Rexroth Indramat GmbH. Ando estimates the SERCON816 ASIC will cost $8.60 each in quantities of 50,000. While this may seem high compared to other low-cost communications interfaces on the market, it's important to compare apples to apples.

Scott Hibbard, VP of technology for Indramat Div. of Mannesmann Rexroth, warns against comparing costs per node of SERCOS to other buses that weren't designed with motion control in mind. "Other buses need additional infrastructure to achieve the performance of SERCOS in motion control applications," explains Hibbard. In one example, Hibbard shows that to avoid the overflow of data when using the CAN (Controller Area Network) ASIC for motion control applications, an additional memory buffer and signal management must be provided. The resulting cost of the CAN endpoint is approximately twice the cost of the SERCOS interface.

The primary difference between SERCOS and other serial fieldbus technologies-apart from its fiber-optic noise immunity-is its determinism, and its clearly defined motion-specific functions, which make it predictable and easy to work with in synchronized motion applications, says Hibbard.

SERCOS grows because engineers want greater machine speeds, higher productivity, part reduction, cost reduction, modular subassembly, shorter set-up times, and freedom from mechanical components such as line shafts.

While other buses try to approximate determinism through brute speed, SERCOS, in contrast, operates efficiently with low overhead that yields a net bandwidth of approximately 75%, and has all functions a motion control interface requires built in. Speed doesn't compensate for deliberate design for motion control requirements. In fact, Ando illustrates how in one test, the 16 Mbit/sec SERCON816 achieves an effective data transfer rate of 11 Mbit/sec, which actually exceeds the 5-10 Mbit/sec effective data transfer rate of 100 Mbit Ethernet. The data transfer rate is fast, Ando explains, because SERCOS endpoints store transmitted data in a memory buffer where a configurable event scheduler provides accurate synchronization.

PC-based CNC . Acknowledging the trend toward use of NT as a real-time controller operating system, the group announced SoftSERCANS, a software interface master function to SERCOS-based drives running on VenturCom's RTX real-time Windows technology. According to Roy Kok, VenturCom VP, it's essentially a SERCOS driver to PC-based soft motion control, analogous to a printer driver on a PC. SoftSERCANS and a PDF-format technical manual are downloadable at no charge from

"SoftSERCANS accomplishes the complete unbundling of software and hardware," explains Ando. "It allows the internal mechanisms of SERCOS to remain transparent to motion control development engineers and simplifies low-level programming of the control." Hibbard emphasizes that SoftSERCANS is not a motion controller, "It's a tool that makes the engineer's job faster and easier. It's a library that extends all the SERCOS IDNs (identification numbers) into the real-time Windows operating system. So it makes it easy for IT to connect the boardroom to the back of the motors and drives on the factory floor." The only hardware requirement is a passive SERCOS card that replaces command signal interface, a diagnostic channel to the drive, and a feedback interface from the drive. The SoftSERCANS card is presently available from several vendors for approximately $250.

Although industry continues to leverage advances in PC technology, and most agree that the PC platform is the future of motion control, it's unlikely we'll see a fiber-optic interface in every PC. "It just wouldn't make sense in the office automation environment on which PC development is focused," says Hibbard.

SERCOS endpoints store transmitted data in a memory buffer (parking lot) so a configurable event scheduler (traffic light) can synchronize communications. The clock frequency of the endpoint determines synchronization accuracy, which is independent of the configured cycle time and the data transfer rate. The end result is sub-microsecond accurate synchronization that improves machine productivity.

Those quick to judge might take this as a strike against SERCOS, however the trend in office automation is to smaller microprocessors and higher speed networks. Two trends increase susceptibility to electromagnetic interference which reinforces the need for fiber-optic networks such as SERCOS on the factory floor.

"As microprocessors shrink in size and networks achieve greater speeds," explains Ando, "both become more susceptible to noise from larger motors and drives. So very soon, office PC hardware won't meet factory-floor requirements for noise immunity, and future automation products will have to use light (fiber optics) on the plant floor."

Other major members . With the likes of Rockwell Automation and Beckhoff Automation LLC jumping on board the SERCOS bus, Indramat isn't the sole automation powerhouse promoting SERCOS anymore. Although Rockwell wouldn't comment for this article because the company is working internally to establish its position on SERCOS, SERCOS North America Managing Director Ron Larson says Rockwell plans to become more active within the SERCOS community both in North America and in Europe, and will be launching SERCOS-based products. In fact, says Hibbard, the IGS (Interests Group SERCOS Interface) in Germany will soon name one of Rockwell's European managers to an additional board position.

Many were surprised when Beckhoff, a strong advocate of its own Lightbus fiber-optic communications interface, supported SERCOS at the Hannover Fair in Germany in March. Here, Beckhoff Industrie Elektronik's Dirk Janssen gave a presentation on how the SERCOS interface extends PC-based control and eliminates the jitter or lag time in real-time Windows extensions. Focusing on SERCOS I/O devices, and using SERCOS in combination with other fieldbus systems for higher I/O-count applications, Janssen went on to announce a SERCOS interface for TwinCAT-Beckhoff's full-featured, PC-based control system and real-time extension. Minneapolis-based Beckhoff Automation LLC President Gerd Hoppe says, "One of our goals is to extend SERCOS I/O capability as a single network architecture for control of I/O and drives on the same network."

Not only is SERCOS picking up the pace in North America and in Europe, but SERCOS Japan, the youngest of the three organizations, is exploding with almost 40 member companies to date. "With little fanfare on our part," says Hibbard, "SERCOS development in Japan just seems to have taken off primarily because so many European and American customers won't accept quotes based on anything but SERCOS."

SoftSERCANS is essentially a SERCOS driver to PC-based soft motion control, analogous to a printer driver on a PC.

"The days of Indramat and Pacific Scientific being the only suppliers of SERCOS drives are over," says Hoppe. For example, a high-performance multi-axis SERCOS controller from Industrial Indexing Systems (Victor, NY) recently hit the market to compliment the company's SERCOS drives. Called the Emerald EMC 2000, its SERCOS, PCI, and DeviceNet interfaces open it up to a wide choice of machine-control philosophies and vendor-independent products. "Just because all the plugs fit together in a motion system," explains IIS President Ed Steiner, "doesn't mean it's going to work. SERCOS is so well defined for motion control that you can be pretty sure that you're going to have interoperability right out of the box, as long as engineers stay away from any optional vendor-specific IDNs that are not within the SERCOS specification."

Competency centers. One hurdle to future SERCOS growth is that up until now, SERCOS developers have had only one conformance testing station in Germany at their disposal. "We've got to make it easier for people to get on board," says Larson. As more companies get involved, cross vendor compatibility issues take center stage. "With all the SERCOS skunkworks going on now all over the world," explains Hibbard, "we need to strengthen our conformance testing." That's why last year the SERCOS community launched a program to set up so called competency centers that will be responsible for providing conformance testing as well as fee-based consulting services for people developing SERCOS products.

As of this writing, the European competency center will be the ISW Institute at the University of Stuttgart in Germany, and SERCOS North America is in the process of negotiating with the University of Akron in hopes it will become the U.S.-based competency center.

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