Networks in Motion

Oct. 3, 2007
Fast Digital Fieldbuses Improve Motion Control.

Not just any arrangement of motors will do when you’re controlling the motion of carpet looms or other kinds of intricately timed machinery. Fast feedback loops, like those available on modern digital fieldbuses, are crucial if such machinery is to stay synchronized at speeds high enough to be competitive in today’s marketplace.

Mohawk Industries Inc., of Calhoun, Ga., can testify to this fact. The producer of carpets and other floor products learned recently just how helpful digital fieldbuses can be. Its engineers are having some installed on eight older cam-driven mechanical looms that the company has at its carpet plant in Landrum, S.C.

The weaving action on an automated loom requires tightly synchronized axes, so one can follow another closely along a strategic profile. If their paths are off by a small amount, the mechanism can jam, and even break. Because the mechanical machines relied on cams for this synchronization in the past, Mohawk had to be vigilant for wear and replace these precisely machined components frequently.

To eliminate this maintenance and the associated downtime, the engineering staff embarked on a project to replace the cams with servo motors, beginning with the most troublesome axes. The plan is to get to the remaining, less-troublesome axes later, after picking the low-hanging fruit. So far, the first phase of the harvest is three-quarters completed, with six looms already retrofitted.

At first, the engineers tried to synchronize the servo drives with a motion-control module installed on the backplane of the controller. The problem with this strategy was that communications between the drives and the controller occurred through the module. “The extra step in the communication process slowed the loop updates,” explains Geoff Owens, one of Mohawk’s project engineers. The result was that the axes on the first retrofitted machines could not keep up with one another.

Owens solved the problem with an MP2300 servo controller and a Mechatrolink motion fieldbus network from Yaskawa Electric America Inc., of Waukegan, Ill. The cam program in the controller generates the trajectory profiles formerly created by the cams and sends them to the servo motors through a deterministic communications network that is synchronized at the systems level, rather than generating and executing them locally. “Instead of sending information to one drive at a time, the controller sends it out all at once and then sends a pulse that synchronizes the axes,” says Owens.

This strategy caused the old 6 to 7 millisecond (msec) loop update time to shrivel to only 0.5 msec, solving the synchronization problem and eliminating the associated downtime. “It has doubled the efficiency of the machines,” says Owens. Moreover, it gives the operator the ability to adjust operating speeds and other parameters on the fly from the controller’s human-machine interface (HMI) without having to adjust each motion independently.

Another advantage of the Mechatrolink  fieldbus is that it an open network that can accommodate a number of devices, including inverters, a mix of motors and a variety of input/output (I/O) devices. Adding and replacing drives, therefore, is relatively easy and inexpensive. Because the fieldbus accommodates as many as 16 axes, the only cost for retrofitting the remaining axes during the second phase of the retrofit will be the cost of the drive itself.

With the first retrofit strategy, the modules could accept only two servo drives. Each loom, therefore, would have needed another module for each pair of drives, in addition to the drives.

“If we have a drive failure, a technician will be able to replace the drive and program it from the HMI,” adds Owens. No special skills for downloading programs from a personal computer (PC) are necessary. “This will save me from getting phone calls in the middle of the night.” This feature has yet to be used, however, because none of the drives have failed since the first machine was retrofitted a year and a half ago.

Mechatrolink is certainly not the only open digital fieldbus that is fast enough for motion control. Builders of numerically controlled machines, for example, tend to favor another called SERCOS (Serial Real-time Communication System). “This fieldbus is known for its strength in the area of motion control and electrical noise immunity due to the fact that it uses fiber optic cabling,” says Skip Hansen, I/O systems product manager at Beckhoff Automation LLC, a controls and automation supplier based in Burnsville, Minn.

He points to CBW Automation, of Fort Collins, Colo., a designer, builder, installer and troubleshooter of high-speed robots and downstream automation for the plastics injection molding industry. The company uses dual-channel SERCOS PCI (Peripheral Component Interconnect) cards in Beckhoff industrial PCs to pass data between servo controllers and bus input/output terminals. For I/O, a bus coupler links eight-channel digital input terminals and eight-channel digital output terminals.

Open Networks

The benefits of the SERCOS-based fieldbus fall into basic categories. First is communications speed. CBW’s customers can scan programs in two msec now, rather than 27-35 msec as they did before. Data transmission speeds are up too. “A 0.1-second change in transmission can mean several thousand dollars to our customers,” says Steve Corwine, at CBW Automation. “It adds up over the course of a day.” He notes that cycling at 5.4 seconds, instead of 5.5 seconds, can mean about 18,500 more parts in 24 hours for a process capable of producing 64 parts per cycle.

The second basic benefit of the SERCOS fieldbus is the elimination of hard wiring between devices. Not only does the builder save money on the associated materials, but it also saves on labor in both assembly and installation start-ups. Because of the Sercos fiber-optic fieldbus and Beckhoff’s distributed I/O terminals, “we have seen about 30 percent labor savings in the manufacture of our systems and 50 percent labor savings in start-up,”
says Corwine.

Another open digital fieldbus being deployed for motion control is Profibus—a network having a user group of more than 1,400 member companies. Like Mechatrolink, it simplifies automation projects. “It supports the requirements of motion control, standard I/Os, and safety I/Os,” explains Jeremy Bryant, a networking specialist at automation vendor Siemens Energy & Automation Inc., of Alpharetta, Ga. “You can run one cable for everything.”

Having only one network to engineer and maintain can save a lot of time and money. Consider just the safety network. “You had to buy safety relays, blocks and contacts in the past,” says Bryant. “Now, it’s in a standard I/O rack with just a safety module with the rack.”

Just as important to DaimlerChrysler Corp. was the ability to install the network in stages when it came time to upgrade the body shop at its Eurostar works in Graz, Austria. Unlike its U.S. counterparts, this plant produces a wide variety of models, each built in much lower volumes than in North America. The line had to continue producing both the current model and various pre-production vehicles. Consequently, a variety of old and new technologies had to work with one another during the installation.

Crucial to the gradual retooling was a distributed architecture built around a Siemens Simatic S7-400 master controller with two Profibus interfaces. Through the first interface, Profibus-DP connects the controller to the distributed I/O devices, Micromaster frequency converters for roller conveyors, and Master-drives for various lifting and swiveling mechanisms. Body-in-white, final assembly and other areas that have more than one controller use DP/DP-couplers to allow the DP networks to exchange data.

The master controller’s second interface connects it to a separate Profibus network that creates a link to the operator control and monitoring terminals. This redundant visualization system allows workers in any area of the body shop to access relevant data from other areas.

Ethernet Is Everywhere

A number of vendors are basing their fieldbuses upon Ethernet, perhaps the most ubiquitous computer network used today. The problem with using this approach for motion control, however, has been that the  speed has been inadequate. Vendors typically need to modify the network to get the necessary performance.

An example is SynqNet, an all-digital motion control network developed by vendor Motion Engineering Inc. (MEI), of Santa Barbara, Calif. This high-speed motion network is based on the physical layer of Ethernet, but it also uses MEI’s data layer and time stamps to streamline communications and deliver data to the drives on it in a deterministic manner. These synchronous communications eliminate most of the jitter that can occur with the isochronous communications defined by the original standard.

The precision and speed offered by this design was crucial for Asyst Technologies Inc., of Fremont, Calif., a builder of equipment for the semiconductor industry. The high value of its customers’ products multiplies the effects of any inefficiency and error in their manufacturing processes. Consequently, Asyst’s engineers would need a digital motion fieldbus if their new combination equipment front-end module (EFEM) and wafer sorter was going to be faster and more efficient than previous models, yet occupy only half the space.

The innovative design needed a motion controller capable of executing the robot’s programmed path and interacting reliably with vision systems, I/O and other feedback devices. The engineers specified an XMP-SynqNet-PCI-based controller to coordinate the axes and custom dual-axis SynqNet servo drives from the Kollmorgen Servotronix Group of Danaher Motion, in Wood Dale, Ill.

Besides offering high-speed, synchronous servo update rates, the network also gives them the ability to have PC-based centralized control, execute client-enabled motion, download firmware configurations and perform remote diagnostics. It also lets the engineers use Danaher’s sophisticated programming, tuning and testing software. “I could excite any axis or two axes simultaneously and see the frequency response of the system,” says Mike Krolak, senior director of controls technology at Asyst.

Past Propriety Protocols

Developments have made Ethernet suitable for motion control—without the proprietary protocols that have been necessary in the past, according to Matheus Bulho, product manager for motion control at Milwaukee-based vendor Rockwell Automation Inc. This means that users should be able to simplify their network architectures with off-the-shelf technology, rather than deploying different networks for motion control, sensors and enterprise communications.

One of these enabling developments is the mechanism for prioritizing packages sent over the network. Another is switching technology that manages the traffic to allow sending and receiving data at the same time in a true duplex fashion. Older Ethernet duplex technology either sent or received data. The last of the developments is a synchronous clock defined by the IEEE 1588 standard promulgated by the Institute of Electrical and Electronics Engineers. “It allows us to synchronize nodes on the network within 200 nanoseconds,” reports Bulho.

Rockwell’s EtherNet/IP fieldbus made strategic sense for W&H Systems Inc., a Carlstadt, N.J.-based supplier of merchandise sorters and other handling equipment for retailers and distribution outlets. “When Rockwell announced its Ethernet-based medium for controlling I/O, we looked into replacing our Profibus technology with it,” says Kevin Kiefer, senior controls engineer at W&H Systems.

The handling-equipment builder had been growing and attracting larger customers that needed bigger conveyor-based systems with hundreds of lanes. “As our sorters became larger, it became more difficult to make our Profibus network fast enough to recognize changes in device states,” explains Kiefer.

The engineering staff tested the new fieldbus initially on a small version of the company’s Reliable Sorting Unit (RSU). The real test, however, came when a huge mail-order house needed an RSU-based sorter with more memory and features than any that W&H had built before. The builder would not have attempted the project with Profibus technology because the program size and physical network medium would have made the device update times too slow to track and sort merchandise correctly, according to Kiefer.

The sorter’s enclosed-track conveyor would have to circulate 615 tilted carrying trays among 330 diverting chutes. A scanner would read the bar codes of the products in the trays, so the appropriate diverter could direct them to holding bins for delivery. “The new automated system needed to be able to track materials throughout the process at rates up to 50 percent faster than the previous system,” says Kiefer. Yet, it had to fit in the same 6,400-square-foot space and provide the house’s maintenance crew easy access to the mechanical components.

The Ethernet-based fieldbus did the job, communicating 10 times faster than the old network. Not only can the new sorter handle 120 pieces per minute, but its simpler architecture also cut engineering and programming costs by 40 percent and installation time by 20 percent. “We came in early,” says Kiefer. It took only two weeks to put this network into motion.

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