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 ...