Applications Bring Credibility to Digital Buses for Motion Control

By Dan Hebert, PE, Senior Technical Editor

Before an automation technology matures, standards, predicted performance and hypothetical cost savings are discussed. Machine builders, robot builders and system integrators typically don't see eye-to-eye with suppliers at this stage of development, mainly because suppliers are touting benefits with few real-world application examples to back up their claims.
That described the state of digital motion buses in 2007, when relatively few motion bus applications existed. Two years later, the landscape has changed dramatically. While there still is confusion on the standards front, the arguments about benefits and costs have crystallized based on application experience.

This has created unanimity of opinion among machine builders, system integrators and suppliers. Most everyone now agrees that digital buses save money and improve performance when compared to their hardwired alternatives. There is also a general consensus that these benefits don't come easily, although implementation is not as difficult as it was in the early days of the technology pioneers.

Wires Everywhere

Few if any automation components have wiring requirements as extensive as a motion controller. From the central controller to the motion controller, discrete signals can include start, stop, reset, table reference pointers and other signals. Analog outputs are typically speed set point and analog type.

From the motion controller to the central controller, discrete signals can include general alarm, alarm codes, executing and position complete. Analog inputs are typically actual speed, torque and other information-based values.

“Digital buses save money and improve performance when compared to their hardwired alternatives. ”

Twenty wires and cables could run between the central and the motion controller. Multiply this wiring by the number of motion controllers within a single machine or robot or material-handling system, and wiring requirements quickly become daunting.

Enter the digital motion bus, which replaces scores of hardwires with one cable—a nice feature for simple systems, but almost a necessity for large and complex machines with many motion controllers.

Wires Be Gone

"Digital buses are an excellent way to communicate between devices in a larger motion control system where hardwiring would result in unneeded complexity or would require more I/O than is feasible," says Jason Woyak, electrical engineer with Advanced Machine Automation (www.advancedmachineauto.com) in Birmingham, Ala. "The sheer volume of data that can be transferred over a digital bus makes it superior to hardwiring," adds Woyak.

Advanced Machine Automation makes a wide variety of machines including presses, feeders and stackers. It also builds custom machinery ranging from a countertop sawing machine or a fire extinguisher filling station to a gluing machine for truck axles.
One vendor agrees with Woyak about wiring savings and also cites other reasons to go digital. "In systems with many connected devices, multi-drop digital interfaces such as Ethernet offer much lower cost of manufacturing than point-to-point wiring schemes," notes Bill Savela, marketing manager at Delta Computer Systems (www.deltamotion.com). "Digital networks have fewer interconnections, and that makes them more reliable since each electrical contact is a potential failure point. When hardware problems do occur, it's much easier to debug and fix systems that are network-based, as compared to those using point-to-point or analog connections."

The maintenance problems solved by motion networks are of particular importance to machine builders. "The reduction in wiring gained by going digital produces a higher MTBF because there are fewer terminations and therefore fewer points of failure," observes Jim Mullins, vice president of engineering at Integrated Motion (www.integratedmotion.com), a system integrator based in Greensboro, N.C.

Many of the systems designed by Integrated Motion use components from B&R Industrial Automation. Not surprisingly, both integrator and supplier are on the same page when it comes to digital bus benefits. "Replacing hardwired control circuits between machine controller, motion controller and drives with a digital bus greatly reduces the wiring, eliminates potential points of errors and increases noise immunity," says Robert Muehlfellner, director of automation technology at B&R (www.br-automation.com).

Features Abound

When things go wrong with a hardwired system, it can be a real challenge to find the source of the problem. For many, the main benefit of going digital is better diagnostics. "With our digital network, the diagnostics at the operator station are outstanding," exclaims Rick Moscarino, vice president of engineering at Bardons & Oliver (www.bardonsoliver.com) in Solon, Ohio.

Bardons & Oliver makes CNC turning machines and rotating head cutoff machines (Image 1) and also is a robotics integrator. "Application status, including motor loads, speeds and warning messages, are at the operator's fingertips," adds Moscarino. "Troubleshooting is much more intuitive, saving time and money for both the end user and OEM. Best of all, this diagnostic information can be integrated into the control system."

Setup can be time-consuming, explains Moscarino. "But as our technicians become trained and as they gain experience, this has become less of an issue," he says. "In the case of a field failure, the restarts have been fairly simple, and we typically can support end users via phone."