As CNC machines become increasingly more integrated into the shop by communications networks at the management level, they also are becoming more integrated at the I/O device level by higher-level bus communications networks. The larger and more complex the machine, the bigger the benefits when using bus communications networks to connect machine I/O devices to the CNC.
Golden E. Herrin
As CNC machines become increasingly more integrated into the shop by communications networks at the management level, they also are becoming more integrated at the I/O device level by higher-level bus communications networks. The larger and more complex the machine, the bigger the benefits when using bus communications networks to connect machine I/O devices to the CNC. The benefits are even greater if the CNC machine is integrated into a cell, if there is a greater variety of devices and if the devices are farther apart.
DeviceNet is one of three higher level protocols based on controller area network (CAN), a bus communications architecture technology developed during the mid-1980s by Robert Bosch GmbH of Germany. CAN's original purpose was to simplify wiring in automobiles and trucks. DeviceNet, smart distributed system (SDS) and CAN Kingdom are all three higher level bus protocols based on ISO 11898 CAN communications protocol. DeviceNet was introduced by Allen-Bradley in 1994. The company developed the application and physical layers, eventually making the specification available to industry.
DeviceNet has emerged as a low-cost, open communications network for connecting intelligent industrial devices such as limit switches, photoelectric sensors, bar code readers, valves, motor starters, variable frequency drives, process sensors, panel displays, operator interfaces and more into a control system. All connections to the devices, including both signal and power, are made through a single four-wire cable system that eliminates expensive wiring, labor and troubleshooting associated with large bundles of wires in hardwired systems. Combining improved communications technology with interchangeability of like components from multiple vendors, industrial devices and device-level diagnostics (which are either not available or at best not easily accomplished in hardwired device) has led DeviceNet to be used extensively in the United States. It also has gained rapid popularity in Europe.
One of the main reasons attributed to its universal acceptance is the formation of the Open DeviceNet Vendors Association (ODVA). This is an independent supplier organization formed in 1995. It now consists of 280 member companies that manage the DeviceNet specification, and support its worldwide growth. ODVA works with vendors providing assistance through developer tools, developer training, compliance testing and marketing assistance. ODVA also publishes a DeviceNet product catalog and supports vendor special interest groups (SIGs) that develop device profiles for specific classes of products.
The SIGs play an important role in keeping the suppliers of DeviceNet products in tune with the needs of industry. As an example, ODVA recently announced the formation of an Alternate Transport SIG to tackle migration of DeviceNet to alternate transport mediums such as TCP/IP, EtherNet, and Asynchronous Serial Links. Companies participating in this task are Control Technology, Cutler-Hammer, Contemporary Controls, DVT Corp., General Motors, OMRON, National Instruments, Rockwell Automation/Allen-Bradley, and University of Warwick-UK.
In addition to SIGs, another strategy that ODVA credits for helping make DeviceNet a success is the number of outside sales professionals working in a wide variety of sales channels to sell DeviceNet. ODVA founding member companies (Rockwell Automation, Cutler-Hammer and OMRON) collectively have over 10,000 outside sales professionals with an additional 5,000 coming from 200 other North American companies marketing DeviceNet. This brings the total sales personnel trained and authorized to sell DeviceNet to over 15,000. According to ODVA, DeviceNet's closest open bus network competitor is Profibus with an estimated 1,000 sales personnel in both its direct and independent sales channels.
The trend is very clear that CNCs will be required to connect to intelligent devices. Will users and machine tool builders all agree on a single protocol for connecting intelligent devices? Probably not. Therefore, the control builder who supports both DeviceNet and Profibus in its controls will enjoy the widest market, and those who have PC-based controls have the best shot at making this a reality.