The Long-Term Strategic Importance of Selecting The Most Viable Ethernet Fieldbus Standard
Selecting The Most Viable Real-Time Ethernet Fieldbus Standard
The Evolution of Real-Time Ethernet Fieldbuses
For many years, Ethernet and the TCP/IP protocol have been used in the manufacturing arena to network control systems, management systems, and manufacturing cells on a manufacturing shop floor, but not for the controlling communications inside the actual machines and equipment. The machine controller itself and the communications to the actuators invariably demand the use of deterministic fieldbus, so TCP/IP is not suitable. Trying to use traditional TCP/IP protocol from machine control to the sensors and actuators has failed due to the inability to satisfy deterministic, real-time demands. It is simple as that.
However, the machine-builder industry (companies that build equipment like CNC, semiconductor, medical imaging, test bench, etc.) could see the value of reusing the hardware components typically used in a TCP/IP network setting. The explosive growth of the Internet created a universal standard for communication cards and cabling. A network interface card (NIC) and a TCP cable cost a mere fraction of the cost of an industrial fieldbus cable and DAQ card. Reusing this hardware could save companies 50% of the cost or more versus a traditional proprietary fieldbus configuration.
The economics of adopting Ethernet as a fieldbus are compelling because Ethernet components offer dramatically lower costs and are universally available. Relying on a proprietary fieldbus, a robot manufacturer, for example, must purchase the entire proprietary motion assembly from a single vendor: IO card at up to $400, proprietary cables at up to $30/linear foot and servo drive and motor at a premium. By using Ethernet-based standard protocol, the IO card can be replaced by the onboard NIC card ($0 additional cost) that comes installed on the PC, the proprietary cables can be replaced by inexpensive CAT5 cables, and the servo drives will be dramatically lower if the standard is strong enough to support multiple vendors. Equipment assembly for Ethernet components is much simpler too. Rather than having cable harnesses that are 4 inches in diameter at the PC interface, a simple CAT5 cable similar to the one that connects to your home PC is far more manageable. These are just a few of the economic benefits for Ethernet.
The industry soon realized that while TCP/UDP/IP protocol could never deliver the real-time, deterministic response required by industrial machine controllers, the ubiquitous and inexpensive hardware – network interface card (NIC) and CAT5 ethernet cables –could be used to deliver the real-time response required by machine control applications. All that was needed was a new real-time protocol that was designed from the ground up to use the physical layers of the hardware, but could deterministically connect and communicate the machine controller to all the sensors and actuators in a machine. In some cases, companies could potentially use some augmenting hardware that delivered the needed determinism.
Since 2001, reputable industrial titans have introduced no less than five different Industrial Ethernet real-time protocols. They have been promoted as potential standards that would enable the reuse of the Ethernet protocol or Ethernet hardware, and promise to lower the cost to build a real-time controller for equipment. This proliferation of so many potential standards stems mainly from the fact that there are clearly different technical approaches to making it possible to reuse Ethernet hardware components like NICs and CAT5 cables to dramatically lower the cost and improve the performance of a machine, while making it possible for messages to be capable of supporting real-time, deterministic applications.
There is another reason that so many potential standards were being promulgated. Leading industrial control manufacturers were attempting to capitalize on the cost savings from Ethernet by extending their own standard to include Ethernet. In doing so, their customers receive some of the benefit from Ethernet but are still locked into the proprietary networks for the long term. Frequently forgotten in these discussions is the fact that it’s not just technical properties such as performance and transfer rates that count, it’s also the soft facts like easy implementation, openness, independence, risk avoidance, conformity, interoperability, long-term availability, and overall distribution that makes a standard gain acceptance and even thrive.
Next, we will examine the key features of the following five real-time Industrial Ethernet protocols, measure their efficacy as a standard real-time Ethernet-based fieldbus, and predict relative long-term viability:
The five:
EtherCAT
EtherNet/IP
Ethernet POWERLINK
PROFINET IRT
SERCOS III
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