Three Reasons to Use Ethernet for Motion Control

Ethernet is becoming an increasingly important network in industrial applications.

When it comes to motion control, Ethernet, fieldbuses, and other technologies such as the Peripheral Component Interconnect have historically competed for the right to handle some of the most demanding workloads in industrial automation and control systems. Motion control applications require determinism (guarantee that the network can deliver the workload to the intended node in a timely manner), which is necessary to ensure position maintenance, which in turn ensures precise stopping of the drive, proper acceleration/deceleration, and other tasks.

Standard IEEE 802.3 Ethernet never met this requirement. Even though full-duplex switching and isolated collision domains eliminated the outdated CSMA/CD data link layer, it lacked predictability. In addition, the high complexity of TCP/IP in the typical stack was not optimized for reliable delivery of real-time traffic. Therefore, fieldbus and PC control architectures with ASIC-based PCI cards have been the common motion control solutions.

Ethernet solutions, from EtherNet/IP® to EtherCAT®, overcome these shortcomings in their own unique ways. While Industrial Ethernet has some other advantages over alternative technologies, it is far from dominant in motion control. Let’s look at three reasons why it can and will become increasingly accepted in the competition in the coming years.

Integrate rather than add complexity

Over time, the interconnection between enterprise IT and the factory floor has increased, resulting in more complex systems that often mix standard Ethernet and Industrial Ethernet with fieldbuses. For example, a machine might utilize:

• SERCOS 1 for communication with servers

• PROFIBUS® for networked variable frequency drives

• SafetyBUS p for fail-safe fieldbus communication

• For DeviceNet connection to sensors

• Ethernet for sending data to end users, accessed through a gateway

Such networks are complex, expensive to set up and maintain. Each protocol requires its own implementation procedures, installers and training. In contrast, Ethernet offers the possibility to converge different networks for motion, safety, etc. onto a cost-effective infrastructure that is easier to wire, widely supported by vendors and adaptable to future requirements.

Ethernet provides the possibility of integrating different networks.

The EtherNet/IP protocol exemplifies how convergence can be put to good use in practice. By using standard Ethernet technologies such as TCP/IP and UDP/IP, supplemented by features such as CIP Sync (for IEEE 1588 Precision Time Protocol synchronization of distributed clocks), integrated switched systems can accommodate both commercial and industrial applications.

Determinism for motion control applications

Motion control relies on precise communication. This precision is supported by using slot-based scheduling, where each device has a schedule for communicating with other devices in the scheduling strategy. These servo drives and controllers calculate their respective timings, from which the ΔT value of the control function can be calculated. However, if the data transmission becomes unpredictable, the results may be lost, so determinism is needed to ensure the stability of the loop.

Ethernet can support demanding motion control applications in factories

The implementation of IEEE 1588 in EtherNet/IP, in some cases through accelerator circuits integrated directly into Intel® silicon, is just one common mechanism used by Ethernet solutions to enforce determinism. EtherCAT’s high-speed real-time processing is another example of how consistent and predictable performance can be achieved in motion control applications. EtherCAT overcomes the strict physical limitations of centralized PCI-based communications, which require fast communication between the machine processing unit and the servo processors but keep the distance short.

“The wide bandwidth, synchronization, and physical flexibility of a distributed processor architecture based on EtherCAT can match the functionality of centralized control with the benefits of a distributed network,” Jason Goerges explained in a 2010 Machine Design article. 3 “In fact, some processors using this approach can control up to 64 highly coordinated axes (including position, velocity, and current loops, as well as commutation) with sampling and update rates of 20 kHz.”

Long-term viability for IIoT

Ethernet has evolved since its inception as a local area network technology. Given that traditional fieldbus components are currently manufactured on a small scale and PCI is facing the risk of becoming an obsolete industrial standard architecture, Ethernet has continued to evolve and is now fully capable of serving the IP-centric Industrial Internet of Things.

Upcoming improvements, such as time-sensitive networking that will complement IEEE 1588 and support the possibility of network convergence, also make Ethernet an ideal choice for current and future motion control. This is not to say that fieldbuses and PCI will die, but as the automation industry moves toward IIoT, Ethernet's advantages will continue to grow.

references

1 Paul Brooks. “EtherNet/IP for Motion Control,” Industrial IP Advantage, October 2015

2 “Successful Motion Control over Standard Ethernet” Industrial Ethernet Book, Vol. 48, No. 71

3 Jason Goerges. “EtherCAT Enables High-Performance Motion Control.” Machine Design, November 2010.